596 DentalUpdate July/August 2017
Dental Implants: An Overview
Abstract: Dental implants are widely used and are considered to be one of several treatment options that can be used to replace missing
teeth. A number of implant-supported treatment options have been used successfully to replace a single tooth and multiple teeth, as well
as a completely edentulous jaw. However, as the number of patients who have dental implants is increasing, dental personnel are more
likely to see patients with implant-supported restorations or prostheses. Nevertheless, dental implants may fail as a result of mechanical
complications, such as screw loosening or due to biological causes like peri-implant diseases. As a result, dental personnel should be able
to recognize these complications and the factors that have negative effects on the success of such implant-supported restorations or
prostheses. Therefore, a basic knowledge of dental implants is necessary for every dental student, hygienist and dentist.
CPD/Clinical Relevance: Maintenance of implant-supported restorations and prostheses requires long-term follow-ups. It is the
responsibility of the patient to maintain good oral hygiene and also of the dental personnel who look after the patient to ensure a durable
restoration and prosthesis.
Dent Update 2017; 44: 596-620
implants is known as peri-implant tissue
and is comprised of soft (mucosa) and
hard (bone) tissues. The peri-implant
soft tissue has similar features to the soft
tissue that surrounds teeth.
of a junctional epithelium and connective
tissue. The junctional epithelium is
attached to the implant and/or abutment
surface through a hemi-desmosomal
attachment. Connective tissue is present
apical to the junctional epithelium and
coronal to the crest of alveolar bone.
Connective tissue fibres are found to be
positioned close to the implant surface
but not attached to it, and predominantly
arranged in a circular manner. Connective
tissue fibres also arise from the crest of
alveolar bone and from the periosteum
and are oriented parallel to the implant/
abutment surface and extend towards
the oral epithelium. Thus, the junctional
epithelium and connective tissue form
a protective seal between the oral
environment and the peri-implant bone
which plays a vital role in the success
of the implant treatment outcome. The
junctional epithelium and the connective
tissue are collectively known as the
biologic width, which is comparable to
that found around teeth.
Abdulhadi Warreth, BDentSc,
MDentSc(TCD), PhD(TCD), MFD RCSI,
Department Restorative Dentistry,
Ajman University, Al–Fujairah Campus,
United Arab Emirates, Najia Ibieyou,
BDentSc, MDentSc(TCD), PhD(TCD),
Postgraduate student, Institute of
Molecular Medicine, Trinity College,
Dublin, Ronan Bernard O’Leary,
Fifth Year Dental Science, Matteo
Cremonese, Third Year Dental Science,
Dublin Dental University Hospital,
Trinity College, Dublin and Mohammed
Abdulrahim, BDentSc MDentSc(TCD),
PhD(TCD), Oral Medicine Department,
Faculty of Dentistry, Benghazi University,
integration is influenced by several factors,
such as implant material, bone quality
and quantity, and the implant loading
As the use of dental implants
has become much more common, dental
personnel are more likely to see patients
who have implant–supported/retained
restorations. Nevertheless, dental implants
are affected by diseases in a similar manner
to teeth and may also fail after several
months or years in service.
it is not unreasonable to suggest that
the implant and the peri-implant tissue
should be examined on a routine basis in
a similar manner to that which is carried
out for periodontal examination.
a deviation from the norm is found, the
treatment may be carried out in practice or
by a specialist, depending on the severity
of the condition. Accordingly, the dentist
should be equipped with basic knowledge
of dental implants. Hence, it is the aim of
this article to provide this basic information
which is needed by every dental student and
Implant-soft tissue interface
The tissue that surrounds
Dental implants (also known as oral or
endosseous implants) have been used to
replace missing teeth for more than half
a century. They are considered to be an
important contribution to dentistry as
they have revolutionized the way by which
missing teeth are replaced with a high
This success depends on the
ability of the implant material to integrate
with the surrounding tissue. However, this
Najia Ibieyou, Ronan Bernard O'Leary, Matteo Cremonese and Mohammed Abdulrahim
July/August 2017 DentalUpdate 597
Implant-bone interface and
For dental implants to succeed,
intimate contact between the peri-implant
bone and the implant surface should
be achieved and maintained. Therefore,
an integration between the implant
surface and the bone is required for
the success of any implant system. This
integration is known as osseointegration,
and is defined as a direct structural
and functional connection between
ordered living bone and the surface of
a load-carrying implant.
microscopy, successful osseointegration
shows direct apposition of bone on
implant surface (Figure 1). However, when
the bone-implant interface is examined
using electron microscopy, the implant
surface is found to be separated from
the surrounding bone by an amorphous
layer, a granular electron-dense layer, or a
layer of uncalcified collagen fibrils
a thickness that ranges from 100 nm to
Nevertheless, this layer appears
not to have a negative impact on the
success of the osseointegration. Inversely,
when the connection between implant
surface and bone is mediated by a layer of
connective tissue, osseointegration fails to
It is important to mention that,
as a result of the absence of periodontal
ligaments between the implant and its
surrounding bone, when the implants are
loaded, they move within the bone due to
bone elastic deformation.
osseointegrated implants cannot be
Several factors are
reported to play a role in obtaining
As an example, poor
bone quality was found to be associated
with a high implant failure rate when
compared with bone of a high quality.
Clinical studies have reported that dental
implants in the maxillary arch (especially
for the posterior maxilla) have lower
survival rates than those in the mandibular
This is usually attributed to the
differences in bone quality between
the two arches.
Bone quality, as
classified by Lekholm and Zarb,
based on radiographic assessment as
well as resistance during the implant
drilling procedure. Accordingly, bone is
categorized into four classes, as described
in Figure 2 and Table 1. Some factors which
affect osseointegration are discussed below
and summarized in Table 2.
Implant placement methods
Surgical implant placement may
be carried out in one- or two-stage methods
(Figure 3). The one-stage method is also
known as the non-submerged method.
Using this technique, the bone is prepared to
receive the implant. The implant is fitted into
the prepared bone (osteotomy). However,
the coronal part of the implant is kept
above the bone crest, protruding through
the soft tissue, and is exposed to the oral
environment during the healing stage.
restoration can be attached immediately
after the implant placement surgery or may
also be delayed.
The advantages of the one-stage
The avoidance of a second surgical
The lack of a micro-gap between the
implant and the abutment at the alveolar
bone crest level, resulting in a less crestal
The prosthetic procedure is simplified and
less chair time per patient is required; and
A non-loaded, immediate, or delay-
Table 1. Classification of bone according to its quality.21
Type I: almost the entire bone is composed of homogeneous compact bone;
Type II: a thick layer of compact bone surrounds a core of dense trabecular bone;
Type III: a thin layer of cortical bone surrounds a core of dense trabecular bone; and
Type IV: a thin layer of cortical bone surrounding a core of low density trabecular bone.
Figure 1. A histological image of bone-implant
interface. Bone formation around the implant
labelled with different chelating agents (fluoro-
chromes). The implant is the large black area.
Figure 2. The classification of bone according to its quality: Class I (A), Class II (B), Class III (C) and Class
598 DentalUpdate July/August 2017
undue trauma which can negatively affect
the healing. However, bone with optimum
quality and quantity is a prerequisite for
this method to be used. Nevertheless,
the method can be clinically successful.
Examples of the implants that can be placed
using the one-stage technique include
the Solid-Screw Implant®
UK, Crawley, W Sussex), AdVent
(Zimmer, FLA, USA) and Single-stage Implant
System® (BioHorizons, AL, USA).
In contrast, the two-stage
method is also known as the submerged
technique (Figure 3). In this method, two
surgical procedures are carried out. The
first surgery involves installing the implant
into the bone, and a cover-screw (also
known as a sealing-screw) is attached to
the implant platform. A countersink bone
preparation that allows for placement of
the implant platform below the bone crest
may be implemented. The countersink
allows the placement of the cover-screw
level with the bone crest. The raised flap is
then repositioned and sutured to conceal
the cover-screw and the implant (Figure
3). After a few months, the second stage
surgery is carried out. In this stage, the
implant site is re-opened, the cover-screw is
accessed and then replaced with a healing
abutment, which is also known as a sulcus
former or transmucosal abutment (Figure 4).
Afterwards, the healing abutment is replaced
with a provisional or final restoration. This
surgical protocol is suitable for use when the
quality of bone is not optimum and when
bone graft materials are used in conjunction
with the implant. Examples of an implant
system used for the two-stage procedure
include the Fixture MK III®
Uxbridge, UK), MAX 2.5®
Implant (Bicon Inc,
Boston, MA, USA) and OSSEOTITE®
Implant (BIOMET 3i, Maidenhead, UK).
It is important to mention that
the cover-screw is used to prevent tissue
growth into the implant or over its platform.
It is attached to the implant using a screw-
driver with a light finger force. It is essential
to confirm that the cover-screw is fully
seated and no gap is left between the cover-
screw and the implant platform. The cover-
screw has a low profile which facilitates
the suturing procedure and allows the two
edges of the cut mucosa to be brought
close together without undue tension. If
there is too much tension, it may deteriorate
and preclude the healing.
healing abutment has a high profile and
protrudes through the peri-implant mucosa
to the oral cavity. Therefore, the healing
abutment is available in different lengths,
depending on the distance between the
implant platform and the surface of the
peri-implant mucosa. It is also available in
a variety of diameters, which is selected
according to the implant diameter. The
cover-screw and the healing abutment are
shown in Figure 4.
Implant stability (lack of mobility)
is divided into primary and secondary. The
primary, also known as initial stability, is
achieved during implant placement surgery.
It is believed that primary stability plays a
vital role in reaching osseointegration, upon
Table 2. Some factors affecting osseointegration.
Bone quality and quantity
Implant surface macro-structure
Implant micro-structure (roughness)
Heat generation during the implant
Implant primary (initial) stability
Figure 3. A schematic presentation of an implant placed according to the one-stage (left) and two-
stage (right) implant placement methods. Note the transmucosal part (the neck) penetrating the peri-
implant mucosa in the one-stage method.
Figure 4. An image of a cover–screw (left) and healing abutments (middle and right)
loaded protocol can be implemented.
One of the drawbacks that
may be associated with this surgical
protocol is that the implant is exposed
to the oral environment, which may lead
to contamination of the surgical site.
Furthermore, the implant may be exposed to
July/August 2017 DentalUpdate 601
which secondary stability depends.
Implant stability is produced
by close contact between the implant and
the host bone. The factors that may affect
primary stability may be categorized into
three factors; those related to surgical site
(local) or related to implant or surgical
method used in placement of the implants.
Local factors, such as bone quality and
volume, may affect the degree of bone-to-
implant contact and consequently affect
primary stability. As an example, larger bone-
to-implant contact fractions were observed
in bone sites of higher density. The implant
factors include shape, length, diameter
and surface texture. For instance, tapered
implants lead to higher insertion torque
values than cylindrical implants, which
was considered to be due to the greater
frictional surface of the tapered implants
and associated with high primary stability
(see below). A surgical technique, such as
that which leads to bone condensation
during implant placement surgery or a
mismatch between the osteotomy and
implant diameter (with the implant diameter
being slightly greater than the osteotomy),
results in satisfactory primary stability.
Also, the use of implants with self-taping
blades results in a lower primary stability in
medium-density bone when compared with
those without such blades.
issue is contradictory.
Secondary stability represents
integration of the implant as a result of new
bone formation through its remodelling.
Therefore, this stability depends on bone
activities and factors that influence such
activities throughout the patient’s life.
The general consensus is that peri-implant
bone is in a continuous active remodelling
state which maintains osseointegration and
provides secondary stability.
It is important to mention that,
when the implant is inserted into the host
bone, spaces may exist in the bone-implant
interface. These spaces are initially filled
with blood that comes from injured blood
vessels, forming a fibrin network which is
the important step towards the formation of
Dental implant types
In the worldwide market,
there is a wide range of dental implant
systems available, but only a few brands
are American Dental Association (ADA)
approved. The most commonly used implant
systems include Nobel Biocare, Straumann,
AstraTech, Bicon, BioHorizon, BIOMET 3i,
Intralock, and Zimmer. All are constructed
on the same basic concepts but there are
differences in the patented technology and
In general, dental implants may
be classified as a one- or two-piece implant.
The one-piece implant
In the first type, the implant and
the abutment are formed as a single solid
unit. In this case, there is no screw-joint
between the implant and the abutment.
The lack of a screw-joint is considered an
advantage as there is no screw-loosening,
dangerous fracturing or micro-motions
between the abutment and the implant.
The one-piece implants may be used when
narrow implants are indicated, such as in the
replacement of the maxillary lateral incisors
and lower incisors, or when bone volume
is limited and the use of standard implants
is not suitable. These types of implants are
installed only with the one-stage implant
placement method. Examples of a one-
piece implant are the one-piece 3.0 Dental
(BioHorizons) and Y-TZP Ceramic
Implant® (Nobel Biocare).
The two-piece implant
The two-piece implant type
consists of an implant to which an abutment
or a restoration/attachment is connected,
usually with a screw. It is more commonly
used than the one-piece implant type. With
this implant type, both the one- and the
two-stage implant surgery protocol can be
Angled implants in which their
coronal part is angled in relation to the
main implant body are also available. These
angled implants are useful in the anterior
region when placing non-angled implants
in their optimum position is not possible. An
example of angled implants is the Co-axisä
implant (the Southern Implants, UK) in
which the neck is at an angle to the long
axis of the implant body. It is useful to use
when the long axis of a prospective implant
is not along the long axis of the potential
restoration. An angled abutment, such as
Regular Neck synOcta® angled abutment
(Straumann), is also available and can be
used to overcome angle mismatching
Implants are also available as
hollow and solid. Hollow implants allow
more contact with bone but are weaker
than solid implants, which makes them
more susceptible to mechanical failure and
fracture. An example of a hollow implant
is the Hollow Cylinder Implants® made by
Straumann and ITI (Basel, Switzerland).
Irrespective of the implant type
and for descriptive purposes, the implant
usually consists of an implant body and
neck. The implant body is the part of the
implant that is buried in the osteotomy.
The coronal part of the implant is denoted
as the neck, through which the abutment/
attachment is connected to the implant.
The coronal part may be smooth (one- and
two-piece) and placed above the crest of the
bone, or roughened (two-piece), in which
the platform is usually placed below or level
with the crestal bone. When the coronal part
is smooth and placed above the crest of the
bone and penetrates peri-implant mucosa,
it is known as the transmucosal part. The
surface of the transmucosal part is usually
highly polished and is available in different
lengths. It may also have a straight or a bevel
profile and may be augmented with micro-
grooves in order to optimize healing around
Placing the smooth (machined)
part of the implant below the bone crest
may lead to its resorption.
fewer crestal bone changes were observed
when the smooth part was located above
the crestal bone level, irrespective of the
implant type; one- or two-piece implants.
Accordingly, it has been recommended that
the smooth-rough border should coincide
with the alveolar bone crest.
Features to consider when
choosing an implant system
Five features can be used to
describe the dental implant body: shape,
surface macro- and micro-structure, length
and diameter. These features are important
when an implant system is chosen.
1. Shape (geometry)
Implant shape may generally
be tapered or parallel (straight-walled).
The tapered type in general has more
primary stability than the parallel type.
602 DentalUpdate July/August 2017
The use of tapered implants results in lateral
compression of bone and increased stiffness
of the interfacial bone, which is reported
to increase the implant primary stability.
Tapered implants were found to require a
higher insertion torque and less insertion
time than parallel implants. A higher
insertion torque gives a better implant
Tapered implants are also
used to avoid damaging the converging
roots of adjacent teeth that bind the
edentulous space and in softer bone, such
as type IV (Figure 2), where primary stability
is not always easy to achieve.
also be used immediately or early after
The use of a tapered
implant with a wide platform achieves
a satisfactory emergence profile of the
2. Surface macro-structure (threads)
The implant macro-structure
is represented as threaded or non-thread
(thread-less). The threaded type is the
most commonly used implant design.
The threads are usually incorporated into
the implant design to improve the initial
stability and dissipate interfacial stress in
a more favourable way. As the threaded
implants provide better mechanical and
biological outcomes, non-thread implants,
such as cylinder (press-fit) implants, are
less likely to be used and are replaced by
the threaded type. Thread features such as
thread depth, thread thickness, face angle,
pitch and helix angle are considered to
be factors that determine the functional
thread surface and affect the biomechanical
load distribution of the implant.
There are three thread shapes
which are most regularly used when a
dental implant is described (Figure 5). These
are V-shaped, square-shaped or reverse
An animal study conducted
by Steigenga and colleagues
the effects of thread type on peri-implant
bone formation. The study showed that
implants with a square thread design had
significantly more bone-implant contact
and greater reverse-torque measurements
than observed when the V-shaped and
reverse buttress thread designs were tested.
A threaded implant may also
be classified as a self-taping or pre-taping
A self-taping implant is an
implant which is designed to make its
own threads as it is being placed into the
prepared osteotomy. On the other hand, in
pre-taped implants, threads are prepared
on the surface of the osteotomy using a
tap drill (taper). The produced threads will
accommodate the threads of the implant.
The pre-taping method is sometimes
recommended, such as in the case of dense
bone (type I and II) (Figure 2). However, pre-
taping implants achieved lower primary
stability than the self-taping implants.
Figure 5. A representation of the most commonly
used implant threads: V-shaped thread (left);
square thread (middle) and a reverse buttress
Figure 6. Bone resorption at alveolar crest occurs after tooth extraction which may preclude the use of
a long implant as the crestal bone has to be trimmed down to maintain at least one millimetre of bone
buccally and lingually at the bone crest region.
Implant length: a long implant should be considered whenever the condition permits.
Implant diameter: ideally, the implant should be approximately the same diameter as
the root of the tooth it is replacing.
a. Wide implant:
i. Poor quality bone;
ii. Limited ridge height with adequate mesio-distal and bucco-lingual width; and
iii. Immediate implant placement (after tooth extraction).
b. Narrow implant:
i. Used to replace maxillary lateral incisors or mandibular incisors;
ii. Limited edentulous space;
iii. Limited ridge width (to avoid ridge augmentation surgery);
iv. When it is not possible to achieve good emergence profile with a wide implant body;
v. Converging adjacent tooth roots.
i. In type IV bone, where primary stability is difficult to achieve;
ii. Narrow or concave bone;
iii. Converging adjacent roots; and
iv. Immediate and early implant placement.
Table 3. Some implant features that should be considered when an implant is selected, and their
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3. Surface texture (micro-structure)
Implant surface texture describes
the roughness of the implant surface.
Therefore, the implant surface is either
smooth (machined) or can be of a variety
of roughness. A rough-surfaced implant
has a larger surface area than that of its
counterpart smooth implant. It is found
to be associated with positive healing of
peri-implant tissue and encourages the
formation of osseointegration.
in surface area distributes forces to which
the implant is exposed in a more favourable
manner. It also provides better primary
stability than that attained when the implant
surface is smooth.
removal torque studies with roughened
implant surfaces have revealed greater
and higher removal
torque values than implants with smoother
In general, two methods for
the alteration of implant surface texture
have been described in the literature:
subtractive and additive methods. In the
subtractive method, the implant surface
is roughened by removal of its surface
materials usually by blasting and/or acid
In the additive method, a
biocompatible material, such as titanium or
hydroxyapatite, is added to the surface
below). Some examples of rough surface
implants include: grit blasting with titanium
oxide produced by Astra Tech (Mannheim,
Germany); Sand-blasted Large-grit Acid-
etched (SLA®) implants from Straumann
(Basel, Switzerland); Acid-etched Implants®
from BIOMET 3i (Florida, USA); and Plasma-
sprayed® (molten titanium sprayed on the
implant surface) produced by Straumann
and Dentsply Sirona Implants (Weybridge,
It is important to note that, if
the rough implant surface is exposed to the
oral environment, it may encourage plaque
accumulation and interfere with its removal,
and subsequently may induce peri-implant
disease (see below).
4. Implant length
Implant length is determined by
the distance between the top surface of the
implant platform and the apex. In general,
the length of the standard implant ranges
from 7−18 mm.
Selection of an implant
of the required length is governed by the
available vertical bone height, width and
quality which will accommodate the implant
(Figure 6). As implant primary stability is a
function of contact between the implant
surface and bone, the longer the implant,
the greater the surface contact and primary
stability. However, the increase in implant
stability does not occur linearly to the
increase of the implant length. For instance,
a 10 mm implant has about 30% more
surface area than a 7 mm implant, while a
13 mm implant has 20% more surface area
than a 10 mm implant.
The bone of the edentulous
ridge may not be sufficient for placing an
implant with the optimum length. Therefore,
several techniques have been suggested to
compensate for the deficiency in the residual
ridge, either before or simultaneously with
implant placement. Among these methods
are guided bone regeneration, block grafts,
sinus lifting procedures, inferior alveolar
nerve repositioning methods, and bone
These surgical methods are
successful and can be used to increase bone
However, they are not without risks
and may lead to several complications and
undesirable treatment outcomes.
encourage the dentist and patient to avoid
such surgical methods and to use short
implants, therefore the implant is installed
with less invasive surgical procedure and
the cost is reduced. Nevertheless, when a
short implant is used, factors that affect the
osseointegration, such as implant shapes,
surface texture, and thread designs, should
be carefully selected to achieve a satisfactory
studies have reported that shorter implants
are unpredictable and fail more frequently
than longer implants.
longer implants had statistically higher
survival rates when compared with shorter
For instance, it has been reported
that survival rates after two years were
93.1% for 5 mm implants and 98.6% for 9.5
Furthermore, short implants
may fail at an earlier stage than standard
as peak failure rates of short
dental implants were 4−6 years, and 6−8
years for the standard implants.
It is important to note that bone
resorption following tooth extraction may
result in the thinning of the alveolar bone
crest, which may preclude placement of
an implant with an adequate length and
diameter, as shown schematically in Figure
6. Therefore, bone mapping and a CT-scan or
Cone-Beam Computed Tomography may be
5. Implant diameter
The implant diameter is
measured from the crest of the widest
thread to the same point on the opposite
side of the implant.
According to the
diameter, implants may be classified as mini
when diameter is ≤2.7 mm; narrow when the
diameter is >2.7 mm but ≤3.75 mm; regular
when it ranges from 3.75−5 mm; and wide
when the diameter is >5 mm.
The implant diameter plays
an important role in the success of oral
implants and has a major impact on the
implant’s ability to withstand occlusal load.
Selecting an implant of a suitable diameter
is governed by the dimensions of the
edentulous space (bucco-lingual and mesio-
distal) (Figure 7), as well as the bone quality.
Moreover, it is also affected by the type of
tooth being replaced.
An increase in the diameter of
an implant is associated with an increase
in its surface area. For instance, increasing
the diameter in a 3 mm implant by 1 mm
increases the surface area by 35% over the
Also, a 3.75 x 10 mm implant
has 61% less surface area than a
6 mm diameter implant of the same
Figure 7. The implant should be placed in the site that was previously occupied by the tooth being
replaced, and surrounded by an adequate amount of bone. Two implants may be used to replace a
molar tooth, which results in the dissipation of the occlusal forces in a satisfactory manner (right).
604 DentalUpdate July/August 2017
length.33 Furthermore, an increase in the
diameter and a change in the threads may
lead to an increase in the implant surface
area of more than 300%. This increase in
the surface area may lessen stresses to
the crestal bone areas and reduce both
crestal bone loss and early loading implant
It is important to mention that,
when the implant is installed, it should be in
close contact with the surrounding bone of
not less than 1 mm thickness on its buccal
and lingual surface, and preferably 1.5 mm
or more between the implant surface and its
adjacent tooth (Figure 7). For instance, when
an implant of 4 mm is selected, the bucco-
lingual and mesio-distal dimensions of the
edentulous space should be a minimum of
6.0 and 7.0 mm, respectively. However, it has
been suggested that, in the aesthetic zone,
maintaining a minimum of 3 mm of bone
between adjacent implants is beneficial,
as bone height as well as the inter-dental
papilla are more likely to be maintained.
Consequently, implants with a smaller
diameter at the implant-abutment interface
may be used when multiple implants are to
The diameter of the roots
is usually estimated at 2 mm apical to
the cemento-enamel junction. With this
measurement, an implant with a diameter
that matches, or is slightly smaller than, the
tooth being replaced is selected. In order
to obtain a restoration with an optimal
emergence profile, the implant platform is
usually placed at about 2 mm apical to the
cemento-enamel of the adjacent teeth. If an
implant is placed deeply below the crest of
bone, the crown height is increased, which
may lead to mechanical failure of implant
components and compromise aesthetic
treatment outcomes. When the implant is
placed more superficially, restoration may be
deemed impossible and aesthetic treatment
outcome is also compromised.
When a molar tooth is replaced,
the use of two implants may be an option, as
dissipation of occlusal loads are favourable.
However, placement of implants close to
each other is associated with difficulty in
obtaining an optimal emergence profile,
interferes with oral hygiene and leads to
chronic inflammation and bone resorption.
Short and wide implants may
be used to compensate for the decrease in
the vertical bone height of the edentulous
space when surgery cannot be considered.
They may also be used when the quality of
the bone bed is not optimal.
can be used to increase implant stability,
thus improving stress distribution within
the surrounding bone.
use of a wide diameter implant may reduce
the stress on the retained screws. Wide
implants are also used for the replacement of
posterior teeth and immediately after tooth
extraction (Table 1).
Several situations do not allow
the use of wide diameter implants
narrow implants are an alternative. For
example, narrow implants are suitable
for replacing maxillary lateral incisors
and mandibular incisors. They are also
suitable when bone quantity is insufficient,
or when the roots of adjacent teeth are
converging. They may also be used with a
removable implant-supported overdenture.
However, the use of an implant with a small
diameter is not without disadvantages,
such as mechanical failure of the implant
component. Furthermore, obtaining a
good emergence profile of the restoration
may also be a problem. Hence, a detailed
examination of each patient’s condition
should be taken before a specific implant is
selected, and alternative treatment options,
such as a fixed (conventional or resin-
bonded) prosthesis, may be considered.
It is important to distinguish
between the implant diameter and platform
diameter as they may not be equal. The
implant platform represents the part of the
implant that is connected to the prosthetic
(abutment) counterpart. Table 3 displays
examples of implant features that should be
considered when an implant is selected.
The most commonly used
materials in dental implants are either bio-
inert, such as commercially pure titanium (Cp
Titanium) and titanium alloy, or bio-active
ceramics such as hydroxyapatite, tri- and
tetra-calcium phosphate and bio-glass.
For more than five decades,
titanium was the most commonly used
material in dental implants due to its bio-
compatibility, as well as its mechanical and
physical properties, such as resistance to
corrosion, high strength and low weight.
Depending on its oxygen content, Cp
titanium may be categorized into four
grades; grade I contains the least oxygen
while grade IV contains the most (0.18%
Titanium alloy consists of 90%
titanium, 6% vanadium, and 4% aluminium
and is classified as grade V.
Titanium is a non-noble metal
which has the ability to form a very adherent
self-repairing and protective surface oxide
layer, which prevents further titanium
corrosion. This layer forms immediately
when the titanium is exposed to oxygen.
The formed oxide layer on Cp titanium is
similar to that which is formed on titanium
Figure 8. A schematic representation of the screw-joint connections: the external connection and the
butt joint (left) and the internal connection and the slip joint (right).
606 DentalUpdate July/August 2017
Titanium dioxide (TiO
) forms the
main constituent of this oxide layer, however,
other oxides, such as Titanium oxide (TiO)
and Titanium pentoxide (Ti
) may also
exist. Incorporation of other chemical
elements, such as carbon, traces of nitrogen
or chlorine, into the oxide layer have been
The release of metallic ions
from the titanium implant surface may
occur and increase as the implant surface
It has been suggested
that ionic release may interfere with the
normal peri-implant bone mineralization
and remodelling, which could lead to
the failure of the implant.
titanium release may induce hypersensitivity
in susceptible patients, which may have an
undesirable impact on implant success.
However, this issue is still debatable
and more clinical and further laboratory
investigations are required.
available literature indicates that Cp titanium
has a long-term successful performance.
In addition, the surface of the titanium
implant, which was previously contaminated
in the peri-implantitis case, was found to
reintegrate with bone which was treated to
remove the contaminant.
Cp titanium and titanium alloys
can make up the entire implant or can be
used as a substrate to which a coating of
bio-active material, such as hydroxyapatite,
To speed up the healing
process and osseointegration, implant
surfaces are coated with ceramics.
ceramics may be bio-active, such as calcium
phosphates, or inert, such as aluminium
oxide and zirconium oxide. Examples of
calcium phosphate coating materials are
hydroxyapatite and fluorapatite.
bio-active ceramics are reported to act as
osseoinductive materials which encourage
and accelerate bone apposition around the
implants. Furthermore, coatings that have
similar properties to that of the extra-cellular
matrix provide a favourable environment for
osteoblasts, osteoclasts and their progenitor
cells, that are responsible for the healing
Therefore, an early and strong
implant stability is achieved and the risk of
implant failure is reduced.
Ceramics are initially used in the
additive methods in which ceramic coatings
are added to the metal implant. However,
high bond strength between the coating
material and the substrate is required to
withstand functional stresses and to avoid
fragmentation of the coating materials.
found that hydroxyapatite mechanical failure
occurs primarily at the interface between
the metal substrate and hydroxyapatite coat
(adhesive failure), irrespective of the implant
design. This may have a negative effect on
the risk for hydroxyapatite-coat degradation
and loosening (delamination) are still a
With improvement in technology,
ceramic materials are extended for use
as implant substrates. This is because
ceramics such the yttrium-stabilized
tetragonal zirconia polycrystalline has
improved mechanical properties, superior
wear and corrosion resistance, with a high
flexural strength. These characteristics
may make them a potential alternative
to conventional titanium implants for
Three types of
zirconia-containing ceramic systems are
most commonly used in dentistry; yttrium-
stabilized tetragonal zirconia poly-crystals,
alumina-toughened zirconia and zirconia-
toughened alumina. However, these non-
metallic materials are expected to replace Cp
titanium and its alloys.
on their systematic review of literature,
Andreiotelli and colleagues
that ceramic, in particular zirconia, implants
are not yet suitable as an alternative to
titanium implants. Nevertheless, they
potentially could be a successful material
for use in implants, but this has not yet
been supported by clinical investigations.
However, ceramics such as zirconia are used
nowadays as abutments and crowns as they
have good clinical outcomes.
It is not unreasonable to
conclude that the prospective implant
should be selected carefully and a restorative
driven approach should be implemented to
avoid an unwanted result.
investigation should be carried out to
guarantee the best possible outcome. The
edentulous area should be viewed in three
dimensions: mesio-distal, bucco-lingual and
corono-apical. The mesio-distal dimension
of the edentulous space should also be
thought of as two interrelated spaces
(inter-radicular and restorative). The inter-
radicular space holds the implant and can
be found between the roots of the two
adjacent. Hence, a precise radiograph image
of the area is important. The restorative
space should be carefully investigated as it
extends between the two adjacent teeth and
accommodates the prospective restoration.
When an implant is put to
function, it is connected with the restorative/
prosthetic components. The connection
type can be classified as internal or external.
In the internal connection systems, the
apical part of the abutment is inserted into
an access hole in the implant platform. In
the external systems, a protrusion located
above the implant platform is inserted into
a recess in the apical part of the abutment
(Figure 8). The connection is also classified as
a slip joint; when there is a space between
opposing mating surfaces, and a friction
fit when such space does not exist. The
connection may be further categorized as a
bevel (conical) joint or a butt joint (Figure 8).
The connection may have
an anti-rotational component, such as
hexagonal, octagonal, cone hex, cylinder
hex, cam tube and pin/slot or be without
an anti-rotational device, such as a cone
(Morse taper). The function of the anti-
rotational component is to stabilize and
prevent abutment rotation.
the connection usually has a screw but is
sometimes screw-less and relies entirely on
the friction fit for its stability, such as Bicon®
dental (Bicon Inc, Boston, MA, USA).
The first implant connection type
used with a dental implant was described
by P-I Brånemark.
It was an external hex,
therefore consisting of six sides, each two
adjacent sides make a 60-degree angle
and had a height of
0.7 mm. The hex was
originally used to carry and insert the
implant into the prepared host bone
(osteotomy). The hex was not aimed for
use as an anti-rotational device, as the
implants were mainly used to restore
completely edentulous dental arches with
implant-supported overdentures with
multiple implants. Consequently, rotational
displacement of the overdenture was not
an issue. However, as the use of dental
implants progressed and extended for use
in replacing single and multiple missing
teeth, the use of a guiding index and an
anti-rotational device is needed. To fulfil this
requirement the original external hexagonal
July/August 2017 DentalUpdate 607
connections were modified and are now
available in different heights including
0.9, 1.0 and 1.2 mm and with various sizes.
Furthermore, several types of internal
connections were also introduced and are
widely used nowadays.
In general, when the connection
is an internal type, the occlusal load is
usually dissipated through the implant
body and the screw is more likely to
be protected from the imposed load.
Loose screws were reported to occur less
frequently with internal connections than
with external ones.
However, the implant
neck should be strong enough to resist
such loads. Nevertheless, when the internal
connection is used with a narrow implant,
the connection is exposed to vertical or
oblique loads. Although the screw itself may
be protected from loading, the implant neck
may not be able to resist such a load and will
as most of the occlusal
forces are transferred to the implant walls.
When the implants and the
restoration/prosthesis are connected
together by a screw, the connection is
known as a screw-joint.
when the single restoration (crown) is screw-
retained, one screw-joint is usually found to
connect the restoration to the implant. When
the restoration is cement-retained, there
is also one screw-joint, but it is between
the abutment and the implant (see below).
The screw-joint is also found with the fixed
implant-supported prosthesis in a similar
way as that described for the cement- and
screw–retained single implant-supported
restoration. In the fixed implant-supported
overdentures (FISOs), there is a screw-
joint between the frame-work and the
implants, whereas in the removable implant-
supported overdentures (RISOs), there is a
screw-joint between the attachment system
and the implant.
The attachment systems
are discussed later in the article. In some
situations when a screw-retained restoration
is used, there may be two screw-joints: one
between the implant and the abutment,
and one between the abutment and the
When the screw is tightened,
there are two opposing forces that act on
the implant platform and the abutment or
restoration/attachment that form the joint.
One of these forces tries to hold the joint
together and is known as the clamping
force. The other force is called the separating
force as it tries to disengage the screw-joint
components away from each other. Hence,
the two forces are acting against each other.
As a tightening torque is applied to the
screw, a tension (pre-loaded) is generated
in the screw. Consequently, the screw shank
and threads are tense and an elastic recovery
is generated, thus creating the clamping
force between the mating surfaces.
To obtain an effective clamping
Figure 9. Measurement of rotational freedom. A
passive fit of the abutment (blue) into a recess
(hexagonal) in the implant platform (a dotted
circle). The space between the two components is
represented by the red area. The rotational freedom
degree during abutment rotation is indicated by the
Figure 10. An intra-oral radiograph showing a single implant-supported crown replacing the right
second molar (a). The cuspal inclinations are lowered and flattened, but the occlusal table is widened
which creates a cantilevering effect and exposes the restoration, the screw and the implant to high
tipping forces that may lead to their mechanical failure. A diagram of an implant-supported restoration;
the implant is oriented so occlusal loading is directed along its long axis (b).
Table 4. Factors that affect screw-joint stability.
1. Implant-abutment interface design/type.
2. Rotational freedom (misfit).
3. Manufacturing allowances (tolerance).
4. The settling (embedment).
5. Repeated opening and closing of the screw.
6. The applied torque value: over and under torqueing the screw.
7. Loading of restoration.
8. Prefabricated metal- and costume-made cylinders.
9. The casting process:
a. Casting alloy;
b. Investment; and
c. The finishing/polishing method.
10. Screw design and materials:
a. Shank or shank-less screws (a shank-less screw is usually less resilient than that with
b. Shape and diameter of screw’s head;
c. Materials from which a screw is made of such as gold, titanium and gold-coated
608 DentalUpdate July/August 2017
force, the tension created in the screw
material should be less than that of the
material’s elastic limit (Young’s modulus)
so no permanent plastic deformation or
screw fracture occurs. Maximum screw-joint
stability can be achieved with a maximum
pre-load when the proportional limit of the
screw is approached. Thus, to obtain this,
the applied torque should be 75% of the
torque required to cause screw permanent
In order to hold the implant
components together, a maximum clamping
force and a minimal separating force are
required. Therefore, the clamping force
overcomes the separating force.
Factors affecting screw-joint
Lack of screw-joint stability is
reflected in loosening of the screw. It is
considered as one of the most common
problems associated with the use of implant-
One of many
factors that play a role in the stability of
the screw-joint is the friction coefficient of
the materials used in the fabrication of the
implant components, such as the abutment,
implant and screw. The friction coefficient
has an effect on the generated pre-loading.
Tightening torque and consequently the
developed pre-load is inversely affected by
the friction between the mating surfaces.
In general, during screw torqueing, friction
occurs between the implant surface and the
opposing abutment surface, between the
head screw and the abutment surface and
between the screw threads (male) and the
implant threads (female). As such, when a
screw is tightened, only 10% of the torque
is converted into screw pre-load, while the
other 90% of the tightening torque is lost as
In order to maximize pre-loading,
the friction between mating surfaces should
be reduced. This can be achieved by coating
the mating surfaces with other materials,
such as carbon film or the screw with
tungsten carbide. This process is known as
dry lubrication and the coating material is
denoted as a dry lubricant. Both carbon and
tungsten carbide coatings were reported to
reduce the friction coefficient and improve
Torq-Tite® abutment screws
(Nobel Biocare, Uxbridge, UK) are made
of titanium alloys and are coated with a
carbon layer and Gold-Tite® abutment screws
(BIOMET 3i) are titanium screws with a
gold-plated surface. Both screw types were
found to be associated with lower friction
coefficients and greater pre-load values than
the conventional gold alloy and titanium
Likewise, higher pre-loads
were associated with gold-coated screws
when compared with that obtained from
screws made of uncoated gold or titanium
alloy for all insertion torques, as well as when
the screws were re-tightened.
Manufacturing tolerance is
another factor that affects the screw-
joint stability. It is defined as unplanned
deviations from the theoretical dimension
of the shaft and its mating recess as some
deviations from a perfect fit are expected,
but not planned. Hence, this indicates an
insignificant value of misfit between the
matting surfaces. This misfit allows for what
is known as rotational freedom (play) to
occur. The rotational freedom is calculated
by the formed angle between the clockwise
and anti-clockwise rotation of the anti-
rotational components of the screw-joints
(Figure 9). The rotational freedom may vary
from 1.6 to 5.3 degrees.
The most stable
and predictable screw-joint may be expected
when the rotational freedom is lower
than two degrees.
Hence, the produced
rotational freedom affects the stability of the
Furthermore, the presence of
a micro-roughness on the implant and
abutment mating surface, which is worn
away as a result of screw torqueing, leads
to what is called settling (embedment
relaxation). Consequently, part of the
clamping force is lost and the screw
becomes loose. The mean loss of pre-load
may be up to 40% of the original pre-load
value 15 hours after screw torqueing.
To reduce the settling effect, it has been
suggested that the implant screws should
be retightened ten minutes after the initial
torque application as a routine clinical
All screw types were reported
to display some decline in pre-load with
repeated tightening. This decline occurs
irrespective of the insertion torque and
As screws lose pre-load
following placement, their re-tightening
is required from time to time during the
The screw pre-load should
be high enough to maintain the joint
integrity and reduce the possibility of the
screw loosening and fracturing.
when excessive torque is applied, slippage
between the screw threads (male) and
the implant internal threads (female)
occurs, which consequently leads to screw
Inversely, too little torque or a
lower torque value which cannot produce
the required screw pre-loading needed to
hold the mating surfaces together exhibits
greater micro-motion at the screw-joint,
which consequently causes screw loosening
and may lead to its fatigue and fracture.
Therefore, it is vital to use the manufacturer’s
recommended tightening torque, which
should be within the elastic range of the
screw’s materials, as mentioned earlier.
It is also essential to ensure consistent
tightening torque values are applied.
Therefore, torque gauges (control) should
be used and manual torqueing should be
It is also important to calibrate
the torqueing devices to obtain consistent
Torqueing the screw should be
carried out carefully and a counter-torque
device should be used to avoid disturbing
the osseointegration. Hence, the use of a
counter-torque device is recommended as
it reduces transmission of the tightening
torque to the implant-bone interface. On
average, about 90% of the recommended
pre-load tightening torque is transmitted
to the implant-bone interface when the
counter-torque device is not used. This value
is reduced to only 10% when the counter-
torque device is used.
Overloading of the restoration
may lead to screw loosening and failure.
Therefore, the occlusion should be adjusted
and occlusal forces should be directed along
the long axis of the implant, whenever
possible (Figure 10). This can be achieved
by construction of a restoration in which
its occlusal morphology is constructed
according to the mechanical principals
that favour this concept. For instance, the
cuspal inclination should be flattened and
the incisal guidance made shallow to avoid
bending moments caused by the lateral
component of the occlusal forces.
occlusal table of the prospective restoration
may be reduced by 30−40% of the tooth
being replaced (Figure 10) and cantilevering
the restoration should be avoided. Use
of an occlusal splint is recommended for
patients with parafunctional habits such as
bruxism. The implant should be placed in
the site that was previously occupied by the
610 DentalUpdate July/August 2017
tooth being replaced, and surrounded by
an adequate amount of bone (Figure 7).
It should also be oriented along the long
axis of the tooth being replaced and within
the occlusal table. However, when a molar
tooth is replaced, the use of two implants
may be considered in order to dissipate the
occlusal loads satisfactorily, as mentioned
earlier (Figure 7).
Some of the other factors that
may affect the screw-joint stability are
displayed in Table 4.
Platform switching concept
This concept was based on
clinical observations where the implant
platform diameter was wider than the
It is assumed that, when this
principle is used, the crestal bone loss
after implant placement is less than when
the implant platform and the abutment
pose a similar diameter.
is theoretically explained on the bases
of moving the micro-gap between the
platform and the abutment inward from
the outer edge and consequently away
from the bone.
It also results in an
increase in horizontal soft tissue dimension,
which may protect the bone crest and
limits its resorption.
It also shifts the stress
between the implant and abutment away
from the cervical bone-implant interface,
which may also help in maintaining the
crestal bone level.
A recent meta-analysis,
including 13 human randomized clinical
trials (RCTs), has shown a significantly less
mean crestal change at platform-switching
implants, compared with when the implant
platform dimensions matches the abutment
(0.49 mm versus 1.01 mm). However, the
use of platform-switch did not preserve
the crestal bone better than when the
switching concept was not used, when
thin mucosal tissues on crestal bone were
Furthermore, the stress within the
screw-joint was found to increase when the
platform-switching concept is implemented.
This may lead to failure of the screw-joint
Therefore, this concept
should be used with substantial care.
Types of restorations/
prostheses for missing teeth
(prostheses) may be used to replace a
single or multiple missing teeth, as well
as completely edentulous mandible and
maxilla. Therefore, when a patient whose
missing teeth were replaced with an implant-
supported restoration attends the dental
clinic, one of the following restoration/
prosthesis is usually present:
An implant-supported single restoration
(crown) (Figure 11);
A fixed implant-supported prosthesis;
A removable implant-supported partial
denture (Figure 12); and
A fixed or removable implant-supported
prosthesis (overdenture) (Figure 13).
Figure 11. A clinical image of a missing upper
right centre incisor (1.1) replaced with a single
cement-retained, implant-supported crown. The
abutment (a) and the restoration (b) is made
of porcelain fused to metal. The papilla failed
to fill the inter-dental space on the mesial and
distal aspect of the restoration. This may have a
negative effect on the aesthetic outcome if the
patient has a high lip-line.
Figure 12. (a, b) Clinical images of multiple
missing maxillary teeth restored with a partial
denture which gains its support/retention from
the teeth, alveolar ridge as well as from an
implant placed in the right canine region. The
fitting surface of the denture showing the patrix
of a locator attachment.
Figure 13. Clinical views of an upper edentulous
maxilla restored with a RISO. (a) Four dental
implants placed in the anterior region. (b)
The implants are connected with a CAD/CAM
designed and fabricated bar. Four locator
attachments (matrices) are attached to the bar.
(c) The fitting surface of the RISO showing the
patrices of the attachment.
July/August 2017 DentalUpdate 611
Treatment options for
replacement of missing teeth with dental
implants are shown in Figure 14.
1. An implant-supported single restoration
When a single tooth is replaced,
the restoration is usually either cemented
to the abutment or screwed to the implant
(Figure 11). This is known as a cement-
retained restoration and a screw-retained
restoration, respectively. As mentioned
earlier, in the cement-retained restoration,
the abutment is attached to the implant
body through a screw-joint and the
restoration is cemented to the abutment
in a similar fashion to that which is used
in the conventional crown. Therefore, the
abutment is used to connect the crown
to the implant. In the screw-retained
implant restorations, the restoration and
the abutment are a single unit which
is attached to the implant directly by a
2. A fixed implant-supported prosthesis (fixed
This is when multiple teeth are
missing and replaced with a prosthesis
that cannot be removed by the patient. In
principle, this type of restoration resembles
that described for a single-implant
supported crown: cement- or screw-retained
3. A removable implant-supported prosthesis
In certain clinical situations,
multiple missing teeth cannot be restored
with a fixed implant-supported restoration.
Instead, they are restored with a removable
prosthesis which is fundamentally similar to
that which is used in replacing a completely
edentulous jaw with a removable implant-
supported overdenture (RISO) (Figure 12). In
this case, in addition to the available teeth,
one or more implants with attachment
systems are usually used. The attachment
systems are discussed later in the article.
4. Implant-supported overdenture for
completely edentulous jaws
When the jaw is completely
edentulous, there are two treatment options
for its restoration; namely a fixed or a
removable implant-supported overdenture
(FISO or RISO). A FISO is when the prosthesis
is permanently fixed to the implants through
screw-joints between the prosthesis and the
This is so it cannot be removed
by the patient. The prosthesis is supported
by several implants (usually four or more).
When such prostheses are indicated, it is a
favourable option for many patients. The
volume of the prosthesis, and consequently
the tissue coverage by the prosthesis, are
reduced. However, this type of prosthesis is
more expensive than removable ones. It also
requires more implants to support and retain
FISOs are of two basic types:
hybrid and porcelain fused to metal.
The hybrid prosthesis is made of a metal
substructure, acrylic and denture teeth. The
porcelain fused to metal prosthesis is made
of a metal substructure and porcelain in a
similar way to that used in the fabrication of
the conventional porcelain-fused-to-metal
restoration. It is more expensive than the
hybrid and is difficult to make, but it is the
better option when the vertical restorative
space is limited.
Conversely, the RISOs are
removable prostheses that can be removed
and replaced by the patients. They are used
in combination with attachment systems
The number of implants
used with the RISOs may be reduced.
For instance, in the case of edentulous
mandible, the number may be reduced to
Figure 14. Treatment options for replacement of missing teeth with dental implants.
Missing multiple teeth
but not adjacent
Completely edentulous jaw
Porcelain fused to metal
612 DentalUpdate July/August 2017
two implants, which are usually placed in
the anterior region of the mandible. The
two-implant supported overdenture option
is recommended as the first-choice standard
of care for an edentulous mandible.
When two-implant supported overdentures
are used, the attachments permit movement
of the overdenture during function and
allow the mucosa of the residual ridge to
be involved in dissipating the imposed
force. Therefore, it is important to note
that, in order to obtain good support
from the residual ridges, the RISOs should
extend to cover the supporting tissues in
a similar fashion as that covered when the
conventional complete denture is used.
The restorations that consist of
crowns or fixed prostheses (bridges), and
that are supported by implants, may be
divided into two types, depending on how
they are connected to the implants; cement-
retained and screw-retained. As mentioned
earlier, in the cement-retained restoration
the abutment is required to connect the
restoration to the implant, while in the
screw-retained restoration the abutment and
the restoration form one unit. In addition,
there are five types of abutments which are
available for use in single and fixed implant-
A summary of
these abutment types can be found in Table
In this case, the retention of
the restoration relies on the retaining
screw. Nevertheless, the restoration can be
removed and/or replaced when required,
without damage or need for a new
restoration. The adaptation between the
They are made of a plastic/wax pattern with/without a metal-machined interface ring;
The pattern is made (wax) or adjusted (plastic) to the required form, shape and angle;
The pattern is then used to create a metal abutment in a similar procedure to the conventional lost-wax technique;
An abutment plastic/wax pattern is attached to the implant analogue, which is submerged in a working cast;
The restoration is then made to fit the abutment also in the conventional method;
UCLA plastic patterns are an example of these types of abutments;
They require an impression of the implant platform.
Pre-machined (prefabricated/ready-made) modifiable metal abutments
They are prefabricated abutments;
They are adjustable and modifiable intra- and extra-orally;
They cannot be used when the implant is placed in an improper position or with improper angulation;
An impression of the abutment, not the implant, is taken using a manufactured impression coping;
The conventional crown and bridge procedures are used when provisional or final restorations are made.
Pre-machined (pre-fabricated/ready-made), non-modifiable metal abutments
They are pre-fabricated abutments that cannot be modified or altered;
The abutment that is suitable for the specific clinical condition is selected;
The abutment is attached to the implant body;
An impression of the abutment, not the implant, is taken using a manufactured impression coping;
The conventional crown and bridge procedures are used when provisional or final restorations are made.
They are made entirely of ceramic;
They are available in ready-made or customizable forms;
They are indicated for use in cases when aesthetics are essential, and when thin biotype gingiva exists so that metal show through is
CAD/CAM milled abutments
They are made from a block of titanium or ceramic;
An implant platform level impression may be required depending on the manufacturers;
A working cast is fabricated then scanned optically to generate exact 3D images of the region;
The information is sent to the milling machine to produce the abutment;
It eliminates certain negative factors that may be associated with the conventional method of abutment fabrication, such as an
improper fit and incorporation of porosity;
This type of abutment is more expensive than the other abutment types.
Table 5. Different abutment types.
July/August 2017 DentalUpdate 613
restoration and the underlying implant is
usually better than that in the case of its
cement-retained counterpart. It can be used
when the vertical restorative space is limited
as the retention depends on the screw, but
is contra-indicated when mouth opening
is limited, as the use of the different tools
required for screwing and torqueing the
screws may not be possible. However, the
use of a screw-retained restoration may be
considered when the implant platform is
situated deep sub-mucosally, as complete
removal of cement is not always possible
when a cement-retained restoration is used.
The screw type is not indicated when the
screw hole is pointed at the labial surface
as this compromises the aesthetics. Hence,
the implant should be placed in its optimal
position and angulation to avoid negative
effects on aesthetics, otherwise an angled
abutment may provide an acceptable
alternative. In the posterior region, the
occlusal morphology of the restoration may
be difficult to obtain as the hole through
which the screw is tightened occupies
a major part of the occlusal table of the
restoration. Furthermore, the access hole
may weaken the porcelain and lead to its
fracturing. It is important to mention that,
if screw loosening of one restoration occurs
in a fixed-implant supported restoration, a
cantilevering effect can arise and put the
other abutment, implants, screw and the
peri-implant bone at risk as they are exposed
to tremendous forces. Also, the screw
loosening is not an unreal problem with the
screw-retained restoration. However, the
ability to retrieve the restoration/prosthesis
easily to allow its cleaning (and of the peri-
implant tissues) is a significant advantage of
The cement-retained restoration
is indicated when mouth opening is
restricted, and when the implant angulation
is not optimal without a major negative
effect on the aesthetic outcome of the
The occlusal morphology
can be easily constructed in the normal way,
as in conventional restorations. The materials
and techniques used for the fabrication
of the cement-retained restoration are
similar to those used in the fabrication of
conventional restorations. The trial stage and
the final cementation procedure are almost
identical to those used in conventional
restorations. However, it may not be possible
to remove the cement-retained restorations
if permanent cementing media is used.
Therefore, restorations have to be cut in
order to remove them. The removal of excess
cement may be not possible, which may
result in soft tissue problems and to peri-
implant disease (see below).
its use should be avoided when the implant-
abutment connection is deeply embedded
sub-mucosally, which may preclude its
removal. Furthermore, removing the
cement is not a predictable procedure and
may cause the abutment/restoration to be
leading to plaque accumulation.
Marginal adaptation between the abutment
and the restoration may also be inferior
to that obtained when the screw-retained
restoration is used. It is also not suitable
when the vertical restorative space is limited,
as retention may be compromised.
The attachment systems
An attachment is defined as a
mechanical device used for the fixation,
retention and stabilization of a dental
It is used with implant-
supported removable partial dentures
and overdentures. The attachment usually
consists of two parts. One part is attached to
the implant, while the other part is attached
to the prosthesis. Five types of attachment
systems are available and compatible with
the main implant systems. The attachment
systems that are commonly used with RISOs
include: bar/clip, balls, locators, magnet
and telescopic crown.
The use of a
bar system allows splinting of two or more
implants together. The other attachment
types may be used individually and also
in combination with the bar system. The
attachments are attached to the implant by
screws, resulting in a screw-joint. Features
of attachment systems used for RISOs are
displayed in Table 6.
Peri-implant tissue response
to bacterial insult and peri-
Despite their high success
rate, implant failures are also reported to
occur. Several factors that have already
been mentioned earlier which influence
such success should be considered when
treatment is planned.
The implant may fail
before it is put to function as a result of its
failure to integrate with the peri-implant
tissue during the healing stage. This type of
failure is categorized as an early failure. The
implant may also lose its integration and
fail at a later stage, months or even years
after implant placement. This is known as
The criteria for dental implant
success are displayed in Table 7.
One of the complications that
is reported to affect the peri-implant tissue
is caused by the inflammatory response of
this tissue to bacteria that forms a biofilm on
the implant surface.
It occurs when the
balance between the host’s defence and the
bacterial load shifts in favour of the bacteria.
This tissue response may be limited to the
peri-implant soft tissues (mucosa) or may
also extend to and affect the peri-implant
bone and lead to its resorption.
Both tissue responses to
bacterial insult are collectively known as
Table 6. Features of attachment systems used for RISOs.
1. The different designs of the attachment systems are used to gain retention, support
and stability of the overdenture.
2. They consist of a matrix (female) and a patrix (male):
The matrix accommodates the patrix; and
The patrix frictionally fits and engages the matrix.
3. The joint that is made between the patrix and the matrix may be rigid (when
no movements exist between the patrix and matrix) or resilient (when there are
4. The involved dental implants are either splinted or non-splinted.
5. A bar is usually used to connect the implants (splinted).
6. Bars may be custom-made, pre-fabricated (ready-made) or CAD/CAM milled.
7. An individual attachment system is usually used in a non-splinted manner or combined
with a bar system.
614 DentalUpdate July/August 2017
peri-implant diseases, and are classified as
peri-implant mucositis or peri-implantitis.
In peri-implant mucositis, the inflammatory
response is not essentially different from
that which occurs in gingiva when it is
exposed to pathogenic bacteria and leads
Therefore, in principle, peri-
implant mucositis resembles gingivitis.
The onset and progression of
mucositis may be affected by a decrease
in the vascularity and an increase in
collagen to fibroblast ratio in the peri-
implant connective tissue, and by the way
they are arranged around the implant
Clinically, peri-implant mucositis
is characterized with bleeding on gentle
probing. It is a treatable disease and the
damage is reversible. However, it may
progress into peri-implantitis if untreated.
There are no major differences in the
bacteria that were found to be associated
with mucositis and peri-implantitis. This may
indicate that mucositis is the origin of peri-
On the other hand, peri-
implantitis occurs when both the peri-
implant mucosa and bone are affected.
It resembles chronic periodontitis in
natural teeth. However, some differences
do exist. For instance, the crestal bone
loss occurs in a circumferential fashion
around the affected implant, unlike bone
resorption seen in chronic periodontitis. The
circumferential shape of the peri-implantitis
lesions may be attributed to the lack of
periodontal ligament, and to the surface
topographies of the involved implants which
facilitate the spread of infection apically
as well as laterally.
The extent and the
composition of cells in the peri-implantitis,
as well as its progression rate, may differ
from that which is commonly seen in
For instance, the
protective connective tissue capsule, which
was found to separate the periodontal lesion
from the alveolar bone around teeth in the
case of chronic periodontitis, does not exist
Therefore, the self-limiting
process is not present around implants,
which may provide an explanation for the
fast development and progression of the
It should be mentioned that
dental implants may fail as a result of these
diseases if they are not treated as they
lead to bone resorption, and eventually
to mobility and failure of the affected
It is important to remember
that resorption of peri-implant crestal
bone occurs within the first year of implant
placement and continues to occur to a lesser
degree afterwards. It occurs irrespective of
the implant placement method (sub-merged
or non-submerged). Based on a 15-year
retrospective study, Adell and colleagues
reported that crestal bone loss during
the healing period and the first year after
connecting the prosthesis, was about 1.5
mm. Thereafter, there was only 0.1 mm bone
loss annually. In another study, an average of
0.9 mm crestal bone was lost during the first
year and no more than 0.07 mm annually in
the following years.
The exact cause of
this bone loss is still debatable. Nevertheless,
the current literature presents several factors
which may contribute to this loss, such as
surgical trauma, reformation of a ‘biologic
width’ and presence of a rough/smooth
interface. However, the factors that are
most commonly cited to cause such bone
resorption are displayed in Table 8.
Role of the patient and the
Each dental implant and
restoration/prosthesis should be evaluated
clinically and radiographically in a similar
manner to the treatment of periodontal
disease. Oral hygiene should be observed
and regular check-ups should be
scheduled. Therefore, after a physiologic
tissue remodelling period and at the time
of prosthesis installation, clinical and
radiographic examinations of the peri-
implant tissue should be carried out and
used as a baseline to monitor any change
in the tissue and to intervene if required.
When any deviation from the norm is
found, intervention is then considered and
carried out. In general, oral hygiene should
be monitored and different oral hygiene
aids should be demonstrated and the
patient encouraged to use them as often as
In general, care for dental
implants has two phases: patient self-care
and professional clinical maintenance
It is the responsibility of the
patient to maintain good oral hygiene.
Patient self-care consists of a daily oral
hygiene procedure in which toothbrush
(manual/powered and single tufted ones),
auxiliary aids such as inter-proximal
brushes, dental floss/tape and mouthrinses
may be used. A combination of these
aids, whenever it is necessary, should be
Table 7. Criteria for dental implant success114
1. That an individual, unattached implant is immobile when tested clinically.
2. That a radiograph does not demonstrate any evidence of peri-implant radiolucency.
3. That vertical bone loss is less than 0.2 mm annually following the implant’s first year of service.
4. That individual implant performance is characterized by an absence of signs and symptoms, such as pain, infections, neuropathies,
paresthesia or violation of the mandibular canal.
Table 8. Factors that may contribute to or cause crestal bone loss.6
1. Bone remodelling after implant placement
2. Reformation of a ‘biologic width’
3. Presence of rough/smooth interface
4. Presence of a micro-gap at implant-abutment/restoration interface
5. Surgical trauma
6. Occlusal overloading
7. A ‘stress shielding’ phenomenon
8. Incomplete removal of luting cement
9. Peri-implant disease
616 DentalUpdate July/August 2017
considered and demonstrated. For instance,
powered toothbrushes, which have different
interchangeable bristle heads (flattened,
rubber cup-like, short- and long-pointed in
shape) that suit different clinical situations
may be used. When they are used properly,
the result is a healthy environment around
the implant. However, it is important
to mention that limiting the number of
auxiliary aids, their simplicity and the time
required for their use are important for
patients’ compliance as they play a vital role
in this aspect.
As already mentioned, dental
implants are affected by and may fail as a
result of the peri-implant disease which
can be detected only by regular clinical
and radiographic examinations. Therefore,
when an implant is affected by the peri-
implant disease, the patient should be made
aware of the situation and a treatment plan
should be implemented and regular follow-
ups arranged. However, there is a lack of
consensus on how peri-implant disease
is treated. Nevertheless, the Cumulative
Interceptive Supportive Therapy (CIST)
protocol that was presented by Lang
may be followed when
peri-implant disease is found. The CIST is
a systemic comprehensive protocol. This
protocol is based on clinical parameters
such as peri-implant pocket depth (PIPD),
bleeding on probing (BoP) and peri-implant
bone loss on which clinical diagnosis
is made. Accordingly, a treatment plan
and continuous follow-up strategy are
constructed. A summary of this protocol
is presented in Table 9. However, the
management of peri-implant diseases is not
within this article’s scope.
Complications associated with
Several biological and
mechanical complications are reported
with the use of dental implants to support/
retain restorations and prostheses. For
instance, screws used to connect different
combinations of the implant-supported
restorations/prostheses may become loose
and need to be retightened or replaced.
Screw loosening may be due to it not being
adequately torqued or over-torqued or due
to micro-movements that occur as a result of
the manufacturing tolerance.
torqued screw fails to deliver the tension
that is required to produce the optimum
clamping force between the screw-joint
components. Re-tightening is there for
required. Screw re-tightening can be easily
achieved when the restoration is a screw-
retained type. However, when the restoration
is cement-retained, cutting the restoration
to gain access to the screw may be the only
solution, especially when permanent cement
is used. When a provisional cement is used,
the use of crown removal may be tried.
When the screw is over-torqued
to a degree which places the screw material
in tensile stress that exceeds its elastic limit,
the screw may be plastically elongated.
This leads to screw loosening or even to its
fracture. In the former situation, the screw
may be replaced, but in the latter situation
the removal of the screw may not be
possible and the treatment is complicated,
which is beyond the scope of this article. To
minimize the occurrence of screw-loosening
or fracture, the recommended torque should
be implemented using a torque driver that
ensures that the right amount of torque is
failure may also occur when the material’s
mechanical properties and/or thickness is
not optimum or when the occlusal design is
not correctly designed. The superstructure
failure may also occur as a result of lack
of passivity when several implants are
connected together. The lack of passivity
may overload the implants and place the
superstructure under tremendous pressure,
that may lead to its failure. To check for
passivity a test called a ‘Sheffield test’ or
a ‘one-screw test’ is usually carried out.
However, the passivity problem may be
avoided by the use of computer-aided
design/computer-aided manufacture (CAD/
Acrylic or porcelain veneer may
also fail when the bulk of these materials
Table 9.The clinical parameters, diagnosis and a summary of the CIST protocol for treatment of peri-implant diseases.123
Clinical parameters Clinical Diagnosis Treatment Protocols
Healthy peri-implant tissues No treatment is needed, just regular check-ups and
enhancement of oral hygiene
Plaque is present
**BoP is present
Mucositis A. Mechanical debridement and polishing using a
rubber cup and non-abrasive paste and regular check-
ups and enhancement of oral hygiene
*PIPD ≤5 mm Mucositis B. Treatment includes treatment A with antiseptic
*PIPD >5 mm associated with bone loss of up to
Peri-implantitis C. Same as treatment B in addition to the use of local
or systemic antibiotic
*PIPD >5 mm associated bone loss >2 mm Severe peri-implantitis D. Same as treatment C combined with surgery
(access flap, resective method or regenerative
*Peri-Implant Pocket Depth; **Bleeding on Probing
July/August 2017 DentalUpdate 617
are inadequate. For instance, when a
limited vertical restorative space does not
allow the use of the optimum thickness
of the material. Depending on the degree
of mechanical damage of the restoration/
prosthesis, fracture of porcelain may be
repaired intra-orally using the Co-Jet® system
(3M ESPE, St Paul, Mn, USA) and composite
resin material. It is considered as a reliable
method for such repairing. Fracture of acrylic
may also be repaired using composite resin
materials. However, when the metal frame-
work is fractured, the only solution is its
RISO attachment failure and
complications are mostly of a mechanical
nature and include:
Fracture of the acrylic base, teeth and
Reduction of retention as a result of wear
of the retentive elements or loosening of
matrices and screws;
Fracture or wear of the clip and matrix;
Fracture of solder joints; and
Dislodgement of the attachments.
Wear of the attachment
component is a problem that may
reduce the overdentures’ retention and,
consequently, a replacement of the worn
attachment becomes a necessity. Less
prosthetic maintenance was required with
the splinted (bar/clip) designs than with
the unsplinted ones.
use of bars may complicate the hygiene
and it may be associated with
a misfit of the framework, which has the
potential to generate unwanted stress on the
attachment, the implant, the retained screw
and also the peri-implant bone.
Relining of the denture is also
required regularly and may need to be
carried out every few years to compensate
for the changes in the alveolar ridge
that may occur. Failures of the implant-
supported fixed dental prosthesis also
occur. The failures include screw loosening
and fracture of the superstructure. Speech
may be affected when tissue loss is severe.
The compensation of lost tissue with
acrylic or porcelain is usually required. This
compensation may lead to an increase
in plaque accumulation and tissue
inflammation as the oral hygiene procedure
is compromised. Meticulous effort from the
patient is required. Calculus deposition once
formed cannot be removed by a daily oral
hygiene. Therefore, professional intervention
is necessary. This intervention consists of
the use of scalers with plastic tips to avoid
scratching the implant components.
Dental implants are widely
used and considered as one of the options
by which missing teeth are replaced. They
are used successfully to replace single
and multiple missing teeth as well as a
completely edentulous jaw. The use of
dental implants are increasing and dental
professionals are more likely to see patients
who have implant-supported restorations/
prostheses. Therefore, basic knowledge
of dental implants is necessary for dental
personnel. Several factors are known to
affect success of any implant system. These
factors may be related to features locally,
such as bone quality and quantity. Other
factors are related to the surgical method
by which an implant is placed or which are
related to the implant system used, such
as length and diameter of the implant.
Furthermore, dental implants are affected by
peri-implant diseases which, if not treated,
can cause the implant to fail. It requires
continuous monitoring, regular check-ups
and may require professional interventions,
the time of intervention being vital.
The success of any implant-
supported restoration/prosthesis is
dependent on the interaction between
the patient and the dental personnel.
Therefore, maintaining good oral hygiene
and committing to regular check-ups are the
responsibility of the patient. On the other
hand, it is the responsibility of the dental
personnel to examine the implants and
the restorations/prostheses clinically and
radiographically. It is also the responsibility
of the dental practitioner to demonstrate
and educate the patient on how to look after
the implant and to tailor check-up recall
visits according to the patient’s needs.
Mechanical failures associated
with implant-supported restorations/
prostheses, such as screw loosening or
fracture and chipping of porcelain veneer
and fracture of the superstructure, are not
uncommon. Loss of retention of the implant-
supported overdenture are common clinical
findings which may make the patient seek
treatment. On the other hand, plaque
accumulation and mucosal hyperplasia
in the per-implant site do not necessarily
promote the patient to look for treatment.
Consequently, professional evaluation and
assessment are required to discover such
conditions. This necessitates recall visits and
check-ups which allow the dental personnel
to intervene in the proper time and to rescue
the implant and its restoration/prosthesis.
Therefore, the dental personnel should be
prepared and able to diagnose and to deal
with such complications and to refer the
patients when required.
The authors would like to thank
Mr Emmet Ryan (Dublin Dental University
Hospital) for providing the images in Figure
11 and Dr Brendan Grufferty (Dublin Dental
University Hospital) for providing the images
in Figure 13.
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