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PRACTICE
dental implants
BRITISH DENTAL JOURNAL, VOLUME 187, NO. 3, AUGUST 14 1999 127
Introduction to dental
implants
Richard Palmer1
Implants have been used to support dental
prostheses for many decades, but they have not
always enjoyed a favourable reputation. This
situation has changed dramatically with the
development of endosseous osseointegrated
dental implants. They are the nearest equiva-
lent replacement to the natural tooth, and are
therefore a useful addition in the management
of patients who have missing teeth because of
disease, trauma or developmental anomalies.
There are a number of dental implant systems
which offer predictable long-term results
backed by good scientific research and clinical
trials. In the first place it may be helpful to clar-
ify some of the commonly used terms in
implant dentistry (Table 1).
Success criteria
It is important to establish success criteria for
implant systems, and for implants to be tested
in well controlled clinical trials. The minimum
success criteria proposed by Albrektsson et al.
(IJOMI 1986; 1: 11) is set out in Table 2.
The most obvious sign of implant failure is
mobility. However, some of the criteria in
Table 2 apply to the overall requirements of an
implant system, but are not as useful when
judging the success of individual implants. This
is well illustrated by considering the radi-
ographic criteria. Bone remodelling occurs in
the first year of function in response to occlusal
forces and establishment of the normal dimen-
sions of the peri-implant soft tissues (See Part
2). The ‘ideal’ bone level is usually judged
against a specific landmark on the implant
(such as the implant/abutment junction) and it
may differ therefore between implant systems
(fig. 6). Subsequently the bone levels are usually
more or less stable, and small changes such as
0.2 mm per annum are impossible to measure
with conventional radiographs. These specified
changes therefore do not apply to individual
implants but to mean (average) changes mea-
sured across a large number of implants. For
example, a detectable change of 1mm or more
may occur at very few implants in contrast to
the majority which remain unchanged or in a
steady state. It is also difficult to stipulate what
level of change in an individual implant over a
given period of time would constitute failure. A
rapid change in bone level may be followed by a
long period of stability. On the other hand, pro-
gressive or continuous bone loss is a worrying
sign of impending failure. An implant with
marked loss of bone may therefore be judged as
‘surviving’ rather than ‘successful’.
Implants placed in the mandible (particu-
larly anterior to the mental foramina) enjoy a
higher success rate than the maxilla (approxi-
mately 95% success for implants in the
Professor of Implant Dentistry and
Periodontology, Guy's Kings and St
Thomas' Medical and Dental School,
London SE1 9RT
© British Dental Journal
1999; 187: 127–132
This first part of a new
series outlines the
salient aspects of
osseointegration,
implant design and
other factors which
contribute to successful
treatment.
1
Basic terminology in implant dentistry
Table 1
Osseointegration A direct structural and functional connection between ordered, living
bone and the surface of a load-carrying implant (Albrektsson
et al.
Acta
Orthopaedica Scand
1981; 52:155 (fig. 1).
Endosseous dental implant A device inserted into the jaw bone (endosseous) to support a dental pros-
thesis. It is the ‘tooth root’ analogue and is often referred to as a ‘fixture’
(fig. 2).
Implant abutment The component which attaches to the dental implant and supports the
prosthesis. A transmucosal abutment (TMA) is one which passes through
the mucosa overlying the implant. A temporary or healing abutment may
be used during the healing of the peri-implant soft tissue before the defini-
tive abutment is chosen (fig. 3).
Abutment screw A screw used to connect an abutment to the implant.
Single stage implant surgery Surgical placement of a dental implant which is left exposed to the oral
cavity following insertion. This is the protocol used in non-submerged
implant systems (fig. 4).
Two stage implant surgery Initial surgical placement of a dental implant which is buried beneath the
mucosa and then subsequently exposed with a second surgical procedure
some months later. This is used in submerged implant systems (fig. 5).
In this part, we will
discuss:
• Success criteria
• Basic guide to
osseointegration:
➤Biocompatibility and
implant design
➤Bone factors
➤Loading conditions
➤Prosthetic
considerations
128 BRITISH DENTAL JOURNAL, VOLUME 187, NO. 3, AUGUST 14 1999
PRACTICE
dental implants
mandible compared with 85 to 90% for the
maxilla with systems such as Branemark,
5 years after loading). An example of the lowest
recorded success rates are for short implants
(7 mm) used in the maxilla to support overden-
tures, especially when the implants are not
joined together. A few studies have now shown
that the overall mean failure rate in smokers is
about twice that in non-smokers. Smokers
should be warned of this association and
encouraged to quit the habit. It should also be
noted that reported mean failure rates are not
evenly distributed throughout the patient pop-
ulation. Rather, implant failures are more likely
to cluster in certain individuals.
Basic guide to osseointegration
Figure 1 shows an histological section of a tita-
nium screw threaded implant which has been
in function in bone for 1 year. There is very
close apposition of bone over most of the
implant surface. It has been proposed that the
biological process leading to and maintaining
osseointegration, is dependent upon a number
of factors which include:
Biocompatibility and implant design
Implants made of commercially pure titanium
have established a benchmark in osseointegra-
tion, against which few other materials com-
pare. Related materials such as niobium are able
to produce a high degree of osseointegration
Figure 1. Histological section of an implant
with bone growing in intimate contact with the
surface. The dense bone which contains a
small medullary space fills the area between
two thread profiles which are 0.6 mm apart.
Figure 2. Three different designs of
endosseous implants being inserted into
prepared sites within the jaw bone.
Scanning electron micrographs of the
implants are shown in Figures 7 to 9.
Figure 2a is a machined threaded
implant of the Branemark design (Nobel
Biocare). Figure 2b is an Astra ST
implant which has a microthreaded
coronal portion, a macro-threaded
apical portion and the surface has been
blasted with titanium oxide. Figure 2c is
an ITI Straumann implant which has a
smooth transmucosal collar, a macro-
threaded body and a plasma sprayed
surface.
a
c
b
Implant design
parameters
• Implant length
• Implant diameter
• Implant shape
• Surface characteristics
Prosthetic
considerations
• The type of prosthetic
reconstruction
• The occlusal scheme
• The number, distribution,
orientation, and design
of implants
• The design and proper-
ties of implant connectors
• Dimensions and location
of cantilever extensions
• Patient parafunctional
activities
BRITISH DENTAL JOURNAL, VOLUME 187, NO. 3, AUGUST 14 1999 129
PRACTICE
dental implants
and in addition, successful clinical results are
reported for some titanium alloys and hydrox-
yapatite coated implants. More recently
resorbable coatings have been developed which
aim to improve the initial rate of bone healing
against the implant surface and then resorb
within a short time frame to allow establish-
ment of a bone to metal contact.
The implant design has a great influence on
initial stability and subsequent function. The
main design parameters are:
•Implant length – implants are generally avail-
able in lengths from about 6 mm to as much
as 20 mm. The most common lengths
employed are between 8 and 15 mm which
correspond quite closely to normal root
lengths.
•Implant diameter — most implants are
approximately 4 mm in diameter. At least
3.25 mm in diameter is required to ensure
adequate implant strength. Implant diameter
may be more important than implant length
in the distribution of loads to the surrounding
bone. Implant diameters up to 6 mm are avail-
able, which are considerably stronger, but they
are not so widely used because sufficient bone
width is not so commonly encountered.
•Implant shape — hollow-cylinders, solid-
cylinders, hollow screws or solid screws are
commonly employed shapes which are
designed to maximise the potential area for
osseointegration and provide good initial
stability (figs 7a–9a). Even minor alterations
in the size and pitch of threads can enhance
the latter property. Screw shaped implants
also offer good load distribution characteris-
tics in function.
•Surface characteristics — the degree of sur-
face roughness varies greatly between differ-
ent systems. Surfaces which are machined,
grit-blasted, etched, plasma sprayed and
coated are available. Figures 7b to 9b show
the characteristics of these surfaces viewed
with the scanning electron microscope,
showing considerable increases in potential
surface area. The optimum surface mor-
phology has yet to be defined, and some may
perform better in certain circumstances. By
Suggested minimum success criteria for dental implants*
Table 2
1. An individual, unattached implant is immobile when tested clinically.
2. Radiographic examination does not reveal any peri-implant radiolucency.
3. After the first year in function, radiographic vertical bone loss is less than 0.2 mm per annum.
4. The individual implant performance is characterised by an absence of signs and symptoms such as
pain, infections, neuropathies, paraesthesia, or violation of the inferior dental canal.
5. As a minimum, the implant should fulfil the above criteria with a success rate of 85% at the end of a
5 year observation period and 80% at the end of a 10 year period.
*after Albrektsson
et al. IJOMI
1986; 1:11
Figure 3. Various forms of implant
abutment are illustrated. Figure 3a
shows ball abutments which are
used to support overdentures.
Figure 3b shows abutments which
are used to support individual
crowns in ‘single tooth
restorations’. The crowns are
cemented on the parallel sided
hexagon. Figure 3c shows four
conical shaped abutments which
are used to support a bridge
superstructure. In this the bridge
would be screwed to the abutments
rather than being cemented. Figure
3d shows some simple cylindrical
healing abutments which are used
during the healing phase of the
mucosa before definitive abutments
are selected.
ab
cd
Figure 4. An implant of the ITI
Straumann type has been
inserted and left protruding
through the mucosa in a one
stage surgical procedure. A
wide screw has been placed
on the top to protect the inner
aspect of the implant until a
definitive abutment is
connected.
130 BRITISH DENTAL JOURNAL, VOLUME 187, NO. 3, AUGUST 14 1999
PRACTICE
dental implants
increasing surface roughness there is the
potential to increase the surface contact with
bone but this may be at the expense of more
ionic exchange and surface corrosion. Bacte-
rial contamination of the implant surface will
also be affected by the surface roughness if it
becomes exposed within the mouth.
Bone factors
The stability of the implant at the time of place-
ment is very important and is dependent upon
bone quantity and quality as well as implant
design. The edentulous ridge can be classified
in terms of shape and bone quality (fig. 10).
Following loss of a tooth the alveolar bone
resorbs in width and height. In extreme cases
bone resorption proceeds to a level which is
beyond the normal extent of the alveolar
process and well within the basal bone of the
jaws. Radiographic determination of bone
quantity and quality is considered in Part 5 and
procedures which can be used to augment bone
in Part 8. The most favourable quality of jaw
bone for implant treatment is that which has a
well formed cortex and densely trabeculated
medullary spaces with a good blood supply.
Bone which is predominantly cortical may offer
good initial stability at implant placement but
is more easily damaged by overheating during
the drilling process, especially with sites more
than 10 mm in depth. At the other extreme,
bone with a thin or absent cortical layer and
sparse trabeculation offers very poor initial
implant stability and fewer cells with a good
osteogenic potential to promote osseointegra-
tion. Success is highly dependent upon a surgi-
cal technique which avoids heating the bone.
Slow drilling speeds, the use of successive incre-
mentally larger sharp drills and copious saline
irrigation aim to keep the temperature below
that at which bone tissue damage occurs
(around 47°C for 1 minute). Further refine-
ments include cooling the irrigant and using
internally irrigated drills. Methods by which
these factors are controlled are considered in
more detail in Part 6 (Basic Implant Surgery).
Factors which compromise bone quality are
infection, irradiation and heavy smoking. The
effects of the latter two are a result of a diminu-
tion of the vascular supply to the bone which
compromises the healing response, a feature
which has been well described in the healing of
fractures.
Loading conditions
Following installation of an implant it is
important that it is not loaded during the early
healing phase. Movement of the implant
within the bone at this stage results in fibrous
tissue encapsulation rather than osseointegra-
tion. This has been compared to the healing of
a fracture where stabilisation of the bone frag-
ments is very important to prevent non-union.
In partially dentate subjects it is desirable to
provide temporary/provisional prostheses
which are tooth supported to avoid early
implant loading. However, in patients who
wear mucosally supported dentures it is gener-
ally recommended that they should not be
worn over the implant area for 1 to 2 weeks.
This also helps to prevent breakdown of the
soft tissue wound. Systems such as Branemark
have advised leaving implants unloaded
beneath the mucosa for around 6 months in
the maxilla and 3 months in the mandible,
mainly because of differences in bone quality.
However, these are largely empirical guide-
lines, and bone quality and implant stability
will vary greatly between individuals, jaws and
sites within jaws. Currently there is no accurate
measure which precisely determines the opti-
mum period of healing before loading can
commence. Bone quality can be assessed by
measuring the cutting torque during prepara-
tion of the implant site. The stability of an
implant and increasing bone-to-implant con-
tact has been quantified using resonance fre-
Figure 5. Exposure of two
implants which have been
buried beneath the mucosa
for a period of 6 months.
Bone has grown over the top
of them and this needs to be
removed before a healing
abutment is connected.
Figure 6. A periapical radiograph of
a single tooth implant. The bone
contacts the implant up to the most
coronal thread. An abutment screw
which is more radio-opaque can be
seen connecting the abutment to the
implant. The crown is all porcelain
and is cemented to the abutment. In
this system (Branemark) the
landmark for measuring the bone
level from is the junction between
implant and abutment.
BRITISH DENTAL JOURNAL, VOLUME 187, NO. 3, AUGUST 14 1999 131
PRACTICE
dental implants
quency analysis. This newly developed non-
invasive research tool measures the stiffness of
the implant at the bone interface. In some cir-
cumstances it has been shown that immediate
loading is compatible with subsequent suc-
cessful osseointegration, providing the bone
quality is good and the functional forces can be
adequately controlled. The latter may involve
placing an adequate number of implants and
connecting them together as soon as possible
with a rigid framework. However, these latter
protocols should be considered experimental
at the present time, and there is much data to
support the more cautious approach advo-
cated by Branemark in ensuring a high level of
predictable implant success. Some systems
employ a single stage approach in which the
implant is installed so that it protrudes
through the overlying mucosa (ie non-sub-
merged), although avoidance of early loading
is equally critical. Following the recommended
healing period (around 3 months) abutments
are connected to the implant to allow con-
struction of the prosthesis. This protocol
therefore avoids further surgery to uncover the
implants. The loading of the implant sup-
ported prosthesis is a further important con-
sideration which will be dealt with in the
following section.
Prosthetic considerations
Carefully planned functional occlusal load-
ing will result in maintenance of osseointe-
gration and possibly increased bone to
implant contact. In contrast, excessive load-
ing may lead to bone loss and/or component
failure. Clinical loading conditions are
largely dependent upon:
The type of prosthetic reconstruction
This can vary from a single tooth replacement
in the partially dentate case to a full arch
reconstruction in the edentulous individual.
Implants which support overdentures may
present particular problems with control of
loading as they may be largely mucosal sup-
ported, entirely implant supported or a com-
bination of the two.
The occlusal scheme
The lack of mobility in implant supported fixed
prostheses requires provision of shallow cuspal
inclines and careful distribution of loads in lat-
eral excursions. With single tooth implant
restorations it is important to develop initial
tooth contacts on the natural dentition and to
avoid guidance in lateral excursions on the
implant restoration. Loading will also depend
upon the opposing dentition which could be
natural teeth, another implant supported pros-
thesis or a conventional removeable prosthesis.
Surprisingly high forces can be generated
through removable prostheses.
Figure 7. This shows a scanning electron
micrograph of a Branemark/Nobel
Biocare implant. Figure 7a shows the
basic thread design and figure 7b a
higher power view of the machined
surface.
ab
Figure 8 shows a scanning electron
micrograph of an Astra ST implant. The
conical neck has a microthread and the
apical part a coarser self tapping
thread (fig. 8a). Figure 8b shows a
higher power view of the blasted
(Tio-blast) surface at the same
magnification as figure 7b.
ab
Figure 9 shows a scanning electron
micrograph of an ITI Straumann solid
screw implant. The polished
transmucosal neck is clearly demarcated
from the plasma sprayed body (fig. 9a).
The thread has a coarser pitch than the
implants shown in figures 7 and 8.
Figure 9b shows the plasma sprayed
surface at the same magnification as
figure 7b and 8b. The increase in surface
area is considerable.
ab
132 BRITISH DENTAL JOURNAL, VOLUME 187, NO. 3, AUGUST 14 1999
PRACTICE
dental implants
The number, distribution, orientation, and
design of implants
The distribution of load to the supporting bone
can be spread by increasing the number and
dimensions (diameter, surface topography,
length) of the implants. The spacing and
3-dimensional arrangement of the individual
implants will also be very important. The so-
called ‘tripod’ arrangement of three implants is
recommended in situations of high load, such
as replacement of molar teeth in the partially
dentate individual.
The design and properties of implant connectors
Multiple implants are joined by a cast or milled
framework. A rigid connector provides good
splinting and distribution of loads between
implants. It is equally important that the con-
nector has a passive fit on the implant abut-
ments so that loads are not set up within the
prosthetic construction.
Dimensions and location of cantilever extensions.
Some implant reconstructions are designed
with cantilever extensions to provide function
(and appearance) in areas where provision of
additional implants is difficult. This may be
caused by practical or financial considerations.
Cantilever extensions have the potential to
create high loads, particularly on the implant
adjacent to the cantilever. The extent of the
leverage of any cantilever should be considered
in relation to the anteroposterior distance
between implants supporting the reconstruc-
tion. The cantilever extension should not exceed
this length and the cross sectional design should
be adequate to prevent flexing.
Patient parafunctional activities
Great caution should be exercised in treating
patients with known parafunctional activities.
Excessive loads may lead to loss of marginal
bone or component fracture.
These factors will be considered in more
detail in Parts 4 and 7.
Conclusion
There are a great many factors to take into
account to ensure predictable successful
implant treatment. There is no substitute for
meticulous attention to detail in all of these
areas. Failure to do so will result in higher fail-
ure rates and unnecessary complications.
Figure 10 shows examples of dental panoramic tomograms of
edentulous jaws. Both show extensive resorption of the
maxillary ridge. There is far less resorption of the mandible in
figure 10a than figure 10b. In the latter case there is reasonable
bone volume in the anterior mandible but resorption close to the
level of the inferior dental canal in the posterior part.
a
b
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