Josef Kučera,*1"Simon"J."Littlewood2 and Ivo Marek3
The main goals of orthodontic treatment
are improve d aesthetics, occlusion and
stability. Achieving long-term stability of the
orthodontic treatment is a signicant challenge
for a number of reasons.1,2 e problem may
be in the denition of long-term stability itself.
If we are aiming to achieve an occlusal result
that is permanent and unchangeable forever,
then both clinicians and patients are likely
to be disappointed. Indeed, this reflects a
general natural tendency in nature for things
to become less organised with time due to the
laws of entropy within biological systems.
Post-treatment changes can arise due to
relapse, dened as the return of teeth towards
their original position,3 but can also relate to
normal age-related changes. Specically, there
is a natural tendency for shortening of the
arches in relation to length and depth, reduced
perimeter, decrease in intercanine width
and subsequently an increase in crowding,
especi ally in the mandibular anterior
segment.4,5,6,7 However, patients and even
clinicians are unable to distinguish between
relapse and normal age changes. We can either
accept these long-term changes as part of the
normal ageing process or resist them with
Rationale behind the use of xed
Many studies have shown that long-term
stability is problematic when retention is
discontinued, with some degree of relapse
inevitable.1,2,9 Previous researchers have tried
to define acceptable levels of relapse after
retention has been stopped. Little etal.10
and Alexander etal.11 have suggested that
irregularity of <3.5mm in the labial segment
may be considered an acceptable level of
relapse; however, some patients would not
tolerate this amount of irregularity. As a
result, the use of long-term retention has been
recommended.12,13 Fixed retainers can be used
for long-term retention with minimal patient
Surveys on contemp orar y retention
protocols show an increase in the use of xed
retainers, with clinicians either preferring
lifelong retention or not dening a precise
time to remove them.12,15,16,17,18 However,
long-term retention has implications for the
maintenance of xed retainers and there are
questions about who is responsible for the care
of these retainers.19 Is the clinician who placed
the retainer responsible for monitoring and
maintaining it? Studies have shown that within
a couple of years in retention, the majority of
the patients stop attending for regular check-
ups for various reasons or seek treatment only
when the problem is apparent.20 This may
lead to problems only being noticed when it
is too late – when relapse has occurred and
re-treatment is required. It has been suggested
that, provided appropriate training and
remuneration is in place, the retainers can be
monitored as part of a patient’s normal regular
dental check-up and specialist advice sought if
problems are noticed.19
According to some studies, xed retainers have
been found to be safe and predictable,21 and
compatible with periodontal health.9 However,
they undoubtedly do have some disadvantages
Encourag es+balanced+discus sion+amon g+the+
with+bon ded+retainers,+focusing+o n+the+possible+
Discusses the factors inuencin g occurrence of
complications and the potential negative eec ts on
retainers is being increasingly used among clinicians. Bonded retainers can provide an ecient and attrac tive method
1Depar tment of Ortho dontics, Clinic o f Dental Medicine,
First M edical Faculty, Char les Universit y, Katerinska
32, Prague 2, 1280 0, Czech Republic ; 2Orthodont ic
Depar tment, St Luke’s Hospi tal, Little Ho rton Lane,
Bradfo rd, BD5 0NA, UK; 3D epartment of O rthodontic s,
Clinic of De ntal Medicine, Palac ky Universit y, Palackeho
700/12, Olomouc , 77900, Czech Rep ublic."
*Corresp ondence to: Josef Ku čera"
Email addr ess: firstname.lastname@example.org
and complications associated with their use,
• Failure of bonded retainer. is may occur
due to debonding of the composite adhesive
from the tooth, failure of the bond between
the wire and the composite, or fracture of
the retainer. Failure rates may be aected
by clinical technique, the choice of retainer
design and material, adhesive used and the
location of the retainer14,22,23,24,25
• Adverse eect on periodontal health26,27,28
• Unwanted to oth movement with the
• Adverse eect on general health.31,32
When long-term or lifelong retention is
indicated, an appreciation of these pitfalls and
complications becomes crucial.
Failure of a bonded retainer is a relatively
common complication (Fig. 1) and may be
due to detachments of the wire on single to
multiple teeth either at the enamel-composite
interface (type1), wire-composite interface
(type2) or retainer fracture (type 3).24,33 e
prevalence of common complications reported
in the literature varies significantly, from
1%34 up to 53%.35 e wide range in reported
prevalence of failure is due to influence
from a number of factors, including: how
the complications or failures are recorded,
categorised and calculated;24,25 sele ction
bias;25 experience of the clinician bonding
the retainer;25,33,36 dierence in wire type and
dimension,21,22,24,35 and adhesive material
used;23,37,38 type of retainer;14,25,28,34 and, possibly
most importantly, the length of observational
period.24,25,39 If bo nded reta iner s are l e in pl ace
for the long term, it is almost inevitable that
one of the common complications will occur
at some stage. It is, however, very important
to note that these complications, even when
reported as a failure, do not always negatively
aect or signicantly reduce the eectiveness
of the retainer, provided that the problems are
recognised soon and the retainer is restored to
its full function (Fig. 2).25
Fac tors that affect failure rates are
e most common complication is detachment
at the enamel-composite interface and this may
be attributed to technical problems during
the bonding procedure, such as moisture
contamination, insucient cleaning of the
enamel surface before bonding or movement
of the wire during the bonding procedure.22,40
Most of the enamel-composite failures will
therefore appear within the rst six months
aer bonding of the xed retainer.22,35,36,41 It has
been shown that failure rates are lower among
more experienced operators who have placed
more bonded retainers.25,33,36,42
There is a lack of consensus among
orthodontists regarding the optimal type of
retainer wire.12,15,16 e most commonly used
materials for xed retainers are either exible
multi-strand stainless steel wires bonded to
all the anterior teeth, or thick monolament
stainless steel, cobalt-chromium or titanium-
moly bdenum wires bonded only on the
canines (Fig. 3).
Fractures of retainer wires are less frequent
than debonds but are more likely to occur
in places where occlusal contact with the
retainer is present, typically between upper
lateral incisors and canines or lower canines
and first premolars.21,24 The probability of
fracture may increase with time due to wire
fatigue or damage by food particles or forces
e thin, exible multi-strand retainers are
available in dierent diameters, cross-sections
Fig. 1 Dierent types of failures of bonded retainers. a) Type1 – detachment on the enamel-composite interface. b) Type2 – gradual wear and
fractures of the adhesive on the wire-composite interface. c) Type 3 – wire fracture
Fig. 2 a, b, c, d) Repair of retainer failure is simple when identied in a timely manner and does
not necessarily reduce the eectiveness of the retainer
and inner organisation of the filaments.
Diameters of the thin flexible wires range
between 0.0155” to 0.0215” with round cross-
section or0.016x0.022” with rectangular
cross-section. e number of laments range
from 3 to 8and these can be organised and
manufactured in dif ferent patterns; for
example, twisted, braided or coaxial. The
mechanical properties of the wire allow
physiological movement of included teeth
while suciently splinting the teeth at the
same time.43 All multi-strand wires seem to
full this criteria; however, small diameter
wires with fewer laments are more susceptible
to damage and have been reported to have
increased failure rate due to lower bond
strength and mechanical instability.12,21,24,29,37
Multi-strand wires also exhibit better retention
of the wire to the composite when compared
to monolament wires.37
e diameter of the thick wires is typically
between 0.025” to 0.036” and these are usually
round in cross-section. These retainers
have lower failure rates when compared
to thin multi-strand wires bonded to all
incisors.14,22,25 ey are sometimes referred
to as ‘fail-safe bonded retainers’ because,
if they become detached from one tooth,
patients will be aware of this and can contact
their orthodontist. Retainers bonded only
to canines are also easier to keep clean.9,21,38
Lower frequencies of failure and thus higher
survival rates have been reported with thick
retainers bonded only to canines. However,
because they are not attached to the incisors,
these teeth are more prone to movement.14,22,44
Dead-so stainless steel wires are also used
due to the ease of use related to the enhanced
degree of formability, but they can be deformed
by the forces of mastication and food particles,
and they do not maintain the intercanine width
Fibre-reinforced composite, bre-reinforced
plastic or even ceramic materials can be
used.28 However, fibre-reinforced retainers
are prone to greater failure rates due to a lack
of exibility.46 More recently, monolament
nickel-titanium wires constructed by CAD/
CAM procedures have been described and are
currently being investigated.47
Retainer wires are attached to teeth by a layer
of adhesive material. Chemically- or light-
cured composite resin materials can be used
and show similar failure rates.23,37 Flowable
light-curing hybrid composite resin materials
with decreased amount of filler particles
are usually preferred among clinicians for
the ease of application, setting ‘on demand’
and polishability.48 Harder and more wear-
resistant posterior composite materials have
also been suggested as adhesive materials for
xed retainers to resist long-term abrasive
wear from tooth brushing and diet. However,
these rigid materials may be more fragile and
therefore also more prone to failure at the wire-
composite interface, and may complicate wire
Failure between the wire and the composite
is less common but may occur as a result of
gradual wear of the composite layer during
mastication, toothbrush abrasion or occlusal
contacts. e prevalence of this wear increases
with time.22,24 e recommended thickness of
the composite layer has been reported to be
between 0.25mm to 1mm.34 icker layers
exceeding 1mm are not believed to provide
additional advantage.37 If long-term retention
is planned, the composite layer needs to be
checked regularly and will occasionally require
repair to restore function and stability.
Failures of xed retainers are more likely
to occur in the upper arch.24,28,41 is may
be due to occlusal factors. Even when the
wire is placed out of occlusion, occlusal
contacts due to functional movements of the
mandible cannot be excluded and these may
lead to gradual wear of the adhesive layer or
Fig. 3 Dierent thin multi-stranded or thick monolamental stainless steel archwires are used
for construction of bonded retainers. a) 0.0175” three-strand twisted wire. b) 0.0215” six-
stranded coaxial wire. c) 0.016x0.022” eight-stranded braided wire. d) 0.036” monolamental
stainless steel wire bonded only to canines. Panel d is courtesy of Dr Gudrun Edman Tynelius
Fig. 4 Adverse eects on gingival health relate to bonding procedures and on the level of oral
hygiene. a) Bonding of the retainer too close to the gingiva will result in plaque accumulation
and gingival hyperplasia. b) Staining and calculus may form around the retainer; regular oral
hygiene recalls are therefore necessary
wire-fatigue fractures.49 Retainers bonded
to incisors are more prone to failure when
compared to canines in the mandibular arch,
while little difference was found between
central and lateral incisors in the maxillary
arch.25 However, the incidence of failures
increases when upper retainers are extended
to the maxillary canines21 and the first
premolars are included in the lowerjaw.50
Negative eect on periodontal health
Because of the increasing acceptance of
long-term or even lifelong retention with
bonded retainers, t he potential negative
eects on periodontal health are worthy of
consideration. Whi le some authors have
reported no negative effect on adjacent
hard and so tissues in the long term,9,21
increased plaque and calculus accumulation
in the vicinity of bonded retainers with no
detrimental eect on the adjacent tissues is
commonly suggested (Fig. 4),26 and worsening
of periodontal health due to higher plaque
and calculus accumulation in association
with the long-term use of bonded retainers
has also been demonstrated.27 e presence
of calculus in gingival regions adjacent to
bonded retainers may also predispose the
patient to an increased incidence of gingival
recessions.51 It is therefore important to place
the retainer at a sufficient distance from
gingiva, to avoid touching the papillae (in
the incisal third of teeth), ensuring that the
adhesive layer is smooth with no undercuts
predispo sin g to pl aque and calc ulu s
retention.21,22,26,27 is might be challenging
in the upper incisor region, where occlusal
contacts with the edges of the mandibular
incisors may force the clinician to position
the retainer closer to the gingiva. Regular
check-ups and hygiene recalls are needed
to monitor the periodontal health of every
patient, particularly in cases of long-term or
Unwanted tooth movement within the retainer
segment can occur even with the retainer
insitu with no previous or apparent failure of
the retainer. Two distinct forms (Fig. 5) can
• Change in torque between two adjacent
teeth (X eect)
• Opposite tipping of contralateral canines
with torquing of the whole anterior segment
The tooth movement in these cases does
not occur in the direction of original tooth
position and thus cannot be classif ie d
as relapse.14,20,29 The prevalenc e of these
unexpected complications has been reported
to be quite low, between 1.1%20 and 5%.29
However, around 50% of patients presenting
with these complic at ions may requ ire
orthodontic re-treatment (Fig. 6).29 The type
of treatment required varies depending on
the extent of the tooth movement, ranging
from a simple clear plastic appliance based
on a set-up model and short-term fixed
appliance-based treatment when single teeth
are involved, to more comprehensive full
fixed appliance re-treatment and periodontal
surgery when larger changes occur.30
The aetiology of these changes remains
unclear; however, the presence of a tooth-
moving force is a prerequis ite. Sp ec if ic
aetiological factors may be related to:52
The most frequently dis cussed factors
include instability of the thin multi-strand
spiral wires (unwinding or untwisting of
the wire),20,24,29,53 insufficient passivity of
the bonded wire or activation during the
bonding procedure,29 type or position of
the retainer wire,52 or activation of the wire
during the retention period due to occlusal
contact, habits or damage by hard food
particles, which may undoubtedly generate
forces capable of tooth movement.22,29,54 With
increasing time, progressive wear of the
adhesive layer occurs;24 consequently, larger
portions of wire are exposed and this increases
the risk of activation by occlusal forces and
forces of mastication.52 e possible role of
unrecognised failure on the wire-composite
interface due to microleakage55 creating a
‘tunnelling eect’ has also been suggested,
allowing torquing to occur around the wire
as a centre of rotation.52
It has also been hypothesise d that patient-
related factors including bad habits and
par afunction, such as chewing on pens,
sagittal and ver tical relapse, relaps e in
intercanine wi dth or anterior vec tor of
occlusal force, may be influential.52 Other
contributing anatomica l factors that may
influence the severity and speed of the
unexpecte d tooth movement may also
be the individual quality and quantity of
adjacent bone and soft tissues. It has been
well documented in the literature that the
cortical plate surrounding the lower incisors
is often very thin or may show frequent
dehi scenc es an d fenestrations even in
untreated subjects,56,57 especially in those
Fig. 5 Two distinct types of unexpected complication associated with bonded retainers
can arise. a, b) X eect presents with change in torque on two adjacent incisors. c, d) Twist
eect presents with opposite tip on contralateral canines and change in torque in the incisor
region. In severe cases, gingival recession may be present and adjunctive periodontal surgical
treatment may be required
with increased vertical dimension,58 and
orthodontic treatment can in some cases
further accentuate the reduction of already
deficient buccal cortical plate.59 The timing
of onset of these complications var ies,
ranging f rom months to over ten years,
and most likely depends on the aetiological
factor.20,29,53 Un doubt edl y, mis takes in
fabrication and insufficient passivity while
bonding the retainer are likely to occur
rapidly in the first months after bonding
if not counteracted by another retention
device, while other aetiological factors may
act slowly and become apparent after several
years of retention. It is difficult to establish
the tim ing of the onse t of unexp ected
complications, particularly as attendance
of patie nt s du rin g rete ntion pha se i s
intermittent and the unwanted movement
may go unnoticed until it reaches a level
that is apparent to the patient, dentist or
Adverse eect on general health
It has been suggested that prolonged retention
with bonded retainers may have an eect on
general health as a result of problems with the
biocompatibility of the wires and adhesives.31
is is controversial and there is little clear
evidence to suggest bonded retainers can aect
general health. Composite resins have been
reported to have potentially negative eects
on general health as they contain bisphenol A
(BPA), possibly resulting in decreased sperm
count and fertility, increased risks of breast and
prostate cancers, diabetes, negative behavioural
effects or altered immune functions.60 The
amounts of BPA leaching from bonded
retainers invivo is negligible, but care must be
taken to reduce the immediate leach of BPA
out of the uncured layer of adhesive by proper
light curing and polishing.32 Most bonded
retainers are made of stainless steel, which
contains 8% nickel. Nickel hypersensitivity is
common in the population, with an increase
in prevalence in recent decades.61 Fortunately,
the frequency and magnitude of allergic
responses to nickel intraorally is signicantly
lower compared to skin.62 Severe nickel allergy
reactions to xed orthodontic appliances are
rare and present extraorally with swelling of
the lips and eczema lesions, and intraorally
with symptoms of burning sensation, loss of
taste and gingival hyperplasia.63 One case of
facial eczema attributed to the use of a xed
retainer has been reported.64
Bonded retainers are generally well tolerated
because they are invisible, xed in place and
appear to be safe in the long term for the vast
majority of patients. However, they can be
associated with failure, such as detachment or
fracture, and rare but potentially signicant
unwanted tooth movement with the retainer
still insitu. Patients with bonded retainers
therefore require regular review. Patients
and orthodontists, but also treating dentists
and hygienists, should be aware of these
complications so that any problems can be
identied and intercepted as soon as possible,
in order to mitigate the development of
potential adverse eects.
Conict of interest
e authors declare no conict of interest.
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