Figure 1a: Bone levels nine years after placement following nerve transposition.
Right side.
Figure 1b: Left side.
Figure 2: Abutments in situ. Three preangled abutments and one engaging shouldered
abutment.
Figure 3: Occlusal view of FPD with access cavities for coping screw to engaging
shouldered abutment and to retaining screws of anterior preangled abutments for
future retrieval.
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Clinical Dilemmas: Restoration of Posterior Quadrants
in the Partially Edentulous Dentition -- 9 Year Retrospective Cumulative Study
Martin Gross, BDS, LDS, MSc
Several clinical dilemmas remain unresolved in planning fixed implant supported
restorations in the partially edentulous dentition. Relevant patient, anatomic,
implant and biomechanical parameters need to be evaluated for each case and risk
factors distinguished from risk indicators. Risk assessment involves the probabilistic
assessment and management of the likelihood of a health event or outcome. In addition
to incomplete knowledge of biomechanical overload and peri-implant disease, there
is insufficient long-term outcome evidence to accurately predict outcomes for a
single patient and implant. A risk factor is causal and its reduction would cause
decreased morbidity incidence and better treatment outcome. Risk indicators exhibit
association, identify high-risk groups or individuals, but whose elimination has
little therapeutic and clinical significance. There is a need for long term outcome
studies to understand early and late implant failures at the subject level, implant
level, implant site level and prosthetic restoration level. At the subject risk
level, medical history, smoking, oral parafunction, oral hygiene, hyperinflammatory
phenotype, bone density, morphology and undefined factors related to cluster factors
are contributory elements. Implant risk includes device morphology surface finish,
coating, length and diameter. Site specific risk includes pocket depth, attached
keratinized mucosa, peri-implant mucositis, bleeding on probing, suppuration, plaque
retentive elements, exposed threads and osteophilic rough surfaces. Biomechanical
and restorative risk includes preload stresses, screw or cement retention, bruxism,
occlusal scheme, implant distribution, number, diameter, crown implant ratio, off-axis
loading, pier abutment cantilever and splinting to teeth.
In most instances the clinician has to make clinical decisions based on insufficient
scientific evidence. Some of the unresolved biomechanical and prosthetic variables
include number and diameter of implants, off axis loading, screw or cement retention,
cantilevers, span length, pier abutments, interposed teeth, splinting to teeth,
crown implant ratio, occlusal pattern and bruxism. This paper presents a 9 year
retrospective study of 205 HA coated cylinder implants restoring posterior quadrants
in the partially edentulous dentition. 205 consecutive HA cylinder implants were
placed by 3 surgeons in 73 patients and restored by one prosthodontist in one practice
with 84 fixed partial dentures (FPDs) between 1990 and 1998. 53% of the implants
were placed in the first 2 years (1990-1991) and around 8% each subsequent year.
55% were placed in the maxilla and 45% in the mandible. 53% were 10mm in length,
37% were 13mm. Short 8mm length distal implants were not used. All cases had anterior
disclusion on the natural teeth and 60% were free end. Either natural teeth or implants
opposed 99% of the FPDs. No immediate failures were recorded. Overall cumulative
implant survival rate was 95% with mean marginal bone loss of less than 2.5 mm in
over 90% of implants (Figures 1a,b). No significant difference in morbidity is seen
between 2 and 3 implant supported FPDs or between maxillary and mandibular implants.
No effects on bone morbidity due to implant diameter, short cantilever, pier abutment
or splinting to teeth were seen. Two cases of tooth intrusion on semi-rigid connection
of teeth to implants occurred out of 16 FPDs. The low incidence of this phenomenon
is consistent with other reports. 98% of superstructures were screw retained indicating
that potential preload stresses did not cause significant bone loss. Isolated cases
with angulation problems were solved prosthetically with combination of screw and
cement retained abutments (Figures 2-3). Bone levels of distal implants were not
significantly different to anterior implants. There is a current tendency to advocate
replacement of one implant per tooth, facilitated by increased predictability of
sinus augmentation, bone augmentation and nerve repositioning (Figures 1a,b). However
the long term success of FPDs supported by 2 and 3 implants as seen in this and
other studies tends to push these options towards the ‘risk indicator’
category. Small samples in this study and other studies also show long term survival
of single implants rigidly connected to adjacent teeth. Certainly more, longer and
wider implants with enhanced bone support reduce risk potential. However, often
financial, anatomical, medical, emotional and age factors preclude their use. Thus
in facing the dilemmas outlined briefly above the clinician should identify the
dilemma, isolate and distinguish risk factors from risk indicators, share the dilemma
with the patient and reach a decision within the framework of informed consent and
evidence based risk assessment when possible.
Failures occurred as cluster failures. Severe lingual implant inclination, overload
in class II dentition and one case of long standing screw loosening were principal
biomechanical factors deemed contributory to the isolated cases of failed implants.
Bruxism did not contribute to increased morbidity, probably due to anterior disclusion
on the anterior natural teeth. Most of the failures occurred after several years
of loading. Peri-implantitis with chronic exudate and bone loss was seen in one
early cluster failure of 3 implants, and subsequent to biomechanical overload after
5-7 years in 4 cases. The incidence of failure modes related to implant form and
surface finish needs to be further clarified.
The high success rate of the vast majority of implants and FPDs restoring posterior
quadrants in the partially dentulous dentition in this single practice consecutive
patient sample supports the validity of the treatment modalities employed.
Many more clinical outcome studies are urgently needed to take the guesswork out
of clinical decision making and define the multiple parameters of risk factors and
risk indicators for effective risk assessment and predictable treatment outcome.
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