September 1, 2001
by Ron Zokol, DMD, DABOI, FADI
Implant dentistry is now well established in our profession as a recognized and validated protocol for replacing missing teeth. While implant dentistry have been in existence for more than 2000 years,1 only in the last 20 years has its success rates found favour with the profession as a whole. A minimum success rate of 85% for 5 years and 80% for 10 years was stated 15 years ago.2 More recently, another author identifies a criteria for success as 90% for 5 years and 85% for 10 years.3
While we can academically review improvements in success rates with a professional pat on the back, those 90% success rates, otherwise known as 10% failures, are insignificant to those patients who experience failures. To those patients, they have 100% failure rates. Those patients and doctors who have experienced failure of large cases will not soon forget the anxiety and trauma of all parties due to the time, expense, and commitment that went into the development of that large case. The longer-term consequences and, arguably the most profound consequences, are reflected in the attitude of both the recipient and the provider of those failed services. The false conclusions that are developed and the misinformation that is circulated cause other people to avoid the treatment that could significantly improve their lives. Indeed, the integrity and the reputation of the dental profession suffer as well.
It is in the best interest of the public, the individual dentist, and the profession as a whole to ensure that the science of this field is properly trained and that the training and skill of its senior providers is certified. The learning curve of the implant dentist is only partially dependent on the history of his or her “normal” dental training. Even the combined postgraduate training of prosthodontics, periodontics, and oral surgery do not provide for the full knowledge base necessary to avoid some of the pitfalls in implant dentistry.
The single most significant difference between implants and natural teeth is in the mechanism in which each transfers force to the supporting bone structure. The rationale for this “Risk Level” protocol is based on the doctor’s ability to recognize the degree to which occlusal forces will influence the restored implant case creating either complications or failure.
Implant cases can be divided into three categories. These categories are separated by the relative role in which the implant teeth support the occlusal system.
1. Risk Level 1
Risk Level 1 (RL-1) cases are those case where the natural teeth within a dental arch are solely responsible for the complete management of the occlusal system. In other words, if the implant teeth were taken out of occlusion, there would be no significant loss to the occlusal system.
2. Risk Level 2
Risk Level 2 (RL-2) cases have dental arches whose occlusal support systems that are completely supported by implants. This does not mean that there are no natural teeth, but it does mean that if the natural teeth are lost, the occlusal support system is not compromised.
3. Risk Level 3
Risk Level 3 (RL-3) cases have dental arches where the occlusal support system that is dependent on the synchronized function between natural teeth and implant teeth.
Risk Level 1 – Occlusal stability is totally stable on natural teeth.
RL-1 cases are those where the natural teeth can physiologically accept the full occlusal load. Hence, the need to have the implant supported prosthesis assist in occlusal function is unnecessary.
In the case of the maxillary right lateral incisor it is rare that this tooth is necessary in the protrusive or lateral guidance of the mandible. It is both appropriate and necessary to ensure that the lateral incisor is out of function during forced protrusive and lateral excursion movements while the teeth are under parafunctional loads.
Risk Level 2: Occlusal stability is totally dependent on implant teeth.
RL-2 cases have sufficient implant teeth support so that there is no dependence on any natural tooth to maintain stability occlusal function (Figs. 4-8).
Risk Level 3: Occlusal stability is dependent on the synchronized function of natural and implant teeth.
Those cases where neither the natural teeth nor the implant teeth individually can support the occlusal function.
A comprehensive dental examination is essential in determining the level of occlusal force that is being developed by our patients. The areas that require our attention include:
The specific areas influencing risk can then be assessed:
1) Missing teeth a. Presently missing b. Requiring removal
2) Stability of retained teeth a. Bone loss b. Mobility c. Expectation of longevity
3) Malocclusion a. Centric relation to Centric occlusion relationship b. Stability of the anterior guidance system c. Ability to regenerate the anterior guidance system
4) Occlusal traumatism a. Parafunctional habits i. Bruxing ii. Clenching b. Masticatory dynamics i. Gender ii. Age iii. Size c. Level and rate of attrition d. Migration of natural teeth
Unfortunately, a detailed examination of each of these influencing factors is beyond the scope of this paper. The point being made is that all of these factors must be taken into account before treatment is started. It is impossible to overstate the importance in understanding the etiological factors that led to the patient’s present condition. Without correcting the conditions that created the problems, the patient is destined to repeat the dental failure, usually, at a faster rate than natural teeth.
In this particular case (Figs. 9-20), this 63-year-old Caucasian male maintained an uncorrected long-term malocclusion created posterior working and balancing interferences leading to secondary localized periodontitis and eventual loss of all posterior support. You will note (Fig. 13) the lack of vertical bone loss and periodontal disease on this retained teeth. The lack of posterior support leads to excessive forces on the anterior teeth creating one or more of these conditions; 1) excessive attrition, 2) mobility, and/or 3) migration. The presence of long-standing removable prostheses does little to prevent this long-term deterioration of the gnathostomatic system.
The restoration of this case with predictable long-term prognosis requires the stability that comes with endosteal root form implants supporting the posterior occlusion. It follows that there is adequate bone volume and morphology to place the implants in their optimal position to ensure axial loading. Accordingly, a right sinus elevation graft (Fig. 15) and a left posterior mandibular monocortical block graft (Fig. 16) harvested from the left ascending ramus provided the foundation for optimally positioned root form implants.
The completion of the prosthetics phase includes leveling of the plane of occlusion, increasing lost vertical dimension of occlusion, and ensuring a mutually protected occlusion. This involves the provision of adequate anterior guidance in protrusion and lateral excursion so that lateral forces on the posterior dentition are prevented.
Another type of RL-3 case involves occlusal traumatism complicated by secondary localized chronic marginal periodontitis in the posterior maxilla. Unlike the previous case, this case demonstrated mobility of all the maxillary anterior teeth as well as advanced attrition (Fig. 21). Note the lack of bone loss in the mandible while the maxilla suffers extensive periodontal breakdown.
This case involves a 42-year-old Caucasian female. She presents with a stable lower arch with the exception of tooth 36 which requires a full crown. Her maxillary teeth have suffered extensive periodontal deterioration, especially in the posterior segments. She has extensive attrition resulting in the loss tooth structure, adequate lateral and protrusive guidance, and an increased tendency to a class III cuspid relationship bilaterally (Figs. 22, 23). The maxillary molars were removed and a transitional removable partial denture was provided. The amount of bone loss around the maxillary anterior teeth and the right premolars left the possibility that they may be retained in the final restoration. To determine if the rest of the maxillary natural teeth could be saved, they were splinted for a period of 6 months (Fig. 24) and then re-evaluated for stability.
In conditions involving mobility of natural teeth where there has been bone loss it is important to determine the potent of those teeth to continue their role in the dental system. Arguably, the lack of adequate anterior guidance was a significant contributing factor to the eventual loss of the posterior teeth. Subsequently, the anterior teeth became mobile as a result of the lack of posterior support. Without re-establishing a stable anterior guidance system with either natural or implant teeth, any implant teeth replacing the lost molars would suffer the same fate, only sooner.
One treatment option is to remove all the maxillary teeth. While this option is the easiest solution, it is certainly not necessarily in the best interest of the patient. Any time the anterior guidance system can be created on stable natural teeth, the combination of the periodontal ligament buffering forces and the aesthetic quality of the natural tooth and gingivae outweigh the advantages of simplicity.
After a six-month period, the splinted anterior teeth were found to be stable in their splinted configuration. The final treatment plan was confirmed to include the remaining natural teeth and they were prepared for crowns and a splinted provisional restoration was provided for a further six months (Figs. 26, 27).
By the time the implants were integrated and ready for their stage 2 surgery, we had confirmed that the residual natural teeth could maintain their periodontal integrity and we continued the prosthetics phase of the treatment plan.
We established the final vertical dimension of occlusion (Fig. 29) and recorded the interocclusal relationship. The case proceeded through the various laboratory and try-in procedures to its conclusion that included a splinted porcelain fused to metal restoration from 15 to 23 (Fig. 30-36).
These last two cases are examples of two variations of Risk Level 3 cases. In both cases, the prognosis for the natural teeth with implant supported restorations was extremely guarded. As well, the implant teeth that were placed would not be able to support the dental system on their own thereby creating the interdependence of natural and implant teeth.
The differences in the last two cases lie in the etiology of their deterioration processes. Both patients exhibited evidence of parafunctional activity influenced in part by an anterior malocclusion resulting in the loss of posterior teeth and the eventual deterioration of the anterior teeth. However, the last case had the additional influence of periodontal disease that accelerated the rate of deterioration noted by the age and sex differences between the two individuals – a 42-year-old female versus a 63-year-old male.
Being able to assess the risk level of a case prior to starting may make all the difference in the world for both the patient and the doctor. Hopefully, the risk levels identified here will assist those in their early learning curve to choose carefully which cases to treat and which cases to refer.OH
Dr. Zokol maitains an implant and prosthodontic practice in Vancouver, B.C.
1.Misch, CE: Contemporary Implant Dentistry, Mosby, p 445, 1993.
2.Albrektsonn T, Zarb GA, Worthington P et al: The long-term efficacy of currently used dental implants: a review and proposed criteria of success, Inc J Oral Maxillofac implants 1:1-25, 1986.
3.Misch, CE: Contemporary Implant Dentistry, 2nd Edition, Mosby, p 22, 1999.
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