Written by Carl E. Misch, BS, DDS, MDS, PhD (hc) Wednesday, 11 August 2010 09:21
In an interview conducted by Dr. Damon Adams, Dentistry Today’s editor-in-chief; Dr. Carl Misch, co-chairman of the Board of Directors of the International Congress of Implantologists, the world’s largest implant organization; and founder of the Misch International Implant Institute; shares his opinions and knowledge on a topic that should hold keen interest for anyone placing dental implants.
Dr. Adams: Can you comment on some of the risk factors when it comes to treatment plans for implant dentistry?
Dr. Misch: Implant dentistry has become the most predictable method to replace missing teeth. The doctor and the patient often have an incentive to do treatment which is faster, easier, and less expensive. As a consequence, treatment planning for implant dentistry is most often driven by the existing bone volume in the edentulous sites. This method is often problematic. In partially edentulous patients, more than 6 mm of bone height is found in 40% of posterior maxillae and 50% of posterior mandibles. This percentage is further reduced to less than 20% of completely edentulous patients in either arch. Therefore, the treatment plan often presented to the patient uses short implants in the posterior region, where the forces are greater and the bone is less dense.
The typical fees associated with treatment in implant dentistry are related to the number of implants and teeth replaced. Hence, a 3-unit fixed partial denture (FPD) supported by 2 implants is one-half the fee of a 6-unit FPD supported by 4 implants. As a result, instead of posterior bone grafts and additional implants supporting a 3-unit FPD with a 4-unit FPD, a distal cantilever is often extended from 2 anterior implants, since more vertical bone is found anterior to the maxillary sinus in the maxilla, or the inferior alveolar nerve and mental foramen in the mandible.
Dr. Adams: Implant surgical success rates are often reported at about 98%. What is the concern then of short implants, soft bone around the implants, or biomechanical stress to the prosthesis?
Dr. Misch: When implants are inserted into abundant bone volume and allowed to integrate for 4 or more months before loading, the surgical success rate is higher than 98%. This success rate is not related to implant number, size, or design. However, when the implant is occlusally loaded with the prosthesis, the failure rate may be greater than 3 to 6 times the surgical failure. For example, a meta-analysis reveals 15% failure rates (with several reports of rates higher than 30% failure) when the implant prosthesis is occlusally loaded with implants shorter than 10 mm, or when they are placed in softer bone. This failure most often occurs during the first 18 months of loading and is called early-loading failure. The primary cause of this complication in implant dentistry is related to biomechanical factors, with too much stress applied to the implant support system.
Mechanical complications of the implant components or prosthesis outnumber surgical failures and many reports are more frequent than early-loading failures. These complications include abutment screw loosening, uncemented prostheses, and porcelain fracture. These complications are found more often in bruxism patients (males) when opposing implant prostheses and with group function occlusion. All of the factors increase the amount of stress on the implant system (occlusal porcelain, cement, implant abutment screw, and implant-bone interface). Hence, mechanical complications are also related to biomechanical factors.
Biomechanical stress may also cause marginal crestal bone loss. Since the implant does not have a periodontal membrane like a tooth, the stress to the implant-bone interface is mostly to the crestal marginal bone. When the stress is beyond the bone physiologic limit, resorption may occur. The bone loss may increase the risk of anaerobic bacteria and peri-implantitis, or the surrounding soft tissues may shrink and result in poor cervical aesthetics. Hence, biomechanical factors can lead to early loading failure, mechanical complications, and/or marginal bone loss around an implant.
Dr. Adams: Cantilevers are often used for implant prostheses. Is this also a risk factor for success?
Dr. Misch: Cantilevers on the prosthesis are stress magnifiers to the implant system. When used in the posterior regions, the greater bite force—up to 5 times greater than the anterior region—is further magnified and may increase the force on the implant system by an additional 3 times. In other words, a bite force of 100 lbs exists on a premolar implant. This force may be magnified by a 200-lb force on a posterior cantilever, which results in a 400-lb force on the anterior premolar abutment, since it is applied to a lever.
Dr. Adams: What is the disadvantage of doing a bone graft to eliminate a cantilever on the prosthesis?
Dr. Misch: In order to eliminate posterior cantilevers, a bone augmentation is often indicated. Most bone augmentation procedures are not as predictable as implant integration in existing bone volumes. Bone augmentation often requires an additional surgery prior to implant placement. Additional training is required to learn bone augmentation procedures, and the learning curve is longer and it is more difficult to become accomplished in these techniques. Complications related to bone augmentation are more common than implant surgery in existing bone volumes and may be more extensive and even debilitating to the patient.
The discomfort following bone augmentation is usually more than that which occurs after implant surgery. An extended healing time of 4 to 9 months may be necessary for the bone graft to mature, compared to implant healing in native bone. The costs associated with bone augmentation are often greater than the fees related to implant insertion. In addition, there are usually more implants and more teeth replaced after bone augmentation compared to situations when implants are inserted into existing volumes of the bone and teeth are cantilevered to the posterior regions. More implants and more teeth replacements further increase the cost to the patient. As a consequence of these considerations, the doctor and the patient are both motivated to use existing bone volumes for implants and restore fewer posterior teeth in the prosthesis. However, this method of tooth replacement has an increase in biomechanical related complications.
Dr. Adams: Can you give an example of increasing a risk factor in an implant treatment plan?
Dr. Misch: An example of the patient and doctor having incentive to perform procedures with higher biomechanical risks is when a patient has 4 teeth missing in a posterior maxillary quadrant (2 premolars and 2 molars), with a pneumatized maxillary sinus cavity. There are typically 2 treatment options: The first is to place 2 implants anterior to the sinus, which supports a 3-unit prosthesis (with a first molar cantilever). A second option is to perform a sinus bone graft, insert 3 implants (in the first premolar, first molar, and second molar position), and to fabricate a 4-unit restoration.
The first treatment option is one-half the fee of the second option, since it doesn’t require a sinus graft and involves fewer implants and fewer teeth replaced. The first option is also faster and easier, since a bone augeneration is not required. The patient undergoes one surgery and therefore experiences less discomfort. However, the second treatment option has 3 to 4 times better chance for long-term success, since it doesn’t cantilever a pontic in the molar region. Since cantilevers increase the biomechanical force to the anterior implants, there is an increased risk of an unretained prosthesis on the first premolar. This is because of a tensile force to the retainer, and cement is 20 times weaker to tension compared to compression. This results with one implant—the second premolar—supporting 3 teeth and the risk of overload and failure. In addition, the first option often has more bone loss from occlusal overload related to the increased biomechanical stress as a result of the cantilever (Figure 1). In addition, a mandibular second molar may erupt past the plane of occlusion with the first option, since it only has one molar, and each protrusive mandibular movement would result in a lateral premature contact on the maxillary prosthesis. This force direction increases the shear force, and may even trigger parafunction. As a result, all complications that are related to biomechanical stress are increased.
Biomechanical related complications often occur within the first few years of function. As a result, the patient expects the dentist to repeat the treatment for no charge. When the first option fails, the second treatment option may be selected, often from a different dentist, which is associated with a greater cost. As a result, the patient is more likely to bring litigation against the first treatment team in order to pay for the additional costs of the second treatment option.
As a consequence of an increased risk of complications in the first treatment option, the fees for this option should be more than the second treatment option. In other words, the fee for services rendered should not only be based upon the sum of the number of implants and teeth in the prosthesis; it should also include the amount of risk associated with the treatment.
Dr. Adams: Do risk factors related to cost also apply to other restorative procedures?
Dr. Misch: A more basic example of charging for risk factors is the treatment for a crown on a maxillary central incisor compared to a mandibular molar. The time and technique for an anterior preparation, impression, and transitional prosthesis is greater than that to restore a mandibular posterior tooth. The risk that an anterior maxillary crown has to be redone because of gingival recession, shade selection, etc, is greater than the mandibular crown. Yet, most dentists charge the same fee for both procedures. The maxillary anterior crown has more risk associated with it; therefore the fee should be greater.
Dr. Adams: How does this risk/cost factor affect the maxilla versus the mandible?
Dr. Misch: When a full-arch fixed implant restoration is the treatment for a maxillary arch, the number of implants is often the same as the mandible. For example, “all in 4” is a common treatment option presented to the profession in either arch, along with similar fees for either arch to the patient. Yet, the maxillary fixed restoration is supported by softer bone. The hardness of the bone is related to its strength. The mandible more often has hard (strong) bone and the maxilla more often has softer bone. In fact, the posterior maxillary bone may be 5 to 10 times weaker than the hard bone of the anterior mandible (Figures 2 to 8).
The maxillary anterior arch receives a force at a 12° to 15° angle during occlusion and up to a 30° angle in excursions. A 15° angled force increases the force component on the implant by 25.9%, and a 30° force increases the force by 50%. This is a biomechanical rationale for the reason why maxillary anterior teeth are larger than mandibular anterior teeth.
The excursive forces in a maxillary restoration come from within the arch to push outside the arch. This force direction on an arch is more detrimental than in the mandible. The mandible receives a force from outside of the arch toward the inside of the arch, which is the mechanism of force the arch was designed to resist.
The maxillary arch usually has shorter implants than the mandible, since the vertical height of bone is less compared to the anterior mandible. The shorter implants have less surface area and higher stresses, especially in soft bone. Therefore, the biomechanical risks associated with full arch maxillary prosthesis is greater than mandibular restoration. A literature review of full-arch prostheses reports a 3-times higher implant failure rate in full-arch maxillary implant fixed restorations compared to full-arch implants and a mandibular restoration.
Aesthetic retreatments and complications are more often observed in the maxillary restoration compared to the mandible prosthesis. The maxillary lip translation during smiling is greater than the mandibular lip movement during speech. The aesthetic requirements of the maxillary restoration must often include the soft tissue drape around the teeth. As a consequence, aesthetic complications most often occur in maxillary versus mandibular prosthesis. The air can escape between the residual ridge and prosthesis and aesthetic requirements for the patients are primarily obtained by the maxillary restoration. In other words, the maxillary full-arch restoration should be treatment planned differently and should cost more than a similar restoration in the mandible (Figures 2 to 8).
Dr. Adams: Can you summarize the risk/cost factors for implant dentistry?
Dr. Misch: The fees associated with implant surgery and prosthetic rehabilitation should reflect the risks related to the treatment. To compensate for the softer bone, higher biomechanical stress, and prosthetic risks, the maxillary arch should more often have bone augmentation (to eliminate posterior cantilevers), more implants inserted, and higher prosthetic fees than a mandibular arch. The treatment plan in implant dentistry should have a biomechanical rationale to decrease stress to the implant system. The fees for an implant treatment plan in the maxilla that has fewer implants and/or cantilevers should be greater than restorations supported by more implants and/or without cantilevers. The risks in dentistry are a factor which should be included in the cost of most all procedures that are associated with greater complications. The implant and associated restoration is not a commodity in which the cost is solely related to the number of implants and prosthetic units.
Dr. Adams: Carl, I want to sincerely thank you so much for your time in sharing your expertise with our readers.
Disclosure: Dr. Misch is a consultant for BioHorizons, Materialize, and DTI Laboratories.
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