“Rules of 10″—Guidelines for Successful Planning and Treatment of Mandibular Edentulism Using Dental Implants

[Anonymous]

Rule No. 3

Anterior/Posterior Distribution of Implants Must be at Least 10 mm for the ISFP

The ISFD was originally envisioned for treatment of mandibular edentulism by using the abundant bone of the mandibular parasymphysis. A cantilever design of the ISFD was inherent to the solution, using multiple anterior implants. The implants must be able to support functional loads at the posterior occlusal contacts via the cantilever. These loads, however, are magnified within the framework and components, and potentially at the implant/bone interface. In the early conceptualization of this therapy, the anterior-posterior distribution of dental implants was recognized as a key factor affecting the incidence of complications in the cantilevered mandibular ISFP. To counteract the imposed bending moments of the loaded cantilever, maximum distribution of implants was recommended. This anterior-posterior distribution of implants is referred to as the “A-P spread.” Clinicians were quick to point out that there were anatomic constraints for implant placement in the parasymphyseal mandible. Compared to curved or V-shaped mandibles, square-shaped mandibles often provide little anterior-posterior dimension anterior to the inferior alveolar nerve (Figure 7 through Figure 9). Additionally, anatomic variations in the inferior alveolar nerve (eg, anterior loop) are not uncommon and can reduce the available A-P spread.

A number of different models have been used to estimate the proper cantilever length in relationship to the A-P spread. These approaches include the use of photoelastic models, piezoelectric strain sensors, and finite element models. The results are diverse and the majority examined the stresses that accumulate at the implant bone interface. Interestingly, the focus on the implant, per se, does not match the clinical situation where implant failures are infrequent and prosthesis complications are more prevalent.

Any discussion of cantilever length requires that: 1) the position of the distal-most implant be anticipated; and 2) the number of teeth to be provided distal to that implant be defined. For the purposes of establishing a concept that meets the needs of most patients, the goal is to have the distal implant in the distal-most location that does not impose on the inferior alveolar nerve, which is generally located in the canine or first premolar region. Further, distal inclination of the posterior implants may place the prosthetic interface even more distal in the first premolar region.

The all-on-four concept generally requires that posterior teeth beyond the first premolar are supported by a cantilever. Efforts to reduce or eliminate the cantilever by distal orientation of terminal implants are advocated. Bridge screws emerging at the first premolar or molar position can be achieved in this way. Malo has reported that this approach is associated with high implant and prosthesis survival over 10 years. In 2011, Malo reported on 245 patients in whom 980 implants were placed and immediately loaded. The cumulative patient-related and implant-related success rates were 94.8% and 98.1%, respectively, at 5 years and 93.8% and 94.9%, respectively, at 10 years.

One other key factor contributing to the overloading of these components is prosthesis misfit. However, the accuracy of contemporary impression materials and the introduction of precision-milled frameworks for the ISFP reduce the prevalence of prosthetic misfit and its impact on the therapeutic outcome. The geometric distribution of the implant-abutment and the abutment-prosthesis interfaces remain the significant features of ISFP therapy that can be clinically managed to reduce complications.

When considering the number of teeth to be provided distal to the canine or first premolar site, the minimal provision of one additional premolar and one molar is sufficient to meet the esthetic and functional requirements of most individuals. These general guidelines can be converted to linear measurements. The average dimension of a mandibular premolar is approximately 6 mm, and that of the mandibular first molar is approximately 10 mm. Thus, a cantilever of 16 mm can suffice to provide function and esthetics. The functional relationship between the cantilever length and the A-P spread (Figure 7 through Figure 9) has been debated. More than 30 years of opinion, experimentation, and calculation have generated an array of suggested solutions. To provide the broadest range of success for the largest set of patients, a conservative estimate of this functional relationship should be selected. For a mandible with no more than four implants to be restored with a rigid framework, utilizing a cantilever length to A-P spread ratio of 1.5:1 has been advised.Thus, for the ideal situation of four implants placed in the parasymphyseal mandible with the distal-most implants located in the first premolar region, a cantilever of approximately 15 mm (one premolar and molar tooth) requires 10 mm of A-P spread. This represents the third Rule of 10.

Applying the Rules of 10

Several key steps are required for using the Rules of 10 in the treatment of mandibular edentulism. The previously mentioned reference points (ie, occlusal plane and osseous crest) must be firmly established. To assure accurate measurement, all treatment should begin with the proper fabrication of complete dentures and verification of ideal tooth position (Figure 10 and Figure 11).

Rule No. 1 requires a volumetric assessment of the edentulous mandible with cone-beam computed tomography (CBCT). However, other important information can be found in the radiographic process, and no radiograph should be made for ISFP treatment-planning purposes without the presence of a radiographic stent. The resultant image(s) should display the location of the planned prosthesis in relation to the mandible.

Rule No. 2 requires that the plane of occlusion is properly located and the appropriate vertical dimension of occlusion is defined. If the patient is also edentulous in the maxilla, this involves the fabrication of ideal maxillary and mandibular dentures. The dentures will define the location of the occlusal plane and mandibular tooth position, where the mandibular denture can be duplicated in radiopaque acrylic for a radiographic stent. The amount of alveolectomy needed can then be determined from the CBCT images.

Rule No. 3 requires an understanding of the anatomy of the edentulous mandible in relationship to the location of the planned prosthetic teeth, as well as the ability to translate this information to the implant placement, generally via a surgical guide. This can be accomplished practically in one of two ways. One method involves evaluation of the CBCT images using 3-D planning software (eg, Materialise Simplant^®, Materialise Dental, http://www.materialise.com; NobelClinician™, Nobel Biocare, http://www.nobelbiocare.com) and then modifying a duplicate denture made from clear acrylic (Figure 10 and Figure 11). The other involves use of a third-party company to fabricate a digital surgical guide.

Conclusion

Dental implant therapy for the edentulous mandible has been successful. Data concerning implant survival is high and reflects the quality and quantity of bone available for osseointegrated implant function. The complications associated with both removable and fixed dental implant prostheses reflect the constraints of current materials and design limitations. The Rules of 10 assure that there is: 1) adequate bone for osseointegration and its long-term success; 2) sufficient dimension for fabrication of an esthetic, comfortable, and robust prosthesis; and 3) proper distribution of imposed forces from occlusal function within the prosthesis, at the implant/abutment screw interfaces, and at the implant/bone interfaces. Following these simple geometric and linear guidelines to treatment planning enables proper implant placement decisions that underscore robust and lasting prosthesis construction.