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A Key Implant Position For A Fixed Prosthesis: The First Molar Rule


August 1, 2009
by Carl E. Misch, BS, DDS, MDS, Ph(h. c.) and Jennifer T. Silc, BS, DDS, MS

Treatment planning for implant dentistry in the posterior regions of the mouth is often driven by the existing bone volume in the edentulous sites. As a consequence, distal cantilevers are extended from anterior implants or shorter implants are placed in the posterior regions of the mouth. 1 In the maxillary posterior region, pterygoid and/or zygomatic implants are considered in inadequate bone situations. 2

The primary causes of complications in implant dentistry are related to biomechanics. 3 For example, early loading failures outnumber surgical healing failures, especially in soft bone, when forces are greater than usual and/or implant sizes are shorter than 10mm.4 The posterior regions of the jaws often have the poorest bone density and the least available bone height, especially in the maxilla. Therefore this region is especially prone to biomechanical complications.

When higher biomechanical stresses are applied to the implant “system”, one or more of the components may have complications. The “system” includes the occlusal porcelain, the prosthesis, the abutment screw, the implant components, the marginal bone (especially at the crestal region), the bone-implant-interface and the implant body. Complications from increased biomechanical stress include porcelain, component or implant body fracture, unretained restorations, crestal bone loss, implant mobility and/or failure. 3

Misch developed a treatment plan sequence to decrease the risk of biomechanical overload to a fixed implant prosthesis consisting of: 1) Prosthesis design, 2) Key implant positions for the prosthesis, 3) Patient force factors, 4) Bone density in the edentulous sites, 5) Implant number beyond the key positions, 6) Implant size, 7) Available bone in the edentulous sites and 8) Implant design3 (Table 1). This article will consider the first molar site as a key implant position for a prosthesis.

Key Implant Positions

Some abutment positions are more critical than others in a fixed prosthesis, in regards to biomechanical force reduction. Misch has developed three general guidelines to determine key implant abutment positions:3 (Table 2)

Cantilevers on the prosthesis should be preferably eliminated. Hence, the terminal abutments in the prosthesis are key abutment positions, especially in partially edentulous patients.

Three adjacent pontics should not be designed in the prosthesis, especially in the posterior regions of the mouth.5

The canine and first molar sites are key abutment positions, especially when additional adjacent teeth are missing.

Cantilevers

The maxillary sinus often invades the first molar site and a sinus graft is often necessary to place an implant of adequate length. In the mandible the posterior bone is lost at a faster rate than the anterior region and the presence of the mandibular canal eliminates the use of available bone below this structure. As a result, a cantilever is often prescribed to replace the first molar.

Cantilevers are force magnifiers. The worst case scenario for a biomechanical treatment plan is to place a cantilever in the molar position, which further magnifies the highest force in the mouth. This is especially important to consider in the maxilla, where the bone density is less. The natural teeth are biomechanically designed relative to shape and size. The natural dentition uses two to three roots splinted together in the first molar region.

The exception to the no cantilever rule may be in the completely edentulous mandible, when enough implants are splinted together with cross-arch stabilization. 1 The mandible has flexure upon opening and torsion with heavy biting on one side. A molar cantilever from a full arch splinted prosthesis may be of benefit under these conditions. However, partially edentulous patients or complete maxillary edentulous patients should not have molar cantilevers.

No Three Adjacent Pontics

In most prostheses designs, three adjacent pontics are contraindicated on implants, just as they are contraindicated on natural abutments. 6,7 The adjacent abutments are subjected to considerable additional force when they must support three missing teeth, especially in the posterior regions of the mouth. In addition, all pontic spans between abutments flex under load. The greater the span between abutments, the greater the flexibility of the metal in the prosthesis. The greater the load, the greater the flexure. This metal flexure places shear and tensile loads on the abutments. Materials (as cement, porcelain, etc.) are weaker to these types of force. 8 The greater the flexure, the greater the risk of porcelain fracture, uncemented prostheses and abutment screw loosening.

A one pontic span exhibits little flexure, under a load. A two pontic span flexes eight times more than a one pontic span, if all other variables are equal. A three pontic span flexes 27 times more than a one pontic span. 9 Hence, not only is the magnitude of the force increased to the adjacent abutments when the prosthesis has three pontics (since they are supporting two abutments and three pontics), but the flexure of the metal increases to a point that the incidence of complications make the treatment plan contraindicated, especially when forces are greater (as in the molar region).

It should be noted the flexure of materials in a long span is more of a problem for implants than natural teeth. Since natural roots have some mobility both apically and laterally, the tooth acts as a stress absorber and the amount of material flexure may be reduced. Since an implant is more rigid than a tooth (and also has a greater modulus of elasticity than a natural tooth) the complications of increased load and material flexure are greater for an implant prosthesis.

Hence, It is even more important to limit pontics in an implant restoration to two missing teeth. In addition, the edentulous span of a missing first molar is 10 to 12 mm, compared to a 7mm span for a premolar. This additional span further increases the associated flexure.

A pterygoid implant may be inserted in the third to forth molar position when the maxillary sinus is in the region of the first and second molar with inadequate bone below this structure. When the maxillary sinus has pneumatized to this point, the more anterior implant site is often in the first premolar region, anterior to the maxillary sinus. As a consequence, three to four pontics are often designed in the prosthesis when a pterygoid implant is used without a sinus graft.

Magnitude of Force

The maximum bite force which a patient applies to the central and lateral incisors is in the range of 25 to 35 lbs. This force is increased to 150 to 250 lbs in the molar region, because of class III lever dynamics and the molar is

closer to the tempero-mandibular joint than the anterior teeth. In addition, more muscle mass in the temporalis and masseter muscles contract when the molars engaged in occlusion, compared to only the central and lateral incisors.

KEY IMPLANT POSITION — FIRST MOLAR SITE

Molar Sites

In the biomechanics of an arch, there are some specific locations which are more important than others. In the dental arch, these positions are represented by the canines and first molars. In fact, the natural dentition in both arches respects this biomechanical position. The natural canine has the greatest root surface area of any anterior teeth, and the molar has the most surface area of any posterior teeth. A fixed restoration replacing one or both of these teeth are at greater risk than most any other restoration in the mouth. The maxillary and/or mandibular lateral incisor is one of the weakest teeth in the mouth and the second molar is often one of the weakest poste
rior teeth.

The first molar is a key implant position. Since the bite force doubles in the molar position compared to the premolar position and is up to five times more than the anterior teeth, the natural dentition increases the diameter of the molar and increases the root number, which results in more than twice the surface area of the rest of the dentition.

When a first molar is missing, especially in the maxilla or a partially edentulous mandible, the key implant positions include the terminal abutments for the prosthesis and the first molar position. For example, in a patient missing the first premolar, second premolar, first molar and second molars, there are three key implant positions needed to restore the full function of the missing teeth: the first premolar and second molar terminal abutments and the first molar pier abutment.

CONCLUSION

The most common complications in implant dentistry are related to biomechanical overload and include implant failure, crestal bone loss, abutment screw loosening, uncemented or unretained prosthesis and implant failure. As a consequence, a logical scenario is to reduce force factors in the treatment plan.

There are key implant positions within a prosthesis which are more critical to reduce force. Of these positions, the position of the first molar is one of the more

important locations. The magnitude of the force is increased at this site. Cantilevers or multiple pontic spans increase biomechanical risk factors. Hence, whenever this tooth is included in the implant restoration, an implant should most often be positioned at this site. This is especially noteworthy in the maxilla and partially edentulous mandibles. OH

Carl E. Misch BS, DDS, MDS, Ph(h. c.), Clinical Professor and Director of Oral Implantology, Dept. of Periodontology and Im plantology, Temple Dental School, Philadelphia, PA. Director, Misch International Implant Institute, Beverly Hills, MI.

Jennifer T. Silc, BS, DDS, MS, Clinical Associate Professor, Department of Periodontology and Implantology, Temple Dental School Private Practice, Schaumburg, IL.

Oral Health welcomes this original article.

REFERENCES

1. Adell R, Lekholm U, Rockler B et al: A 15 year study of osseointegrated implants in the treatment of the edentulous jaws. Int J Oral Surg 10: 387-416, 1981.

2. Jensen J, Sinder-Peterson S, Olive AJ: Varying treatment strategies for reconstruction of maxillary atrophy with implants: Results in 98 patients. J Oral Maxillofac Surg. 52:210-216, 1994.

3. Misch CE. Stress treatment theorem for implant dentistry in Contemporary Implant Dentistry, CE Misch editor, 3rd edition, Mosby/Elsevier, 2008, 68-91

4. Goodacre CJ, Bernal G, Rungcharassaeng K, Kan JYK. Clinical complications with implant and implant prostheses. J Prosthet Dent, 90: 121-132, 2003.

5. Misch CE. Treatment Plans related to Key Implant Position: Three Adjacent pontic rule. Oral Health 16-20, 2007

6. Shillingburg HI, Hobo S, Lowell D, et al: Treatment planning for the replacement of missing teeth. In Fundamentals of fixed prosthodontics, Shillingburg HI, Hobo S, editors: ed 3, Chicago, IL Quintessence, 1997.

7. Rosenstiel SF, Land MF, Fujimoto J. In Contemporary Fixed Prosthodontics, CV Mosby 2nd ed. St. Louis, 1995.

8. Bidez MW, Misch CE. Clinical biomechanics in implant dentistry. In Contemporary Implant Dentistry, Carl E. Misch, Editor, C. V. Mosby, St. Louis, MO, 2nd edition; 303-316, 1999.

9. Smyd ES: Mechanics of dental structures: guide to teaching dental engineering at undergraduate level, J Prosthet Dent 2:668-692, 1952.

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The primary causes of complications in implant dentistry are related to biomechanics

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The greater the span between abutments, the greater the flexibility of the metal in the prosthesis. The greater the load, the greater the flexure.

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The maxillary and/or mandibular lateral incisor is one of the weakest teeth in the mouth and the second molar is often one of the weakest posterior teeth

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The most common complications in implant dentistry are related to biomechanical overload