Key Implant Positions: Treatment Planning Using the Canine and First Molar Rules

Historically, treatment planning for implant dentistry was mainly driven by the existing bone volume in the edentulous sites. As a result, distal cantilevers were extended from anterior implants or shorter implants were placed in the posterior regions of the mouth. A second historical phase of treatment planning has since developed based upon soft-tissue aesthetics. In this scheme, in order to improve the interproximal papillae, implant positions in aesthetic regions are limited and replaced by ovate pontics.
The primary causes of complications in implant dentistry are related to biomechanics.¹ 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 10 mm. 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 include porcelain, component or implant body fracture, unretained restorations, crestal bone loss, implant mobility and/or failure.² Misch developed a treatment plan sequence to decrease the risk of biomechanical overload 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 design (Table 1).² This article will consider the canine and first molar sites as key implant positions for a prosthesis.

GUIDELINES FOR KEY IMPLANT POSITIONS

With regard to biomechanical force reduction, some abutment positions are more critical than others in a fixed prosthesis. There are 3 general guidelines to determine key implant abutment positions:² (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.
-The canine and first molar sites are key abutment positions, especially when additional adjacent teeth are missing.

BIOMECHANICS MUST BE CONSIDERED
In the biomechanics of an arch, there are some specific locations that are more important than others. In the premaxilla of the dental arch, these positions are represented by the canines. 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. A fixed restoration replacing a canine is at greater risk than most any other restoration in the mouth. The maxillary and/or mandibular adjacent incisor is one of the weakest teeth in the mouth and the first premolar is often one of the weakest posterior teeth.
A traditional fixed prosthetic axiom indicates that it is contraindicated to replace a canine and 2 or more adjacent teeth.^3,4 Therefore, if a patient desires a fixed prosthesis, implants are required whenever the following adjacent teeth are missing in either arch: (1) the first premolar, canine and lateral incisor; (2) the second premolar, first premolar, and canine; and (3) the canine, lateral, and central incisors (Figure 1).

Figure 1. When the maxillary right canine, lateral incisor, and central incisor are missing; the key implant positions are the canine and central incisor.	Figure 2. When the maxillary first premolar, canine, lateral incisor, and central incisor are missing; the key implant positions are the first premolar, canine and central incisor.
Figure 3. A patient missing the right maxillary first premolar to the right central incisor. There is inadequate bone in the key implant sites.	Figure 4. Blocks of bone from the mandibular symphysis are fixated to the edentulous site to augment the key implant regions.
Figure 5. After 5 months of healing for the bone graft, endosteal implants are inserted into the key implant positions: the first premolar, the canine, and the central.	Figure 6. A 4-unit fixed implant prosthesis is cemented to the implants after 4 months of healing.
Figure 7. When the first premolar, second premolar, first molar and second molar are missing: the key implant position are the first premolar, first molar and second molar.	Figure 8. Implants are inserted into the posterior left mandible, in a patients missing 4 adjacent teeth. The key implant positions are the first premolar, first molar and second molar.
Figure 9. A splinted 4-unit fixed prosthesis is cemented onto the implant abutments after 5 months.	Figure 10. A panoramic radiograph from Figures 8 and 9.

Whenever the canine and 2 adjacent teeth are missing, implants are required to restore the patient because (1) the length of the span is 3 adjacent teeth, (2) the lateral direction of force during mandibular excursions increases the stress against the implant support system, and (3) the magnitude of the bite force is increased in the canine region compared to the anterior region. Therefore, under these conditions, at least 2 key implant positions are required to replace these 3 adjacent teeth, including the canine (especially when one of the terminal abutments is the canine position).
In most prostheses designs, 3 adjacent pontics are contraindicated on implants, just as they are contraindicated on natural abutments.⁴ The adjacent abutments are subjected to considerable additional force when they must support 3 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 is the flexibility of the metal in the prosthesis. The greater the load, the greater the flexure will be. This metal flexure places shear and tensile loads on the abutments.⁵ The greater the flexure, the greater will be the risk of porcelain fracture, uncemented prostheses, and abutment screw loosening.
A one pontic span exhibits little flexure under a load.6 A 2 pontic span flexes 8 times more than a one pontic span, if all other variables are equal. A 3 pontic span flexes 27 times more than a one pontic span. Hence, not only is the magnitude of the force increased to the adjacent abutments when the prosthesis has 3 pontics (since they are supporting 2 abutments and 3 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 canine 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. Since it is contraindicated for 3 posterior pontics in a natural tooth fixed prosthesis, it is even more important to limit pontics in an implant restoration to 2 missing teeth.

DIRECTION OF LOAD AND MAGNITUDE OF FORCE
Angled loads to implant bodies increase the force to the implant system. A 15≤ angled force increases the amount of force by 25.9%, while a 30 ≤ angled force increases the force 50% more compared to an axial force in the long axis of the implant.⁷ As a consequence, the risk of abutment screws loosening in the implant system is increased, an increased risk of crestal bone exists, the implant body may fracture and/or the implant may become mobile.
This is especially noteworthy in the maxilla, since the force starts inside the arch and is pushed outside of the arch. An arch is stronger when the force is applied from the outside to inside (as in a mandibular situation). Since angled forces magnify the amount of the force to the implant system and in the maxilla the forces are directed to the outside of the arch during mandibular excursions. Most maxillary anterior prostheses should limit the number of pontics in the restoration.
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 90 to 100 lbs in the canine region. This is because of Class III lever dynamics and the canine is closer to the temporomandibular joint than the anterior teeth. In addition, more muscle mass in the temporalis and masseter muscles contract when the canine is engaged in occlusion, compared to the central and lateral incisors.

KEY IMPLANT POSITIONS
When the second premolar, first premolar and canine are missing, the key implants positions are the canine and the second premolar. When canine, lateral incisor, and central incisor are missing; the key implant positions are the canine and central incisor. When the 3 adjacent teeth are the first premolar, canine, and lateral incisor; the key dominant implant positions are the first premolar and the canine. When the lateral incisor is wider than 6 mm, an implant is also placed in this area to avoid a cantilever. However, when the lateral incisor is less then 6 mm the first premolar and canine may replace the 3 teeth. These implant positions result in an anterior cantilever to replace the lateral incisor. However, since the smaller than average lateral incisor is in the anterior region with the least bite fore, the cantilever is of limited negative impact. In addition, the occlusion is modified so no occlusal contact is present on the lateral incisor pontic in centric occlusion or excursions of the mandible.
When there are multiple missing teeth, and the canine edentulous site is a pier abutment position, the canine position is a key implant position to help disclude the posterior teeth in mandibular excursions. Hence, when 4 or more adjacent teeth are missing, including a canine and at least one adjacent posterior premolar tooth, the key implant positions are the terminal abutments, the canine position, and additional pier abutments which limit the pontics spans to no more than 2 teeth (Figures 2 to 6).
The first molar is also a key implant position. The bite force doubles in the molar position compared to the premolar position in both maxilla and mandible. The natural dentition increases the diameter of the molar as a consequence of the increased force and the first molar has twice the surface area of a premolar tooth. In addition, the edentulous span of a missing first molar is 10 to 12 mm, compared to a 7-mm span for a premolar.
The maxillary sinus often invades the first molar site and a sinus graft is often necessary to place an implant of adequate length. As a result, a cantilever is often prescribed to replace the first molar. 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. Remember, the natural dentition uses 3 roots splinted together in the first molar region.
When a first molar is missing, 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 3 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 (Figures 7 to 10).

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 that are more critical to reduce force. Of these positions, the positions of the canine and first molar are 2 of the more important locations. Both the magnitude of the force is increased and the direction of force is modified at these arch locations. Hence, whenever these teeth are included in the implant restorations, an implant should be positioned in these sites.

References

1. Goodacre CJ, Bernal G, Rungcharassaeng K, et al. Clinical complications with implants and implant prostheses. J Prosthet Dent. 2003;90:121-132.
2. Misch CE. Stress treatment theorem for implant dentistry. In: Contemporary Implant Dentistry. 3rd ed. St. Louis, Mo: Mosby Elsevier; 2008: 68-91.
3. Rosenstiel SF, Land MF, Fujimoto J. Contemporary Fixed Prosthodontics. 2nd ed. St. Louis, Mo: Mosby Elsevier; 1995.
4. Shillingburg HT Jr, Hobo S, Whitsett LD, et al. Treatment planning for the replacement of missing teeth. In: Fundamentals of Fixed Prosthodontics. 3rd ed. Chicago, Ill: Quintessence Publishing Co; 1997.
5. Bidez MW, Misch CE. Clinical biomechanics in implant dentistry. In: Misch CE, ed. Contemporary Implant Dentistry. 2nd ed. St. Louis, Mo: Mosby; 1999:303-316.
6. Smyd ES: Mechanics of dental structures: guide to teaching dental engineering at undergraduate level. J Prosthet Dent. 1952;2:668-692.
7. Misch CE, Bidez MW. Occlusal considerations for implant supported prostheses: implant-protective occlusions. In: Misch CE, ed. Dental Implant Prosthetics. St. Louis, Mo: Mosby; 2005:472-510.

Dr. Misch is clinical professor and director of Oral Implantology, Department of Periodontology and Implantology, Temple Dental School, Philadelphia, Penn. Dr. Misch is also director, Misch International Implant Institute, Beverly Hills, Mich. He can be reached at (248) 642-3199 or via e-mail info@misch.com.

Disclosure: Dr. Misch is the Chief Scientific Advisor for BioHorizons Dental Implants. 

Dr. Silc is clinical associate professor, Department of Periodontology and Implantology, Temple Dental School, Philadelphia, Penn and in private practice, Schaumburg, Ill. She can be reached at (847) 605-0280 or reached via e-mail at jtsilc@yahoo.com.

Disclosure: Dr. Silc reports no conflict of interest.