In the past, treatment planning for implant dentistry was primarily driven by the existing bone volume in the edentulous site. As a consequence, distal cantilevers were often extended from anterior implants to support a full arch implant prosthesis, rather than performing bone augmentation. A second historical phase of treatment planning then developed, based upon esthetics. In this scheme, implant positions were primarily controlled by the soft tissue contours of the teeth replaced.
Pontics often replace missing teeth, and ovate pontics are easier to develop than esthetic adjacent implant crowns. However, 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 10mm. Screw loosening and uncemented restorations are also complications which cause more prosthetic problems and more often than esthetic consequences of treatment. Hence, biomechanical issues should be a primary issue in developing an implant treatment plan.
Misch developed a treatment plan sequence to decrease the risk of biomechanical overload consisting of: 1) Prosthesis design, 2) Patient force factors, 3) Bone density in the edentulous sites, 4) Key implant positions 5) Implant number, 6) Implant size, 7) Available bone in the edentulous sites and 8) Implant design.2 (Table 1) This article will consider the key implant positions for a prosthesis, when three adjacent teeth are missing in the mouth.
THREE ADJACENT PONTICS
The number of pontics for a prosthesis is related to several factors, including the strength of the abutments (Fig. 1). Since implants seem to be more rigid than natural teeth, the number of pontics with implants is often increased compared to natural teeth. This is an incorrect assumption. In fixed prosthesis design, three adjacent pontics in the posterior regions of the mouth are contraindicated with natural abutments3 (Fig. 2). The adjacent abutments are subjected to considerable additional force when they must support three missing teeth, especially in the posterior regions of the mouth. Likewise, the occlusal force distributed to five posterior teeth should also not be supported by only two implants. The posterior regions have greater bite forces than the anterior regions. In addition, the posterior regions most often have less bone density (less strength) and less volume of bone, compared to the anterior regions. Hence, to compensate for the higher occlusal loads to the prosthesis, especially in reduced strength of bone and smaller implant size, more than two implants are indicated when replacing five adjacent teeth.
In addition to the additional force distributed to the abutments, all pontic spans between abutments flex under load.4 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 and most all materials (i.e. cement, porcelain) are weaker to these types of force.5 The greater the flexure, the greater the risk of biomechanical complications (Fig. 3).
A one pontic span exhibits little flexure, eight microns or less with precious metal under a 25 lb load. A two pontic span flexes eight times more than a one pontic span, all other variables equal. A three pontic span flexes 27 times more than a one pontic span.3 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 posterior region) (Fig.5).
It should be noted the flexure of materials in a long span is more of a problem for implants than natural teeth.6 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.7 Hence, three posterior pontics are contraindicated for a natural abutment fixed prosthesis, it is even more important not to design three pontics in an implant restoration. Angled forces magnify the amount of the force to the implant system, hence, most maxillary anterior prostheses should also limit the number of pontics in the restoration.
The complications of a fixed prosthesis with three pontics include an uncemented prosthesis, porcelain fracture, and abutment screw loosening. The uncemented restoration most often first affects one abutment (the one with least retention or greater lateral force). Once the restoration becomes uncemented from the first abutment, the prosthesis acts as a cantilever on the other implant. A cantilever magnifies the force and the retained implant abutment has a greater risk of crestal bone loss, failure of the implant or fracture of an abutment screw or implant body, especially in the posterior region (which has a greater bite force).
The span of the pontics in the ideal implant treatment plan should be limited to the size of two premolars, which is 13.5mm to 16mm. When a molar is one of the teeth missing between existing teeth, the missing molar space may be 10 to 14mm long. Therefore, when the span is greater than 14 mm, two pontics should be considered to replace the molar.8 As a consequence, when a second premolar and first molar is missing, this span is often treatment planned to replace three teeth, rather than two (Fig. 6). In other words, a missing tooth span is often related to the missing number of roots in the mandible and number of buccal roots in the maxilla.
This is especially appropriate for greater patient forces (i.e. moderate to severe parafunction) or softer bone types (i.e., D3 and D4) (Figs. 8-6). As a result of these guidelines, an edentulous arch missing 14 natural teeth may have 18 potential implant sites.
To limit the effect of the complications of three adjacent premolar size pontics, additional key implant positions are indicated in prostheses missing more than four adjacent roots. Hence, when four to 14 missing adjacent teeth are to be replaced, key implant positions are located in the terminal abutments and additional pier or intermediary abutments are indicated to limit the pontic spans to two premolar size pontics or less.
Following this rule, a five to seven premolar size unit prosthesis has three key abutments (2 terminal and 1 pier) (Fig. 7). An eight to 10 premolar size unit prosthesis has four key implant positions (2 terminal and 2 pier). An 11 to 13 unit prosthesis has five key abutments (2 terminal and 3 pier) and a 14 unit prosthesis has six key abutment positions.
IMPLANT NUMBER
In addition to these key abutments, additional abutments are usually needed to address force factors and/or bone density (Figs. 8-11). Rarely is the force factor situation favorable and bone density ideal enough to be fulfilled with solely key abutments for a fixed prosthesis. The more teeth missing, the more often additional implants are required.
SUMMARY
Complications in implant dentistry most often are related to biomechanical factors. There are several methods to decrease these risks and key implant positions are an effective means. Three adjacent pontics are contraindicated in the posterior region of the mouth for a natural tooth supported fixed prosthesis. This prosthetic axiom is even more critical for implants, since the abutments are more rigid. This article suggests the key implant positions when more than four adjacent teeth are missing. Additional implants are also required in most clinical situations.
Dr. Misch is Clinical Professor and Director of Oral Implantology, Temple University, Philadelphia, PA and Director of Misch International Implant Institute, Beverly Hills, MI.
Oral Health welcomes this original article.
REFERENCES
1.Goodacre CJ, Bernal G, Rungcharassaeng K, Kan JYK. Clinical complications with implant and implant prostheses. J Prosthet Dent, 90: 121-132, 2003.
2.Misch CE., Consideration of Biomechanical Stress in Treatment with Dental Implants, Dentistry Today 25 (5) 80-85, May 2006.
3.Shillingburg HT, Hobo S, Lowell D, et al: Treatment planning for the replacement of missing teeth. In Shillingburg HI, Hobo S, editors: Fundamentals of fixed prosthodontics ed 3, Chicago, IL Quintessence, 1997.
4.Smyd ES: Mechanics of dental structures: guide to teaching dental engineering at undergraduate level, J Prosthet Dent 2:668-692, 1952.
5.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.
6.Misch CE, The evaluation of natural teeth adjacent to implant sites. Contemporary Implant Dentistry 2nd Ed., Carl E. Misch (ed), Mosby, St Louis, MO. Pg. 151-161, 1999.
7.Bidez MW, Lemons JE, Isenberg BR: Displacements of precious and nonprecious dental bridges utilizing endosseous implants as distal abutments. J Biomed Mater Res 20:785-797, 1986.
8.Misch CE. Prosthetic Considerations. In: Contemporary Implant Dentistry, Carl E. Misch, Editor, C.V. Mosby, St. Louis, MO, 2nd edition; 198; 1999.
Table 1
Misch stress theorem
Sequence of treatment plan
Prosthesis Design
Patient Force Factors
Bone Density (D1, D2, D3, D4)
Key Implant Position
Implant Number
Implant Size
Available Bone
Implant Design
