Occlusion is important for the load distribution; the lesser resilience of the implant segments relative to the adjacent teeth
may force the implants to take the major share. If cuspal contact is allowed, higher cusp inclination leads to higher relative magnitude of the transverse force component and the position of the lateral excursive contact determines the position of the force; the more lateral the contact, the greater the leverage. By centering the occlusal contacts, the lever arm will be reduced. Careful consideration of the design of the occlusal surfaces and the contact pattern is therefore important for limiting the stress on implant and bone. As the basic design of the occlusal surfaces is created by the dental technician, it is important for him or her to be involved at an early stage of treatment.
Furthermore, it is important to diagnose any parafunctional habits. Such habits may contribute to bending overload, as they increase both load magnitude and frequency. Natural teeth or veneering material should be considered indicators of increased loading.
Given the above background, it is possible to present guidelines, mainly from a biomechanical point of view, for designing implant-supported restorations. First, it is important to visualize the design of the final prosthetic reconstruction with regard to dimension, occlusal contacts, and function. Next, consideration must be given to the anatomical limitations related to implant number and position, and how these match the expected support requirements. There should be a balance between demands and possibilities at the initial stage of the treatment planning. In the case of a three-unit prosthesis, the ideal situation from a biomechanical point of view is three implants placed in a slightly curved configuration, with the middle implant offset a minimum of 2 to 3 mm in the buccolingual direction. This tripod implant configuration allows the load response to bending forces to be mostly axial, minimizing the stress level. It can be estimated that the stress level will be reduced approximately 50% by tripodization, compared to a straight-line configuration with the same number of implants. The tripod should be related to where the fixture heads penetrate the anchoring bone. A slight inclination of the implant may be a useful way to achieve such placement.
If only two implants can be placed, they should preferably be placed as end supports, eliminating extensions. A three-unit prosthesis with one cantilever pontic doubles the stress level at the implant closest to the extension, in comparison to the situation in which the pontic is placed between the two implants. As two implants cannot prevent a possible bending moment, it is important to diagnose the patient's functional habits and, if needed, reduce the consequences of any parafunction. Centering the occlusal contacts and eliminating lateral excursive engagements should be considered. There could be as much as a sixfold stress difference between the most adverse load situation (two implants with an extended pontic) and the most optimal one (tripod placement) due only to implant support. Considering occlusal control possibilities would lead to even greater difference between the worst and the best situation.
Currently, the single-tooth molar replacement supported by a single implant has limited long-term clinical documentation. As this situation has a high susceptibility to bending overload, the occlusion should be developed so that only centric contacts exist. Due to the potential high level of stress, fixtures with a diameter of 4 mm or more are recommended for singlemolar replacements. Bruxism, clenching, and the periodontal condition of the adjacent teeth should be carefully evaluated, as such conditions may contraindicate single-implant molar restorations. In many situations, two implants might be needed to sufficiently support a single molar.
To a large extent, the biomechanical considerations for implants follow simple mechanical rules, based on the leverage principle. By considering the patient's functional behavior, limiting the extension of the prosthesis, and controlling the occlusal pattern and contacts, possible overload situations can be minimized. Placing the implants in position and number like tooth roots rather than bridge posts combines biomechanical and esthetic aspects harmoniously. The main biomechanical recommendations for implant treatment in the posterior positions can be listed as follows:
- Strive for a minimum of three implants with one offset a minimum of 2 to 3 mm to provide for axial loading.
- If only two fixtures can be placed, eliminate cantilevers and minimize buccolingual prosthesis contact and cuspal inclinations.
- Slight implant inclination may be used to position the fixture head for optimal support.
- The use of only one free-standing implant to support a fully functioning molar should be approached cautiously. In addition, the occlusion should be centered and light, and a fixture diameter of at least 4 mm should be used.
- Occlusal forces may be difficult to control in situations of reduced periodontal tissue support around neighboring teeth. Treatment planning of such cases should be done cautiously.
- Increased risk of exceeding component and/or bone strength is present in patients with parafunctional habits. Optimal implant support, elimination of cantilevers, and minimization of occlusal contacts should be striven for in these situations.
- Sufficient precision of the fit between prosthesis and abutment and appropriately tightened screw joints are important factors for obtaining the full mechanical strength of components.
- Observation of screw loosening or fracture should be looked upon as overload indicators and should lead to immediate evaluation and correction of the cause to avoid further complications.