Implant-protected occlusion: A biomechanical rationale
This article discusses the prosthetic and fundamental biomechanical considerations and presents the concept of "implant-protected occlusion." The mobility and shock absorption of a natural tooth versus an implant is vastly different. The fibrous interface surrounding the natural teeth acts as a viscoelastic "shock absorber,² serving to decrease the magnitude of stress to the bone at the crest. The implant does not have such an apparatus and therefore cannot dissipate stresses and strains. The width of almost every tooth is greater than the width of the implant to replace that tooth. The greater the width of the implant, the less the magnitude of stress that is transmitted to the surrounding bone. Implants generate more inhomogeneous stress and strain profile at the crest of bone compared to a tooth in similar mechanical loading conditions. If an initial load of equal magnitude and direction is placed on both an implant and a natural tooth, the implant will bear a higher proportion of the load. The implant-protected occlusion concept protects the implant-bone crestal region. Occlusal contact in centric occlusion on implant prostheses when natural teeth are adjacent requires a reduced initial mechanical load on the implants. Light force and thin articulating paper are first used to evaluate centric relation of the occlusal contacts. The implant crowns are relieved, which places heavier contact on the adjacent natural teeth. A greater force is applied to the articulating paper, with equal regions established on the implant-supported crowns and natural teeth. Excursions are evaluated after CR occlusion has been corrected. Many occlusal schemes in which the natural teeth oppose each other suggest the anterior teeth disclude the posterior teeth in excursions. Anterior, compared to posterior, bite force measurements provide evidence that the stomatognathic system elicits less force when the posterior teeth are not in contact. So occlusion opposing fixed prostheses or natural teeth should disclude the posterior segments.
A similar scenario is used when anterior implants and teeth disclude the posterior dentition in excursions. Light force and thin articulating paper are first used to ensure that no implant crown contact occurs during the initial lateral movement. A heavier force during excursions is balanced to provide similar occlusal contacts on both anterior implants and natural teeth. The implant orientation influences the direction of the load. The primary component of the occlusal force should be directed along the long axis of the implant body and not at an angle or following an angled abutment post. An angled load to the implant increases the compressive forces at the crest of the ridge on the opposite side of the implant where the force is directed. The greater the angle of the force to the long axis, the greater the potentially damaging load at the crest of bone. The greater the crown height, the greater the resulting crestal moment with any lateral component of force which develops as a consequence of an angled load. Implant-protected occlusion attempts to eliminate or reduce all shear loads to the implant-bone interface. Shear forces are increased with an angled load to the implant body. The implant-protected occlusion attempts to eliminate lateral or angled loads to an implant-supported prosthesis. Premature occlusal contacts also result in localized lateral loading of the opposing contacting crowns. Elimination of this is especially important when habitual parafunction is present because the magnitude and duration of the force is increased. This is more important in implants that are less mobile than natural teeth and cannot effectively dissipate force.
Another important parameter is the adequate surface area of support for the load transmitted to the prosthesis. Implants that have decreased surface area or are angled have increased stress and strain magnitude in the interfacial tissues. This can be reduced by placing an additional implant in the region of concern. Wider root-form implants have a greater area of bone contact at the crest than do narrow implants. Therefore, the mechanical stress at the crest is less with wider implants. The implant-protected philosophy indicates additional implants when smaller diameter implants are used. When narrow implants and/or short implants are used, the effect of moment loads should be reduced by less offset loads, shorter cantilever, and minimal horizontal forces. The wider the occlusal table, the more frequently offset contacts occur during mastication or parafunction. The narrower the implant body, the greater the influence of occlusal table width and offset loads. Therefore, the implant-protected occlusion dictates that the width of the occlusal table be directly related to the width of the implant body. The proportions of an implant crown that are unsupported by an axial-positioned implant should be recontoured if possible not to receive occlusal loads. The mechanical stresses generated by a patient onto an implant prosthesis are primarily influenced by occlusal contact. The primary goal of implant-protected occlusion is to maintain the occlusal load transferred to the implant within the physiologic limits of each patient. [M.C.R.]
Misch, C., and M. Bdiez, Compendium, 11:1330, 1994