Transverse Force, Lever Arm, and Bending Moment
During chewing, forces are generated on the prosthesis. If a chewing force is acting along the axis of an implant (axial force), the stress will be evenly distributed through the implant cross-section and the fixture threads. This means a high load-bearing capacity for the implant and supporting bone. However, if the force, or a component thereof, is acting in a transverse direction relative to the implant axis, it will result in a bending moment on the implant. In bending, only a small portion of the cross-section of the implant will counteract the load, and the bone will be loaded mainly at the terminal portions of the fixture, giving rise to higher stress levels in both implant and bone.
The bending moment is defined as the force multiplied by the lever arm (the orthogonal distance) between the direction of force and the cross-section; the larger this lever arm, the larger the bending moment and the larger the stress. Thus, the acting force itself may be of reasonable magnitude, but the forces needed to counteract the bending may be excessive due to the leverage effect. Therefore, axial loading most likely is preferable.
The full-arch restoration is based on the use of multiple implants, positioned on a curved line dictated by the residual jawbone. This curved line creates an inherently high capacity to counteract a transverse force with axial forces; any potential bending around a line combining any two implants will effectively be counteracted by the axial forces of implants placed off of that line. When restoring posterior edentulous spaces in partially dentate jaws, however, the implants often are placed in a more linear configuration and the compensating effect of an offset implant is thus not automatically produced; the straighter the alignment, the greater the potential bending of the implants. For the one or two-implant-supported prostheses, no such compensating effect can be produced. Posterior implant-supported pros-theses are, therefore, more frequently exposed to bending moments.
The leverage effect as described above can be caused by a variety of geometric conditions. An extension beyond the implant support, or implants offset buccolingually relative to the prosthesis are commonly encountered examples of such leverage situations. One particular condition is the single-implant molar replacement, in which the dimension of the tooth crown is substantially larger than the diameter of the fixture, leading to possible bending in all directions.