Spine biomechanics: fundamentals and future.

where M is the bending moment, F is the force applied, and D is the distance from the point of force application to the axis of rotation (moment arm length). This is perhaps best depicted in Figure 11.1. Using this equation, one can determine the bending moment applied in any given clinical circumstance. The bending moment has substantial clinical significance. The application of a bending moment results in the concentration of stresses that, in turn, increase the chance of failure at the site of maximum bending moment and force application. In the situation depicted in Figure 11.1, the site of maximum bending moment application is located at or near the ventral vertebral body (at the time of spinal column failure in the case of trauma). After the initiation of failure ventrally (due to the concentration of stresses induced by the applied bending moment), such failure usually propagates dorsally. In this example, all points ventral to the instantaneous axis of rotation (IAR) come closer together and all points dorsal become farther apart. The IAR at the moment of impact/failure is, in fact, located in the ventral/dorsal plane of the vertebral body in which the height of the vertebral body is equal to the rostral and caudal neighboring vertebral bodies. All points ventral to this point came closer together, whereas all points dorsal became further apart. Spine surgeons have understood for years that there are fundamentally six mechanisms by which we can exert leverage on the spine to correct or prevent deformity and structural failure. These are 1) distraction, 2) three-point bending, 3) tension-band fixation, 4) fixed moment arm cantilever beam fixation, 5) non-fixed moment arm cantilever beam fixation, and 6) applied moment arm cantilever beam fixation. The use of cantilevers, in the form of screws attached to rods or plates, has greatly and positively affected the spine surgeon’s ability to stabilize the spine and prevent or correct deformity. Implant fracture, however, occasionally occurs (Fig. 11.2). Such a fracture always occurs at the point of maximum stress application. Hence, the surgeon either “asked” too much of the implant or fusion failed to ensue, thus fatiguing the implant at its most vulnerable point (the point of maximum stress application). Dynamic spine fixation for cervical spine applications was popularized approximately one decade ago. By offloading the implant from axial forces (by allowing the telescoping or subsidence of the spine to passively occur), while providing stability in rotation, flexion and extension, and lateral bending, such devices found clinical utility. Advantages of axial implant off-loading include the provision of bone healing enhancing axial loads to the interbody bone graft, while minimizing loads applied to the implant. Henceforth, the stresses applied to the implant are diminished as well. In spite of the advantages associated with dynamic fixators, constructs still failed. Spine surgeons, thus, continued their quest to optimize spine stabilization via spinal instrumentation. They designed screws that were strengthened in the region of maximum stress application, thus decreasing the incidence of failure and shifting potential failure points to different locations along the spinal implant or screw, locations that expose the implant to a diminished chance for failure due to an enhanced ability to resist potential failure inducing loads and induced stresses (Fig. 11.3). In addition to the aforementioned enhanced and bolstered implants, new surgical strategies have increasingly become more popular. These include the utilization of intermediate fixation points (screws placed into intermediate vertebral bodies in multi-segmental constructs) to add a threepoint bending fixation force application. The addition of three-point bending forces to the construct enhances construct stability and diminishes the chance for fatigue failure at the bone-metal interface, as depicted in Figure 11.4, A–D. The application of three-point bending forces (Fig. 11.5, A and B) Copyright © 2006 by Lippincott Williams & Wilkins 0148-703/06/5301-0098