Clinical Biomechanics of the Spine
A clear understanding of biomechanical principles is essential in the treatment of orthopedic and spinal disorders. Charnley designed a smaller than anatomical femoral head to attain low-friction arthroplasty, with less wear and tear. White and Panjabi1 put forward a landmark definition of instability of thoracolumbar burst fracture, combining clinical and biomechanical perspectives, as inability to maintain structural integrity under physiological load to prevent progression of neurological deficit and pain. Mechanical back pain due to disc degeneration is worse with sitting, as opposed to claudication pain from spinal stenosis, which is worse after standing. This was explained by Nachemson in 1964, and was endorsed later by Wilke, who measured intradiscal pressure greater in sitting posture than standing. Screws, rods, and various interbody cages have been designed and used to achieve successful spinal fusion. Withdrawal of cylindrical interbody cages, marking the end of “cage rage” in the late 1990s, resulted from failure caused by the smaller footprint of the implant-bone interface. Adjacent segment degeneration after successful rigid fusion achieved by instrumented fusion with screws and rods led to the development of several nonfusion stabilization devices. Lack of a biomechanical basis in design led to early clinical failure of many such devices. An early design of a lumbar disc prosthesis, AcroFlex, which was made of a rubber cushion between metal endplates, was clinically effective in the short term but did not last long, with chemical disintegration of the rubber material. Subsequent designs for the disc prosthesis focused more on restoration of physiological quality and quantity of motion, but lack of attention to shock absorption compromised long-term success. Newer designs are focusing not only on restoration of physiological motion but also on shock absorption at the same time. The simple straight Harrington rod required a lot of biomechanical work behind the design. Restoration of sagittal balance in spinal deformity surgery and unstable spondylolisthesis (Figure 1) are areas of current interest in spine biomechanics. The human body is not a mathematical entity, that is, in treating patients with spine pain, 2 + 2 does not always equal 4. However, continued exploration and understanding of mathematical and bioengineering principles will ensure the discovery of new solutions and more effective ways of helping patients.