Evolving Biomechanics Issues in Orthopedic Surgery

Biomechanics has always been a major factor in the study of orthopedic surgery. Understanding the mechanics of the function and failure of musculoskeletal tissues is essential to determining how to protect, heal, and improve those tissues. To know the range of forces on different tissues involved in various activities is to be able to know how strong to make implants or braces. While determining those forces is not an exact science, it is a necessary part of orthopedic science.

For several reasons, most orthopedic biomechanics studies utilize cadaver tissue. It is easy to obtain and behaves similarly to normal tissue. The processing of cadaver tissue does make it somewhat stiffer than normal tissue and cadaver tissue is typically from the elderly, so those factors must be considered.

While every facet of orthopedic surgery is currently under scrutiny concerning biomechanics of failure and treatment of failure, certain areas are of more urgent examination. Osteoporosis is a major problem around the world and is getting worse as we are becoming more sedentary, living longer, and getting less calcium and vitamin D to strengthen our bones. Biomechanics plays a critical role to understand how bone weakens as bone density decreases. When bone-building drugs are given, we need to understand how the bone properties change regarding stiffness, elasticity, and whether weak points are created. In terms of soft musculoskeletal tissues, aging leads to increased stiffness and decreased ability to tolerate sudden loads—altered viscoelasticity.

"While every facet of orthopedic surgery is currently under scrutiny concerning biomechanics of failure and treatment of failure, certain areas are of more urgent examination"

Why this occurs, how it can be modified, and how it can be protected against are intense areas of interest.

An area with equal concern is Sports Medicine. Injuries in sport are common and equipment design like helmets, pads, and braces rely on accurate biomechanics data to predict success. Orthopedic sports researchers are using accelerometers, pressure-sensing shoe inserts, and other sensors to analyze specific parameters. Motion-capture devices and high-speed cameras are used to analyze performance and technique. The orthopedic surgeon is the medical practitioner most likely to be involved in these studies. As an example, the US Figure Skating team gets tested at the Olympic Training Center in Colorado Springs. Jump height, rotation speed, torque, and peak landing forces all factor into performance and to injury patterns. Mechanical analysis can predict who will succeed and who might get injured. Another example is anterior cruciate ligament (ACL) injury and reconstruction. We are learning that the ligament fails when tension forces in the ligament exceed certain levels. These levels can be exceededwhen the knee is stressed in certain actions. Learning these actions has allowed us to devise specific exercise techniques to avoid those actions. After ACL reconstruction, most knees begin to develop arthritis within 10 years. Understanding the force distribution within the normal knee and ACL-reconstructed knee has helped us improve our operative technique to achieve better biomechanics after surgery.

A third area of intense interest is joint replacement. At present, most joint replacements last only 15-20 years. Minimizing the peak forces throughout the full range of joint motion will allow the high molecular weight cross-linked polyethylene bearing surface to last longer. Using other materials such as ceramics or metals for bearing surfaces requires even more precise placement and force distribution.

Prosthetic limbs have surged in use over the past 20 years as we have been involved in multiple wars with fewer deaths, but more injuries to limbs. Maximizing prosthetic limb function while minimizing weight and pressure complications continues to evolve.

In summary, all of orthopedic surgery requires biomechanical analysis and incorporation of sound mechanical principles. The more sophisticated we become, the more essential these data become.