Knee Arthroplasty to Maximize the Envelope of Function

Summary

The knee functions as a type of biological transmission whose purpose is to accept and transfer a range of loads between and among the femur, patella, tibia, and fibula without causing structural or metabolic damage. Arthritic knees are like living transmissions with worn bearings that have limited capacity to safely accept and transmit forces. A new method of representing the functional capacity of the knee and other joints is the “envelope of function”, a load and frequency distribution that delineates the range of loads a given joint can sustain while still maintaining homeostasis of all tissues. The purpose of joint replacement surgery, therefore, is to maximize the envelope of function for a given joint as safely and predictably as possible.

A fundamental principle of all orthopedic treatment is to restore, as much as possible, normal musculoskeletal function. Following minor trauma to a previously normal joint such as the knee (e.g., contusion, mild medial collateral ligament sprain), the process of healing — the result of over 400 million years of vertebrate evolutionarily designed molecular and cellular mechanisms [1] — is most often accomplished without the necessity of any therapeutic intervention. True restoration to the full preinjury functional status is expected and most often occurs. With more substantial trauma to the knee, such as occurs with a complete rupture of the anterior cruciate ligament treated with a reconstruction, restoration to the full pre-injury physiological functional status is more problematic and often does not occur despite modern surgical techniques [2–4].Even well-reconstructed knees have unfortunately demonstrated the development of early arthrosis if the joint is exposed to sufficiently high levels of loading, such as occurs with soccer and other similar pivoting sports. One can say that the pre-injury functional capacity of such an anterior cruciate ligament reconstructed knee has not been fully restored.

In the case of knees with advanced degenerative arthrosis which undergo joint replacement surgery, the principle of functional restoration may be more properly stated as maximization of the functional capacity of the knee. As effective as current joint replacement techniques are at achieving pain relief and often associated increases in muscle strength and control, knees that have had joint replacement surgery do not replicate the functional status of a healthy, uninjured, adult joint. No one with a total knee replacement, for example, should run marathons or play tackle football. Since the goal of total knee replacement surgery is to maximize joint function, what, then, is the function of the knee?