The knee as a biologic transmission with an envelope of function: a theory.

The knee can be characterized as a complex set of asymmetrical moving parts acting together as a living biologic transmission. The purpose of this system is to accept, transfer, and dissipate loads generated at the ends of the long mechanical lever arms of the femur and tibia. In this analogy, the various ligaments represent sensate adaptive linkages, the articular cartilages represent bearings, and the menisci, mobile sensate bearings within the transmission. The muscles represent both living engines providing motive forces, and brakes and dampening systems under complex neurologic control mechanisms. The range of load that can be applied across an individual joint in a given period without supraphysiologic overload or structural failure can be termed the envelope of function. This range of homeostatic loading can be graphed with increasing applied loads on the vertical axis and the frequency of loading on the horizontal axis. At least 4 categories of factors together determine the envelope of function for a given joint including anatomic, kinematic, physiologic, and treatment factors. This theory of joint function can result in a more rational clinical approach to treating patients with knee injuries and other orthopaedic conditions. Orthopaedic therapy should be designed to maximize the envelope of function of musculoskeletal systems with the least degree of risk.