Sensorimotor control of knee stability. A review

Traditionally, the concept of joint stability considered the displacement ( or subluxation) of two bones relative to each other as the measurement inde x, and attributed the preservation of such stability in its physiologic ran ge to the various ligaments associated with the joint. Although the ligamen ts are indeed the major restraints of any joint, the significant contributi on of the musculature toward joint stability bad been grossly overlooked or neglected until the last 15 years. The value and importance of muscular ac tivity in that role becomes immediately apparent if one performs even a sup erficial functional comparison of muscles and ligaments. Ligaments are pass ive viscoelastic structures, whereas muscles are dynamic viscoelastic organ s. The viscoelastic effects of the ligaments are activated and applied stri ctly upon the geometric and kinematic configuration of the joint traversing through its range of motion according to fixed force-displacement relation ships. The musculature, however, can apply passive viscoelastic effects to the joint when not active (passive tone) and variable dynamic viscoelastic effects when contracting under voluntary or reflexive control, and at any d esirable point in the range of motion and in response to joint speed, exter nal load, gravity, pain, and so forth, while executing the functional objec tive of the movement set by the individual. Preservation of joint stability cannot be ascribed to the ligaments alone, but should be considered as a s ynergistic function in which bones, joint capsules, ligaments, muscles, ten dons, and sensory receptors and their spinal and cortical neural projects a nd connections function in harmony. The objective of this report is to first review the anatomy and physiology of the various mechanoreceptors and their neural pathways about the joint, and describe some of the current concepts of the reflex arcs elicited by su ch receptors, with special emphasis on biomechanical outcomes relative to s tability. The role of the musculature in maintaining stability while contro lling joint motion is then reviewed, with data obtained from experiments pe rformed on humans and animals. Finally, some clinical findings from patient s with anterior cruciate ligament deficiency using a brace that simulates t he ligament-muscle functions is described.