The role of joint load in knee stability
Markolf, K.L.; Bargar, W.L.; Shoemaker, S.C.; Amstutz, H.C.
Journal of Bone and Joint Surgery. American Volume 63(4): 570-585
1981
ISSN/ISBN: 0021-9355 PMID: 7217123 Document Number: 178668
Fresh cadaver knees were tested at full extension and at 20.degree. of flexion in specially designed fixtures that allowed tibiofemoral contact forces of as much as 925 N (207 lb) to be applied to the knee while joint movement was not restricted. Force vs. displacement responses for anterior-posterior and medial-lateral movement of the tibia and moment vs. rotation responses for varus-valgus angulation and tibial torsion were recorded with and without load before and after medial and lateral meniscectomy. When joint contact force was applied the loading apparatus could be adjusted so that the knee was in equilibrium and there was no tendency for the tibia to flex, extend or subluxate anteriorly or posteriorly with respect to the femur. Position of static equilibrium (called the neutral position) was represented graphically as the origin of the response curves. Slope of the response curve as it passed through the neutral position (neutral stiffness) is an important descriptive parameter for small movements about this equilibrium position. Laxity, the displacement (or rotation) of the tibia with respect to the femur at specified force (or moment) levels, also is a quantity of clinical interest. While a tibiofemoral contact force was applied, the anterior-posterior, medial-lateral, varus-valgus and torsional stiffnesses at the neutral position always increased and the corresponding laxities always decreased. Similar increases in joint stability associated with increased joint load were observed after medial and lateral meniscectomy. While a tibiofemoral contact force was applied to the knee, the joint was protected against varus-valgus angulation. In this situation varus-valgus angulation of the tibia could not occur until the condylar lift-off moment (the force applied to the tibia multiplied by its distance from the condylar pivot point) overcame the protective joint-load moment (the resultant joint force multiplied by its distance to the condylar pivot). Once this protective joint-load moment was exceeded condylar lift-off and ligament stretch occurred. Joint load generated by gravitational, dynamic, or muscular forces is an important protective mechanism that avoids ligament strain. After medial and lateral meniscectomy in the fully extended unloaded knee, anterior-posterior neutral stiffness and varus-valgus neutral stiffness were reduced significantly and varus-valgus laxity was increased significantly. When the knee was in 20.degree. of flexion, internal and external rotation of the tibia decreased anterior-posterior laxity and increased anterior-posterior neutral stiffness. Hyperextension of the knee increased neutral stiffness and decreased laxity during the varus-valgus test. Load on the knee has an important stabilizing effect on tibiofemoral motions because it limits displacements and rotations, protecting the ligaments from excessive strains produced by external forces and moments. Removal of the menisci often makes a knee looser in the unloaded condition but does not affect the stability of the loaded knee.