Transection of the anterior cruciate ligament (ACL) in animals has been shown to induce osteoarthritis (OA) in the knee. Joint instability and changes in knee loading are believed to initiate OA. However, muscular forces which ultimately determine knee loading have not been quantified in vivo in an ACL transection model. Consequently, changes in knee loading following ACL transection are not known, and the relationship between changes in knee loading and the pathogenesis of OA remains unclear. This study investigated changes in knee loading following ACL transection in the cat. The specific objectives were: a) to measure patellar tendon and gastrocnemius forces, electromyographic activity in knee extensor and flexor muscles, external hindlimb kinematics, and ground reaction forces (GRFs); b) to quantify changes in patellofemoral contact forces using a planar model of the patella with patellar tendon forces and patellofemoral geometry as input; and c) to determine if changes in GRFs following ACL transection are representative of changes in knee loading.
Measurements were performed in both hindlimbs during unrestrained locomotion of six animals. In four animals, measurements were performed before and five, seven, and nine days following uni-lateral ACL transection. In two animals, measurements were performed in the intact hindlimbs, four days after sham surgery, and five days after ACL transection. The muscular forces, knee extensor EMGs, GRFs and knee range of motion were not altered by sham surgery but were decreased in the ACL-deficient limbs and increased in the contralateral limbs in all animals following ACL transection. Patellofemoral contact forces in the ACL-deficient hindlimbs were decreased by about 30% following ACL transection. While the relative reduction in peak vertical GRFs and peak patellar tendon forces was similar, the detailed time-history of knee loading cannot be inferred from externally measured forces. For the first time, knee extensor forces have been measured following ACL transection in an experimental model of joint injury. The results of this study suggest that ACL transection causes a general unloading of the knee. The decreased knee loading immediately following ACL transection is speculated to play an important role in triggering degenerative responses of the knee.