Primary and coupled motions in the intact and the ACL-deficient knee: an in vitro study in the goat model.

Quadrupeds are commonly used as animal models to study healing of anterior cruciate ligament (ACL) reconstructions. While rabbits, dogs, goats, and sheep have been used, goats and sheep are increasingly being employed because of the larger joint size that facilitates surgery, ease of availability, and lower expense to maintain in the farm environment. In spite of this, little is known about the function of the ACL in controlling primary and coupled motions in the quadruped. We report here on the measurements of these motions in goats, with the application of anterior-posterior forces, varus-valgus moments, and internal-external moments in the intact and ACL-deficient knee. Sectioning the ACL caused significant increases in primary anterior translation, and in varus-valgus and internal rotations. The increases in anterior translation were similar in amount and dependence on flexion angle to those seen in human knees. The increase in varus averaged 7 degrees and did not depend on flexion angle, whereas the increase in valgus was significant only in the flexed knee. The increases in internal tibial rotation were greatest in extension, whereas the increases in external rotation were small and independent of flexion angle. When the ACL was cut, coupled internal rotation increased with an anterior force as well as a valgus moment. Large increases were seen in coupled anterior translation with the application of varus and valgus moments, whereas smaller increases were seen with internal and external moments. These findings demonstrate that the ACL restrains multiple motions in the goat knee. This study also provides baseline data for future studies of ACL reconstruction.