Abstract
Clinically, anthropometry is widely used for assessing joint laxity in patients with ACL injury. However, whether statically measured ligament laxity is related to performance of knee joint is still unclear. Elasticity of ligaments and contraclility of muscles across knee joint are primary contributors of the resultant forces on sagittal plane. During a change of direction maneuver (cutting), the forces generated from those structures provide stability to the joint. PURPOSE: To examine the relatioinship between joint resultant forces and ligament laxity during cutting in subjects with unilaterally reconstructed ACL. METHODS: 7 subjects (2 male, 5 female, age 19–25 yrs) with unilaterally reconstructed ACL were recruited for the study. Joint laxity was measured by a certified physical therapist using KT-2000®. Subjects were instructed to perform cutting moves 45 degrees to both sides during running. Corresponding legs were used for pushing off to the intended direction (e.g., right leg pushing to cut to the left). Kinematic and kinetic data were collected using Hawk® digital motion capturing system (Motion Analysis®) and two Bertec® forceplates. Bilateral comparisons of variables were made using paired t- tests. Correlations between ACL laxity and joint forces were assessed with regression analysis. RESULTS: The laxity of the ACL was significantly higher in reconstructed knee (7.21±3.80mmvs. 5.33±2.64mm, P=0.045). There was no significant bilateral difference between legs in peak ground reaction force generated from pushing off (P=0.247). A trend was found for the healthy knee to have a higher normalized joint resultant force in A-P direction compared with the reconstructed side (1.044+0.37BW vs. 0.832+0.15BW, P=0.079). We failed to find a significant correlation between measured ACL laxity and resultant A-P joint force. CONCLUSIONS: The results showed that non-operated and ACL reconstructed knee could push off with comparable amount of force. However, ACL reconstructed knee has higher laxity, but generates a lower anterior-posterior directed joint resultant force. This might suggest that a different muscle activation stratedy is used to minimize the load on ACL in reconstructed knee. The decrease in A-P force cannot be explained by the measured amount of ligament laxity.
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