Estimating muscle forces and joint contact forces in the anterior cruciate ligament deficient knee

Date
2011
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University of Delaware
Abstract
Many of the estimated 200,000 individuals who sustain anterior cruciate ligament (ACL) injuries annually in the U.S. experience characteristic knee instability with daily activities (.noncopers.). The risk for developing knee osteoarthritis (OA) increases after injury, and within 20 years half of ACL-injured patients will have radiographic OA, significant pain and functional limitation. The noncoper gait strategy has been described in terms of aberrant movement and altered muscle activity, but neither muscle forces nor joint loads have been investigated. Further characterization of their neuromuscular strategy and knee joint loading will ultimately expand our understanding of the onset and progression of knee OA after ACL injury. Therefore, the first aim of this work was to identify the well-described decreased injured limb peak extensor moment in a group of noncopers and to identify the muscle force adaptation which drove that gait asymmetry. We used an EMG-driven modeling approach to partition flexor and extensor group forces during normal walking. We found that noncopers accomplished the decreased peak moment with decreased muscle force from both flexor and extensor muscle groups. This strategy may be explained by muscle weakness which frequently accompanies ACL injury, or by apprehension, low confidence and fear of further injury. The second aim of this work was to determine whether medial and lateral joint contact forces are altered acutely after ACL rupture. Medial and lateral compartment loads were calculated using muscle forces obtained from EMG-driven modeling. Noncopers unloaded their injured knees. This adaptation may cause cartilage deconditioning, resulting in an increased potential for cartilage insult with the resumption of normal or excessive loading. The lower muscle force in the injured limb of these patients affirms the existence of neuromuscular asymmetries following ACL injury. The loading analyses provide a basis from which to compare longitudinal changes in knee joint loading.
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