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Instability, Laxity, and Physical Function in Patients With Medial Knee Osteoarthritis

LC Schmitt, PT, PhD, is Postdoctoral Research Fellow and Physical Therapist, Sports Medicine Biodynamics Center and Division of Occupational Therapy and Physical Therapy, Cincinnati Children's Hospital Medical Center, University of Cincinnati College of Medicine, Cincinnati, Ohio. At the time of the study, she was a doctoral candidate in the Program in Biomechanics and Movement Science, Department of Physical Therapy, University of Delaware, Newark, Delaware.
GK Fitzgerald, PT, PhD, OCS, is Associate Professor, Department of Physical Therapy, School of Health and Rehabilitation Sciences, University of Pittsburgh, Pittsburgh, Pennsylvania.
AS Reisman, MD, ATC, is Team Physician, Student Health Services, and Associate Professor, Department of Physical Therapy, University of Delaware. At the time of the study, he was affiliated with Casscells Orthopaedics and Sports Medicine, Wilmington, Delaware.
KS Rudolph, PT, PhD, Assistant Professor, Program in Biomechanics and Movement Science, Department of Physical Therapy, University of Delaware, 301 McKinly Lab, Newark, DE 19716 (USA).

krudo{at}udel.edu

Background and Purpose: Studies have identified factors that contribute to functional limitations in people with knee osteoarthritis (OA), including quadriceps femoris muscle weakness, joint laxity, and reports of knee instability. However, little is known about the relationship among these factors or their relative influence on function. The purpose of this study was to investigate self-reported knee instability and its relationships with knee laxity and function in people with medial knee osteoarthritis (OA).

Participants: Fifty-two individuals with medial knee OA participated in the study.

Methods: Each participant was classified into 1 of 3 groups based on reports of knee instability. Limb alignment, knee laxity, and quadriceps femoris muscle strength (force-generating capacity) were assessed. Function was measured with the Knee Injury and Osteoarthritis Outcome Score (KOOS) and a stair-climbing test (SCT). Group differences were detected with one-way analyses of variance, and relationships among variables were assessed with the Eta² statistic and hierarchical regression analysis.

Results: There were no differences in alignment, laxity, or strength among the 3 groups. Self-reported knee instability did not correlate with medial laxity, limb alignment, or quadriceps femoris muscle strength. Individuals reporting worse knee instability scored worse on all subsets of the KOOS. Self-reported knee instability scores significantly contributed to the prediction of all measures of function above that explained by quadriceps femoris muscle force, knee laxity, and alignment. Neither laxity nor alignment contributed to any measure of function.

Discussion and Conclusion: Self-reported knee instability is a factor that is not directly associated with knee laxity and contributes to worse function. Further research is necessary to delineate the factors that contribute to self-reported knee instability and reduced function in this population.

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