Instability, laxity, and physical function in patients with medial knee osteoarthritis

Abstract

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 Eta(2) 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.

Figure 1.

Tibiofemoral joint alignment was determined by the angle formed by the intersection of the mechanical axis of the femur and the mechanical axis of the tibia. The figure shows an angle of less than 180 degrees, indicating varus alignment.

Figure 2.

Setup for varus stress radiograph on left lower extremity, with corresponding radiograph (top). For the varus stress radiograph (shown), a consistent 150-N force was applied to the medial knee joint line. For the valgus stress radiograph (not shown), the force was applied to the lateral joint line. Calculation of medial laxity (bottom).

Figure 3.

Markers represent average scores on subsets of the Knee Injury and Osteoarthritis Outcome Score (KOOS) questionnaire (Pain, Symptoms, Quality of Life [QOL], Activities of Daily Living [ADL], Sport/Recreation [Sport]), lines represent 95% confidence intervals, brackets indicate significant group differences at *P≤.05, circle=I_f group (participants with knee instability that affects function), square=I_m group (participants with mild knee instability that does not affect function), triangle=I₀ group (participants with no knee instability).

Figure 4.

Results of hierarchical regression analysis showing the relative contribution (R² value) of each variable (vertical bars) in the prediction of scores on subsets of the Knee Injury and Osteoarthritis Outcome Score (KOOS) questionnaire (Pain, Symptoms, Quality of Life [QOL], Activities of Daily Living [ADL], Sport/Recreation [Sport]) and on the stair-climbing test (SCT). Asterisk indicates that the addition of the independent variable to the regression yielded a significant change in the R² value at P<.05. Med Lax=medial laxity, I_KOS score=self-reported knee instability score, MVIC=normalized maximal voluntary isometric contraction of quadriceps femoris muscle.