Quadriceps strength and osteoarthritis progression in malaligned and lax knees.

BACKGROUND Quadriceps muscle strengthening is a common goal in the management of knee osteoarthritis. In healthy knees, strength protects against new osteoarthritis. In arthritic knees, greater strength may protect joints and thereby delay osteoarthritis progression. Alternatively, in certain joint environments, such as malalignment or laxity, greater strength may translate into damaging joint reaction forces. The relationship between quadriceps strength and progression of knee osteoarthritis may differ according to these factors. OBJECTIVE To determine whether greater quadriceps strength is associated with greater probability of tibiofemoral osteoarthritis progression in malaligned knees and in high-laxity knees. DESIGN Prospective, longitudinal cohort study. SETTING Academic medical center. PARTICIPANTS 237 persons with primary knee osteoarthritis, definite tibiofemoral osteophytes, and at least some difficulty with knee-requiring activity. Two hundred thirty completed the 18-month evaluation. The current study primarily involved those without advanced osteoarthritis in either knee (n = 171). MEASUREMENTS Quadriceps strength, knee laxity and alignment, and osteoarthritis progression. RESULTS The predicted probability of tibiofemoral progression was 0.153 (95% CI, 0.100 to 0.228) in high-strength knees and 0.098 (CI, 0.061 to 0.155) in low-strength knees. In malaligned knees, high strength was associated with a significant increase (P = 0.03) in the likelihood of progression (predicted probability, 0.406 [CI, 0.226 to 0.615] vs. 0.187 [CI, 0.081 to 0.375] in high-strength vs. low-strength knees). Strength was also associated with increased likelihood of progression in high-laxity knees (P = 0.003 when high laxity was defined as > or =6.75 degrees). The probability of patellofemoral progression did not differ between high- and low-strength knees in the full sample or within subsets. CONCLUSIONS Greater quadriceps strength at baseline was associated with increased likelihood of tibiofemoral osteoarthritis progression in malaligned knees and lax knees. Subset-specific approaches beyond strengthening exercises should be developed to enhance joint-protective muscle activity.