Quadriceps strengthening does not protect Malaligned and Lax Knees from subsequent progression of Knee Osteoarthritis (OA)
Quadriceps strengthening is often recommended to patients with knee osteoarthritis. In fact, some data have suggested that weak quadriceps muscles predisposes to the onset of knee OA. Sharma et al (Ann Intern Med 138:613, 2003) investigated whether greater quadriceps strength is associated with slower progression of knee osteoarthritis(OA), particularly in patients with malaligned or lax knees where muscle forces may increase stress on localized areas of cartilage.
Methods: 171 participants with K-L grade 2 or 3 primary knee OA, definite tibiofemoral osteophytes, and at least a little functional difficulty underwent an 18-month evaluation. Quadriceps strength and knee laxity and alignment in both knees were measured at baseline. Semi-flexed radiographs of both knees were obtained at baseline and 18 months to measure OA progression in the medial and lateral compartments, as well as the patello-femoral articulation. 14 knees were excluded because of prior knee replacements. Body mass index (BMI), age, disease severity, and physical activity were also assessed as possible confounding variables.
Results: Greater quadriceps strength at baseline did not protect participants from subsequent OA progression. After adjusting for potential confounders, the predicted probability of tibiofemoral OA progression was slightly greater in knees with higher quadriceps strength compared to knees with lower strength (0.153 [95% CI, 0.100 to 0.228] vs. 0.098 [CI, 0.061 to 0.155], respectively; P=0.09).
OA progression was substantially more likely in high-strength versus low-strength knees that were malaligned (>5 degrees), 50% vs. 26.3%, respectively. This corresponded to an increased predicted probability of OA progression in high-strength maligned knees compared to low-strength malaligned knees (0.406 [CI, 0.226 to 0.615] vs. 0.187 [CI, 0.081 to 0.375], respectively; P=0.03). In normally aligned knees (< 5 degrees), quadriceps strength had no effect on OA progression.
Additionally, in knees with significant laxity (>5.75 degrees), OA progression was also more likely in high-strength knees versus low-strength knees (24.4% vs. 15.4%), corresponding to an increased predicted probability of OA progression in knees with high-strength high laxity versus low-strength high laxity (0.178 [CI, 0.078 to 0.357] vs. 0.074 [CI, 0.032 to 0.164], respectively; P=0.05). This effect of strength also held true for low laxity knees, 19.2% vs. 14.3%, respectively.
Conclusion: These results cause one to question the assumption that quadriceps strengthening will improve the course of OA in all patients. The effect of strength may differ between joint sites according to the local environment. Muscle forces that are beneficial in the nondiseased knee may have adverse effect in an osteoarthritic knee, especially in malaligned and lax knees.
Editorial Comment: This study has potentially far reaching and important implications. To date, we have assumed that exercise is beneficial for patients with knee OA, based on improvements in function, pain, and muscle strength in short term studies. The current study by Sharma calls this into question, at least for patients with significant structural changes in the knee as manifested by malalignment and/or laxity. Quads strengthening may alter the local environment of the knee in a detrimental way, perhaps disturbing a protective balance between knee extensors and flexors.
Further study in this area is definitely needed. Until then, we must be cautious about recommending intense quadriceps strengthening in knee OA patients with preexisting structural damage.