To determine whether the presence of varus thrust at baseline increases the risk of progression of medial tibiofemoral osteoarthritis (OA), whether knees with thrust have a greater adduction moment, whether thrust has any additional impact on top of static varus, and whether thrust is associated with poor physical function outcome.
Two hundred thirty-seven patients with knee OA (definite osteophytes and symptoms) underwent baseline gait observation to assess varus thrust and full-limb radiography to assess alignment. Sixty-four of these 237 patients also underwent quantitative gait analysis to determine the maximum knee adduction moment. Two hundred thirty patients (97%) returned for followup at 18 months. At baseline and 18 months, the 230 participants had semiflexed, fluoroscopically confirmed knee radiographs (with progression defined as worsening of medial joint space grade); self-reported and performance-based measures of function were also assessed. Logistic regression with generalized estimating equations was used to estimate odds ratios (ORs) for medial OA progression, after excluding knees that were not at risk for progression.
Varus thrust was present in 67 of 401 knees. Thrust increased 4-fold (age-, sex-, body mass index-, and pain-adjusted OR 3.96, 95% confidence interval [95% CI] 2.11-7.43) the odds of medial progression, with some reduction after further adjustment for varus alignment severity. In varus-aligned knees, thrust increased the odds of OA progression 3-fold (adjusted OR 3.17, 95% CI 1.60-6.31). In the gait substudy, the adduction moment was greater in knees with a thrust compared with knees without a thrust. Having a thrust in both knees versus neither knee was associated with a 2-fold increase in the OR for poor physical function outcome (P not significant).
Varus thrust is a potent risk factor, identifiable by simple gait observation, for disease progression in the medial compartment, the most common site of OA involvement at the knee. Varus thrust may also predict poor physical function outcome. Varus thrust increased the odds of progression among varus-aligned knees considered separately, suggesting that knees with a thrust are a subset of varus-aligned knees at particularly high risk for progression of OA.