Frequency of varus and valgus thrust and factors associated with thrust presence in persons with or at higher risk of developing knee osteoarthritis

Knee osteoarthritis (OA), a leading cause of functional limitation and disability in older persons, is believed to result from local mechanical factors acting within the context of a systemic susceptibility. Varus thrust is a local mechanical factor shown to be associated with a 4-fold increase in the odds of medial tibiofemoral OA progression over an 18 month follow-up period (1). Varus thrust is visualized during gait as the dynamic worsening or abrupt onset of varus (bow-leg) alignment as the limb accepts weight (stance phase), with a return to less varus and more neutral alignment during lift-off and the non-weightbearing (swing) phase of gait. A varus thrust indicates dynamic instability of the knee in the frontal plane and an acute increase in load transmitted to the medial compartment with each step of walking. Valgus thrust is the worsening or abrupt onset of valgus (knock-knee) alignment visualized during the stance phase, with a return to less valgus (more neutral) alignment during lift-off and the swing phase of gait. In theory, a valgus thrust increases load transmitted to the lateral tibiofemoral compartment, potentially contributing to lateral OA disease onset or progression. While valgus thrust is thought to be less common than varus thrust, the frequency of varus thrust and valgus thrust in persons either with knee OA or at risk to develop it is not known for any racial group. In the Johnston County Osteoarthritis Project, African-Americans were more likely than Caucasians to have lateral joint space narrowing (2), a finding that was not explained by differences in static varus-valgus alignment (3); a greater frequency of valgus thrust in African-Americans could help to explain a predisposition to lateral compartment involvement. The factors associated with varus or valgus thrust presence have not been identified. A thrust is a visible manifestation of excessive motion of one tibiofemoral surface in relation to the other under the dynamic conditions of gait. Thrust presence may represent capsuloligamentous damage and/or insufficient neuromuscular stabilization, conditions that may precede OA or develop or worsen as a result of disease. The capsular changes from static varus-valgus alignment may predispose to thrust development. Knee injury may primarily or secondarily damage stabilizing joint tissues. In theory, greater axial load from obesity may intensify a thrust in knees with compromised frontal-plane stability. Quadriceps weakness or pain-induced quadriceps inhibition may impair mechanisms of neuromuscular stabilization. To address these gaps in knowledge relating to varus and valgus thrust, we undertook an ancillary study to the Osteoarthritis Initiative (OAI) in which thrust was assessed by trained examiners. Here we analyze thrust data from African-Americans and Caucasians, the two largest racial groups in the OAI sample. The objectives of our study were: 1) to determine the frequency of varus thrust and valgus thrust in African-Americans and Caucasians for persons at high risk for but without radiographic OA in either knee and for persons with radiographic OA in one or both knees; and 2) to identify factors associated with the presence of varus and valgus thrust in persons without radiographic OA and with radiographic OA in one or both knees.