Is end-stage lateral osteoarthritic knee always valgus? Mechanical alignment analysis and radiographic severity assessment.

BACKGROUND: We hypothesized that not all persons with end-stage lateral osteoarthritis (OA) have valgus malalignment and that full extension radiographs may underreport radiographic disease severity. The purpose of this study was to examine the demographic and radiographic features of end-stage lateral compartment knee OA.

MATERIALS AND METHODS: We retrospectively studied 133 knees in 113 patients who had undergone total knee arthroplasty between June 2008 and August 2010. All patients had predominantly lateral idiopathic compartment OA according to the compartment-specific Kellgren-Lawrence grade (KLG). The mechanical axis angle (MAA), compartment-specific KLG and joint space narrowing (JSN) of the tibiofemoral joint at extension and 30° of knee flexion, tibia vara angle, tibial slope angle, body mass index, age, and sex were surveyed.

RESULTS: End-stage lateral compartment knee OA has varus (37.6 %), neutral (22.6 %), and valgus (39.8 %) MAA on both-leg standing hip-knee-ankle radiographs. KLGs at 30° of knee flexion (fKLG) were grades 3 and 4 in all patients. However, for KLGs at full extension (eKLG), 54 % of all patients had grades 3 and 4. The others (46 %) showed grades 1 and 2. We observed significant differences in lateral compartment eKLG/eJSN (2.3/2.3 mm in varus, 2.5/1.9 mm in neutral, 2.9/1.6 mm in valgus, p = 0.01 and 0.03, respectively), tibia vara angle (4.9° in varus, 4.1° in neutral, 3.0° in valgus, p < 0.01), and medial compartment eKLG/eJSN (2.1/3.1 mm in varus, 2.0/3.4 mm in neutral, 1.8/4.3 mm in valgus, p < 0.01 and 0.01, respectively) between MAA groups, except for the tibial slope angle (9.7° in varus, 10.1° in neutral, 9.8° in valgus, p = 0.31).

CONCLUSION: Varus alignment was paradoxically shown in approximately one-third of those with end-stage lateral knee OA on both-leg standing hip-knee-ankle radiographs. Films taken in full extension underreported the degree of OA radiographic severity.

LEVEL OF EVIDENCE: Level IV, observational study.