Valgus Stress Radiographs Predict Lateral-Compartment Cartilage Thickness but Not Cartilage Degeneration in Varus Osteoarthritis.

BACKGROUND: Intact cartilage in the lateral compartment is an important requirement for medial unicompartmental knee arthroplasty. This study sought to determine how measurements of joint space width in the lateral compartment on valgus stress radiographs compare to cartilage thickness as measured with a precise needle test, and whether cartilage thickness is a predictor of cartilage degeneration.

METHODS: A consecutive series of 100 knees undergoing total knee arthroplasty for end-stage varus osteoarthritis was studied. Twenty-eight knees were retrospectively excluded because not all data were available, leaving 72 knees (61 patients; mean age, 67 years [49-87]). On calibrated valgus stress radiographs, lateral-compartment joint space width was measured. During surgery, osteochondral samples of the distal lateral femur and the lateral tibia plateau were harvested. Cartilage thickness and histology were assessed. Cartilage thickness of tibia and femur was defined as lateral-compartment cartilage thickness.

RESULTS: Lateral-compartment joint space width on valgus stress radiographs and lateral-compartment cartilage thickness correlated well (rs  = 0.671, P < .001). However, no correlation of cartilage histology according to the osteoarthritis cartilage histopathology assessment system, and cartilage thickness on the lateral tibia plateau (rs  = -0.060, P = .614) and cartilage thickness on the distal lateral femur (rs  = -0.128, P = .282) was observed.

CONCLUSION: Valgus stress radiographs can assess combined cartilage thickness in the lateral compartment of the knee. Cartilage thickness, however, is a poor predictor of cartilage degeneration.