Joint Space Narrowing in the Osteoarthritic Knee

Background: When patients with knee osteoarthritis (OA) are referred to an orthopaedic specialist for care, standard evaluation includes anterior-to-posterior (AP) weight-bearing (WB) radiographs to best evaluate the degree of arthritis and residual joint space, which can tailor treatment recommendations. Primary care guidelines do not require radiographs for their diagnosis; if performed, they are often non–weight-bearing (NWB). This study assessed the significance of joint space narrowing (JSN) between NWB and WB knee radiographs in patients evaluated for OA. We also compared demographic data with the measure of JSN. Methods: This was a prospective quantitative study that followed an experimental design comparing the JSN of the same AP NWB knee with WB knee radiographs. Two blinded fellowship-trained orthopaedic adult reconstruction surgeons and 1 musculoskeletal radiology fellow performed the joint space measurements. Data were analyzed using the Student t test and χ2 test as appropriate. Results: The mean JSN between WB and NWB was 1.20 mm (95% confidence interval 0.93-1.49 mm, p < 0.0001). Interrater reliability between NWB and WB measurements was 0.87 and 0.93, respectively. As body mass index (BMI) increased in our population, the amount of JSN decreased. There were significantly higher rates of JSN in those who were not obese compared with those who were. Conclusions: Weight-bearing radiographs are better able to evaluate JSN than NWB. BMI had a negative correlation with JSN in our population. Level of Evidence: II.