Severe Knee Osteoarthritis Patients Show More Femoral Coronal Bowing Than Moderate Knee Osteoarthritis Patients – a Study Using Three Dimensional Computed Tomography

Purpose: Anatomically femoral bone has bowing. Conventionally, femoral bowing was evaluated using two dimensional (2D) radiography. We have reported a novel method to evaluate accurate three dimensional (3D) femoral bowing using computed tomography (CT) at OARSI 2016 meeting. There is still limited information about 3D femoral bowing of patients with knee osteoarthritis (OA). Typically, 2D femoral bowing has been evaluated to perform total knee arthroplasty (TKA) for OA knee, however 2D images often involves measurement error due to malposition of legs. Many reports suggested femoral bowings affected the likelihood of bone cut error in TKA, so precise measurement of femoral bowing is required. Purpose of this study was to evaluate the difference between the 2D radiographic images and 3D CT in examination of femoral coronal bowing to indicate superiority of 3D femoral bowing analysis of knee OA pateints. Methods: Femoral CT images and full-leg anterior-posterior radiographic images of 74 patients (15 men, mean age 51.7, and 59 women, mean age 67.0) with knee OA were enrolled in this study. The xyz-coordinate system was introduced in the 3D femoral bone CAD-model, which was reconstructed based on CT-data with imaging software (Mimics 14.0). Reference plane of the coordinate system was defined with three bony landmarks, medial/lateral epicondyles and center of femoral head, and XYZ planes were defined as shown in Fig.1. The cross-sectional contours of femoral canal (cancellous/cortical border) were extracted along Z axis. The range of the cross-sectional slicing was set between the lesser trochanter and the distal end of the epiphysis. For each extracted cross-sectional contour, a least-square fitted ellipse was calculated. A least-square line was fitted to centers of the cross-sectional ellipses. Proximal and distal anatomical axes were calculated with proximal and distal half of the ellipse data, respectively. The angle between these two axes was measured and defined as total bowing. Directions of lateral/anterior bowing were defined as plus directions. Proximal and distal axes were made to project to these YZ planes of the coordinate system, and angles of coronal bowing were examined. Similarly, two axes were made to project to these XZ planes, and angles of sagittal bowing were examined. (Fig.1) Next, the femoral bone on full-leg radiograph films was divided into two parts by the center line of the femoral canal. Proximal and distal axes were set grossly, and the angle between these axes was measured and the results were compared as coronal bowing and sagittal bowing on reconstructed 3D imaging. We also compared the difference in femoral bowing between two OA groups, a moderate OA group (Kellgren-Lawrence [K–L] grade 0∼2, n = 43) and severe OA group (K–L grade 3, 4 n = 31). Results: The average 2D coronal bowing was 0.6 ± 3.2 (−6.6–8.1) degrees, and the absolute value of the difference between 2D and 3D coronal bowing was 1.7 ± 1.5 (0.1–6.4) degrees in OA knees. Averages of 2D coronal, 3D coronal, and 3D sagittal bowing for moderate OA group and severe OA group are shown in Table 1. Statistically, severe OA group showed greater coronal bowing compared to moderate OA group. Correlation coefficients between 2D coronal bowing and 3D coronal bowing, and between 2D coronal bowing and 3D sagittal bowing, were 0.70 (P < 0.01) and 0.36 (P < 0.01) in all OA knees, 0.72 (P < 0.01) and 0.17 (P = 0.28) in moderate OA knees, and 0.57 (P < 0.01) and 0.42 (P = 0.018) in severe OA knees (Fig.2). Conclusions: We have shown severe OA had greater coronal bowing compared to moderate OA. The difference in femoral coronal bowing between 2D radiography and 3D CT was average 1.7 degrees, and can be as much as 6.4 degrees in OA knees. Our data show 2D femoral bowing analysis has limited accuracy in severe knee OA and would lead to patient outliers in TKA (Fig.2D). Thus, for precise TKA, we recommend preoperative planning and analysis of femoral bowing using 3D CT.Table 1Comparison of average bowing between moderate OA group and severe OA groupModerate OA (Degrees, n = 43)Severe OA (Degrees, n = 31)P value2D coronal bowing−0.6 ± 2.7 (−6.6–6.3)2.0 ± 3.1 (−3.5–8.1)0.000243D coronal bowing−0.7 ± 2.0 (−4.2–5.6)0.9 ± 2.2 (−2.6–6.8)0.00123D sagittal bowing8.2 ± 1.8 (4.9–12.6)9.1 ± 2.2 (6.1–14.4)0.051 Open table in a new tab