Are there any patho-anatomical features that might predispose to primary knee OA? We investigated the 3D geometry of the load bearing zones of both distal femur and proximal tibias, in varus, straight and valgus knees. We then correlated these findings with the location of wear patches measured intra-operatively. Patients presenting with knee pain were recruited following ethics approval and consent. Hips, knees and ankles were CT-ed. Straight and Rosenburg weight bearing X-Rays were obtained. Excluded were: Ahlbäck grade “>1”, previous fractures, bone surgery, deformities, and any known secondary causes of OA. 72 knees were eligible. 3D models were constructed using Mimics (Materialise Inc, Belgium) and femurs oriented to a standard reference frame. Femoral condyle Extension Facets (EF) were outlined with the aid of gaussian curvature analysis, then best-fit spheres attached to the Extension, as well as Flexion Facets(FF). Resected tibial plateaus from surgery were collected and photographed, and Matlab combined the average tibia plateau wear pattern. Of the 72 knees (N=72), the mean age was 58, SD=11. 38 were male and 34 female. The average hip-knee-ankle (HKA) angle was 1° varus (SD=4°). Knees were assigned into three groups: valgus, straight or varus based on HKA angle. Root Mean Square (RMS) errors of the medial and lateral extension spheres were 0.4mm and 0.2mm respectively. EF sphere radii measurements were validated with Bland-Altman Plots showing good intra- and interobserver reliability (+/− 1.96 SD). The radii (mm) of the extension spheres were standardised to the medial FF sphere. Radii for the standardised medial EF sphere were as follows; Valgus (M=44.74mm, SD=7.89, n=11), Straight (M=44.63mm, SD=7.23, n=38), Varus (M=50.46mm, SD=8.14, n=23). Ratios of the Medial: Lateral EF Spheres were calculated for the three groups: Valgus (M=1.35, SD=.25, n=11), Straight (M=1.38, SD=.23, n=38), Varus (M=1.6, SD=.38, n=23). Data was analysed with a MANOVA, ANOVA and Fisher's pairwise LSD in SPSS ver 22, reducing the chance of type 1 error. The varus knees extension facets were significantly flatter with a larger radius than the straight or valgus group (p=0.004 and p=0.033) respectively. In the axial view, the medial extension facet centers appear to overlie the tibial wear patch exactly, commonly in the antero-medial aspect of the medial tibial plateau. For the first time, we have characterised the extension facets of the femoral condyles reliably. Varus knees have a flatter medial EF even before the onset of bony attrition. A flatter EF might lead to menisci extrusion in full extension, and early menisci failure. In addition, the spherical centre of the EF exactly overlies the wear patch on the antero-medial portion of the tibia plateau, suggesting that a flatter medial extension facet may be causally related to the generation of early primary OA in varus knees