Abstract: Objective　To investigate the morphological features of circumferential proximal cortex in pertrochanteric fractures by CT-images，and providing structural basis and anatomical reference for the cortical support reduction in pertrochanteric fractures.　Methods　Eighty cases of AO/OTA 31-A1 and A2 pertrochanteric fractures were enrolled in this study. The CT scans of the cases were collected and the 3D-CT images were reconstructed by using the Mimics software. Fractures were reconstructed and simulated by 3D-CT. With the angle between the femoral shaft axis and the head-neck axis maintained at 130 degrees, the circumference of remnant circumferential cortex, width of its anterior part, lateral part, posterior part, and angle of the line between anterior and posterior broken-ends were measured.　Results　Twenty-one cases belong to type A1, the average circumferential cortex length was 88.7 mm; anterior, lateral and posterior circumferential cortex widths were 36.9 mm, 36.9 mm and 27.3 mm, respectively; and broken-ends line angle was 16.2°. 59 cases in type A2, the average circumferential cortex length was 60.0 mm; anterior, lateral and posterior circumferential cortex widths were 32.3 mm, 27.4 mm and 9.2 mm, respectively; and broken-ends line angle was 40.2°. The length of circumferential cortex in type A1 was significantly longer than that in A2 (P<0.01). On the whole, the posterior cortex had the largest variation, while anterior cortex was much more stable and showed the lowest variation (C.V were 75.5% and 20.0%).　Conclusions　There are significant differences between type A1 and A2 in circumferential cortical morphology of the proximal femur. The posterior cortex width is small and highly variable. While the anterior cortex width is much more longer and basically stable, which could be used as the main structure for the cortical support reduction.

Key words: Pertrochanteric fracture, 　Circumferential proximal cortex;　Anterior cortex;　Posterior cortex;　Cortex support reduction;　CT measurement