Three-dimensional morphologic features of Asian atypical femur and clinical implications of cephalomedullary nail fixation: Computational measurement at actual size.

PURPOSE

(1) To analyze three-dimensional (3D) morphologic features of atypical femur fracture at actual size without projection error; and (2) to provide clinical implications of cephalomedullary nail (CMN) fixation by separating the medullary canal and the cortex.

MATERIALS AND METHODS

Fifty-nine atypical femurs (opposite non-fractured femurs) were reconstructed as 3D models with medullary canal by importing CT data into Mimics® software. A reference line was drawn from the tip of the greater trochanter to the center of the intercondylar notch and used for classifying bowing grade according to the centerline of medulla. Proximal and distal straight lines (length of 60 mm, diameter of 1 mm) were placed in the centerline of medulla. Acute angles between the two straight lines were measured as lateral and anterior bowing. The acute angle by straight line and reference line was measured as proximal and distal bowing in both AP and lateral view. The diameter of curve (DOC) of medulla along the posterior border was measured.

RESULTS

The anterior bowing of all femur on lateral view was an average of 13.82° (range, 6.2°-31.1°, SD 3.91), and the values of proximal and distal anterior bowing were an average of 7.82° (range, 2.7°-14.3°, SD 2.23) and 6.0° (range, 2.2°-16.8°, SD 2.31), respectively. The lateral bowing of all femur on AP view was an average of 5.49° (range, 0.1°-17.3°, SD 4.48), and the values of proximal and distal anterior bowing were an average of 3.64° (range, 0.1°-11.3°, SD 2.70) and 2.48° (range, 0-7.4°, SD 1.98), respectively. The medullary canal was changed to be straightened and more bowed anteriorly. Concerning the lateral bowing grade of entire diaphysis, it was grade -I for 15 femurs, grade 0 for 21 femurs, grade I for nine femurs, grade II for five femurs, and grade III for nine femurs. Regarding anterior bowing grade, it was grade II for 14 femurs and grade III for 45 femurs. The average diameter of medullary canal was 1276.3 ± 232.25 mm.

CONCLUSION

Asian atypical femur had three morphologic features: (1) straightened medullary canal, (2) positive values of lateral bowing, and (3) significantly increased ante-curvature of medullary canal. Considering that conventional CMN had about 4° of mediolateral angle, the disparity between implant and medullary canal might be clear. Thus, new nail design for Asian atypical femur that could increase the radius of curve (ROC) but decrease the mediolateral angle needs to be introduced.