Mismatch of AO anatomically shaped distal humeral plate with humeral shaft forward flexion angulation in adult Chinese population

Abstract

LCP extra-articular distal humerus plate (LCPEA) designed by AO has been introduced as an anatomically shaped plate that improves the results of surgical fixation in extra-articular distal humeral shaft fractures. However, no study analyzed whether LCPEA matches humeral shaft forward flexion angulation (FFA). The aims of this study were to evaluate the morphological discrepancies between LCPEA and the humeral shaft FFA in Chinese cadaveric dried adult humeri and to propose a further design of pre-contoured plates to accommodate the FFA. Forty-four Chinese cadaveric dried adult humeri were used for this study. An eight-hole LCPEA was applied to the posterior aspect of the distal humerus according to the contour. Mismatches between the bone and the plate were recorded. The distance between the inner surface of the plate and the underlying humeral dorsal cortex was measured at the sites of mismatch. The humeral shaft FFA was measured from the intersection angle between tangent lines placed on the dorsal aspect of the 1/3 distal humeral shaft and the dorsal ridge of the 2/3 proximal humeral shaft. The location of the apex of the FFA was determined by measuring the distance from the most distal point of trochlea of humerus to the point of intersection of the FFA tangent lines. The distance was defined as forward flexion distance (FFD). Mismatch was found at the level of proximal 3-6 holes of LCPEA with an average distance of 6.9 ± 3.1 mm (range 2.3-14.0 mm) at the tip of the plate. The FFA was present in all specimens. The average FFA was 8.2° ± 2.2° (range 4°-13°), the average FFD was 99.9 ± 9.6 mm (range 79.2-117.9 mm), and the average ratio of FFD to humerus length was 0.33 ± 0.03 (range 0.27-0.39). A rather consistent pattern of mismatch was found at the proximal part of LCPEA. An attempt to fit the plate to the bone at this level may cause a gap of the fracture at the opposite cortex. The main reason for the mismatch is the existence of the humeral shaft FFA. LCPEA is usually made a bend of about 8° between the fourth and the fifth dynamic-compression portion of the combination hole in the distal-to-proximal direction.