Helical plating versus straight plating and intramedullary nailing of proximal third humeral shaft fractures: a biomechanical comparative study

Proximal humeral shaft fractures are commonly treated with long straight plates or intramedullary nails. Helical plates might overcome the downsides of these techniques as they are able to avoid the radial nerve distally. The aim of this study was to investigate in an artificial bone model: (1) the biomechanical competence of different plate designs and (2) to compare them against the alternative treatment option of intramedullary nails. Twenty-four artificial humeri were assigned in 4 groups and instrumented as follows: group1 (straight 10-hole-PHILOS), group2 (MULTILOCK-nail), group3 (45°-helical-PHILOS) and group4 (90°-helical-PHILOS). An unstable proximal humeral shaft fracture was simulated. Specimens were tested under quasi-static loading in axial compression, internal/external rotation and bending in 4 directions monitored by optical motion tracking. Axial displacement (mm) was significantly lower in group2 (0.1±0.1) compared to all other groups (1: 3.7±0.6; 3: 3.8±0.8; 4: 3.5±0.4), p<0.001. Varus stiffness in group2 (0.8±0.1) was significantly higher compared to groups1+3, p≤0.013 (1: 0.7±0.1; 3: 0.7±0.1; 4: 0.8±0.1). Varus bending (°) was significantly lower in group2 compared to all other groups (p<0.001) and group4 to group1, p=0.022. Flexion stiffness in group1 was significantly higher compared to groups2+4 (p≤0,03) and group4 to group1, p≤0,029 (1: 0.8±0.1; 2: 0.7±0.1; 3: 0.7±0.1; 4: 0.6±0.1). Flexion bending (°) in group4 was higher compared to all other groups (p≤0.024) and lower in group2 compared to groups1+4, p≤0.024. Torsional stiffness remained non significantly different, p≥0.086. Torsional deformation in group2 was significantly higher compared to all other groups, p≤0.017. Shear displacement remained non significantly different, p≥0.112. From a biomechanical perspective, helical plating with 45° and 90° may be considered as a valid alternative fixation technique to standard straight plating of proximal third humeral fractures. Intramedullary nails demonstrated higher axial and bending stiffness as well as lower fracture gap movements during axial loading compared to all plate designs. However, despite similar torsional stiffness they were associated with higher torsional movements during internal/external rotation as compared to all investigated plate designs.