Biomechanical evaluation of two intramedullary nailing techniques with different locking options in a three-part fracture proximal humerus model.

Background: Osteosynthesis of unstable proximal humerus fractures still remains challenging. The aim of this study was to investigate two intramedullary nailing techniques with different locking options in a three-part fracture model and prove whether two new fixation concepts, introducing additional locking screw-in-screws inserted through the head of the proximal screws, and a calcar screw, provide better stability.

Methods: A biomechanical testing model for three-part proximal humerus fractures including cyclic axial loading with increasing peak load and simultaneous pulling forces at the rotator cuff was used to test 12 pairs of human cadaver humeri, assigned to four groups and instrumented with either Targon PH (T1) or MultiLoc PHN in 3 different configurations (standard M1; two additional screw-in-screw M2; one additional calcar screw and two screw-in-screw M3).

Findings: Initial range of motion in internal-external rotation and mediolateral translation was smallest in M3 (1.82°; 0.11mm), biggest in T1 (3.63°; 0.51mm) and significantly different between these two groups (p=0.02 and p=0.04, respectively). M3 showed minimum head migration along the nail and varus tilting after 5000 cycles (0.31mm; 0.20°) and 10000 cycles (1.59mm; 0.34°). M2 and M3 performed better than M1 and T1 regarding varus collapse. The highest number of cycles to failure was observed for M3 (20733) and the lowest for T1 (10083) with significant difference between these two groups (p=0.04).

Interpretation: The configuration with two screw-in-screw and a calcar screw was superior in most aspects. The screw-in-screws were found to contribute against varus collapse. Both new fixation concepts could provide better stability in proximal humerus fractures.