The objective of this study was to evaluate the biomechanical effect of an additional unlocked calcar screw compared to a standard setting with three proximal humeral head screws alone for fixation of an unstable 2-part fracture of the surgical neck.

The additional calcar screw improves stiffness and failure load.

Fourteen fresh frozen humeri were randomized into two equal sized groups. An unstable 2-part fracture of the surgical neck was simulated and all specimens were fixed with the MultiLoc^®-nail. Group I represented a basic screw setup, with three locked head screws and two unlocked shaft screws. Group II was identical with a supplemental unlocked calcar screw (CS). Stiffness tests were performed in torsional loading, as well as in axial and in 20° abduction/20° adduction modes. Subsequently cyclic loading and load-to-failure tests were performed. Resulting stiffness, displacement under cyclic load and ultimate load were compared between groups using the t-test for independent variables (α=0.05).

No significant differences were observed between the groups in any of the biomechanical parameters. Backing out of the CS was observed in three cases.

The use of an additional unlocked calcar screw does not provide mechanical benefit in locked nailing of an unstable 2-part fracture of the surgical neck.

Level III. Experimental biomechanical study with human specimen.