Complex distal humerus fractures-comparison of polyaxial locking and nonlocking screw configurations-a preliminary biomechanical study.

Objective: The investigation hypothesized that in current anatomical precontoured plates, angular stability plays only a minor role for the efficacy of the osteosynthesis at the distal humerus.

Methods: An AO C2.3 fracture model was simulated and osteosynthesis performed with plates positioned in parallel. System rigidity and median fatigue limit were analyzed in artificial bones and the cycles to failure in cadaver specimens. Loads were applied in anterior-posterior direction (75° flexion) and axial direction (5° flexion). Four composite bone groups were investigated as follows: (1) 2.7 mm polyaxial locking screws, (2) 3.5 mm polyaxial locking screws, (3) 3.5 mm polyaxial locking screws and a gap bridging screw, and (4) 2.7 mm nonlocking screws. Two cadaver groups were investigated with 3.5 mm diameter polyaxial locking (5) versus nonlocking screws (6).

Results: There were no differences in stiffness found between the locking versus nonlocking constructs in artificial (1) versus (4) and in cadaver bones (5) versus (6). The larger screw diameter of 3.5 mm in combination with a gap bridging screw significantly increased construct stiffness by 25% (3). The median fatigue limit was significantly increased using larger screw diameters (2) and a gap bridging screw (3). In cadaver bones, the polyaxial locking screws constructs (5) resisted higher peak loads and more cycles until failure compared with nonlocking constructs (6).

Conclusions: System stiffness increases with larger screw diameters and becomes significant with additional gap bridging screws in artificial bones. The use of polyaxial locking screws in anatomical adapted plates becomes more important in poor bone quality.