Dynamic locking screws in proximal humeral plate osteosynthesis demonstrate superior fixation properties: a biomechanical study.

PURPOSE

Angular stable implants reduced the complication rate in the treatment of humeral head fractures. But the failure rate is still high. To further reduce the risk of cut-out, cement augmentation of screws was introduced. A reason for failure of plate osteosynthesis might be the extremely high stiffness of the screw-plate interface leading to a loss of reduction and cut-out of screws. A more homogeneous distribution of the forces on all screws may avoid secondary dislocation. We hypothesize that dynamic osteosynthesis minimizes screw loosening and results in a higher load to failure than standard locking screws.

METHODS

Twelve paired human humerus specimens were analysed. A standardized three-part fracture model with a metaphyseal defect was simulated. Within each pair of humeri, one was fixed with a Philos plate and standard locking screws (LS), whereas the other humerus was fixed with a Philos plate and dynamic locking screws (DLS). A cyclic varus-bending test or a rotation test with increasing loading force was performed until failure of the screw-bone-fixation.

RESULTS

In the varus bending test, pairs failed by screw loosening in the humeral head. The LS-group reached 2901 (601-5201) load cycles until failure, while the DLS-group failed after 3731 (2001-5601) cycles. This corresponds to a median loading of 195 N for the LS-group and 235 N for the DLS-group (p = 0.028). In the rotation test the LS-group reached a median of 1101 (501-1501) load cycles until failure of fixation occurred, while the DLS-group failed after 1401 (401-2201) cycles (p = 0.225).

CONCLUSIONS

Plate fixation using dynamic locking screws for the treatment of proximal humerus fractures demonstrated more load cycles until failure compared to standard locking plate osteosynthesis.