Schuhli augmentation of plate and screw fixation for humeral shaft fractures: A laboratory study

Objectives: Schuhli locking nuts provide a mechanism to lock 4.5-millimeter bone screws to a standard dynamic compression plate (DCP plate). It has be en proposed that Schuhlis can provide increased fixation stability in areas of a proximal cortical defect or osteopenic bone and may keep screws from loosening and backing out from the plate. A biomechanical study was perform ed to investigate the effect of Schuhli augmentation of a ten-hole broad DC P plate for fixation of a simulated humeral shaft fracture versus standard DCP plate fixation. Design: Biomechanical cadaver study. Intervention: Six pairs of cadaveric humeri from elderly individuals were t ested in offset axial loading, torsion, and four-point bending to obtain lo ad versus deformation curves and baseline mechanical properties. Each pair of humeri was then instrumented with a ten-hole broad DCP plate on one side and a DCP plate augmented with Schuhlis at each screw hole on the contrala teral side. All screws were placed in cortical bone. The constructs were re tested in all three modalities. The humeri were then cycled in torsion for 1,000 cycles and retested in all three modalities. Each humerus was then lo aded to failure in torsion to determine the ultimate load and rotational di splacement. Main Outcome Measurements: Resistance to displacement was determined from t he load versus deformation curves in each testing modality before and after cycling; these data were normalized to the intact values determined prior to instrumentation. Paired Student's t tests were performed to determine st atistically significant differences between the two modes of fixation. Results: There were no significant differences in stability between the two fixation techniques in all three testing modalities both before and after cycling. However, the Schuhli augmented constructs sustained significantly greater loads and rotational deformations prior to failure. Conclusions: In this model of humeral shaft fractures in the elderly, the a ddition of Schuhlis did not significantly change the mechanical stability o f plate and screw fixation. However, load and angular deformation at failur e were significantly,greater in the Schuhli augmented specimens.