• J A Simon, M G Dennis, F J Kummer, and K J Koval.
• Department of Orthopaedic Surgery, Hospital for Joint Diseases Orthopaedic Institute, New York, New York 10003, USA.
• J Orthop Trauma. 1999 Mar 1;13(3):196-9.

ObjectivesSchuhli locking nuts provide a mechanism to lock 4.5-millimeter bone screws to a standard dynamic compression plate (DCP plate). It has been 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 performed to investigate the effect of Schuhli augmentation of a ten-hole broad DCP plate for fixation of a simulated humeral shaft fracture versus standard DCP plate fixation.DesignBiomechanical cadaver study.InterventionSix pairs of cadaveric humeri from elderly individuals were tested in offset axial loading, torsion, and four-point bending to obtain load 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 contralateral side. All screws were placed in cortical bone. The constructs were retested 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 loaded to failure in torsion to determine the ultimate load and rotational displacement.Main Outcome MeasurementsResistance to displacement was determined from the 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 statistically significant differences between the two modes of fixation.ResultsThere 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.ConclusionsIn this model of humeral shaft fractures in the elderly, the addition of Schuhlis did not significantly change the mechanical stability of plate and screw fixation. However, load and angular deformation at failure were significantly greater in the Schuhli augmented specimens.

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