Retrograde nailing of humeral shaft fractures: A biomechanical study of its effects on the strength of the distal humerus

Objectives: The purpose of this study was to evaluate the loss of strength in the distal humerus that resulted from the creation of two different entry portals used for retrograde humeral nailing. Design: Nine pairs, treated as blocks of size two, of fresh frozen humeri from individuals free of musculoskeletal disease were randomly divided into three groups in a balanced incomplete block experimental design. Intervention: The specimens were tested intact (control) or with an entry portal drilled in either the distal metaphyseal triangle or the proximal slope of the olecranon fossa. Two of these three conditions were applied to each pair of the bones. Therefore, three pairs of bones accommodated testing of all possible combinations of the two treatments. All specimens were tested in torsion at a rate of 30 degrees per second until failure or fracture. Results: The creation of an entry portal reduced the ultimate torque to 63 percent of that of the intact specimens (p = 0.044) and the energy absorbed to failure to 27 percent of that of the intact specimens (p = 0.039). The metaphyseal entry portal reduced the torque to failure to 71 percent of that of the intact specimens (p = 0.143) and the energy absorbed to failure to 37 percent of that of the intact specimens (p = 0.073), The olecranon fossa entry portal reduced the torque to failure to 55 percent of that of the intact specimens (p = 0.035) and the energy absorbed to failure to 18 percent (p = 0.058) of that of the intact specimens. Conclusions: Surgeons should be aware of the loss of strength in the distal humerus after retrograde humeral nailing. This is especially important when prescribing postoperative mobilization in which the upper extremities will be used for weight-bearing in either transfers or ambulation.