Biomechanical analysis of humeral diaphyseal segmental defect fixation.

With disseminated malignancies, segmental defects of the humeral diaphysis may occur from surgical resection or as a complication of failed prior internal fixation. This study directly compared the biomechanical properties of a second generation titanium modular intercalary humeral spacer (segmental defect replacement prosthesis) with those of a modern locked humeral nail combined with methylmethacrylate (intramedullary nail) or with an intercalary allograft spacer (allograft nail composite) for fixation of segmental defects of the humeral diaphysis. Eighteen matched pairs (36 specimens) of fresh-frozen humeri were prepared in a standard fashion to create a 5-cm middiaphyseal defect and were divided randomly into three groups of 12 specimens each, using three different reconstructive methods. Specimens were tested in external torsion to failure on a Materials Testing System machine. The segmental defect replacement specimens had statistically greater peak torque (mean, 41.4 N-m) and stiffness (mean, 2.1 N-m/ degrees) than the intramedullary nail specimens (mean peak torque, 23.1 N-m) (mean stiffness, 1.6 N-m/ degrees) or the allograft nail composite specimens (mean peak torque, 12.4 N-m) (mean stiffness, 0.6 N-m/ degrees). The intramedullary nail specimens also had a statistically greater peak torque at failure and stiffness than the allograft nail composite specimens. For segmental defects of the humeral diaphysis, reconstruction with a cemented metallic intercalary spacer provides significantly greater immediate stability than interlocked intramedullary nail fixation supplemented with segmental methylmethacrylate or intercalary allograft reconstruction.