OBJECTIVES: Obtaining stable fixation in cases of long bone non-union with segmental bone defects can be challenging. Bone quality is often sub-optimal. Locking plates and structural allografts have both been used clinically in these cases. The objective of this study was to determine the biomechanical characteristics of three constructs that have been employed in this context. METHODS: A biomechanical study was performed using 3rd Generation Composite Femurs as specimens. A diaphyseal segmental defect was created and fixed with one of three constructs: (1) lateral locking plate (LP); (2) lateral non-locking plate and medial allograft strut (S); (3) lateral non-locking plate and intramedullary fibula allograft (F). The "allografts" were fashioned from 3rd generation composite bones. Axial, torsional and bending stiffness as well as load to failure were determined using a materials testing machine. RESULTS: Overall, construct S was the stiffest and construct LP was the least stiff. Construct F had intermediate characteristics. Axial load to failure for construct S (6108N) and for construct F (5344N) was significantly greater than for construct LP (2855N). CONCLUSION: When maximal stiffness is desired, a construct with a structural allograft should be chosen over a locking plate. However, biological and anatomic factors must also be taken into account when using these constructs clinically.