Biomechanical evaluation of retrieved massive allografts: preliminary results

Allograft bone is the primary source of graft material for large skeletal defects. No study has determined the physical characteristics of such grafts after various periods of time in vivo for incorporation and remodeling. The purpose of this pilot study was to obtain allograft tissue and biomechanically evaluate the tissue to assess allograft bone material properties. The mechanical properties of the retrieved allograft tissue were compared to allograft bone prior to transplantation. Histological analysis of the retrieved allograft tissue is currently underway to correlate degree of incorporation, allograft porosity, and microfracture density with allograft material properties. After allograft retrieval, radiographs were used to plan sectioning for histological and biomechanical analyses. Rectangular sections of uniform dimensions (50 x 3 x 3 mm) were mapped and machined from the bulk specimens. The samples were loaded in bending in the medial to lateral direction using a 4-point bending fixture to obtain flexural elastic modulus and breaking strength. Preconditioning was applied to each specimen by cycling through 5 submaximal loading cycles (maximum deflection = 1% specimen length). After preconditioning, the specimens were loaded to failure at a rate of 1 mm/min. Retrieved specimens consisted of 1 tibia, 2 femurs, and 2 humeri ranging from 2 to 13 years in vivo. Two control tibia specimens were also tested. Assuming that material properties of cortical bone are consistent regardless of skeletal site, the preliminary data indicates that allograft modulus and strength decline with time in vivo. Testing and analysis of more specimens continue in order to corroborate these initial results.