Biomechanical comparison of callus over a locked intramedullary nail in various segmental bone defects in a sheep model.

BACKGROUND: Little has been written about the size of a bone defect that can be restored with one-stage lengthening over a reamed intramedullary nail. MATERIAL/METHODS: Sixteen adult female sheep were divided into four main groups: a simple osteotomy group (group I) and three segmental defect groups (1-, 2-, and 3-cm gaps, groups II-IV). One intact left tibia from each group was also used as the non-osteotomized intact control group (group V). In all cases, the osteotomy was fixed with an interlocked Universal Humeral Nail after reaming to 7 mm. Healing of the osteotomies was evaluated after 16 weeks by biomechanical testing. The examined parameters were torsional stiffness, shear stress, and angle of torsion at the time of fracture. RESULTS: The regenerate bone obvious in x-rays in the groups with 1- and 2-cm gaps had considerable mechanical properties. Torsional stiffness in these two groups was nearly equal and its value was about 60% of the stiffness of the simple osteotomy group. Gradually decreasing stiffness was observed as the osteotomy gap increased. No significant differences were found among the angles of torsion at fracture for the various osteotomies or the intact bone. These results showed that the group with 1-cm gap had 65% of the shear stress at failure of the simple osteotomy group. CONCLUSIONS: The authors believe that there is evidence indicating that intramedullary nailing could be a reasonable option when one-stage lengthening of a long bone by 1 or 2 cm is contemplated.