Purpose: Little has been written about the size of bone defect that could be restored with one-stage lengthening over a reamed intramedullary nail. The aim of this study was to investigate the mechanical properties of the callus created at gaps of various sizes in sheep tibiae treated with reamed intramedullary nailing.

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 gap; groups I to III). 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 7 mm reaming. The osteotomized site was closed in layers including the periosteum without additional bone grafting and the limb was protected with long soft cast for 5 weeks postoperatively. 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: Samples with a simple osteotomy or 1 cm gap were fractured distally to the callus zone, whereas samples with 2 and 3 cm gaps were fractured at the callus zone or at distal metaphysis. The regenerate bone obvious in the x-rays in the group of 1 cm and 2 cm gap had considerable mechanical properties. Torsional stiffness in these two groups was nearly similar and its value was about 60% of the stiffness of the simple osteotomy group. A gradually decreased stiffness was observed as the osteotomy gap increased. There was a decrease in maximum shear stress from simple osteotomy to osteotomy with a fracture gap of 3 cm. No significant differences were found among the angles of torsion at fracture for the various osteotomies or the intact bone. Our results showed that the group of 1 cm gap had the 65% of the shear stress at failure of the simple osteotomy group.

Conclusion: We believe that there is evidence indicating that intramedullary nailing would be a reasonable option when one-stage lengthening of a long bone of 1 or 2 cm is contemplated.