Effects of torsional rigidity on fracture healing: strength and mineralization in rat femora.

OBJECTIVE: To assess the effects of torsional rigidity and dynamization on fracture healing in a medullary nailed rat femoral model. STUDY DESIGN: Randomized study in male Wistar rats with a diaphyseal osteotomy/fracture. METHODS: Reamed cannulated nailing was performed in sixty rats. One group (twenty rats) received unlocked nails (UL group) and another group received nails with proximal and distal locking that was dynamized after twenty days (DL group). The third group was statically locked (SL group). A fourth group of twenty rats served as the control group. After randomization, the femurs of ten rats in each group at six and twelve weeks were studied clinically, radiologically, and biomechanically, and bone mineralization was measured by dual-energy x-ray absorptiometry (DEXA). RESULTS: Radiographs in two planes revealed clearly visible fractures at six weeks, and at twelve weeks bridging callus was apparent in all three treatment groups. The callus area in the UL group was significantly larger at six weeks than in the other groups, and at twelve weeks the UL and DL groups had larger callus areas than the SL group. Biomechanically, UL nails had reduced maximum bending load at six and twelve weeks, while DL nails had increased fracture energy at six weeks compared with SL nails. Bone mineral content and bone mineral density in the callus segment and diaphysis were increased with DL nails at six weeks. CONCLUSION: This animal study indicates that (a) interlocked nailing has a beneficial effect on bone healing and (b) although dynamization may have a beneficial effect on the quality of early bone healing, (c) dynamization does not increase the rate of bony union. The clinical implication is that routine early dynamization of locked femoral fractures may not be indicated, reserving dynamization to cases of delayed union.