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 osteot omy/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 gro up of twenty rats served as the control group. After randomization, the fem urs of ten rats in each group at six and twelve weeks were studied clinical ly, radiologically, and biomechanically, and bone mineralization was measur ed by dual-energy x-ray absorptiometry (DEXA). Results: Radiographs in two planes revealed clearly visible fractures at si x weeks, and at twelve weeks bridging callus was apparent in all three trea tment groups. The callus area in the UL group was significantly larger at s ix 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 r educed maximum bending load at six and twelve weeks, while DL nails had inc reased fracture energy at six weeks compared with SL nails. Bone mineral co ntent and bone mineral density in the callus segment and diaphysis were inc reased 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 do es not increase the rate of bony union. The clinical implication is that ro utine early dynamization of locked femoral fractures may not be indicated, reserving dynamization to cases of delayed union.