The effect of implant axial and torsional stiffness on fracture healing

A study was performed to compare the mechanical properties of healing transverse femoral osteotomies fixed with either (a) plates of high or low axial stiffness (compression or slotted plates) but equivalent bending and torsional stiffnesses, or (b) rods of high or low torsional stiffness (solid or slotted rods) but equivalent bending and axial stiffnesses. Compression and slotted plates were implanted contralaterally in one group of adult mongrel dogs and solid or slotted rods were implanted contralaterally in a second group as fixation for transverse osteotomies. We found that the compression-plated femora regained strength and stiffness earlier than the slotted-plated femora and healed with less callus formation. The femora fixed with rods healed with no significant differences in properties at any time interval. Also, the femora fixed with rods had mechanical properties close to those of the slotted-plated femora and lower than those of the compression-plated femora at the same time intervals. We concluded that contact compression caused the differences in the healing results and was more important than differences in torsional or axial rigidity of the implant.