Experimental study of effect of stress-relaxation bone plate on fracture healing

To study the change of the stress shielding rate of stress-relaxation plate in vivo and its influence on fracture healing. Methods: The diaphyses of bilateral tibias in 70 New Zealand rabbits were osteotomized and fixed with stress-relaxation plates (SRP, the SRP group) and rigid plates (RP, the RP group), respectively. The fracture healing process in these 2 groups was investigated by radiography, light and polarized light microscopy and biomechanical test at 2 to 48 weeks postoperatively. Results: Early after fixation the stress shielding rate was about 70% in both groups. While in the SRP group the stress shielding rate decreased gradually as time passed, which was significantly lower than that of the RP group (P< 0.05 ) by the end of the 8th postoperative week, and stabilized at the level of about 27% at 36-48 weeks after fixation. Abundant external callus associated with the formation of cartilaginous callus could be observed in the SRP group at 2-4 weeks postoperatively. The transformation of the callus into the lamellar bone began at 8-12 weeks, the collagen gradually arranged in order, and the mechanical nature of the united bone was gradually strengthened, too. In the RP group, the external callus was scarce at the early stage of fracture healing, and the callus remodeling at the late stage of fracture healing was dominated by bone absorption. The ultimate bending strength (UBS) was only 57.95 % of that of the normal by 48 weeks. Conclusions: The decrease of the stress shielding rate of SRP in vivo was well interrelated with the time of fixation. The application of SRP could promote the callus formation and bone reconstruction thus to favor the recovery of the mechanical strength of the united bone.