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

OBJECTIVE: To study the change of the stress shi elding rate of stress-relaxation plate in vivo and its influence on fracture he aling. METHODS: The diaphyses of bilateral tibias in 70 New Zealand ra bbits were osteotomized and fixed with stress-relaxation plates (SRP, the SRP g roup) and rigid plates (RP, the RP group), respectively. The fracture healing pr ocess in these 2 groups was investigated by radiography, light and polarized l ight microscopy and biomechanical test at 2 to 48 weeks postoperatively. RESULTS: Early after fixation the stress shielding rate was abo ut 70% in both groups. While in the SRP group the stress shielding rate decrease d gradually as time passed, which was significantly lower than that of the RP gr oup (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. Abund ant external callus associated with the formation of cartilaginous callus could be observed in the SRP group at 2-4 weeks postoperatively. The transformation o f 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 s trengthened, too. In the RP group, the external callus was scarce at the early s tage of fracture healing, and the callus remodeling at the late stage of fractur e 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 i n vivo was well interrelated with the time of fixation. The application of SRP c ould promote the callus formation and bone reconstruction thus to favor the reco very of the mechanical strength of the united bone.