To study the influence of a stress-relaxation plate on the remodeling of callus and cortex under the plate Methods The bilateral tibia diaphysis of New Zealand rabbit were osteotomized and fixed with stress-relaxation plate (SRP) and rigid plate (RP), respectively Polarized light microscopy and transmission electron microscopy (TEM) were used to study the remodeling of callus and the cortex under the plate from 4 to 24 weeks postoperatively Results Polarized light microscopy: the structural changes of callus and cortex beneath the plate are similar in the SRP and RP groups at the early postoperative stage, manifesting an alignment disorder of collagen fibers with a weak birefringence in the callus and absorption cavities in the cortex under the plate After the twelveth postoperative week, the SRP group showed callus starting to transform to lamellar bone and absorption cavities in the cortex under the plate becaming smaller By contrast in the RP group the absorption cavities in the callus and cortex under the plate became larger and the whole layer of cortex was cancellated TEM: the active osteoclasts appeared in both SRP and RP groups in the period from 4 to 8 weeks postoperatively In the SRP group, many functionally active osteoblasts could be seen on the surface of the bone, while in the RP group, the osteoblasts were not very active By 24 weeks postoperatively, the shape of osteocytes were normal but the number of the osteoclasts were small in the SRP group In the RP group, the osteoclasts became more active and osteocytic osteolysis was manifested Conclusions Fixation with SRP not only enhanced callus remodeling, but also abated the degree of osteoporosis in the cortex under the plate This approach may lead to an improved osteosynthetic apparatus
