Assessment of bone union/nonunion in an experimental model using microcomputed technology

Background: High-resolution microcomputed tomography ((μCT) is one of the most recent technical developments to visualize and quantify primarily cancellous bone. Regarding bone formation, μCT is becoming increasingly important, although its reliability has not yet been evaluated. Our study had two goals: to develop a reproducible nonunion model and to determine the efficacy of μCT for the assessment of bone healing in this model. Methods: The designed fracture model in the rat simulates secondary fracture healing. After plate fixation to the femur, diaphysis transverse middiaphyseal osteotomy was performed with a reciprocating saw, resulting in a 0.38-mm gap with a defect of bone and periosteum corresponding to the thickness of the blade. Proximally and distally to this gap, the periosteum was preserved. Thus, three separate zones were defined: proximal femur diaphysis with periosteum, gap, and distal femur diaphysis with periosteum. In the nonunion group (NM group), a model of impaired bone healing (nonunion), silicone foil was wrapped around the femur diaphysis to block any influence from surrounding tissue. Coverage of the bone repair site by thigh muscles was designed for a model of bone union (M group). Four weeks postoperatively, callus formation was determined by conventional anterior-posterior and lateral plain radiographs. Ten weeks later, a second x-ray series was done as the clinical standard evaluation method. Afterward, specimens were harvested for μCT examination (two-dimensional and three-dimensional [3D]). Biomechanical testing was carried out to determine fracture healing. Results: Our model is highly reproducible and results in bone nonunion in five out of six cases (83.3%). In determining fracture site, plain radiographs the least reliable method in comparison to the biomechanical testing which is the most accurate reference method. In contrast, μCT (the 3D reconstruction) showed significant correlation (r = 1) to the results assessed by biomechanical testing, whereas μCT was correct in 100%. We found bone healing in five out of six animals in the M group verified by μCT (in accordance to biomechanical data). In the M group, significantly enhanced bone formation (50%) (p = 0.008) was observed within the osteotomy site (i.e. within the gap), but there was no difference in periosteal bone formation between the groups proximally and distally to the gap. Interestingly, we did not find statistically significant differences in mineralization. Conclusion: We conclude that μCT with 3D reconstruction is the optimal method diagnostic tool in fracture healing, especially in nonunion. Furthermore, direct coverage of the fracture site by muscle flaps results in a mineralized enhanced bone formation within the osteotomy site (i.e. within the gap). Skeletal muscle coverage hypothetically might have osteogenic augmentation potential, thus being able to prevent pseudoarthrosis.