Comparative Empirical Study On The Open Osteotomy Model And Closed Fracture Model Of Rat Femur

Two distinct animal models, open osteotomy and closed fracture, are used to study fracture healing research of the long bone shaft. A smooth cut surface with controlled fracture line can be consistently reproduced with osteotmy. But someone reported that the influencing factors for fracture healing will be increased with the open osteotomy model due to the instability of the fracture site, more serious damage of the soft tissue closed to the fracture site during the operation, and the thermal damage of the cortial bone. Conversely, the closed fracture produces an interdigitate surface which is more stable. The closed fracture model can be truly reveal the process of the fracture healing because of the stability of the fracture site with minimal initial displacement, less influencing factors and minimal soft tissue damage. At present, the closed fracture model is widely used for studying the early and intermediate stage of fracture healing in abroad. However, in domestic, the open osteotomy model is still used frequently. Now, we conduct this study to compare the two kinds of fracture model in order to provide the reference for choosing animal model during the fracture healing research.Object: To provide the reference for choosing animal model during the fracture healing research by comparing the process of model building and fracture healing of the open osteotomy model and closed fracture model.Method:1 Comparing the process of model building: 40 Spragu-Dawley rats were divided into open group and closed group randomly. All animals in the open group received open osteotomy on the right midshaft fumer. Then the osteotomy were immobilized by retrograde Kirschner wire.While the animals in close group received the right midshaft femoral fracture by a simple apparatus following the retrograde Kirschner wire immobiling. Calculate the average modeling time, achievement ratio. Then, at 2, 3, 4, and 8 weeks after operation, 5 animals of each group were sacrificed. Fracture healing was assessed by using 2-D radiography for callus bridging.2 Comparing the difference process of fracture healing between the open osteotomy model and closed fracture model: 80 Spragu-Dawley rats were divided into open group and closed group randomly. The operation methods have been mentioned above. At 30 minutes, and 1 week after operation, 4 animals of each group were sacrificed. Fracture healing was assessed by radiography and optical microscope for callus microstructure. At 2, 3, 4, and 8 weeks after operation , 8 animals of each roup were sacrificed, 4 animals of which were used for the radiography and optical microscope observation, and the rest were choosed for biomechanics test.Results:1 In the open group, the average model building time is 17 minutes, the achievement ratio is 90%, the ratio of malunion and nonunion is 40%.While in the closed group, the average model building time is 12 minutes,the achievement ratio is 85%,and none of the malunion or nonunion were found.2 The radiographic scores of callus in the open group is lower at 2, 3, 4 weeks after operation compared to the closed group(p<0.05).At 8 weeks after operation ,the radiographic scores of callus in the open group is still lower than that in the closed group, but the difference has no statistical significance(p>0.5).3 Assessment of the histologic sections stained by Hematoxylin and eosin confirmed that a smooth cut surface was reproduced with osteotmy at 30 minutes after operation. While,in the closed group, the fracture surface looks like interdigitate. At 1 week and 2 weeks, there was little mineralized callus in open group, but the specimen in the closed group already can be found apparente mineralized callus. At 3 weeks, the mineralized callus was increased in both groups. But the mineralized callus was less in the open group compared to the closed group. At 4 weeks, while the mineralized callus in the open group was sill large, the callus in the closed group become decreasing. At 8 weeks, all the specimen of each group achieved bone union. But the medullary cavity of the fracture site in the open group was still obstructed, which was different from the closed group.4 All femora failed at the fracture site after 4-point bending test. The mechanical parameters continuously increased throughout whole experimental period. At 2, 3, and 4 weeks, all parameters in the open group were inferior to that of closed group (p<0.05). At 8 weeks, no statistically significant difference was shown in Max-load, Stiffness and Energy between the two groups.Conclusion:1 Compared to the closed fracture model, open osteotomy model's model building time is longer and the ratio of malunion or disunion is higher.2 Due to the more influence factor to fracture healing ,the fracture healing process of open osteotomy model was delayed compared with the closed fracture.3 The fracture healing process of the open osteotomy model is hysteresis,and the change of biomechancs parameter of it in the ealy stage of fracture healin process is not significant, so open osteotomy model is not suitable for the early and intermediate process of the fracture healing research.