Abstract: BACKGROUND: In order to solve the complications of external fixation screws, various modified screws have emerged, but there is a lack of suitable animal models and evaluation methods for validating their efficacy, especially for simulating real loads.  
OBJECTIVE: To construct a simple, rational and efficient model that can be used to validate the efficacy of external fixator screws in Sprague-Dawley rats and establish the corresponding evaluation methods.
METHODS:  Thirty male adult Sprague-Dawley rats underwent the right femoral osteotomy, followed by fixation with a homemade external fixator system. Routine X-ray films were taken on the day after operation and 1, 2, 4, and 6 weeks after operation. During this period, the activity of rats was observed. At 6 weeks after operation, the rats were euthanized, and the right thigh femur with screw specimens were taken for general observation, biomechanical measurement, micro-CT, three-dimensional reconstruction analysis of the screw path and histological observation. 
RESULTS AND CONCLUSION: The modeling time of external fixation model of rat femoral fracture was (20±5) minutes. The general condition and activity of rats were good during the experiment. In general, 28 (93%) of the 30 femoral specimens with screws achieved bone healing; 1 case (3%) had poor pseudoarthrosis formation with severe screw loosening accompanied by large osteolysis around the proximal screw path; 1 case (3%) had mild screw loosening combined with severe screw path infection; 1 case (3%) only had severe screw path infection. The overall satisfactory rate of the model was 90% (27/30). Radiological findings revealed that the right femoral screws were fixed in place without loosening, the bone morphology was restored, the broken end was well aligned, no free body was seen around the broken end, and the right hip and knee were in position at 6 weeks after operation. Micro-CT three-dimensional analysis showed that body mineral density, bone volume fraction, and the number of bone trabeculae around the internal screw path were (0.426 7±0.088 7) mg/cm2, (3.125 2±1.004 2)%, and (0.000 3±0.000 1) /μm respectively, at 6 weeks after operation. Intra-model group distribution and normality test were performed, and all the data tended to be stable and normally distributed. Hematoxylin-eosin staining showed that the area around the screw path was repaired by a large amount of proliferating granulation tissue, and new bone tissue was visible at the edges. Von Kossa staining revealed the presence of extensive black or brownish-black calcium deposition around the screw path. Biomechanically, the screw extraction force was (130.416 7±35.008 0) N, which tended to be generally stable and normally distributed (P=0.245 6). To conclude, the validation model and evaluation method for the efficacy of external femoral fixator screws in Sprague-Dawley rats can be evaluated from three aspects: histology, imaging and biomechanics, and the screw path can be comprehensively evaluated. The design is scientific and reasonable, the procedure is simple, and the success rate is high. Therefore, this validation model has good value for scientific research and clinical application.

Key words: screw, femur, external fixation, Sprague-Dawley rat, animal model