Development of new treatment methodologies for bone nonunion requires validated experimental models for their application into tissue engineering approaches. Critical-sized bone defect (CSBD) models identify the smallest size at which tissue defects fail to completely heal during the experimental period. The purpose of this study was to systematically determine a CSBD in rat femurs using external fixation. Thirty Fischer 344 rats were equally divided into six groups. Bone defects of 1, 2, 3, 4, 5, and 6 mm were created in the diaphysis of the femur, before external fixation. Defects were evaluated radiographically at weekly intervals and histologically at the predefined healing period of 8 weeks. Defects of 4 mm or more failed to achieve union in our model. Nonunions were characterized by the absence of a bridging callus and filling of the defect with fibrous and muscle tissue. Radiographically, bone ends in nonunions were rounded up to 4 weeks postsurgery. Our systematic approach for identification of a CSBD in rat femurs, revealed that externally fixed defects of 4 mm and larger, failed to heal within the 8-week time frame.
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