Controlled induction of a pseudarthrosis: a study using a rodent model

OBJECTIVES: This study aimed to test the hypothesis that under standardized mechanical and biologic conditions, the process of indirect bone repair in a rodent species could be manipulated to form a reproducible, atrophic, fibrous pseudarthrosis. DESIGN: The model used comprised a mid-diaphyseal, transverse osteotomy in the rat femur, stabilized via a precision miniature external fixator, a constant axial fixation stiffness being defined by a specific frame geometry. MAIN OUTCOME MEASUREMENTS: The repair process for both 0.5-mm and 3.0-mm gap osteotomies was characterized using radiography, dual-energy x-ray absorptiometry, histologic assessment of standardized longitudinal sections, and postmortem mechanical testing. RESULTS: Healing of the defect was highly reproducible, bone union being attained at around 5 weeks postoperatively with a 0.5-mm gap. Increasing the gap width to 3.0 mm resulted consistently in a pseudarthrosis. CONCLUSION: These two reproducible patterns of repair can now be used to elucidate the underlying molecular mechanisms controlling the extent and progression of connective tissue differentiation in indirect bone repair without the additional variable of a nonstandardized mechanical environment.