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DEVELOPMENT OF AN IN VIVO MODEL OF BLOOD FLOW AND CALLUS FORMATION DURING FRACTURE REPAIR

Musculoskeletal Research and Education Unit, Musgrave Park Hospital, Belfast, BT9 7JB

Introduction: Angiogenesis is essential during bone formation. Many studies have looked at the developing vascular network during normal and abnormal bone growth, using histological, immunohistological and contrast-radiological techniques; however all require sacrifice of animals to obtain tissue samples for examination and consequently chronological investigation of angiogenesis is not possible. We have endeavoured to produce an animal model, whereby quantitative assessment of blood flow, and callus formation across a fracture gap, can be repeatedly assessed.

Methods: The model is an adaptation of a 4-pin externally fixated murine femoral fracture previously developed in this department. Three extra conduits have been drilled onto the fixator cross-bar, such that it now links with an x-ray jig and implantable optical cable. The x-ray jig permits repeated lateral x-rays whereas the optical cable which is implanted adjacent to the fracture gap and connected to a laser, measures blood flow using the principle of the Doppler shift of light. Ten mice underwent surgery. Doppler readings and x-rays were taken on the day of surgery and subsequently at days 1, 2, 4, 8, 12, 16, 24 and 32.

Results: Fracture gap pixel density was seen to rise steadily and plateau at day 24, with significant statistical differences between the day of surgery and early time points, and then again between these early time-points (days 2, 4 and 8) and the late time-point day 24. Blood flow was noted to fall following the day of surgery and then slowly increase, with a rapid rise in flow at day 8 until day 16, when levels began to fall again to resting levels.

Conclusion: The data correlates with previous histo-morphological work performed in this department and also with early results from immunohistochemical studies. The above graph for blood flow conforms to that expected in a murine model of fracture healing, with a short initial drop in flow followed by a large rise as angiogenesis follows chondrocyte hypertrophy at the end of the first week, leading to callus formation. This in vivo model may be used to assess the effects on angiogenesis and callus formation of osteogenic compounds and investigate possible antiangiogenic mechanisms of action of medications such as NSAIDs that are known to be detrimental to fracture repair.

Correspondence should be addressed to Mr Carlos Wigderowitz, Honorary Secretary BORS, University Dept of Orthopaedic & Trauma Surgery, Ninewells Hospital & Medical School, Dundee DD1 9SY.