Rabbit distal femoral critical size bone defect model: Vascular anatomy, surgical technique, peri-operative husbandry and biological response of empty and autograft-filled defects

Introduction Critical size bone defects do not heal spontaneously and are often used preclinically to evaluate the in vivo response to materials, devices or bone graft substitutes. Results may inform product development or satisfy regulatory body requirements, whilst also, providing clinically relevant information that cannot be obtained by in vitro or in silico experimentation. The New Zealand White (NZW) Rabbit distal femoral condylar defect in skeletally mature animals is a common model used in the development and evaluation of bone graft materials. When performed with appropriate consideration of the many pre, peri and post-operative factors the morbidity with this model is low. We present our experience with this model to provide a technical reference including the local vascular anatomy, surgical technique, peri-operative husbandry in large series between 2017–2022. The typical healing responses of empty and autogenous corticocancellous bone-filled defects is provided to assist in illustrating the model. Materials and methods The local vascular anatomy was characterised ex vivo with anatomical dissection and intra-vascular injection of methylene blue, and an iohexol contrast study with micro-computed tomography. The in vivo animal model is described, highlighting the surgical approach, technique for defect creation and perioperative animal husbandry considerations. Examining outcomes from animals operated on over a 5-year period between 2017 and 2022 at our institution, the biological response following defect creation is also described qualitatively for empty defects (negative control) and autograft-filled defects (positive control) at 6 and 12-weeks post-operatively, with paraffin and polymethyl methacrylate (PMMA) histology and micro-computed tomography (µCT) endpoints. Results 2040 individual defects were created in 1020 rabbits confirmed to be skeletally mature with radiographs. Anaesthetic and intra-operative complications occurred with a very low incidence (1/ 1020 animals {0.1%}and 1/ 2040 defects {0.05%}, respectively). No post-operative complications impacting the healing outcomes or necessitating removal from the study occurred. Empty defects functioned as critically sized defects over a 12-week period, whilst autograft implanted into defects had incorporated into the surrounding cancellous architecture at 12 weeks. Closure of the cortical bone surface on the medial aspect of the femur was restored at 12 weeks, but not 6 weeks. Conclusion In vivo models provide invaluable, irreplaceable preclinical data; however, such high-impact studies must have a sound ethical basis and be conducted with animal welfare and scientific rigour at the forefront of study design and execution.