Abstract: BACKGROUND: In most species, a bone defect that is longer than 1.5 or 2 times its diameter can be considered a critical bone defect, and when the bone defect volume reaches the critical value, it cannot heal on its own. Currently, there is no uniform standard for the size of critical-sized defects in the osteoporotic femoral condyle of rabbits.
OBJECTIVE: To establish a rabbit model with different sizes of bone defects in the osteoporotic femoral condyle and to determine the critical-sized defects of osteoporotic femoral condyle in rabbits.
METHODS: Thirty-six 3-month-old female New Zealand white rabbits were randomly divided into ovariectomy group (n=30) and sham operation group (n=6). Rabbits in the ovariectomy group underwent bilateral ovariectomy to establish an osteoporosis model, and then femoral condyle bone defect models of different diameters (diameters were 4, 5, 6, and 7 mm, and depths were 8 mm) were further established; rabbits in the sham operation group did not undergo ovariectomy. At 8 and 12 weeks after modeling, 3 rats were randomly selected from each group for CT scanning and three-dimensional reconstruction to evaluate the healing of bone defects. Afterwards, samples were taken for gross observation and hematoxylin-eosin staining to observe the growth of new bone in the femoral condyle bone defect area.
RESULTS AND CONCLUSION: (1) All rabbits survived and moved well after modeling of osteoporosis and femoral condyle bone defect. (2) At 12 weeks after osteoporosis modeling, dual-energy X-ray absorptiometry results showed that the bone mineral density of lumbar vertebrae in ovariectomy group was significantly lower than that in sham operation group (P < 0.05). Hematoxylin-eosin staining showed that the bone trabeculae in the ovariectomy group became thinner and sparse. The proportion of bone tissue area in the ovariectomy group was significantly lower than that in the sham operation group (P=0.00). Micro-CT results showed that the bone tissue parameters of the femoral condyle in the ovariectomy group were significantly different from those in the sham operation group (P < 0.05), and the ovariectomy group showed obvious characteristics of osteoporosis. (3) CT showed that the bone defect in the 4 mm and 5 mm diameter groups was basically completely repaired at 12 weeks after surgery. There was more new bone tissue in the 6 mm diameter group, but the central part of the bone defect was not completely repaired. A small amount of new bone tissue grew in the 7 mm diameter group, and the bone defect was obvious. (4) Gross observation at 12 weeks after surgery showed that the femoral condyle bone defect in the 4 mm and 5 mm diameter groups was completely repaired. Obvious depression was seen in the bone defect area of the 6 mm and 7 mm diameter groups, and the bone defect was not completely repaired. (5) Histological observation at 12 weeks after surgery showed that the bone defect area of the 4 mm and 5 mm diameter groups was completely filled with new bone, and the trabecular structure was irregular; while there were new trabeculae in the periphery of the 6 mm and 7 mm diameter groups, and the bone defect in the central area was still obvious. (6) The results showed that during the 12-week experimental observation period of osteoporotic femoral condyle defects in rabbits, under the condition of the same defect depth of 8 mm, femoral condyle defects with a diameter ≥ 6 mm could not heal on their own, while femoral condyle defects with a diameter < 6 mm were completely repaired. A diameter of 6 mm and a depth of 8 mm can be used as the critical bone defect value of osteoporotic femoral condyle in rabbits.

中国组织工程研究杂志出版内容重点：组织构建；骨细胞；软骨细胞；细胞培养；成纤维细胞；血管内皮细胞；骨质疏松；组织工程

Key words: critical bone defect, osteoporosis, femoral condyle, model, rabbit, Micro-CT, DXA, bone tissue engineering