A new experimental model to study healing process of metaphyseal fracture.

There are few researches for the healing of metaphyseal fractures; moreover, the animal models to study the metaphyseal fractures are usually made by the oscillating saw osteotomy without reliable fixation, which is not in accordance with our current clinical practice. In this study, we established a new model to observe the healing process of metaphyseal fractures.Eighteen New Zealand rabbits were used in the study. The fracture model was created by splitting the medial tibial plateau in rabbits, then reset, and fixed with compression screws. At 1, 2, 3, 4, 6, and 8 weeks postoperatively, the tibial specimens were collected; firstly, a general observation and an X-ray examination of the specimens was done, and then they were embedded in methylmethacrylate and cut into sections with hard tissue slicer. The sections were stained with Giemsa reagent and examined under light microscopy.There was no fracture displacement in the tibial specimens of all time points, except for one showing a collapse. No external callus formation could be observed by X-ray and general examination. After 1 week of the operation, the fracture gap was filled by mesenchymal tissue; 2 weeks postoperatively, a large number of woven bones were formed; from the third week onwards, the woven bone began to turn into lamellar bone, and new trabecular structure began to form. In all of the slices, no obvious chondrocytes formed in fracture areas; thus, there was no endochondral ossification.This model was an ideal fracture animal model and suitable for the study of metaphyseal fracture healing. The X-ray and histological images demonstrated that metaphyseal fracture healing was a process of direct bone healing through intramembranous bone formation under the conditions of minor trauma, good reduction, and firm fixation.