The aim of this study was to examine the therapeutic potential of locally transplanted MSCs or osteoprogenitor cells (OPCs) in delayed unions. Autologous MSCs were cultured in DMEM or osteogenic medium. A femoral osteotomy was created in rats and stabilized with an external fixator. Except for the Control-group (C-group), a delayed union was induced by cauterization of the periosteum and bone marrow removal. After 2 days, these animals received an injection of DMEM in the gap containing MSCs (MSC-group), OPCs (OPC-group) or no cells (Sham-group). Histomorphometrical analysis showed significant differences in the fraction of mineralized bone, cartilage and connective tissue between the C- and the Sham-group after 2 (p=0.001) and 8 weeks (p≤0.009). After 2 weeks, the MSC- and OPC-groups developed a larger cartilage fraction (each p=0.019) compared to the Sham-group. Biomechanical testing after 8 weeks demonstrated a significantly lower torsional stiffness (p=0.001) in the Sham-group compared to the C-group. Both the MSC and OPC groups showed a higher torsional stiffness than the Sham-group with statistically significant differences (p< 0.002) in the OPC-group. Locally applied MSCs and OPCs slightly improved the healing in this model. The MSCs were less effective compared to the OPCs. The less than expected healing improvement of both cell treatments may be related to an unfavourable microenvironment at the application time. An explanation for the superior outcome of the OPCs might be that the OPCs may be protected by macroscopically visible matrix at the transplantation time point.

Introduction: Currently used small animal models of a critical size defect do not sufficiently simulate the biologically unreactive situation in an atrophic non-union. Furthermore, models using intramedullary nails are of little, and poorly standardised, biomechanical stability. This is a characteristic known to promote callus formation though, rather leading to a hypertrophic non-union.

The aim of this study was to establish an atrophic non-union model in the rat femur under well defined biomechanical conditions and with minimised interactions between the processes in the healing zone and the implant by using external fixation.

MATERIALS AND METHODS: 80 male Sprague Dawley rats were randomly divided into two groups (non-union vs. control). All animals received an osteotomy (app. 0.5 mm gap) of the left femur, stabilised with a custom made external fixator. In the non-union group the periosteum was cauterised 2mm distal and proximal of the osteotomy, and the bone marrow was removed. X-rays were performed once weekly. Animals were sacrificed at 14 or 56 days post-operation. At both time points the femurs of 16 animals of each group underwent histological/histomorphometrical and immunhis-tochemical analyses (PMMA or paraffin embedding). Additionally at 56 days 8 animals of each group were tested biomechanically. The maximum torsional failure moment and the torsional stiffness were determined in relation to the intact femur. Post-mortem x-rays were evaluated in a descriptive manner.

RESULTS: At 14 days the histology and radiology showed considerable mineralised periosteal callus in the control group, while the non-union group only showed very little periosteal callus, distant to the osteotomy. At 56 days the control group was completely, or at least partially, bridged by mineralised callus. The non-union group did not show a bridging of the osteotomy gap in any of the animals, moreover the bone ends were resorbed and the gap widened. The relative mean torsional stiffness was significantly larger (p< 0.001) in the control group compared to the non-union group (136.2±34.5% vs. 2.3±1.2%). In the non-union group no maximal torsional failure moment could be detected for the osteotomised femurs. In the control group it was 134.2±79.1%, relative to the intact femur.

DISCUSSION: The cauterisation of the periosteum and the removal of the bone marrow, in combination with a high stiffness of the external fixator may create an atrophic non-union under well defined biomechanical conditions and with minimised interactions between the healing zone and the implant. This model will allow better standardised investigations on the subject of atrophic non-unions.