Mast cells (MC), the tissue-based effector cells in allergic diseases, have many functions. Within bone tissue, they have been linked with new blood vessel formation and marrow fibrosis and it has been proposed that they are capable of promoting osteoclastic bone resorption. MC numbers are known to increase in a number of osteolytic conditions e.g. osteoporosis, hyperparathyroidism and periodontitis. In fracture callus, too, large numbers of MC are present, especially during the onset of remodelling where it is believed they may be responsible for osteoclast recruitment and/or differentiation. The aim of this study was to look for further evidence of mast cell (MC) involvement in pathological bone resorption. MC activity was assessed in tissue sections of osteolytic conditions including Paget’s disease of bone, rheumatoid arthritis and fibrous dysplasia together with several benign and malignant bone tumours. MCs were identified by toluidine blue staining and by immunostaining with a commercial antibody against MC tryptase.

Extensive infiltration of mast cells was observed in fibrous dysplasia, rheumatoid arthritis and Paget’s disease of bone and mast cell accumulation was seen at the bone resorbing margin of a number of enlarging bone tumours including osteosarcoma, giant cell tumour of bone, osteoma and osteoid osteoma.

MCs, along with other inflammatory cells, are known to accumulate at the margins of soft tissue tumours where they are thought to promote tumour growth. The current findings are consistent with a similar role for mast cells in the primary bone tumours examined. In each of the conditions studied, an additional role for MC may be that of promoting bone lysis. MC are known to contain numerous factors including TNF-alpha and IL-1, which are potent stimulators of osteoclast formation and activity.

It is concluded that MCs may contribute to the fibrosis, angiogenesis and increased bone resorption seen in certain metabolic bone diseases. MC activity may also be an important factor contributing to the lysis that occurs in numerous other pathological situations including at the margins of aggressive primary bone tumours and skeletal metastases, leading to the expansion of these lesions.

Atrophic non-unions are usually attributed to impaired blood supply but the events that lead to atrophic non-union remain poorly understood. Recent studies^1,2 have shown that vascularity is not reduced in established non-unions but these studies have not examined vascularity at an early stage. The aims of this study were to: 1) develop and validate a clinically relevant small animal model of atrophic non-union and 2) test the hypothesis that the vessel density of atrophic non-unions reaches that of normal healing bones but at a later time point.

Twenty eight adult female Wistar rats underwent application of a novel circular frame external fixator to the right tibia under general anaesthesia. The fixator construct was standardised, with eight needles that were drilled through the skin into the proximal and distal metaphyses of the tibia. An osteotomy was performed with a 1mm burr under irrigation. The periosteum was removed on 14 of the 28 animals using a scalpel and the intramedullary canal was curetted. Both insults were performed proximally and distally for a distance equivalent to 1 diameter of the tibia. A 1mm gap was introduced at the osteotomy site and the wound was closed. Once the animal had recovered it was allowed unrestricted weight bearing. Anteroposterior X rays were performed every 2 weeks. Animals were killed at 1, 3, 8 and 16 weeks. Callus areas were measured from X rays using an image analysis system. The average callus area was calculated for each rat every 2 weeks as an indicator of callus production. Specimens were fixed, decalcified, embedded in paraffin wax and 6 ìm sections were stained with H& E. Vascularity was assessed immunohistochemically with monoclonal antibody against smooth muscle actin. The total number of blood vessels in the interfragmentary gap was counted.

At 8 and 16 weeks post-osteotomy all animals where stripping and curetting had been performed went on to an atrophic non-union. All animals where this was not performed went on to unite successfully. Histological observations support these radiological findings. Significantly less callus formed in the non-unions than in those that united. There were significantly fewer vessels in the non-unions at week 1 compared to the controls but, by 8 weeks the blood vessel density in the established atrophic non-unions had reached the same level as the vessel density during normal healing.

An atrophic non-union model that closely resembles the clinical situation has been developed and validated in rats. The results support the hypothesis that the number of vessels in atrophic non-unions reaches the same level as in those that unite but at a later time point. It is concluded that diminished vessel density within the first 3 weeks may prevent fractures from uniting.

During fracture repair, a number of growth factors and cytokines are present at elevated levels at the fracture site such as Transforming Growth Factor Beta (TGF-), Fibroblast Growth Factor (FGF) and Platelet Derived Growth Factor (PDGF). The aim of the study was to investigate the presence of these growth factors in healing fractures and fracture non-unions, in order to test the hypothesis that atrophic non-unions express a lower level of growth factors than hypertrophic non-unions and healing fractures.

Biopsies were taken from the fracture site of 23 patients (mean age 46) with uninfected non-unions, 12 patients with hypertrophic (mean 13.8 months after fracture) and 11 patients with atrophic (mean 16.5 months after fracture). A comparison group of biopsies from early fracture callus (one to four weeks after fracture) in five patients with healing fractures was also included. Five-micron paraffin sections were immunohistochemically stained for TGF-, FGF-II and PDGF. Growth factors were then assessed in six different cell types.

Fibroblasts, endothelial cells and macrophages were found to express TGF-, FGF-II and PDGF in all three-fracture groups. Osteoblasts, osteoclasts and chondrocytes were not present in the healing fracture group. The growth factor expression in osteoblasts, osteoclasts and chondrocytes in the non-union groups were found to be variable, however, the expression of these growth factors appeared to be less in the atrophic non-unions than hypertrophic non-unions.

The expression of these growth factors was found to be less in the atrophic non-union group than the hypertrophic non-union group in osteoblasts, osteoclasts and chondrocytes. These results may have relevance for new therapies that can be aimed at delivering growth factors to treat fracture non-unions. By further investigation of the differential expression of these growth factors it may be possible to determine which factors are likely to stimulate fracture healing.