Heterotopic ossification (HO) of the hip after injury to the central nervous system can lead to joint ankylosis. Surgery is usually delayed to avoid recurrence, even if the functional status is affected. We report a consecutive series of patients with HO of the hip after injury to the central nervous system who required surgery in a single, specialised tertiary referral unit. As was usual practice, they all underwent CT to determine the location of the HO and to evaluate the density of the femoral head and articular surface. The outcome of surgery was correlated with the pre-, peri- and post-operative findings.

In all, 183 hips (143 patients) were included of which 70 were ankylosed. A total of 25 peri-operative fractures of the femoral neck occurred, all of which arose in patients with ankylosed hips and were associated with intra-articular lesions in 18 and severe osteopenia of the femoral head in seven. All the intra-articular lesions were predicted by CT and strongly associated with post-operative complications.

The loss of the range of movement before ankylosis is a more important factor than the maturity of the HO in deciding the timing of surgery. Early surgical intervention minimises the development of intra-articular pathology, osteoporosis and the resultant complications without increasing the risk of recurrence of HO.

Purpose: Few studies have been devoted to neurogenic paraosteoarthorpathy (PAOn). We characterised the expression of genes specific for osteoblastic and chon-drocytic phenotypes using the osteomy wedge and non-mineralised tissue near the osteotome.

Material and methods: Osteotomy fragments and non-mineralised tissue near the osteotomy were obtained during surgery performed in 25 patients. The explants were cultured for 56 days. We searched for the messenger RNA of the principal markers of osteoblastic, chon-drocytic, and adipocytic phenotypes, as well as certain specific proteins. Serial cryotome sections were stained for histology and immunolabelling tests.

Results: Cells issuing from the osteotomy fragment and neighbouring tissues formed structures that miner-alised in culture. The following osteoblast markers were observed: alkaline phosphatase (bone isoform), osteo-calcin, Cbfa1, type 1 collagen; for chondrocytes: type II collagen, aggrecane; type X collagen as well as VEGT demonstrating the presence of hypertrophic chondrocytes.The adipocyte-specific transcription factor PPAR 2 was also found in the two cultures. The proportions and chronological expression of these markers were slightly different for the two tissues. Ex vivo study demonstrated the typical sequence of enchondral type bony formation from non-osseous cell populations.

Discussion: This work provided the first characterisation of non-mineralised tissue near osteotomy. It also provided clear indications concerning the history of ectopic bone formation. The osteochondrogenic potential of connective tissue lying close to an osteotomy has not been reported previously. The persistence of this potential could explain recurrence after resection. The observation that this potential is suppressed in vivo but expressed in vitro opens a new avenue of research concerning the mechanisms controlling bone formation.

Conclusion: The culture model developed in this study provides a means of studying factors determining the outcome of cell populations implicated in the formation of neurogenic paraosteoarthropathies.