Since several spinal conditions are currently treated with spinal fusion, alternatives to autogenous bone graft in spinal surgery have been under study for many years. Results have shown that, compared to other non.-spinal conditions, such as filling bone cavities, spinal fusion, in particular posterolateral fusion, is much more challenging due to the reduced area of the graft bed. As a result, most of the bone substitutes are still under investigation and their effectiveness in the clinical setting has yet to be demonstrated.

In recent years the authors analysed several bone graft substitutes using an animal model which has been widely used in experimental spinal fusion. In particular, porous ceramics have been used alone or with osteoin-ductive material such as fresh bone marrow or cultured mesenchymal stem cells. The results of these studies have shown that with ceramic alone a percentage of solid fusion similar to that with autogenous bone graft cannot be achieved. However, compared to the latter, more favorable results have been obtained when ceramics are loaded with mesenchymal stem cells. The addition of fresh bone marrow to ceramics also increases the fusion rates; however, in this case new bone formation was mainly found in the peripheral portions of the graft and to a lesser extent than when cultured mesenchymal stem cells were used.

Aternatives to autogenous bone graft for spinal fusion have been investigated for many years. It has been shown that osteoconductive materials alone do not give a rate of fusion which is comparable to that of autogenous bone graft. We analysed the effectiveness of porous ceramic loaded with cultured mesenchymal stem cells as a new graft material for spinal fusion in an animal model.

Posterolateral fusion was carried out at the L4/L5 level in 40 White New Zealand rabbits using one of the following graft materials: porous ceramic granules plus cultured mesenchymal stem cells (group I); ceramic granules plus fresh autogenous bone marrow (group II); ceramic granules alone (group III); and autogenous bone graft (group IV). The animals were killed eight weeks after surgery and the spines were evaluated radiographically, by a manual palpation test and by histological analysis.

The rate of fusion was significantly higher in group I compared with group III and higher, but not significantly, in group I compared with groups II and IV. In group I histological analysis showed newly formed bone in contact with the implanted granules and highly cellular bone marrow between the newly formed trabecular bone. In group II, thin trabeculae of newly formed bone were present in the peripheral portion of the fusion mass. In group III, there was a reduced mount of newly formed bone and abundant fibrous tissue. In group IV, there were thin trabeculae of newly formed bone close to the decorticated transverse processes and dead trabecular bone in the central portion of the fusion mass.

In vitro cultured mesenchymal stem cells may be loaded into porous ceramic to make a graft material for spinal fusion which appears to be more effective than porous ceramic alone. Further studies are needed to investigate the medium- to long-term results of this procedure, its feasibility in the clinical setting and the most appropriate carrier for mesenchymal stem cells.