Aims: Aims of this study was to perform a quantitatively evaluation of newly formed bone, vascular density (VD) and their correlation in animal model of posterolateral spinal fusion based on skeletal stem cells (SSCs) combined with a coral.

Methods: 15 rabbits received cell-material constructs, 15 rabbits were sham-operated (decortication of transverse apophyses), 15 rabbits received material alone. After 6 months the animals were sacrified. We performed a semi-quantitative and quantitative histologycal analysis of the fusion mass. To assess the VD, sections of the fusion mass were immunolabelled for alpha-smooth muscle actin as a vascular marker.

Results: No complete fusion was observed in all groups and no bone was formed in the interapophyseal region. Aboundant newly formed bone was observed in the peri-apophyseal regions in 60% of cases. The quantitative analysis showed a significantly higher amount of bone and VD in animals treated with cells and/or biomaterial alone compared to sham (p< 0.05). Periapophyseal VD and new bone formation was significantly higher compared to interapophyseal region in all groups (p< 0.05). Positive correlation exist between newly formed bone and vascolar density (p = 0,0009).

Conclusions: Interapophyseal region is scarcely vascolarized. The study shows a positive correlation between VD and osteogenesis. The inadequacy of staminal cells could be related with the poor survival after the implant. For the use of stam cells in the APL are necessary more studies in order to clarify the survival and in situ differentiation of the grafted cells in short and mid term.

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.