USE OF A COLLAGEN-HYDROXYAPATITE MATRIX IN SPINAL-FUSION - A RABBIT MODEL

Study Design. The efficacy of a specially designed mineralized bovine collagen matrix as a carrier for more marrow stem cells was studied in a rabbit posterolateral spinal fusion model. Objectives. To determine if bone marrow cells added to Healos matrix will lead to fusion rates , biomechanical properties, and histologic properties comparable with those of fusions using autologous iliac crest bone graft; and to deter mine if the addition of preservative-free heparin to anticoagulate the bone marrow during harvest will adversely affect the fusion rate. Sum mary of Background Data. Although the development of new preparations of osteoinductive agents has advanced rapidly in recent years, the car rier systems that have been used in their application have received le ss attention. The composition and structure of the matrix used are key components affecting the ability of the matrix to function as a scaff old on which cells can migrate, adhere, proliferate, and form bone. Th e composition and design of matrix components also determine the abili ty of osteoinductive agents to influence local and hematogenously deri ved osteogenic precursor cells, which migrate to or are brought into t he fusion site. Thus, the properties of the carrier can affect the beh avior and efficacy of the osteoinductive agent that is used. The autho rs studied the properties of a new mineralized collagen matrix called Healos, which has been engineered specifically for spinal fusion appli cation. Methods. Forty-four adult female New Zealand white rabbits wer e divided into five groups. Groups 1-4 underwent bilateral intertransv erse fusion between L5 and L6. The fusions were augmented with either autologous iliac crest bone graft, Healos matrix alone, Healos matrix mixed with autologous bone marrow, or Healos matrix combined with hepa rinized autologous bone marrow. At 8 weeks after surgery, the fusions were characterized radiographically, histologically, and biomechanical ly. The rate of fusion was determined by radiographic analysis. The fi fth group consisted of two animals whose bone marrow was aspirated fro m their tibias and femurs and then sent for determination of total nuc leated cell count. Results. At 8 weeks, the radiographically determine d fusion rate for autologous bone graft was 75% (9/12 animals), compar ed with 100% (10/10 and 9/9 animals) for groups in which fusions were done by using Healos matrix augmented with bone marrow (P less than or equal to 0.1). Matrix used alone yielded a fusion rate of 18% (2/11 a nimals, P less than or equal to 0.006). Histologically, the most matur e bone was seen in the group augmented with autologous iliac crest gra ft, followed in decreasing order by the groups augmented with Healos w ith heparinized bone marrow, Healos with unheparinized bone marrow, an d Healos alone. Biomechanically, the group augmented with autologous g raft had the highest mean stiffness, followed by the groups augmented with Healos with heparinized bone marrow, Healos with untreated bone m arrow, and finally Healos matrix alone. However, the differences in st iffness between groups were not statistically significant with the num ber of spines tested. Conclusions. These results show that Healos is a n osteoconductive matrix that can be a useful carrier in the biologic and mechanical environment of a posterolateral intertransverse fusion site. In combination with bone marrow, it produces fusion rates that a re comparable with those of autologous bone graft. However, it must be combined with an osteoinductive or osteogenic agent to ensure reliabl e fusion rates and alone cannot produce reliable osteogenesis. The Hea los matrix was not compared with other commercially available matrices currently in use. Therefore, the efficacy of Healos relative to these other materials could not be determined.