Porous tricalcium phosphate and transforming growth factor used for anterior spine surgery

Harvesting autologous bone graft from the iliac crest is associated with co nsiderable secondary morbidity. Bone graft substitutes such as porous ceram ics are increasingly used for spinal surgery. This paper presents the resul ts of an animal study in which beta -tricalcium phosphate (beta -TCP) bone substitutes were used for anterior spinal surgery in sheep and baboons. The presented baboon study also investigated the effect of impregnating the ce ramic material with transforming growth factor (TGF). In the first study, u sing the sheep model, a stand-alone instrumented anterior fusion was perfor med. The animals were randomized into three treatment groups: autologous bo ne, beta -TCP granules, and sham group. The results were analyzed biomechan ically and histologically at three survival intervals: 8, 16 and 32 weeks. An additional animal group was added later, with ceramic pre-filled implant s. In the second study, a baboon model was used to assess the osteointegrat ion of a 15-mm-diameter porous beta -TCP block into the vertebral body. The experiment was partially motivated by a new surgical procedure proposed fo r local bone graft harvest. Three treatment groups were used: beta -TCP plu g, beta -TCP plug impregnated with TGF-beta3, and a sham group with empty d efect. The evaluation for all animals included computer tomograms at 3 and 6 months, as well as histology at 6 months. In the sheep model, the mechani cal evaluation failed to demonstrate differences between treatment groups. This was because massive anterior bone bridges formed in almost all the ani mals, masking the effects of individual treatments. Histologically, beta -T CP was shown to be a good osteoconductor. While multiple signs of implant m icromotion were documented, pre-filling the cages markedly improved the his tological fusion outcomes. In the baboon study, the beta -TCP plugs were co mpletely osteointegrated at 6 months. For the group that used ceramic plugs impregnated with TGF-beta3, no incremental advantage was seen as a result of this particular application. However, TGF-beta3 is a potent growth facto r at a very low dose. Not only does it speed up the ceramic material resorp tion, but it is also responsible for massive regional new bone formation. M ore experiments are required to better understand the biological effects of this growth factor in relation to bone formation, and to be able to take c linical advantage of them.