VIDEO-ASSISTED LATERAL INTERTRANSVERSE PROCESS ARTHRODESIS - VALIDATION OF A NEW MINIMALLY INVASIVE LUMBAR SPINAL-FUSION TECHNIQUE IN THE RABBIT AND NONHUMAN PRIMATE (RHESUS) MODELS

Study Design. Cadaveric anatomic and in vivo survival animal studies w ere performed to develop a new arthrodesis technique for the lumbar sp ine. Objectives. To examine the feasibility, efficacy, and safety of a minimally invasive lumbar intertransverse process arthrodesis techniq ue using an osteoinductive growth factor (rhBMP-2) delivered in a coll agen sponge carrier. The technique was first developed using a rabbit model and modified for the nonhuman primate (rhesus monkey), a larger animal with the most similar bone biology to humans. Summary of Backgr ound Data. The morbidity of conventional posterolateral lumbar intertr ansverse process arthrodesis includes graft donor site morbidity; para spinal muscle devascularization, denervation, and scarring; and nonuni on in up to 35% of patients. Minimally invasive anterior lumbar interb ody arthrodesis techniques have been developed, but these often requir e a metal implant and carry risks to major vessels and development of retrograde ejaculation. A minimally invasive technique for posterolate ral intertransverse process arthrodesis has not been described previou sly. Methods. In Part 1, we examined seven New Zealand white rabbits a nd five rhesus monkeys at necropsy and during nonsurvival surgeries to determine the best access routes and to develop special instruments f or video-assisted lateral intertransverse process arthrodesis. In Part 2, 38 New Zealand white rabbits underwent L4-L5 intertransverse proce ss arthrodesis: the ''OPEN'' group (n = 16) underwent a standard open muscle-splitting approach using rhBMP-2 (bone morphogenetic protein) a nd collagen as a bone graft substitute: the ''video-assisted control'' group (n = 6) underwent video-assisted lateral intertransverse proces s arthrodesis with the collagen implant only (no growth factor);and th e ''video-assisted-BMP'' group (n = 16) underwent video-assisted later al intertransverse process arthrodesis with rhBMP-2 and collagen as th e graft material. In Part 3, rhesus monkeys (n = 4) underwent video-as sisted lateral intertransverse process arthrodesis using rhBMP-2 and c ollagen after laminectomy of L4-L5. Results. In Part 1, we identified expedient, minimally invasive routes to the intertransverse process in terval appropriate for each species: an intermuscular approach for the rabbit and a lateral, extramuscular approach for the rhesus monkey. I n Part 2, all rabbits in the OPEN and video-assisted-BMP groups achiev ed solid intertransverse process lumbar fusions when assessed at 10 we eks. There were no neurologic impairments nor any difference between t he two groups in the frequency of postoperative infection or other com plications. None of the animals in the video-assisted-control group sh owed evidence of fusion. In Part 3, exposure, decortication and grafti ng with rh-BMP-2 and collagen was accomplished successfully in all fou r monkeys through the video-assisted minimally invasive approach witho ut complications. Conclusion. Video-assisted lateral intertransverse p rocess arthrodesis is a feasible, effective, and safe method of lumbar spinal fusion in the rabbit and rhesus monkey. Use of this arthrodesi s procedure will minimize the morbidity of paraspinal muscle denervati on and devascularization seen with open intertransverse process fusion techniques, and the use of an osteoinductive growth factor will elimi nate the problem of graft donor site morbidity and possibly increase t he chances for successful fusion.