Spinal fusion surgery is often needed for patients with spinal deformity, instability, traumatic injuries, spinal cord injuries, and patients requiring revision spinal surgery. The accomplishment of a successful spinal fusion depends upon the stability, good blood supply and and osteoinductive (ability of the patient to initiate bone growth) and osteoconductive (bone graft platform on which to grow) elements.
The prospect of a successful fusion depends on four distinct processes: osteogenesis (bone forming cells), osteoinduction, osteoconduction and lack of motion or stability. Osteogenesis describes the process of new bone formation occurring at a cellular level (i.e., stem cells). Osteoinduction is the recruitment and growth of cells that are key for new bone formation; an osteoinductive material must contain one or more growth factors that can induce new bone growth by osteoblasts (bone-forming cells). Osteoconduction is defined as the apposition of growing bone to the three-dimensional surface of the graft structural scaffold with the graft providing the framework for the ingrowth of the tissue.
The use of autogenous bone graft (a patient’s own bone graft), due to its intrinsic osteogenic, osteoinductive and osteoconductive properties, makes it an optimum graft source for a spinal fusion. In addition modern surgical techniques provide early stability through the use of spinal instrumentation; nonetheless, long-term stability is dependent on the development of a solid biologic fusion.
Principles of Bone Grafting in Spinal Fusion
The patient must have multiple factors to obtain a solid spinal fusion. The presence of an adequate blood supply is essential and may be minimally disrupted through careful surgical manipulation of the surrounding bony and soft tissue elements. Preserving an intact blood supply maintains oxygen as well as the nutrients necessary to preserve cell growth, promote new blood vessel growth, and form a fusion.
Systemic illnesses (e.g.: diabetes, chronic renal failure) can also have an adverse impact on fusion rates by impairing vascular ingrowth. Additionally, cigarette smoking, most likely as a result of the effect of nicotine on vascular supply [2], and malnutrition, are associated with a general impairment of wound healing and may also have an adverse effect on fusion healing. The type and position of spinal bone grafts affect spinal fusion. Anterior interbody (between the bone bodies) structural grafts are exposed to local compressive forces. Precise contouring and fit may affect ultimate fusion success or failure. On the contrary, a posterior applied bone graft is placed along the tension side of the spinal column. Due to the absence of local compressive stimuli, bone graft incorporation is not as directly affected by local biomechanical factors. The lack of stability of this grafting technique and the presence of excessive strain may lead to inadequate neovascularization into the graft and ultimately lack of a successful arthrodesis. Bone forming cells (osteogenesis) is an absolute requirement for a successful fusion. A patient’s own bone (autograft) directly provides stem cells (all powerful cell lines) and bone forming cells (osteoblasts). Also, osteoinductive factors such as bone morphogenetic proteins (BMPs) play a critical role in bone formation by stimulating new cells to grow. [3].
Osteoconductive substitutes (collagen, ceramics, polymers, coral, and allograft bone) may be used as scaffolds to facilitate bone formation. They provide a lattice within the developing fusion mass for new blood vessels and migrating stem cells [4]. The graft also has to be biocompatible (patient does not reject it) [5].
Conclusions
Obtaining a successful spinal fusion can be extremely challenging. Careful patient selection and surgical technique are paramount and remain the most important variables in obtaining satisfactory outcomes in a majority of patients. Unfortunately, a failed spinal fusion continues to represent a considerable source of unsatisfactory clinical outcomes. Using autograft bone has previously provided the most reliable means of achieving a rapid and robust fusion mass, but pain with taking one’s own bone (harvest) has often overshadowed positive clinical results from the primary operation. Numerous alternatives to autograft bone have been developed. Some recombinant BMPs and advances in gene therapy may provide fusion enhancing capacity that matches or even exceeds the performance of autograft. The recent development of newer and more effective composite grafts has developed a new and exciting field of research in regards to biomaterials and biotechnology, promising a more effective treatment for patients requiring spinal fusion.
Sources:
1) Boden SD. Biology of lumbar spine fusion and use of bone graft substitutes: present, future, and next generation. Tissue Eng. 2000 Aug;6(4):383-99
2) Boden SD, Schimandle JH. Biology of Lumbar Spine Fusion and Bone Graft Materials. In: International Society for Study of the Lumbar Spine Editorial Committee, eds. The Lumbar Spine, 2nd ed. Philadelphia: WB Saunders, 1996:1284-1306
3) Boden SD. Overview of the biology of lumbar spine fusion and principles for selecting a bone graft substitute. Spine 27(16S);2002:S26-S31
4) Simon SR, ed. Orthopaedic Basic Science. 2nd ed. Rosemont, Ill: AAOS;1994:284-293
5) Vaccaro AR. The role of the osteoconductive scaffold in synthetic bone graft. Orthopedics. 2002 May;25(5 Suppl):s571-8