CHAPTER 14 Biological Approaches to Spinal Instrumentation and Fusion in Spinal Deformity Surgery Praveen V. Mummaneni, MD, Scott A. Meyer, MD, and Jau-Ching Wu, MD T he use of autografts, allografts, and synthetic materials as bone substitutes in spinal surgery continues to increase as the population ages. Each of these materials has advantages and disadvantages. Autografts have previously been consid- ered the gold standard for their excellent graft incorporation without the risk of rejection or disease transmission. However, their drawback is limited supply and the potential for donor- site morbidity. Allografts are an acceptable alternative because of the relative abundance, availability of desired sizes and shapes, and elimination of procurement-related morbidities. Advances in processing techniques and strict guidelines for allograft donor screening have reduced the risk of disease transmission. However, some manufacturing and sterilization processes can compromise the mechanical strength and biological properties of the allograft. Moreover, allografts may still, albeit rarely, elicit an inflammatory response, be rejected, or transmit disease. Synthetic materials are an emerging and increasingly popular option. However, their ability to incorporate into the host tissue remains uncertain. In addition, the host immune responses are not well elucidated. The biological approaches to spinal instrumentation and fusion must be tailored to meet the specific needs of each clinical scenario. 1 PRINCIPLES OF BONE FUSION AND BIOMECHANICS Bone is a biologically dynamic tissue that is always in an active state of deposition, resorption, and remodeling. The process of bone fusion is regarded as a process of healing in which metabolically active cells, matrices, and minerals are integrated into a rigid framework. Hormones, biomechanics, physical activities, nutritional status, and medical comorbid- ities all exert mutual influences on this process. For example, smoking has been shown to impede bone fusion in spinal surgery. 2,3 Osteoporosis may cause fusion failure owing to unbalanced bone resorption and metabolism. Moreover, patients with diseases such as ankylosing spondylitis often have spontaneously fused spines as a consequence of overreactive inflammation and healing, resulting in brittle deformities. Bone fusion depends on the biological activities of bone healing. Three major physiological processes directly in- fluence the quality and rapidity of graft incorporation: osteogenesis, osteoinduction, and osteoconduction. Osteogenesis is new bone formation through cellular proliferation by osteoblastic activity, which requires the presence of bone-forming cells, osteoprogenitor cells or osteogenic precursor cells. Fresh autogenous bone grafts or bone marrow cells have great osteogenic potential because they contain viable cells. Osteoinduction is the stimulation of precursor cells to differentiate into mature bone cells. Bone grafts usually contain elements with such properties, or supplements can be added. The most powerful osteoinductive effects come from bone morphogenetic proteins (BMPs). Materials like demin- eralized bone matrix (DBM) also are osteoinductive. Osteoconduction refers to the appropriate 3-dimensional scaffolds into which viable bone cell growth and neo- vascularization can take place easily. This process is dependent on the physical properties of the material such as porosity and pore size, architecture, and structural stiffness. Materials like ceramics are designed for this property. One caveat is that osteoconductive materials alone, like ceramics without autograft, have little potential in achieving a solid fusion. Bone tissue itself adapts to the load it is placed under in a dynamic biomechanical process, as described by the German surgeon Julius Wolff in the 19th century. Wolff’s law suggests that certain amounts of loading are helpful to induce bone remodeling to achieve arthrodesis (eg, interbody spinal fusion). 4 AUTOGRAFTS Autograft bone provides the 3 properties for successful bone fusion: osteogenesis, osteoinduction, and osteoconduc- tion. Autograft contains viable osteoblasts and osteoprogenitor cells ready for osteogenesis. Endogenous BMPs convey osteoinduction in autografts. Moreover, autografts have similar bone quality and perfect biocompatibility and provide no antigenicity. Autografts remain the gold standard for fusion procedures. 5 Copyright Ó 2011 by The Congress of Neurological Surgeons 0148-396X 110 Clinical Neurosurgery  Volume 58, 2011