Autograft/Allograft/Cage/Bone Morphogenetic Protein

This chapter focuses on the different types and features of autograft, allograft, cages, and bone morphogenetic proteins in revision lumbar spine surgery. These constitute major contributors to a solid spinal arthrodesis through osteoinductivity, osteoconductivity, and osteogenicity. Autograft is defined as the patient’s own bone and is usually harvested from the iliac crest. Its tricortical surface provides structural support when used as posterior lumbar interbody grafts until bony fusion occurs. An alternative to autograft is allograft, which is harvested from cadaveric tissue donors. It comes in a variety of forms including fresh-frozen, freeze-dried allograft, or demineralized bone matrix (DBM). Both fresh-frozen and freeze-dried grafts can serve as structural allografts. DBM is created by acid extraction of bone minerals while preserving proteins that constitute the bone matrix. It is readily available but does not provide structural support. Cages are used when the intervertebral disc is removed and can help achieve a more solid arthrodesis, indirect neural decompression, restoration of lordosis, and mechanical stability. The ideal cage is rigid enough to maintain stability but with similar elasticity to bone to prevent subsidence. Most cages are made of titanium alloys or polyetheretherketone (PEEK). They are available in different shapes and some models are expandable. Bone morphogenetic proteins (BMPs) promote bone formation by inducing mesenchymal stem cells to differentiate into osteoblasts. Their use is US Food and Drug Administration (FDA)-approved for anterior lumbar interbody fusion (ALIF) but is associated with some adverse effects, which is why it is important to use BMPs judiciously.