Biologics in Spine Fusion Surgery

Although there are several indications and various techniques used for spine fusion surgery, the fundamental aim is to augment or restore spinal stability by achieving solid bony union between two or more vertebral motion segments. Regardless of the clinical setting, successful achievement of an arthrodesis can be a major predictor of outcome. For example, in the treatment of lumbar degenerative spondylolisthesis, improvements in patient outcomes are seen throughout the fusion process and are maintained after confirmed fusion. In patients with thoracolumbar spinal deformity, successful fusion leads to significantly higher quality-of-life outcome scores relative to those with a pseudarthrosis. Finally, in the cervical spine, patients who achieve a solid fusion after anterior cervical discectomy and fusion (ACDF) experience significantly greater improvements in pain and disability outcomes relative to those who fail to fuse. Augmentation with biologics is one strategy aimed at increasing the rate of successful fusion.

From a biological perspective, spinal fusion is challenging because the goal is to achieve bone growth in an environment not originally developed for this purpose (i.e., the disc space, intertransverse space), which necessitates modification of the biological microenvironment to stimulate and support the process. Biologics can enhance specific cellular and molecular activity, which helps to accomplish arthrodesis by facilitating osteoinduction, osteoconduction, or osteogenesis. Bone healing is a cellular process that requires critical thresholds of bone-forming osteoblasts, osteoprogenitor cells, local vascular tissues, and the cells involved in the inflammatory response. The osteogenic potential of a graft material refers to its ability to directly provide the local fusion bed with these primary osteoblasts or responding cells. Osteoinduction describes the stimulation of cellular events that transform previously uncommitted stem cells into those that will form new bone. Osteoinductive stimuli may also facilitate angiogenesis or vascular ingrowth to supply the fusion site with the cells, growth factors, and nutrients required for fusion. Osteoconduction is the local environmental support of these cells and enhancement of their functions. An osteoconductive material acts as a scaffold that permits the migration, attachment, differentiation, and proliferation of osteogenic cells and the other cells contributing to the bone healing response, facilitating subsequent bone and vascular ingrowth. This chapter identifies and critically evaluates the available biological agents used in spine surgery to facilitate bone fusion.

Care to preserve the local blood supply is important because, in addition to providing essential nutrients to the healing tissue, it can be a potential source of osteoprogenitor cells. Preparation of the host bone surface is key because the local bone can be an excellent source of osteogenic cells and osteoinductive signals, while also providing an osteoconductive surface for new bone growth.

Finally, the location of fusion is a major factor that must be considered. Boden emphasized this at the time that he first characterized the biology of fusion in the lumbar spine, making it clear that the results of healing for a grafting material in one region of the spine should not necessarily be extrapolated to other regions. This is because the fusion environment differs substantially in different locations throughout the spine. For example, the interbody fusion environment includes a large bony surface area providing a wide cancellous bed for graft contact, excellent vascularity of well-exposed end plates, and compression loading of graft material, all of which enhance the potential for fusion. The posterior intertransverse fusion environment is much more challenging owing to the limited surface area for healing, the large defect size between transverse processes which must be bridged, the lack of external support in fixating graft materials, and the influence of tensile forces.