Silicon Nitride for Spinal Arthrodesis

Spinal fusion surgery eliminates the intervertebral disc space between adjacent vertebral bodies by filling the gap with a material to promote bony fusion of adjacent spinal segments (i.e., vertebral bodies). As documented by well-designed clinical trials, this type of surgery produces dramatic relief of pain and functional improvements for the treatment of degenerative disc disease, degenerative spondylolisthesis, and spinal stenosis. The global burden of back pain from degenerative disorders of the spine is increasing, both in disability, associated dollar amounts, and loss of productivity. Biomaterials used in spinal fusion include bone autograft and allograft, metal alloys, polymers, and silicon nitride. This chapter delves into the historical context of various biomaterials employed in interbody fusion devices (spacers or cages). The discussion centers on the advantages and disadvantages associated with each material selection. These spacers function similarly to shims, inserted between vertebral bodies to maintain spinal height and promote new bone formation, ultimately leading to fusion of the affected segments. Desirable properties in spinal fusion biomaterials include biocompatibility, bioactivity as manifested by predictable bone healing, a modulus of elasticity comparable to native bone, amenability to all forms of radiographic imaging, requisite strength to withstand the axial and shear loads of the spinal column during daily activities, and resistance to bacterial colonization and infection of the implant. Although relatively new, silicon nitride is a promising alternative biomaterial for spinal fusion implants. While lacking extensive surgeon experience and long-term clinical data, ongoing research suggests that this could be the preferred material in the future for spinal arthrodesis purposes.