11.3.2\ Bone Graft Materials

11.3.2.1\ Discussion

Several options are available for promoting bone fusion, including autologous, allograft, and synthetic bone grafts. Autologous bone grafts are often harvested from the iliac crest, rib, or local lamina, and spinous process (Fig. 11.24). Alternatively, a trephine system can be used to obtain a core of cancellous bone from an adjacent vertebral body, which leaves a cylindrical defect in the anterior portion of the vertebral body and pedicle (Fig. 11.25). Allografts are derived from cadavers and are available as bone chips or cylinders from fibula or rib and retain some bony structure (Fig. 11.26). Ultimately, an uninterrupted bony bridge should form across the vertebral bodies and facet joints as the bone graft fusion matures (Fig. 11.27).

The main types of synthetic bone graft substitutes that are used during spine surgery include ceramics, demineralized bone matrix, and composite materials. Demineralized bone matrix consists of non-collagenous proteins, bone growth factors, and collagen, which are intended to stimulate bone healing. These materials are radiolucent and difficult to visualize directly on imaging. Demineralized bone matrix is available in powder form or as putty that can be used to fill voids (Fig. 11.28). Sometimes demineralized bone

a

matrix is combined with bone grafts, which has attenuation intermediate between medullary and cortical bone. Ceramics include calcium sulfate, hydroxyapatite, tricalcium phosphate, or a combination of hydroxyapatite and tricalcium phosphate that are available in the form of pellets, pastes, or cement. These materials are denser than native bone. Composite materials such as moldable morsels contain mixtures of ceramic and collagen or other demineralized bone matrix components. The mineralized component (i.e., calcium phosphate) provides compressive strength and a substrate for bone formation, while the collagen contributes tensile strength and promotes hemostasis at the surgical site. On CT, such materials appear as grainy foci of heterogeneous attenuation (Fig. 11.29).

Recombinant bone morphogenic protein (BMP) is often added to bone graft agents in order to promote fusion. This substance promotes bone resorption or osteolysis. Despite this finding, fusion typically progresses and matures within 2 years. In fact, BMP expedites arthrodesis. On imaging, BMP-induced osteolysis appears as multiple cystic spaces in the endplate adjacent to the implant (Fig. 11.30). BMP is also known to form an excessive inflammatory response with excessive fluid collections in the early ­postoperative period, sometimes even leading to undesired bone formation within the spinal canal.

b

Fig. 11.24  Autologous bone graft harvested from the iliac crest. Axial CT of the spine (a) shows bilateral cortical and cancellous bone fragments with sharp edges

(encircled). Demineralized bone matrix was also applied. Axial CT at a lower level (b) shows the iliac donor site packed with hemostatic agent (arrow)

Fig. 11.25  Local vertebral body bone harvest. Axial CT image shows cylindrical defect in the anterior vertebral body (arrow)

Fig. 11.27  Mature bone graft fusion. Coronal CT image shows a solid fusion mass that bridges the left L4 and L5 vertebrae from transverse process to transverse process (arrow)

Fig. 11.26  Allograft bone chips. Axial CT image shows numerous cubes of cancellous cadaveric bone grafts within an adjacent to the laminectomy site. Many of the chips show a trabecular bone structure and contain air

Fig. 11.28  Demineralized bone matrix. Sagittal CT image shows amorphous hyperattenuating material filling the space of partially collapsed vertebral bodies secondary to prior osteomyelitis (arrow)

Fig. 11.30  Recombinant BMP-induced osteolysis. Sagittal (a) CT image shows rounded lucencies (arrow) along the inferior endplate of the vertebral body above the

interbody fusion. Sagittal T2-weighted MRI (b) shows high-intensity foci (arrows) within the endplates adjacent to the interbody fusion material