11.3.12  Interbody Fusion

11.3.12.1\ Discussion

The goal of lumbar interbody fusion with prosthetic devices is to provide stability while promoting bony ingrowth. Many materials and devices have been used for this purpose, including bone threaded bone graft dowels or femoral rings, metal cages, and polymer cages. Femoral ring grafts are cylindrically shaped and inserted into the intervertebral disc space via anterior lumbar interbody fusion, posterior lumbar interbody fusion, or transforaminal lumbar interbody fusion approach (Fig. 11.63). A major disadvantage of such allograft device is the risk of disease transmission. Wide varieties of metal cages have been and continue to be developed. The first-­generation Bagby and Kulich (BAK) and second-­generation Ray threaded fusion cages are cylindrical, hollow, porous, threaded, titanium alloy cages that can be screwed into position in the intervertebral disc space (Fig. 11.64). The more recent third-generation LT-CAGE has been widely used in North America and has a trapezoidal, tapered configuration that provides increased surface area for bone growth and facilitates restoration of lumbar lordosis (Fig. 11.65).

a

More recent interbody fusion devices are mainly composed of polyether ether ketone (PEEK) or biocompatible high-density carbon fiber. These materials are radiolucent, which facilitates visualization of the bone graft-­vertebral body endplate interface. The devices also contain press-fit titanium markers in order to demarcate the boundaries of the device on radiographs. Many designs are in use, but generally are rectangular with grooves in order to promote vertebral body attachment. There are a variety of approaches that can be used for interbody fusion (Figs. 11.66, 11.67, 11.68, 11.69, 11.70, and 11.71 and Table 11.2).

Imaging can be used to assess the position of the implants, which should be located at least 2 mm anterior to the posterior wall of the vertebral body. Another role of imaging following interbody fusion surgery is to assess fusion versus pseudarthrosis. Radiographs with lateral flexion and extension views can be used for this purpose, although the accuracy is highly dependent upon precise positioning and the type of implant. Rather, CT is the modality of choice for evaluating interbody fusion, although the streak artifact from the early stainless devices can obscure adjacent bone formation. Early bone healing can often be appreciated at 3 months and is usually nearly complete at 6 months after surgery.

b

Fig. 11.68  XLIF. Axial (a) and coronal (b) CT images show the metallic markers of the XLIF device, which is positioned in the intervertebral space. There is bone graft

material within the device. Axial T1-weighted (c) MRI shows the XLIF device as low signal intensity with a “figure of 8” shape

Fig. 11.70  Stalif. Frontal radiograph (a) and sagittal CT image (b) show that the device composed of both radiolucent and metallic parts, including titanium screws that

enter the anterior vertebrae above and below. Note the absence of additional hardware. As such, the device “stands alone”