11.4\ Dynamic Stabilization

Devices of the Spine

11.4.1\ Total Disc Replacement

11.4.1.1\ Discussion

Total disc replacement is an option for treating degenerative disc disease, in which motion at the operated level is preserved. In theory, this technique results in less load transfer to adjacent levels compared with fusion. Several types of disc replacement are available, including one-piece implants and implants with singleor double-­ gliding articulations with metal-on-metal or metal-on-polymer bearing surfaces (Figs. 11.72, 11.73, and 11.74). For example, the Charite® device is a three-part device consisting of two cobalt-chromium-molybdenum (CoCrMo) alloy endplates and a radiolucent ultra-high-molecular-­ weight polyethylene core. The radiolucent core is delineated by a small radiopaque metal wire. There are small “teeth” along the device endplates in an attempt to anchor the device in the vertebral endplates and limit migration. The endplates are coated with porous plasma-sprayed titanium and with calcium phosphate to promote bony ingrowth. ProDisc is also a three-­part disc arthroplasty device with two metallic endplates and a radiolucent inlay that snaps onto the lower endplate to prevent extrusion. The device is secured in the adjacent vertebral bodies by ridged keels along both of the endplates (Fig. 11.75).

Migration of the disc prosthesis is not uncommon and usually occurs in the anterior direction (Fig. 11.76). Oversizing and positioning the prosthesis too far anteriorly in the disc space are risk factors for migration. Extrusion of the polyethylene inlay in the Charite device is a similar complication and can occur if the wire surrounding the core fractures in three-component prostheses. It is important to obtain cross-sectional imaging

for these complications in order to exclude impingement upon the adjacent great vessels.

Subsidence of total disc prostheses into the adjacent vertebral body occurs in about 3% of cases in the lumbar spine and can be asymptomatic or cause recurrence of pain. Risk factors include osteopenia, an oblique approach versus an anterior approach, removal of too much of the endplate, and the use of an undersized prosthesis. Total disc subsidence is most readily identified on lateral radiographs and sagittal or coronal CT reconstructions (Fig. 11.77).

The frequency of heterotopic ossification after total disc replacement ranges from 1.7% to 7.7%. Heterotopic ossification along the lateral aspect of the device often leads to ankylosis and limited range of motion, while the heterotopic bone anterior to the device is usually of no significance. Annular ossification can also occur, and this actually decreases the risk of device dislocation. On imaging, partial or complete bone formation across the disc space can be identified (Fig. 11.78).

Vertebral body fracture has been reported in less than 1% of cases with the Charite device and tends to involve the endplate (Fig. 11.79). In contrast, devices with a keel, such as ProDisc, are prone to the characteristic vertical split fractures. Short vertebral body heights predispose to fracture with these models. Prosthesis-associated fractures should be evaluated with CT to assess for spinal canal stenosis and associated findings, such as device migration.

Finally, it is important to consider disease at adjacent levels when patients present with pain following total disc replacement. Adjacent-level disc degeneration is perhaps the most common explanation in such situations. This outcome may result from increased stress on, or hypermobility of, adjacent segments. If nothing is apparent, MRI should be performed to evaluate the discs, although susceptibility effects from the hardware can degrade detail of the adjacent levels as well (Fig. 11.80).

Fig. 11.72  Prestige cervical spine total disc prosthesis. Flexion (a) and extension (b) views of the cervical spine show the range of motion of the device. Sagittal (c) and

coronal (d) CT images show the device positioned within the disc space, secured by two rows of corrugated keels

Fig. 11.75  ProDisc-L total disc prosthesis. Lateral view of the lumbar spine (a) demonstrates proper positioning of ProDisc-L, which features serrated keels perpendicular to

the endplates. Photograph of ProDisc-L (b) (Courtesy of Synthes, West Chester, PA)

Fig. 11.76  Total disc prosthesis anterior migration. Initial postoperative lateral radiograph (a) shows satisfactory positioning of the C3–C4 total disc prosthesis. Lateral

radiograph obtained at 4 postoperative months (b) shows interval anterior migration of the C3–C4 device (arrow)

Fig. 11.77  Total disc prosthesis subsidence. Initial postoperative lateral radiograph (a) shows satisfactory positioning of the total disc prosthesis at C5–C6. Routine

follow-up imaging at 6 weeks (b) shows that the superior endplate of the C5–C6 device has subsided into the inferior endplate of C5 (arrow)

Fig. 11.78  Total disc prosthesis with heterotopic ossification and ankylosis. Sagittal CT image shows bone (arrow) spanning the disc space at the level of the prosthesis

Fig. 11.79  Total disc replacement vertebral fracture. The patient presented to the emergency department with severe back pain. The patient underwent implantation of Charite® at L5–S1 about 6 months earlier. Sagittal CT image shows a small fracture involving the posterior-­ inferior corner of the L5 vertebral body (arrow)

Fig. 11.80  Adjacent-level disc herniation after total disc replacement. The patient presented approximately 1 year after implantation of Charite® at the L5–S1 disc space level with persistent back pain. Sagittal CT image (a) shows degenerative disc disease at L4–L5 with vacuum disc phenomenon. The CT is otherwise limited at the level of the prosthesis due to the beam-hardening artifact from

the device (*). However, axial sagittal T2-weighted MR image (b) shows a central and right subarticular recess disc extrusion resulting in right L5 nerve root compression (arrow) despite the susceptibility artifact from the hardware. This was new from an MRI of the lumbar spine obtained 6 months earlier