11.4.2\ Nucleus Pulposus

Replacement

11.4.2.1\ Discussion

Nucleus pulposus (partial disc) replacement surgery is a minimally invasive technique that is indicated for younger patients with persistent disc herniation and prior discectomy. The two main types of nucleus replacement devices include elastomeric and mechanical nuclei. Elastomeric nuclei can be composed either of

a

c

hydrogel or nonhydrogel materials and preformed or injectable. The mechanical nuclei can be either oneor two-piece.

The NUBAC is a mechanical two-piece nucleus pulposus replacement device that is composed of two endplates of PEEK-OPTIMA with an inner ball-and-socket articulation. The large contact area results in low subsidence risk. The device is radiolucent because of PEEK (polyether ether ketone), but has a characteristic of radiopaque makers (Fig. 11.81).

b

d

Fig. 11.81  NUBAC. Frontal radiograph (a) shows three radiopaque markers from the nucleus pulposus in the L4– L5 disc space (encircled). The remainder of the device is

radiolucent. Axial (b) and coronal (c) CT MIP images show the device centered within the disc space. Photograph of NUBAC (d)

11.4.3\ Posterior Dynamic

Stabilization Devices

11.4.3.1\ Discussion

Interspinous spacers are designed for minimally invasive treatment of neurogenic claudication. In this condition, symptoms like radicular pain, sensation disturbance, and loss of strength in the legs are relieved with flexion, presumably by decreasing epidural pressure and increasing the cross-­sectional area of the spinal canal. The devices are typically placed into the interspinous space at the affected level. The implantation is less invasive than spinal fusion procedures, leaves the ALL and PLL intact, and does not preclude removal and additional spinal surgery.

Various designs for interspinous spacers are commercially available, including the Isobar, X-Stop, DIAM, and coflex, among others.

The Isobar incorporates radiopaque metallic posterior pedicle screws with limited motion provided by either a mobile screw head or mobile rods (Fig. 11.82). The mobile portion of this particular device consists of a stack of metal rings that allow for flexion and extension and a dampener that allows for limited axial loading. The dampener and rings are contained in a bell-­ shaped structure on the rod.

The X-Stop has an H-shaped configuration consisting of two metal plates positioned lateral to the spinous processes (Fig. 11.83). The interspinous ligament is conserved in order for this device to be effective.

The DIAM (device for intervertebral assisted motion) spinal stabilization system consists of a silicone core covered by a polyester sleeve. Three mesh bands hold the core in place. Two of the bands encircle the adjacent spinous processes, while a third encases the supraspinous ligament. The silicone device and bands are radiolucent, but radiopaque markers are placed along the superior-lateral aspect of the device (Fig. 11.84). As with other interspinous devices, loss of correction ensues over time, although this does not appear to affect clinical outcome.

The coflex is a saddle-shaped device composed of titanium that requires resection of the interspinous and supraspinous ligaments (Fig. 11.85). The wings are crimped onto the adjacent spinous processes. This device is inserted in conjunction with the central laminectomy.

Reported complications specific to interspinous prosthesis implantation include overcorrection, spinous process fractures (Fig. 11.86), and device dislocation (Fig. 11.87). These complications are not very common and may be predisposed by underlying anatomic variations.

Fig. 11.83  X-Stop. Frontal radiograph (a) shows an X-Stop device in position L4–L5, which appears as H-shaped metallic hardware interposed between the spi-

nous processes. Photograph X-Stop (b) (Courtesy of Medtronic Sofamor Danek USA)

576

a

b

Fig. 11.84  DIAM. Coronal CT image (a) shows the DIAM interspinous spacer device with bilateral high attenuation markers on the sides. Photograph of the DIAM device (b) (Courtesy of Medtronic Sofamor Danek USA)

D.T. Ginat et al.

Fig. 11.86  Periprosthetic fracture. Sagittal CT image shows lucency (arrow) in the spinous process to which the interspinous prosthesis is attached. The bones are diffusely osteopenic and there is anterolisthesis at the operated level

Fig. 11.87  Dislocated device. Lateral radiograph shows disengagement of the device from the interspinous space into the soft tissues posterior to the spinous processes