ProDisc-C Cervical Artificial Disk

Gerard K. Jeong, Frank P. Cammisa, and Federico P. Girardi

Since its original description in the 1950s by Cloward and Robinson and Smith, anterior cervical diskectomy and fusion (ACDF) has become the standard for operative treatment of radiculopathy and myelopathy due to one- to two-level cervical degenerative disk disease.^1,2 However, the detrimental effects of arthrodesis on adjacent motion segments at longer follow-up have been demonstrated. Hilibrand et al observed the occurrence of symptomatic adjacent segment disease at a relatively constant incidence rate of 2.9% per year on a cumulative basis during the 10-year postoperative follow-up.³ Using survival analysis methods, the authors predicted that 25.6% of the patients who had an anterior cervical arthrodesis would have new disease at an adjacent level within 10 years after the operation. Other investigators have reported that an estimated 7 to 15% of patients ultimately require a secondary procedure at an adjacent level.^4,5 Whether the rate of adjacent symptomatic disk disease is accelerated due to the increased stress placed on the adjacent motion segments, particularly the subjacent motion segment, following successful fusion remains controversial.⁶ Due to the long-term effects of fusion on adjacent motion segments, motion preservation and nonfusion technologies have met renewed interest.

The early clinical experience with a variety of cervical disk prostheses is growing. The premise behind artificial vertebral disk implantation is that abnormal motion will be corrected, the intervertebral space height will be restored, the physiological curvature and instantaneous axis of rotation will be normalized, the corrected normal intervertebral motion will be maintained over time, and the patients will experience pain relief and return of function.

The ProDisc-C cervical disk prosthesis is a relatively new device with limited clinical experience compared with its counterparts such as the Bryan (Medtronic Sofamor Danek, Memphis, TN), Cummins (or Bristol, Frenchay), or Prestige (Medtronic) disk prostheses.^7–17 At present, the ProDisc-C is under the U.S. Food and Drug Administration Investigational Device Exemption (IDE) study. This chapter reviews the specific features, theoretical advantages/disadvantages, and biomechanical considerations of the ProDisc-C implant (Synthes, Inc., West Chester, PA). The chapter also discusses the early clinical results from case series and from the ongoing multicenter, prospective, randomized, controlled clinical trial comparing the safety and effectiveness of the ProDisc-C cervical implant to ACDF in the treatment of symptomatic cervical disk disease. Some of the early device-related complications reported in a few European case series are described here as well.

ProDisc-C Cervical Disk Prosthesis

The ProDisc-C cervical disk is a metal-on-polymer implant (Fig. 9–1). It consists of two forged cobalt-chromium-molybdenum (CoCrMo) alloy end plates and an ultra high molecular weight polyethylene (UHMWPE) inlay element, which is fixed to the inferior prosthetic end plate. The UHMWPE wear rate was 40- to 50-fold less than typical wear rate shown for hip and knee prosthesis in multiaxis simulator testing. The metal end plates are plasma sprayed with titanium and have two vertical fins for fixation in the end plates. A keel cutting chisel with a safety block mechanism is used to prepare the host bone for the keeled end plates. The keels, a design unique to the ProDisc-C cervical prosthesis and the ProDisc lumbar prosthesis, provide immediate fixation of the prosthetic end plate in each adjacent vertebral end plate, whereas the surface roughness of the plasma-sprayed titanium end plate surface provides not only a high coefficient of friction to augment immediate implant fixation but also long-term stability through bony ongrowth.

Figure 9–1 ProDisc-C cervical prosthesis is a semiconstrained, ball and socket design with fixed axis of rotation. It consists of two forged cobalt-chromium-molybdenum alloy end plates and an ultra high molecular weight polyethylene inlay element, which is fixed to the inferior prosthetic end plate. The metal end plates have two vertical fins for immediate fixation in the end plates and are plasma sprayed with titanium for long-term fixation through osseointegration.

The ProDisc-C is a semiconstrained, ball and socket design with fixed axis of rotation. Whether the semiconstrained nature of the ProDisc-C may provide a more optimal balance between motion preservation, ease and consistency of implantation, and neurological fitness than a nonconstrained disk prostheses is unknown.

The theoretical advantages of the ProDisc-C device are (1) the absence of anterior plate-like fixation/hardware common to many other disk prostheses, (2) immediate stability provided by the end plates’ keel fixation, and (3) the possibility of multilevel application.

The theoretical concerns of the ProDisc-C may be divided into concerns inherent with any motion-preserving disk prosthesis and concerns specific to the ProDisc-C device. General concerns include revision capabilities, ectopic bone formation, autofusion, and persistent or recurrent neural compression, which is inherent with any motion-preserving procedure because posterior osteophytes may not resorb as they do in arthrodesis. Device-specific concerns include the possibility of a more difficult revision procedure and signifi-cant host bone loss because extraction of the keeled device may sacrifice a substantial portion of host bone, especially when performed at multiple levels. Sagittal-splitting vertebral body fracture following preparation or implantation of the keeled prosthesis is also a concern, especially in multilevel applications. Theoretical concerns regarding UHMWPE wear and osteolysis are also specific to the ProDisc-C because many of the available cervical disk prostheses employ a metal-on-metal design.

Lastly, the theoretical concern of the dynamics and kinematics of the ProDisc-C has been raised. The instantaneous axis of rotation (IAR) in the normal cervical spine is not fixed and dependent upon the specific motion performed. In the normal cervical spine, the location of the IAR varies depending on the type of motion. In flexion-extension, the IAR lies along the anterior border of the vertebral body; in axial rotation, it lies in the center of the vertebral body; whereas in lateral bending, the IAR translates in the superior-inferior direction. Whether the ProDisc-C, a semiconstrained device with a fixed axis of rotation, can adequately replicate or reproduce the motion of the normal motion segment remains a topic of investigation.

A recent biomechanical study has demonstrated that the kinematics of the ProDisc-C are similar to those in the normal cervical motion segment. Despite having a fixed center of rotation, the ProDisc-C prosthesis was found to reasonably preserve the physiological distribution of the IAR, which in turn may decrease loosening forces and facet joint strain.18 Interestingly, the investigators found that, despite the level of constraint in the prosthesis, there appeared to be some translational motion in the IAR depending on the specific motion, which suggests the influence of other anatomical stabilizing structures as well as the ability of the disk to accommodate such motion.

Recent studies have also demonstrated that the dynamics of the ProDisc-C closely replicate those in the normal motion segment. In a multilevel human cadaveric model, DiAngelo et al demonstrated that the ProDisc-C implant did not alter the motion patterns of flexion, lateral bending, and axial rotation at either the instrumented level or adjacent segments compared with the harvested condition.¹⁹ The only significant difference between the ProDisc-C and harvested spine conditions occurred in extension, in which only 57% of motion of the harvested spine was obtained with the cervical prosthesis.

The authors also demonstrated that a simulation of a one-level fusion significantly reduced motion at the surgical site, for which it was compensated by increased motion at adjacent segments. This increased adjacent segment motion may accelerate degeneration of adjacent disk segments. As a result, they argued that use of a prosthetic total disk replacement device to treat symptomatic degenerative cervical disk disease may minimize or alleviate adjacent segment disease associated with fusion surgery.

Sliva et al also demonstrated that the kinematics and range of motion (ROM) of the operated and adjacent motion segments were fairly well preserved following one-level disk replacement with the ProDisc-C prosthesis compared with the harvested, native condition.²⁰

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