By: Kern Singh, MD
Introduction
Spinal fusion is a versatile and effective option in the management of instabilities, deformities, and painful spinal conditions. An increasing body of biomechanical and clinical evidence suggests that the relative immobility of fused spinal segments alters stress transfer leading to adjacent level degeneration. The development of non-fusion spinal prostheses has been driven by increasing concerns of these arthrodesis-related morbidities including graft-site harvest, pseudarthrosis and adjacent level degeneration (ALD). Motion-sparing implants offer some theoretical advantages over fusion; however, judicious use of these products with careful patient selection is warranted until outcome studies can demonstrate their efficacy.
Total Disc Replacement
Intervertebral disc replacements may be indicated for patients with degenerative disc disease at one or two levels of the spine. In order to avoid complications that may arise from artificial disc replacement surgery, careful patient selection by the treating surgeon is paramount.
Cervical Disc Replacement
Although fusion at single or multiple levels of the cervical spine allows for decompression of neural structures and restoration of disc height, it has been shown to alter the normal biomechanics of the cervical spine potentially leading to adjacent-segment degeneration. Cervical total disc replacement has 
emerged as a promising alternative for the management of disc herniations. The primary goals of cervical TDR are to remove the offending disc while restoring disc height and segmental motion.
Current metal-on-metal designs include the Prestige Disc (Medtronic Sofamor Danek, Memphis, TN). The Prestige uses a stainless steel metal-on-metal configuration. Although metal-on-metal designs are associated with the potential for debris and increased systemic concentrations of metal ions, the purpose of this design is to avoid the increased particulate debris found in metal-polymer designs.
Lumbar Disc Replacements
The ideal lumbar TDR would function as a physiologic replacement for the human intervertebral disc. It would assume the role of the nucleus pulposus and anulus fibrosis complex. In preserving the lumbar spine’s range of motion, the lumbar TDR would also need to transmit and absorb loads across the disc space between the vertebral bodies.
Indications for lumbar disc arthroplasty include back pain from degenerative disc disease in patients who have failed a protracted course of nonsurgical treatment. Spine-specific contraindications include spondylolysis, spondylolisthesis, posterior element disease, central or lateral recess stenosis, fixed deformity, osteoporosis, or infection.
Posterior Stabilization Devices
Posteriorly implanted, motion-limiting devices that reduce but do not eliminate movement have been widely investigated in Europe for the treatment of mechanical back pain and spinal stenosis. In contrast to the principles of TDR, whereby painful disc tissues are ablated or unloaded directly by the implant, posterior stabilization devices leave the pain-generating disc tissues in situ but restrict certain types of motion and alter load transfer through the functional spinal unit. Posteriorly-based, dynamic devices include pedicle systems with tethers between the intra-pedicular anchors and interspinous process devices.
Pedicle-Based Systems
A dynamic stabilizing device ideally establishes pain-free segmental motion and withstands the physiologic static and dynamic loads of the spine. These devices work because they restrict movement to a zone or range where normal or near-normal loading may occur.
The DYNESYS (Zimmer, Warsaw, IN) is similar to the Graf system, with the addition of compression-resistant polycarbonate urethane sleeves around the prosthetic ligaments. The motion segment can be stabilized in a more neutral alignment instead of lordosis. The prosthetic ligaments provide restraint to flexion, whereas the sleeves prevent excessive lordosis, bear compressive loads, and unload the disc when the patient’s musculature exerts a net lordotic moment on the functional spinal unit. Three potential problems are introduced by the DYNESYS system. First, the compressive sleeves limit the amount of lordosis that can be achieved, and if placed with excessive distraction, the implant becomes kyphogenic. Second, compressive loads on the sleeves produce bending moments on the pedicle screws that could lead to breakage or loosening. Lastly, the introduction of compression sleeves significantly increases the rigidity of the construct thereby questioning its role in preventing adjacent level degeneration.
Interspinous Process Devices
The first-generation interspinous process implant for non-rigid, lumbar stabilization was developed in 1986. Over the past twenty years, several designs of interspinous process devices have been developed; however, the clinical indications for their use remain poorly defined. The clinical results with these devices have been reported only in anecdotal case reports with varied indications and incomplete patient follow-up making meaningful interpretation difficult.
Conclusion
The ability of the intact spine to provide mobility and stability over an extended period of time while withstanding repetitive loading is a remarkable feat. In many patients, however, degenerative diseases of the spine eventually cause significant pain and disability. Fusion remains one of the must useful and versatile tools in the spine surgeon’s armentarium. The emergence of nonfusion implants that can relieve pain, restore motion, and endure repetitive loads may provide an attractive option to arthrodesis. Optimally designing these motion-sparing prostheses requires an understanding of the complex pathophysiology, anatomy and kinematics associated with altered spinal biomechanics and pain syndromes. Judicious use of these implants is warranted until further clinical studies can validate their safety and efficacy.
Figure Legend
Figure 1: A lateral radiograph demonstrating a Charite disc replacement in situ.
Figure 2: A disassembled Charite disc demonstrating the two metal alloy end-plates and the inner UHMWPE core (Picture courtesy of Depuy Spine, Raynam, MA)
Figure 3: Schematic of a Charite disc demonstrating biconcave nature of the metal end-plates. (Picture courtesy of Depuy Spine, Raynam, MA)
Figure 4: A picture of the PDN (Raymedical Inc., Bloomington, MN). The device is composed of a hydrogel pellet that is surrounded by a polyethylene layer. (Picture courtesy of Raymedical Inc., Bloomington, MN)
Figure 5: A. Axial image of a cadaveric specimen with the NuCore injectable nuclear material. B. Lateral image of a cadaveric specimen with the NuCore injectable nuclear material. (Pictures courtesy of Spine Wave, Shelton, CT)
Figure 6: Picture of the DIAM inserted into the proper interspinous location. (Picture Medtronic Sofamor Danek, Memph
