Stress analysis of the interface between cervical vertebrae end plates and the Bryan, Prestige LP, and ProDisc-C cervical disc prostheses: an in vivo image-based finite element study.

STUDY DESIGN: Segmental motion and bone-implant interface stresses were analyzed at C5-C6 levels with Bryan, Prestige LP, and ProDisc-C cervical disc prostheses using an image-based finite element modeling technique. OBJECTIVE: To predict stress patterns at the interface between prosthesis and lower vertebral end plate to better understand the underlying mechanisms of subsidence and how the load transfer pattern of each disc design affects segmental motion. SUMMARY OF BACKGROUND DATA: Subsidence is one of the most commonly reported device-related complications in intervertebral disc arthroplasty. Although clinical outcomes have been reported regarding many types of cervical prostheses, few reports have analyzed the effects of stress from cervical artificial discs to the vertebral end plate. METHODS: Three-dimensional voxel finite elements were built for C5-C6 spine unit based on computed tomography images acquired from a patient with indication for cervical disc arthroplasty. Models of facet joints and uncovertebral joints were added and artificial disc designs were placed in the intervertebral disc space. Static analyses were conducted under normal physiologic loads in flexion, extension, and lateral bending with precompression. RESULTS: Bryan disc recovered highest range of motion (4.75 degrees ) due to the high elastic nucleus, and therefore imposed the lowest stresses superior to C6. The ProDisc-C and Prestige LP discs caused high stress concentrations around their central fins or teeth, and may initiate bone absorption. Analysis of Prestige LP disc may indicate possible subsidence posteriorly caused by the rear-positioned metal-to-metal joint. CONCLUSION: Rigidity of the cores ("nuclei") in Prestige LP and ProDisc-C prostheses guarantee initial maintenance of disc height, but high contact stress takes place at the bone-end plate interface if they are improperly placed or undersized. Anchorage designs add an additional factor that may increase propensity of subsidence, indicated by the high contact stress occurring at the end plate flanges of Prestige LP, and at midline keel fixation on the end plate of ProDisc-C. Although Bryan disc differs in these 2 concerns, it also creates much larger displacement during motion with more variation in disc height that may theoretically increase the load sharing of facet and/or uncovertebral joints compared to more rigid artificial discs.

Bone ingrowth in retrieved Bryan Cervical Disc prostheses.

STUDY DESIGN: Explant analysis for bone ingrowth of retrieved cervical disc prosthesis in chimpanzees and humans. OBJECTIVES: To assess the bone ingrowth into retrieved Bryan Cervical Discs. SUMMARY OF BACKGROUND DATA: Bone ingrowth in cervical disc prosthesis has not been documented in the literature. METHODS: Chimpanzee: Two chimpanzees underwent placement of the Bryan disc at C3-C4 and 3 months later had explantation and interbody fusion. Human: Two patients had removal of their Bryan disc and interbody fusion for failure to resolve symptoms at 8 and 10 months. The explants were analyzed for bone ingrowth. RESULTS: Chimpanzee: Histologic analysis showed bony ingrowth through the interstices of the porous coating and apposition ranging from 10% to 50% of toluidine blue-stained sections. New ingrowth, rather than bony impaction, was confirmed with fluorochrome-labeled sections Human: Bone ingrowth was a mean of 30.1% (12% SD). No difference was observed between peripheral, intermediate, or central locations. CONCLUSIONS: Adequate bony apposition was found in all primate device-to-vertebral body interfaces. Human retrievals also demonstrated significant ingrowth in all four surfaces. This compares with hip and knee arthroplasty percent ingrowth rates of 10% to 30%. All implants had stable fixation judged by radiographs and at the time of implant removal.

Kinematic analysis of the cervical spine following implantation of an artificial cervical disc.

STUDY DESIGN: Prospective cohort study. OBJECTIVE: To assess the biomechanical profile of the cervical spine following cervical arthroplasty. SUMMARY OF BACKGROUND DATA: Spinal arthroplasty offers the promise of maintaining functional spinal motion, thereby potentially avoiding adjacent segment disease. Disc replacement may become the next gold standard for the treatment of degenerative cervical spine disease, and must be studied rigorously to ensure in vivo efficacy and safety. METHODS: A total of 20 patients underwent single or 2-level implantation of the Bryan artificial cervical disc (Medtronic Sofamor Danek, Memphis TN) for treatment of cervical degenerative disc disease producing radiculopathy and/or myelopathy. Lateral neutral, flexion, and extension cervical radiographs were obtained before surgery and at intervals up to 24 months after surgery. Kinematic parameters, including sagittal rotation, horizontal translation, change in disc height, and center of rotation (COR), were assessed for each spinal level using quantitative motion analysis software. RESULTS: Motion was preserved in the operated spinal segments (mean range of motion 7.8 degrees) up to 24 months following surgery. The relative contribution of each spinal segment to overall spinal sagittal rotation differed depending on whether the disc was placed at C5-C6 or C6-C7. Overall cervical motion (C2-C7) was moderately but significantly increased during late follow-up. Sagittal rotation, anterior and posterior disc height, translation, and COR coordinates did not change significantly following surgery. The COR was most frequently located posterior and inferior to the center of the disc space. CONCLUSIONS: The Bryan artificial cervical disc provided in vivo functional spinal motion at the operated level, reproducing the preoperative kinematics of the spondylotic disc.

The Bryan Cervical Disc: wear properties and early clinical results.

BACKGROUND CONTEXT: The rationale for motion preservation by disc replacement after anterior cervical discectomy is to diminish long-term morbidity secondary to adjacent segment degeneration. However, these disc prostheses will be subject to wear and its possible adverse consequences. PURPOSE: Assess the in vitro and in vivo wear properties of the Bryan Cervical Disc and the early clinical results. STUDY DESIGN: In vitro mechanical testing, caprine animal model and prospective European human trial. OUTCOME MEASURE: In vitro mechanical testing used American Society for Testing and Materials standards to measure wear rates and debris. Caprine results were analyzed by histopathologic analysis by a blinded veterinary pathologist. Clinical outcomes were measured by Odoms's criteria, independent radiologic analysis and Short Form-36. METHODS: In vitro wear testing in cervical spine simulator for up to 40 M cycles. Wear rates were determined and particles analyzed microscopically. The in vivo inflammatory response was studied in goats that had discectomy at C4-C5. They were sacrificed between 3 and 12 months and histopathologically were compared with controls who had fusions with titanium plates. A prospective clinical trial of 136 patients with minimum 1-year follow-up was analyzed using validated questionnaires and radiographs. RESULTS: In vitro wear averaged approximately 1.76% by weight at 10 M cycles and 18% at 40 M cycles. Wear debris were present in the periprosthetic tissues in 4 of 11 animals without inflammatory response. The early clinical results were satisfactory in over 90% of patients. CONCLUSION: The in vivo and in vitro wear properties are satisfactory for the expected duration of life of the prosthesis. The early clinical results are satisfactory and equal to fusion.

Intervertebral disc arthroplasty.

STUDY DESIGN: Review article of current knowledge of disc arthroplasty. OBJECTIVES: To review the rationale for disc replacement, the general principles of design, and early clinical results. SUMMARY OF BACKGROUND DATA: Disc arthroplasty is an emerging treatment for patients with disc degeneration. Its theoretical advantages are to maintain motion, decrease the incidence of adjacent segment degeneration, avoid complications related to fusion, and allow early return to function. METHODS: Literature review of currently implanted prostheses or those undergoing investigation. RESULTS: At this time, the theoretical advantages are unproven clinically but have been confirmed in biomechanical and kinematic investigations. Multicenter studies of both cervical and lumbar prostheses have shown short-term results equivalent to fusion. Neurologic complications and failures have been rare. Prosthetic subsidence and long-term wear will most likely be potential failure mechanisms. Thus far, with the exception of nucleoplasty, these problems have not been observed. CONCLUSIONS: The early results are satisfactory, but the basic premise that motion preservation will diminish adjacent segment degeneration is yet unproven. Long-term results are unavailable and failure modes are unknown. Before implantation, the surgeon and patient must understand the experimental nature of the devices.

A comparison of simulator-tested and -retrieved cervical disc prostheses. Invited submission from the Joint Section Meeting on Disorders of the Spine and Peripheral Nerves, March 2004.

OBJECT: Total joint arthroplasties most commonly fail because the implant becomes worn and a host inflammatory response subsequently develops. Both the material response to the biological environment and the host response to the device must be thoroughly evaluated to establish the efficacy of cervical arthroplasty. Analyses of devices explanted in humans allow evaluation of both responses. Hypothetical wear rates can be determined by comparing in vivo wear with simulator-derived wear. The purpose of this study was to perform explant analyses involving the Bryan and Prestige discs and compare these results with those obtained using spine simulators. METHODS: Of the approximately 5500 patients treated with the Bryan disc, 11 have undergone explantation of the device. Six of these devices were analyzed for dimensional and chemical changes. Three of the approximately 300 implanted Prestige discs were retrieved, and two were examined microscopically. Histological specimens were assessed for wear particles and host inflammatory response. Additionally, the extent of simulator-produced wear was compared with that demonstrated in the retrieved specimens. CONCLUSIONS: The simulator-generated results predict adequate wear-related characteristics for both the Bryan and Prestige prostheses for a minimum of 40 years. Comparison of data with those of the retrieved specimens indicates that the wear was more minimal than predicted in simulators by five- to 10-fold. In no instance did the revisions result from failure of the device due to a reaction to wear debris, fracture, polymer oxidation, or metal corrosion. The inflammatory response seen in the periprosthetic tissues was minimal and not characteristic of inflammatory responses in failed diarthrodial joint arthroplasties.

Wear analysis of the Bryan Cervical Disc prosthesis.

STUDY DESIGN: In vitro wear testing of the Bryan Cervical Disc prosthesis was performed in a cervical spine simulator. The biologic response was assessed in chimpanzee and goat animal models. OBJECTIVE: Determine the wear characteristics of the Bryan disc. SUMMARY OF BACKGROUND DATA: Large joint arthroplasties fail most commonly by wear and consequent formation of particulate material, which induces an inflammatory response. Therefore, measuring the wear characteristics of the new spinal disc replacements is important. METHODS: Six prosthetic assembles were tested to 10 or 40 million cycles by load and motion and 3 additional assemblies were tested by load only in a cervical spine simulator. Any debris was examined using ASTM standards. The local biologic response to the prosthesis was examined in two chimpanzees. Nine goats were used to assess the biologic response in both local and distant tissues. Arthrodesis was performed on three additional control goats that received an allograft and an anterior cervical plate. RESULTS: Wear results: cervical spine simulators that applied the loads and motions associated with activities of daily living produced wear particulate at a rate of 1.2 mg per million cycles. Device height decreased 0.02 mm per million cycles with approximately 77% of this decrease due to gradual creep of the nucleus under the constant compressive load. Particles generated were granular in shape with a mean feret diameter of 3.9 microm. All animals tolerated placement of the Bryan disc. Wear debris was present in the periprosthetic and epidural spaces in some animals. However, no significant inflammatory response was observed. No wear material was found distant from the implant in draining lymph tissue, the liver, or the spleen. CONCLUSIONS: The Bryan disc has satisfactory wear characteristics and does not produce a significant inflammatory response.