Artificial Disc Versus Fusion: A Prospective, Randomized Study With 2-Year Follow-up on 99 Patients

Rick C. Sasso, MD; Joseph D. Smucker, MD; Robert J. Hacker, MD; John G. Heller, MD

Disclosures

Spine. 2007;32(26):2933-2940. 

In This Article

Materials and Methods

A total of 115 patients were enrolled and followed prospectively at 3 centers involved in a multicenter, FDA IDE trial for the Bryan cervical disc prosthesis. Patients with symptomatic, cervical radiculopathy or myelopathy refractory to nonoperative interventions were randomized in a 1:1 ratio to a single-level ACDF with allograft and plate (control group) or single-level cervical arthroplasty with the Bryan cervical disc prosthesis (investigational group). Preoperative imaging studies included plain radiographs, magnetic resonance imaging (MRI), and computed tomography (CT). The inclusion criteria were single-level cervical degenerative disc disease causing radiculopathy or myelopathy in skeletally mature patients (21 or older) from C3-C7. Patient's had to fail conservative care for 6 weeks (except for myelopathy cases needing immediate attention). Patients required a Neck Disability Index (NDI) score of ≥30%. The most important exclusion criteria were the presence of significant anatomic deformity, such as moderate to advanced spondylosis, radiographic signs of subluxation (>3.5 mm), or angulation (>11°), or previous cervical procedures at the operative level.

Surgical technique was similar in both groups to the point of interbody fusion/arthroplasty. A standard Smith-Robinson approach was made to expose the symptomatic level. The same technique for discectomy and decompression was used for both groups. The uncovertebral joints were left in place unless a portion of it was the cause of neural compression; then just the portion compressing the nerve was removed. Endplate preparation for ACDF was completed with a high-speed burr and an appropriately sized CORNERSTONE SR fibular allograft (Medtronic Sofamor Danek, Inc., Memphis, TN) was placed in the prepared interspace. All ACDF patients underwent anterior cervical plating with the ATLANTIS VISION Cervical Plate System (Medtronic Sofamor Danek, Inc.).

The Bryan cervical disc prosthesis (Medtronic Sofamor Danek, Inc.) is a 1-piece, biarticulating, metal-on-polymer, semiconstrained device with fully variable instantaneous axis of rotation that is not dependent on supplemental fixation.[13,14] It has a unique polyurethane sheath that is designed to contain wear debris and prevent soft tissue ingrowth. Each endplate is porous coated to promote bony ingrowth for long-term device stability. In May 2002, this implant became the first cervical artificial disc replacement performed in the United States. The Bryan disc is comprised of a polyurethane polymeric nucleus sandwiched between 2 titanium alloy clamshell-shaped endplates.[14] There are 2 bearing surfaces, 1 at each nucleus-endplate interface. Because the device is unconstrained (internally) throughout the physiologic range of motion, coupled motions of angulation and translation exist. The polyurethane sheath is attached to the endplates with titanium wires forming a closed compartment. This sheath may promote formation of a surrounding pseudocapsule with time. Sterile saline lubricant is injected into this compartment before implantation and titanium alloy seal plugs seal the compartment. Anterior flanges on each shell prevent posterior migration of the implant. An insertion device engages a hole in each flange to allow easy control of the disc during implantation.

Preparation of the endplates for arthroplasty was accomplished in the standard technique. The Bryan disc milling technique creates 2 concave surfaces via a milling jig stabilized by table mounted retractors. Sizing of the Bryan cervical disc was determined with a combination of templates and preoperative radiographic studies including CT. The center of the disc space was determined intraoperatively by a jig that defines the uncovertebral joints and finds the center. With knowledge of the center of the disc space, a milling fixture was anchored to the vertebral bodies. This fixture controlled the cutting tools, which mill the endplates to the exact geometry of the device endplates providing immediate stability.

Insertion of the TDR was accomplished under lateral fluoroscopy to assure adequate depth. Before inserting the Bryan disc, the implant was filled with saline as an initial lubricant. The prosthesis was then placed into the milled interspace. Before closure of the incision, appropriate placement of a TDR was confirmed with anteroposterior and lateral fluoroscopic imaging.

Preoperative demographic data, surgical data, and outcomes data were collected on all patients. Clinical outcome tools included: NDI, Arm Pain Score (VAS), Neck Pain Score (VAS), and SF-36. Outcome assessments were made before surgery and at 6 weeks, 3 months, 6 months, 12 months, and 24 months. Radiographic angular motion at the target level was tracked on digital radiographs using quantitative motion analysis software (QMA, Medical Metrics) to calculate the functional spinal unit motion parameters tool by 2 blinded, trained observers.

Primary outcome measures were the pain and functional assessment data using patient self-report instruments, the NDI and SF-36 questionnaires, as well as numerical rating scales (VAS) for neck and arm pain. Radiographic measures were included as secondary endpoints. Investigational patients were evaluated with respect to maintenance of functional spinal unit height, implant subsidence, anteroposterior implant migration, and angular motion at the implanted and adjacent disc spaces.

For continuous variables, statistical comparisons between the treatment groups were performed by using analysis of variance (analysis of variance) and for categorical variables, the Fisher exact test was used. A paired t test was used to assess the statistical significance of postoperative score change from the preoperative in SF-36, NDI, and neck and arm pain measures. It was also used for analysis of motion scores at the target level when change from preoperative angulation was recorded.

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