Prognostic Factors Related to Motion Dynamics Following Cervical Arthroplasty With a Bryan Disc: Average 2-Year Follow-Up

DISCUSSION

Since the 1960s, various clinical experiences with cervical disc arthroplasty have been published, and recently, several motion- preserving cervical prostheses have become commercially available. The Bryan cervical disc was developed in the early 1990s,⁸ and its clinical use was first described by Goffin et al.⁷ This cervical disc is a single-piece, metal-on-polymer prosthesis comprised of a polycarbonate/polyurethane core between two porous coated endplate shells, all encapsulated by a polymer sheath. It is a bi-articulating, unconstrained device with a fully variable, instantaneous axis of rotation.

The early clinical reports of this disc prosthesis were encouraging. Goffin et al.⁷ described the preliminary clinical results of 60 patients who underwent cervical arthroplasty using this disc. According to their results, relief of preoperative symptoms was observed in 86% at 6 months postimplantation without measurable device subsidence. Duggal et al.⁹ assessed early clinical outcomes for 30 Bryan disc prostheses in 26 patients using the physical component scores (PCS) of the short-form health survey SF-36 and NDI scores. They reported a statistically significant improvement in mean NDI scores (from 18.7 to 4.7) and in SF-36 PCS scores (from 34.58 to 46.28), at a mean follow-up point of 12 months postoperatively. Our study also demonstrates successful clinical results. Immediately postoperatively, mean VAS and NDI scores were significantly improved, and these were maintained at the point of final follow-up.

Several studies have discussed the radiological outcomes following insertion of the Bryan disc prosthesis. Pickett et al.¹⁰ noted that motion was preserved in operated spinal segments up to 24 months following arthroplasty in 20 patients who underwent single or 2-level implantation. Mean postoperative ROM at the index level was 8.04° during early follow-up and 8.92° during late follow-up, which were not significantly different from the preoperative value of 8.89°. Duggal et al.⁹ reported that mean postoperative ROM at the index level was 7.8° at late follow-up time points, which was not significantly different from the preoperative mean value of 10.1°. Shim et al.¹¹ stated that average ROM increased significantly after surgery (6.7° to 8.5°) in 61 patients, and Fong et al.¹² noted that the preoperative average ROM at index segments increased postoperatively (from 7° to 8°). These studies confirm that motion preservation can be achieved and maintained.

In the present study, FSU ROM at index segments also showed increases in segmental motion, from 6.96° to 8.93° at the point of final follow-up. These studies revealed that the age of the patients and the preoperative segmental ROM are valuable parameters for predicting postoperative FSU ROM, although these factors do not relate to the clinical outcome. As the age of the patient increased, cervical spondylosis including disc degeneration and facet arthrosis also were advanced, which contributed to decreased segmental motion. Results of these studies indicate that preoperative segmental motion is an important prognostic factor for cervical disc replacement in terms of motion preservation. If the segmental ROM is compromised preoperatively, arthroplasty would not be a very good option from the perspective of motion-preserving surgery.

Although contrary reports also exist,^{7, 9, 13} sagittal malalignment of the cervical spine has been mentioned after Bryan disc prostheses placement.^{11, 12, 14, 15} Pickett et al.¹⁰ noted a postoperative segmental kyphosis. According to their results, a reduced segmental angle was observed in 49% of 94 operated segments. Shim et al.¹¹ reported radiological results at an average follow-up of 6 months after Bryan cervical arthroplasty in 61 patients. It was found that mean segmental angles became more kyphotic after surgery (from □ 0.7° of kyphosis to □ 1.3°), but without statistical significance. Johnson et al.16 also alluded to similar segmental kyphosis, which demonstrated a loss of overall lordosis of 2.7 degrees. In our study, mean FSU angle decreased from 2.85° ± 3.27° to 1.03° ± 5.74° at 1 month postoperatively (P = .012) with statistical significance and then was maintained at final follow-up.

Sears et al.¹³ recently suggested that the occurrence of segmental kyphosis after Bryan cervical arthroplasty tends to depend on surgical technique. Possible factors that contribute to the development segmental kyphosis include amount of bone removal, angle of prosthesis insertion, prosthesis length, and loss of preoperative disc space height. Among these, the loss of preoperative disc space height is the most reliable predictive factor.

Loss of disc height depends on the amount of bone removed from the anterior aspect of superior vertebra and the amount of distraction achieved with the sagittal wedge. Although disc heights at index segments showed no statistically significant changes at the point of final follow-up in the present study, Fong et al.¹² postulated that the milling procedure used to fit for the Bryan disc prosthesis might have a basic problem. The milling trajectory should follow the line perpendicular to the tangent drawn along the posterior cortices of the adjacent rostral and caudal vertebrae. However, in the majority of patients, the normal disc angle is not perpendicular to the posterior tangent line, which results in asymmetric endplate milling. In our opinion, kyphotic malalignment could be mainly related to excessive preparation of endplates during the milling procedure. Excessive milling and asymmetric milling of endplates result in inaccurate implantation and sequential subsidence of the treated level and segmental kyphosis. Accordingly, in order to reduce these problems, the milling procedure must be approached carefully.

In the present study, despite decreases in FSU angles, preoperative global angles increased significantly at final follow-up. This may have been caused by a compensatory increase in the lordotic angles of upper and lower non- treated segments. Mean global ROM was lower at 1 month postoperatively, but almost reached the preoperative level at final follow-up. According to Pickett et al.,¹⁰ global ROM increased from a preoperative mean of 47.2° to 56.1° during late follow-up. They speculated that relief of neck pain during late follow-up allowed for increased neck movement in patients who had been symptomatic before surgery. In the present study, range of motion changes in upper and lower adjacent segments were minimal, which means that cervical arthroplasty with prostheses like the Bryan disc could minimize mechanical stress to adjacent levels. However, comparison studies with ACDF will be necessary to definitively determine the advantages of cervical disc arthroplasty.