Design limitations of Bryan disc arthroplasty

Background context: Disc arthroplasty is gaining momentum as a surgical procedure in the treatment of spinal degenerative disease. Results must be carefully scrutinized to recognize benefits as well as limitations. Purpose: The aim of this study was to investigate factors associated with segmental kyphosis after Bryan disc replacement. Study design/setting: Prospective study of a consecutively enrolled cohort of 10 patients treated in a single center using the Bryan cervical disc prosthesis for single-level segmental reconstruction in the surgical treatment of cervical radiculopathy and/or myelopathy. Radiographic and quality of life outcome measures. Methods: Static and dynamic lateral radiographs were digitally analyzed in patients undergoing Bryan disc arthroplasty throughout a minimum 3-month follow-up period. Observations were compared with preoperative studies looking for predictive factors of postoperative spinal alignment. Results: Postoperative end plate angles through the Bryan disc in the neutral position were kyphotic in 9 of 10 patients. Compared with preoperative end plate angulation there was a mean change of -7° (towards kyphosis) in postoperative end plate alignment (p=.007, 95% confidence interval [CI] -6° to -13°). This correlated significantly with postoperative reduction in posterior vertebral body height of the caudal segment (p=.011, r²=.575) and postoperative functional spine unit (FSU) kyphosis (p=.032, r²=.46). Despite intraoperative distraction, postoperative FSU height was significantly reduced, on average by 1.7 mm (p=.040, 95% CI 0.5-2.8 mm). Conclusions: Asymmetrical end plate preparation occurs because of suboptimal coordinates to which the milling jig is referenced. Although segmental motion is preserved, Bryan disc arthroplasty demonstrates a propensity towards kyphotic orientation through the prosthesis likely as a result of intraoperative lordotic distraction. FSU angulation tends towards kyphosis and FSU height is decreased in the postoperative state from lack of anterior column support. Limitations of Bryan cervical disc arthroplasty should be carefully considered when reconstruction or maintenance of cervical lordosis is desirable.