Effects of shell angle on clinical and radiological outcomes after single-level cervical disc arthroplasty with Prestige LP prosthesis

Background: Cervical disc arthroplasty (CDA) using Prestige LP disc has been widely used for cervical degenerative disease. Postoperative shell angle (SA) is the angle between the upper and lower prosthesis endplate and previous studies have reported a significant correlation between prosthesis SA with postoperative biomechanical stress. Due to the lack of reference range in SA, a larger discrepancy exists in postoperative SA among the patients after CDA with Prestige LP disc. Methods: A total of 160 patients who had been subjected to single level CDA with Prestige-LP were enrolled. SA was defined the angle between the upper and lower prosthesis endplate and patients were assigned into parallel (SA 0 degrees-3 degrees) and lordotic (SA > 3 degrees) groups according to the immediate postoperative SA. Patients were followed up for a mean duration of 27.78 +/- 5.13 months. Clinical, radiographic and complication data were compared among groups. Results: 90 patients were assigned in the parallel group while 70 patients were included in the lordotic group. The mean immediate postoperative SA in the parallel and lordotic groups were 1.64 +/- 0.08 degrees and 4.88 +/- 1.20 degrees, respectively (p < 0.05). FSU angle in the parallel group at 1 year and last follow-up were 2.6 +/- 2.1 degrees and 2.1 +/- 2.0 degrees, respectively, with a change value of 0.68 +/- 2.28 degrees. In the lordotic group, FSU angle at 1 year and last follow-up were 3.3 +/- 2.5 degrees and 3.1 +/- 2.2 degrees, respectively, with a change value of 0.70 +/- 1.43 degrees. At 1 year, the FSU angle in the parallel group was significantly low than in the lordotic group (p < 0.05). Besides, average inter vertebral space heights (ISH) at 6 months, 1 year and last follow-up were 6.81 +/- 0.38 mm, 6.79 +/- 0.3 7 mm, 6.78 +/- 0.36 mm in the parallel group and 6.64 +/- 0.24 mm, 6.32 +/- 0.19 mm, 6.30 +/- 0.19 mm in the lordotic group, respectively (all p < 0.05). There were no significant differences in clinical outcomes and complications between the groups (p > 0.05). Conclusions: Immediate postoperative SA has no significant effects on clinical outcomes and complications after CDA with Prestige-LP. However, it has a positive correlation with postoperative FSU angle and a negative correlation with ISH maintenance.