Does Fusion Affect Anterior Bone Loss in Adjacent Cervical Disc Arthroplasty in Contiguous Two-Level Hybrid Surgery?

Objective

To investigate whether anterior bone loss (ABL) in cervical discarthroplasty (CDA) in 2-level hybrid surgery (HS) is affected by adjacent fusion in vivo compared with 1-level CDA alone.

Methods

A total of 180 patients undergoing either a 1-level CDA or contiguous 2-level HS were retrospectively reviewed. The clinical and radiographic outcomes were collected preoperatively and at routine postoperative intervals of 1 week, 3, 6, and 12 months, and at the last follow-up. The initial and postoperative radiographs were compared to determine the incidence and degree of ABL.

Results

ABL was identified in 68.7% of CDA cases (37.9% mild, 34.8% moderate, and 27.3% severe) and 44.0% of HS cases (54.1% mild, 27.0% moderate, and 18.9% severe). Sex, age, bone mineral density, operation time, blood loss, postoperative alignment, and range of movement at the arthroplasty segment were not related to the incidence of ABL. According to the logistic regression analysis results, ABL showed a significant correlation with the surgery type and body mass index. However, there was no significant difference in the incidence and degree of ABL with or without an adjacent fusion level. Compared with preoperative values, clinical outcome scores significantly improved after surgery in both the HS and CDA groups. No definite clinical effect associated with ABL was found.

Conclusions

ABL was common in both CDA and HS. Although HS had a lower incidence rate and degree than did CDA, the fusion location in HS did not affect the ABL of adjacent CDA.

Introduction

Cervical disc arthroplasty (CDA) has been accepted as an alternative to traditional anterior cervical discectomy and fusion (ACDF) in the treatment of cervical degenerative disc disease (CDDD). Previous studies have shown that CDA provides similar clinical outcomes to ACDF but has the advantages of theoretically maintaining the cervical range of motion (ROM), reconstituting the disc height, preventing adjacent segment degeneration, and allowing the earlier recovery of cervical function.1, 2, 3 In recent years, considering that the spondylotic spine is often associated with multiple-level disease, ACDF involving more fused levels has been found to lead to greater loss of mobility at operative levels, and the benefits from CDA for adjacent segments may be more essential.⁴ However, strict indications and higher medical cost may limit the application of CDA to treat multilevel CDDD.5, 6, 7 In this regard, as a combination of CDA and ACDF, hybrid surgery (HS) that can be tailored to each level allows segmental motion preservation at the index levels and minimizes hypermobility at adjacent levels; therefore, it has become an alternative procedure for the treatment of multilevel CDDD, with satisfactory outcomes.6, 7, 8, 9 However, some adverse outcomes of CDA have recently emerged in HS, including anterior bone loss (ABL), heterotopic ossification, prosthesis migration and subsidence, and segmental kyphosis.9, 10, 11

ABL was defined as nonprogressive early periprosthetic vertebral bone loss commencing within 6 months after surgery at the arthroplasty level.¹² In 2011, Tumialan et al.¹³ reported vertebral bone resorption after CDA in a case study, and this phenomenon has since received increasing attention. The cause of ABL remains unknown, and the clinical implications vary. Several biomechanical studies have shown that mechanical strains are responsible for the development of periprosthetic bone loss.14, 15, 16 In addition, Kim et al.¹⁷ conducted a retrospective clinical study and reported that the anterior flange of the Bryan artificial disc might result in stress shield effects, indicating that the strains and stresses are important factors in ABL occurrence. Heo et al.¹⁸ reported that the spinal biomechanical environment may be influenced by stressful mechanical change on the vertebral bodies after CDA, with subsequent development of bone loss.

In addition, in vitro cadaveric studies have compared the biomechanical effects between cervical disc replacement and fusion using hybrid constructs^8,19 and reported that CDA placed adjacent to the fusion was subjected to a more challenging biomechanical environment, especially higher intervertebral stress compared with stand-alone CDA. Our previous clinical study¹¹ showed that fusion in 2-level HS partially affected the behavior of the adjacent disc prosthesis compared with CDA alone. More importantly, Kieser et al.¹⁰ reported that there was a relationship between the number of surgical levels and severe ABL. This result was similar when including the hybrid procedures. However, no previous reports have specifically discussed the occurrence of ABL in HS. The primary aim of the present study was to investigate whether the ABL in contiguous 2-level HS was affected by adjacent fusion in vivo compared with CDA alone.