Evidence-based Analysis of Adjacent Segment Degeneration and Disease After ACDF: a Systematic Review

Background Context: Follow-up studies of patients undergoing anterior cervical discectomy and interbody fusion (ACDF) have demonstrated varying degrees of radiographic degeneration at adjacent levels, with most cases being asymptomatic (adjacent segment degeneration, ASDeg) and far fewer being symptomatic (adjacent segment disease, ASDz). Controversy remains as to whether these conditions are related to altered biomechanics or represent the natural history of cervical spondylosis at the adjacent segment.

Purpose: To provide an evidence-based analysis of the peer-reviewed literature on clinical studies of ASDeg and ASDz after ACDF.

Study Design/setting: Systematic review of existing literature.

Methods: The MEDLINE database was queried for clinical studies reporting ASDeg and/or ASDz after ACDF. Articles written in the English language with a minimum follow-up of 2 years were independently reviewed and analyzed by two authors, and the level of evidence was assigned. Data were pooled to generate summary outcomes and organized by number of levels, technique, and graft/implants.

Results: Of the 238 articles returned from the MEDLINE database query, 14 met inclusion criteria. An average of 168 patients was enrolled per study with an average follow-up of 106.5 months. Graft materials, cage design, plate fixation system, and length of fusion varied widely. Additionally, no clear standard was seen for radiographic assessment modalities (eg, plain lateral radiograph, flexion-extension radiographs, computed tomography, or magnetic resonance imaging). Validated clinical outcome measures were used in 43% (6/14) of the studies. The average incidence of ASDeg was 47.33% (459.14/970) with a range from 16% to 96%. The frequency-weighted average for ASDz was 11.99% (263.70/2,199) with a range from 1.80% to 36.00%. Follow-up ranged from 24 to 296 months with no reliable commonalities, which prohibited a meta-analysis.

Conclusions: This review highlights the heterogeneous methodology of the peer-reviewed literature on ASDeg and ASDz after ACDF and the paucity of high-level clinical data published on these conditions. Despite the low level of evidence to define the incidence of ASDeg and ASDz, it is clear that radiographic ASDeg is more common than symptomatic ASDz, indicating that adjacent segment pathology remains subclinical in a large subset of patients. This analysis underscores the need for standardized radiographic measures in the assessment of ASDeg and validated clinical outcome measures for ASDz after ACDF. Consistent methodology and multi-surgeon collaboration may improve the quality of clinical data on ASDeg and ASDz and elucidate the true etiology and incidence of these conditions.