How to Choose the Measurement Methods for Cage Subsidence Following Anterior Cervical Discectomy and Fusion?

The methods for measuring cage subsidence from radiographs after anterior cervical discectomy and fusion were not standardized and are typically categorized into 3 approaches. The reliability and clinical correlation of these methods have not been fully elucidated. This study aims to evaluate 3 methods based on radiographs for measuring cage subsidence following anterior cervical discectomy and fusion and identify the most reliable one.

The patients with complete and clear lateral cervical radiographs were included in this study to assess subsidence. The cage migration into endplate height (ΔCMH), change in disc space height (ΔDH), and change in middle segment height (ΔMSH) were separately used to represent cage subsidence. The differences between the 3 measurement methods were compared. Interclass correlation coefficients were calculated to assess the reliability of the 3 measurement methods. Spearman's correlation coefficient was used to assess the clinical relevance of these 3 methods.

A total of 105 patients with 142 levels were included in the study. There were significant differences between the ΔDH and the ΔCMH/ΔMSH methods, but not between the ΔCMH and ΔMSH methods. All methods presented good reliability, with the ΔMSH method showing the highest intra- and inter-observer interclass correlation coefficients. The subsidence values measured by ΔMSH showed good correlation with clinical outcomes, while those measured by the ΔDH and ΔCMH methods demonstrated poor clinical correlation.

The ΔDH method and the ΔCMH/ΔMSH method are not interchangeable within a single study. We recommend the ΔMSH method for measuring subsidence due to its relatively better reliability and clinical correlation.