A fusion classification system for the advent of bioactive interbody implants

Background: Historically, intervertebral fusion required bone bridging for stability. Bioactive interbody devices enable bone integration at the endplate–implant interface, prompting an updated radiographic assessment and classification system that recognizes implant adhesion as contributing to fusion. Aim: This study aims to develop and evaluate a reliable computed tomography (CT)-based grading system for assessing spinal fusion and stability associated with interbody devices, including factors such as osseointegration, implant type, and bone connectivity. Methods: A novel fusion grading system was developed, incorporating both bone and fusion consolidation through and around the device and the apposition of bone to the implants. A total of 10 cases with one-year postoperative CT scans were provided for the survey. Surveys were administered twice, more than two weeks apart and in a different order. Inter- and intra-rater reliability were assessed using the intraclass correlation coefficient (ICC). Results: A total of 51 spine surgeons participated, of whom 37 completed both survey rounds. For all 51 participants, the ICC was 0.618 (95% confidence interval = 0.435–0.834; p < 0.001), and the mean intra-rater reliability was 0.778 (standard deviation = 0.211). Four raters had outlier intra-rater scores, with an average of 0.259. After removing these four raters (10.8% of the cohort) and analyzing the remaining 89.2% of raters, the ICC was 0.628 (95% confidence interval = 0.445–0.841; p < 0.001), and the mean intra-rater reliability was 0.831 (standard deviation = 0.125; p ≤ 0.015). Conclusions: The novel CT-based classification system showed moderate-to-good inter-rater and good-to-excellent intra-rater reliability in clinical cases. Future work should use it to assess the relative contributions of on-growth, in-growth, and bone healing around bioactive interbody devices. Relevance for patients: This new classification system may be utilized to account for the stabilizing effects of bone in-growth at the implant interface when studying fusion with bioactive implants.