Radiological changes in anterior cervical discectomy and fusion with cage and plate construct: The significance of the anterior spur formation sign

Study Design: Retrospective review. Objective: To analyze the changes of fusion process and radiological parameters during the postoperative period after anterior cervical discectomy and fusion with cage and plate construct (ACDF-CPC). Summary of Background Data: Because of its well-reported efficacy, plate augmentation has been performed to avoid the various complications associated with the cage-alone procedure. The radiological changes at the fusion site after ACDF-CPC have yet to be fully explored. Methods: Seventy-eight patients (122 fusion sites) who underwent ACDF-CPC were observed at 6 weeks and at 3, 6, 12, and 24 months postoperatively. Fusion status was classified into 3 categories: Type I (pseudoarthrosis), Type II (borderline), and Type III (fusion). Changes at the fusion site were described through radiological parameters at each follow-up time point. In addition, the ability of the radiological parameters to predict fusion rates was analyzed. Results: The fusion process after ACDF-CPC progresses slower when compared with the standard procedure utilizing autograft. Fusion between bone graft chips begins at 6 weeks post surgery. At 3 months, initial bone bridging between graft and host bones begins to form. Anterior spur formation occurs at 3 to 6 months, and "kissing" lesions form at 6 to 12 months. Bony incorporation is achieved at 1 to 2 years. Persistent or newly developed Type I at the 1-year follow-up exhibited significantly higher pseudoarthrosis rates in comparison with rates determined at the 3- and 6-month time points. Among 29 subsidence cases, 9 of the 16 (56.3%) cases that exhibited anterior spur formation eventually achieved fusion, whereas 2 of the 13 (15.4%) cases that did not exhibit anterior spur formation eventually achieved fusion. In cases that demonstrated anterior spur formation, the fusion rate was significantly higher than in cases without it (P = 0.016). Conclusion: The fusion process after ACDF-CPC progresses slower than the standard procedure utilizing autograft. Cage subsidence of greater than 2 mm, a radiolucent defect, or a halo sign are poor prognostic signs indicating a high probability for pseudoarthrosis when detected radiographically after 1 year postoperatively. The anterior spur formation sign and "kissing" lesion, on the contrary, represent signs for eventual successful fusion.