Cage height is more important than surface area for preventing subsidence in multilevel anterior cervical discectomy and fusions

The relationships between subsidence and cage dimension and surface area in multilevel anterior cervical discectomy and fusion (ACDF) remain unclear in the current literature. This study aimed to analyze the risk factors influencing cage subsidence following multilevel ACDFs, and the relationship between cage subsidence and cage dimensions, surface area, patient-reported outcomes, and fusion rates. Patients who had undergone primary multilevel ACDFs with a minimum follow-up duration of 1 year were enrolled in this study. Cage subsidence was evaluated using lateral radiographs and defined as a decrease in disc height ≥3 mm. The patients were classified into the cage subsidence and no cage subsidence groups. Neck Disability Index (NDI) scores were recorded at baseline and every clinic visit. Fusion rates were evaluated at the final follow-up. Eighty-five patients (mean age, 58.6 years; mean follow-up, 12.7 months) were included. The cage subsidence rates were 5%, 12%, 23%, and 31% at the 6-week, 3-month, 6-month, and 1-year follow-up, respectively. The demographic, perioperative, and radiographic parameters were not different between the two groups. Cage type, width, depth, and surface area did not influence the subsidence rates. Cage height was positively correlated with the depth of cage subsidence (p < 0.001, R = 0.263). A cage height ≥8 mm was associated with a higher risk of cage subsidence (odds ratio: 3.9, 95% confidence interval: 1.8–8.5, p < 0.001). An SPA ≥ 4° of the distal screw was identified as a risk factor for cage subsidence at the distal level. The overall fusion rate was 88.5% in both groups. NDI scores did not differ preoperatively or at follow-up. Cage height and cage subsidence were significantly correlated in ACDF patients. A cage height ≥8 mm may be a risk factor for cage subsidence. A cage with a larger surface area may not provide protection against subsidence.