Incidence and clinical relevance of cage subsidence in anterior cervical discectomy and fusion: a systematic review

BACKGROUND: The placement of intervertebral cages in anterior cervical discectomy (ACDF) supposedly maintains foraminal height. The most commonly reported cage-related complication is subsidence, although it is unknown whether a correlation between subsidence and clinical outcome exists. AIM: To assess the incidence and relevance of subsidence. METHODS: Literature searches were performed in PubMed, MEDLINE, Embase, Web of Science, COCHRANE, and CENTRAL. The inclusion criteria were as follows: ≥ 20 patients, ADCF with cage, subsidence assessed, and primary data. Risk of bias was assessed using adjusted Cochrane checklists. RESULTS: Seventy-one studies, comprising 4784 patients, were included. Subsidence was generally defined as ≥ 3-mm loss of height comparing postoperative intervertebral heights with heights at last follow-up. Mean incidence of subsidence was 21% (range 0–83%). Of all patients, 46% of patients received polyether-ether-ketone (PEEK) cages, 31% received titanium cages, 18% received cage-screw-combinations, and 5% received polymethyl-methacrylate (PMMA) cages. Patients treated with cage-screw-combinations had significantly less subsidence than patients treated with PEEK, titanium, or PMMA cages (15.1% vs. 23.5% vs. 24.9% vs. 30.2%; p < 0.001). Thirteen studies assessed clinical outcome in relation to subsidence; the majority did not find a significant correlation. Only four studies correlated subsidence to cage size and/or height; no correlation was established. CONCLUSIONS: Subsidence in ACDF with cages occurs in 21% of patients. The risk for subsidence seems lower using PEEK or titanium cages or adding screws. Whether subsidence affects clinical outcome is not satisfactorily evaluated in the available literature. Future studies on this correlation are warranted in order to establish the additional value of the interposition of a cage in ACDF.