Table 1.

Characteristic of the included studies

Author	Year	Year of onset	Follow-up time	Inpatient		Outpatient		Matched factors
				Events	Total	Events	Total
Adamson [16]	2014	2006–2013	90 days	270	274	621	629	ACDF level and other disease
Liu [18]	2009	August 2005–May 2007	62.4 days (mean)	60	64	45	45	ACDF level and operation condition
McGirt [27]	2015	2005–2011	30 days	630	650	781	792	Age, other disease, operation condition and ASA grades
Silvers [21]	1996	–	1.3 (outpatient) and 1.6 (inpatient) years	52	53	49	50	None
Stieber [22]	2005	1998–2002	21 days	49	56	27	30	Age
Trahan [24]	2011	November 2005–April 2009	6 h	58	58	58	59	Age
Walid [25]	2010	–	–	562	578	96	97	Age and other disease