Bohl DD, Mayo BC, Massel DH, Iantorno SE, Ahn J, Basques BA, Grauer JN, Singh K. Incidence and Risk Factors for Pneumonia After Posterior Lumbar Fusion Procedures: An ACS-NSQIP Study. Spine (Phila Pa 1976). 2016 Jun; 41(12):1058-1063.