To prescribe early trismus therapy, prognostic factors influencing the restricted mouth opening should be identified first. Our aim is to present an overview of these factors in patients with head and neck cancer.

PubMed, Cochrane, EMBASE, and CINAHL were searched using terms related to head and neck cancer and mouth opening.

Risk of bias was assessed using the “Quality in Prognosis Studies” tool. A best evidence synthesis was performed.

Of the identified 1418 studies, 53 were included. Three studies contained a prognostic multivariate model for a restricted mouth opening.