Retention systems for extraoral maxillofacial prosthetic implants: a critical review - Cobein, Marcus Vinicius; Coto, Neide Pena; Crivello Júnior, Oswaldo; Lemos, José Benedito Dias; Vieira, L. M; Pimentel, M. L; Byrne, H. J; Dias, Reinaldo Brito (British Journal of Oral and Maxillofacial Surgery, 2017)