Comparison of external and internal implant-abutment connections for implant supported prostheses: a systematic review and meta-analysis - Lemos, Cleidiel Aparecido Araujo; Verri, Fellippo Ramos; Bonfante, Estevam Augusto; Santiago Júnior, Joel Ferreira; Pellizzer, Eduardo Piza (Journal of Dentistry, 2018)