Suginaka Riegel lock denture eliminates the need for a retentive projection. 
The riegel and swing-lock attachments are excellent retainers in terms of secure retention, but the former requires a great volume of abutment tooth reduction and the latter has problems with aesthetics and abrasion.  In contrast, the RPI and RPA clasps have been highly appreciated as excellent retainers for abutment tooth protection, but any retentive forces are needed.  Thus, the purpose of the study is to design a retainer that made the use of these merits.  The form of the retainer (named Suginaka Riegel lock retainer) for this purpose is based on the RPPA, for which a lingual arm is provided.  In applying this technique, the buccal arm corresponding to the Akers-type buccal arm in the RPPA utilizes the undercut area below the survey line, ranging from the distal to the mesial corner.  This buccal arm extends from the metal tooth that turns the hinge placed in the buccal denture border.  The Suginaka Riegel device placed in the denture base locks the metal tooth in place.  Utilizing the deeper undercut below the survey line provides secure retention while eliminating the risk of adverse forces being exerted on the abutment tooth during denture function or insertion and removal.  This new, additional form of the Suginaka Riegel lock denture allows the abutment tooth to provide latch effect-derived retention solely by preparation of the rest seat and guide plane.  This allows use on a healthy tooth and on a tooth for which prosthodontic treatment has already been completed.