The RPI system of clasping is used to minimize the denture caused stresses on the abutment teeth, In particular with cases with free end saddle(s). The system aims to have as little an impact as possible on the remaining teeth. For that purpose, the RPI system is normally used with the “skeletal design” option for a particular partial denture. The skeletal design has very limited coverage on the lingual surfaces of the remaining natural teeth. Added to that, the RPI system places very little metal parts of the denture on the abutment teeth surfaces. That keeps the gingival margins of the natural teeth exposed to the self cleansing actions of the tongue and saliva.
The mechanics of the RPI also aim to exert minimal amount of torquing (tilting) forces on the abutment teeth. The placement of rests and their positioning makes a marked difference to the way a denture will behave under load. Normally, we would place the rest on the mesial side of the abutment in the case of a free end saddle denture. A rest placed on the distal side of the abutment leads to greater amount of rotation of the major connector. More importantly, it can cause the clasp to act as an extracting tool. Or even worse, the whole denture can slip off the tooth in a distal direction, causing great injury to the oral tissues.
The proximal plate is usually limited to the distal width of the tooth. It provides the mesio-distal stability to the denture; As well as the limit of the saddle where the acrylic and teeth are fixed. The proximal plate can also provide a certain degree of retention by mean of friction (without engaging an undercut). These plates are also a major component determining the path of insertion, and giving the clasps their effectiveness in the undercuts they engage.
The I bar clasp is utilized to provide direct retention and better aesthetics because of its minute surface area coverage. If the clasp crosses the gingival margin at a right angle (90°), then its interruption of that margin is confined to a very small distance (1-2mm). Its impact on the tooth itself is also very small, leaving almost negligible areas for food traps and germ harboring.
These three components of the RPI system when connected to a major connector rigidly will provide a very low impact, low maintenance mean of stability retention and support.
Define and explain RPI in relation to a RAPD.
List the advantages of the RPI design compared to a Lingual plate design.