Prosthodontics & Occlusion · single question
In a distal extension RPD, why is the occlusal REST placed on the MESIAL surface of the abutment tooth?
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Correct answer: C. To prevent tipping/torqueing of the abutment tooth (RPI design)
✓ C) Mesial rest prevents abutment tooth tipping and torqueing (RPI design principle).
In distal extension RPDs (Class I, II Kennedy), the MESIAL REST is placed because: During function, the distal extension saddle sinks into tissue (tissue-ward movement). With a DISTAL rest: the clasp arm would dig into the abutment creating a lever → tipping force. With a MESIAL rest (RPI clasp): as the saddle sinks, the I-bar and proximal plate DISENGAGE from the tooth → no tipping force → kinder to abutment. This is the biomechanical advantage of the RPI (Rest-Proximal plate-I bar) design. Distal rest = torque; Mesial rest = no torque.
In distal extension RPDs (Class I, II Kennedy), the MESIAL REST is placed because: During function, the distal extension saddle sinks into tissue (tissue-ward movement). With a DISTAL rest: the clasp arm would dig into the abutment creating a lever → tipping force. With a MESIAL rest (RPI clasp): as the saddle sinks, the I-bar and proximal plate DISENGAGE from the tooth → no tipping force → kinder to abutment. This is the biomechanical advantage of the RPI (Rest-Proximal plate-I bar) design. Distal rest = torque; Mesial rest = no torque.
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