Prosthodontics & Occlusion · single question
Same RPD case. To improve the prognosis of the RPD design, which modification should be made?
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Correct answer: B. Change the rest from distal to mesial
In RPD design, a MESIAL rest on an abutment adjacent to a free-end saddle is preferred over a distal rest because: (1) Mesial rest transmits forces along the long axis of the tooth (better biomechanics); (2) Distal rest creates a fulcrum effect — when occlusal load is applied to the free-end saddle, a distal rest acts as a fulcrum that TORQUES the abutment tooth distally and apically (PDL widening = evidence of this torque). Changing to a mesial rest improves load distribution and protects the abutment.
- A. INCORRECT. Splinting teeth 20 and 21 could help distribute forces but is more invasive (requires crowns) and doesn't address the fundamental biomechanical problem of the distal rest/fulcrum effect.
- B. CORRECT. For a distal extension (Class I or II) RPD, the MESIAL rest is biomechanically superior to the distal rest. A mesial rest directs forces more along the tooth's long axis and reduces the torquing/fulcrum effect that damages the abutment's PDL.
- C. INCORRECT. Keeping the distal attachment perpetuates the torquing forces that are causing the PDL widening and mobility — this would worsen the prognosis.
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