A mandibular distal-extension removable partial denture has been in service for several years. The terminal abutment, a second premolar carrying a rest and clasp assembly at its distal, now shows widening of the periodontal ligament space along the distal aspect of the root, deepened distal probing and increasing mobility. What is the most likely explanation for this pattern?

Correct answer: D. Torquing of the abutment by the distal-extension base rotating about a distal rest

• A. A single tooth deteriorating out of step with the rest of the mouth points to a LOCAL cause, and here the local cause is handed to you in the stem — a prosthesis loading that specific tooth in a specific direction. Generalized disease produces generalized findings.
• B. Pulpal necrosis produces a radiolucency at the APEX or at a lateral canal, not a uniform widening of the PDL space along the length of the root. The two can coexist in a combined lesion, but the described pattern is that of occlusal trauma.
• C. The periodontal ligament space does not widen as a normal consequence of ageing. Widening is a response to increased or altered functional load and is a genuine pathologic finding.
• D. CORRECT. A distal-extension base is supported at one end by a tooth and at the other by resilient mucosa, which is roughly an order of magnitude more displaceable. Under load the base sinks posteriorly and ROTATES about the fulcrum line. When the rest is placed on the DISTAL of the terminal abutment, that rest becomes the fulcrum and the clasp arm drives the tooth distally — a tipping, torquing force. The periodontium responds to that repetitive lateral load exactly as described: a WIDENED PDL space, most pronounced on the side toward which the tooth is being tipped, with increased mobility. This is why RPI and RPA designs place the rest on the MESIAL of a distal-extension abutment, so the base rotates away from rather than into the tooth.

Note: Occlusal trauma widens the PDL and increases mobility, but on its own it does NOT cause attachment loss — that requires bacterial plaque. When you see PDL widening plus a deep pocket plus mobility on an RPD abutment, read it as two processes stacked: trauma from occlusion superimposed on plaque-induced periodontitis. Both have to be addressed.