Forces Transmitted to Abutment Teeth in Distal Extension Removable Partial Denture—Impact of Indirect Retention

Background/Objectives: Stability, retention, and support are removable partial denture (RPD) biomechanical principles. The literature shows contradictory opinions on indirect retention in RPDs, but no solid scientific evidence exists. This in vitro research aims to analyze indirect retainers’ (IRs) influence on forces transmitted to abutment teeth of a Kennedy Class I mandibular RPD. Methods: Bilateral distal-extension mandibular RPDs—differing only in the presence or absence of an IR on tooth 44 (IR model vs. nonIR model, respectively)—were installed on an acrylic master model. Tensile forces were applied perpendicularly to the occlusal plane on the longest free-end saddle’s distal aspect. Electronic speckle pattern interferometry (ESPI) measurements were obtained with and without an IR. The three-dimensional out-of-plane displacements of both models were acquired. Results: Abutment teeth 46 and 47 contralateral to the longest distal extension suffered more deformation under displacement forces when an IR was used. In turn, the IR’s influence on the deformation values of the abutment tooth 34 adjacent to the larger edentulous area depended on the intensity of the tensile force exerted: low-intensity forces resulted in reduced deformation, while higher-intensity forces resulted in higher deformation. Conclusions: This study’s findings indicate that indirect retention promotes better tensile force distribution in the existent teeth. However, they also question the IR’s role in protecting abutment teeth against excessive torque forces. This study’s preliminary results highlight the need for research on indirect retention principles using new methodologies, namely, in silico and ex vivo studies, and their experimental and clinical validation.