Effects of posterior ridge resorption and attachment wear on periimplant strain in mandibular two-implant-supported overdentures

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Abstract

Statement of problem.

Progressive resorption of the posterior residual ridge and attachment wear increase the rotational movement of mandibular 2-implant-supported overdentures (IODs). Limited information is available regarding the biomechanical effects of rotational movement on anterior implants during mastication.

Purpose.

The purpose of this in vitro investigation was to analyze the effects of posterior ridge resorption and attachment wear, using simulated IODs to examine periimplant strain changes under dynamic loading conditions.

Material and methods.

Two dental implants were positioned in the canine regions of a mandibular edentulous cast. Two mandibular edentulous models were fabricated from the cast by using a fixture-level impression technique, and IODs reinforced with a cobalt-chromium cast framework were prepared using the laboratory models. Two different types of stud attachments (Locator and O-ring) were connected to each paired model and to the IOD. Using a dynamic load of 100 N, continuous stress-strain changes were recorded under 3 conditions: the original condition of the paired laboratory models and the IODs, following a 1-mm reduction of the posterior residual ridge support, and after performing a fatigue test to simulate attachment wear after reduction of the posterior ridge support. After these measurements, a scanning electron microscope (SEM) was used to analyze attachment wear.

Results.

In all implants, the 1-mm posterior ridge reduction and attachment wear procedure did not remarkably elevate tensile forces compared with their original condition. All stress-strain curves showed phased strain changes caused by the rotational movement of the IODs. After the fatigue test, the shape of the stress-strain curve changed from a smooth curve to a polygonal line compared with that in the previous records. The Locator matrices showed more wear than the other attachment components.

Conclusions.

Neither attachment wear nor an increase of the IOD rotational movement caused by an approximately 1-mm decrease in the posterior mucosal support led to significant changes in the periimplant strain in IODs.

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