Freedom of rotation of selected overdenture attachments: an in vitro study

Attachments for mandibular overdentures may not allow for adequate freedom of rotation.

The purpose of this study was to subject 3 commonly used stud types of resilient attachments to rotation and to measure and compare the degree of rotation that occurs before the attachments bind.

Three stud attachments (ERA, Locator Extended Range, O-Ring System) with their corresponding metal housings were evaluated, first for a single implant system and secondly for a 2-attachment parallel implant system for the mandibular arch. Both the manufacturers' most and least retentive components were tested in each system. In the Locator system, the medium retention (3 lbs) for the standard is the most retentive of the extended range. A total of 60 test specimens, 5 per group, were fabricated. The attachments were connected to a 115-mm rod and attached to a testing device composed of a base, implant analogs, and a vertical plate holding graph paper. A free-sliding plastic block, which supported the metal rod parallel to the base, was removed to release the rod. The distance the rod fell was photographed and input into a computer with software designed to measure linear movement. Three-way ANOVA with heteroscedasticity consistent standard errors was used for data analyses (α=.05).

Significant differences found among the most retentive components showed that the Locator had a greater freedom of rotation than the ERA (P=.001). The most retentive components in all systems showed that 1 attachment had a greater freedom of rotation than 2 (P=.005). When testing the least retentive components, all systems were significantly different (P<.003), with the freedom of rotation ranking being O-Ring System > Locator Extended Range > ERA. For all 3 systems, the least retentive components showed the most freedom (P≤.008).

The least retentive components offer greater rotation than the most retentive components for all attachment systems tested and for both single and double attachments. Using the most retentive components, the Locator Extended Range attachment allows greater rotation than does the ERA for both single and double attachments. Using the least retentive components, the ORS attachment allows the most rotation and ERA the least for both single and double attachments. Using the most retentive components, one attachment allows greater rotation than does 2 attachments for the attachment systems evaluated.