Retentive Behavior of Locator versus Ball Attachments on Parallel versus Non-Parallel Implants

Several factors determine the retention force in removable implant-retained overdentures using prefabricated ball- or locator-type attachment systems. In this context, it was the goal of this in vitro study to examine the effect of implant angulation and female part alignment. Two model situations with two parallel or 12° tilted implants were fabricated onto which locator or ball attachments could be mounted. Simulated prostheses (n = 5) were made as antagonist parts and the assemblies were positioned in a universal testing machine for repeatedly (three times per female attachment) quantifying retention force. Statistical analysis was based on Shapiro–Wilk tests, Levene tests, ANOVAs, Tukey’s HSD tests and Welch t-tests, with the level of significance set at p < 0.05. With tilted implants, the retention force of locators was significantly diminished (p < 0.004) by at least 21%, while with ball attachments, a maximum reduction of 8% was noted, with only yellow inserts showing a significant difference (p = 0.040) compared with the parallel situation. Not aligning female retentive components on tilted implants for achieving a common path of insertion in ball anchors had only a minor effect on retentive force (6.5% increase as compared with aligned female parts), which was not statistically significant (p = 0.100). Not being able to establish a common path of insertion in locator attachments affects retention force. Ball anchors allow for aligning female retentive components, but due to the spherical structure of the male component this seems not even to be necessary.