# Retentive Behavior of Locator versus Ball Attachments on Parallel versus Non-Parallel Implants
## 4 Discussion  
This study, transferring an existing patient situation to an in vitro model, evaluated the effect of implant angulation and attachment type on initial retention force. Given the clinicians' preference [2] of rather using locators than ball anchors as prefabricated attachment systems and acknowledging the relevance of implant angulation as a critical factor for the success of implant-retained overdentures [5; 8], this in vitro study compared locators and ball anchors. Locators placed on tilted implants consistently showed lower retention values as compared with the situation of parallel implants while inserts with increasing retention values could still be differentiated. Alignment of female retentive parts was not possible with locator attachments, which may be seen as a disadvantage, ultimately causing a higher rate of maintenance [16]. The reduction in retention force seen in tilted situations can only be explained by the female attachment parts not fully embracing the male components in a divergent implant configuration. This seems to be consistent with a report by [PERSON] and coworkers [8], and this may also hold true for so-called extended-range female components. The ball anchors used here showed only a very minor effect of implant angulation on retention force. Not adjusting the female attachment part to achieve a common path of insertion also only had a minor effect on retention force, making the ball anchor a more forgiving device as compared with the locator. This may be seen as one reason for ball anchors to require a lower rate of maintenance as compared with locators [16]. The ball anchors used here represented a much smaller component as compared with the locator allowing for greater flexibility in terms of implant positioning and angulation. However, retention has to be realized and maintained by matching attachment surfaces, which are approximately 23% smaller as compared with locators. It can only be speculated how much effect the smaller component size would have on longevity, which is affected by several factors, including [18] tooth brushing [19], implant angulation [12; 13], as well as the material of the female component [6]. From a technical point of view, smaller  
Figure 5: Virtual assemblies of locator and ball anchors used in this study show the total surface areas of the male components (**a**) and the regions used for retaining female attachment parts (**b**). components are advantageous as the removable restorations become less bulky and less prone to fractures. Previous work in this field has shown that combinations of different materials, i.e., male attachment parts made from titanium and female parts made from plastic, were advantageous with respect to loss of retention [20]. The following limitations have to be considered when interpreting the findings presented. A simplistic but well-controllable in vitro study design was chosen in order to focus on one isolated parameter, i.e., implant angulation and its effect on retention. Other than using a parallel implant situation, only one specific tilted orientation of implants, which was based on a real patient, has been considered. Based on a retrospective measurement in a profile projector, a 12\({}^{\circ}\) divergence in the mesial-distal direction was determined. From a clinical point of view, deviations in the anterior-posterior direction are more common as a result of bone resorption in the anterior mandible. It can only be speculated how an alternative misalignment would have affected the study outcomes. The experiment was run under dry conditions, which certainly does not represent the clinical conditions [21; 22], and hence, no conclusions should be drawn on the long-term behavior of retentive plastic inserts [22] based on this experiment. A recent study analyzing the surface topography of attachment parts showed that the chemical composition of the surrounding medium also had a considerable impact on retention [23].