Axial and Nonaxial Retention Forces of Different Designs of Milled Bar Attachments for Maxillary Implant Overdentures: An In Vitro Study

Purpose: To compare the axial and nonaxial retention forces of different milled bar attachment designs for maxillary implant overdentures.
Materials and Methods: Four implants were placed in the canine and second premolar areas of an edentulous maxillary ridge model and connected to a cobalt-chromium milled bar either with or without Locator attachments. According to the type of bar and overlying housing, the following groups (n = 10 each) were investigated: group 1 (MWM) = milled bar without attachments and metal housing; group 2 (MWP) = milled bar without attachments and PEEK housing; group 3 (MAM) = milled bar with Locator attachments and metal housing; and group 4 (MAP) = milled bar with Locator attachments and PEEK housing. Axial and nonaxial (anterior, posterior, and lateral) retention forces were measured both at baseline and after wear simulation, then compared between groups and dislodging directions.
Results: MAM showed the highest axial (53.20 ± 2.28 N) and nonaxial (anterior [33.80 ± 1.48 N], posterior [37.60 ± 2.07 N], and lateral [34.40 ± 1.67 N]) retention forces at baseline, followed by MAP, then MWM, and MWP (P < .001). MAP showed the highest axial (42.80 ± 2.28 N) and nonaxial (anterior [24.00 ± 1.58 N], posterior [29.40 ± 2.07 N], and lateral [27.80 ± 1.64 N]) retention forces after wear simulation, followed by MAM, then MWP, and finally MWM (P < .001). MAM showed the highest axial (25.25 ± 2.45 N) and nonaxial (anterior [28.29 ± 4.03 N], posterior [24.40 ± 3.25 N], and lateral [25.55 ± 1.65 N]) retention loss, followed by MWM, then MAP, and finally MWP (P < .001). For all groups, the highest retention forces were noted with axial dislodging, followed by posterior dislodging, then lateral dislodging, and finally vertical dislodging (P < .001).
Conclusion: Milled bars with PEEK housings and Locator attachments for maxillary implant overdentures were associated with the highest axial and nonaxial retention forces after wear simulation, while milled bars with metal housing and no attachments showed the lowest forces. Milled bars with metal housing and attachments showed the highest retention loss, while milled bars with PEEK housing with no attachments showed retention gain.