title: Retention forces of spherical attachments as a function of implant and matrix angulation in mandibular overdentures: an in vitro study.
abstract: BACKGROUND
  Nonparallel implants, when used to retain overdentures, may present a restorative challenge, particularly when using attachments. Premature wear of the components and loss of retention may be observed, resulting in increased maintenance.
  
  
  OBJECTIVE
  The purpose of this study was to compare the retentive behavior, over time, of spherical attachments when used in nonparallel and parallel implant scenarios in a cyclic testing mode.
  
  
  METHODS
  Thirty sets of 2-implant-supported overdenture models were evaluated: 4.0 x 13-mm implants (Astra-Tech Osseospeed) with 1.5-mm ball abutments (Astra-Tech) were used as the intraoral analog to the implants, and spherical attachments (Preci Clix) were used as the overdenture analog. Five different attachment and implant-abutment complex angulations were evaluated. Angulation was determined by deviation from the vertical reference plane. The groups consisted of the following: Group 0-0, 0-degree implants/0-degree attachments; Group 10-0, 10-degree implants/0-degree attachments; Group 15-0, 15-degree implants/0-degree attachments; Group 10-10, 10-degree implants/10-degree attachments; and Group 15-15, 15-degree implants/15-degree attachments. The specimens were subjected to cyclic loading (3500 cycles). Peak and valley retention loads were recorded at the first pull and then after every 100 cycles; therefore, 36 data points per specimen were recorded. Nonparametric analyses followed by post hoc analyses were conducted to test for differences in median peak load among groups (alpha=.05).
  
  
  RESULTS
  Peak load to dislodgment values for all groups ranged from 11.43 N to 23.56 N. Group 0-0 had the highest median retention value overall, 21.3 N, and Group 15-15 had the lowest median value, 17.3 N. Nonparametric analyses showed significant differences between Groups 0-0 and 15-15 (P=.014); and 10-0 and 15-15 (P=.002).
  
  
  CONCLUSIONS
  Within the limitations of this study, it was observed that there was a decrease in retention in the groups with 30-degree divergent implants and divergent attachments compared to the groups with parallel implants and parallel attachments. In general, retention varied from 11 N to 23 N, and attachment retention stabilized after initial loss in most groups.
authors.affiliation: Prosthodontics Department, University of Texas Dental Branch at Houston, TX 77030, USA. Sergio.Ortegon@uth.tmc.edu
authors.forename: Sergio M
authors.initials: SM
authors.lastname: Ortegón
authors.forename: Geoffrey A
authors.initials: GA
authors.lastname: Thompson
authors.forename: John R
authors.initials: JR
authors.lastname: Agar
authors.forename: Thomas D
authors.initials: TD
authors.lastname: Taylor
authors.forename: Dimitri
authors.initials: D
authors.lastname: Perdikis