title: Retentive force of stress-breaking attachments on maxillary implant overdentures.
abstract: OBJECTIVE
  The purpose of this study was to investigate the effect of the hardness of resilient materials applied to the female parts of specialized ball attachments, and the diameter of ball abutments on the retention of an implant overdenture.
  
  
  METHODS
  A cast chrome-cobalt framework was mounted onto a maxillary edentulous model. Eight different ball abutments, with diameters of 2.5, 2.75, 3.0 and 3.25 mm, and with undercut (UC) of 0.5 and 1.0 mm, which was the difference between the ball diameter and the neck diameter, were screwed into implants which were connected to resilient materials with four different degrees of hardness (58, 68, 78 and 90) conforming to ISO7619. A universal testing machine was used to test the 32 different attachments (crosshead speed 3 mm/min). Three tests of 10 consecutive pull separations were done in both vertical and oblique directions. After 10 consecutive pull separations, the inner surfaces of the resilient materials were examined by a scanning electron microscope (SEM). The measured retentive forces were analyzed statistically using ANOVA and Tukey's tests.
  
  
  RESULTS
  Both the hardness of the resilient materials and the diameter of the ball abutments significantly affected the retentive force (P<0.05).
  
  
  CONCLUSIONS
  The designs with the diameter of the ball abutments ranging from 2.75 to 3.0 mm and UC of 0.5 mm, and with the hardness of the resilient materials ranging from 68 to 78, were considered to be suitable for this attachment system.
authors.affiliation: Complete Denture Prosthodontics, Department of Masticatory Function Rehabilitation, Division of Oral Health Sciences, Graduate School, Tokyo Medical and Dental University, Tokyo, Japan. ohore@tmd.ac.jp
authors.forename: Kei
authors.initials: K
authors.lastname: Ohya
authors.forename: Manabu
authors.initials: M
authors.lastname: Kanazawa
authors.forename: Shunsuke
authors.initials: S
authors.lastname: Minakuchi