Influence of casting modification on the retention of implant crowns

Objective: Due to adverse implant inclination, the use of pre-angled or custom abutments may be necessary in the restorative phase. These may have a compromised resistance and retention form resulting in loosening/loss of the restoration. The objective of this study was to investigate whether extending a casting into the screw access channel improves retention on implant abutments with various modifications. Methods: Replace Select Straight abutments (Nobel Biocare) received set modifications (buccal wall removal at 15, 22 and 30 degrees) using a milling machine, controls were unmodified. Two castings were fabricated for each of the modified abutments, one with and one without an extension into the screw access channel. The casting without an extension was cemented having either occluded the channel with PTFE or protected the screw head with PTFE and allowed Tempbond to fill the channel. The casting with an extension was cemented with Tempbond having protected the screw head with PTFE. Following cementation with Tempbond under standardised conditions, the castings were pulled off the abutments using an Instrom machine and the peak removal force was recorded. Results: (1) Extending the casting into the screw access channel significantly (p<0.001) increased the peak load of removal compared with PTFE or Tempbond in the channel. (2) There was no significant difference in peak load of removal when the screw access channel was filled with either PTFE or Tempbond (p = 1.000). (3) Modification of the abutment had a significant effect (p<0.001) on peak removal force except between an unmodified abutment and one with a 15 degree modification. Conclusion: The increased load required to remove a casting which extends into the screw access channel of an implant abutment may compensate for any loss in retention which occurs through modification of the abutment.