In vitro effect of load cycling on metal-ceramic cement- and screw-retained implant restorations

Statement of problem In implant prosthodontics, restorations can either be cement- or screw-retained. Aside from passivity of fit, esthetics, and retrievability, the occlusal opening of the screw-access hole (SAH) is a major difference between the 2 types of restorations. In veneered fixed partial dentures (FPDs), the SAH forms a discontinuity of the ceramic layer and may be an origin for chipping fractures. Purpose The objective of this study was to investigate whether more chipping fractures occur during dynamic loading on the occlusal surface of screw-retained ceramic-veneered implant FPDs than in cement-retained restorations. Material and methods Ten cement-retained and 10 screw-retained ceramic veneered 5-unit FPDs were manufactured for a 3-implant situation. A masticatory simulator was used for loading the occlusal surface of the FPDs to the implant positions for 20,000 cycles with a force of 100 N applied for 1 second followed by 1 second of no loading. For analysis, 3 clinicians evaluated the FPDs using a light microscope and a dental probe. The numbers of chipping fractures found were analyzed by means of a generalized linear model with Poisson response (α=.05). Results The investigators found significantly more chipping fractures in the group of screw-retained FPDs, the rate ratio of screw-retained FPDs versus cement-retained FPDs being greater than 1 with a P value of .0023. Conclusion The SAH of screw-retained implant FPDs forms a weak point of the ceramic layer.