Background: Axial displacement is a unique biomechanical feature of internal conical connection (ICC) implants. ICC abutments lacking structures for vertical stop might sink axially into the inner surface of the implant. Axial displacement is caused by factors — machining tolerance and the settling and wedge effects. Machining tolerance and the settling effect are inevitable phenomena in both external and ICC implants. However, the wedge effect, which induces concentrated axial compressive force in the direction of abutment sinking, is observed only in ICC implants. Although axial displacement in screw-type prostheses has been reported previously, no information is available regarding axial displacement in cement-retained prostheses. Differences in implant–abutment connection type have been known to result in variations in axial displacement in various clinical situations. Three typical implant–abutment connections—the external-butt (EXT), internal-conical (INT), and internal-butt (INT-Butt) connections—have been known to provide satisfactory results in clinical practice.
Aim/Hypothesis:The purpose of this study was to quantitatively evaluate axial displacement in three different implant–abutment connections in cement-retained monolithic zirconia and polymethylmethacrylate (PMMA) provisional prostheses with the computer aided design–computer aided manufacturing (CAD-CAM) custom abutments.
Materials and Methods: Three types of implants with different connections—external and internal butt connection implants and an internal conical connection implant—were evaluated. In each implant group, implant replicas were embedded vertically and angulated at 15°. Sixty titanium custom abutments were fabricated using a CAD-CAM system. The geometries and surface morphologies of custom and ready-made
abutments were comparatively evaluated by scanning electron microscopy. Micro-gaps in abutment connections were scanned by
optical microscopy during screw tightening, re-tightening, and cyclic loading. Zirconia and provisional acrylic prostheses were fabricated and evaluated for internal gap by an indirect silicone technique. Cemented prostheses on abutments were mounted on a universal testing machine and subjected to 250 N sine wave cyclic loads. Cumulative axial displacement was measured at loading
periods of 3, 10, 100, and 1,000,000 and analyzed by repeated measures analysis of variance (ANOVA). Internal gap width and net
change in axial displacement were analyzed by multi-way ANOVA.
Results: There were no significant differences in internal gap widths of prostheses according to implant or prosthesis type. However,
measurement site and implant angulation were influencing factors on internal gap. Surface geometries and morphologies of custom abutments varied according to the implant–abutment connection. Machining tolerance control and precision fit were difficult to achieve with the custom CAD-CAM abutment. The internal and external conical connection implants exhibited the greatest and lowest axial displacements, respectively. Acrylic prostheses exhibited greater axial displacement than zirconia prostheses. The vertically positioned implant group exhibited greater axial displacement than the angulated implant group in the early cyclic loading periods. Axial displacement in the late cyclic loading period was relatively low.
Conclusions and Clinical implications:
Custom abutments for cement-retained prostheses exhibited greater axial displacement than screw-retained prostheses. Axial displacement mostly occurred in the early loading period and was self-limited. Axial displacement should be managed by provisional restoration, with consideration of implant connection type and angulation, abutment fabrication method, and occlusal force of the patient.

Axial displacements of cemented prosthesis in different implant-abutment connections