Digital Implant Prosthetics: CAD/CAM Workflow, Materials for Implant Restorations and Prosthetic Connections

The digital workflow in implant prosthetics has replaced traditional polysulfide/polyvinylsiloxane elastomer impressions as the reference technique in high-volume practices. Intraoral scanning with scan bodies placed on implants (or on analogs at the lab stage) allows capturing the three-dimensional position of the implant abutment with precision depending on scanner technology, scan body geometry, and operator technique. Papaspyridakos et al.'s systematic review (Int J Oral Maxillofac Implants, 2016) documented average intraoral scan deviations compared to open-tray impressions of 8-45 µm for single implants and 50-120 µm for partial arches — clinically acceptable values for single-unit restorations and worth careful consideration for multi-implant frameworks where error accumulates.

The scan body is the device that transfers the implant abutment's spatial position to the digital scan. Each implant system has dedicated scan bodies, whose geometry is registered in the CAD/CAM software library (Exocad®, 3Shape Dental System®, Cerec®). The quality of the fit between the scan body and the implant abutment — no play, no rocking during scanning, matte surface to avoid reflection artifacts — affects final accuracy. In cases with angled implants or in hard-to-access posterior areas, scanning with a digitized impression coping (scan rim) or a two-stage approach (intraoperative scan + extraoral scan on a physical model with analogs) can ensure better accuracy than direct intraoral scanning.

Materials for implant restorations are distinguished by clinical application and mechanical characteristics. High-translucency monolithic zirconia (HT-Zirconia, Y₂O₃ content 4-5 mol%, flexural strength 700-900 MPa) is the material of choice for single posterior restorations and multi-unit frameworks, combining adequate mechanical strength (superior to traditional feldspathic ceramic) with acceptable aesthetics and documented biocompatibility. Multilayer (gradient) zirconia allows making monolithic crowns without layering firing, with a cervical-to-incisal translucency gradient simulating the natural tooth. PEEK (polyether ether ketone) is used as a material for long-term implant provisionals and for secondary implant frameworks, with an elastic modulus (3-4 GPa) closer to cortical bone (15-20 GPa) than titanium (110 GPa) or zirconia (200 GPa) — a characteristic theoretically favorable for load transfer to peri-implant bone.

The implant-restoration prosthetic connection is the critical biomechanical point of the entire implant prosthetic system. Internal cone-morse connections (11°, 8°) leverage the frictional force between overlapping conical surfaces to create a seal resisting microbial leakage and reducing abutment micromovement — the main factor in peri-implant crestal bone loss in the first years of function. The external hexagon connection (original Brånemark) is biomechanically disadvantageous for single restorations: the flat-on-flat assembly offers no intrinsic resistance to lateral loads, which are entirely absorbed by the abutment screw, with a high risk of loosening. Abutment screw tightening torque — specified by the manufacturer based on design (25-35 N/cm for titanium screws, 15-20 N/cm for gold screws) — must be applied with a calibrated torque wrench after confirming abutment fit, with radiographic verification of implant-abutment contact.

Designing the emergence profile in CAD software — the shape of the restoration in the implant-to-soft-tissue transition zone — is where the quality of an implant restoration is decisively distinguished from simply "putting a crown on an abutment". The ideal emergence profile is concave in the submucosal portion (to avoid compressing the mucosa and allow circulation) and progressively widens toward the coronal contour to reach the natural tooth's shape at the free gingival margin. CAD design allows optimizing this profile mathematically, but clinical verification with the implant provisional — which "trains" the tissue before the definitive restoration — remains the gold standard for assessing individual mucosal response.

Prosthetic complications on implant restorations — screw loosening (incidence 5-15% at 5 years for single implants in aesthetic areas), framework fracture (<2% for correctly sized zirconia frameworks), and veneering ceramic chipping (12-30% at 10 years for layered zirconia crowns vs. <5% for monolithic ones) — have documented frequency in systematic reviews with 10-year follow-up. The shift toward monolithic HT-zirconia restorations for posterior areas significantly reduces chipping risk compared to layered ceramic-on-zirconia or porcelain-fused-to-metal crowns, with acceptable aesthetic trade-offs in non-aesthetic areas. Maintaining screw torque at every annual check-up visit is recommended — even for cone-morse connections, where torque checking prevents bacterial buildup in the abutment-implant micro-gap.