This study will investigate the use of highly esthetic, second and third generation multi-layer zirconia crown materials to restore removable partial denture abutment teeth. All performed treatment will be the standard of care and to the usual and customary standards used in United states dental clinics for crown and RPD procedures. Treatment subjects will have RPD abutment teeth restored with either Noritake Katana STML (anterior teeth) or HTML (premolars and molars) zirconia crowns, and periodically evaluated for 60 months following RPD delivery. Outcomes will be compared to a similarly treated control group restored with metal, metal-ceramic, or a combination of metal and metal-ceramic crowns.

Following informed consent, subjects will be randomly assigned using an internet program (https://www.randomizer.org/) to either a treatment (zirconia abutment tooth crown) or control (metal or metal-ceramic abutment tooth crown) group. Using information from the oral examination, articulated diagnostic casts will be evaluated , the RPD design confirmed, and the need for a crown on one or more abutment teeth verified. Enrolled subjects will be given oral hygiene instruction at the beginning of the study. They will also be instructed to brush their teeth twice daily using an OTC fluoride dentifrice of their choice. They will also be asked to use a 0.05% NaF oral rinse for 1 minute daily.

Abutment teeth must be in function with the opposing arch and vital at the beginning of the study. Vitality will be be determined using a synthesis of history, percussion, palpation and pulp testing. Pulp tests will be conducted using cold and an EPT, and the facial, lingual and occlusal (incisal) surfaces of all abutment teeth will be tested for responsiveness. If abutment tooth vitality is confirmed, teeth requiring surveyed crowns will be prepared and restored using standardized clinical and laboratory guidelines.

Abutment teeth restored with metal and zirconia crowns will be prepared and restored using the following clinical and laboratory guidelines. The margin will be a circumferential chamfer prepared to a depth of 0.5 mm with a rounded internal line angle and a 90° cavosurface exit angle. Margin height will be at or slightly coronal to the free gingival margin where possible. Axial surfaces will be prepared with a total occlusal convergence of \>6° but not to exceed 20°. Incisal and facial surfaces will be reduced 0.7-1.0 mm. Functional surfaces will be reduced to 1.0 mm of opposing tooth clearance with the exception of under rest seats, where opposing tooth clearance will be 2.0 mm. At completion, the prepared tooth should have a height to base ratio of 0.4. If inadequate retention and resistance form is identified following preparation of axial walls, supplementary grooves will be added, the number and location of which are at the discretion of the investigator. Final impressions will be made using PVS in a custom tray (Extrude) and poured in type V dental stone (Jadestone). Following fabrication, the working cast will be articulated, tripoded, and the die(s) sectioned, trimmed and scanned (3Shape D2000 laboratory scanner or equivalent). The crown(s) will be waxed to full contour. Rest seats, undercuts and guide planes will then be developed in wax. Once the waxup has been surveyed and finalized, it will be secured to the scanning platform and a new scan performed with the waxup in place. The data file with the die scan will be merged with the file that contains the waxup. The merged file will then be transmitted to a designated production facility where the zirconia crown(s) will be milled. Canines will be milled using Noritake Katana STML zirconia and premolars and molars will be milled using Noritake Katana HTML zirconia. Once the crown(s) are returned and the margins, contacts, occlusion and contours clinically verified, they will be luted using a self-adhesive resin-based cement (RelyX Unicem 2).

Porcelain-fused-to-metal (PFM) crowns will be prepared and restored using a standard protocol utilizing the following guidelines. Posterior crowns will have metal occlusal surfaces with the porcelain-metal junction on the occlusal surface at half the distance between the central groove and the buccal cusp tip. Mesial, distal and lingual surfaces will be in metal, and the crown will have a disappearing metal margin on the facial surface. The facial preparation from mesiofacial to distofacial line angles will be a heavy chamfer or modified shoulder 1.0-1.2 mm in depth with a rounded internal line angle and a 90° cavosurface exit angle. Mesial, distal and lingual chamfer margins will be prepared to a horizontal depth of 0.5 mm. Facial margin height will be at or slightly apical to the free gingival margin. Mesial, distal and lingual margin height will be at or slightly coronal to the free gingival margin if possible. Functional surfaces will be prepared with opposing tooth clearance of 1.5 mm with the exception of under rest seats where opposing tooth clearance will be 2.0 mm. Nonfunctional cusp reduction will be 1.0 mm. Final impressions will be made using PVS (Extrude) in a custom tray and poured in type V dental stone (Jadestone). Following working cast fabrication it will be articulated, tripoded, and the die(s) prepared for conventional laboratory crown fabrication. Conventional (all-metal and PFM) surveyed crowns will be fabricated using noble and high-noble casting alloys and PFM crowns will use feldspathic porcelain as a veneering ceramic. Once the crown(s) are returned and the margins, contacts, occlusion and contours clinically verified, they will be luted using a self-adhesive resin-based cement (RelyX Unicem 2).

Qualifying RPD designs may be Kennedy class I-IV with up to two modification spaces, and will be designed using a standardized protocol. Maxillary major connectors may consist of a complete palatal plate, modified palatal plate, anterior-posterior palatal strap or palatal strap. Mandibular major connectors will consist of either a lingual plate or a lingual bar. Frameworks will be fabricated from nickel-chrome alloy (Ticonium), the denture bases acrylic resin (Lucitone 199), and artificial teeth will be DENTSPLY Trubyte IPN Portrait. To meet the definition of an RPD abutment tooth it must host a direct retainer consisting of an occlusal or cingulum rest, a proximal plate and a retentive clasp. Reciprocation must be provided in the form of a plate, reciprocating clasp or minor connector and rest. The plan for occlusion will be based upon the number and distribution of remaining natural teeth. If an arch opposing the RPD is edentulous and restored by a removable complete denture, then natural and artificial teeth will be arranged in bilateral balance. If anterior guidance is present on natural teeth in both arches it will be preserved so that artificial RPD teeth contact opposing teeth in maximum intercuspation only.

Clinical assessments, procedures and annual examinations will be performed in the University of Kentucky College of Dentistry second, third, and fourth floor student clinics. Clinical procedures will be performed by third and fourth year dental students, and clinical supervision for these procedures will be provided by licensed, calibrated investigators.