A Prospective Clinical Trial Evaluating Outcomes of Surveyed Zirconia Crowns

Zirconia has shown to be a suitable substitute for metal ceramic crowns when comparing survival, biological and technical complications. Evidence demonstrating the outcomes of surveyed zirconia crowns on removable partial denture abutments is unavailable. Zirconia as a substitute for metal ceramic is being currently used for fabricating surveyed crowns although quantitative data evaluating its efficacy is sparse. The purpose of this prospective clinical trial is to evaluate clinical outcomes when zirconia is used as a full coverage restoration on removable partial denture abutments. The study will be designed as a 3-5 year prospective observational trial in the University of Toronto undergraduate clinic in a controlled environment. Outcomes measures will include survival, biological and technical complications. Descriptive statistics will be used to summarize patient characteristics and outcomes and for categorical variables frequency and percentages will be reported. A level of significance 0.05 will be used for inferential analysis, with p-values \< 0.05 reported as statistically significant. It is expected that zirconia will demonstrate similar outcomes to traditional metal ceramic surveyed crowns.

Research data on the survival of zirconia crowns as RPD abutments is limited although anecdotally these are being used regularly by dentists in clinical practice. To our knowledge a clinical trial evaluating the outcomes of surveyed monolithic zirconia crowns as RPD abutments has not been investigated previously. We propose to evaluate the outcomes of surveyed zirconia crowns as RPD abutments in a prospective trial and compare the survival, biological and technical complications with alternative materials historically used for surveyed crowns.

Aims of Project

1. To evaluate biological and technical complications of surveyed zirconia crowns on RPD abutments for 3- 5 years following insertion.
2. To evaluate survival of surveyed zirconia crowns for 3- 5 years following insertion.

Materials and Methods

Research Design and Overview: The study will be designed as a 3-5 year prospective observational trial in the University of Toronto undergraduate clinic in a controlled environment where procedures are standardized for case selection and execution. All patients requiring surveyed crowns in the academic years 2024 and 2025 will be approached to participate in the trial. Patients will be recalled every year for 3-5 years following insertion and a check list will be used to assess the pre insertion and outcome variables. Clinical coordinators supervising students and clinical examiners performing outcome assessments at yearly follow up will be trained and their measurements calibrated for inter and intra observer variability to ensure uniformity. The research proposal will be submitted to the University Research Oversight and Compliance office for ethics approval.

Participant recruitment: Patient recruitment will be from the patient population that presents to the undergraduate dental clinic requiring removable dental prosthesis to replace missing teeth. Selection of abutment teeth for zirconia surveyed crowns will be similar to that currently utilized when selecting abutment teeth to be restored with either porcelain fused to metal or full metal surveyed crowns for tissue and tooth supported cast removable partial dentures. These will include but are not limited to abutment teeth which require full coverage due to large restorations, posterior endodontically treated teeth with inadequate marginal ridge support and abutment teeth which require axial modification for retentive and or stability elements of the denture.

Sample Size Calculation: This is an exploratory study and will include all patients in the undergraduate clinic requiring surveyed crowns. We expect approximately 25-50 surveyed crowns per year, based on numerical data for metal ceramic surveyed crowns done in previous years.

Methods: To avoid the inherent fractures associated with veneered zirconia surveyed crowns, monolithic zirconia a combination of 3Y-TZP and 5Y-TZP (IPS e.max ZirCAD Prime, Ivoclar AG) with a flexural strength of 1200MPa will be utilized. Tooth preparation will include occlusal reduction of 1.0 to 1.5mm and rounded line angles for fabrication of the rest seats within the zirconia crown and buccal and lingual reduction of 1 to 1.5mm to allow for adequate buccal and lingual contour to accommodate retentive clasp arms and minor connectors within the crown. Impressions for zirconia crowns will be made with polyvinyl siloxane in a full arch custom tray. The models will be surveyed to produce CAD CAM crowns on RPD abutments which will incorporate design elements such as heights of contour and rest seats. Once cemented and following cast removable partial denture insertion, patients will be followed every year up for 3-5 years to assess outcomes.

Data Collection:

Independent Variables: Variables to be collected will include demographic data, age, sex and smoking status. Abutment tooth, reason for crown (including endodontic treatment, large restoration, replacement crown, change of axial morphology, tooth sensitivity), pulp vitality, history of previous endodontic treatment, presence of post core if any, type of core, status of opposing tooth whether present or absent or opposed by removable or fixed prosthesis, presence or absence of proximal contacts and type of cement used at insertion. Post insertion variables will include any and all follow up dates, and procedures performed at follow up.

Statistical Analysis: Descriptive statistics will be used to summarize patient characteristics and outcomes. For categorical variables, the investigators will report frequency and percentages. Numerical variables will be summarized using mean, standard deviation (or median/IQR for skewed distributions). For primary clinical outcomes, the investigators will also report 95% confidence intervals.

Sample size permitting, the investigators may also examine the association between baseline/demographic patient characteristics and outcomes using inferential analysis (chi-square tests, logistic/linear regression models). A level of significance 0.05 will be used for inferential analysis, with p-values \< 0.05 reported as statistically significant.

Eligibility Criteria:
Inclusion Criteria:

* Inclusion criteria: Patient recruitment will be from the adult patient population that presents to the undergraduate/graduate prosthodontic dental clinic requiring new tooth or tissue supported removable cast dental prostheses to replace missing teeth for the following situations:

  * In Kennedy Class 1 and 2 tisuue supported RPD's-Terminal abutments 3,4,5,6
  * In Kennedy Class 3 and 4 tooth supported RPD's- any abutment teeth selected will need to be documented if selected as abutments for a surveyed crown.
  * Selection of abutment teeth for zirconia surveyed crowns will be similar to that currently utilized when selecting cast RPD abutments to be restored with either porcelain fused to metal or full metal surveyed crowns for tissue and tooth supported cast RPD's. Identifiable data will include:
  * RPD abutment teeth which may benefit from full coverage due to large restorations.
  * Posterior endodontically treated teeth with inadequate marginal ridge support and
  * Abutment teeth which require axial modification for RPD retentive and or stability elements.
  * In situations where esthetic improvement is desirable
  * Reasons for selection wil be documented.

Exclusion Criteria:

* Crowns that need to be splinted; patients that decline a surveyed crown; patients under the age of 18 years; patients that decline to participate in the study.