Accuracy of Subgingival Finish Line Reproduction in Intraoral Scanning Versus Conventional Impressions: A Prospective Clinical Study With and Without Gingival Retraction

HYPOTHESIS:

The null hypothesis states that the accuracy of subgingival finish line capture is not influenced by the type of impression technique used or the application of subgingival retraction. The research hypothesis states that differences exist in the accuracy of intraoral scanners depending on the impression technique used and the application of gingival retraction.

AIM OF THE STUDY:

The current study aims to evaluate the accuracy of intraoral scanners in recording subgingival finish lines by evaluating the three critical parameters, i.e. the depth and width of the finish line and the extent of gingival retraction of the prepared tooth with and without gingival retraction for fabrication of fixed prostheses in comparison with the conventional impression techniques.

OBJECTIVES:

•       To evaluate the accuracy of intraoral scanner in relation to the width of finish line with and without gingival retraction.

•       To evaluate the accuracy of intraoral scanner in relation to the depth of finish line with and without gingival retraction.

•       To evaluate the amount of gingival retraction.

 METHODOLOGY:

Study Design:

A Randomised Controlled Trial

 Study setting: Goa Dental College and Hospital

 Study population:

Patients indicated for single full crown prosthesis with subgingival finish line

Inclusion Criteria:

1. Age – between 18-50 years
2. Requirement for a single full-coverage crown on a mandibular first molar
3. Periodontally healthy (Whole mouth bleeding sites <10%, full mouth plaque score (FMPS) <20%, no loss of attachment)
4. General good systemic health

 Exclusion Criteria:

1. Presence of active infection or severe periodontal inflammation

2.No restorations in cervical 1/3rd of teeth

3. History of chronic systemic disease

4. Current or past smoking habit

5. Presence of parafunctional habits

 Permissions to be taken:

Written consent will be obtained from patients participating in the study

  Methodology in brief

Tooth preparation and temporization

Pretreatment radiographs and intraoral photographs will be obtained for all participants. A polyvinyl siloxane putty index will be fabricated for the construction of temporary crowns. Tooth preparation will be performed according to the planned restoration, with a standardized shoulder finish line placed on the buccal surface with all finish lines located 0.5–1.0 mm subgingivally.

The prepared abutments will be temporized with a bis-acrylic resin using non-eugenol temporary cement for a duration of 2 weeks. To avoid tissue fatigue, a minimum interval of 7 days will be maintained between post-displacement impressions within the same participant, following a Latin block design. Periodontal health will be re-evaluated and confirmed before impression making. Four impression techniques will be performed: digital without retraction (DWR), digital with retraction (DR), conventional without retraction (CWR), and conventional with retraction (CR)

Impression procedures

Conventional impressions will be made using polyvinyl siloxane material with a two-phase, two-step technique using a sulcus tip. A polyethylene sheet will be used as a spacer. Casts of conventional impressions will be scanned with a laboratory scanner and STL files generated.

(b)

Intraoral scans

Digital impressions will be obtained using an intraoral scanner following the manufacturer’s recommended protocol. The scanning sequence includes the maxillary arch, the mandibular arch, and bite registration. All scans will be saved in standard tessellation language (STL) format.

Gingival retraction procedure

A knitted gingival displacement cord will be placed in the gingival sulcus below the preparation finish line. The cord will be pre-soaked in a hemostatic agent and inserted using a gingival cord. A single-cord technique will be employed.

 Measurements and data collection

All STL files will be processed by a single dental master technician using Exocad software. All four STL files will be superimposed using best fit algorithm and sectioned mesiodistally and buccolingually. Measurements were performed at three locations—mid-buccal (B), mesial (M), and distal (D)—by a blinded evaluator. Three linear distances will be recorded: (1) between the most proximal apical and the most distal points of the finish line; (2) between the finish line and the most subgingival part of the tooth; and (3) between the most distal point of the finish line and the gingiva.

Expected Outcome:

The primary outcome is that the accuracy of intraoral scanner will be comparable to conventional impression in recording the subgingival width, depth and amount of retraction. Gingival retraction will improve the accuracy.