The influence of undercut depths on the accuracy of casts poured from irreversible hydrocolloid impression materials

Vidya Latchmi Mudliar; Kai Chun Li; Ludwig Jansen van Vuuren; John Neil Waddell

doi:10.1016/j.heliyon.2019.e03143

The influence of undercut depths on the accuracy of casts poured from irreversible hydrocolloid impression materials

Heliyon. 2020 Jan 14;6(1):e03143. doi: 10.1016/j.heliyon.2019.e03143. eCollection 2020 Jan.

Authors

Vidya Latchmi Mudliar¹, Kai Chun Li², Ludwig Jansen van Vuuren², John Neil Waddell²

Affiliations

¹ Department of Oral Rehabilitation, School of Dentistry and Oral Health, College of Medicine, Nursing and Health Science, Fiji National University, Suva, Fiji.
² Sir John Walsh Research Institute, Faculty of Dentistry, University of Otago, PO Box 56, Dunedin 9054, New Zealand.

Abstract

Objectives: To evaluate the elastic recovery of a typical irreversible hydrocolloid impression material in terms of the amount of undercut identified and additional thickness of spacer provided before custom tray fabrication.

Methods: This study surveyed the undercut depths of the teeth surfaces and lingual sulci of one hundred partially dentate mandibular dental casts which were subsequently scanned and the data exported as STL files. A typical cast from each undercut category (0.5mm, 1mm, 1.5mm, 2mm, 2.5mm, 3mm, 4mm, 5mm) was 3D printed. Three custom trays were constructed with spacer thicknesses representing 20%, 30% and 50% compression against the wall of the custom trays on removal, impressions recorded, and casts fabricated in dental stone. Measurements for elastic recovery of irreversible hydrocolloid were done using an internal caliper and a digital caliper. All measurements were done by the principal investigator. Non-parametric analyses were used for comparisons of the undercut values between the 3D printed and poured casts to determine the amount of elastic recovery of the irreversible hydrocolloid impression material.

Results: There was no statistically significant difference in measurements between the original 3D printed casts and the poured casts, irrespective of percentage compression (p $>$ 0.05).

Conclusions: In removable partial denture construction, casts should be blocked out parallel to the path of insertion and then an additional 3mm of spacer applied as a standard technique before custom tray fabrication to ensure elastic recovery of the irreversible hydrocolloid impression.

Clinical implications: In severe undercut situations, accurate impressions can be taken using irreversible hydrocolloid by blocking out all undercut surfaces on the teeth and lingual sulci before adapting additional three-millimetre wax spacer on casts when constructing custom trays thereby ensuring accurate and reliable impressions for removable prosthesis construction.

Keywords: Accuracy of alginate impressions; Anatomy; Dental materials; Dentistry; Materials characterization; Materials science; Prosthetic dentistry; Special tray design; Tooth undercuts; Viscoelastic recovery.