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

Abstract

Abstract Background: Making accurate partial dentures is one of the greatest challenges in the treatment of patients with partial edentulous. Positional teeth movements create undercuts that must be accurately recorded in the impression material. Distortion of irreversible hydrocolloid (alginate) impressions has been reported as one of the several possible causes of an inaccurate master casts. Prostheses constructed on inaccurate master casts will not fit intraorally. Objectives: I. To measure and compare undercuts of teeth adjacent to the extraction sites and the lingual sulcus in partially dentate mandibular master casts. II. To evaluate the elastic recovery of a typical alginate impression material in terms of the amount of undercut identified and additional thickness of spacer provided prior to custom tray fabrication. III. To make recommendations of the thickness of spacer required when designing custom trays for taking definitive alginate impressions for partially dentate mandibular cast in relation undercuts depth. Materials and Methods: The undercut depths of the teeth and lingual sulci of partially edentulous mandibular dental stone casts (n=100) of adults, being routinely treated in a dental laboratory, were measured prior to the casts being digitally 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 and three custom trays were constructed for each cast with spacer thicknesses representing 20%, 30% and 50% compression of impression material against the wall of the custom tray on removal, impressions made, and casts fabricated in dental stone. Comparative measurements between the original 3D printed and stone casts were done by a single investigator. Reliability of measurements was assessed by means of Cohen’s kappa scores. Descriptive statistics were used to calculate the measurement of undercuts by age, gender and teeth surfaces. An Independent samples Mann-Whitney U test and Kruskal-Wallis test of the dataset were used to make comparisons of the undercut values between the original 3D printed casts and poured casts from the three compression impressions for each category of undercut, to establish the amount of elastic recovery of the alginate impression material. Results: For objective 1, the difference in teeth undercut depths compared by teeth surfaces and age were statistically significant ≤ 0.001). For objective 2, there was no statistical difference between the casts poured from the three compression impressions compared with the original 3D printed casts irrespective of percentage compression. Conclusion: In RPD construction, primary casts should be blocked out parallel to the path of insertion and then additional 3mm of spacer applied as a standard technique prior to custom tray fabrication to ensure elastic recovery of the alginate impression