The design and use of special trays in prosthodontics: guidelines to improve clinical effectiveness

Medical Device Regulations and custom-made device documentation: Ten frequently asked questions and their answers

Dental professionals who practice in the UK must follow the General Dental Council (GDC) Standards, one of which is to "find out about laws and regulations that affect your work and follow them". Dental professionals manufacture custom-made devices (CMDs) and the legislation that governs these devices has changed. Medical devices manufactured within the European Union (EU) were previously subject to Council Directive 93/42/EEC (Medical Devices Directive [MDD]), which was given effect in UK law by The Medical Devices Regulations 2002 (Statutory Instrument 2002/618 [UK MDR 2002]). Regulation (EU) 2017/745 (Medical Device Regulation [EU MDR]) was scheduled to replace the MDD on 26 May 2020, but this was postponed for one year due to the coronavirus (COVID-19) pandemic. In preparation for the UK's departure from the EU, the EU MDR was largely transposed into The Medical Devices (Amendment etc.) (EU Exit) Regulations 2019 (Statutory Instrument 2019/791 [UK MDR 2019]), which amended the UK MDR 2002, and was expected to come into effect on 1 January 2021 but a further amendment, The Medical Devices (Amendment etc.) (EU Exit) Regulations 2020 (Statutory Instrument 2020/1478 [UK MDR 2020]), removed these provisions. As of 1 January 2021, medical devices in Great Britain are subject to either the UK MDR 2002 (as amended) or the EU MDR (until 30 June 2023) while those in Northern Ireland must be manufactured in accordance with the EU MDR. This paper provides the answers to some key questions regarding the documentation that must be supplied with CMDs following these changes.

Custom double tray for a special impression technique for flabby maxillary residual ridge: Case report

The construction of a removable dental prosthesis for patients with compromised residual alveolar ridges is a challenge for prosthodontists. Flabby anterior ridges and hypermobile tissues in completely edentulous arches require special considerations during prosthetic management, especially when natural dentations remain on the opposite side. Previous studies have revealed that the displacement of flabby tissue can be reduced during impressions by controlling the applied forces via changes in factors such as the tray design, scraping of impression trays, impression material, window technique, and seating velocity of the impressions. However, there may still be some forces applied during impression or there is no even space because there are no trays supporting the vinyl polysiloxane (VPS) impression material in the open window area. Using a custom double tray with even gap between these trays and injecting light body impression material may eliminate these forces and provide accuracy due to even space for the impression material. This article is a clinical report of a patient who presented with an anterior flabby maxillary edentulous ridge opposing the remaining anterior natural teeth. A custom double tray was fabricated using the principle of the window technique. The gap between the double trays allows mucostatic impressions of flabby ridge tissue to be made with accuracy. The maxillary single denture, which was made with a custom double tray, satisfied the patient.

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

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.

Technique for adapting a spacer for a custom impression tray

A method of adapting a spacer for the custom trays used to make a definite impression for complete dentures is presented. The technique can be used under a variety of conditions and offers several advantages.

Comparative evaluation of pressure generated on a simulated maxillary oral analog by impression materials in custom trays of different spacer designs: An in vitro study

INTRODUCTION

Literature reveals that masticatory load on denture bearing tissues through complete dentures should be maximum on primary stress bearing areas and least on relief area in accordance with the histology of underlying tissues. A study to validate the existing beliefs was planned to compare the pressure on mucosa using selective pressure technique and minimal pressure technique, with the incorporation of two different impression materials utilizing the pressure sensors during secondary impression procedure.

MATERIALS AND METHODS

The study was performed using a maxillary analog. Three pressure sensors were imbedded in the oral analog, one in the mid palatine area and the other two in the right and left ridge crest. Custom trays of two different configurations were fabricated. The two impression materials tested were light body and zinc oxide eugenol. A total of 40 impressions were made. A constant weight of 1 kg was placed, and the pressure was recorded as initial and end pressures.

RESULTS

A significant difference in the pressure produced using different impression materials was found (P < 0.001). Light body vinyl polysiloxane produced significantly lesser pressure than zinc oxide eugenol impression materials. The presence of relief did affect the magnitude of pressure at various locations.

CONCLUSION

All impression materials produced pressure during maxillary edentulous impression making. Tray modification is an important factor in changing the amount of pressure produced. The impression materials used also had a significant role to play on the pressures acting on the tissues during impression procedure.

CLINICAL IMPLICATION

Light body VPS impression material may be recommended to achieve minimal pressure on the denture bearing tissues in both selective as well as minimal pressure techniques.

Comparison of the surface roughness of gypsum models constructed using various impression materials and gypsum products

Background/purpose

This study compared the surface roughness of gypsum models constructed using various impression materials, gypsum products, and storage times before repouring.

Materials and methods

Three alginate impression materials, four commercial silicone impression materials, and three types of gypsum product (MG crystal rock, Super hard stone, and MS plaster) were used. Impression materials were mixed and poured into five plastic rings (20 mm in diameter and 2 mm high) for each group, and the surfaces of the set gypsum product models of 63 groups, which were poured immediately, and 1 hour and 24 hours later, were assessed using a surface roughness tester. One-way ANOVA and Bonferroni's comparison tests were used for the statistical analyses.

Results

The surface roughness: (1) was greater for most specimens constructed from alginate impression material (2.72 ± 0.45–7.42 ± 0.66 μm) than from silicone impression materials (1.86 ± 0.19–2.75 ± 0.44 μm); (2) differed with the type of gypsum product when using alginate impression materials (surface roughness of Super hard stone > MG crystal rock > MS plaster), but differed little for silicone impression materials; and (3) differed very little with the storage time before repouring.

Conclusion

The surface roughness of stone models was mainly determined by the type of alginate impression material, and was less affected by the type of silicone rubber impression material or gypsum product, or the storage time before repouring.

Variations in tooth preparations for resin-bonded all-ceramic crowns in general dental practice

OBJECTIVE

To investigate variations in tooth preparations for resin-bonded all-ceramic crowns (RBCs) in general dental practice (GDP).

DESIGN

Laboratory-based retrospective analysis of dies for RBCs.

SETTING

General dental practices in the UK and Ireland (2000).

METHODS

A sample (n = 132) of laboratory models containing 180 tooth preparations for RBCs, featuring work from different general dental practitioners was obtained from four commercial dental laboratories. Aspects of the preparations were quantified and compared with accepted criteria defined following a review of the literature.

RESULTS

The teeth found to be most frequently prepared for RBCs were maxillary incisors (41%). Margin positions were variably positioned with 29% of the preparations on the buccal aspect having subgingival margins. There were many tooth preparation dies for low fusing RBCs (47%) and Chameleon Fortress RBCs (62%) demonstrating overpreparation in the mesiodistal plane. The majority of the margins (84% buccally and 79% lingually) of the dies examined exhibited appropriate shoulder or chamfer finishes. Of the Chameleon Fortress preparations analysed, 86% had been underprepared occlusally. 42% of the teeth had been prepared with no regard to tooth morphology and demonstrated just one plane of reduction. The majority (93%) of the clinicians failed to provide any information regarding the shade of the prepared tooth stump.

CONCLUSIONS

On the evidence of this survey of this sample of general dental practitioners' work, it was found that relevant guidelines for the preparations of RBCs are not being fully adhered to.