Introduction

Over the recent decades, there has been an ongoing worldwide trend towards increased life expectancy both at birth and as a percentage increase in the older adult population (^{Balachandran et al., 2020}). Internationally, there are reports that in some Balkan countries complete edentulism can be as high as 78% of the adult population (^{Petersen et al., 2005}). In Chile, the average tooth loss in 65-74 year-old population currently stands at 15.8 teeth per person; with 69.8% of them being partially edentulous and 29.1% being completely edentulous (Chilean Ministry of Health (MINSAL), 2017).

Because financial constraints and systemic conditions faced by some patients contraindicate the use of dental implants (^{Hwang & Wang, 2006}), complete removable dental prostheses continue to be a suitable therapeutic alternative (^{Carlsson, 2010}). A critical step in the construction of these devices is the impression of the edentulous ridges. Several techniques (^{Rao et al., 2010}) have been proposed for this purpose; though there are still a number of controversies in this regard (^{Petropoulos & Rashedi, 2003}). Most dental schools and pertinent texts (^{Petropoulos & Rashedi, 2003}) teach and describe the socalled “traditional method” (^{Rao et al., 2010}), which advises taking a definitive open-mouth impression, using a custom tray made from an initial impression. Additionally, relief in the stress bearing areas should be fashioned, as well as peripheral extensions and border molding (dual impression technique - Boucher technique or its modifications -associated with the selective pressure philosophy) (^{Petropoulos & Rashedi, 2003 ; Rao et al., 2010}). The above-mentioned method claims that this technique provides better clinical results and greater patient satisfaction as compared to a “simplified method” in which a prosthesis is created from the preliminary impression taken with a stock tray (^{Heydecke et al., 2008}). Its proponents maintain that there is evidence indicating that this technique saves time and cost without affecting clinical results or patient satisfaction (^{Duncan & Taylor, 2001}; ^{Jagger, 2006}).

The “patient satisfaction” variable is related to the his/her perception of the treatment, and is measured along a rating scale. The most widely used scales are the subjective Oral Health-related Quality of Life (OHRQol) (^{Kawai et al., 2010}; ^{Ye Ye & Sun, 2017}; ^{Albuquerque et al., 2020}) and the Oral Health Impact Profile (OHIP) (^{Ye Ye & Sun, 2017}; ^{Albuquerque et al., 2020}; ^{Nuñez et al., 2013}) which is based on functional limitations, physical pain, psychological discomfort, physical disability, psychological disability, social disability, and handicap. Another relevant variable is the “clinical quality”, understood as the dentist’s clinical criteria regarding the prosthesis (^{Ye Ye & Sun, 2017}) and its fulfillment of retention, support and stability requirements. Other variables assessed using question-naires with binary responses and numerical rating scales (^{Schott et al., 2010}; ^{Cunha et al., 2013}) are “masticatory performance,” defined as the objective measure- ment of the degree of grinding to which food can be subjected in an established number of masticatory cycles (^{Schott et al., 2010}; ^{Cunha et al., 2013}); and “masticatory capacity,” a subjective parameter which measures the individual’s assessment of their mastication (^{Schott et al., 2010}).

The purpose of this review is to explore the scientific evidence reported in specialized publications comparing the results of the “traditional” (functional impression) and “simplified” methods used in making complete removable dental prostheses, considering patient satisfaction, clinical quality, performance, and masticatory capacity, among others.

Materials and methods

MEDLINE databases (MESH criteria) were used to conduct two searches. The first search was conducted using the following combination of keywords and search operators: “impression techniques” OR “dental impression” AND “traditional method” AND “simplified method” and their corresponding terms in Spanish, i.e., “técnica de impresión” OR “impresión dental” AND “método tradicional” AND “método simplificado”. The second search employed the combined terms “impression techniques” OR “dental impression” AND “selective pressure” AND “mucostatic,” and the corresponding terms in Spanish.

Four criteria were defined for inclusion in this study: (a) that the studies had been published between 2000 and the first quarter of 2020 (date on which the search was conducted); (b) that they were randomized clinical trials, critical reviews, or systematic reviews comparing both methods regarding the impression-taking process for making a complete removable dental prosthesis; (c) that both the abstract and the article were available online; and (d) that they were written in English and/or Spanish.

The lead author, who conducted the searches, selected the articles based on a review of their abstracts using a checklist of inclusion and exclusion criteria. A summary table of the selected articles was generated in chronological order (see Table 1). As neither a statistical analysis of the results of the selected articles nor an exhaustive search of those studies not available online was performed, this paper is classified as a critical appraisal of evidence.

Results

A total of 23 publications, including critical or systematic reviews and randomized clinical trials, met all of the inclusion criteria (see Table 1). In 16 randomized clinical trials (^{Duncan & Taylor, 2001}; ^{Kawai et al., 2005}; ^{Jagger, 2006}; ^{Heydecke et al., 2008}; ^{Kawai et al., 2010}; ^{Galaz et al., 2012}; Núñez, 2013; ^{Cunha et al., 2013}; ^{Omar et al., 2013}; ^{Jo et al., 2015}; ^{Mengatto et al., 2017}; ^{Ceruti et al., 2017}; ^{Kawai et al., 2018}; ^{Lira-Oetiker et al., 2018}; ^{Tripathi et al., 2019}; ^{Albuquerque et al., 2020}), involving more than 800 patients, no significant differences were reported in terms of masticatory performance controlled at 3 months (^{Cunha et al., 2013}; ^{Albuquerque et al., 2020}), 6 months (^{Mengatto et al., 2017}; ^{Albuquerque et al., 2020}), and 10 years posttreatment (^{Kawai et al., 2018}); masticatory capacity at 3 months (^{Heydecke et al., 2008} ; ^{Albuquerque et al., 2020}) and 6 months (^{Mengatto et al., 2017}; ^{Albuquerque et al., 2020}); the same was true for patient satisfaction assessed at 1 month (Núñez, 2013; ^{Omar et al., 2013}; ^{Jo et al., 2015}), 3 months (^{Jagger, 2006}; ^{Kawai et al., 2005}; ^{Omar et al., 2013}; ^{Albuquerque et al., 2020}), 6 months (^{Kawai et al., 2005}; ^{Jagger, 2006}; ^{Kawai et al., 2010}; Núñez, 2013; ^{Lira-Oe-tiker et al., 2018}; ^{Albuquerque et al., 2020}) and 10 years (^{Kawai et al., 2018}); and also with respect to clinical quality assessed at 3 months (^{Kawai et al., 2005}; ^{Omar et al., 2013}) and 6 months (^{Kawai et al., 2005}; ^{Kawai et al., 2010}; ^{Lira- Oetiker et al., 2018}) respectively. By comparison, the favorable results for the “simplified method” include the following: fewer clinical sessions required for manufacturing the prosthetic device (^{Duncan & Taylor, 2001}; ^{Ceruti at al., 2017}), without undermining retention (^{Galaz et al., 2012}); nor increasing the number of subsequent check- ups (^{Duncan & Taylor, 2001}; ^{Albuquerque et al., 2020}); overall satisfaction, stability and aesthetic appearance at 3 months post-installation (^{Heydecke et al., 2008}), and less resorption of the mandibular residual ridge in individuals with decreased bone density, as assessed by CT scan, one year after installation of the prostheses (^{Tripathi et al., 2019}).

One article reported that for the 24 subjects evaluated the patient satisfaction rating was higher for the traditional method (^{Jo et al, 2015}).

The selected critical reviews (^{Carlsson, 2009}; ^{Carlsson, 2010}; ^{Carlsson et al., 2013}; ^{Uram-Tuculescu & Constantinescu, 2017}; ^{Jayaraman et al., 2018}) and systematic reviews (^{Regis et al., 2016}; ^{Ye Ye & Sun, 2017}), reported that there is no evidence to support that one impression technique provides better results than the other in the long-term follow-ups (^{Carlsson, 2009}; ^{Jayaraman et al., 2018}), nor significant differences between both methods in terms of: performance and masticatory capacity (^{Regis et al., 2016}; ^{Ye Ye & Sun, 2017}); patient satisfaction (^{Carlsson, 2009}; ^{Carlsson, 2010}; ^{Carlsson et al., 2013}; ^{Regis et al., 2016}; ^{Ye Ye & Sun, 2017}; ^{Uram-Tuculescu & Constantinescu, 2017}) and clinical quality (^{Carlsson, 2009}; ^{Carlsson, 2010}; ^{Carlsson et al., 2013}; ^{Regis et al., 2016}; ^{Ye Ye & Sun, 2017}; ^{Uram- Tuculescu & Constantinescu, 2017}).

Discussion

The traditional impression method for a complete removable dental prosthesis involves taking two impressions: a preliminary one, generally with irreversible hydrocolloid and a stock impression tray, and following casting creating a model in which an individual tray is made. This tray is then used to record the three-dimensional contours of the denture with a thermoplastic molding compound, followed by a second impression, usually with silicone elastomers or zinquenolic paste (^{Petropoulos & Rashedi, 2003}; ^{Rao et al., 2010}; ^{Carlsson et al., 2013}; ^{Jo et al., 2015}).

However, a so-called simplified method has also been proposed and used, advocating restoring patient functionality and aesthetics, and also optimizing dental care. It postulates the use of the preliminary impression, without need for a second one or assembly in a semi-adjustable articulator (^{Duncan & Taylor, 2001}; ^{Heydecke et al., 2008}; ^{Carlsson, 2010}; ^{Carlsson et al, 2013}; ^{Jo et al., 2015}; ^{Ye Ye & Sun, 2017}; ^{Kawai et al., 2018}). This procedure offers comparative ad- vantages such as reducing costs, materials, number of clinical sessions and laboratory time, thereby providing access to the treatment for the most disadvantaged population, without sacrificing biofunc- tional and prosthodontic principles (^{Duncan & Taylor, 2001}; ^{Owen, 2004}; ^{Kawai et al., 2005}; ^{Cunha et al., 2013}; ^{Carlsson et al., 2013}). A considerable number of studies report that there are no significant differences between the two methods in terms of patient satisfaction, clinical quality, performance and masticatory capacity, when these variables were controlled for over the short and long term (^{Duncan & Taylor, 2001}; ^{Kawai et al., 2005}; ^{Carlsson, 2010}; ^{Omar et al., 2013}; ^{Cunha et al., 2013}; ^{Ye Ye & Sun, 2017}; ^{Mengatto et al., 2107}; ^{Ceruti et al., 2017}; ^{Kawai et al., 2018}; ^{Lira- Oetiker et al., 2018}).

Based on the consistency of the evidence presented, the simplified method appears as an acceptable protocol for the manufacturing and quality control of prosthetic devices (^{Carlsson, 2010}; Carlsson & Omar, 2010; ^{Omar et al., 2013}; ^{Carlsson et al., 2013}).

It seems that the prevailing approach in the literature, undergraduate teaching, and specialized practice as to the need for and the “superiority” of using the traditional method, in detriment of the simplified method, should be reconsidered. While some particular clinical situations may benefit from the application of the traditional method, the simple and in- expensive one-step procedure - the simplified method - has shown that it fulfills the needs of the vast majority of totally edentulous patients (^{Petropoulos & Rashedi, 2003}; ^{Kawai et al., 2010}; ^{Carlsson et al., 2013}; ^{Kawai et al., 2018}). Regarding the advisability of exclusively teaching the traditional method at the undergraduate level, we suggest to revise the dental curriculum- including the simplified method (^{Duncan & Taylor, 2001}; ^{Kawai et al., 2018}; ^{Carlsson et al., 2013}), while highlighting the benefits that this change will have providing wider dental coverage, especially for patients from disadvantaged communities.

Conclusions

According to the available information, the traditional method of taking impressions for complete removable dental prostheses does not provide significantly superior clinical results when compared to those obtained using the simplified method in terms of clinical quality, patient satisfaction, performance and masticatory capacity.

Considering the limitations of this study, which arise mainly from the methodology employed, and despite the fact that some special clinical situations may Benefit from other combinations of materials and techniques, the reported scientific evidence supports the appropriateness of the simplified approach in fulfilling the prosthetic needs for the majority of completely edentulous patients.

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