Introduction

Functional and esthetic rehabilitation of a completely or partially edentulous patient includes a battery of treatment modalities varying in the degree of invasiveness and reversibility along with the risks and benefits. In the past, patients had low expectations for the esthetic and functional outcomes of conventional removable dental prostheses. However, nowadays these expectations have changed leading to more patients demanding the esthetic aspect of their prostheses.

Particularly for the patients facing partial edentulism, the metal clasps of removable partial denture prostheses (RPDPs) in the esthetic area seem to be undesirable for both esthetic and psychological reasons with an increasing number of them avoiding and disliking their use (1, 2).

In response to this shift in expectations and demand, manufacturers and dental healthcare providers (dentists, dental technicians) have developed innovative, esthetic, natural-looking materials and methods for fabricating removable dental prostheses.

From the 1950s thermoplastic resins have been available and can be used to fabricate RPDPs or removable complete denture prostheses (RCDPs), (3-7). Many types of thermoplastic materials like nylon (polyamides), polyesters (polyethylene terephthalate), polycarbonates, acrylics (polymethyl methacrylate), polypropylenes and acetal resin (polyoxymethylene) can be used to fabricate flexible RPDPs (8, 9). From the aforementioned materials, nylon, acetal resin, polypropylene and acrylic resin are the most widely used (1).

Thermoplastic RPDPs have been mostly known as flexible dentures but other commonly used terms are non-clasp dentures, metal-free dentures, clasp free dentures and non-metal clasp dentures (2). Dentures made ​​of these materials should show some advantages over the conventional acrylic ones. Since the materials are flexible, undercuts of the alveolar ridges can be engaged for improved retention and insertion in the mouth is much easier, which is especially important in cases of microstomia (e.g. scleroderma). Since the materials are resistant to plastic deformation and fracture, the denture base may be thinner than in classic acrylic dentures. Also, there is no risk of any allergic reaction (no residual monomer) with improved esthetics due to the transparency of the material that reflects the shade of the mucosa and the absence of metal clasps which makes dentures almost invisible in the mouth (10).

Flexible RPDPs typically do not contain any metal elements (unless a metal structure is combined with thermoplastic retentive elements) and tooth colored clasps are an esthetic option. Even though flexible RPDPs have been available to the dental profession for almost 65 years and have received much attention in the dental advertisements over the past two decades there is almost no evidence-based information in the relevant literature concerning clinical performance, follow-up or incidence data for these prostheses (1, 8). Most relevant articles focus on case or case series reports and evaluation of physicomechanical properties of thermoplastic materials (11-14).

The clinical performance and patient’s acceptance of nylon RPDPs and RCDPs compared to acrylic resin ones were evaluated for 18 months in two studies (15, 16).

In the first study only two cases of midline fracture were reported, and mastication, phonetics and tolerance found to be improved with flexible maxillary dentures opposing natural mandibular teeth. Patients also reported gradual fading of color and teeth debonding. In the second among 18 patients who replaced their acrylic resin prostheses with flexible ones, they denoted 100% preference for the flexible dentures in all functional parameters evaluated, such as mucosal irritation, halitosis, fractures and comfort.

Demographic or incidence data of flexible RPDPs were reported in a survey of 5 dental laboratories in Wisconsin within a 4-month period. The incidence of flexible RPDPs was 5.2% of 903 RPDPs of all types fabricated (17).

From the aforementioned, the lack of data in the relevant literature is obvious and information on attitudes, knowledge and awareness of dentists towards flexible RPDPs. The purpose of this study was to investigate through a questionnaire the knowledge, attitudes and possible differences in the use of flexible RPDPs among dentists in Greece and Croatia.

Material and methods

An English questionnaire of 16 questions was originally created online using Polldaddy’s survey tool (www.polldaddy.com) and tested for its apprehension, precision, clarity and homogeneity by a number of native English speaking Greek dentists. Following the necessary corrections, the final questionnaire was translated by bilingual professionals into Greek and Croatian language which was again tested and changed accordingly for best language adaptation. This process included two way translations (18). The URLs of the two questionnaires, also created online by Polldaddy’s survey tool were sent by email to nearly 4000 dentists of each country, in order to have a response number of about 370 dentists, a predefined sample size based on a 5% confidence interval (CI) and 95% confidence level. The number of 4000 represents the total number of dentists in Croatia and the 1/3 of active dentists in Greece, from most of its major areas. For both surveys permission was given by the respective authorities (Greek Dental Association, Ethics Committee of the School of Dental Medicine, University of Zagreb).

The created online surveys contained dichotomous and polytomous (nominal and ordinal) closed-ended questions, and a start message informing the participants about the aims of the study, its confidentiality and anonymity. Most of the questions were signed as mandatory, rank ordered questions were randomized and only one response per computer was allowed. The English questionnaire is given in Table I.

Progress and response rate of the survey was monitored and 2 reminding emails were sent before the reach of the final sample units number, in a period of 3 months. Both cross-sectional surveys took place and completed about the same period of time. Collected data were evaluated for their accuracy and consistency and analyzed statistically by chi-square tests at a=.05 level of significance.

Results

378 dentists from Greece (5% CI) and 304 from Croatia (5.4% CI) participated in the study. A small number of respondents skipped at least one non-obligatory question and for this reason the percentages were based on the actual number of respondents for each question. Table II shows details for gender, age, years in practice, specialty, instruction (taught) and provision of flexible dental prostheses of all respondents. Statistical analysis (chi square test) revealed significant differences between the two target samples in respect to most of the above parameters, except for the specialists and the instructed to flexibles respondents (Table II).

137 dentists from Greece and 56 from Croatia responded positively for providing flexible RPDPs as part of their treatment options offered (Table II). Their profile and differences between the two countries are shown in Table III. Statistical analysis for all the providers together, indicated no significant difference between genders (P>.05), but significant differences between age groups (P<.01), years in practice (P<.05), specialization (P <.001), and instruction on flexible prostheses (P <.001) (Table III). Analysis between the two countries showed differences for gender and age groups (P<.01) but no differences between the more experienced (over 11y), specialized or instructed groups (P >.05).

The preference of respondents for RPD type was recorded as weighted ranks. RPDPs with metallic framework had the lowest (best) score (1.40), flexibles the second lowest (2.14) and acrylics the highest (worst) (2.45). The reasons for provision of flexible RPDPs were also ranked and the weighted scores showed that “more comfort for the patient” was the reason with the lowest score (2.85), followed by “better esthetics” (2.91), “less fabrication time” (3.00), “allergy to metal” (3.06) and “less cost” (3.16). Statistical analysis showed that preferences for RPDPs were significantly different (P<.001), but reasons for deciding the flexible RPDPs were not (P>.05). No significant differences between the two countries were noted, either for the preferences (P>.05) or for the reasons (P>.05).

The reasons according to respondents, for providing and replacing flexible RPDPs are shown in Table IV. Most flexibles were recommended by the dentist, and planned to be used as both provisional and permanent appliances. The material mainly used for the fabrication of flexible RPDPs was polyamide (87% in Greece and 37% in Croatia). Their performance after 1 year in place was satisfying for the dentists and the problems they presented during their functioning in the mouth were related to discoloration of the base, fracture of clasps and debonding of teeth. Nearly half of the respondents relate the replacement of the flexible RPDPs to problems in abutments and more than 50% replaced after 2 years of use. Table IV also shows the differences between the two countries (P_Gr-Cr) along with the differences between the groups of the same question (column superscripts).

Discussion

This study investigated the attitudes and knowledge about flexible RPDPs among dentists in two European countries, namely Greece and Croatia. The results revealed that only 1 out of 6 of all respondents were instructed about the flexible RPDPs concept, but 1 out of 3 do provide a flexible prosthesis as an alternative to the conventional metallic or acrylic one. Provision of flexibles was found to be associated with age, years of practice, specialization and instruction to flexibles of the respondent practitioners but not with their gender. A difference was found between the two countries in the provision of flexibles, but not in the percentage of instructed practitioners. The difference in the provision is higher in Greece than in Croatia for both genders, for ages over 45, for those with more years in practice, for general practitioners and for not instructed respondents. Among all, 1 out of 10 seemed to prefer flexibles in place of metallic RPDPs and the reasons were comfort, esthetics and cost for their patients. No significant differences for the preferences and the reasons were noticed between the two countries, except for comfort.

Gender did not play a significant role in the provision of flexibles, even within the countries. Percentages for both genders were higher in Greece than in Croatia and this can be explained by the fact that provision of flexibles in general was higher in Greece than in Croatia (36.8% and 18.9% respectively). Age played a significant role in the provision of flexibles but not within countries. Older practitioners provide flexibles in higher percent, meaning that younger practitioners are more reluctant to adopt new techniques. Years in practice seems to follow the same general pattern as age. Greek respondents present also a significant increase with age. If years in practice means more experience for the respondents, then practitioners with more experience seem to provide flexible prostheses more easily than the less experienced. This is also reported by Hill et al (1). Specialization was found to play a negative role in the provision of flexibles since more general practitioners than specialists provided flexible prostheses to their patients. Because there is still not enough clinical evidence for the use of flexibles, promotional literature may affect more general practitioners to adopt this treatment method, as Hill et al. also noted (1). Although this does not apply to the Croatian sample, where both groups provide flexible prostheses in the same percentage, an explanation can be given by the difference of instructed respondents contained in the specialists (20.4%) and not-specialists (12.9%) groups. Instruction plays a significant role in the provision to flexibles, as is discussed below.

Instructed practitioners providing flexibles are 3 times more than not instructed. This is actually expected, since not-instructed are more reluctant to recommend and provide flexibles due to the absence of experience and education.

In general, 1out of 3 respondents prefer flexibles and acrylic prostheses in place of metallic ones and this is in agreement with the results of Pun et al. (17) survey. Flexible prostheses were decided on differently in the two countries. The decision in Greece was made mostly by the dentists whereas in Croatia the patients seemed to affect the decision equally to dentists. This is probably due to some differences in the provision of dental care between the two countries. Flexibles planned to be used primarily as provisional prostheses in Greece and as permanent in Croatia, although in total, permanent and provisional use was found equal. The main reasons for providing flexible prostheses were comfort for the patient, esthetics and cost, with almost the same percentage between respondents and with only small differences between the two countries. The problems related to the flexibles were discoloration of the base, fracture of clasps and debonding of teeth. Problem percentages were the same in both countries. The reasons for replacing flexible prostheses were reported to be related in a decreasing order with problems in abutment teeth, denture base material and finally the supporting tissues. Satisfaction after 1 year in place of flexibles was high in both countries. However, the satisfaction was higher in Croatia than in Greece.

From the abovementioned, it is evident that there are differences between the two countries in the use of flexible prostheses. Instructions for their use are given by promoting literature provided by manufacturing companies, dental laboratories and not by academic institutions. That could be the reason why younger, less experienced dentists do not recommend flexible prostheses. Also, little evidence exists on their clinical usage, performance and duration. For these reasons, clinical studies are urgently needed to investigate their value in long term performance and patients’ satisfaction along with techniques overcoming material inherited problems.

Conclusion

This survey on the use of flexible RPDPs indicated differences between the two countries in the number of dentists using, selecting and providing these dentures as an alternative to metallic RPDPs for their patients. Only 1/5 to 1/3 of the practitioners provide flexible RPDPs for their patients but over 75% of them were satisfied with their performance after 1 year. Age, years in practice and instruction of the practitioner are all related to the provision of these dentures, while comfort, esthetics and cost were the main reasons for deciding flexible RPDPs for their patients. Base discoloration, clasp fracture and tooth debonding were their main problems in a decreasing order.