Introduction

Prosthodontics is a branch of dental medicine dealing with restoration of lost teeth and soft and hard tissues of the oral cavity, in order to establish and preserve the function of the masticatory system, as well as patients’ appearance and health. The purpose of prosthodontics is to maintain functional integrity of the stomatognathic system (1). The study of dental medicine is an integrated undergraduate and graduate university study for obtaining the academic degree of doctor of dental medicine. In the Republic of Croatia, the School of Dental Medicine in Zagreb (SFZG) is the only school of dental medicine that enrolls 90 students per year and lasts for 12 semesters (2). The course in prosthodontics is designed on a six-semester basis (from 5^th semester) and continues throughout the 10^th semester. It is divided into preclinical courses – Preclinical and Laboratory Removable Prosthodontics (PLMP) and Preclinical and Laboratory Fixed Prosthodontics (PLFP) which last for two semesters, and clinical courses - Removable Prosthodontics 1 and 2 (MP) and Fixed Prosthodontics 1 and 2 (FP), which last for four semesters (3-8).

In the preclinical courses during two semesters dental medicine students are taught about basic biomedical and technological knowledge and skills underlying clinical and laboratory work in the conventional prosthetic treatment of completely dentate and partially or completely edentulous patients. They can acquire basic knowledge and skills for specific clinical and laboratory procedures in fabrication of removable and fixed restorations. Knowledge and skills are acquired through lectures, seminars and preclinical exercises on casts and phantoms. Preclinical courses prepare for work with patients in clinical courses. After passing exams in preclinical courses, the acquired knowledge and skills are applied, expanded and developed during four semesters of clinical courses, through lectures, seminars and clinical practicals in small groups of students under the guidance of teachers and assistants. Clinical practicals are performed on patients (3-8).

As prescribed in the catalogue of knowledge and skills in Removable Prosthodontics 1 and 2 and Fixed Prosthodontics 1 and 2 of the study program of Dental Medicine at the School of Dental Medicine, University of Zagreb, students are expected to be capable of carrying out individually certain phases of fabrication of all types of removable and fixed prosthetic restorations. Dental students should possess a thorough knowledge of the anatomy and physiology of the stomatognathic system. They are expected to master the skill of taking medical history from a patient. Also, they should be able to make diagnoses and prosthetic treatment plans. They should be skilled in taking impressions in order to determine the correct relationship between the jaws. They should be familiar with the materials used, types of trays used, correct use of articulators, as well as with indications, contraindications and clinical phases of certain types of prosthetic restorations (3-8). Besides, it is important for students to develop the capability of self-criticism and self-assessment of their own skills, which will encourage them in lifelong learning, self-improvement and gaining new clinical experience (8-10).

The aim of the study was to analyze dental medicine students' subjective perception of learning, knowledge and skills in prosthodontics and to determine how their perception changed during their education and through experience gained in clinical work. Furthermore, the aim of the study was to determine whether socio-demographic factors (gender and secondary education) influenced students' perceptions.

Materials and Methods

The study was conducted at the School of Dental Medicine in Zagreb in the academic year 2015/16. It was approved by the Ethics Committee and carried out by means of an anonymous survey drawn up only for this purpose. The participants were 4th, 5th and 6th-year-students and they were recruited during the lectures. Their participation was voluntary. The survey consisted of four parts. In the first part, general information about the subjects were collected (age, gender, secondary school degree, year of study, passed preclinical prosthetic courses and grades in preclinical prosthetic courses) and clinical experience of the participants (the number of taken impressions and fabricated fixed and removable restorations). The second part of the survey examined students' subjective perception of learning, and the third part examined students' personal opinions about their own abilities and skills for performing prosthetic treatment. Knowledge in the fourth part was tested by simple questions from the field of fixed and removable prosthodontics and YES/NO/DO NOT KNOW types of answers were used.

The perception of students' own skills was assessed by a visual analog scale (from minimum to maximum). The values ​​of the visual scale were converted into numerical values in such a manner that the minimum value of the visual scale was associated with the numerical value of 0 (zero), and the maximum value of the visual scale with the numerical value of 5 (five). The numerical value of each individual answer to study questions was determined in a way that the distance on the visual scale between the value of zero and value of the given answer was measured by means of a slide caliper with an accuracy of 0.1 cm. A total length of the visual scale was 5 cm. The distances between the answer values from minimum values were measured by three researchers, and the arithmetic mean of all three measurements was taken as the final value, e.g. the answer 3.6 cm far from the minimum value on the visual scale obtained the numerical value of 3.6. The visual scale was used as a measuring tool in this way in order to try to avoid suggestive influence of numerical values on subjects' answers.

The data were organized in Microsoft Excel files (Microsoft Inc., USA) and were statistically analyzed using SPSS 12.0 software (IBM Inc., USA). Descriptive statistics methods (mean and standard deviation) were used to present the obtained data. The ci-square test, t-test for independent samples and analysis of variance were used for establishing the differences between the two groups. If the tests used determined statistically significant differences, further testing was conducted by means of Tukey's post hoc test. Correlations between individual variables were expressed by the Pearson's and Spearman's correlation coefficients. The values ​​of p <0.05 were considered statistically significant.

Results

The study included 168 subjects (response rate was 71.3%): in their 4th- (62%), 5th-(72.6%) and 6th (79.4%) year of undergraduate studies (79.4%) at the School of Dental Medicine, University of Zagreb. With respect to subjects' secondary education, it was found out that 88.7% of them completed a grammar school providing advanced secondary education, while 11.1% completed secondary medical school (nurses/technicians, dental laboratory technicians). As for their parents, 36.8% of the respondents had at least one parent employed in health care. The average number of subjects was 8.3 ± 1.6 per group for clinical practicals.

The influence of socio-demographic factors on clinical experience is shown in Table 1. Previous clinical experience positively correlated with respondents’ age, year of study and number of completed semesters in prosthodontics (p<0.05). The subjects with completed dental laboratory technician school took a significantly larger number of impressions (p<0.05) and fabricated a significantly larger number of fixed restorations (p<0.01). Male subjects took statistically significantly larger number of impressions (p<0.01) and fabricated a larger number of fixed restorations (p<0.05) than female subjects.

Table 2 shows the students’ perception of learning and its relationship with previous clinical experience. The results revealed that subjects who completed the school for dental laboratory technicians reported a statistically significantly higher level of agreement with the following statements: "In my opinion removable and fixed prosthodontics should be taught as one course of study“(p<0.05) and "I believe to have enough knowledge to recognize by myself technician's and my own faults“ (p<0.05) than subjects who completed grammar schools and secondary medical schools.

The analysis of the relationship between previous clinical experience and perception of learning pointed to a statistically significant correlation between the number of impressions taken, fixed and removable restorations fabricated and satisfaction with the chosen faculty, perception of usefulness of learning and lectures from preclinical courses and the statement "I believe to have enough knowledge to recognize by myself technician's and my own faults“, The number of subjects in the group expressed a significantly negative correlation with the statement "I am satisfied with the faculty I chose", "I consider exercises in clinical prosthodontics as useful“ and "I think that the number of lectures in PLMP, PLFP, MP and FP is sufficient" (p<0,05).

Table 3 shows perception of one's own knowledge and skills. A statistically significant correlation in perception of knowledge and skills according to the year of study was established. 5th- and 6th-year students showed a statistically higher level of agreement with the majority of statements compared to 4th-year-students.

Students who completed dental laboratory technician school showed a statistically significant higher level of agreement (p<0.05) with all statements except with the statement "I have a good knowledge of anatomy and physiology of the stomatognathic system" compared to other students. 5th- and 6th-year students showed a statistically significant higher level of agreement with the statement "I know how to choose and use gingival retraction cord“ (3.1 ± 1.4 and 3.3 ± 1.3 to 2.1 ± 1.4), "I am skilled in preparation for single rooted teeth and individual core-up fabrication" (2.2 ± 1.3 and 2.5 ± 1.3 to 1.4 ± 1.2), "I am skilled in direct preparation for temporary crowns"(2.6 ± 1.5 and 2.3 ± 1.5 to 1.7 ± 1.4) and "I am skilled in permanent fixation of restorations by non-adhesive cementation technique "(2.8 ± 1.3 and 2.8 ± 1.5 by 1.7 ± 1.4), compared to 4th-year students. There was no statistically significant difference between 4th- and 5th-year students and 6th- and 5th-year students. 6th-year students showed a statistically significant higher level of agreement with the statement "I am familiar with indications, contraindications, and clinical phases of acrylic crown fabrication" (3.3 ± 1.3 to 2.6 ± 1.2), and "I am familiar with indications, contraindications and clinical phases of jacket crown fabrication"(3.3 ± 1.3 compared to 2.3 ± 1.2), compared to 4th-year students. There was no statistically significant difference between 4th- and 5th-year students and 6th- and 5th-year students. 6th-year students showed a statistically significant higher level of agreement with the statement "I am familiar with indications, contraindications, and clinical phases of metal-ceramic crown fabrication" in relation to 4th- and 5th year students (3.4 ± 1.1 compared to 2.9 ± 1.3 and 2.4 ± 1.2). Statistically significant difference was also established between 4th- and 5th-year students (2.9 ± 1.3 compared to 2.4 ± 1.2).

Personal knowledge was assessed according to the number of correct answers per student. The average number of correct answers per student amounted to 15.2 ± 2.9 (min.=3, max = 20). 6th-year students showed significantly higher levels of personal knowledge (16.6 ± 2.3) compared to 5th- and 4th-year students (14.6 ± 2.6 and 14.9 ± 3.1). There was no difference in the level of personal knowledge between 4th- and 5th students. No differences either in the level of personal knowledge between female and male subjects (p=0.343) or regarding secondary school education (p=0.534) were established. The number of correct answers increased along with the year of study, i. e. 6th-year students gave the highest number of correct answers to most questions (16/20). The percent of correct answers ranged from 20.1% to 96.4% (Table 4).

The relationship between personal knowledge and perception of learning, knowledge and skills showed statistically significant correlations (Table 5). Also, the relationship between personal knowledge and previous clinical experience was examined by Spearman's correlation coefficient. A statistically significant correlation between personal knowledge and number of fabricated fixed restorations was established (p>0,001).

Discussion

Life expectancy has increased rapidly due to the improvement in the living standard. In consequence, the percentages of elderly people have been increasing over years, thus increasing the need for prosthetic restorations (11-13). In 1999, Waldman et al (14) stated that the need for prosthetic restorations has increased proportionally with older age, lower socio-economic standard of living and lower level of education. In order for students to meet the growing need for prosthetic restorations, it is necessary for them to acquire high quality theoretical and practical knowledge during the course of their study, which will set the foundation for their future clinical work. It is also important that students develop the ability of self-criticism, i.e. self-assessment of their own work, which is acquired through work and experience (14, 15).

During the integrated six-year study program, dental students gain extensive theoretical knowledge of dental medicine. Yet they lack clinical experience. Since there is a gap between the contents learned and the contents applied not only in Croatia but also in other countries, it is necessary to introduce a change that would allow a balance between these two factors (16). Since courses in Prosthodontics, within the study of dental medicine, in other countries begin at earlier years of study, without separating fixed from removable prosthodontics, our questionnaire comprised the following statements: "I think that removable and fixed prosthodontics should be taught as one course" and "I think that courses in removable and fixed prosthodontics should be held at an earlier year of study", with the purpose of finding outdental medicine students' opinions about it. Preclinical and special clinical practicals are held in small groups of students with an average of 8.3 ± 1.6 members, which corresponds to the size of groups of other dental medicine study programs in the world (15-18).

Similar results were obtained by Brand et al who conducted research on students' assessment of materials and methods in learning of fixed prosthodontics among several dental medicine study programs in Europe. (19) The results show that dental medicine courses of study which start earlier with preclinical learning have higher ratings in terms of preparedness of students for clinical work, as well as a higher overall rating of courses in fixed prosthodontics. It is interesting to notice that assessment of preparedness of students for clinical work, as well as overall assessment of quality of learning did not correlate with the year of study of the students who participated in the study.

Women's interest in medical sciences, including dental medicine, has increased during the last few decades. Kelsey et al (20) claimed that in America in the 1970s there were 1% of female students on average among dental medicine students, while today women make up 38% to 40% of them. Men showed greater clinical experience and their skills were assessed as being better, while there were no statistically significant differences regarding the theoretical knowledge. Among other socio-demographic factors which affect students' perception of knowledge, attention should be paid to a possible impact of parents employed in health care. However, such an impact was not statistically significant in our study. Park et al (21) obtained identical results, and they found that this factor had no influence on preclinical and clinical work. The impact of completed secondary education was positively graded in the assessment of clinical practice and perception of students' own skills. During their education in a school for dental laboratory technicians, students acquire the basics of laboratory removable and fixed prosthodontics, which provides a good basis for further academic education of doctors of dental medicine. Beier et al (22) examined the impact of prior medical education to the success in studying. The impact was observed in relation to the success in the entrance examination, clinical work and clinical courses, and success in final examination. It was confirmed that students with previous medical education were more successful in entrance examinations, better in clinical courses and had better grades at final exams, especially in prosthodontics and oral and maxillofacial surgery. They also found that the subjects' gender had no impact on the results. The results of our study largely correspond to their study.

Henzi et al (18) investigated students' perception of the quality of learning and concluded that despite the fact that we live in this modern age, the present disadvantages are similar to those that were observed fifty years ago. Advantages and disadvantages of learning are best known to students, and they can point to them in the best manner. It is therefore necessary to establish proper ways of communication, appropriate cooperation with responsible persons and create viable solutions. Based also on some other research, e.g. by Nikzad et al (23), it is clear that in dental medicine, including prosthodontics, more practical work should be introduced, students' independence should be encouraged and confidence strengthened from the very beginning of their study. Nikzad and colleagues investigated the impact of video demonstrations of tooth preparations for metal-ceramic and temporary crown fabrication in order to perform the same treatment procedures during clinical practicals. They found that the students from groups that had previously watched the video were more successful in implementation of clinical procedures, but only in those segments that were closely related to the laboratory part of the procedure. They also noted a moderate impact of previous videos on stress reduction and self-confidence increase in students and concluded that live demonstration on patients are among best ways of acquiring previous clinical experience required for independent work of students on patients under supervision of their teachers.

Chambers and LaBarre (24) investigated the impact of students' perceptions of acquired knowledge about laboratory techniques and procedures in removable prosthodontics on their further learning and acquiring clinical knowledge of removable prosthodontics. On two students' tasks, they compared self-assessment of students' knowledge with grades given by teachers, and they concluded that self-assessment was a better predictor of future learning than teachers' grades. Velayo et al (25) reached similar conclusions. They compared grades for preclinical and corresponding clinical courses of two generations of dental medicine graduates and drew conclusions about statistically significant correlations. Barrero et al (26) examined 3rd- and 4th-year students in relation to their assessment of their personal preparedness for clinical work, and they found that less than 60% of students felt ready to apply preclinical knowledge in their clinical work. The results of the abovementioned studies correspond with the results of our study. They point to the need to change not only the content of preclinical courses in terms of learning methods, materials and skills, but also in terms of ways of transferring knowledge, suggesting greater use of video and group learning methods (team-based learning), as well as learning oriented toward problem solving (problem-based learning). Shina et al (27) also confirmed the necessity of changes. They studied retrospectively two groups of graduates, each consisting of four generations in the period from 2007-2011 and 2012-2016. Between these two groups, there was a change in the curriculum of their course of study and students were surveyed with respect to their experiences and perception of teaching in different branches of dental medicine, including prosthodontics. Besides, their attitude toward choosing future specialization was observed. The results showed an increase in satisfaction of former students with the level of knowledge in prosthodontics acquired during their study, but also a decrease in interest for specialized training in prosthodontics.

Necessary changes of the curriculum in prosthodontics primarily refer to team-based learning, with its advantage in relation to traditional methods of teaching. The study by Takeuchi et al (28) on Japanese students confirmed the benefits of team-based learning in teaching prosthodontics. The group of students who attended team-based learning classes showed greater satisfaction with the quality of learning compared to traditional methods of teaching. The main advantage of such a method is that it encourages active discussion among the students attending team-based learning classes. Furthermore, students of team-based learning classes showed statistically significant better final exam results, which confirms the benefits of team-based learning compared to traditional methods of teaching. Similar results were obtained by Echeto et al (29), with students of the University of Florida (USA). They compared two generations of students of removable prosthodontics. Students of generation 2013 were taught by means of traditional methods, and students of generation 2014 were taught by means of team-based learning. The results showed statistically significant difference in the exam results in favor of the team-based learning group of students (23.7% more „pass“ grades in the exam compared to the traditionally taught group of students). In addition, the team-based learning group of students obtained a significantly higher grade in the final exam. Based on all these results, it can be concluded that the team-based learning method is better than traditional methods of teaching, and corresponding changes of the curriculum should be introduced. However, it becomes clear that major change will take a long time. This topic is still insufficiently explored in Croatia, and there are only limited comparable data or findings available from other countries.

Conclusions

This study provides us with insight into dental medicine students' subjective assessments of their knowledge and skills in prosthodontics. The results of the study show that:

Students who completed dental laboratory technician school program took a significantly larger number of impressions and fabricated a significantly larger number of fixed restorations; Male subjects took a significantly larger number of impressions and fabricated a larger number of fixed restorations; Clinical experience (number of impressions taken and removable and fixed restorations fabricated) increased with the number of years of study and completion of semesters in prosthodontics; 5th- and 6th-year students showed a statistically higher level of agreement with most statements about perception of knowledge and skills compared to 4th-year students. The number of correct answers from the assessment increased with the year of study, and 6th-year students had the largest number of correct answers to most questions. A positive correlation between the perception of learning and previous clinical experience was established.

It is believed that the results of this study will contribute to the improvement of education and quality of work of dental students and future doctors of dental medicine, thus encouraging and motivating them to continue with good work. They are expected to put further efforts into learning during their university study, thus improving their competence for independent work.