A successful prosthesis, accepted by a patient, is that prosthesis that functions effectively during mastication and speech and is adequately retained on the edentulous arches. Precise recording, marking, and carving of the posterior palatal seal (PPS) is an important step for the retention of the maxillary denture and overall treatment of the edentulous patient. Thus, the significance of the distal extent of the upper complete denture needs to be adequately performed. ^{1 , 2}

Many researches were aimed to enhance the retention of the maxillary denture by enhancing the denture base adaptation to the supporting structures. ^{3 , 4} Even the disinfection of dentures may have an influence on the dimensional stability and overall retention of the complete dentures or the damaging effects on denture base material itself. ^{5 , 6} Such attempts enhanced the adaptation but did not eliminate the need for a PPS. The purpose of the PPS is to offer retention for a well-adapted upper denture and this is provided by an appropriate seal to reduce the effect of shrinkage from polymerization setting of the denture base material. In addition, the presence of the PPS prevents food accumulation between the posterior border of the denture and the soft palate, reduces gag reflex, and renders the extension of the posterior border of the denture more comfortable to the touch of the tongue. ⁷ Adding additives to the denture base may enhance the strength of the denture base but the presence of the PPS itself can add extra strength for the posterior part of the upper complete denture. ^{7 , 8}

The preparation of the PPS should be performed with adequate understanding of palatal anatomy and the displaceability of the mucosa. ⁹ The dentist should be responsible for recording, marking, and carving of the PPS and this responsibility and the completion of this step should not be shifted to the dental technician. ^{1 , 2 , 9}

Skinner ¹⁰ stated that “The most effective addition to increase retention is the post dam”. In spite of the recommendations through previous researches concerning the importance of the PPS during denture manufacturing, the preparation and position was often ignored by dentists and dental students. ¹¹ Thus, a significant number of dentures were unsuccessful due to inadequate carrying out of the PPS. ¹²

Adams ¹³ investigated the methods for the PPS employed by clinicians and dental students during construction of complete dentures and determined how many carryout this step by themselves. He revealed that the location and measurements of the post-dam were overlooked or forgotten by the practitioners and dental students and placed in the hands of technicians. This showed the deficiency in attention given to the significance of the post-dam by the practitioners and dental students. There was a need for research directed to analyzing the reason for the decline in interest by students and experience dental clinicians for this step.

A survey was prepared by Sunil and Jain ¹² to investigate registering the PPS among dentists and dental students. The survey showed that they had satisfactory knowledge of the anatomical landmarks associated with the PPS, but were poorly associated with the size, shape, location, and compressibility of the mucosa of the PPS. This reflected the need for highlighting the importance of the PPS and its role in complete denture retention and the satisfaction of the patient.

Therefore, this study was aimed to evaluate the knowledge of Iraqi dentists with different academic degree for locating the vibrating lines, if Iraqi dentists mark and/or carve the post-dam, and what were the reasons for not recording the post-dam by the dentists themselves. The null and the alternative hypotheses for the study are shown with description in Table 1.

This study was approved by the Committee for Ethics / College of Dentistry, University of Baghdad (project no. 357221, date: June 7, 2021) and was conducted from October 2022 to November 2022 in accordance with the Helsinki Declaration of Ethical Principles for Medical Research with Human Subjects. The consent form was distributed with the google form and gathered electronically. The consent form and the patient information sheet were written as recommended by the Committee for Ethics/College of Dentistry, University of Baghdad and the participants were aware of the aims of the research and the confidentiality of their personal information as they accepted to fill the google form questionnaire.

The questionnaire was shared as a google form online in different groups for Iraqi dentists with different academic degree on social media applications (WhatsApp and Telegram). The first part of the questionnaire consisted of the personal information for the participant including the age, sex, years of experience after graduation and academic degree; doctorate (Ph.D.), master's (M.Sc.), higher diploma (H.D.D.), and bachelor's (B.D.S). The second part consisted of four questions related to the study, as shown in Table 2. The data was analyzed by using IBM SPSS Statistics for Windows, Version 19.0. Armonk, NY: IBM Corp https://www.ibm.com/products/spss-statistics. for frequency distribution. In addition, a multinomial logistic regression analysis model was used to examine the association between the dentists' ability to detect the vibrating line, including age, years of experience after graduation, gender, and academic degree.

The Iraqi dentists with different academic degree who participated in the survey were 118 in number. Among the participants, 86 were female and 32 were male with an age range between 22 to 46 years.

Most of the Iraqi dentists (90.7%) who participated in the survey stated that they know how to locate the vibrating lines, anterior and posterior, during maxillary removable denture construction and they answered by selecting “yes”, whereas 9.3% answered by selecting “No” ( Table 3).

Most of the participants (84.7%) answered that they “dentist” mark the post-dam on the master cast during maxillary removable denture construction while (15.3%) stated that the “dental technician” marks it ( Table 4).

More than half of the participants surveyed (52.5%) reported that they “dentist” carved the post-dam on the master cast during maxillary removable denture construction ( Table 5), while (47.5%) of the participates stated that the “dental technician” did it.

The percentage of participants who stated that they did not mark the post-dam on the master cast was 30.5% ( Table 6) and depend on the dental technician. While, the fear from denture failure and loss of retention was 25.4%, followed by 24.6% forget to carve before sending to dental laboratory, 10.2% have no information about carving, and 9.3% difficult to carve.

A multinomial logistic regression model was used to assess the understanding of Iraqi dentists' for locating the vibrating lines, whether they mark and/or carve the post-dam, and why the dentists did not record the post-dam themselves. However, only the academic degree showed a significant association. Age, experience after graduation, gender, and academic degree do not affect the participants' knowledge of locating the anterior and posterior vibrating lines during maxillary removable denture construction ( Table 7).

Being a B.D.S and a H.D.D dentist will reduce the chance of marking the post-dam on the master cast during maxillary removable denture construction by 18 and 19 units, respectively compared to dependence on the dental technician for this step ( Table 7).

Being a B.D.S and a H.D.D dentist will increase the chance of carving the post-dam on the master cast during maxillary removable denture construction by 19 units compared to dependence on the dental technician for this step ( Table 8).

One of the essential steps in treatment of edentulous patients is the construction of an appropriate denture with esthetically, functionally, and biologically acceptable features. The retention of the prosthesis is crucial to achieve these goals and the PPS plays a significant role in obtaining these features. ²

The step for introducing the PPS in the prosthesis should be done by the dentist and should not be imposed on the dental technician. ^{1 , 10}

This study’s results revealed that Iraqi dentists stated that they had the knowledge for the location of the vibrating line and they marked it in the patient’s mouth. However, most of those who didn’t mark and/or carve the post-dam most likely depended on the dental technician and fear from denture failure and loss of retention.

The first alternative hypothesis was accepted for this study. Results showed that the highest percentage (90.7%) of the Iraqi dentists stated that they had the knowledge on the location of the vibrating lines. This may be related to the fact that the position of the vibrating lines had been taught in detail during the study in Iraqi Colleges of Dentistry. This result agreed with finding of Sunil and Jain ¹² who stated that the practitioners had an acceptable amount of knowledge. In their study they found that over half of the participants surveyed (57%) stated that they had knowledge on how to locate the vibrating lines, while 43% stated that they had no knowledge.

The second null hypothesis was rejected and the alternative hypothesis was accepted. Most of the participants (84.7%) stated that they mark the post-dam on the master cast during maxillary complete denture construction. This could be due to the confidence and adequate knowledge they have for this step. In addition, this step is essential to transfer the vibrating lines form the patients mouth to the maxillary cast and was a mandatory step that the students must do when constructing a complete denture during the college study in the Iraqi Colleges of Dentistry. Adams ¹³ findings showed contradictory results in which most of dentists didn’t mark the post-dam on the maxillary cast. Also, our finding regarding the dentist carving the post-dam also disagreed with Adams ¹³ since almost half of the participants (52.5%) reported that the “dentist” carves the post-dam during maxillary removable denture construction. Therefore, the third null hypothesis was rejected and alternative hypothesis was accepted. Krysinski and Prylinski ¹⁴ suggested that the shape and depth of the PPS was influenced by the clinical experience and the ability to evaluate the mucosal compressibility in the post-dam area, as well as the practitioner’s hand-carving abilities. These findings were in accordance with this study, in which 25.4% of the participant didn’t mark the post-dam on the master cast, may be due to fear of denture failure and loss of retention, while 10.2% of the participants had no information and 9.3% considered carving as a difficult procedure. Therefore, the fourth alternative hypothesis was accepted.

Within the limitations of this study, most dentists with different academic degree stated they were familiar with the location of the vibrating lines, and they marked the post-dam. However, carving the post-dam was still a matter of subjective experience and they depending on dental technicians for this step. The marking of the post-dam may be related to several different reasons and the most common reason was that they depended on the dental technician in marking and/or carving the post-dam on the master cast followed by the fear from denture failure and loss of retention.