This study was the first in Yemen to evaluate the quality of fixed prosthesis impressions and to determine the factors that affect errors present in impressions. It is very important for dentists to self-evaluate the impressions after taking them as a fundamental step for the clinical success of fixed prosthesis. In this study, the impressions were evaluated according to these criteria: errors in finish line errors, retention of material to tray, errors in the preparation area, gingival retraction, errors in silicone impression technique, blood in impression and retention of material to the tray.
This study showed that 97% of impressions received by the dental laboratories had at least one detectable error which is in agreement with previous studies [14,15,16,17].
Marginal detail is the most critical aspect of the impression. Failure to record the appropriate details of the finish line of the preparation will result in incorrect prosthetic fit. The errors in the finish line were considered critical errors. In this study 152 out of 165 (92.1%) impressions were had at least one visible error in the finish line. This finding was higher than Rau's study in which the finish line area had at least one visible error in 55% of the evaluated impressions .
According to the present study, (69.1%) of impressions had bubbles in the finish line. This percentage was in agreement with Samet’s study (40.4%) . Bubbles occurs as a result of air entrapment during mixing of material, tray-loading, syringing or tray placement and can negatively affect the fit of the prosthetics [24, 26]. Voids in this study were detected in (43.6%) of impressions evaluated. Voids usually larger and less sharp in definition and occurs due to fluid accumulation . This result was consistent with Samet’s study (50.7%) in Israel and more than Rau’s study in North Carolina, USA (24.8%) [14, 17].
Pull or fold in the finish line was detected in (32.7%) of impressions evaluated. It is often produced at the gingival aspect when impression material pasts its working time (no longer in its most fluid state) or when the impression material fails to adapt to the teeth . Tear in the finish line was presented in (17.6%) of impressions (Fig. 1b). Marginal tears can result when a syringing material with inadequate tear strength is used, using a light body PVS in a thin deep sulcus, or the impression is removed prior to the complete setting of the syringing material . The tissue over the finish line was detected in (11.5%) of impressions evaluated (Fig. 1a). This result was less than Rau’s study (49.09%) . That is due to the difficulty of detecting this error because most of the finish line types in evaluated impressions were knife-edge and it is difficult to detect in impression to determine if the tissue is covering the finish line.
Accurate impressions of the margins can only be anticipated with appropriate gingival displacement, margin design, margin placement, and moisture control. In tooth-supported fixed prosthesis, impression making requires an accurate record of the prepared finish line area, especially in cases where the preparation margin is located at the same level of gingiva or sub-gingiva [27,28,29]. In this study, the adequate gingival retraction was adequate only in (8.5%) of impression evaluated.
The result of this study presented that errors in the preparation area were detected in (53.9%) of impression evaluated. The most common error in the preparation area was bubbles (34.5%) followed by voids (27.9%) as shown in Fig. 1c. Air bubbles are resulted during mixing, while voids are resulted due to moisture or debris on the oral tissues . This percentage was in agreement with Rau's study  in which voids percentage was (13.3%). Voids may be large enough to affect the long-term success of the luting cement, which must now fill a wider space. The prosthetic material may also be thinner than recommended. This can be more critical when using all-ceramic materials, as they require minimum thicknesses to perform as expected 
The contaminated impression is considered a principle possible route of spread of infection from the patient in the clinic to dental technicians . Disinfection of impression materials should be mandatory to prevent cross-infection. The impression should be rinsed with water and then disinfected . The present study found that blood was in (52.1%) of impressions evaluated. This finding was in the same line with a study in Sudan in which blood was clear in (68.9%)  and higher than that in North Carolina, USA in which (14.70%) of impression was soiled with blood .
The results of this study indicated that the most generally used impression materials by the general practitioners for their crown and bridgeworks were alginate impression materials. These practices weren't concurrent with the practice worldwide where the most commonly used materials by the general practitioners for their crown and bridgeworks were elastomeric impression material [14, 16, 17, 20]. Alginate is not accurate enough for fixed partial dentures but used for partial framework impressions or provisional restorations [34, 35]. In addition, usually the alginate impressions were sent to the laboratory covered by wet paper; this method is not considered ideal because impression can easily absorb water from the wet paper and consequently deform before pouring .
PVSs are the impression materials of choice for fixed prosthodontics. However, in this study Addition silicone only account for (20.6%). The widespread use of alginate may be related to their cheap price, lack of knowledge about the proper use of silicone impression materials and/or to the dentist’s lack of knowledge about their limitations.
This study was reported that all of the silicone impressions were recorded using the putty wash technique. The most commonly used impression techniques for putty wash are one-step and two-step techniques. The result of this study showed that out of 81 silicone impressions, (80.2%) were two-step techniques and (19.8%) were the one-step technique. Hung et al.  and Idris et al.  concluded that impression accuracy is technique independent and the differences between techniques were not considered to be clinically important. On the other hand, other studies stated that the impression technique is a significant factor in determining the accuracy of the impression [37,38,39,40]. The one-step putty-wash technique requires less chair-side time. The two-step putty-wash technique produces some more precise castings . The two-step putty-wash technique has been reported to be more accurate than the one-step putty-wash technique [37,38,39, 41] because there is uniform space for the light-body material to polymerize and the details are captured by the light-body material only [37, 40, 41].
The most common error in the silicone impression technique was heavy-bodied materials exposure through wash material in (69.1%) of impressions evaluated, and this finding was in agreement with Samet's study (44%) . This error in double-step technique may indicate that there is no uniform space for the light body either before putty impression taking or during cutting space for the light body after putty impression taking. In the single-step technique, this error due to the putty tends to push the light-body wash away from the prepared tooth .
Lack of wash materials in the finish line area represent the third most common errors in silicone impression technique (7.9%) (Fig. 1D), this finding was in agreement with Rau’s study in North Carolina, USA (16%) . The light body materials are able to record fine detail of 25 µm or less , but the putty materials, in general, cannot record fine detail at the 25-µm level and are required only to reproduce the detail of 75 µm .
In order to increase the accuracy of final impressions, the dimensional stability of an impression tray is also a contributing factor. Trays should have good stability along a period of time and do not portray any permanent deformation between impression taking and pouring stage . The use of a soft plastic stock tray cannot be considered an ''observable defect,’’ although there is a likelihood for inaccuracies due to the flexible nature of these trays [15, 44]. The result of this study presented that (92.1%) of impression was recorded using a plastic tray as found in previous studies [14, 17]. The widespread use of such trays may be related to their cheap cost and/or to the dentist’s lack of knowledge about their limitations.
In general, most of the impressions evaluated were had at least one error. There are many reasons for the high incidence of unacceptable impressions sent to the laboratories. It could either be clinicians' factors, material properties factors or patient factors. As for clinicians, it is either due to the lack of knowledge and experience, poor manipulation of the impression material, lack of attention to details, low awareness on the need for critical self-evaluation , early removal of the impressions prior to complete setting which means that it is possible that impressions are often removed from the mouth when the dentist ‘‘feels’’ that the material has polymerized, ignoring the polymerization time recommended by the manufacturer . Or even financial constraint could be the possible underlying reasons for these unacceptable impressions being sent to the laboratory .
Dentists involved in this study are graduated from different Yemeni dental schools which have varying crown and bridge prerequisites, and requirements for graduation. In general, teaching of crown and bridge is started from the third year as “fixed prosthodontics (I)” in which a preclinical annual course is taught. In this course, the dental students are required to perform different type of preparations “all metal, metal-ceramic, all ceramic and partial veneer crown “on artificial acrylic teeth, also they taught to perform the one step and two step putty wash technique by using condensation silicone.
In the following fourth year, “fixed prosthodontics (II)” is taught, which is a clinical annual course in which students have to perform many simple single crown cases. Finally, on the fifth year of the dental program, clinical fixed prosthodontics is taught as “fixed prosthodontics (III)” in which students are required to perform three or four unite bridges, custom made post and fiber post. The students are required to record final impression by using condensation silicone and due to the high cost of addition silicone, many dental schools do not provide this material for students.
Teaching the fixed prosthodontics is considered highly qualified but unfortunately many dentists do not keen to update their information after graduation. Also, the absence of control by the medical council may contribute to these unacceptable impressions.
There is a tendency for errors of silicone impression technique to be more in female dentists than males. This finding may be explained by that the number of male dentists is more than females, and thus their error percentage decreased. This result may need more studies to prove or disprove it.
There was an increased percentage of errors occurred in silicone impression technique among dentists with less than 10 years of experience with a significant difference. Lack of wash materials in finish line area and Heavy bodied materials exposure through wash material were more in dentists with more than 10 years of experience years with a significant difference too. This can be explained by that experience years is not a factor that influences the quality of impression or a larger number of samples may be necessary to study this association.
This study presented that there was non-significant association between errors in the FL and in the preparation area and dentist’s gender or years of experience. A larger number of samples may be necessary to study this association.