Different Types of Finish Lines, Gingival Retraction Methods and Impression Techniques Used During Single Crown Preparation

Ahmed Hilal Sheriff K1, Nivedhitha M S*2, Rakshagan V3 1Saveetha Dental College and Hospitals, Saveetha Institute of Medical and Technical Science, Saveetha University, Chennai-77, Tamil Nadu, India 2Department of Conservative Dentistry & Endodontics, Saveetha Dental College and Hospitals, Saveetha Institute of Medical and Technical Science, Saveetha University, Chennai-77, Tamil Nadu, India 3Department of Prosthodontics, Saveetha Dental College and Hospitals, Saveetha Institute of Medical and Technical Science, Saveetha University, Chennai-77, Tamil Nadu, India


INTRODUCTION
Finish line refers to the border of the preparation where the prepared tooth structure meets the unprepared surface of the tooth; line of demarcation, terminal portion of the prepared tooth. The adaptation of a restored tooth to the abutment tooth can be one of the most important factors that affect restoration prognosis (Komine et al., 2007). Factors such as inish lines in luence the marginal adaptation of the crown restorations (Alkumru et al., 1988;Syu, 1993;Shearer et al., 1996). Location of inish lines can be either subgingival, supragingi-val or equigingival. Different types of inish lines are knife/feather edge, shoulder, bevelled shoulder, chamfer. Although tooth preparation is part of the daily routine, dentists select the cervical inish lines based mainly on their experience and personal choice (Ravinthar and Jayalakshmi, 2018). Rounded shoulders and chamfer have been advocated by the manufacturers of various ceramic systems.
The retraction of the gingival tissue is a long established technique. It can be de ined as a process of de lection of the marginal gingiva away from the tooth. The gingival retraction is to allow access for the impression material beyond the margins and to create space for impression material to be suf iciently thick. Gingival retraction should be mandatory prior to impression so as to expose the prepared tooth surfaces (Goldberg et al., 2001;Ramamoorthi et al., 2015;PradeepKumar et al., 2016). Gingival retraction measures fall into one of four major categories (a) mechanic (b) chemomechanical (c) rotary gingival curettage and (d) electro-surgical methods (Benson et al., 1986). The mechanical aspect involves placing of a string into the gingival sulcus to displace the marginal gingiva physically. The chemo-mechanical aspect is basically treatment of the string that will induce temporary shrinkage of the tissues. Of these four categories; the chemo-mechanical method is most widely used (Donovan et al., 1985;Ramanathan and Solete, 2015;Siddique, 2019).
Although crown preparation is a common factor in daily practice, little is known about the prevalence of different techniques that may exist regionally or by each practitioner. Transfer of an accurate replication of the patient's hard and soft tissue to the dental laboratory is important. Most dentists have experienced the results of making a poor impression. The ability to identify and analyze inaccurate impressions and to understand how to avoid them is key to successful restoration. There are various techniques for making single crown impressions. The advancement in material and development of technique is a critical factor (Boulton et al., 1996;Caputi and Varvara, 2008;R and Ms, 2019). Various impression techniques are available but most commonly employed technique may be single step or two-step impression technique. The impression technique may be performed with putty, light body putty, medium body or heavy body rubber base impression material. Since impressions replicate both the teeth and the gingiva, success is based on understanding the anatomy of the periodontal tissues, creating an accurate and decipherable preparation (especially at the inish line), using the correct impression material and appropriate tech-niques (Chiche et al., 1994;Millar, 2001;Anusavice et al., 2003;Rajendran et al., 2019).
This study aimed to determine various types of inish lines, gingival retraction and impression techniques incorporated during single crown preparation in Saveetha Dental College.

MATERIALS AND METHODS
This was a retrospective study regarding different inish lines, gingival retraction methods and impression technique incorporated during single crown preparation done in Saveetha Dental College and Hospitals among patients from June 2019 to March 2020. The approval for this university setting was obtained from the Institutional Review Board.
The sample size of this study was found to be 812 patients with a mean age of 35.27 11.37, of which 453 of them were males and 359 of the patients were females. These patient details were obtained from a software system known as DIAS (Dental Information Archiving Software) which is exclusive for Saveetha Dental College and Hospitals. The data was extracted and tabulated in the Microsoft Excel based on the parameters required.

Inclusion Criteria
Patients treated with Single Crowns.

Exclusion Criteria: Incomplete Data
Once the case details have been obtained, the data was then extracted and tabulated based on the parameters which are age, gender, tooth no., inish line, gingival retraction, impression technique etc.

Statistical analysis
Once the results have been tabulated based on the parameters, the data was then exported to SPSS software. Associations of the parameters were done to detect the signi icance in SPSS. P-value less than 0.05 was considered to be statistically signi icant. Graphs were added to represent the association between the parameters.

RESULTS AND DISCUSSION
Total sample size of this study was 812 patients, out of which 252 of the patients were treated for mandibular molars and 632 of the tooth preparations had shoulder inish lines. The gingival retraction method used was a mechanical gingival retraction method which comprised 794 teeth and 554 of the gingival retraction was done using "2 cords (000+1)". 2 stage putty wash technique was commonly employed during impression taking in tooth preparation of 746 teeth. Chi-square test done for association between tooth no and inish lines, gingival retraction methods, no. of cords used and impression technique did not show any statistical signi icance (p>0.05) while association of inish line with no. of cords and type of impression showed statistical signi icance (p<0.05).
Graph 1: Bar graph representing the association between the site of the teeth and inish lines Graph 2: Bar graph representing the association between the site of the teeth and gingival retraction methods As a whole procedure during crown preparation each step is important starting from tooth preparation to impression making for successful treatment to be achieved. From this study, it can be inferred that the most commonly treated teeth for single crown are mandibular molars, meaning mandibular molars were the most commonly endodontically treated teeth requiring a single crown. These results coincided with a study done by Ahmed and Rahman (2009) in which he stated that mandibular molars were the most common teeth that required restora-Graph 3: Bar graph representing the association between the site of the teeth and cords used during retraction Graph 4: Bar graph representing the association between the site of the teeth and type of impression techniques tion post endodontic treatment (Ramesh et al., 2018;Janani et al., 2020;Jose et al., 2020).
This study is one of a kind as we have studied the association of tooth no. with inish lines, gingival retraction methods, impression techniques. Tables 1, 2, 3 and 4 & Graphs 1, 2, 3 and 4 show the association between teeth and inish lines, gingival retraction methods, no. of cords used, impression techniques, in which we can see that mandibular molars were commonly treated with shoulder inish lines (22.91%). Mechanical gingival retraction (30.30%) with 2 cords (000+1) (22.04%) was used and 2 step putty wash technique (28.57%) was commonly employed during single crown preparation.        Graph 5: Bar graph representing the association between the inish lines and gingival retraction methods Graph 6: Bar graph representing the association between the inish lines and no. of cords most commonly used inish line (76.23%) followed by chamfer inish line (=18.35%) and the least being feather edge inish line (0.37%). Mechanical retraction was commonly employed in shoulder inish lines which was followed by chamfer inish lines. Chemical, chemo-mechanical retraction methods were only used during shoulder and chamfer inish line preparations among which chemical method of retraction was mostly employed in shoulder inish line preparation and chemo mechanical method of retraction had an equal distribution among the two inish lines while laser retraction method was only used with chamfer inish line.
Tables 6 and 7 and Graphs 6 and 7 showed association of inish lines with no of cords used and different impression techniques. 2 cords (000+1) was Graph 7: Bar graph representing the association between the inish lines and impression techniques used seen highest (55.30%) followed by 2 cords (00+1) (17.98%) among shoulder inish lines. Among chamfer inish lines, 2(000+1) was the highest (10.71%) and 2 cords (0+2) the least (2.96%) among chamfer inish lines. As for impression technique, 2 stage putty wash technique was the most common impression technique followed for most of the single crown preparations and they were speci ically common among the shoulder inish lines (72.29%) and they were least among the feather edge and radial shoulder inish lines (0.37%). The 1 stage monophase impression technique was the least followed by the dentists and was employed in chamfer inish line preparations with a percentage rate of 0.12%.
Different inish line designs have been advocated for tooth preparations of ceramic crowns. All anterior restorations are fabricated with shoulder inish line margin where aesthetics is of primary concern and this coincides with our study (Edelhoff and Sorensen, 2002;Manohar and Sharma, 2018;Teja and Ramesh, 2019). The type of inish line used in the preparation of the teeth in luences the fabrication of the restoration and the inal outcome of the treatment.
Clinicians place retraction cords by using cord packing instruments. Serrated round end cord packing instruments are generally used with braided cord since small indentations in the instrument's head sink in the cord and prevent the instrument slippage and further trauma to the epithelial attachment. Non-serrated lat end instruments are applied in twisted cords with sliding motion (Hansen et al., 1999;Prasad et al., 2011;Noor et al., 2016). Gingival sulcus can be enlarged by placing a cord into the sulcus and leaving it in place for a period of time. It can either be single or double cord, they alone cannot control the sulcular hemorrhage hence used in conjunction with medicaments.
The use of a single retraction cord often provides inadequate gingival retraction. The dual-cord technique in which the irst cord remains in the sulcus reduces the tendency for the gingival cuff to recoil and partially displace the setting impression material (Cloyd and Puri, 1999). Results from a survey showed that most of the dentists (98%) used mechanical gingival retraction with double cord packing method (48%) and 44 percent using a single-cord technique (Hansen et al., 1999;Albaker, 2010;Nasim and Nandakumar, 2018). These results coincide with our study, where mechanical gingival retraction was the most commonly employed retraction method by the dentists during single crown preparations which was followed by chemical retraction method.
There are several impression techniques and protocols suggested; in the 2 stage putty wash technique, high viscosity material is used for a preliminary impression, lower viscosity material used for inal impression. Many studies have suggested that 2 stage putty wash technique is widely accepted, adopted and offers good accuracy (Nissan et al., 2000;Caputi and Varvara, 2008;Dugal et al., 2013;Hussainy, 2018;Nissan et al., 2002). This accuracy of impression is of utmost importance during tooth preparation and fabrication of a crown; (Kumar and Antony, 2018) the impression taken after tooth preparation not just replicates the tooth structure but also the anatomical structures which gives us an understanding and a clear negative picture of the tooth prepared for the fabrication of the crown.
In Table 1, it shows that the lower molars were commonly treated with shoulder inish line (n=176) followed by upper molars and premolars (n=135) and then upper anteriors(n=134). Chi-square test was done and was found to be statistically not signi icant [Chi-square value= 28.598; p=0.096 (p>0.05)].
In Graph 1, X-axis represents the site of the teeth among the patients and Y-axis represents the number of teeth receiving single crown. From this graph, we can infer that shoulder inish lines (Red) were commonly employed and lower molars had the highest percentage compared to the other sites. Chamfer inish lines (Blue) were also used among the patients and it was also highest among the lower molars. Chi-square test was done and was found to be statistically not signi icant [Chi-square value= 28.598; p=0.096(p>0.05)].
In Table 2, it shows that the mechanical gin-gival retraction was commonly used in lower molars(n=246) and the least was seen among lower anteriors (n=9). Chi-square test was done and was found to be statistically not signi icant p=0.804 (p>0.05)]. In Graph 2, X-axis represents the site of the teeth among the patients and Y-axis represents the number of teeth receiving single crown. From this graph, it can be seen that mechanical retraction (Blue) was commonly employed and lower molars had the highest rate compared to the other sites. Chi-square test was done and was found to be statistically not signi icant p=0.804(p>0.05)].
In Table 3, it shows that the most commonly used cords were "000" and "1" size cords during single crown preparation while the size "0" and "2" were not commonly used during the preparation. The size "000+1" was commonly employed in lower molars while the sizes "0+2" were commonly employed for upper molar teeth. Chi-square test was done and was found to be statistically not signi icant p=0.109 (p>0.05)]. In Graph 3, X-axis represents the site of the teeth among the patients and Y-axis represents the number of teeth receiving single crown. From this graph we can infer, 2 cords (000+1) (Red) were commonly used during retraction of the gingiva and lower molars (22.04%) had the highest rate compared to the other sites. The 2 cord(0+2) (Blue) were commonly used among the upper molars. Chisquare test was done and was found to be statistically not signi icant p=0.109 (p>0.05)].
In Table 4, it shows that the most commonly used impression technique was "2 stage putty wash" technique(n=746) followed by "1 stage putty wash"(n=65) which was common among the lower molars, and the least being "special tray+monophase" technique(n=1). Chi-square test was done and was found to be statistically not significant [Chi-square value= 6.266; p=0.792(p>0.05)]. In Graph 4, X axis represents the site of the teeth among the patients and Y-axis represents the number of teeth receiving single crown. From this graph, it can be seen that 2 stage putty wash (Red) was widely used among most of the teeth and had the highest incidence among the lower molars. Special tray+monophase technique(green) was used only among the upper molar teeth. Chi-square test was done and was found to be statistically not signi icant [Chi-square value= 6.266; p=0.792(p>0.05)].
In Table 5, it shows that mechanical gingival retraction method was commonly employed among the different inish lines, shoulder(n=619) being the highest followed by chamfer(n=149) and the least being feather edge inish line(n=3 In Table 6, it shows that the retraction cord '000+1' was commonly used and was seen mostly with the shoulder inish lines(n=449) followed by chamfer(n=87 In Table 7, It shows that the 2 stage putty wash technique was mostly used(n=746) among all the teeth but most common among the shoulder inish lines(n=587) followed by chamfer inish lines(n=138). The 1 stage putty wash technique(n=65) was also used during single crown preparation but not as much as 2 stage putty wash technique. The special tray+monophase technique was employed only in the teeth with chamfer inish line and not in any other inish lines. Chi-square test was done and was found to be statistically signi icant [Chi-square value= 20.576; p=0.008(p<0.05)]. In Graph 7, X axis represents the different inish lines used and Y-axis represents the number of teeth receiving single crown. From this bar graph, it can be seen 2 stage putty wash technique (Red) was mostly used among the shoulder inish lines followed by chamfer inish lines. The special tray+monophase technique (green) was employed only in the teeth with chamfer inish line and not in any other inish lines. Chi-square test was done and was found to be statistically signi icant [Chi-square value= 25.863; p=0.001(p<0.05)].

Study limitations
This study was done in a small population among the patients who required a single crown in Saveetha Dental College and Hospitals in a given period of time.

Future Scope
This study can be done in a larger population among the patients without any speci ic time period. Other parameters like material of choice for the crown in single crown preparations can be included in future studies.

CONCLUSION
Within the limit of the study, the most widely used inish lines were shoulder inish line and mechanical gingival retraction method was commonly employed with 2 stage putty wash impression technique.