|Year : 2012 | Volume
| Issue : 1 | Page : 19-26
The effect of retentive groove, sandblasting and cement type on the retentive strength of stainless steel crowns in primary second molars - An in vitro comparative study
MM Veerabadhran1, V Reddy2, UA Nayak3, AP Rao2, MA Sundaram4
1 Department of Pedodontics and Preventive Dentistry, Vivekananadha Dental College and Hospital for Women, Trichengode, India
2 Department of Pedodontics and Preventive Dentistry, Rajah Muthiah Dental College and Hospital, Annamalai University, Chidambaram, Tamil Nadu, India
3 Department of Pedodontics and Preventive Dentistry, Mahatma Gandhi Dental College and Hospital, Jaipur, Rajasthan, India
4 Department of Pedodontics and Preventive Dentistry, PMS College of Dental Science and Research, Trivandrum, Kerala, India
|Date of Web Publication||3-May-2012|
M M Veerabadhran
18/112, Main Bazaar, Kotagiri 643 217, The Nilgiris, Tamil Nadu
| Abstract|| |
Purpose: This in vitro study was conducted to find out the effect of retentive groove, sand blasting and cement type on the retentive strength of stainless steel crowns in primary second molars. Materials and Methods: Thirty-two extracted intact human maxillary and mandibular primary second molars were embedded in aluminum blocks utilizing autopolymerising acrylic resin. After tooth preparation, the 3M stainless steel crown was adjusted to the prepared tooth. Then weldable buccal tubes were welded on the buccal and lingual surfaces of each crown as an attachment for the testing machine. A full factorial design matrix for four factors (retentive groove placement on the tooth, cement type, sandblasting and primary second molar) at two levels each was developed and the study was conducted as dictated by the matrix. The lower and upper limits for each factor were without and with retentive groove placement on the tooth, GIC and RMGIC, without and with sandblasting of crown, maxillary and mandibular second primary molar. For those teeth for which the design matrix dictated groove placement, the retentive groove was placed on the middle third of the buccal surface of the tooth horizontally and for those crowns for which sandblasting of the crowns are to be done, sandblasting was done with aluminium oxide with a particle size of 250 mm. The crowns were luted with either GIC or RMGIC, as dictated by the design matrix. Then the retentive strength of each sample was evaluated by means of an universal testing machine. The obtained data was analyzed using ANOVA for statistical analysis of the data and 't'- tests for pairwise comparison. Results: The mean retentive strength in kg/cm 2 stainless steel crowns luted with RMGIC was 19.361 and the mean retentive strength of stainless steel crowns luted with GIC was 15.964 kg/cm 2 with a mean difference of 3.397 kg/cm 2 and was statistically significant. The mean retentive strength in kg/cm 2 of stainless steel crowns, which was not sandblasted, was 18.880 and which was sandblasted was 16.445 kg/cm 2 with a mean difference of 2.436 kg/cm 2 . These results were again statistically significant. Conclusion: It was found that the crowns luted with resin-modified glass ionomer cements (RMGIC's) offered better retentive strength of crowns than glass ionomer cements (GIC) and stainless steel crowns which were cemented without sandblasting showed higher mean retentive strength than with sandblasting of crowns. The presence of groove did not influence the retentive strength of stainless steel crowns.
Keywords: Aluminium oxide, glass ionomer cement, resin-modified glass ionomer cement, retentive groove, retentive strength, sandblasting, stainless steel crown
|How to cite this article:|
Veerabadhran M M, Reddy V, Nayak U A, Rao A P, Sundaram M A. The effect of retentive groove, sandblasting and cement type on the retentive strength of stainless steel crowns in primary second molars - An in vitro comparative study. J Indian Soc Pedod Prev Dent 2012;30:19-26
|How to cite this URL:|
Veerabadhran M M, Reddy V, Nayak U A, Rao A P, Sundaram M A. The effect of retentive groove, sandblasting and cement type on the retentive strength of stainless steel crowns in primary second molars - An in vitro comparative study. J Indian Soc Pedod Prev Dent [serial online] 2012 [cited 2013 Jun 18];30:19-26. Available from: http://www.jisppd.com/text.asp?2012/30/1/19/95570
| Introduction|| |
The restoration of multisurface involved carious lesion in primary molars with conventional restorative materials has been a great challenge to the dentist. Ever since the introduction of stainless steel crowns by Engel  and later Humphery  in 1950, the premature loss of teeth due to rapidly progressing caries has been decreased, whenever these crowns are used.
A number of factors have been tried to increase the retention of these crowns. Some modified the tooth preparation , and some compared various luting cements so as to find out which cement gave maximum retention. Over the years, stainless steel crowns have been cemented with red copper phosphate cement,  zinc phosphate cement, ,, zinc polycarboxylate cement, , zinc oxide eugenol cement and glass ionomer cement. , Although some good luting agents were available in the past, the development and use of innovative, adhesive, esthetic restorative materials have led to the introduction of new types of cements. Some of the recently developed luting agents like the resin-modified glass ionomer cements represent significant advances in physical properties with higher strength that may result in better clinical performance of the luted restoration.
Croll  suggested cutting vertical grooves around the prepared tooth crown's periphery increased the surface area and perhaps enhanced crown retention by providing resistance against any rotational forces during mastication. However, the efficacy of this preparation versus a conventional preparation has not been demonstrated.
Sandblasting the interior surface of the stainless steel crowns by aluminium oxide may also aid in additional retention of the stainless steel crowns but this effect is also not well documented.
The present study was undertaken with the following aims and objectives
- To evaluate whether retentive groove placement on the tooth influences the retentive strength of stainless steel crowns.
- To find out the influence of cement type on the retentive strength.
- To find out the efficacy of sandblasting of the crowns on the retentive strength.
- To assess whether any difference in retentive strength of stainless steel crowns is present between maxillary and mandibular primary second molar.
- To find out the interaction of any two factors mentioned above.
| Materials and Methods|| |
Four variables namely 1) retentive groove placement on the tooth 2) type of cement, 3) sandblasting of crowns and 4) primary second molar were considered to evaluate the retentive strength of stainless steel crowns. In the four parameters, the respective lower and upper limits for the retentive groove placement were chosen as without and with retentive groove placement and for the cement type it was glass ionomer cement (GIC) and resin-modified glass ionomer cement (RMGIC). The lower and upper limits for the other two variables were: For sandblasting of crowns, it was without and with sandblasting, and for primary second molar it was maxillary and mandibular tooth. For the convenience of recording and processing the experimental data, the lower and upper levels of the parameters were coded as -1 and +1, respectively [Table 1].
Developing the experimental design matrix
Due to the narrow range of factors, it was decided to use a two-level full factorial design. The [Table 2] shows the 16 sets of coded conditions (-1 or +1) used to form the design matrix 2 4 factorial design. The evaluation of retentive strength was done as per the condition dictated by the design matrix [Table 2].
Conducting the experiment as per design matrix
Sixteen extracted, intact human primary mandibular second molars (8 maxillary and 8 mandibular) with minimal occlusal caries, if any, were collected and stored in distilled water at room temperature (23°C). These teeth were then hand scaled and cleaned, to remove soft tissue debris after which, horizontal notches were prepared on the roots for better anchorage of the tooth in the autopolymerizing acrylic resin. Each tooth was embedded in autopolymerizing acrylic resin exposing the whole crown till the cementoenamel junction using aluminium blocks measuring a height of 6 cm, length and width of 1.7 cm.
The appropriate crown for a particular tooth was selected by a trial and error procedure with respect to mesiodistal width and cervicoocclusal height of each tooth. Pretrimmed, precontoured stainless steel crowns were selected (3M Co.). The occlusal surfaces of all teeth were reduced uniformly about 1.4 mm (diameter of the bur) by using a straight fissure bur [No: SF21 (Mani)]. This was established by placing depth-orientation grooves at the cuspal heights. The proximal surfaces were prepared with a No. 104R tapered fissure bur (Shofu), by removing all mesial and distal undercuts without leaving any ledge. All the sharp angles were rounded. After preparation of the tooth, the selected crown was trimmed, contoured and crimped. The proper fit of the crown was checked and confirmed by means of an explorer. The contour and crimping of crowns were established by contouring and crimping pliers. The crowns were then removed and weldable buccal tubes were welded on the buccal and lingual surface of all crowns to facilitate an attachment for the universal testing machine.
For those teeth for which the design matrix dictated groove placement, groove was prepared horizontally on the middle third of the buccal surface with a depth of 1 mm and a length of 4 mm corresponding to the diameter and the length of a straight fissure bur (No: SF 41 - Mani). For those crowns for which the design matrix indicated sandblasting of crowns, sandblasting was done with aluminium oxide particles - particle size of 250 mm (Korox® 250-blasting material out of 99.6% aluminium oxide).
All the teeth were cleaned with pumice and water before cementation. The crowns were luted with either GIC (GC Fuji I - GC Dental Industrial Corp., Tokyo, Japan) or RMGIC (FujiCem™ - GC Dental Corp., Tokyo, Japan) as indicated by the design matrix for the respective specimen. Both the cements were mixed according to manufacturer's instructions at room temperature. They were loaded into the crown (2/3 rd of the crown) and each crown was seated with finger pressure. After initial set, excess cement was removed from the crown-tooth interface using an explorer. The teeth were then stored in prepared artificial saliva and incubated at 37°C for 24 hrs.
Recording the results viz. retentive strength
The retentive strength was evaluated using a universal testing machine (Unitek 94100, FIE, India) [Figure 1] . After stabilization of the specimen on the machine, load was applied which was gradually increased from zero reading to a point until the cemented crowns showed first dislodgement and the corresponding value was noted from the graphical representation present in the testing machine computer monitor. The same procedure was repeated for all the other specimens. The applied load was directed parallel to the long axis of the tooth during crown removal, with a crosshead speed of the machine 0.05 inch/minute.
|Figure 1: Specimen undergoing retentive strength testing in universal testing machine (Unitek 94100)|
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The retentive strength was calculated by the formula:
Retentive strength=load/area and expressed in terms of kg/cm 2 .
The surface area of each crown was determined by cut opening the crown and their surfaces were developed on graph sheets and the area of these developed surfaces was determined by counting the squares on these developed areas. In order to obtain estimate of the experimental error in this two factor experiment, the entire set of experiment was replicated again using another 16 teeth. Hence, the total sample size was 32 teeth.
The obtained data was analyzed using ANOVA (Analysis of Variance) for statistical analysis of the data and 't'- tests for pairwise comparison. (SPSS statistical package was used).
| Results|| |
Retentive strength for each combination of factors obtained during the first and repetitive experiment and the descriptive statistics for retentive strength are shown in [Table 3] and [Table 4], respectively
In the main effects, only sandblasting of the crowns and cement type had a statistically significant effect on retentive strength of stainless steel crown and in two factor interactions, three combinations namely groove and cement, groove and sandblasting and cement and molar were statistically significant [Table 5].
The mean retentive strength in kg/cm 2 stainless steel crowns luted with RMGIC was 19.361 and the mean retentive strength of stainless steel crowns luted with GIC was 15.964 kg/cm 2 with a mean difference of 3.397 kg/cm 2 and was statistically significant [Table 6]. The results clearly indicate that RMGIC's have a better retentive strength than GIC and thus type of cement definitely influenced the retentive strength.
The mean retentive strength in kg/cm 2 of stainless steel crowns, which was not sandblasted, was 18.880 and which was sandblasted was 16.445 kg/cm 2 with a mean difference of 2.436 kg/cm 2 favoring the crowns, which were not sandblasted [Table 6]. These results were again statistically significant.
The pairwise comparison of two factors interaction between groove and cement showed a mean retentive strength in kg/cm 2 of stainless steel crowns with groove on the tooth, luted with GIC to be 17.132 and luted with RMGIC to be 17.979 [Table 7]. On the other hand, the mean retentive strength of stainless steel crowns without groove on the tooth, luted with GIC was 14.796 kg/cm 2 and luted with RMGIC was 20.744 kg/cm 2 and was statistically significant [Table 7]. This clearly indicates that when there is no groove, RMGIC behaves better than GIC.
|Table 7: Pairwise comparison – two factor interactions - Groove and Cement|
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The pairwise comparison of two factor interactions namely groove and sandblasting showed a mean retentive strength of stainless steel crowns which was not sandblasted, with the groove on the tooth to be 17.334 kg/cm 2 and for the crowns which was sandblasted to be 17.778 kg/cm 2 [Table 8]. When there is no groove on the tooth, the mean retentive strength in kg/cm 2 of stainless steel crowns without and with sandblasting were 20.427 and 15.112, respectively, and was statistically significant [Table 8]. This shows that when groove is not given, the crowns, which were not sandblasted, offered better retention.
|Table 8: Pairwise comparison -Two factor interactions-Groove and Sandblasting|
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The pairwise comparison of two factor interactions namely type of cement and molar was also calculated. The mean retentive strength of stainless steel crowns for mandibular primary second molar luted with GIC and RMGIC were 17.454 kg/cm 2 and 18.124 kg/cm 2 , respectively [Table 9]. The mean retentive strength in kg/cm 2 of stainless steel crowns for maxillary primary second molar luted with GIC and RMGIC were 14.475 and 20.599, respectively, [Table 9] and was statistically significant, indicating that RMGIC behave better in the maxillary tooth.
|Table 9: Pairwise comparison – Two factor interactions - Cement and Molar|
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| Discussion|| |
In the past, studies on retentive strength of stainless steel crowns were done based on the cement type and tooth preparation. But the effect of sandblasting the interior of the stainless steel crowns and placement of groove on the primary tooth on retentive strength were not done. All the factors (cement type, sandblasting, retentive groove) may be independent or may interact with each other. If they are not independent, separate single factor experiments may not give results, which can be intercompared. To make the intercomparison possible, a two-level factorial experiment was done. The method of designing a two-level factorial design matrix is dealt with elsewhere.  In factorial experiments, not only the different levels of each factor, the main effects (-1 and +1) can be compared but also their interaction can be studied. Even when the factors are independent, that is when there is no interaction, a factorial experiment is more economical in the sense that a smaller sample size will give the same efficiency in a factorial experiment compared to single factor experiments. 
One of our results was consistent with the results of the study done by Mitchell et al.  where they compared the maximum loads and modes of failure of four glass-ionomer cements during post-pull-out strengths. They found that Vitremer, a resin-modified glass ionomer cement offered a higher retention when compared to glass ionomer cements. Although the method of that study and the present study was different, the purpose of both the studies was to determine better glass ionomer cements for retention. The probable reason for increase in retentive strengths of crowns luted with RMGIC can be attributed to the modified composition and low solubility. Fujicem is a hybrid ionomer and two setting reactions takes place. The initial setting reaction of the material is by the polymerization of methacrylate (2-hydroxy ethyl methacrylate). The slow acid-base reaction is ultimately responsible for the unique maturing process and final strength.
The other statistically significant factor that affected the retentive strength of stainless steel crowns was the sandblasting of crowns. The idea of roughening the interior of the crown came from a clinical study done by Garcia-Godoy  where he roughened the interior of the crown with a high-speed bur to create a more retentive surface. However, the authors failed to report the significance of this, on the retention of stainless steel crowns. Taking this into consideration, in this study instead of roughening the inside of the crown with a high-speed bur, the inside of the crowns were sandblasted with 250 m aluminium oxide particles uniformly. The high-speed bur was avoided for fear of perforating the crown.
In our study, sandblasting of the crowns increased the bond between metal-cement interface but reduced the retentive strength of the stainless steel crowns because of the cohesive failure within the cement. O' Connor et al.  found that microblasting the internal surface of cast crowns with 50 mm aluminium oxide significantly improved the retention of castings luted with zinc phosphate. On the other hand, a study done by Worley et al.  observed that the sandblasting of the crowns had no significant effect on retention. Our results are not consistent with the earlier studies , because in these studies zinc phosphate cement which provides mechanical rather than chemical retention was used. But in this study both the cements used provided chemical retention. As the sandblasting of the stainless steel crowns provided decreased retentive strength, it can be suggested that the manufacturers surface pretreatment of the crowns are sufficient when glass-ionomer cement and resin-modified cements are used.
In our study, the presence of groove that was thought to increase the surface area did not have a significant effect on retention. This finding is consistent with the statement made by Rosenstiel et al.  "Adding grooves or boxes to a preparation with a limited path of withdrawal does not markedly affect retention because the surface area is not increased significantly. However, where the addition of a groove limits the paths of withdrawal, retention is increased."
Usually in fixed prosthodontics, the grooves are placed proximally and studies done with the proximal grooves , found that addition of grooves produced a significant increase in retention of complete cast crowns. On the other hand, one study done by Potts et al.  found that the addition of grooves to partial veneer preparations did not significantly augment retention. The authors suggested that a groove adds little to the total surface area of the preparation and the current study results are in accordance with this. Croll  in his clinical technique has placed vertical striations on the buccal surface of the tooth. In this study, the groove was neither placed proximally nor vertically on the buccal surface, instead groove was placed horizontally exactly on the middle third of the buccal surface of the tooth. Proximal surfaces were not selected because very often caries destroys these surfaces in primary molars. Since the crown removal was done parallel to the long axis of the tooth, it was assumed that placing groove horizontally rather than vertically might offer more retention.
In our study, since the buccal and lingual reduction was not done on the tooth, maxillary and mandibular tooth was also taken as a variable so as to find out whether any difference is present in the natural contours of these teeth. It was found out that although the natural contours on the buccal and lingual surfaces were left intact for both maxillary and mandibular primary second molars, there was no significant difference between maxillary and mandibular primary second molars in their retentive strengths.
One of the main aims of the study was to find the pairwise comparison of two factor interactions, which has not been studied before. When the interactions between any two factors were considered, only three combinations were statistically significant. They were groove and cement, groove and sandblasting and cement type and molar.
The results of pairwise comparison of two factor interactions namely groove and cement clearly indicated that when there is no groove, RMGIC offered better retentive strength than GIC.
The results of pairwise comparison of two factor interactions namely groove and sandblasting showed that when groove was not given, the crowns which were not sandblasted, offered better retention.
The results of pairwise comparison of two factor interactions namely type of cement and molar indicated that RMGIC offered better retentive strength in the maxillary tooth.
| Conclusions|| |
From the present study, it can be concluded that,
Considering the interaction of factors, which were statistically significant,
- Presence of groove did not influence the retentive strength of stainless steel crowns.
- RMGIC's offered better retentive strength of crowns than GIC.
- Stainless steel crowns which were cemented without sandblasting showed higher mean retentive strength than with sandblasting of crowns and this difference was statistically significant.
- There is no statistically significant difference in the retentive strength of stainless steel crowns in maxillary and mandibular primary second molar.
- When there is no groove, RMGIC offered better retentive strength than GIC while RMGIC and GIC offered the same retentive strength in the presence of groove.
- When groove is not placed, the crowns which were not sandblasted offered better retentive strength than the crowns which were sandblasted while in the presence of groove with or without sandblasting offered the same retentive strength.
- RMGIC's offered better retentive strength than GIC in maxillary primary second molars while the retentive strength was the same for GIC and RMGIC in mandibular primary second molars.
On the basis of this study, resin-modified glass ionomers can be recommended for the cementation of stainless steel crowns for better retention. The placement of the groove that was thought to increase the surface area did not increase the retentive strength and the manufacturer's surface pretreatment of the stainless steel crowns is sufficient to enhance crown retention. However, further studies with a larger sample size should be carried out in this direction.
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[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7], [Table 8], [Table 9]