Oral Conditions And Tooth Morphology Biology Essay

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Polyvinyl siloxane impression materials have the best fine detail reproduction and elastic recovery of all available materials. Because there is no by-product to the setting reaction, they possess remarkable dimensional stability and are odorless and tasteless and pleasant for patients. They are provided in wide range of viscosities, rigidities, and working and setting times, so they can be used in a variety of clinical situations.

Impression techniques can be categorized as monophase or dualphase. Techniques that use monophase materials are accomplished in a single-step procedure, using materials with a medium viscosity to allow the material itself to record the finer details while avoiding the slumping of the material in the tray. Techniques that use dual-phase materials such as the putty and light-body may be accomplished in one or two steps (one-step and two-step putty/ light-body techniques). The one-step putty/light-body technique requires less chair-side time. In the two-step putty/ light-body technique, the details are recorded by the light-body material only. [1]

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A number of factors are critical in achieving optimum accuracy from an elastomeric impression material. One of the most important factors is control of the cross-sectional thickness and the provision for a relatively uniform bulk of impression material. [2]

The problem of accuracy of impressions has been stressed recently in a clinical study, which reported that over 89% of the impressions investigated had 1 or more observable errors; thus, a more critical evaluation of impressions on the part of the dentist is recommended [3]

Several factors can influence the quality of impressions, including the impression technique[4-8] the impression material[5,9] the bulk of material[10-12] and others.[5,13,14]

Therefore the objective of this study was to evaluate the dimensional accuracy of casts obtained from single step and double step polyvinyl siloxane putty wash impression techniques and in the double step impression technique 0.5mm, 1mm and 1.5mm spacer thicknesses were used to evaluate that which one gives maximum linear accuracy.

Materials and instruments:

A mild steel master model containing 2 complete-crown, tapered abutment preparations was made. This model had two dies which simulated a clinical crown preparation with 6 degrees total taper. The occluso - gingival length of the die was 8 mm. the width of the die is 6mm and the base of the each die was 2 mm in height. Cross grooves were provided on the occlusal surfaces 1mm in depth to serve as reference points for making measurements(Fig. 1). The two dies were then welded onto a horizontal metal platform measuring 120mm-40mm (figure 2). A distance of 28 mm was maintained between the dies at the occlusal level. Two metallic stumps were fixed on either side of the horizontal metal platform for proper orientation of the perforated metal tray. Grooves were made on the platform for the escaping of the light body. A mark was made near the base of one die to recognize the right and left die after pouring the cast.

This was then used as the definitive standardized model for the comparison of the impression techniques in this study.All of the impressions were made in customarily made perforated metal trays (Figure 2). These were fabricated maintaining a space of 7 mm for the impression material between the abutment preparation and the inner tray surface. Holes of 2 mm diameter were provided for mechanical retention. The perforations were provided for the mechanical retention. Additional retention was provided using a tray adhesive(3M, ESPE VPS tray adhesive). Four holes were provided for fitting of the vertical extensions on the base for proper orientation of the tray during impression making and to keep the path of insertion and removal same for all the impressions. Metallic spacers were fabricated with 0.5mm, 1mm and 1.5mm thicknesses (Figure 3).0.5 mm, 1mm and 1.5mm spacer caps were milled in polymethyl methacrylate resin and then they were cast in Ni-Cr alloy. Three wax pattern were fabricated for spacer plates with 0.5mm, 1mm and 1.5mm thickness and then were cast in Ni-Cr alloy(Figure 4). All the metallic components were fabricated from mild steel and with the exception of the inner surface of the tray and carrier, were chrome plated to avoid rusting. 15 Impressions of the stainless steel model were made for each of the 4 techniques, - single step putty light body impression technique and two step putty light body impression technique with 0.5mm, 1mm and 1.5mm of spacer thickness.

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Impressions were made with addition-reaction silicone impression material(3M ESPE Seefeld; Germany )Putty (Express TM XT Putty soft) and Light body (Express TM XT Light body)

The putty material was mixed with fingertips for 30 seconds until the color was uniform, and all of the other materials were dispensed with an automatic mixing syringe (AutoMix; Dentsply Intl) Care was taken to maintain a working time of three minutes for putty and three and half minutes for light body impression material as recommended by the manufacturer.

All the impressions were allowed to set on the master model for twice the recommended setting time in the mouth. This was in order to compensate for the polymerization occurring at room temperature (25 oC ± 2 oC) rather than mouth temperature (32 oC ± 2 oC) in accordance with ADA specification No 19 [15-17]

Proper care was taken to check metal to metal contact of the tray and the base to confirm exact seating of the tray.

The impressions were poured after thirty minutes because this would approximately be the elapsed time before an impression could be poured in a clinical situation

Group I - single step putty light body impression technique

Equal quantities of the base and catalyst of putty impression material (Express TM XT Putty soft, 3M ESPE) were mixed and loaded into the impression tray, while the light body material (Express TM XT Light body,3M ESPE)was injected over the abutment preparations on the master model with the help of mixing gun. Once the light body material was injected, the tray loaded with putty was seated over the master model. All the impressions were allowed to set on the master model for twice the recommended setting time in the mouth (six minutes for putty and light body). Metal to metal contact was established to ensure the complete seating of the tray and it was held in place with gentle finger pressure. (Figure 5)

Group II - two step putty light body impression technique with 0.5 mm of spacer

0.5mm Ni-Cr spacer caps and the in between spacer plates were placed over the abutments .Equal quantities of the base and catalysts of putty impression material were mixed and then it was loaded into the impression tray. This tray was then seated over the master model till the material set. The tray was then removed from the master model, spacer caps and spacer plates were removed from the model and then the light body material was injected over the abutment preparations on the master model with the help of mixing gun. Once the light body material was injected, the tray with putty was again seated over the master model. All the impressions were allowed to set on the master model for twice the recommended setting time in the mouth (six minutes for putty and light body). Metal to metal contact was established to ensure the complete seating of the tray and it was held in place with gentle finger pressure.

Group III - Two Step putty light body Impression Technique with 1mm of spacer thickness

1mm Ni-Cr spacer caps and the spacer plate were placed over the abutments .Equal quantities of the base and catalysts of putty impression material were mixed and then it was loaded into the impression tray. This tray was then seated over the master model till the material set. The tray was then removed from the master model, spacer caps and spacer plates were removed from the model and then the light body material was injected over the abutment preparations on the master model with the help of mixing gun. Once the light body material was injected, the tray with putty was again seated over the master model. All the impressions were allowed to set on the master model for twice the recommended setting time in the mouth (six minutes for putty and light body). Metal to metal contact was established to ensure the complete seating of the tray and it was held in place with gentle finger pressure.

Group IV - Two Step putty light body Impression Technique with 1.5 mm of spacer thickness

1.5mm Ni-Cr spacer caps and the in between spacer plate were placed over the abutments. Equal quantities of the base and catalysts of putty impression material were mixed and then it was loaded into the impression tray. This tray was then seated over the master model till the material set. The tray was then removed from the master model, spacer caps and spacer plates were removed from the model and then the light body material was injected over the abutment preparations on the master model with the help of mixing gun. Once the light body material was injected, the tray with putty was again seated over the master model. All the impressions were allowed to set on the master model for twice the recommended setting time in the mouth (six minutes for putty and light body). Metal to metal contact was established to ensure the complete seating of the tray and it was held in place with gentle finger pressure.

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All the impressions were stored for thirty minutes and then poured in type IV dental stone (Zermack technical elite rock). A ratio of 22 ml water: 100 gm die stone was used as recommended by the manufacturer. The die stone was manually mixed with water. A thin mix was initially painted on the impression surface using a camel hair brush. The remaining stone mix was vibrated into the impression and the stone models so formed were allowed to set for one hour before they were separated from the impressions. A flat base was then made for each of the stone models using a custom made acrylic base former. (Fig 6)

Measurements

The master models as well as the stone models were laser scanned using laser scanner 'LaserDenta' and the virtual image was obtained.

This image was then opened with the computer software Rhino 3D and interabutment and intraabutment measurements were made and compared.

Measuring procedure

The measurements of interabutment, abutment height, faciolingual and mesiodistal distances were made from the master and the stone models.

The various distances measured were designated as follows (figure 7).

Mesiodistal distances

AB (die 1)

CD (die 2)

Faciolingual distances

EF (die 1)

GH (die 2)

Abutment heights

EI (die 1)

GJ (die 2)

Inter - abutment distances

KL (between die 1 and die 2)

All the distances were calculated using Rhino 3D computer software. Each distance was measured three times and the mean value was calculated. The measurements of the master model and the stone models were tabulated and statistically analyzed. Table 1,2,3,4 and 5 lists the distances measured for master model in group 1,2,3,4, respectively

Results-

Table 6 and 7 lists the mean of distance measurements across various groups and the distribution of various distance measurements across various groups in comparison with master model

P-values were obtained by simple't' test with reference values by master model.

P-value less than 0.05 was considered to be statistically significant from the reference value by master model.

P-value greater than 0.05 was considered to have better accuracy with the reference standard.

Mesiodistal distances:

AB distance was found to be more accurate in Groups III (P-value =0.537) and IV (P-value =0.463).

CD distance was found to be more accurate in Groups I (P-value = 0.998), III (P-value =0.307) and IV (P-value =0.285).

Faciolingual distances:

EF distance was found to be more accurate in Groups I (P-value =0.089), III (P-value =0.703) and IV (P-value =0.404).

GH distance was found to be more accurate in Groups I (P-value = 0.522), III (P-value =0.356) and IV (P-value =0.100)

Abutment heights:

EI height was found to be more accurate in Groups II (P-value =0.490) and III (P-value =0.733).

GJ height was found to be more accurate in Group III (P-value = 0.303) only.

Inter - abutment distance:

KL distance was found to be more accurate in Groups II (P-value =0.274) and III (P-value = 0.821).

Overall Group III relatively Better accuracy compared all other study groups.

From the (bar charts 1, 2, 3, 4),also it is evident that the group III stone casts produced by using the two step double mix putty wash impression technique with 1mm of spacer thickness produced casts that showed the least dimensional variations from the master model for most of the distances measured

Discussion:

In the present study, the accuracy of 4 different impression techniques was investigated. For all groups, significant variation in dimensions were observed when compared with the stainless steel model. This observation may also be explained by an expansion of stone material.

The concept that a similar expansion rate is expected for all specimens would avoid any bias in the comparisons of the accuracy of each impression technique.

Accuracy of impression resulting from the one-step putty-wash technique is controversial. Some authors found that there was no difference in accuracy between one step and two step techniques[18] while others criticized several potential disadvantages with one step technique [19,20] The one-step putty/light-body technique has also been criticized because of the uncontrolled bulk of the light-body material [20]

These disadvantages include lack of control of the bulk of wash material and the high risk of capturing portions of the prepared margin in putty material rather than lower viscosity material. Most putty viscosity materials have inadequate fine detail reproduction for this purpose. [20]

This present study was designed to determine the impression technique that displays the maximum linear dimensional accuracy for polyvinyl siloxane putty wash impression technique by assessing the linear dimensional change occurring along the various axes of tooth preparation in a partial arch impression. The accuracy of the single step and double step putty wash impressions and double step with three different spacer thicknesses (0.5mm, 1mm, 1.5mm) were compared .Thus, the null hypothesis of no difference between the master model and stone models and the accuracy of the four impression techniques was tested at α=0.05.

An advantage of the two-step double impression technique is that the impression of the teeth can be captured with the wash material. The two-step putty / light-body technique has been reported to be more accurate than the one-step putty/ light body technique

For group I casts, uneven results were obtained. There was decrease in mesiodistal distance of die 1 (AB) by 0.11mm as compared to the master model. The faciolingual distances showed an increase in distance of die 1 (EF) by 0.06mm and decrease in dimension of die 2 (GH) by 0.03mm from the master model. The height of die 1(EI) was more by 0.14mm than the master model, whereas the height of die 2 (GJ) was smaller by 0.05mm. The inter-abutment distance KL between die 1 and die 2 was found to be less than the master model by 0.50mm which was statistically insignificant.

The putty / wash one step technique for addition silicones was criticized by Chee and Donovan who reported several deficiencies with this approach [9]

Clinically, smaller die dimensions would result in castings that are too small or too tight

In this situation, laboratory procedures should not only compensate for the cement thickness (20μm - 40μm) and casting shrinkage of metal but also for the decreased width of the die by using a suitable die relief method. In the single step technique, the small amount of variation in the dimension can be compensated by one coat of die spacer which has been shown to vary from 8μm - 40μm [21]

For group II casts, the results showed a decrease in the mesiodistal distances (AB, CD) as compared to the master model varying between 0.1mm to 0.12mm. The faciolingual distances (EF, GH) also showed a decrease in dimension from the master model by 0.14mm. Die 1 and die 2 showed a negligible decrease in height (EI, GJ) by 0.04mm and 0.16mm respectively. Interabutment (KL) distance is decreased by 0.10mm than the master model

The dies produced were smaller than the master model for all the distances measured. The decrease in the mesiodistal dimension and buccolingual dimension was attributed to the unrestricted polymerization shrinkage of the setting material towards the center of the mass in the interproximal areas.

For group III casts, the results showed a decrease in the mesiodistal distances as compared to the master model varying between 0.02mm and 0.07mm. The faciolingual distance was found to be less than the master model in die 1 (EF) by 0.01mm and increase in die 2 (GH) by 0.02mm. The height of die 1 (EI) and die 2 (GJ) were increased by 0.01mm and 0.03mm respectively. The inter-abutment distances KL between die 1 and die 2 was decrease by 0.01mm.

All the dimensions except for EI and GJ were less compared to master model.

However, even though there was an uneven die size variation, the range of discrepancy from the master model was small.

The common observation in group I, II, and III was that the interabutment distance decreased, though there was no statistical significance.

For group IV casts, the results showed a decrease in the mesiodistal distances AB, CD as compared to the master model varying between 0.03mm to 0.04mm. The faciolingual distances (EF, GH) showed an increase in dimension from the master model varying between 0.02mm to 0.04mm. The height of die 1 and die 2 (EI) and (GJ) was found to be more than the master model by 0.11mm to 0.09mm respectively. The inter-abutment distances between die 1 and die 2 (KL) was more than the master model by 0.22mm.

In general, the dies produced were oversized for all the distances measured except for mesiodistal measurements. The dimensional variation may have occurred probably due to the higher thickness of light body.

Overall, it was seen that the accuracy of group III casts were within the accepted range with P value more than 0.05mm. Our findings show that group III (two step putty wash impression technique with 1mm spacer ) produced the most accurate result in the anteroposterior and vertical dimensions, followed by group IV (two step putty wash impression technique with 1.5mm spacer) in the anteroposterior dimension, and group II (two step putty wash impression technique with 0.5mm spacer) in the vertical dimension. Group I (single step putty wash impression technique) produced the least accurate results in all dimensions.

The accuracy of group III (double step with wash thickness of 1mm) could be attributed to a controlled amount of bulk of impression material, adhesive systems and low polymerization contraction with the heavy-body material.

By diminishing the volume of the polymerizing material at each stage, the final contraction will also be reduced, and the accuracy of the impression can be improved. Therefore, careful control of the bulk of the light body impression material has been advocated because it affects the accuracies of the stone casts.

Conclusion-

Within the limitations of this study the following conclusions can be drawn:

Overall, the two step double mix putty wash impression technique with 1mm of spacer yielded casts that showed the least dimensional variation from the master model as compared to the single step putty wash impression technique.

Two step double mix putty wash impression technique with wash thickness of 0.5mm and 1.5mm showed less dimensional variation from master model as compared to the single step putty wash impression technique

One step putty wash impression technique produced casts that showed the greatest dimensional variation in all the distances.

Casts produced from the one step double mix impression technique and two step double mix impression technique varied significantly from each other for mesiodistal, faciolingual and vertical dimensions of the distances measured.