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An in vivo investigation of seating and removal forces associated with recording impressions in dentate patients
An in vivo investigation of seating and removal forces associated with recording impressions in dentate patients M. S o t i r i o u , D D S , M S c , P h D , a a n d J. A. H o b k i r k , B D S , P h D b
Eastman Dental Institute for Oral Health Care Sciences, University of London, London, United Kingdom This in v i v o s t u d y e x a m i n e d forces exerted d u r i n g the r e c o r d i n g of i m p r e s s i o n s of d e n t a t e m a x i l l a r y arches. This s t u d y c o n s i s t e d of 10 subjects; four i m p r e s s i o n materials in m e t a l stock trays a n d d i s p o s a b l e plastic trays w e r e used. The load rate a n d the m a x i m u m i n s e r t i o n a n d r e m o v a l forces w e r e m e a s u r e d w i t h strain g a u g e s m o u n t e d on the tray h a n d l e s . The results i n d i c a t e d c o n s i d e r a b l e v a r i a t i o n s in forces a n d load rates related to material, subject, a n d tray type. L o w e r forces w e r e m e a s u r e d w i t h the d i s p o s a b l e trays. T h e greatest m e a n p e a k r e m o v a l forces w e r e 36.3 N w i t h a m e t a l t r a y / e l a s t o m e r c o m b i n a t i o n a n d 19.9 N w i t h the s a m e m a t e r i a l u s e d in a plastic tray. (J PROSTHET DENT 1995;74:455-62.)
T h e forces required to insert and remove an impression from an edentulous mouth have been considered by some investigators. 1-3 The effects of these forces on displacement of the denture-bearing mucosa were highlighted by Manderson et al. 4 and Koran 5, who demonstrated that the oral mucosa and periodontal ligament behave viscoelastically under load. Such loads could occur during the recording of impressions; indeed, this is the intention in some procedures. Once the tissues are displaced, their recovery to the original position may take several hours. The forces associated with impression recording depend on the properties of the impression and tray materials, the tray design, the method used to seat and remove the completed impression, and the shape and consistency of the soft tissues. The insertion and removal forces for an impression of a dentate arch are potentially more complex. These forces depend on additional factors such as the size and shape of the clinical crowns, the spacing and angulation of the teeth, the protrusion of anterior teeth, and the presence of irregularities such as abrasion cavities. In an in vitro study, Collard et al. 6recorded the dynamic stresses encountered when impressions were removed from stylized geometric models that incorporated different undercuts. The stylized models used both metal and polymeric trays and a photoelastic material similar to a mercaptan rubber-based impression material. Their results indicated the relation between the degree of undercut, impression bulk, tray deformation, and stress in the impression material. 6
aFormer Postdoctoral Research Student, Department of Prosthetic Dentistry. bProfessor and Head of Department of Prosthetic Dentistry. Copyright 9 1995 by The Editorial Council of THE JOURNALOF PROSTHETICDENTISTRY. 0022-3913/95/$5.00 + 0. 10/1/67266
NOVEMBER 1995
An in vitro study that recorded forces exerted during removal of an impression in a custom tray by two different methods 7 used a dentiform model and custom-made trays, which were removed with either a single or three-point load application in a test machine. The results revealed that high removal forces (224.3 N and 514.0 N) were required for this experimental model, and it may be questioned whether such large forces could be used clinically. There is, however, little data available on the forces involved in the seating and removal of impressions in dentate patients. An awareness of these forces is relevant to the design of impression trays, the formulation and testing of impression materials and their adhesives, and clinical practice, in that such forces may be responsible for tooth and soft tissue displacement. It was the objective of this study, the first of a series, to investigate the hypothesis that the magnitude and rate of change of forces associated with impression recording in dentate subjects are related to impression material and tray design. MATERIAL
AND
METHODS
Clinical investigations of the recording of impressions may be complicated by intersubject variations, which may require large numbers of patients for statistically valid results. The alternative of recording multiple impressions from smaller numbers of patients is usually not feasible for ethical and practical reasons. For the purposes of this initial study it was decided to standardize on one operator and to investigate several different impression procedures using two types of tray and a panel of 10 young dentate subjects. An initial pilot investigation had shown that intrasubject repeatability of the forces involved in recording impressions by one operator with one technique was high; however this could not be repeated on a larger scale for the reasons outlined. Although custom-made trays are considered optimal, Burton et al. s stated that the rigidity of metal stock trays
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Fig. 1. Impression trays used. Waterproofed strain gauges are visible on handles.
provides maximal support for the impression material. For this reason, and because of the standard design of such trays and their common use, it was decided to use a stock metal impression tray and a disposable plastic tray for this study. The stock metal tray is often used for infection control and is similar to those used by Burton et al. s The trays chosen were (1) a nonperforated stainless steel rimlock tray (ASA, INOX, England) and (2) SOLO single patient impression trays (J and S Davis Ltd., Davis Healthcare Services Ltd., U. K.), which have large perforations. The metal tray was cleaned and sterilized between impressions, and a fresh disposable tray was used for each impression procedure. The manufacturer of the disposable tray provides a reusable cranked metal handle, which is inserted into a preformed slot on the anterior aspect of the tray, and this, modified to act as a force transducer, was used with the tray and was sterilized between impressions. The range of impression materials that could be investigated is considerable, and for this initial study it was decided to use several different techniques so as to encompass a range of possible forces. The materials selected were as follows. 1. Alginate (Alginoplast, Bayer Dental, Leverkusen, Germany); adhesive (De Trey Fix Tray adhesive for alginates, lot No. NL 23, Dentsply Ltd., De Trey Division, Addlestone, Weybridge, Surrey, U. K.). 2. Polysiloxane of medium viscosity (Xantopren H, Green, BaseCatalyst, Bayer Dental); adhesive (Universal Adhesive, lot No. 1012K (Bayer Dental). 3. Polyvinyl siloxane, putty (Reprosil HF putty, vinyl polysiloxane, base and catalyst, Dentsply Ltd.); adhesive (Universal Adhesive, lot No. 9301-086, Coltene UK, Ltd., Burgess Hill, West Sussex, U. K.). 4. Same as No.3, but with a light wash: Extrude polyvinyl siloxane impression material, type I wash (low viscosity), ISO type I I I A (Kerr UK, Ltd., Peterborough, U. K.). Eight impressions were recorded for each group with each combination of material and technique, once for each type of tray, a total of 80 impressions (Table I). No vent
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grooves were cut in the putty impression before recording with the light-bodied wash, and although this may have contributed to hydraulic locking of the impression, it was considered that the difficulty of cutting standardized vents would have negated the benefits of the study. A recommended adhesive was used for each material and was dried for 10 minutes at room temperature before the impression was recorded. Each impression material was handled according to the manufacturer's instructions. After the impression had been taken, the metal tray was cleaned with a special adhesive-dissolving solution (De Trey Fix Solvent, Dentsply Ltd., De Trey Division) before it was sterilized prior to the recording of another impression. This was done by placing the trays in a disinfectant solution overnight (Cidex, Long Life sterilizing/disinfecting system, Johnson and Johnson Ltd., Dental Care Division, Slough, U. K.) after which they were rinsed under running tap water before they were used again. The forces applied to the impression trays via their handles were recorded by means of two linear resistance strain gauges (type 4/120/PC 11 120 Ohm Polyimide backed, Tinsley Strain Measurements Co., Londonderry, Northern Ireland, U. K.) (Fig. 1). These gauges were placed opposite each other on the upper and lower aspects of the tray handle for the nondisposable trays and on the similar surfaces of the one metal handle that was used with the disposable trays. The strain gauges were placed with their long axes parallel with the long axis of the handle. The strain gauges were glued to the handles with M-BOND 200 adhesive and catalyst (Measurements Group UK Ltd., Basingstoke, Hants, U. K.) and were then waterproofed with Araldite Rapid epoxy resin (Ciba-Geigy, Plastics and Adhesive Co., Duxford, Cambridge, U. K.) to protect the circuit from moisture and to prevent any damage when the trays were immersed in the sterilizing solution. The two strain gauges on each tray handle were wired in a half bridge mode. The orientation of the gauges allowed measured bending of the handle in an anteroposterior plane only by detecting the strain as a result of applied loads. The gauges were linked with fine wire to an electronic amplifier (type C56, Sangamo Weston Controls Ltd., Sussex, U. K.) which was, in turn, connected to a chart recorder (Linseis, L 2007, Inkjet Recorder, Linseis GmbH, Duxford, U. K.). The impression trays were calibrated statically as transducers before and after the experiment by placing the handle horizontally in a bench-mounted vise and then applying forces to the middle of the tray with a series of weights. A similar procedure was used for the disposable tray handle with a tray mounted on it. Loads were applied with the trays facing in the upward direction and inverted to enable accurate measurement of forces in both directions. A typical calibration curve is illustrated in Fig. 2 and shows the effective performance of the system as installed to demonstrate its linearity within the range of forces detected. The transducer system was found effective and ro-
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AMPLIFIED GAUGE OUTPUT (V) 4 2 0 -2 -4 -40
-30
I
I
I
J
I
I
-20
-10
0
10
20
30
40
APPLIED LOAD (N) Fig. 2. Calibration curve for reusable handle with fitted strain gauges used with Solo trays. H a n d l e calibrated and m o u n t e d on tray with loads applied to center of impression area.
Table I. S u m m a r y of impression tray/material combinations Impression procedure
Type of impression t r a y
Alginate (irreversible hydrocolloid)
Condensationcured silicone elastomer
Poly-vinyl siloxane, putty
Poly-vinyl siloxane, p u t t y + lowviscosity w a s h
Metal, box design Disposable plastic, box design
X X
X X
X X
X X
Table II. S u m m a r y of data obtained from recordings of force exerted d u r i n g seating and removal of various impressions Mean force (in newtons) and lead r a t e (N/second) Initial insertation Tray type
Maximum i n s e r t a t i o n
Maximum removal
I m p r e s s i o n material
Force
N/sec
Force
N/sec
Force
N/sec
Metal
Alginate
Plastic
Alginate
Metal
Xantopren elastomer
Plastic
Xantopren elastomer
Metal
Putty
Plastic
Putty
Metal
Elastomeric wash
Plastic
Elastomeric wash
10.8 (5.4) 6.6 (2.8) 12.0 (5.3) 10.6 (2.1) 23.3 (7.6) 15.4 (4.2) 13.8 (4.3) 11.8 (4.3)
5.5 (2.0) 4.7 (2.0) 6.0 (1.5) 5.8 (2.9) 13.7 (5.6) 9.9 (3.1) 9.5 (4.3) 4.9 (1.9)
16.3 (5.8) 9.9 (3.0) 19.5 (4.9) 14.5 (3.7) 35.7 (9.3) 20.9 (3.5) 20.4 (4.7) 15.2 (4.9)
2.9 (1.5) 2.2 (1.3) 3.8 (2.1) 3.1 (1.6) 7.2 (3.1) 5.0 (1.5) 5.4 (4.4) 2.5 (1.0)
29.6 (8.9) 13.7 (9.3) 36.3 (15.3) 19.9 (7.5) 40.6 (10.9) 10.9 (4.6) 39.5 (15.3) 17.6 (10.2)
8.4 (4.5) 6.5 (5.0) 13.6 (5.8) 5.8 (2.0) 13.6 (8.5) 6.9 (3.8) 14.2 (6.2) 7.4 (3.8)
Standard deviationsare shown in parentheses.
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FORCE (N)
'~I 30
20
10
ii
i i.i i i i i 10
A
20
TIME (sec)
FORCE (N) 50
40 84
30
20
10
B
10
20
TIME (sec) Fig. 3. Typical recording of insertion force (A) and typical recording of removal force (B) demonstrate the initial and final seating and removal forces.
bust, and it was capable of resolving differences of 0.01 N, although, for the purposes of this study, data are presented to 0.1 N. This design of the transducer system, however, is only capable of measuring the resultant forces equivalent to a load applied in the center of the tray. A more accurate description of the loads in different parts of the tray would have required a more complex multichannel device to record forces in various locations, such as anterior and posterior. However, such a system was considered inappropriate for this initial study. After calibration, standard clinical procedures were used to make the impressions in sequence (Fig. 3). During this process the applied forces
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were recorded on the chart recorder, which was out of the operator's field of vision. When the impression tray was inserted and removed from the mouth, the tray was held with the thumb and the index finger of the right hand of the operator, and the necessary forces were applied only on the handle of the tray. Therefore, when the tray was inserted and removed from the mouth, the force was applied only to one region on the tray handle. RESULTS The results of the experiment were in the form of a series of charts, and an example of one of these is shown in
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INITIAL SEATING FORCE (N) 50 40 30
O0 0
ALGINATE
ELASTOMER
PUTTY
WASH
I M P R E S S I O N MATERIAL/TRAY
[] METAL[] DISPOSABLE J Fig. 4. Average initial seating force for each impression technique. (Vertical bars represent standard deviations for each value.)
MAXIMUM REMOVAL FORCE (N) 50
30
100
-
~
ALGINATE ELASTOMER PUTTY IMPRESSION MATERIAL/TRAY
WASH
[[~METAL []DISPOSABLE l Fig. 5. Average peak seating force for each impression technique. (Vertical bars represent standard deviations for each value.)
Fig. 2. The recording sequence followed a typical pattern in which an initial peak seating force and a final peak seating force could be observed. The applied force fell to a plateau while the material set, followed by a recording of the removal force. It was noted that, as the impression was rotated around the distal molars during removal, this force had the same direction as the seating force as measured by the transducer. From the recordings it was possible to identify the peak initial insertion force and load rate, the peak insertion force and load rate, and the peak removal force and load
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rate (Table II). Table II also presents the standard deviations of the means. It was evident from the data that there were large intersubject variations within each impression procedure, although a pilot study demonstrated more intraoperator consistency with one subject. This made detailed statistical analysis inappropriate in view of the small number of subjects studied; therefore, the results have only been considered from the viewpoint of descriptive statistics. This finding has implications for the design of further studies. Figs. 4 through 9 graphically depict summaries of the data.
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INITIAL LOAD RATE (N/sec) 16 14 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
20 ALGINATE
ELASTOMER
PUTTY
WASH
IMPRESSION MATERIAL/TRAY • METAL [ ] DISPOSABLE
Fig. 6. Average peak removal force for each impression technique. (Vertical bars represent standard deviations for each value.)
MAXIMUM REMOVAL FORCE (N) 50 40 30
/........ ~ ~
//~I/ ,/~
20 10
ALGINATE
ELASTOMER
PUTTY
WASH
IMPRESSION MATERIAL/TRAY [ ] METAL [ ] DISPOSABLE
Fig. 7. Average initial load rate for each impression technique. (Vertical bars represent standard deviations for each value.)
DISCUSSION The impression technique used in this study was the insertion and removal of the impression tray from the oral cavity by application of the necessary force only on one point, the handle of the tray. Many operators prefer to apply force at multiple points around the tray, although in an in vitro study, Dixon et al. 7 found that this technique was associated with greater removal forces than when a single point of application was used. The results of this study indicate that the range of forces applied when an impression
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was removed from the mouth was considerably lower than those reported in a previous investigation. When the plastic tray was used, the recorded insertion and removal forces were lower than those found for a metal tray. Although the lower seating forces may have been related to the perforations in the tray (indeed this would be an argument for the routine use of perforated trays), they did not explain the lower forces of removal. Possible explanations are that the operator tended to use lower forces to avoid breaking the tray or that the tray itself flexed to al-
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FINAL LOAD RATE (N/sec)
14 12 t .. .. ....... .. .. .. .. ....... .. .. .. .. ....... .. .. .. .. ....... .. .. .. ......... .. .. .. ......... .. .. .. ......... .. .. .. ......... . . . . . 10
0
.........................................................
ALGINATE
ELASTOMER
PUTTY
WASH
IMPRESSION MATERIAL/TRAY
[] METAL[] DISPOSABLE Fig. 8. Average load rate during final seating of each of impression/tray combinations. (Vertical bars represent standard deviations for each value.)
MAXIMUM REMOVAL LOAD RATE (N/sec) 16 1214 ...............
0-
i
ALGINATE ELASTOMER PUTTY IMPRESSION MATERIAL/TRAY
WASH
[] METAL [] DISPOSABLE Fig. 9. Average load rate during removal of each of impression/tray combinations. (Vertical bars represent standard deviations for each value.)
low easier removal over undercuts. This is supported by the study of Collard et al., 6 which commented on the effect of tray flexure during impression removal, and Burton et al. s indicated that distorted models can be produced when some medium-viscosity elastomers are used in conjunction with certain types of plastic disposable trays. Burton et al. 8 also noted that they found no significant difference in flexibility between Solo trays and custom-made acrylic resin trays. In this investigation, the Solo trays sometimes failed during removal at the second stage of the two-stage impression technique and when a medium-viscosity elas-
NOVEMBER 1995
tomer was used. The recorded removal forces at the time of failure ranged from 11.0 N to 26.S N, with fracture occurring in the housing of the reusable handle. When the nonperforated metal tray was used, the impression adhesive failed in some cases during removal at the second stage of the two-stage impression technique, although the manufacturer's recommendations and investigators' suggestions from previous studies had been followed. 9-13 Although this was unexpected in view of the findings of Grant and Tjan 12 on the bond strength of the elastomeric adhesives, it should be recognized that the
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t r a n s d u c e r only measures applied forces, and local forces associated with levering of the tray downward around the distal molars could be m u c h greater. This suggests t h a t there m a y be benefit in u s i n g a m u l t i c h a n n e l t r a n s d u c e r system capable, as a m i n i m u m , of determining forces anteriorly a n d posteriorly on each side. The m e a n peak removal forces for the metal tray were similar for all elastomeric materials/techniques (Xantopren 36.3 N; Reprosi140.6 N; and Extrude 39.5 N), whereas there were considerable differences for the plastic trays (Xantopren 19.9 N; Reprosil 10.9 N; a n d Extrude 17.6 N). These differences m a y reflect distortion of the tray during removal, which would allow the impression material to flex over undercuts. This was expected to be of more relevance in dealing with the impression materials, which were stiffer w h e n set, a n d is supported by the finding t h a t the m a x i m u m insertion and removal forces, and also the load rates, for the elastomeric materials were greater when compared with those needed for the irreversible hydrocolloid for either tray. Although detailed geometric analysis of the dental arches was not done, the greater removal forces appear to be generated with large i n t e r d e n t a l undercuts a n d w h e n protrusion of the incisors was present. The peak insertion forces for these subjects were not proportionally greater, which was expected because these forces are more likely to be related to tray spacing, rate of seating, bulk and viscosity of impression material, and the presence of perforations. The m e a n peak load rates during the insertion a n d the removal of the irreversible hydrocolloid impressions were slightly greater for the metal tray compared with the plastic trays. This would be expected in view of the probable effects of tray a n d impression material distortion. The generation of relatively high forces, which might be expected in seating a wash impression, were demonstrated i n this study where the greatest initial seating load rate and m a x i m u m seating force were associated with this technique. Removal forces above the value of 45 N become uncomfortable for the p a t i e n t and there is a risk of d a m a g i n g soft tissues or opposing teeth if no precautions are taken. I n view of the pivotal role of impression procedures i n restorative dentistry, a role t h a t cannot be replaced by computerized s c a n n i n g techniques where tissue displacem e n t is required, it is i m p o r t a n t to have a good unders t a n d i n g of the forces involved. The development of impression trays t h a t incorporate transducers offers the potential not only for i n vivo investigations, b u t also the use of such techniques as t r a i n i n g aids.
CONCLUSIONS 1. Forces recorded on the metal t r a y were greater t h a n those applied on the disposable trays. For the metal tray the m e a n peak removal forces were between 36.3 N and 40.6 N w h e n the elastomeric impression materials of dif-
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SOTIRIOU AND HOBKIRK
ferent viscosity were used, and for the irreversible hydrocolloid the m e a n peak force was 29.6 N. 2. For the disposable plastic trays the m e a n peak removal forces were considerably lower a n d ranged from 14.7 N to 19.9 N when elastomeric impression materials of different viscosity were used, and the m e a n peak force for the irreversible hydrocolloid was 13.7 N. 3. Load rates recorded on the metal tray were greater t h a n those applied on the disposable trays. Substantial differences were observed w h e n elastomeric materials were used and may reflect tray distortion. The greater the viscosity of the material, the greater the load rate. 4. W h e n a nonperforated metal tray was used in a double impression technique, the impression tray adhesive may not have been reliable during the second removal from the mouth, especially when the removal forces applied were 50 N a n d higher. We are grateful to Mr. J. Kelleway for his assistance with the technical aspects of this project and to the colleagues who acted as subjects for the investigation.
REFERENCES
1. Douglas WH, Wilson HJ. A study of the zinc oxide-eugenoltype impressionpastes--pressures involvedin takingimpressions.Br Dent J 1964;116:34-6. 2. Frank RP"Analysis~ pressures pr~ duringmaxillaryedentuleus impressionprocedures. J PROSTHETDENT1969;22:400-13. 3. Frank RP. Controllingpressures duringcompletedentureimpressions. Dent Clin North Am 1970;14:453-70. 4. Manderson RD, Wills DJ, Picton DC. Biomechanics of denturesupportingtissues. In: Phillips RW, ed. Proceedingsof the secondinternational prosthodonticcongress.St Louis:C V Mosby,1979:98-101. 5. Koran A III. Impressionmaterials for recordingthe denture bearing mucosa. Dent Clin North Am 1980;24:97-111. 6. CollardEW, Caputo AA, StandleeJP, Trabert KC. Dynamicstresses encountered in impressionremoval. J PROSTHETDENT1973;29:498506. 7. DixonDL, BreedingLC, MoseleyJP. Customimpressiontrays. Part II: removal forces. J PROSTHETDENT1994;71:316-8. 8. Burton JF, Hood JA, Plunkett DJ, Johnson SS. The effects of disposable and custom-madeimpressiontrays onthe accuracyofimpressions. J Dent 1989;17:121-3. 9. Chai JY, Jameson LM, Moser JB, Hesby RA. Adhesiveproperties of several impression material systems: part II. J PROSTHETDENT 1991;66:287-92. 10. MacSweenR, Price RB. Peel bond strengths of five impressionmaterial tray adhesives. J Can Dent Assoc 1991,57:654-7. 11. DavisGB,MoserJB, BrinsdenGI. The bondingpropertiesofelastomer tray adhesives. J PROSTHETDENT1976;36:278-85. 12. Grant BE, TjanAH. Tensileand peel bondstrengths oftray adhesives. J PROSTHETDENT1988;59:165-8. 13. BombergTJ, GoldfogelMH, HoffmanW Jr, BombergSE. Considerations for adhesion of impression materials to impression trays. J PROSTHET DENT 1988;60:681-4.
Reprint requests to: DR. JOHN A. HOBKIRK DEPARTMENT OF PROSTHETICDENTISTRY EASTMANDENTALINSTITUTEFOR ORAL HF~LT~ CARE SCIENCES 256 GRAY'S INN ROAD LONDON WC1X 8LD UNITED KINGDOM
VOLUME 74 NI.rMBER 5
Eastman Dental Institute for Oral Health Care Sciences, University of London, London, United Kingdom This in v i v o s t u d y e x a m i n e d forces exerted d u r i n g the r e c o r d i n g of i m p r e s s i o n s of d e n t a t e m a x i l l a r y arches. This s t u d y c o n s i s t e d of 10 subjects; four i m p r e s s i o n materials in m e t a l stock trays a n d d i s p o s a b l e plastic trays w e r e used. The load rate a n d the m a x i m u m i n s e r t i o n a n d r e m o v a l forces w e r e m e a s u r e d w i t h strain g a u g e s m o u n t e d on the tray h a n d l e s . The results i n d i c a t e d c o n s i d e r a b l e v a r i a t i o n s in forces a n d load rates related to material, subject, a n d tray type. L o w e r forces w e r e m e a s u r e d w i t h the d i s p o s a b l e trays. T h e greatest m e a n p e a k r e m o v a l forces w e r e 36.3 N w i t h a m e t a l t r a y / e l a s t o m e r c o m b i n a t i o n a n d 19.9 N w i t h the s a m e m a t e r i a l u s e d in a plastic tray. (J PROSTHET DENT 1995;74:455-62.)
T h e forces required to insert and remove an impression from an edentulous mouth have been considered by some investigators. 1-3 The effects of these forces on displacement of the denture-bearing mucosa were highlighted by Manderson et al. 4 and Koran 5, who demonstrated that the oral mucosa and periodontal ligament behave viscoelastically under load. Such loads could occur during the recording of impressions; indeed, this is the intention in some procedures. Once the tissues are displaced, their recovery to the original position may take several hours. The forces associated with impression recording depend on the properties of the impression and tray materials, the tray design, the method used to seat and remove the completed impression, and the shape and consistency of the soft tissues. The insertion and removal forces for an impression of a dentate arch are potentially more complex. These forces depend on additional factors such as the size and shape of the clinical crowns, the spacing and angulation of the teeth, the protrusion of anterior teeth, and the presence of irregularities such as abrasion cavities. In an in vitro study, Collard et al. 6recorded the dynamic stresses encountered when impressions were removed from stylized geometric models that incorporated different undercuts. The stylized models used both metal and polymeric trays and a photoelastic material similar to a mercaptan rubber-based impression material. Their results indicated the relation between the degree of undercut, impression bulk, tray deformation, and stress in the impression material. 6
aFormer Postdoctoral Research Student, Department of Prosthetic Dentistry. bProfessor and Head of Department of Prosthetic Dentistry. Copyright 9 1995 by The Editorial Council of THE JOURNALOF PROSTHETICDENTISTRY. 0022-3913/95/$5.00 + 0. 10/1/67266
NOVEMBER 1995
An in vitro study that recorded forces exerted during removal of an impression in a custom tray by two different methods 7 used a dentiform model and custom-made trays, which were removed with either a single or three-point load application in a test machine. The results revealed that high removal forces (224.3 N and 514.0 N) were required for this experimental model, and it may be questioned whether such large forces could be used clinically. There is, however, little data available on the forces involved in the seating and removal of impressions in dentate patients. An awareness of these forces is relevant to the design of impression trays, the formulation and testing of impression materials and their adhesives, and clinical practice, in that such forces may be responsible for tooth and soft tissue displacement. It was the objective of this study, the first of a series, to investigate the hypothesis that the magnitude and rate of change of forces associated with impression recording in dentate subjects are related to impression material and tray design. MATERIAL
AND
METHODS
Clinical investigations of the recording of impressions may be complicated by intersubject variations, which may require large numbers of patients for statistically valid results. The alternative of recording multiple impressions from smaller numbers of patients is usually not feasible for ethical and practical reasons. For the purposes of this initial study it was decided to standardize on one operator and to investigate several different impression procedures using two types of tray and a panel of 10 young dentate subjects. An initial pilot investigation had shown that intrasubject repeatability of the forces involved in recording impressions by one operator with one technique was high; however this could not be repeated on a larger scale for the reasons outlined. Although custom-made trays are considered optimal, Burton et al. s stated that the rigidity of metal stock trays
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Fig. 1. Impression trays used. Waterproofed strain gauges are visible on handles.
provides maximal support for the impression material. For this reason, and because of the standard design of such trays and their common use, it was decided to use a stock metal impression tray and a disposable plastic tray for this study. The stock metal tray is often used for infection control and is similar to those used by Burton et al. s The trays chosen were (1) a nonperforated stainless steel rimlock tray (ASA, INOX, England) and (2) SOLO single patient impression trays (J and S Davis Ltd., Davis Healthcare Services Ltd., U. K.), which have large perforations. The metal tray was cleaned and sterilized between impressions, and a fresh disposable tray was used for each impression procedure. The manufacturer of the disposable tray provides a reusable cranked metal handle, which is inserted into a preformed slot on the anterior aspect of the tray, and this, modified to act as a force transducer, was used with the tray and was sterilized between impressions. The range of impression materials that could be investigated is considerable, and for this initial study it was decided to use several different techniques so as to encompass a range of possible forces. The materials selected were as follows. 1. Alginate (Alginoplast, Bayer Dental, Leverkusen, Germany); adhesive (De Trey Fix Tray adhesive for alginates, lot No. NL 23, Dentsply Ltd., De Trey Division, Addlestone, Weybridge, Surrey, U. K.). 2. Polysiloxane of medium viscosity (Xantopren H, Green, BaseCatalyst, Bayer Dental); adhesive (Universal Adhesive, lot No. 1012K (Bayer Dental). 3. Polyvinyl siloxane, putty (Reprosil HF putty, vinyl polysiloxane, base and catalyst, Dentsply Ltd.); adhesive (Universal Adhesive, lot No. 9301-086, Coltene UK, Ltd., Burgess Hill, West Sussex, U. K.). 4. Same as No.3, but with a light wash: Extrude polyvinyl siloxane impression material, type I wash (low viscosity), ISO type I I I A (Kerr UK, Ltd., Peterborough, U. K.). Eight impressions were recorded for each group with each combination of material and technique, once for each type of tray, a total of 80 impressions (Table I). No vent
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grooves were cut in the putty impression before recording with the light-bodied wash, and although this may have contributed to hydraulic locking of the impression, it was considered that the difficulty of cutting standardized vents would have negated the benefits of the study. A recommended adhesive was used for each material and was dried for 10 minutes at room temperature before the impression was recorded. Each impression material was handled according to the manufacturer's instructions. After the impression had been taken, the metal tray was cleaned with a special adhesive-dissolving solution (De Trey Fix Solvent, Dentsply Ltd., De Trey Division) before it was sterilized prior to the recording of another impression. This was done by placing the trays in a disinfectant solution overnight (Cidex, Long Life sterilizing/disinfecting system, Johnson and Johnson Ltd., Dental Care Division, Slough, U. K.) after which they were rinsed under running tap water before they were used again. The forces applied to the impression trays via their handles were recorded by means of two linear resistance strain gauges (type 4/120/PC 11 120 Ohm Polyimide backed, Tinsley Strain Measurements Co., Londonderry, Northern Ireland, U. K.) (Fig. 1). These gauges were placed opposite each other on the upper and lower aspects of the tray handle for the nondisposable trays and on the similar surfaces of the one metal handle that was used with the disposable trays. The strain gauges were placed with their long axes parallel with the long axis of the handle. The strain gauges were glued to the handles with M-BOND 200 adhesive and catalyst (Measurements Group UK Ltd., Basingstoke, Hants, U. K.) and were then waterproofed with Araldite Rapid epoxy resin (Ciba-Geigy, Plastics and Adhesive Co., Duxford, Cambridge, U. K.) to protect the circuit from moisture and to prevent any damage when the trays were immersed in the sterilizing solution. The two strain gauges on each tray handle were wired in a half bridge mode. The orientation of the gauges allowed measured bending of the handle in an anteroposterior plane only by detecting the strain as a result of applied loads. The gauges were linked with fine wire to an electronic amplifier (type C56, Sangamo Weston Controls Ltd., Sussex, U. K.) which was, in turn, connected to a chart recorder (Linseis, L 2007, Inkjet Recorder, Linseis GmbH, Duxford, U. K.). The impression trays were calibrated statically as transducers before and after the experiment by placing the handle horizontally in a bench-mounted vise and then applying forces to the middle of the tray with a series of weights. A similar procedure was used for the disposable tray handle with a tray mounted on it. Loads were applied with the trays facing in the upward direction and inverted to enable accurate measurement of forces in both directions. A typical calibration curve is illustrated in Fig. 2 and shows the effective performance of the system as installed to demonstrate its linearity within the range of forces detected. The transducer system was found effective and ro-
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AMPLIFIED GAUGE OUTPUT (V) 4 2 0 -2 -4 -40
-30
I
I
I
J
I
I
-20
-10
0
10
20
30
40
APPLIED LOAD (N) Fig. 2. Calibration curve for reusable handle with fitted strain gauges used with Solo trays. H a n d l e calibrated and m o u n t e d on tray with loads applied to center of impression area.
Table I. S u m m a r y of impression tray/material combinations Impression procedure
Type of impression t r a y
Alginate (irreversible hydrocolloid)
Condensationcured silicone elastomer
Poly-vinyl siloxane, putty
Poly-vinyl siloxane, p u t t y + lowviscosity w a s h
Metal, box design Disposable plastic, box design
X X
X X
X X
X X
Table II. S u m m a r y of data obtained from recordings of force exerted d u r i n g seating and removal of various impressions Mean force (in newtons) and lead r a t e (N/second) Initial insertation Tray type
Maximum i n s e r t a t i o n
Maximum removal
I m p r e s s i o n material
Force
N/sec
Force
N/sec
Force
N/sec
Metal
Alginate
Plastic
Alginate
Metal
Xantopren elastomer
Plastic
Xantopren elastomer
Metal
Putty
Plastic
Putty
Metal
Elastomeric wash
Plastic
Elastomeric wash
10.8 (5.4) 6.6 (2.8) 12.0 (5.3) 10.6 (2.1) 23.3 (7.6) 15.4 (4.2) 13.8 (4.3) 11.8 (4.3)
5.5 (2.0) 4.7 (2.0) 6.0 (1.5) 5.8 (2.9) 13.7 (5.6) 9.9 (3.1) 9.5 (4.3) 4.9 (1.9)
16.3 (5.8) 9.9 (3.0) 19.5 (4.9) 14.5 (3.7) 35.7 (9.3) 20.9 (3.5) 20.4 (4.7) 15.2 (4.9)
2.9 (1.5) 2.2 (1.3) 3.8 (2.1) 3.1 (1.6) 7.2 (3.1) 5.0 (1.5) 5.4 (4.4) 2.5 (1.0)
29.6 (8.9) 13.7 (9.3) 36.3 (15.3) 19.9 (7.5) 40.6 (10.9) 10.9 (4.6) 39.5 (15.3) 17.6 (10.2)
8.4 (4.5) 6.5 (5.0) 13.6 (5.8) 5.8 (2.0) 13.6 (8.5) 6.9 (3.8) 14.2 (6.2) 7.4 (3.8)
Standard deviationsare shown in parentheses.
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FORCE (N)
'~I 30
20
10
ii
i i.i i i i i 10
A
20
TIME (sec)
FORCE (N) 50
40 84
30
20
10
B
10
20
TIME (sec) Fig. 3. Typical recording of insertion force (A) and typical recording of removal force (B) demonstrate the initial and final seating and removal forces.
bust, and it was capable of resolving differences of 0.01 N, although, for the purposes of this study, data are presented to 0.1 N. This design of the transducer system, however, is only capable of measuring the resultant forces equivalent to a load applied in the center of the tray. A more accurate description of the loads in different parts of the tray would have required a more complex multichannel device to record forces in various locations, such as anterior and posterior. However, such a system was considered inappropriate for this initial study. After calibration, standard clinical procedures were used to make the impressions in sequence (Fig. 3). During this process the applied forces
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were recorded on the chart recorder, which was out of the operator's field of vision. When the impression tray was inserted and removed from the mouth, the tray was held with the thumb and the index finger of the right hand of the operator, and the necessary forces were applied only on the handle of the tray. Therefore, when the tray was inserted and removed from the mouth, the force was applied only to one region on the tray handle. RESULTS The results of the experiment were in the form of a series of charts, and an example of one of these is shown in
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INITIAL SEATING FORCE (N) 50 40 30
O0 0
ALGINATE
ELASTOMER
PUTTY
WASH
I M P R E S S I O N MATERIAL/TRAY
[] METAL[] DISPOSABLE J Fig. 4. Average initial seating force for each impression technique. (Vertical bars represent standard deviations for each value.)
MAXIMUM REMOVAL FORCE (N) 50
30
100
-
~
ALGINATE ELASTOMER PUTTY IMPRESSION MATERIAL/TRAY
WASH
[[~METAL []DISPOSABLE l Fig. 5. Average peak seating force for each impression technique. (Vertical bars represent standard deviations for each value.)
Fig. 2. The recording sequence followed a typical pattern in which an initial peak seating force and a final peak seating force could be observed. The applied force fell to a plateau while the material set, followed by a recording of the removal force. It was noted that, as the impression was rotated around the distal molars during removal, this force had the same direction as the seating force as measured by the transducer. From the recordings it was possible to identify the peak initial insertion force and load rate, the peak insertion force and load rate, and the peak removal force and load
NOVEMBER
1995
rate (Table II). Table II also presents the standard deviations of the means. It was evident from the data that there were large intersubject variations within each impression procedure, although a pilot study demonstrated more intraoperator consistency with one subject. This made detailed statistical analysis inappropriate in view of the small number of subjects studied; therefore, the results have only been considered from the viewpoint of descriptive statistics. This finding has implications for the design of further studies. Figs. 4 through 9 graphically depict summaries of the data.
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INITIAL LOAD RATE (N/sec) 16 14 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
20 ALGINATE
ELASTOMER
PUTTY
WASH
IMPRESSION MATERIAL/TRAY • METAL [ ] DISPOSABLE
Fig. 6. Average peak removal force for each impression technique. (Vertical bars represent standard deviations for each value.)
MAXIMUM REMOVAL FORCE (N) 50 40 30
/........ ~ ~
//~I/ ,/~
20 10
ALGINATE
ELASTOMER
PUTTY
WASH
IMPRESSION MATERIAL/TRAY [ ] METAL [ ] DISPOSABLE
Fig. 7. Average initial load rate for each impression technique. (Vertical bars represent standard deviations for each value.)
DISCUSSION The impression technique used in this study was the insertion and removal of the impression tray from the oral cavity by application of the necessary force only on one point, the handle of the tray. Many operators prefer to apply force at multiple points around the tray, although in an in vitro study, Dixon et al. 7 found that this technique was associated with greater removal forces than when a single point of application was used. The results of this study indicate that the range of forces applied when an impression
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was removed from the mouth was considerably lower than those reported in a previous investigation. When the plastic tray was used, the recorded insertion and removal forces were lower than those found for a metal tray. Although the lower seating forces may have been related to the perforations in the tray (indeed this would be an argument for the routine use of perforated trays), they did not explain the lower forces of removal. Possible explanations are that the operator tended to use lower forces to avoid breaking the tray or that the tray itself flexed to al-
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16
FINAL LOAD RATE (N/sec)
14 12 t .. .. ....... .. .. .. .. ....... .. .. .. .. ....... .. .. .. .. ....... .. .. .. ......... .. .. .. ......... .. .. .. ......... .. .. .. ......... . . . . . 10
0
.........................................................
ALGINATE
ELASTOMER
PUTTY
WASH
IMPRESSION MATERIAL/TRAY
[] METAL[] DISPOSABLE Fig. 8. Average load rate during final seating of each of impression/tray combinations. (Vertical bars represent standard deviations for each value.)
MAXIMUM REMOVAL LOAD RATE (N/sec) 16 1214 ...............
0-
i
ALGINATE ELASTOMER PUTTY IMPRESSION MATERIAL/TRAY
WASH
[] METAL [] DISPOSABLE Fig. 9. Average load rate during removal of each of impression/tray combinations. (Vertical bars represent standard deviations for each value.)
low easier removal over undercuts. This is supported by the study of Collard et al., 6 which commented on the effect of tray flexure during impression removal, and Burton et al. s indicated that distorted models can be produced when some medium-viscosity elastomers are used in conjunction with certain types of plastic disposable trays. Burton et al. 8 also noted that they found no significant difference in flexibility between Solo trays and custom-made acrylic resin trays. In this investigation, the Solo trays sometimes failed during removal at the second stage of the two-stage impression technique and when a medium-viscosity elas-
NOVEMBER 1995
tomer was used. The recorded removal forces at the time of failure ranged from 11.0 N to 26.S N, with fracture occurring in the housing of the reusable handle. When the nonperforated metal tray was used, the impression adhesive failed in some cases during removal at the second stage of the two-stage impression technique, although the manufacturer's recommendations and investigators' suggestions from previous studies had been followed. 9-13 Although this was unexpected in view of the findings of Grant and Tjan 12 on the bond strength of the elastomeric adhesives, it should be recognized that the
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t r a n s d u c e r only measures applied forces, and local forces associated with levering of the tray downward around the distal molars could be m u c h greater. This suggests t h a t there m a y be benefit in u s i n g a m u l t i c h a n n e l t r a n s d u c e r system capable, as a m i n i m u m , of determining forces anteriorly a n d posteriorly on each side. The m e a n peak removal forces for the metal tray were similar for all elastomeric materials/techniques (Xantopren 36.3 N; Reprosi140.6 N; and Extrude 39.5 N), whereas there were considerable differences for the plastic trays (Xantopren 19.9 N; Reprosil 10.9 N; a n d Extrude 17.6 N). These differences m a y reflect distortion of the tray during removal, which would allow the impression material to flex over undercuts. This was expected to be of more relevance in dealing with the impression materials, which were stiffer w h e n set, a n d is supported by the finding t h a t the m a x i m u m insertion and removal forces, and also the load rates, for the elastomeric materials were greater when compared with those needed for the irreversible hydrocolloid for either tray. Although detailed geometric analysis of the dental arches was not done, the greater removal forces appear to be generated with large i n t e r d e n t a l undercuts a n d w h e n protrusion of the incisors was present. The peak insertion forces for these subjects were not proportionally greater, which was expected because these forces are more likely to be related to tray spacing, rate of seating, bulk and viscosity of impression material, and the presence of perforations. The m e a n peak load rates during the insertion a n d the removal of the irreversible hydrocolloid impressions were slightly greater for the metal tray compared with the plastic trays. This would be expected in view of the probable effects of tray a n d impression material distortion. The generation of relatively high forces, which might be expected in seating a wash impression, were demonstrated i n this study where the greatest initial seating load rate and m a x i m u m seating force were associated with this technique. Removal forces above the value of 45 N become uncomfortable for the p a t i e n t and there is a risk of d a m a g i n g soft tissues or opposing teeth if no precautions are taken. I n view of the pivotal role of impression procedures i n restorative dentistry, a role t h a t cannot be replaced by computerized s c a n n i n g techniques where tissue displacem e n t is required, it is i m p o r t a n t to have a good unders t a n d i n g of the forces involved. The development of impression trays t h a t incorporate transducers offers the potential not only for i n vivo investigations, b u t also the use of such techniques as t r a i n i n g aids.
CONCLUSIONS 1. Forces recorded on the metal t r a y were greater t h a n those applied on the disposable trays. For the metal tray the m e a n peak removal forces were between 36.3 N and 40.6 N w h e n the elastomeric impression materials of dif-
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SOTIRIOU AND HOBKIRK
ferent viscosity were used, and for the irreversible hydrocolloid the m e a n peak force was 29.6 N. 2. For the disposable plastic trays the m e a n peak removal forces were considerably lower a n d ranged from 14.7 N to 19.9 N when elastomeric impression materials of different viscosity were used, and the m e a n peak force for the irreversible hydrocolloid was 13.7 N. 3. Load rates recorded on the metal tray were greater t h a n those applied on the disposable trays. Substantial differences were observed w h e n elastomeric materials were used and may reflect tray distortion. The greater the viscosity of the material, the greater the load rate. 4. W h e n a nonperforated metal tray was used in a double impression technique, the impression tray adhesive may not have been reliable during the second removal from the mouth, especially when the removal forces applied were 50 N a n d higher. We are grateful to Mr. J. Kelleway for his assistance with the technical aspects of this project and to the colleagues who acted as subjects for the investigation.
REFERENCES
1. Douglas WH, Wilson HJ. A study of the zinc oxide-eugenoltype impressionpastes--pressures involvedin takingimpressions.Br Dent J 1964;116:34-6. 2. Frank RP"Analysis~ pressures pr~ duringmaxillaryedentuleus impressionprocedures. J PROSTHETDENT1969;22:400-13. 3. Frank RP. Controllingpressures duringcompletedentureimpressions. Dent Clin North Am 1970;14:453-70. 4. Manderson RD, Wills DJ, Picton DC. Biomechanics of denturesupportingtissues. In: Phillips RW, ed. Proceedingsof the secondinternational prosthodonticcongress.St Louis:C V Mosby,1979:98-101. 5. Koran A III. Impressionmaterials for recordingthe denture bearing mucosa. Dent Clin North Am 1980;24:97-111. 6. CollardEW, Caputo AA, StandleeJP, Trabert KC. Dynamicstresses encountered in impressionremoval. J PROSTHETDENT1973;29:498506. 7. DixonDL, BreedingLC, MoseleyJP. Customimpressiontrays. Part II: removal forces. J PROSTHETDENT1994;71:316-8. 8. Burton JF, Hood JA, Plunkett DJ, Johnson SS. The effects of disposable and custom-madeimpressiontrays onthe accuracyofimpressions. J Dent 1989;17:121-3. 9. Chai JY, Jameson LM, Moser JB, Hesby RA. Adhesiveproperties of several impression material systems: part II. J PROSTHETDENT 1991;66:287-92. 10. MacSweenR, Price RB. Peel bond strengths of five impressionmaterial tray adhesives. J Can Dent Assoc 1991,57:654-7. 11. DavisGB,MoserJB, BrinsdenGI. The bondingpropertiesofelastomer tray adhesives. J PROSTHETDENT1976;36:278-85. 12. Grant BE, TjanAH. Tensileand peel bondstrengths oftray adhesives. J PROSTHETDENT1988;59:165-8. 13. BombergTJ, GoldfogelMH, HoffmanW Jr, BombergSE. Considerations for adhesion of impression materials to impression trays. J PROSTHET DENT 1988;60:681-4.
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