177
Australian Dental Journal, June, 1977 Volume 22, No. 3
An elementary study of deformation of molar teeth during amalgam restorative procedures* J. Gordon Bell, B.D.S. (N.Z.)
ABSTRACT-A technique using an optical comparator was employed to examine the dimensional changes resulting from cavity preparation, application of the matrix and packing with amalgam in three extracted molar teeth. The changes have been reported and the nature of the change discussed. A modification in the use of the matrix and amalgam packing technique is suggested.
(Received f o r publication February. 1976)
Introduction
Investigations of amalgam restorations have covered the leakage occurring at the restoration tooth interface', photoelastic studies of stress within the restorationz, and more recently creep of dental amalgam3. Deformation of the tooth crown under axial load has been established, with the consequential effect of Class V cavities restored with amalgam4. This study is concerned with the changes in shape of a tooth crown during restoration using a molar tooth and a mesio-occlusal-distal cavity. Such a cavity preparation divides the enamel cap
Financial assistance for the project was received from the New Zealand Dental Research Foundation Board.
* Holliger
H H.-Penetration of restoration margins. A fluor;sc&t dye study. Dent. Radi. Phot., 40:1, 9-14 (Jan.) 1961. ZMahler, D. B.-Analysis of stresses in a dental amalgam restoration. J. D. Res., 37:3. 516526 (June) 1958. SEspervik S. and Sorensen, S. E.-Creep of dental :n$iam. 'Scand. J. D. Res., 83:4, 245-253 (July-Aug.) 17,J.
4
Hood J A,-Experimental studies on tooth deformation: sties; distribution in Class V restorations. New Zealand D. J., 68:312, 116-131 (Apl.) 1972.
of a tooth in two halves, except for the continuous band of enamel circumferentially below the distal and mesial gingival margins of the preparation. A method of measuring changes in shape, if any, is described using four optical datum points located one on each cusp of a molar tooth and it was employed to measure any change in shape of a molar tooth during restoration procedures in the cutting of a mesio-occlusal-distal cavity, placement of a matrix band, packing of amalgam, initial carving, removal of the matrix band and the final carving. Materials and methods
The method of measurement selected was to use a Nikon Optical Comparator with X 10 and X 20 lenses (Fig. 1). To establish optical datum points on the four cusps of a molar tooth, a beam of light was directed to these points, reflecting a pin-point of light upon the screen, which was adjusted by the micrometer adjustments of the platform to the intersection of the grid wires (Fig. 2). The micrometer allows adjustment to 0.0001 in.
Australian Dental Journal, June, 1977
178 Where necessary, ‘slip blocks’ (0.9 in, 1.0 in and 2.0 in) were inserted between the platform and the micrometers to position the teeth in relation to the grid on the screen. Pairs of caries-free molar teeth from the same patient, one control, one experimental, were mounted at right angles to the beam of light to
Fig. 2.-Nikon Optical Comparator with X 10 lens, closer view, showing occlusal surface of tooth projected to the screen with one datum point visible in upper right quadrant of grid wires. Fig. 1.-Nikon Optical Comparator showing light source on left with heat absorbent glass supported by clamp with electric fan below.
The teeth were stored in salinc within the room all operative procedures were carried out in ambient conditions of 20-21OC and 55 to 65 per cent humidity. Time was allowed for the teeth to reach the ambient temperature after removal from saline and operative procedures. H{here
To minimize dehydration the teeth were stored in N-saline from time of extraction until mounted in glass blocks 2 in x 2 in x in using low fusing metal (Melottes); subsequently they were returned to the normal saline between various manipulative procedures.
+
Fig. 3.-One experimental and one control tooth mounted in glass block held by rubber band to supporting frame which is clamped to the micrometer adjustable platform.
receive an equal amount of heat, if any, from the light source (Fig. 3). Heat absorbent glass was interposed between the source of light and the teeth and a fan placed below this glass to circulate air and at the same time direct an airflow over the platform on which the teeth were placed (Fig. 2).
Optical datum points established on each cusp by cementing with cyanoacrylate cement (Permabond) one Microbead Glas-shot number MS-XH, nominal size 0.0116 in-0.0069 in. The beam of light on the bead produced a pinpoint of light which was reflected to the screen. This pin-point of light was brought to the intersection of the grid-lines on the optical comparator. The angle of the light beam found to produce the best pin-point reflection was as close to vertical as the machine would allow. Once positioned, this light source was not altered during the complete investigation of all teeth.
Australian Dental Journal, June, 1977 Optical datum points were also established on the surface of the blocks to check for rotation of the block during experimental procedures. The supporting frame was clamped to the platform. Three ball bearings at the apices of an
Fig. 4.-Diagram of glass block showing position of optical datum points ABCD on cusps of control tooth, EFGH on cusps of experimental tooth, and direction of prepared cavity. Also the datum points WXY on the surface of the glass block to check for rotation on repositioning.
equilateral triangle support the base of the glass block, whilst the right angled vertical support has two ball bearings on one arm and one ball bearing on the other. The glass block was held firmly in place with a rubber band. All datum points were lettered in the same manner, Fig. 4, and readings in 0.0001 in were from 0 to 1.0 along the line from W to X and 0 to 1.0 in along the line Y to X. A standard cavity form was prepared, the amalgam packed and carved so that the buccal and palatal cavity walls did not encroach upon the optical datum points or their cement. The cavity outline was cut with a HiDi FG 541 diamond, the walls smoothed with a cylindrical finishing bur, the pulpal and gingival floors were smoothed with an endcutting bur 959 which was also used to simulate the use of a gingival margin trimmer. A round bur No. 3 was used to remove caries (simulated) and the interproximal retention was accentuated using a tapered fissure bur No. 700 at the bucco-axial and palato-axial line angles. The normal operating amount of cooling water was used with the diamond bur and steel burs were rotated at slow speeds. The cavity was well washed with warm water, thoroughly cleaned and dried prior to placing the amalgam.
179 The matrix band holder selected was an Ash Siqveland with a wide band, to which was attached a slipping spring-load clutch, capable of adjustment to a selected tension**. As the stress app:ied to a tooth through the matrix band is empirical, according to the operator’s judgement and the tooth under treatment, some degree of standardization was considered necessary. The matrix band holder was placed, for convenience of observation, adjacent to the tooth surface datum points E and F. The cavity was not lined, amalgam was proportioned in the ratio of one to one using S.S. White New True Dentalloy. Condensation was completed in three minutes after trituration by hand with pestle and mortar. A Premier amalgam carrier was used to convey the amalgam to the cavity, which was packed with Ash No. 1, 2 pluggers, and a plastic instrument Ash No. 64 for the packing of the retention grooves. The whole restoration was subject to final packing using No. 2 point in a Dentatus mechanical packing handpiece and carved with a Ward No. 2 carver. The management of the blocks and recording of results was as follows:- Slip blocks for the vertical component 0.9 in was used when measuring datum points W, X, G, H, and 2.0 in only for Y. For G, H, C and D 1.0 in was used (vertical or horizontal as reflected on the vertical screen). Readings:1. All datum points were recorded with W, X and Y being repeated to check for accuracy and repeatability. 2. Block removed and replaced after cavity preparation. 3. Matrix band placed without removing block. 4. Amalgam restoration placed with block removed then replaced. 5. Matrix band removed without removing block. 6. Block replaced after 23 hours. 7. Block replaced after 29 hours. 8. Block replaced after 47 hours. Of five sets of teeth used in this study, two were lost due to key datum points becoming detached during the experiment, tooth No. 6 was the only one to have eight recordings completed, tooth No. 3 had no time lapse movements recorded, and tooth No. 2 had a repositioning error between initial measure and cavity preparation but was acceptable thereafter.
**Devised by engineers of No. 1 T.T.S.. R.N.Z.A.F., Hobsonville. New Zealand.
180
Australian Dental Journal, June, 1977 Results
The results of readings one to eight were recorded and the results for teeth No. 2,3,6 were further analyzed by a graphic method to demonstrate and calculate changes in tooth dimension.
TABLE1 Sum of tneasurements, centroids to base of recrangle, for readings 1-8 Reading
1
2 3 4 5 6 7 8
Tooth 2
8.0000 8.6407 8.5625 8.9063 8.1250 8.7656 8.1719
Tooth 3
6.0000 7.1719 13.203 1 9.5156 9.7501
Tooth
These photographs showed the minimal extent of the cavity preparation in all directions. Discussion
It is emphasized that the results recorded are only the movements of four optical datum points attached to the cusps of a molar tooth, measured in two dimensions. Preparation of the cavity caused the tooth to change shape, as a result either from heat generated by the cutting instrument or of the release of internal stress inherent within the tooth. Pressure from the matrix band, which it was assumed, would draw the buccal and lingual halves of the tooth together, actually caused an outward movement of these sides. The tension of this band, being concentrated at the gingival margin and at the points of contact on the buccal and lingual surfaces of the teeth, could exert pressure on the mesial and distal surface angles of the preparation causing the buccal and lingual halves to move outward. It can be assumed that the buccal and lingual halves are by nature a composite bar of enamel and dentine. The bulk of dentine remaining between the mesial and distal axial walls will resist compression from the buccal and lingual halves. The position of the pulp chamber appears to have no influence on distribution. The changes recorded when the amalgam was packed are due to the alteration of pressure upon the matrix band. The amalgam was packed according to convention and added tension to the matrix band especially at the occlusal level, thus reducing the end pressure on the cavity preparation and allowed the buccal and lingual halves to move together. On removal of the matrix band the buccal and lingual halves moved outwards; it may be assumed that partial recovery of the original shape would be expected when external tension on the tooth was relieved. Changes in shape will arise from subsequent internal pressures generated by the setting reaction of the amalgam as well as the inherent property of the tooth to recover. Diminution of movement over a period of 47 hours would indicate a resolution of all stress induced whether externally or internally, from the matrix band, packing of amalgam, setting reaction of amalgam and pressures exerted upon the teeth. When a cavity is prepared and a matrix band applied, changes in shape may occur in tooth tissue remote from the cavity preparation. The application of this type of matrix band, through friction, produces a torsion effect greatest
li:!(ll:: 7.6563 7.5938 8.4844
i8.6094 :::::
From measurement of coordinates to 0.0001 in
Analysis of results The original datum points (reading one) were used as a base for comparison and were plotted to coincide with the angles of a rectangle. A symmetrical figure was chosen to facilitate cornparison with other readings when plotted. Using the results of each subsequent reading as co-ordinates a quadrilateral was constructed for each set of readings, 2 to 8. This irregular quadilateral could be compared with the original rectangle to show changes in dimension. The variations in readings of the fixed datum points and the control tooth were also plotted to the same scale and were used to provide a figure of change which was applied to the main diagram to modify movements of the datum points due to observation and experimental factors. A centroid was calculated for each quadrilateral and the dimensional changes shown by these quadrilaterals was measured by the distance from the centroid to each of the horizontal sides of the base line rectangle. The sum of the two measurements obtained from the data for readings two to eight is presented in Table 1. Photographs of the restored teeth No. 2,3,6, were made from the mesial, distal and occlusal aspects after all measurements were completed. Sections cut through teeth No. 2 and 6 as nearly coincidental as possible with the position of the optical datum points were photographed. Finally a section cut through the mesial lingual aspect as near the mid-point mesio-distally as could be judged was photographed.
Australian Dental Journal, June, 1977 adjacent to the movable end of the band. This may induce stress within the cuspal tooth tissue quite independent of observed movement of the buccal and lingual halves of the tooth. Obviously the placing of the matrix band cannot be repeated accurately. The points and surfaces are such that the direction of movement cannot be anticipated, especially when the matrix band holder is set to tighten the gingival edge more than the occlusal edge of the matrix band. Conclusion
From these observations are several important assumptions. 1. Heat generated during cavity preparation may structurally alter enamel and dentine and contribute to leakage at the interface of restoration and tooth. 2. If internal stress is relieved during cavity preparation this may contribute to weakening the enamel at the time of restorative procedures. 3. When the matrix band is tightened, a line of stress may be induced, along which a fracture could occur later. This may account for fractured cusps. 4. A marginal leakage may be induced on removal of the matrix band, although it may be reduced later by amalgam expansion. 5. Compression of tooth tissue of 0.001 in could be significant (dentinal tubule 0.0001 in). Pulpal changes could be induced as a result of these con-
181
ventional procedures, because of the hydrostatic changes which could be induced in the tubules and odontoblasts. 6. When an MOD cavity is cut in a tooth, the enamel is literally divided in two, being only continuous circumferentially below the gingival margins on the mesial and distal aspects. This enamel may be fractured if the compression of the matrix band is excessive. The practice of packing amalgam in a compound cavity such as an MOD above the contact point then partially releasing the tension on the matrix band before final packing to ensure a tight contact may not only have that desired effect but also release the induced matrix band stress upon the tooth tissues. In this way some of the changes observed could be avoided in practice. Acknowledgements
The author wishes to acknowledge the advice and encouragement of Professor J. le B. Warren and Brigadier J. Ferris Fuller; also the willing assistance and advice of Mr W. R. Beasley, Senior Mechanical and Production Engineer, Auckland Industrial Development Division, Department of Scientific and Industrial Research, Dr John Meikk and the staff of same department, who made the supporting frame for the glass blocks. Hall Road, Kerikeri, Bay of Islands, New Zealand.
Australian Dental Journal, June, 1977 Volume 22, No. 3
An elementary study of deformation of molar teeth during amalgam restorative procedures* J. Gordon Bell, B.D.S. (N.Z.)
ABSTRACT-A technique using an optical comparator was employed to examine the dimensional changes resulting from cavity preparation, application of the matrix and packing with amalgam in three extracted molar teeth. The changes have been reported and the nature of the change discussed. A modification in the use of the matrix and amalgam packing technique is suggested.
(Received f o r publication February. 1976)
Introduction
Investigations of amalgam restorations have covered the leakage occurring at the restoration tooth interface', photoelastic studies of stress within the restorationz, and more recently creep of dental amalgam3. Deformation of the tooth crown under axial load has been established, with the consequential effect of Class V cavities restored with amalgam4. This study is concerned with the changes in shape of a tooth crown during restoration using a molar tooth and a mesio-occlusal-distal cavity. Such a cavity preparation divides the enamel cap
Financial assistance for the project was received from the New Zealand Dental Research Foundation Board.
* Holliger
H H.-Penetration of restoration margins. A fluor;sc&t dye study. Dent. Radi. Phot., 40:1, 9-14 (Jan.) 1961. ZMahler, D. B.-Analysis of stresses in a dental amalgam restoration. J. D. Res., 37:3. 516526 (June) 1958. SEspervik S. and Sorensen, S. E.-Creep of dental :n$iam. 'Scand. J. D. Res., 83:4, 245-253 (July-Aug.) 17,J.
4
Hood J A,-Experimental studies on tooth deformation: sties; distribution in Class V restorations. New Zealand D. J., 68:312, 116-131 (Apl.) 1972.
of a tooth in two halves, except for the continuous band of enamel circumferentially below the distal and mesial gingival margins of the preparation. A method of measuring changes in shape, if any, is described using four optical datum points located one on each cusp of a molar tooth and it was employed to measure any change in shape of a molar tooth during restoration procedures in the cutting of a mesio-occlusal-distal cavity, placement of a matrix band, packing of amalgam, initial carving, removal of the matrix band and the final carving. Materials and methods
The method of measurement selected was to use a Nikon Optical Comparator with X 10 and X 20 lenses (Fig. 1). To establish optical datum points on the four cusps of a molar tooth, a beam of light was directed to these points, reflecting a pin-point of light upon the screen, which was adjusted by the micrometer adjustments of the platform to the intersection of the grid wires (Fig. 2). The micrometer allows adjustment to 0.0001 in.
Australian Dental Journal, June, 1977
178 Where necessary, ‘slip blocks’ (0.9 in, 1.0 in and 2.0 in) were inserted between the platform and the micrometers to position the teeth in relation to the grid on the screen. Pairs of caries-free molar teeth from the same patient, one control, one experimental, were mounted at right angles to the beam of light to
Fig. 2.-Nikon Optical Comparator with X 10 lens, closer view, showing occlusal surface of tooth projected to the screen with one datum point visible in upper right quadrant of grid wires. Fig. 1.-Nikon Optical Comparator showing light source on left with heat absorbent glass supported by clamp with electric fan below.
The teeth were stored in salinc within the room all operative procedures were carried out in ambient conditions of 20-21OC and 55 to 65 per cent humidity. Time was allowed for the teeth to reach the ambient temperature after removal from saline and operative procedures. H{here
To minimize dehydration the teeth were stored in N-saline from time of extraction until mounted in glass blocks 2 in x 2 in x in using low fusing metal (Melottes); subsequently they were returned to the normal saline between various manipulative procedures.
+
Fig. 3.-One experimental and one control tooth mounted in glass block held by rubber band to supporting frame which is clamped to the micrometer adjustable platform.
receive an equal amount of heat, if any, from the light source (Fig. 3). Heat absorbent glass was interposed between the source of light and the teeth and a fan placed below this glass to circulate air and at the same time direct an airflow over the platform on which the teeth were placed (Fig. 2).
Optical datum points established on each cusp by cementing with cyanoacrylate cement (Permabond) one Microbead Glas-shot number MS-XH, nominal size 0.0116 in-0.0069 in. The beam of light on the bead produced a pinpoint of light which was reflected to the screen. This pin-point of light was brought to the intersection of the grid-lines on the optical comparator. The angle of the light beam found to produce the best pin-point reflection was as close to vertical as the machine would allow. Once positioned, this light source was not altered during the complete investigation of all teeth.
Australian Dental Journal, June, 1977 Optical datum points were also established on the surface of the blocks to check for rotation of the block during experimental procedures. The supporting frame was clamped to the platform. Three ball bearings at the apices of an
Fig. 4.-Diagram of glass block showing position of optical datum points ABCD on cusps of control tooth, EFGH on cusps of experimental tooth, and direction of prepared cavity. Also the datum points WXY on the surface of the glass block to check for rotation on repositioning.
equilateral triangle support the base of the glass block, whilst the right angled vertical support has two ball bearings on one arm and one ball bearing on the other. The glass block was held firmly in place with a rubber band. All datum points were lettered in the same manner, Fig. 4, and readings in 0.0001 in were from 0 to 1.0 along the line from W to X and 0 to 1.0 in along the line Y to X. A standard cavity form was prepared, the amalgam packed and carved so that the buccal and palatal cavity walls did not encroach upon the optical datum points or their cement. The cavity outline was cut with a HiDi FG 541 diamond, the walls smoothed with a cylindrical finishing bur, the pulpal and gingival floors were smoothed with an endcutting bur 959 which was also used to simulate the use of a gingival margin trimmer. A round bur No. 3 was used to remove caries (simulated) and the interproximal retention was accentuated using a tapered fissure bur No. 700 at the bucco-axial and palato-axial line angles. The normal operating amount of cooling water was used with the diamond bur and steel burs were rotated at slow speeds. The cavity was well washed with warm water, thoroughly cleaned and dried prior to placing the amalgam.
179 The matrix band holder selected was an Ash Siqveland with a wide band, to which was attached a slipping spring-load clutch, capable of adjustment to a selected tension**. As the stress app:ied to a tooth through the matrix band is empirical, according to the operator’s judgement and the tooth under treatment, some degree of standardization was considered necessary. The matrix band holder was placed, for convenience of observation, adjacent to the tooth surface datum points E and F. The cavity was not lined, amalgam was proportioned in the ratio of one to one using S.S. White New True Dentalloy. Condensation was completed in three minutes after trituration by hand with pestle and mortar. A Premier amalgam carrier was used to convey the amalgam to the cavity, which was packed with Ash No. 1, 2 pluggers, and a plastic instrument Ash No. 64 for the packing of the retention grooves. The whole restoration was subject to final packing using No. 2 point in a Dentatus mechanical packing handpiece and carved with a Ward No. 2 carver. The management of the blocks and recording of results was as follows:- Slip blocks for the vertical component 0.9 in was used when measuring datum points W, X, G, H, and 2.0 in only for Y. For G, H, C and D 1.0 in was used (vertical or horizontal as reflected on the vertical screen). Readings:1. All datum points were recorded with W, X and Y being repeated to check for accuracy and repeatability. 2. Block removed and replaced after cavity preparation. 3. Matrix band placed without removing block. 4. Amalgam restoration placed with block removed then replaced. 5. Matrix band removed without removing block. 6. Block replaced after 23 hours. 7. Block replaced after 29 hours. 8. Block replaced after 47 hours. Of five sets of teeth used in this study, two were lost due to key datum points becoming detached during the experiment, tooth No. 6 was the only one to have eight recordings completed, tooth No. 3 had no time lapse movements recorded, and tooth No. 2 had a repositioning error between initial measure and cavity preparation but was acceptable thereafter.
**Devised by engineers of No. 1 T.T.S.. R.N.Z.A.F., Hobsonville. New Zealand.
180
Australian Dental Journal, June, 1977 Results
The results of readings one to eight were recorded and the results for teeth No. 2,3,6 were further analyzed by a graphic method to demonstrate and calculate changes in tooth dimension.
TABLE1 Sum of tneasurements, centroids to base of recrangle, for readings 1-8 Reading
1
2 3 4 5 6 7 8
Tooth 2
8.0000 8.6407 8.5625 8.9063 8.1250 8.7656 8.1719
Tooth 3
6.0000 7.1719 13.203 1 9.5156 9.7501
Tooth
These photographs showed the minimal extent of the cavity preparation in all directions. Discussion
It is emphasized that the results recorded are only the movements of four optical datum points attached to the cusps of a molar tooth, measured in two dimensions. Preparation of the cavity caused the tooth to change shape, as a result either from heat generated by the cutting instrument or of the release of internal stress inherent within the tooth. Pressure from the matrix band, which it was assumed, would draw the buccal and lingual halves of the tooth together, actually caused an outward movement of these sides. The tension of this band, being concentrated at the gingival margin and at the points of contact on the buccal and lingual surfaces of the teeth, could exert pressure on the mesial and distal surface angles of the preparation causing the buccal and lingual halves to move outward. It can be assumed that the buccal and lingual halves are by nature a composite bar of enamel and dentine. The bulk of dentine remaining between the mesial and distal axial walls will resist compression from the buccal and lingual halves. The position of the pulp chamber appears to have no influence on distribution. The changes recorded when the amalgam was packed are due to the alteration of pressure upon the matrix band. The amalgam was packed according to convention and added tension to the matrix band especially at the occlusal level, thus reducing the end pressure on the cavity preparation and allowed the buccal and lingual halves to move together. On removal of the matrix band the buccal and lingual halves moved outwards; it may be assumed that partial recovery of the original shape would be expected when external tension on the tooth was relieved. Changes in shape will arise from subsequent internal pressures generated by the setting reaction of the amalgam as well as the inherent property of the tooth to recover. Diminution of movement over a period of 47 hours would indicate a resolution of all stress induced whether externally or internally, from the matrix band, packing of amalgam, setting reaction of amalgam and pressures exerted upon the teeth. When a cavity is prepared and a matrix band applied, changes in shape may occur in tooth tissue remote from the cavity preparation. The application of this type of matrix band, through friction, produces a torsion effect greatest
li:!(ll:: 7.6563 7.5938 8.4844
i8.6094 :::::
From measurement of coordinates to 0.0001 in
Analysis of results The original datum points (reading one) were used as a base for comparison and were plotted to coincide with the angles of a rectangle. A symmetrical figure was chosen to facilitate cornparison with other readings when plotted. Using the results of each subsequent reading as co-ordinates a quadrilateral was constructed for each set of readings, 2 to 8. This irregular quadilateral could be compared with the original rectangle to show changes in dimension. The variations in readings of the fixed datum points and the control tooth were also plotted to the same scale and were used to provide a figure of change which was applied to the main diagram to modify movements of the datum points due to observation and experimental factors. A centroid was calculated for each quadrilateral and the dimensional changes shown by these quadrilaterals was measured by the distance from the centroid to each of the horizontal sides of the base line rectangle. The sum of the two measurements obtained from the data for readings two to eight is presented in Table 1. Photographs of the restored teeth No. 2,3,6, were made from the mesial, distal and occlusal aspects after all measurements were completed. Sections cut through teeth No. 2 and 6 as nearly coincidental as possible with the position of the optical datum points were photographed. Finally a section cut through the mesial lingual aspect as near the mid-point mesio-distally as could be judged was photographed.
Australian Dental Journal, June, 1977 adjacent to the movable end of the band. This may induce stress within the cuspal tooth tissue quite independent of observed movement of the buccal and lingual halves of the tooth. Obviously the placing of the matrix band cannot be repeated accurately. The points and surfaces are such that the direction of movement cannot be anticipated, especially when the matrix band holder is set to tighten the gingival edge more than the occlusal edge of the matrix band. Conclusion
From these observations are several important assumptions. 1. Heat generated during cavity preparation may structurally alter enamel and dentine and contribute to leakage at the interface of restoration and tooth. 2. If internal stress is relieved during cavity preparation this may contribute to weakening the enamel at the time of restorative procedures. 3. When the matrix band is tightened, a line of stress may be induced, along which a fracture could occur later. This may account for fractured cusps. 4. A marginal leakage may be induced on removal of the matrix band, although it may be reduced later by amalgam expansion. 5. Compression of tooth tissue of 0.001 in could be significant (dentinal tubule 0.0001 in). Pulpal changes could be induced as a result of these con-
181
ventional procedures, because of the hydrostatic changes which could be induced in the tubules and odontoblasts. 6. When an MOD cavity is cut in a tooth, the enamel is literally divided in two, being only continuous circumferentially below the gingival margins on the mesial and distal aspects. This enamel may be fractured if the compression of the matrix band is excessive. The practice of packing amalgam in a compound cavity such as an MOD above the contact point then partially releasing the tension on the matrix band before final packing to ensure a tight contact may not only have that desired effect but also release the induced matrix band stress upon the tooth tissues. In this way some of the changes observed could be avoided in practice. Acknowledgements
The author wishes to acknowledge the advice and encouragement of Professor J. le B. Warren and Brigadier J. Ferris Fuller; also the willing assistance and advice of Mr W. R. Beasley, Senior Mechanical and Production Engineer, Auckland Industrial Development Division, Department of Scientific and Industrial Research, Dr John Meikk and the staff of same department, who made the supporting frame for the glass blocks. Hall Road, Kerikeri, Bay of Islands, New Zealand.
