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Frictional resistance and dental prosthetics
FRICTIONAL
RESISTANCE AND DENTAL PROSTHETICS
ROYAL L. NORMAN, D.D.S. Elmhurst, Ill.
F
RICTION IS DEFINED in mechanics as “the resistance to relative motion between two bodies in contact.“l In nearly all dental procedures, we are confronted in part by this relationship, either directly or indirectly. If the action of the mandible was strictly hinge-type, with no lateral or protrusive movements in function, such resistances would be negligible. However, lateral and protrusive movements do exist in varying degrees, and habitual tooth contacts do occur in these movements. Proprioceptive interception may prevent excessive degrees of opposing tooth contact in natural dentitions during mastication. However, the performance of this proprioceptor mechanism in dentures is greatly impaired, with a resulting over-all increase in friction. The observed patterns of wear in natural and artificial teeth indicate that there is enough relative motion in contact to bring into account the problem of frictional resistance. Excessive stimulation may exceed the physiologic tolerance of the tissue with resultant pathologic changes. Therefore, factors which can cause over-stimulation of teeth and supporting structures must be reduced. Frictional resistance between artificial teeth may likely be such a factor. These forces of friction have to be absorbed as they occur. If forces of friction combine with other forces of mastication to exceed the tolerance of supportive tissues, changes in that tissue will occur. DIRECTION
OF FRICTIONALFORCES
Frictional resistance in the mouth would be expressed primarily in a horizontal or lateral direction, depending on cuspal inclinations and contact areas. Lateral forces directed at the natural teeth can be most destructive. In instances where natural teeth have become intolerable to normal function through various pathologic and/or physiologic processes, forces of friction become more pertinent. The effects of frictional resistances are more apparent in extensive removable partial or complete dentures than in other phases of dentistry. The natural teeth and supporting structures can no longer be responsible for absorbing any normal or abnormal forces. Soft tissues which were never intended to support artificial replacements are now required to absorb the necessary forces that arise during mouth function. Occlusally directed pressures must be minimized in the use of dentures. Proportionately, lateral and protrusive contact pressures are reduced with a comparable reduction in frictional resistance. More surface is usually available to resist vertical forces than is available to resist lateral forces because of the anatomic contour of the 45
46
NORMAN
J. Pros. Jnn:Feb.,
Den. 1961
residual ridges. However, with resorption d the ritlges, less surface rcniains to i-esist lateral forces and proportionately more surface is created to resist vertical forces. The resulting imbalance decreases the ability of the tissues to absorb lateral pressures. Chronic irritation of the ridges and dislodgment of the dentures can then occur. Individual habits of chewing may produce excessive lateral forces, even when ideal ridge relationships exist, and result in chronic irritation or loss of retention of the restoration. If no clinical signs of irritation are present, subclinical evaluations might indicate that the physiologic tolerance of the ridges has been exceeded. Possibly, the resorptive processes of the edentulous ridge are incited in this manner with subsequent atrophy as a secondary factor. Many dentists use porcelain teeth in constructing complete dentures. Yet few dentists seem unaware of the effective results of opposing acrylic resin to porcelain teeth for some patients, As a possible explanation for this occurrence, experiments were performed to determine whether or not the materials commonly encountered in prosthetic procedures display any marked differences in frictional resistance. hfhTERIAIL3
A platform was constructed to hold the various parts of the testing mechanism (Fig. 1). The support arm could move both vertically and horizontally. Inherent resistance of the apparatus was minimized by: (1) paralleling the thread, the arm, and the stationary platform to the base platform; (2) polishing and lubricating all moving parts; and (3) maintaining a 90 degree angle between the thread and the support arm. The five materials chosen for testing of frictional resistance were : ( 1) enamel (extracted teeth), (2) vacuum-fired porcelain, (3) acrylin resin denture teeth, (4) a hard gold, and (5) a chrome cobalt alloy. PREI’ARATION
FOR TESTING
Flat surfaces, 10 mm. or more in diameter, were ground on each of the materials to be tested. The inside surface of an abrasive rubber wheel, 3 inches in diameter, was used to establish these essentially flat planes and eliminate any visual cuts or scratches from the surfaces, (Further polishing could have made the metals react more favorably and alter the uniformity of the experiment.) Five points, one of each material tested, were ground and polished with a rubber wheel to small rounded tips approximately 1 mm, in diameter. All edges or defects were eliminated to develop a comparable contact area when the point was placed against a flat surface. Each of the test materials with the flat surfaces was separately mounted in wax for each test phase. This formed a stationary platform of the test materials. The testing points were attached separately in wax for each test phase underneath the movable weight platform (Fig. 1). This platform was removable, and a common weight was duplicated with each change in test points by either adding or removing wax to compensate for the difference in the weight of the test points.
~,oll$l~r ‘1” Ll
FRICTIONAL
RESISTANCE
AND
DENTAL
PROSTHETICS
47
A drop of saliva was used as a lubricating agent between the test point and the flat surface when they were in the proper relationship. Since there were measurable differences between dry and wet surfaces, a wet surface was employed to simulate mouth conditions. Saliva from the same person was used for all tests. The temperature of the specimen affected the readings. Therefore, on each series of tests the surfaces of the point and flat surface were cleaned, and a new specimen was attached and allowed to cool to room temperature. Subsequent tests showed that this method caused no appreciable variations in the results.
Fig. 1.-A testing device is used to determine the frictional resistance of different materials used as occlusal surfaces for dental restorations. 1, The weight platform for the application of horizontal forces. 2, The silk connecting thread. 3, The pulley. 4, The movable weight platform for applying vertical forces. 5, The supporting arm. 6, The control lever, 7, The stop. 8, The stationary platform and testing surface. 9, The test point. 10, The base platform.
TESTING
PROCEDURES
Weights were added to the weight platform to supply the horizontal force until the resistances of the contacts were overcome. The lever was used as an indirect source of control of the platform to eliminate adverse movements of the hand. The line of contact of the opposing materials, measuring approximately 8 mm. before the stop, was used as the testing area. The criterion for establishing a base upon which all tests could be compared was passage of the pointed tip completely across the testing surface. At no point before the end of the planned movement could a resting pause be maintained. If questioned defects on the testing surfaces hampered the movements of the pointed tips, alternate areas were chosen. Weights measuring from 0 to 25 grams at 5 gram intervals were applied to the vertical weight platform to complete each testing phase. In some of the tests, movement of the weight platform occurred without applying a horizontal weight. Vertical weights were then initially added until a stationary position of the platform was obtained. These weights are recorded in parentheses in Table I.
48
J. Pros. Jan:Feb.,
NORAlAN
TABLE
I.
FRICTION
HORIZONTAL
VERTICAL FORCE ON THE PLATFORMS
RESISTANCE
FORCE
ON
THE
CONTACT
POINTS
-
-/
1 ENAMEL
GM.
PORCELAIN (GM.1
(GM.1
2.1 3.8 5.2 6.8 8.9
0.7 2.8 4.9 6.9 9.4 12.0
3.3 5.3 7.7 12.9 10.1
25
0.7 2.5 4.5 6.8 9.0 10.9
1.5 3.7 8.1 12.7 16.9 20.1
0.7 20 3 7 6.1 96 11.4
3
0.4
06
0.5 Enamel
Acrylic
I
O 5
Porcelain
resin 7.4 92 11.4
1 Chrome cobalt alkly
ACRYLIC RESIN (GM.)
I
a.; 8.8 6.8
10.4
u:;’
0.2 1.2 2.3 3.6 4.8 6.0
1.8
2.9 4.0 5.0
y’
‘;:;’
2.2 3.7 5.0 6.9
CHROME COBALT 4LLOY (GM.)
GOLD
j i
(GM.1
-___
--__-
1: 1.5
Den. 1964
1.8 2.8 3.8 4.7
1.1
0 1.4
/ (
2
10.8
0.9 5
~
44 5.7
,
7.0 0
1.7
1.:
I-
3.5 4.6
(2.9, 04
3.1
: i
4.9 6.7
7.8
--
0.7 3.5 6.0 8.6 11 4 14.9 -__-0.5 2.4 4.4 6.3 8.3 10.2 ___0.4 2.3 4.8 7.2 8.5
(1.2)
1: 3 4 4.7
0.8 3.0 5.4 7.2 9.1 10.4
-1
--0.1
1.3 2.7 4.2 5.2 6.3
‘3 1.6 2.7 3.7
5.1
__-~ ‘;:;’ 1.6
2.9 4.3 5.5
(6 7) 0.6 1.0 2.2 3 3
-. RESULTS
Marked differences were found in the frictional resistances between the materials tested (Table I). The highest resistance in relation to the other materials tested was the combination of porcelain against porcelain. The lowest frictional resistance was found to be between the chrome cobalt alloy point against the chrome cobalt alloy surface. Most combinations of materials seemed to react to increased vertical forces proportionately. Exceptions were for some materials with acrylic resin components, although even these did not vary much, as a rule. With enamel-against-enamel measurements representing a base line for the frictional resistance, only the following combinations of materials had resistances equal to or less than natural tooth resistances : ( 1) enamel against gold or chrome
Volume .N11ml1er
14 1
l~RICTIONAI.
RESISTANCE
AND
DENTI\L
PROSTHETICS
49
cobalt alloy, (2) porcelain against gold or chrome cobalt alloy, (3) gold against gold or chrome cobalt alloy, and (4) chrome cobalt alloy against chrome cobalt alloy. The combinations of materials having frictional resistance equal to or more than natural tooth resistances were : (1) porcelain against porcelain, enamel or acrylic resin, and (2) acrylic resin against acrylic resin, enamel, gold, or chrome cobalt alloy. The porcelain-against-porcelain contacts demonstrated the highest frictional resistance (55 per cent increase over that of enamel against enamel). The combinations of porcelain against gold, and porcelain against chrome cobalt alloy showed a 34 and 60 per cent decrease, respectively, as compared to the frictional resistance of enamel against enamel. These same combinations also decreased the resistance as compared with porcelain against porcelain by 69 and 77 per cent. The alloy-against-alloy contacts showed a remarkable (88 per cent) reduction of frictional rmesistanceas compared to porcelain against porcelain. These calculations in per cent were made by averaging the two results obtained for each dissimilar combination of materials. (An example would be a porcelain tip against an acrylic resin platform and an acrylic resin tip against a porcelain platformj. The per cent of difference between the compared items was then calculated for each given weight load. The resulting six percentages, representing the complete range of the experiment, were then averaged into one mean per cent of change. DISCUSSION
The degree of error in the procedure employed is best described by comparing the two results obtained when unlike materials were measured. For example, the gold point against a porcelain surface under 25 grams vertical load required 7.8 grams force to displace it horizontally. The porcelain point against a gold surface under 25 grams vertical force required 5.0 grams to displace it horizontally. This 2.8 gram discrepancy was the maximum recorded under a 25 gram vertical load. However, the average difference for all combinations utider this load was 1.3 grams. Under a vertical load of 0 grams the average was 0.48 gram. The average of 0.48 and 1.3 is 0.89; therefore, a variation of plus or minus 0.89 potentially exists for each figure contained in Table I. However, because of the wide variety in the materials tested, it is more acceptable to establish the degree of error in each specific combination rather than to use the average for all materials. Under increasing weight loads, softer and more fragile points might break down against a hard platform or vice versa, and, therefore, not give comparable contact measurements of resistance. The same problem would occur in the mouth and these results would be more directly applicable. Nevertheless, there is enough difference in the results that, even allowing for the greatest possible error, the result would not be appreciably altered. With application of the results of these tests intraorally, considerable differences in resistance could be noted. For esample, assume a given vertical force is applied to dentures with opposing porcelain teeth and it requires 10 pounds of force to move one denture over the other denture in a horizontal direction. Dentures
50
NORMAN
J. Pros. Den. Jan-Fel,., 1004
employing occluding surfaces oi chrome cobalt alloy would rccluire only 12 l)er cent, or 1.2 pounds, of force to accomplish the same movement Under the rather extreme horizontally directed forces of the dentures with all porcelain teeth, it is logical to assume that proprioceptive impulses from the supporting tissues would interpret and, therefore, reduce vertical forces or omit horizontal movements. If this were true, then the efficiency of the dentures would be reduced. However, proprioceptive interpretation of edentulous ridge conditions would not be achieved to the same degree of perfection as with natural teeth and, therefore, cannot be relied upon to control the probable destructive forces in complete dentures. The results of the frictional resistance tests can be applied to the natural teeth. It was demonstrated that frictional resistance can be reduced from that of natural tooth enamel. Unstable teeth with unhealthy periodontal support may require a reduction of the occlusal forces as a part of their treatment. For example, occlusal splinting of teeth would be improved by using a chrome cobalt alloy in the splint. The use of splints that do not involve the occluding surfaces would be a second choice. The type of materials used could mean success or failure in long-span fixed partial dentures or such dentures that are supported by weakened teeth. All factors that cause overstressing the supporting structures of teeth must be reduced. Therefore, the use of materials demonstrating minimal frictional resistances will reduce the forces exerted on the tissues that support most dental restorations. SUMMARY
A series of tests was conducted to determine differences in the frictional resistance of materials used to form occlusal surfaces of dental restorations. A wide range of frictional resistance existed among the materials tested. Porcelain-against-porcelain contacts required the greatest displacement force of the materials tested. This force was nearly twice the amount required to displace a similar contact between porcelain and acrylic resin and almost three times that between two contacting surfaces of natural tooth enamel. In all of the tests, a chrome cobalt alloy, either opposing itself or in combination with other materials, was consistently lowest in the relative frictional resistances measured. CONCLUSIONS
1. Frictional resistance of opposing tooth surfaces is a measurable force and must be accounted for in prosthodontics, as well as in conditions involving natural dentitions. 2. The employment of opposing porcelain teeth in prosthetic restorations will result in relatively high frictional forces in a lateral direction as compared with other materials. 3. The success of some dentures which utilize acrylic resin teeth in opposition to porcelain teeth may result from a marked decrease in frictional resistance. 4. Reductions of frictional resistance to a point well below that of the resistances occurring in the natural teeth can be achieved by employing gold or chrome cobalt alloys on the occluding surface of dental restorations.
\‘olun1e 14 Su111lm 1
FRICTIONAL
RESISTANCE
AND
DENTAL
PROSTHETICS
51
5. The development of esthetic posterior teeth made of friction-reducing materials on the occIusa1 surfaces would encourage their use in dental restorations. REFERENCE
1. Webster’s New International Co. p. 1008. 103 HAVEN ELMHURST,
RD. ILL.
Dictionary,
ed. 2, Springfield.
Mass.,
1958, G.
& C. Merriam
RESISTANCE AND DENTAL PROSTHETICS
ROYAL L. NORMAN, D.D.S. Elmhurst, Ill.
F
RICTION IS DEFINED in mechanics as “the resistance to relative motion between two bodies in contact.“l In nearly all dental procedures, we are confronted in part by this relationship, either directly or indirectly. If the action of the mandible was strictly hinge-type, with no lateral or protrusive movements in function, such resistances would be negligible. However, lateral and protrusive movements do exist in varying degrees, and habitual tooth contacts do occur in these movements. Proprioceptive interception may prevent excessive degrees of opposing tooth contact in natural dentitions during mastication. However, the performance of this proprioceptor mechanism in dentures is greatly impaired, with a resulting over-all increase in friction. The observed patterns of wear in natural and artificial teeth indicate that there is enough relative motion in contact to bring into account the problem of frictional resistance. Excessive stimulation may exceed the physiologic tolerance of the tissue with resultant pathologic changes. Therefore, factors which can cause over-stimulation of teeth and supporting structures must be reduced. Frictional resistance between artificial teeth may likely be such a factor. These forces of friction have to be absorbed as they occur. If forces of friction combine with other forces of mastication to exceed the tolerance of supportive tissues, changes in that tissue will occur. DIRECTION
OF FRICTIONALFORCES
Frictional resistance in the mouth would be expressed primarily in a horizontal or lateral direction, depending on cuspal inclinations and contact areas. Lateral forces directed at the natural teeth can be most destructive. In instances where natural teeth have become intolerable to normal function through various pathologic and/or physiologic processes, forces of friction become more pertinent. The effects of frictional resistances are more apparent in extensive removable partial or complete dentures than in other phases of dentistry. The natural teeth and supporting structures can no longer be responsible for absorbing any normal or abnormal forces. Soft tissues which were never intended to support artificial replacements are now required to absorb the necessary forces that arise during mouth function. Occlusally directed pressures must be minimized in the use of dentures. Proportionately, lateral and protrusive contact pressures are reduced with a comparable reduction in frictional resistance. More surface is usually available to resist vertical forces than is available to resist lateral forces because of the anatomic contour of the 45
46
NORMAN
J. Pros. Jnn:Feb.,
Den. 1961
residual ridges. However, with resorption d the ritlges, less surface rcniains to i-esist lateral forces and proportionately more surface is created to resist vertical forces. The resulting imbalance decreases the ability of the tissues to absorb lateral pressures. Chronic irritation of the ridges and dislodgment of the dentures can then occur. Individual habits of chewing may produce excessive lateral forces, even when ideal ridge relationships exist, and result in chronic irritation or loss of retention of the restoration. If no clinical signs of irritation are present, subclinical evaluations might indicate that the physiologic tolerance of the ridges has been exceeded. Possibly, the resorptive processes of the edentulous ridge are incited in this manner with subsequent atrophy as a secondary factor. Many dentists use porcelain teeth in constructing complete dentures. Yet few dentists seem unaware of the effective results of opposing acrylic resin to porcelain teeth for some patients, As a possible explanation for this occurrence, experiments were performed to determine whether or not the materials commonly encountered in prosthetic procedures display any marked differences in frictional resistance. hfhTERIAIL3
A platform was constructed to hold the various parts of the testing mechanism (Fig. 1). The support arm could move both vertically and horizontally. Inherent resistance of the apparatus was minimized by: (1) paralleling the thread, the arm, and the stationary platform to the base platform; (2) polishing and lubricating all moving parts; and (3) maintaining a 90 degree angle between the thread and the support arm. The five materials chosen for testing of frictional resistance were : ( 1) enamel (extracted teeth), (2) vacuum-fired porcelain, (3) acrylin resin denture teeth, (4) a hard gold, and (5) a chrome cobalt alloy. PREI’ARATION
FOR TESTING
Flat surfaces, 10 mm. or more in diameter, were ground on each of the materials to be tested. The inside surface of an abrasive rubber wheel, 3 inches in diameter, was used to establish these essentially flat planes and eliminate any visual cuts or scratches from the surfaces, (Further polishing could have made the metals react more favorably and alter the uniformity of the experiment.) Five points, one of each material tested, were ground and polished with a rubber wheel to small rounded tips approximately 1 mm, in diameter. All edges or defects were eliminated to develop a comparable contact area when the point was placed against a flat surface. Each of the test materials with the flat surfaces was separately mounted in wax for each test phase. This formed a stationary platform of the test materials. The testing points were attached separately in wax for each test phase underneath the movable weight platform (Fig. 1). This platform was removable, and a common weight was duplicated with each change in test points by either adding or removing wax to compensate for the difference in the weight of the test points.
~,oll$l~r ‘1” Ll
FRICTIONAL
RESISTANCE
AND
DENTAL
PROSTHETICS
47
A drop of saliva was used as a lubricating agent between the test point and the flat surface when they were in the proper relationship. Since there were measurable differences between dry and wet surfaces, a wet surface was employed to simulate mouth conditions. Saliva from the same person was used for all tests. The temperature of the specimen affected the readings. Therefore, on each series of tests the surfaces of the point and flat surface were cleaned, and a new specimen was attached and allowed to cool to room temperature. Subsequent tests showed that this method caused no appreciable variations in the results.
Fig. 1.-A testing device is used to determine the frictional resistance of different materials used as occlusal surfaces for dental restorations. 1, The weight platform for the application of horizontal forces. 2, The silk connecting thread. 3, The pulley. 4, The movable weight platform for applying vertical forces. 5, The supporting arm. 6, The control lever, 7, The stop. 8, The stationary platform and testing surface. 9, The test point. 10, The base platform.
TESTING
PROCEDURES
Weights were added to the weight platform to supply the horizontal force until the resistances of the contacts were overcome. The lever was used as an indirect source of control of the platform to eliminate adverse movements of the hand. The line of contact of the opposing materials, measuring approximately 8 mm. before the stop, was used as the testing area. The criterion for establishing a base upon which all tests could be compared was passage of the pointed tip completely across the testing surface. At no point before the end of the planned movement could a resting pause be maintained. If questioned defects on the testing surfaces hampered the movements of the pointed tips, alternate areas were chosen. Weights measuring from 0 to 25 grams at 5 gram intervals were applied to the vertical weight platform to complete each testing phase. In some of the tests, movement of the weight platform occurred without applying a horizontal weight. Vertical weights were then initially added until a stationary position of the platform was obtained. These weights are recorded in parentheses in Table I.
48
J. Pros. Jan:Feb.,
NORAlAN
TABLE
I.
FRICTION
HORIZONTAL
VERTICAL FORCE ON THE PLATFORMS
RESISTANCE
FORCE
ON
THE
CONTACT
POINTS
-
-/
1 ENAMEL
GM.
PORCELAIN (GM.1
(GM.1
2.1 3.8 5.2 6.8 8.9
0.7 2.8 4.9 6.9 9.4 12.0
3.3 5.3 7.7 12.9 10.1
25
0.7 2.5 4.5 6.8 9.0 10.9
1.5 3.7 8.1 12.7 16.9 20.1
0.7 20 3 7 6.1 96 11.4
3
0.4
06
0.5 Enamel
Acrylic
I
O 5
Porcelain
resin 7.4 92 11.4
1 Chrome cobalt alkly
ACRYLIC RESIN (GM.)
I
a.; 8.8 6.8
10.4
u:;’
0.2 1.2 2.3 3.6 4.8 6.0
1.8
2.9 4.0 5.0
y’
‘;:;’
2.2 3.7 5.0 6.9
CHROME COBALT 4LLOY (GM.)
GOLD
j i
(GM.1
-___
--__-
1: 1.5
Den. 1964
1.8 2.8 3.8 4.7
1.1
0 1.4
/ (
2
10.8
0.9 5
~
44 5.7
,
7.0 0
1.7
1.:
I-
3.5 4.6
(2.9, 04
3.1
: i
4.9 6.7
7.8
--
0.7 3.5 6.0 8.6 11 4 14.9 -__-0.5 2.4 4.4 6.3 8.3 10.2 ___0.4 2.3 4.8 7.2 8.5
(1.2)
1: 3 4 4.7
0.8 3.0 5.4 7.2 9.1 10.4
-1
--0.1
1.3 2.7 4.2 5.2 6.3
‘3 1.6 2.7 3.7
5.1
__-~ ‘;:;’ 1.6
2.9 4.3 5.5
(6 7) 0.6 1.0 2.2 3 3
-. RESULTS
Marked differences were found in the frictional resistances between the materials tested (Table I). The highest resistance in relation to the other materials tested was the combination of porcelain against porcelain. The lowest frictional resistance was found to be between the chrome cobalt alloy point against the chrome cobalt alloy surface. Most combinations of materials seemed to react to increased vertical forces proportionately. Exceptions were for some materials with acrylic resin components, although even these did not vary much, as a rule. With enamel-against-enamel measurements representing a base line for the frictional resistance, only the following combinations of materials had resistances equal to or less than natural tooth resistances : ( 1) enamel against gold or chrome
Volume .N11ml1er
14 1
l~RICTIONAI.
RESISTANCE
AND
DENTI\L
PROSTHETICS
49
cobalt alloy, (2) porcelain against gold or chrome cobalt alloy, (3) gold against gold or chrome cobalt alloy, and (4) chrome cobalt alloy against chrome cobalt alloy. The combinations of materials having frictional resistance equal to or more than natural tooth resistances were : (1) porcelain against porcelain, enamel or acrylic resin, and (2) acrylic resin against acrylic resin, enamel, gold, or chrome cobalt alloy. The porcelain-against-porcelain contacts demonstrated the highest frictional resistance (55 per cent increase over that of enamel against enamel). The combinations of porcelain against gold, and porcelain against chrome cobalt alloy showed a 34 and 60 per cent decrease, respectively, as compared to the frictional resistance of enamel against enamel. These same combinations also decreased the resistance as compared with porcelain against porcelain by 69 and 77 per cent. The alloy-against-alloy contacts showed a remarkable (88 per cent) reduction of frictional rmesistanceas compared to porcelain against porcelain. These calculations in per cent were made by averaging the two results obtained for each dissimilar combination of materials. (An example would be a porcelain tip against an acrylic resin platform and an acrylic resin tip against a porcelain platformj. The per cent of difference between the compared items was then calculated for each given weight load. The resulting six percentages, representing the complete range of the experiment, were then averaged into one mean per cent of change. DISCUSSION
The degree of error in the procedure employed is best described by comparing the two results obtained when unlike materials were measured. For example, the gold point against a porcelain surface under 25 grams vertical load required 7.8 grams force to displace it horizontally. The porcelain point against a gold surface under 25 grams vertical force required 5.0 grams to displace it horizontally. This 2.8 gram discrepancy was the maximum recorded under a 25 gram vertical load. However, the average difference for all combinations utider this load was 1.3 grams. Under a vertical load of 0 grams the average was 0.48 gram. The average of 0.48 and 1.3 is 0.89; therefore, a variation of plus or minus 0.89 potentially exists for each figure contained in Table I. However, because of the wide variety in the materials tested, it is more acceptable to establish the degree of error in each specific combination rather than to use the average for all materials. Under increasing weight loads, softer and more fragile points might break down against a hard platform or vice versa, and, therefore, not give comparable contact measurements of resistance. The same problem would occur in the mouth and these results would be more directly applicable. Nevertheless, there is enough difference in the results that, even allowing for the greatest possible error, the result would not be appreciably altered. With application of the results of these tests intraorally, considerable differences in resistance could be noted. For esample, assume a given vertical force is applied to dentures with opposing porcelain teeth and it requires 10 pounds of force to move one denture over the other denture in a horizontal direction. Dentures
50
NORMAN
J. Pros. Den. Jan-Fel,., 1004
employing occluding surfaces oi chrome cobalt alloy would rccluire only 12 l)er cent, or 1.2 pounds, of force to accomplish the same movement Under the rather extreme horizontally directed forces of the dentures with all porcelain teeth, it is logical to assume that proprioceptive impulses from the supporting tissues would interpret and, therefore, reduce vertical forces or omit horizontal movements. If this were true, then the efficiency of the dentures would be reduced. However, proprioceptive interpretation of edentulous ridge conditions would not be achieved to the same degree of perfection as with natural teeth and, therefore, cannot be relied upon to control the probable destructive forces in complete dentures. The results of the frictional resistance tests can be applied to the natural teeth. It was demonstrated that frictional resistance can be reduced from that of natural tooth enamel. Unstable teeth with unhealthy periodontal support may require a reduction of the occlusal forces as a part of their treatment. For example, occlusal splinting of teeth would be improved by using a chrome cobalt alloy in the splint. The use of splints that do not involve the occluding surfaces would be a second choice. The type of materials used could mean success or failure in long-span fixed partial dentures or such dentures that are supported by weakened teeth. All factors that cause overstressing the supporting structures of teeth must be reduced. Therefore, the use of materials demonstrating minimal frictional resistances will reduce the forces exerted on the tissues that support most dental restorations. SUMMARY
A series of tests was conducted to determine differences in the frictional resistance of materials used to form occlusal surfaces of dental restorations. A wide range of frictional resistance existed among the materials tested. Porcelain-against-porcelain contacts required the greatest displacement force of the materials tested. This force was nearly twice the amount required to displace a similar contact between porcelain and acrylic resin and almost three times that between two contacting surfaces of natural tooth enamel. In all of the tests, a chrome cobalt alloy, either opposing itself or in combination with other materials, was consistently lowest in the relative frictional resistances measured. CONCLUSIONS
1. Frictional resistance of opposing tooth surfaces is a measurable force and must be accounted for in prosthodontics, as well as in conditions involving natural dentitions. 2. The employment of opposing porcelain teeth in prosthetic restorations will result in relatively high frictional forces in a lateral direction as compared with other materials. 3. The success of some dentures which utilize acrylic resin teeth in opposition to porcelain teeth may result from a marked decrease in frictional resistance. 4. Reductions of frictional resistance to a point well below that of the resistances occurring in the natural teeth can be achieved by employing gold or chrome cobalt alloys on the occluding surface of dental restorations.
\‘olun1e 14 Su111lm 1
FRICTIONAL
RESISTANCE
AND
DENTAL
PROSTHETICS
51
5. The development of esthetic posterior teeth made of friction-reducing materials on the occIusa1 surfaces would encourage their use in dental restorations. REFERENCE
1. Webster’s New International Co. p. 1008. 103 HAVEN ELMHURST,
RD. ILL.
Dictionary,
ed. 2, Springfield.
Mass.,
1958, G.
& C. Merriam