strength of resin developed used with cast metal restorations Beth
Reilly,
DDS,a
Elaine
L. Davis,
PhD,b
Robert
by four bonding B. Joynt,
agents
DDS,C
and Joseph Quevedod State University of New York at Buffalo, School of Dental Medicine, Buffalo, N.Y. The evolution of the acid etch technique has made possible a more conservative approach to the fabrication of cast metal restorations. The resin bonding technique, however, places a greater burden for success on the selection of a bonding agent. This study examined the shear bond strength durability of cast metal restorations bonded to tooth structure with one of four metal adhesive bonding agents. Results indicated stronger bonds for restorations cemented with Panavia EX bonding agent than with any of the other bonding agents tested, both with and without exposure to thermal stress. Although it was one of the easier materials with which to work, Panavia EX bonding agent requires the additional step of applying an agent to preveut oxygen contact in the setting process. (J PROSTHET DENT 1992;68:53-5.)
ince the introduction of the acid etch technique for the fabrication of resin bonded cast metal restorations, the practitioner has had a means of restoring form and function in a more conservative fashion. It is now no longer necessary to remove sound tooth structure for the sake of developing adequate retentive form for cast restorations. The resin bonding technique, however, places a much greater demand on the selected bonding agent. Traditional preparations provide primary retention from the length and parallelism of walls. Although the configuration of the preparation remains a critical factor in the success of a resin-bonded casting, the bonding agent plays a far more important role in providing primary retention. A number of bonding agents have been developed to meet the demands of these new restorative systems.1-4 The introduction of micromechanical resin bonding systems5-s has provided a reliable means of attaching restorations to tooth structure. These systems depend on the etching of nonprecious alloys to lock the bonding agent to the casting. Recently, bonding agents ‘2 lo have been introduced that might adhere chemically to a sandblasted metal surface. These bonding agents greatly simplify framework preparation before cementation. The purpose of this study was to evaluate the shear bond strength of three metal adhesive bonding agents as compared with a bonding agent requiring etching. Durability was measured by shear bond strength testing after exposure to thermal stress.
Supported in part by U.S. Public Health Service grant No. DE7106-11. %structor, Department of Operative Dentistry. bAssistant Professor, Departments of Operative Dentistry and Behavioral Sciences. CAssistant Professor, Department of Operative Dentistry. dSenior dental student. 10/l/37284
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
MATERIAL
AND
METHODS
The bonding agents evaluated were Panavia EX (J. Morita USA, Inc., Tustin, Calif.), GoldLink (Den-Mat Corp., Santa Maria, Calif.), Comspan (L.D. Caulk Co., Milford, Del.), and Geristore (Den-Mat Corp.). Panavia EX and Geristore are adhesive resins, whereas Comspan is formulated for use with an electrolytically prepared casting surface. GoldLink is a bonding material developed for the repair of fractured porcelain and resin facings for fixed cast metal frameworks. Immediately after extraction, third molars were placed in a solution of phosphate-buffered saline solution containing an antibiotic-antimycotic agent. The teeth were cleaned and examined under a dissecting microscope, and only those free of defects were retained. A total of 32 teeth were used. Twelve surfaces were prepared for each of the four bonding agents, at each of two thermocycling times (0 and 7200 cycles). Four mesial, four distal, and four buccal surfaces were used for each bonding agent at each thermocycling time, for a total of 96 surfaces (three surfaces per tooth). A two-piece break-apart form (Saml-Kup, Lake Bluff, Ill.) was used to mount specimens. With a small piece of beading wax, each molar was centered on the base portion of the form, with its long axis perpendicular to the base. Self-polymerizing resin was poured into the form to the level of the cementoenamel junction. The form was then submerged in room-temperature deionized water to dissipate the heat of the setting resin. Approximately 1 hour later specimens were removed from the forms and returned to deionized water for a minimum of 24 hours. Specimens were kept in deionized water and removed only long enough to complete necessary procedures. The base of each specimen was trimmed on a lathe to expose a cross-section of the root, allowing contact with the platen of the testing machine and transmission of forces entirely through tooth structure. The base was trimmed so
53
REILLY
I. Summary of analysis of variance results
Table
Source
MS
F
1375.38 34.14 47.91 24.50
56.13* 1.39 1.95
df
Material Thermal stress Interaction Residual Total
3
1 3 88
95
*p < 0.01.
II. Group and marginal mean kilogram shearing forces (n = 12 per group)
Table
Thermocycles Material
Panavia Geristore GoldLink Gomspan Average
0 27.50 16.06 16.64 12.54
7200
(5.71) (1.69)
(3.09) (3.69)
32.42 16.52 14.75 13.83
Average*
29.96
(6.04) (6.24) (5.67) (5.45)
16.29 15.70
13.19
19.38
18.19
Standarddeviationsin parentheses. *Means connected by bracket Keuls multiple comparison;
do not differ significantly 0.05 significance level).
(Student
Newman-
that it was flat and perpendicular to the long axis of the tooth. The buccal, mesial, and distal surfaces of the teeth were prepared with a medium grit, barrel-shaped diamond bur. The diamond was changed at the completion of every 24 surface preparations. Preparations were standardized by means of a jig consisting of a mounted handpiece positioned so that the long axis of the diamond was perpendicular to the flat base of the jig. An air-water spray was used with the high-speed handpiece. A stage, coated with a lubricant to facilitate movement, was used to position the tooth in relation to the diamond, assuring uniform preparation of all surfaces. Each surface was then examined at 7~ under an optical microscope to ensure that it was uniform and perpendicular to the base. Preparations were limited to enamel; no dentin was involved. Circular wax patterns were made from relief wax with a custom brass-cutting device to assure that all wax patterns were of a uniform size (3 mm in (diameter and 1 mm in thickness). The circular pattern was sprued with a piece of g-gauge sprue wax, centered and attached with sticky wax, and mounted on the sprue base. Completed wax patterns (12 on each sprue base) were cast in Litecast (Williams Dental, Amherst, N.Y.), a nickel-chromium alloy developed for use with the resin-bonded casting technique. Castings were prepared in accordance with the manufacturer’s instructions. After casting, the disks were cut from the sprues with a separating disk and sandblasted with aluminum oxide, at a distance of 0.5 cm 54
ET AL
from the nozzle, for 20 seconds. The surface of each casting was then rinsed with an air-water spray and examined under the optical microscope to insure uniformity. Castings to be cemented with GoldLink bonding agent received further surface treatment with a No. 56 carbide bur in a highspeed handpiece, held with the long axis of the bur at a 45degree angle to the surface. A cross-hatched pattern was developed with this procedure. Castings to be used with Comspan bonding agent were etched twice for 5 minutes (total etch time 10 minutes) with an etching gel (Dentecon Inc., Los Angeles, Calif.). The preparation and handling of bonding agents were done in accordance with the manufacturers’ instructions. All disks were placed in Dry Bond material (Den-Mat Corp.) for 30 seconds to eliminate any surface contaminants and to assure a dry surface. The prepared enamel surfaces were etched with a 37 % phosphoric acid etchant (Den-Mat Corp.) for 15 to 30 seconds,‘l then washed and dried. Each mounted tooth was positioned in a vise and placed on a surveyor. A thin layer of bonding agent was placed on the surface of the disk, and a drop of bonding agent was placed on the tooth surface. The disk was then positioned on the tooth surface with forceps. A weight of 1.125 kg was applied to each disk, and after 20 seconds the weight and excess adhesive were removed. For Panavia EX a layer of Oxyguard material (J. Morita USA) was applied to cover the exposed bonding agent at the periphery of the casting and was left in place for a minimum of 3 minutes. This created the anaerobic environment necessary for complete set of the bonding agent. After cementation all specimens were placed in a 37’ C oven at 100% humidity for a minimum of 24 hours. Half the specimens were then tested for shear strength on the Instron universal testing machine (Instron Corp., Canton, Mass.). Shear testing was accomplished with a knife-edge testing instrument attached to the Instron crosshead, which engaged the specimen at the castingtooth surface interface. Shear force was applied to each specimen at a crosshead speed of 0.01 cm/min. The force necessary to dislodge the casting from the tooth was measured and recorded in kilograms. Thermal stressing was done for the remaining specimens in each group by subjecting them to 7200 cycles on a thermocycling machine. One cycle lasted 1 minute, and consisted of 15 seconds each at 37” C, 54’ C, and 4’ C. Immediately after thermocycling, specimens were tested for shear strength.
RESULTS A two-way univariate analysis of variance was conducted to determine differences among bonding agents and between levels of thermal stress (0 and 7200 cycles) in shear force necessary to dislodge castings. Results (Table I) indicated a significant main effect for bonding agent (Fs,ss = 56.13, p < 0.01). There was no significant effect for thermal stress (Fi,ss = 1.39, p > 0.05) and no significant inter-
JULY
1992
VOLUME
68
NUMBER
1
RESIN
STRENGTH
IN CAST
METAL
RESTORATIONS
action (Fs,ss = 1.95, p > 0.05). Group means are reported in Table II. A multiple comparison test (Student NewmanKeuls) revealed a significant difference in mean shear strength between Panavia EX and all other bonding agents (p < 0.05). No other bonding agent differences were found. Setting and mixing times, ease of use, and total time from mix to set were also noted and recorded during specimen preparation. Mixing times ranged from a minute or less (Panavia EX and Geristore) to approximately 20 minutes (Cornspan). Mixing times for Comspan reflect the time required for two 5-minute metal etches. Setting times varied between 3 and 5 minutes for Panavia EX, Geristore, and Comspan. GoldLink is light activated and therefore set in considerably less time. Panavia EX required an additional 3 to 4 minutes for the use of Oxyguard material, for a total setting time of approximately 6 to 7 minutes after mixing. CLINICAL
IMPLICATIONS
Although Panavia EX showed favorable bond strength results, it requires an additional step to prevent oxygen inhibition of the bonding agent. Some clinicians may find this procedure too time consuming. SUMMARY Results indicate stronger bonds for Panavia EX than the other bonding agents tested, both with and without exposure to thermal stress. Except for the extra step to prevent oxygen inhibition of the bonding agent, Panavia EX was relatively easy to use, requiring a total preparation time of less than 10 minutes. These results indicate that Panavia EX provides greater bond strength than other resin bonding agents and that this bond, under the conditions of this study, is a durable one.
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
REFERENCES 1. Pegoraro LF, Barrack G. A comparison of bond strengths of adhesive cast restorations using different designs, bonding agents, and luting resins. J PROSTHETDENT 1987;57:133-8. 2. Thompson VP, Grolman KM, Liao R. Bonding of adhesive resins to various non-precious alloys [Abstract]. J Dent Res 1985;64:314. 3. Watanabe F, Powers JM, Lorey RE. Bond strengths of resin cements to gold and nickel alloys [Abstract]. J Dent Res 1987;66:206. 4. Marizlello CP, Kerschbaum B, Hinz R, et al. Experiences with resinbonded bridges and splints: a retrospective study. J Oral Rehabil 1987; 14:251-60. 5. Livaditis GJ, Thompson VP. Etched castings: an improved retentive mechanism for resin-bonded retainers. J PROSTHET DENT 1982;47:52-8. 6. Shen G, Forbes J, Boettcher R, Dvivedi N, Morrow R. Resin bonded bridge bond strength using a cast mesh technique [Abstract]. J Dent Res 1983;62:221. PC. Resin bonded bridge tensile strength utilizing porous 7. Moon patterns [Abstract]. J Dent Res 1984;63:320. 8. Hood AM, Brockhurst P, Harcourt J. The bond strengths of various adhesives used for Maryland bridges. Aust Dent J 1989;34:449-53. 9. Tanaka T, Nagata K, Takeyama M, Atsuta M, Nakabayashi N, Masuhara E. 4-META opaque resin: a new resin strongly adhesive to nickelchromium alloy. J Dent Res 1981;60:1697-706. 10. Omura I, Yamauchi J, Harada I, Wada T. Adhesive and mechanical properties of a new dental adhesive [Abstract]. J Dent Res 1984;63:233. 11. Gilpatrick RO, Ross JA, Simonsen RJ. Resin to enamel bond strengths with variable etching times [Abstract]. J Dent Res 1989;68:345. Reprint
requests
to:
DR. ELAINE L. DAVIS DEPARTMENT OF OPERATIVE DENTISTRY SCHOOL OF DENTAL MEDICINE STATE UNIVERSITY OF NEW YORK AT BUFFALO BUFFALO, NY 14214
Contributing author Xin Yi Yu, DMD, Assistant Professor, Department of
Operative Dentistry.
55
Reilly,
DDS,a
Elaine
L. Davis,
PhD,b
Robert
by four bonding B. Joynt,
agents
DDS,C
and Joseph Quevedod State University of New York at Buffalo, School of Dental Medicine, Buffalo, N.Y. The evolution of the acid etch technique has made possible a more conservative approach to the fabrication of cast metal restorations. The resin bonding technique, however, places a greater burden for success on the selection of a bonding agent. This study examined the shear bond strength durability of cast metal restorations bonded to tooth structure with one of four metal adhesive bonding agents. Results indicated stronger bonds for restorations cemented with Panavia EX bonding agent than with any of the other bonding agents tested, both with and without exposure to thermal stress. Although it was one of the easier materials with which to work, Panavia EX bonding agent requires the additional step of applying an agent to preveut oxygen contact in the setting process. (J PROSTHET DENT 1992;68:53-5.)
ince the introduction of the acid etch technique for the fabrication of resin bonded cast metal restorations, the practitioner has had a means of restoring form and function in a more conservative fashion. It is now no longer necessary to remove sound tooth structure for the sake of developing adequate retentive form for cast restorations. The resin bonding technique, however, places a much greater demand on the selected bonding agent. Traditional preparations provide primary retention from the length and parallelism of walls. Although the configuration of the preparation remains a critical factor in the success of a resin-bonded casting, the bonding agent plays a far more important role in providing primary retention. A number of bonding agents have been developed to meet the demands of these new restorative systems.1-4 The introduction of micromechanical resin bonding systems5-s has provided a reliable means of attaching restorations to tooth structure. These systems depend on the etching of nonprecious alloys to lock the bonding agent to the casting. Recently, bonding agents ‘2 lo have been introduced that might adhere chemically to a sandblasted metal surface. These bonding agents greatly simplify framework preparation before cementation. The purpose of this study was to evaluate the shear bond strength of three metal adhesive bonding agents as compared with a bonding agent requiring etching. Durability was measured by shear bond strength testing after exposure to thermal stress.
Supported in part by U.S. Public Health Service grant No. DE7106-11. %structor, Department of Operative Dentistry. bAssistant Professor, Departments of Operative Dentistry and Behavioral Sciences. CAssistant Professor, Department of Operative Dentistry. dSenior dental student. 10/l/37284
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
MATERIAL
AND
METHODS
The bonding agents evaluated were Panavia EX (J. Morita USA, Inc., Tustin, Calif.), GoldLink (Den-Mat Corp., Santa Maria, Calif.), Comspan (L.D. Caulk Co., Milford, Del.), and Geristore (Den-Mat Corp.). Panavia EX and Geristore are adhesive resins, whereas Comspan is formulated for use with an electrolytically prepared casting surface. GoldLink is a bonding material developed for the repair of fractured porcelain and resin facings for fixed cast metal frameworks. Immediately after extraction, third molars were placed in a solution of phosphate-buffered saline solution containing an antibiotic-antimycotic agent. The teeth were cleaned and examined under a dissecting microscope, and only those free of defects were retained. A total of 32 teeth were used. Twelve surfaces were prepared for each of the four bonding agents, at each of two thermocycling times (0 and 7200 cycles). Four mesial, four distal, and four buccal surfaces were used for each bonding agent at each thermocycling time, for a total of 96 surfaces (three surfaces per tooth). A two-piece break-apart form (Saml-Kup, Lake Bluff, Ill.) was used to mount specimens. With a small piece of beading wax, each molar was centered on the base portion of the form, with its long axis perpendicular to the base. Self-polymerizing resin was poured into the form to the level of the cementoenamel junction. The form was then submerged in room-temperature deionized water to dissipate the heat of the setting resin. Approximately 1 hour later specimens were removed from the forms and returned to deionized water for a minimum of 24 hours. Specimens were kept in deionized water and removed only long enough to complete necessary procedures. The base of each specimen was trimmed on a lathe to expose a cross-section of the root, allowing contact with the platen of the testing machine and transmission of forces entirely through tooth structure. The base was trimmed so
53
REILLY
I. Summary of analysis of variance results
Table
Source
MS
F
1375.38 34.14 47.91 24.50
56.13* 1.39 1.95
df
Material Thermal stress Interaction Residual Total
3
1 3 88
95
*p < 0.01.
II. Group and marginal mean kilogram shearing forces (n = 12 per group)
Table
Thermocycles Material
Panavia Geristore GoldLink Gomspan Average
0 27.50 16.06 16.64 12.54
7200
(5.71) (1.69)
(3.09) (3.69)
32.42 16.52 14.75 13.83
Average*
29.96
(6.04) (6.24) (5.67) (5.45)
16.29 15.70
13.19
19.38
18.19
Standarddeviationsin parentheses. *Means connected by bracket Keuls multiple comparison;
do not differ significantly 0.05 significance level).
(Student
Newman-
that it was flat and perpendicular to the long axis of the tooth. The buccal, mesial, and distal surfaces of the teeth were prepared with a medium grit, barrel-shaped diamond bur. The diamond was changed at the completion of every 24 surface preparations. Preparations were standardized by means of a jig consisting of a mounted handpiece positioned so that the long axis of the diamond was perpendicular to the flat base of the jig. An air-water spray was used with the high-speed handpiece. A stage, coated with a lubricant to facilitate movement, was used to position the tooth in relation to the diamond, assuring uniform preparation of all surfaces. Each surface was then examined at 7~ under an optical microscope to ensure that it was uniform and perpendicular to the base. Preparations were limited to enamel; no dentin was involved. Circular wax patterns were made from relief wax with a custom brass-cutting device to assure that all wax patterns were of a uniform size (3 mm in (diameter and 1 mm in thickness). The circular pattern was sprued with a piece of g-gauge sprue wax, centered and attached with sticky wax, and mounted on the sprue base. Completed wax patterns (12 on each sprue base) were cast in Litecast (Williams Dental, Amherst, N.Y.), a nickel-chromium alloy developed for use with the resin-bonded casting technique. Castings were prepared in accordance with the manufacturer’s instructions. After casting, the disks were cut from the sprues with a separating disk and sandblasted with aluminum oxide, at a distance of 0.5 cm 54
ET AL
from the nozzle, for 20 seconds. The surface of each casting was then rinsed with an air-water spray and examined under the optical microscope to insure uniformity. Castings to be cemented with GoldLink bonding agent received further surface treatment with a No. 56 carbide bur in a highspeed handpiece, held with the long axis of the bur at a 45degree angle to the surface. A cross-hatched pattern was developed with this procedure. Castings to be used with Comspan bonding agent were etched twice for 5 minutes (total etch time 10 minutes) with an etching gel (Dentecon Inc., Los Angeles, Calif.). The preparation and handling of bonding agents were done in accordance with the manufacturers’ instructions. All disks were placed in Dry Bond material (Den-Mat Corp.) for 30 seconds to eliminate any surface contaminants and to assure a dry surface. The prepared enamel surfaces were etched with a 37 % phosphoric acid etchant (Den-Mat Corp.) for 15 to 30 seconds,‘l then washed and dried. Each mounted tooth was positioned in a vise and placed on a surveyor. A thin layer of bonding agent was placed on the surface of the disk, and a drop of bonding agent was placed on the tooth surface. The disk was then positioned on the tooth surface with forceps. A weight of 1.125 kg was applied to each disk, and after 20 seconds the weight and excess adhesive were removed. For Panavia EX a layer of Oxyguard material (J. Morita USA) was applied to cover the exposed bonding agent at the periphery of the casting and was left in place for a minimum of 3 minutes. This created the anaerobic environment necessary for complete set of the bonding agent. After cementation all specimens were placed in a 37’ C oven at 100% humidity for a minimum of 24 hours. Half the specimens were then tested for shear strength on the Instron universal testing machine (Instron Corp., Canton, Mass.). Shear testing was accomplished with a knife-edge testing instrument attached to the Instron crosshead, which engaged the specimen at the castingtooth surface interface. Shear force was applied to each specimen at a crosshead speed of 0.01 cm/min. The force necessary to dislodge the casting from the tooth was measured and recorded in kilograms. Thermal stressing was done for the remaining specimens in each group by subjecting them to 7200 cycles on a thermocycling machine. One cycle lasted 1 minute, and consisted of 15 seconds each at 37” C, 54’ C, and 4’ C. Immediately after thermocycling, specimens were tested for shear strength.
RESULTS A two-way univariate analysis of variance was conducted to determine differences among bonding agents and between levels of thermal stress (0 and 7200 cycles) in shear force necessary to dislodge castings. Results (Table I) indicated a significant main effect for bonding agent (Fs,ss = 56.13, p < 0.01). There was no significant effect for thermal stress (Fi,ss = 1.39, p > 0.05) and no significant inter-
JULY
1992
VOLUME
68
NUMBER
1
RESIN
STRENGTH
IN CAST
METAL
RESTORATIONS
action (Fs,ss = 1.95, p > 0.05). Group means are reported in Table II. A multiple comparison test (Student NewmanKeuls) revealed a significant difference in mean shear strength between Panavia EX and all other bonding agents (p < 0.05). No other bonding agent differences were found. Setting and mixing times, ease of use, and total time from mix to set were also noted and recorded during specimen preparation. Mixing times ranged from a minute or less (Panavia EX and Geristore) to approximately 20 minutes (Cornspan). Mixing times for Comspan reflect the time required for two 5-minute metal etches. Setting times varied between 3 and 5 minutes for Panavia EX, Geristore, and Comspan. GoldLink is light activated and therefore set in considerably less time. Panavia EX required an additional 3 to 4 minutes for the use of Oxyguard material, for a total setting time of approximately 6 to 7 minutes after mixing. CLINICAL
IMPLICATIONS
Although Panavia EX showed favorable bond strength results, it requires an additional step to prevent oxygen inhibition of the bonding agent. Some clinicians may find this procedure too time consuming. SUMMARY Results indicate stronger bonds for Panavia EX than the other bonding agents tested, both with and without exposure to thermal stress. Except for the extra step to prevent oxygen inhibition of the bonding agent, Panavia EX was relatively easy to use, requiring a total preparation time of less than 10 minutes. These results indicate that Panavia EX provides greater bond strength than other resin bonding agents and that this bond, under the conditions of this study, is a durable one.
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
REFERENCES 1. Pegoraro LF, Barrack G. A comparison of bond strengths of adhesive cast restorations using different designs, bonding agents, and luting resins. J PROSTHETDENT 1987;57:133-8. 2. Thompson VP, Grolman KM, Liao R. Bonding of adhesive resins to various non-precious alloys [Abstract]. J Dent Res 1985;64:314. 3. Watanabe F, Powers JM, Lorey RE. Bond strengths of resin cements to gold and nickel alloys [Abstract]. J Dent Res 1987;66:206. 4. Marizlello CP, Kerschbaum B, Hinz R, et al. Experiences with resinbonded bridges and splints: a retrospective study. J Oral Rehabil 1987; 14:251-60. 5. Livaditis GJ, Thompson VP. Etched castings: an improved retentive mechanism for resin-bonded retainers. J PROSTHET DENT 1982;47:52-8. 6. Shen G, Forbes J, Boettcher R, Dvivedi N, Morrow R. Resin bonded bridge bond strength using a cast mesh technique [Abstract]. J Dent Res 1983;62:221. PC. Resin bonded bridge tensile strength utilizing porous 7. Moon patterns [Abstract]. J Dent Res 1984;63:320. 8. Hood AM, Brockhurst P, Harcourt J. The bond strengths of various adhesives used for Maryland bridges. Aust Dent J 1989;34:449-53. 9. Tanaka T, Nagata K, Takeyama M, Atsuta M, Nakabayashi N, Masuhara E. 4-META opaque resin: a new resin strongly adhesive to nickelchromium alloy. J Dent Res 1981;60:1697-706. 10. Omura I, Yamauchi J, Harada I, Wada T. Adhesive and mechanical properties of a new dental adhesive [Abstract]. J Dent Res 1984;63:233. 11. Gilpatrick RO, Ross JA, Simonsen RJ. Resin to enamel bond strengths with variable etching times [Abstract]. J Dent Res 1989;68:345. Reprint
requests
to:
DR. ELAINE L. DAVIS DEPARTMENT OF OPERATIVE DENTISTRY SCHOOL OF DENTAL MEDICINE STATE UNIVERSITY OF NEW YORK AT BUFFALO BUFFALO, NY 14214
Contributing author Xin Yi Yu, DMD, Assistant Professor, Department of
Operative Dentistry.
55
