Effect of repair surface design, repair material, and processing method on the transverse strength of repaired acrylic denture resin John E. Ward, DDS, MSD,a Peter C. Moon, PhD,b Robert and Carol L. Behrendt, BS, MSd Virginia

Commonweal.th University

School of Dentistry,

A. Levine,

DDS,”

Richmond, Va.

The transverse strengths of blocks of denture base acrylic resin repaired with autopolymerizing monomer and polymer and autopolymerizing monomer and heat-cured polymer were measured with a three-point bending test. Three repair joints were studied: butt, round, and 46-degree bevel. Three processing methods were used: bench cure, hydroflask with hot water for 10 minutes, and hydroflask with hot water for 30 minutes. The strengths of repairs made with round and 45-degree bevel joint designs were similar and significantly greater than those with a butt joint design. The strengths of repairs processed in a hydroflask for 10 minutes and 30 minutes were similar and significantly greater than those cured on the bench top. There was no difference in the strength of repairs made with autopolymerizing monomer and polymer and autopolymerizing monomer and heat-cured polymer. (J PROSTHET DENT 1992;67:815-20.)

R

epair of a fractured denture base should match the original material in strength and color. In addition, the repair should be easily, quickly, and inexpensively performed. Repair techniques described in the literature are similar except for the preparation of the fractured edgesl-* and the processing method. In 1970 Harrison and Stansbury4 reported on the effect of three types of repair surface designs on the transverse strength of repaired acrylic resin. They found the rounded design surface to be superior to the rabbet and butt designs. Other literature5-lo has addressed the repair of a fractured denture base, but no clinically relevant conclusions can be made. Repair joint designs other than those investigated by Harrison and Stansbury are advocated but have not been investigated. A hydroflask method of processing autopolymerizing acrylic resins has been introduced by several companies,* but the physical properties of materials processed in this manner have not been investigated. In addition, the problem of matching the shade of a heat-

This study was partially supported by an AD Williams Student Summer Fellowship, Virginia Commonwealth University, Medical College of Virginia. aAssociate Professor, Department of Removable Prosthodontics. bAssociate Professor, Department of Restorative Dentistry, Division of Dental Materials. CSummer Fellow, Department of Removable Prosthodontics. dStatistical Consultant, Department of Biostatistics. *Dacol hydroflask, Benchmark Products Inc., Houston, Texas; Dentu-Repair kit, Karl Schumacker Dental Instrument Co., Philadelphia, Pa.; and Denture curing unit, Huggins Engineering Co., Santa Clara, Calif. 10/l/35788

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a

b

C Fig. 1, Repair Butt,

joint surface contours (h) 45-degree bevel, (c) rounded.

investigated.

(a)

cured denture base with autopolymerizing repair materials has not been studied. The purpose of this investigation was to evaluate the effect of some joint surface contours, repair materials, and processing methods on the transverse strength of repaired acrylic resin. The butt, rounded, and 45-degree bevel joint

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Fig. 2. Repair index. Fig. 3. Sample marked on each side of centering mark. Fig. 4. Prepared sample secured in its repair index.

surface contours were investigated (Fig. 1). An autopolymerizing repair resin and heat-curing polymer repair material were evaluated. Some repair materials were processed on the bench top and others in a hydroflask.

MATERIALS

AND

METHODS

Test samples of heat-cured acrylic resin (Lucitone, L.D. Caulk Co., Milford, Del.) were prepared in the 65 x 20 x 2.5 mm size. Controls comprised 15 of these samples. They were prepared by conventional compression molding techniques and processed at 16Y F for 9 hours. The samples were finished by sanding with 400-grit wet-dry sandpaper (Wetordry Tri-M-Ite paper, A weight, 3M Company, St. Paul, Minn.) held on a flat surface (glass slab). After finishing, the samples were stored in water for a minimum of 2 weeks before testing the transverse strength in threepoint bending. A device for a three-point bending test was constructed; it consisted of a base with brass supporting pivot stands secured at each end. The central loading pivot for the test was a small metal rod 0.10 inch in diameter with a rounded end contacting the center of the sample. The samples were tested to failure on an Instron Universal testing instrument (Instron Corp., Canton, Mass.) with a crosshead speed of 0.5 in/min.

816

To make repair samples that compared with the control samples, the bottom and sides of the samples to be repaired were placed into impression plaster (Snow White No. 2, Sybron/Kerr Manufacturing Co., Romulus, Mich.) to form a repair index (Fig. 2). The sample and index were numbered on corresponding ends to allow realignment in the original position. Each repair sample was placed in the holding device with the numbered end oriented in the original direction. A centering mark was made by the contact of the loading rod with the sample using articulating paper. Lines were drawn perpendicular to the long edge of the repair sample %6 inch on each side of the centering mark (Fig. 3). The sample was cut at these lines with a separating disk, and the center section was discarded. A carbide bur (No. 560, S. S. White, Philadelphia, Pa.) was used to prepare one of the joint surface contours selected for testing, a rounded butt or joint with a 45-degree angle. The butt joint was prepared perpendicular to the surface of the sample and smoothed with sandpaper. The 45-degree bevel joint was prepared by measuring a distance equal to the thickness of the sample prepared as for the butt joint, drawing a line parallel to the prepared edge at this distance from the prepared edge, and trimming the angle freehand. The rounded joint was prepared by measuring and draw-

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Fig. 6. Repair sample that has been sanded flat on glass slab.

Fig. 5. Hydroflask.

ing a line parallel to the prepared edge at a distance equal to the sample thickness from the prepared edge on the top and bottom of the sample. Next, a bevel one fourth this distance was machined on both sides of the sample. The edge was rounded by pumicing. For all samples the space between the edges to be repaired when placed in the repair index was l/s inch. The repair indices were soaked in water for at least 10 minutes to substitute for a separating medium. The excess water was shaken off, and the repair indices were allowed to dry for a few minutes. The repair samples were correctly oriented in their respective repair indices, and impression plaster was placed on the ends of the sample and index to stabilize the combination during the repair procedure (Fig. 4). The repair was made by wetting the prepared sample surface with autopolymerizing monomer and then using the sprinkle-on monomer-polymer technique until the joint space was slightly overfilled to allow for polymerization shrinkage and finishing. As soon as the surface of the repair material lost its glaze, it was coated with petrolatum if it was to be bench cured, or it was placed in the hydroflask (Dacol hydroflask, Benchmark Products Inc., Houston, Texas) containing water at 135O to 140° F, and 60 lb/in2 water pressure was applied by the C-clamp. ‘The two repair materials tested were (1) autopolymerizing monomer and polymer (L. D. Caulk Co., Milford, Del.) and (2) autopolymerizing monomer with heat-curing polymer (Lucitone, L. D. Caulk Co.). The repaired samples were processed on the bench top at ambient temperature and pressure and in the hydroflask for 10 and 30 minutes. Two hydroflasks were used in this study. They were calibrated by replacing the top with a

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Fig. 7. Articulating paper used to mark point of contact of loading pivot with repaired area.

brass plate to which a pressure gauge was attached. The plate formed a tight seal with the body of the flask. A known pressure was applied, and the hydroflask’s pressure gauge was set (Fig. 5). The hydroflasks did not maintain the initial pressure but lost pressure at approximately the same rate (18 lb/in2 over a lo-minute period, 26 lb/in2 over a 30minute period). The repaired samples were recovered from their indices after 48 hours and stored in water. The samples were sanded smooth using wet-dry sandpaper on a flat surface (Fig. 6). The 12 samples were tested to fracture as previously described for each test condition except for a few that were damaged during divesting. Articulating paper was used to indicate the point of contact of the loading pivot with the center of the repaired area (Fig. 7). 817

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Table

ET AL

Test groups

I.

Processing Repair

Processing repair material

Contour

group

Control

None

A

Butt

B

Butt

C

Butt

D

Butt

E

45-degree bevel

F

Bound

G

45-degree bevel

H

45-degree bevel

I

45-degree bevel

Table

II.

Ultimate

three-point

Method

Heat-cured monomer Heat-cured monomer Autopolymerizing monomer Autopolymerizing polymer Autopolymerizing monomer Heat-cured polymer Autopolymerizing monomer Heat-cured polymer Autopolymerizing monomer Heat-cured polymer Autopolymerizing monomer Autopolymerizing polymer Autopolymerizing monomer Autopolymerizing polymer Autopolymerizing monomer Heat-cured polymer Autopolymerizing monomer Heat-cured polymer Autopolymerizing monomer Autopolymerizing polymer

bending

fracture

Heat-cured

Tie

165“ F

9 hr

Hydroflaak

10 min

Hydroflaek

10 mm

Hydroflaek

30 min -

Bench Hydroflask

10 min

Hydroflaek

10 min

Hydroflaak

10 min

Hydroflaek

30

mm -

Bench

load (pounds) Strength

Sample 1 2

3 4 5 6 I

8 9

10

11 12

Control

32.31 33.81 21.41 25.74 28.07 29.96

32.16 28.21 31.58 32.87 33.67 25.14

A

B

C

D

E

F

19.80 21.35 13.08 19.61 21.57 10.69 11.22 11.38 16.30 15.87 13.08

21.75 12.02

7.43 13.41 8.61

23.25 23.58

22.62

18.85

12.09 15.69 15.93 17.67 16.84 14.56 19.83 14.11 21.72 14.50

11.79 15.90 15.07 15.57 11.16 13.73 14.79 15.94 11.67 19.14 24.48 14.54

H

I

23.92

20.28 15.10 16.67

G

20.11 22.12 16.35 22.72

16.07 10.46

22.60 18.82 20.18 21.75 25.51 17.06 24.26 24.27

23.54 28.57 24.75 24.75 25.16 24.63 21.84 23.77 24.51 22.51

22.93 *

* *

*

9.13 7.66 9.09 9.79 9.95

7.83 8.99

15.97 24.11

23.98 22.92

24.51 22.11 22.09

27.40 24.25 24.25 25.57 23.77 24.04 26.09 25.96 * *

13 14 15

28.62

Mean

29.96

16.07

16.39

15.32

9.87

21.78

22.04

24.40

25.03

2.57

3.92

3.18

3.49

2.41

2.96

2.10

1.72

1.20

Standard deviation

18.39 18.77 18.97 21.07 18.43 19.07 21.63 * *

29.95

29.81 18.64

1.84

*Because of damage during divesting, sample was not tested.

RESULTS The test groups are listed in Table I. The fracture loads for the three-point bending test are listed in Table II and graphically represented in Fig. 8. The failure types are recorded in Table III. A one-way analysis of variance was used to test for dif-

818

ferences in the nine groups studied. The means were found to be significantly different with a p value of 0.0001. To determine exactly which groups were different, the Duncan multiple-range test was performed. The major result was that the butt joint means were statistically significantly lower than those using the 45-degree bevel joint and the

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III.

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Failure types

4ol

Failure Adhesive

Group

A B C D E F G H I

Cohesive

12

12 12 12 5 4

I I 10

8

1

10

rounded joint. In addition, within both groups the means for the bench-processed material were significantly lower than the means for material processed in the hydroflask at the p value of 0.05), indicating that the effects of process and joint repair were additive. There was no significant difference for materials used for the butt joint when processed by the hydroflask for 10 minutes, but there was a significant difference for the 45-degree bevel materials at p < 0.05 when processed in the same manner. The effect may he explained by the difference in failure type for the butt and 45-degree bevel repairs as shown in Table III. Cohesive failure of the repair material indicates that a sufficient bonding to the repair surface has been achieved and that the only way to improve the bond is to increase the strength of the repair material. CONCLUSIONS The transverse bend strength of the butt joint was significantly less than that of the rounded or 45-degree bevel joint. For the butt joints, most of the failures occurred at the interface of the original and repair materials-adhesive failure. For the 45-degree bevel and rounded joints, the fracture was predominantly through the repair materialcohesive failure.

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ABCDEFGHI Fig. 8. Ultimate three-point bending fracture loads (in pounds).

The transverse bend strengths of the round and 45-degree bevel joints were statistically similar. Since it is easier to prepare and finish a beveled joint than a rounded joint, the 45-degree bevel joint is preferred clinically. The advantage of the 45-degree bevel and rounded repairs is to shift the mode of fracture from a weak, adhesive interfacial fracture for the butt repair to a stronger cohesive fracture of the repair material in the 45-degree bevel and rounded repairs. The geometry of 45-degree bevel and rounded repairs increases the interfacial bond area and shifts the interfacial stress pattern more toward a shear stress and away from the more damaging tensile stress exerted on the butt repair during flexure. Processing the repair material in a hydroflask produced transverse bend strengths significantly greater than allowing the repair material to process on the bench top. However, there was no significant difference between processing the repair material in the hydroflask for 10 and 30 minutes. This may be clinically significant since it may decrease the time required to make a denture repair. However, the strength of the repair was not measured immediately after recovery from the hydroflask. REFERENCES 1. Zarb GA, Bolender CL, Hickey JC, Carlsson GE. Boucher’s prosthodontic treatment for endentulous patients. 10th ed. St Louis: CV Mosby, 1990:588-89. 2. Sowter JB. Dental laboratory technology. 1st ed. Chapel Hill: University of North Carolina, 1968:133. 3. Sharry JJ. Complete denture prosthodontics. 3rd ed. New York: McGraw-Hill, 1968:329. 4. Harrison WM, Stansbury BE. The effect of joint surface contours on the transverse strength of repaired acrylic resin. J PROSTHET DENT 1970; 23:464-74. 5. Guide to Dental Materials and Devices. 8th ed. Chicago: American Dental Association. 1976.

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6. Smith DC. The acrylic denture-mechanical evaluation mid-line fracture. Br Dent J 1961;110:257-67. 7. Ware AL, Docking AR. The strength of acrylic repairs. Aust J Dent 1950;54:27-32. 8. McCrorle JW, Anderson JN. Transverse strength repairs with self-curing resins. Br Dent J 1960,109:364-66. 9. Sanford JW, Burns CL, Paffenharger GC. Self-curing resin for repairing dentures: some physical properties. J Am Dent Assoc 1955;51:30115. 10. Osborne J. Transverse tests on denture base materials. Br Dent J 1949;86:64-7.

The knife-edge women Ichiro Douglas Harvard

Nishimura, A. School

tendency DDS,

Atwood, of Dental

DMSC,~

MD, Medicine,

11. Peyton FA, Anthony DH. Evaluation of dentures ent iechnics. J PROSTHET DENT 1963;13:269-82. Reprint

requests

Ryuji

Hosokawa,

DDS,

PhD,b

by differ-

to:

DR. JOHN E. WARD VIRGINIA COMMONWEALTH SCHOOL OF DENTISTRY 521 N. 11~~ ST. ROOM 301, Box 566 RICHMOND, VA 23298-0566

in mandibular

processed

ET AL

UNIVERSITY

residual

ridges

in

and

DMDC Boston,

Mass.

To investigate the bone resorption pattern of the residual alveolar bone, the morphologic change that occurred in mandibles was analyzed with standardized lateral cephalographs of 30 edentulous patients (15 women and 15 men). The longitudinal morphologic changes were measured at the sagittal sections of the mandibular bony contour at the symphysis area on superimposed cephalographic tracings. To quantify the morphologic change, a knife-edge index (KEI) was developed as the area change divided by the height change. Geometrically, the higher value of KEI represents the greater tendency to become a narrow residual ridge. The KEI values were statistically higher in the women than in the men (p < 0.002). In addition, the value of KEI seems to correlate with osteopenic change at the center point of the body of the second vertebra @ < 0.01). The continuous bone resorption activity in the edentulous mandible of women seems to be emphasized at the labial and lingual surfaces of the residual alveolar bone, resulting in a knife-edge type of residual ridge. (J PROSTAET DENT 1992;67:820-6.)

F

ollowing the extraction of teeth, the bony socket and adjacent soft tissue undergo a series of tissue repair reactions including acute inflammation, rapid restoration of epithelial integration, and connective tissue remodeling. Histologic evidence of active bone formation in the bottom of the socket and bone resorption at the edge of the socket are seen as early as 2 weeks after the tooth extraction, and the socket is progressively filled with newly formed bone in about 6 m0nths.l Rapid bone remodeling subsides by this

Presented in part at the International Association search meeting, Montreal, Quebec, Canada. aAssistant Professor of Prosthetic Dentistry. bResearch Associate in Prosthetic Dentistry and fellowship for Japanese Junior Scientist, Japan motion of Science. CProfessor Emeritus of Prosthetic Dentistry.

10/l/35085

820

for Dental

Re-

Recipient of the Society for Pro-

time but continuous bone resorption may persist at the external surface of the crestal area of the residual alveolar bone, resulting in considerable morphologic changes of bone and overlying soft tissues over the years.2 This phenomenon has been described as the reduction of residual ridges (RRR).3 The patients with severe RRR experience significant alteration in mid to lower facial contour,4 and have much difficulty in accepting the subsequent prosthodontic treatments.5 To document RRR by means of a standardized method, the lateral cephalometric radiograph has shown that there are significant individual variations in the rate and amount of bone resorption and in the morphologic changes of the residual alveolar bone.6-s Despite recent efforts to explain the significant differences in the rate of RRR observed in different patients, no etiologic factors are proven to date.g In most previous studies, the rate of RRR or the degree of residual alveolar bone atrophy was measured as the linear

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