Improving mouth guards

rds th L. Shaull, The University

of Iowa, College of Dentistry,

CDT,b Bseant

Iowa City, Iowa

guards and ~~~~~ials were tested to provide i~fo~~~t~~~ far a tive yet more ~o~fortable product. Ethylene-vinyl acetate materials varying in thickness and stiffness were tested for their ~ocba~~~a~~

ore protective

he use of athletic mouth guards in various contact sports higbly recommended because of their ability to absorb shock to the mouth, protecting it from serious injuries.*-g An effective mouth guard should have the following characteristics: (1) It must fit the mouth of the individual. (2) It must afford adequate protection. (3) It must stay in place comfortably and securely without impinging on or irritating tke soft tissues. (4) It must allow efficient mouth breathing and speec The American Society for Testing classifies mouth guards or mouth s: I, stock; II: mouth-formed; and III, custom-fabricated over a model.1° Five basic types of materials are used for making mouth guardsI In the order of most to least frequently used, they are (1.)~o~y~~y!~cetate-polyethylene or ethylene-,vinyl ac(EVA) copolymer, (2) polyvinylchloride, (3) natural er, (4) soft acrylic resin, and (5) polyurethane. Previous studies have been directe mainly toward EVA copolymers, the materials most widely used for all three types of mouth guards. Going et al. I1 studied the physical and mechanical properties of the materials used for mouth guards in terms of tensile strength, elongation, tear strength, hardness, rebound, penetration, and water absorption. As these researchers pointed out: the first three measurements give information on a material’s durability in use, the next tbree me~su~em~~ts theoretically describe the degree of protection provided by the materials, and the last parameter possibly indicates the material’s long-term stability in

aPr~f’e~~~r, Department of Biomedical Engineering. “Certified CStudent, eAssociate Copyright

Dental Technician, Department of Family Dentistry. Department of Biomedkal Engineering. Professor, Departmant of Pediatric Dentistry. 3 1994 by the Editoria! Council of THE JOURNAL OF

2n aqueorms condition.

The sesuits

o:.' tix

r *;sst~ ~3x3EVA

copolymers are given in Table I, The.?ewere no ~~ni~caIIt d8erences in the parameters measured iimong the three types of mouth guard materials jEVil, pulyviny1 chloride, and pol~~retha~ej in these atudie~~ 90 we gouped tbem together. It sboufd be noted that tke ~~?~c~~~~~~~ of vinyl acetate can be varied; consequently, Ae properties of the EVA copolymers vary (Table Ii). The mc~e vinyl acetate is copolymerized, the more flexible, stretchable, softer, and tougher, yet w a lower softening temperarureS t rial becomes. st cQrnrner~~1~~ avdabie a5nt are made of 28 % vinyl acetate-pol\;ej;hylerte (de~~~~~~e~. in this studwd as 28% EVA) copolymer. The actual performance of the mouth guard in viva depen bite” because it is shaped into the mouth structure by thermoplastic change of the material after it is ptit ix boiiing water (for about 10 seconds), is put k cold WJ.tW (about 2 secords), 2nd the teeth are closed on ir in the mouth. The most desirable mouth guard. is ihe custom-made mouth guard. The advantages of the cu%om-n&e mouth. guard over the boil-and-bite mouth guard are that it provicles a better fit and better protect;on. Is is also more comfortable and more e~t~etica~l~ pleasing. The cost, ho is 10 to LOOtimes more than that of the ~lQ~i~~-~~~te~ 2nd it requires more time to make. There are no national or ~n~e~~a~~~~~~ standards to folStandard 4”697-&P specifies only standards for care and use of mouth guards; many products and practices may not provide the intended protection from injury, Indeed, in a pre?iminary study ofi~i.i;e-shelf, mot&~-

THE

Table

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Tensile strength (psi)

EVA Polyvinylchloride Polyurethane

II.

1596 + 656 1576 t 921

1030

Tear strength (lb/in)

Elongation (%)

890 -t 93 354 t 95 400

Hardness Rex A (1 set)

177 ir 24 253 k 136 200

82.6 2 3.5 73.6 k 15.5 82

Rebound (%)

water absorption (%)

Penetration (inches)

50.4 t 3.8 24.0 k 11.5 58.9

0.179 + 0.024 0.210 + 0.089 0.195

0.48 + 0.37 0.67 L?Z0.24 0.61

Variation of properties of polyethylene by copolymerization with polyvinylacetate

Amount polyvinylacetate 0

9 18 28

of (%)

Tensile strength (psi)

2100 2100 2700 2100

Elongation (a)

550 600 750 800

Flexural modulus (psi)

23000 12000 6900 2300

formed (boil-and-bite), commercial products lost most of their thickness in the occlusal surfaces (99 % and 70 % for two different companies’ products) when manufacturer’s instructions were followed. l3 In another in-house study, custom-made mouth guards decreased in thickness more than 25 % , even though the mouth guards tested were made by a skilled technician. In some surfaces, thickness decreased more than 50%. These results led to an investigation of ways to retain the thickness in the occlusal surfaces, where energy absorption on impact may be the most critical in preventing teeth-mouth guard-teeth contact damages. Of greatest concern is the type II boil-and-bite mouth guard, which may provide a false sense of protection to the wearer because of the severe decrease in thickness on the occlusal surface. Unless drastic improvements are made, these mouth guards should not be promoted to the customers as they are now. This study investigated the efficacy of applying a reinforcing material on the occlusal surfaces so that the thickness would not decrease substantially after fabrication in either custom-made or mouth-formed (boil-and-bite) mouth guards. The reinforcing materials should have a higher softening temperature and be harder than the matrix material to resist great deformation during molding. Ideally, the reinforcing materials should bond chemically to the matrix material to prevent delamination during molding and actual use. For example, a 9 % EVA copolymer may be used to reinforce a 28% EVA sheet. The relative amount of the reinforcing materials and configuration should be determined according to the parameters previously described. MATERIALS The tests were made with two different EVA materials supplied by a commercial vendor (Dental Resources, Inc.,

374

ET AL

Properties of mouth guard materials

Material

Table

PARK

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StitTness (psi)

Hardness (ShoreA)

16500 11600 7200 1900

98 95

91 83

impact

Tensile (foot-lb/inch3)

315 400 505 565

Density (gm/em3)

Softening temperature (“C)

0.92 0.93 0.941 0.951

97 73 64 47

Delano, Minn.). The materials tested were EVA copolymer sheets of four different thicknesses (1,1.5,2, and 4 mm) and Proform sheets (4 mm thickness; Dental Resources), a laminate of EVA with harder material in the middle (Fig. 1). A colored, heart-shaped piece of harder material in the middle of the original sheet is intended to reinforce the mouth guard after fabrication from behind the anterior maxillary teeth. The material, supplied in 5 x 5-inch prefitted sheets, is molded to a cast of the mouth to yield a custom-made mouth guard. Off-the-shelf, mouth-formed (boil-and-bite) mouth guards, (Safetgard Corp., Golden, Colo.) were purchased locally (in Iowa City), and preliminary studies were made to compare their performance with that of the custom-made mouth guard. TESTS Thickness

measurements

To examine variability in thickness of the finished product, uniformity of the original stock materials is essential. The thickness of the sheets was measured with a precision caliper. A total of 16 uniformly distributed points were measured on each sheet. The thicknesses were then analyzed statistically by use of Minitab software (Minitab Statistical Software, Minitab Inc., State College, Pa.) so that trends in the flat sheets of polymer could be determined. Water absorption, density, temperature measurements

and transition

Water absorption tests were done at room temperature (25OC)and body temperature (37’C) with ASTM standard D70 at 24- and 48-hour intervals. The density of each sample was obtained with a pyknometer and the following equation according to ASTM standard D7014: Density = W,/[W,- (W,, - W,)] 111

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\

12.7

---.-_^-

-.--.

--b_l

j

I Urhon

Pig. 2. Dimensions of tensile lesi, specimens.

Fig, 1. Diagram of polymer sheet. Heart-shaped piece represents colored, harder material of Proform sheet.

where W,?is the dry weight of the sample, a/v, is the weight of water and sample in the specific gravity bottle, and W,,, is the weight of water and sample in the specific gravity bottle. Tbe transition temperature was measured with a differential scanning calorimeter (IX%4 model; Perkin Elmer Co., Norwalk, Conn.). The heating and cooling rates were 40” C per minute. Tensile

Fig. 3. Sagram of ball-drop tez3tsetiip. hClU$ing force transducer, video camera, and digit,d ~5ewas~ei;te TB-. corder.

tests

A standard tensile specimen, as prescribed by ASTM standards, was used for the tensile test. The dimensions of the tensile specimen are shown in Fig. 2. The tensile specimens were tested in a hydraulically controlled materials testing machine (model 812; MTS Corp., Minneapolis, Minn.), and each thickness of the sheet was tested five times, From the recorded force versus elongation data, stress-strain curves for each specimen were plotted. From these plots, material properties, such as elastic modulus, were calculated.

The impact tests provided information on peak impact forces observed and the amount of the energy lost on impact. Two balls were used to vary the speed and amount of impact force. A ‘“small” (I-inch) stainless steel ball that massed 66.8 gm was dropped from 33.76 inches (maximum drop mass allowable for the apparatus). A “large” (2-inch) stainless steel ball that massed 473.4 gm was dropped from 10 inches. The information obtained from the force transducer was used to determine the highest recorded force value observed for each material and sheet thickness. A schematic diagram of the test is illustrated in Fig. 3. The sheet was laid on top of a force transducer and the force of the impact was recorded wir,h a computer. The impact was digitized with Codas eoftware (Dataq Instruments, Inc., Akron, O‘hio) so that a graph that displayed the impulse

could be plotted. This graph r&a;- the:: iniagr~:ed with Simpson’s rule to determine 9~ tmmziiked i.mpuise through the polymer sheet. Peak %q~cr iuree vahxes vxre recorded. Carbon paper was inserted between the sheet and transducer to estimate the area of hmpa:~ This area was used to caiculate the transmitted irnpacc Jtress, In eonjunction with the impulse measuremen~s~ the rebound height of the ball after impact was recor&z? with a video camera and digital video cassette mxxdrr. The meaw.pmerits were applied along with the maad of the ball to Newton’s eqtiations of moticn, and the &pact energy on the plate was calculated:

Mass of the ball, initial height, and gravity were ajl held constant. The dissipated energy would thsn be the amount of energy transmitted from the ball to rtc polymer and force transducer. This would simulate rhe energy traasmitted from a blow to the trouthpfece and t,eetb. t&kg

of

The actua! thicknesses of the cc:ni:~~t SIWP of the occlusal surfaces are probably more imporcwn~ than the . , begmnmg macerial thicknesses. The same precision eaiipers were used to measure the thicknesses id the EVA sheet

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3.80 THICKNESS

(mm)

3.75

3.66

1

,

I

I

I

2

3 PLATE

Fig. 4. Thickness sheet.

variations

for five Proform

Protorm,4mm I

4

I

5

NUMBER

sheets. Overall thickness varies with each

STRESS Pa)

200

100

0

300

STRAIN (%) Fig.

5. Comparison

of stress versus strain relationships

materials before and after they were molded for the custom-made and for the mouth-formed (boil-and-bite) mouth guards.

RESULTS Thickness

AND DISCUSSION measurements

Fig. 4 illustrates the results of thickness measurements of the Proform plates. It demonstrates that the thicknesses vary from plate to plate enough to be statistically significant (p < 0.05). Stock plates of EVA material of different thicknesses demonstrate the same trend. This variation suggests that the original manufacturing processes are inconsistent. For more consistent thicknesses in custom-fabricated mouth guards, better quality control of stock material must be enforced.

376

of EVA specimens.

Water absorption, density, temperature measurements

and transition

Results of the density, transition temperature range, and water absorption measurements are compiled in Table III. The water absorption tests were completed for all sizes of EVA material, but only the results of the 4 mm EVA thickness are shown here for comparison with the Proform material. The water absorption measured in these tests is about twice as high as the values obtained by others, as given in Table I. Water absorption also increases with increased temperature to 37” C by about 30 %, as expected. It is not yet clear how much water absorption affects impact energy absorption and tear resistance against chewing of the mouth guard. The density values obtained are similar to those available in the literature. The glass-transition tem-

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e & Gomparison terials.

Table

911. Summary

of stress versus strain relationship

for 4 mm EVA and Proform

of results for the density, glass-transition

temperature,

PPOpCXty

EVA (n = 5)

peratures ranged widely, from 83” to 97” e, for both EVA and Proform materials. Even the low values of the transition temperature are much higher than the value given in the Table II (47’ G for 28% EVA). This difference may be caused by differences in measurement methods and materials, although both materials came from the same manufacturer.

Tbe results of the tensile tests are given as stress versus strain with the Ieast cross-sectional area of the tensile specimen. A summary of the tests is plotted in Fig. 5. The effect of the thickness variation can be seen easily; the thinner material exhibits the higher -modulus. This is because of t,he increased surface to volume ratio. Fig. 6 provides a comparison of the 4 mm EVA and the 4 mm Proform materials. The Proform has a higher modulus and yield strength than does the EVA.

The results for both the large and small balls are listed in Table IV and plotted In Figs. 7 and 8. Fig. 7 reveals the trends for all materials, indicating that the 4 mm EVA had

Table

material

IV.

and water absorption

Peak

impact.

forces

!::a

tests

.-II __I1_x^_ -,s. PFoEocmin = 5)

ohxt~ec:

*of eacii

and thickness

the lowest impact force in the smal L d ro1.3*;~d the Preform material had the lowest force va!ue for the Ssrge hdi drq~. The considerable amount of variation wiShin the lest, however, statistically eiimEnates these riiscinctions. The graph more accurately displays their similar performance in the impact force test. Repiottin, g 4J the tramsmitied force versus thickness with log-Log of the vaiiles gives a hnear relationship, as shown in Fig. 8. The rciati.onship cm be expressed as follows:

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ET AL

0

1500

* 1000

0

FORCE (Ibf)

0 500

Logo Ball

0 Small Ball

0

0

a 0

0 0

0

0 4mm Proform

Et*

EL

MATERIAL

Fig. ‘7. Summary of highest impact force values observed for each material for both ball sizes.

2.8 LOG FORWIW) 2.6 2.4

0

0.2

0.4

0.6

0.8

LOG THICKNESS(mm)

Fig. 8. Log-log plot of transmitted

where A and B are constants. As the thickness increases logarithmically, the transmitted impact force decrease logarithmically. With the thinner sheets, marked deformation occurred; the big ball even caused destruction of the 1 and 1.5 mm sheets. By placing a carbon-backed paper between the force transducer and polymer sheet, the area through which the force was transmitted was estimated. These areas were then used to calculate the impact stress for the large and small ball drops. The results were plotted in Fig. 9, which is similar to Fig. 7. The same linear relationship between log thickness and log transmitted stress was obtained, with a high level of correlation. The 4 mm EVA material also distributed the impact force over a wider area than did the Proform material. This was expected because EVA deforms more easily than does the Proform material.i5-i7

378

force versus thickness

of EVA sheets.

Thickness measurements mouth guards

after

fitting

of

The results of the thickness measurements on the top and side of the custom-made mouth guard and the occlusal surfaces of the mouth-formed (boil-and-bite) mouth guards are summarized in Table V. The larger decrease in the thickness on the side compared with the top arises because more vacuum suction is applied to the side than the top. The much larger decrease in the thickness of the mouthformed (boil-and-bite) mouth guard results from the uncontrolled biting force by the wearer of the mouth guard during the biting process for fitting the wearer’s mouth structure. Obviously, the resulting mouthpiece gives less than satisfactory protection. It is also a concern in the trade that thinner mouth guard materials (2 mm) are currently being promoted. Obviously, thinner mouth guards provide

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Fig, 8. Thickness versus energy loss on impact same as 4 mm EVA.

Tabae V. Net thickness changes after molding for~ming of boil-and-bite mouthguards Type

of mnutb

guard

Custom-fabricated Mouth-formed

foor EVA material.

Froform

;xrformes

and

Location

Thickness decrease (76)

TOP Side Top

50* 70 and 991

25*

less ;)rotection th.an do the thicker ones but are more comfortable. Tn view of the fact that almost one quarter to one half of the thickness decreases during the molding process, however, it. is necessary to establish a minimum postmolding mouth guard thickness. The same standard should be applied more rigorously to mouth-formed (boil-and-bite) mouth guards because it is more difficult to control the thickness during the fitting process with these mouth guards. Modification in the mouth guard material is proposed so that the minimum thickness can be mainduring the molding and fitting processes (Fig. IO).

Testing of material properties of commercially available mouth gu.ard polymers has led to new perspectives. Both the EVA and Proform materials performed similarly in the mechanical tests, as well as in measurements of water absorption and d.ensity. The water absorption at body temperature was mu& higher than at room temperature. It has been speculated that higher water absorption may also help absorb more impact energy, but this conjecture has not been verified. The ball-drop impact tests have led to the conclusion that the thicker mouth guard is more effective in with-

ig. ItI guard.

Schematic

diagram

iilus;ra?i.ui;

rucjideti mouth

standing a biow to the mouth. ‘Ih33r; ~L?p:sshow that the thicker materials are more effective in ebso,rbing ~rnpa~t energy than the thinner stock sizes. W&n large impact forces were applied, the thinner sheets sb.:xved marked deformation at the side of the impact: in SOE>-IP ins?mces, the integrity of the sheet was destroyed I ‘A 4 rvm sheet is probably a better choice for optimal mouth guard thickness. Although the 4 mm sheet is a more eifec?.5ve thickness, whether EVA or Proform material is ber,tcrr cannot be conciusively decided. Both the 0 mm EVA and the 4 mm &oform material have low impact forces, wI:r, the EVA tending to distribute the force OYCF a kxrgur ami, h:s decreasing the transmitted stress. The I?‘:‘% maWri8.l also

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transferred the least amount of impact energy from both the small and large balls. These results suggest that the 4 mm EVA material performed better than the Proform material in impact tests alone. Other tangible parameters, however, such as chew and tear resistance, manufacturability, and dimensional stability, have not been scrutinized. More interesting results would be obtained by making sandwich sheets with harder materials, such as 99 % acetate in the middle with 28% acetate on the outside (as suggested in our invention disclosure). la This configuration may be altered to give the maximum protection to the mouth and provide the most comfort by making the mouth guard thinner without compromising its effectiveness. We Dental letics, helped

gratefully acknowledge the help of Mr. Douglas Murphy, Resources, Inc., and Mr. Robert Howlsby, Director of AthUniversity of Iowa. Mr. H.J. Lee and Mrs. N.L. Robinson to run part of the experiments.

REFERENCES

protectors to cranial pressure and deformation. d Am Dent Assoc 1967;74:735-40. Godwin WC, Craig RG. Stress transmitted through mouth protectors. ,I Am Dent Assoc 1968;77:1316-20. Turner CH. Mouth protectors. Br Dent J 1977;143:82-6. Chaconas SJ, Caputo AA, Bakke NK. A comparison of athletic mouthguard materials. Am J Sports Mcd 1985;13:193-7. American Society for Testing and Materials. Standard practice for care and use of mouthguards. ASTM P697-80. Philadelphia: reapproved 1986. ASTM. Going RE, Loehman RE, Chan MS. Mouthguard materials: their physical and mechanical properties. d Am Dent Assoc 1974;89:132-8. Elvax resins, Du Pant de Nemours. Wilmington, Del.: E.I. Du Pont C Co., Inc 1989. Park JB, Shaull KL, Donly KJ. Methods of improved mouthguards. Patent disclosure. University of Iowa, Dee 1992. ASTM standard test method for specific gravity and density of semisolid bituminous materials. Book of ASTM standards. Philadelphia: American Society for Testing and Materials, reapproved 7990. Wong EW, White RC. Development of a shock absorbing biomedical elastomer for a new total elbow replacement design. Biomater Med Dev Artif Organs 1979;7:283-90. de Wijn JR, Vrijhoef MMA, Versteegh PA, Stassen HP, Linn EW. A mechanical investigation to the functioning of mouthguards. In: Huiskes R, Van Campen D, De Wijn, eds. The Hague: Nijhoff, 1982:451-8. Chen CP, Lakes RS. Design of viscoelastic impact absorbers: optimal material properties. Tnt ,I Solids Structures 1990;26:1313-28.

7. 8. 9. 10.

11. 12. 13. 14.

15.

16.

17.

1. Bureau of Dental Health Education, Council on Dental Materials and Devices, Mouth protectors: 11 years later. J Am Dent Assoc 1973;86: 1365.7. 2. Picozzi A. Mouth protectors. Dent Clin North Am 1975;19:385-7. 3. Godwin WC, Craig RG, Lang RR, Powers JM. Mouth protectors in junior football players. Phys Sports Med 1982;10:41-8. 4. Stenger JM, Lawson BA, Wright JM, Ricketts J. Mouthguards: protection against shock to head, neck and teeth. J Am Dent Assoc 1964;69:27381. 5. Wehner PJ, Henderson D. Maximum prevention and preservation: an achievement of intraoral mouth protectors. Dent Clin North Am 1965;9:493-8. 6. Hickey JC, Morris AL, Carlson LD, Seward TE. The relation of mouth

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CONTRIBUTING Todd

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AUTHOR BS, Student, Department of Biomedical

Engineering.

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