Dental Materials Journal 2014; 33(4): 551–556

Combined analysis of shock absorption capability and force dispersion effect of materials with different impact objects Ruman Uddin CHOWDHURY1, Hiroshi CHUREI1, Hidekazu TAKAHASHI2, Takahiro WADA3, Motohiro UO3, Shintaro FUKASAWA1, Keisuke ABE1, Sharika SHAHRIN1 and Toshiaki UENO1

1 Department of Sports Medicine/Dentistry, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, Japan 2 Department of Oral Biomaterials Engineering, Faculty of Dentistry, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113- 8549, Japan 3 Department of Advanced Biomaterials, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, Japan Corresponding author, Toshiaki UENO; E-mail: [email protected]

The aims of the present study were to investigate the shock absorption capability and force dispersion effect of mouthguard (MG) materials using load cell and film sensors. Two kinds of MG materials, ethylene vinyl acetate and polyolefin, were chosen for this study. When impact forces of approximately 5,000 N were applied on the MG materials using a round flat-nosed rod and a bluntly pointed rod, peak intensities were measured using the load cell sensor while peak stresses and impressed stress distribution areas were measured using the film sensor. Combined analysis using both load cell and film sensors clearly showed the shock absorption properties and force dispersion effects of different MG materials with different impact object shapes. Therefore, impact analysis involving a combined use of these sensor systems was useful and reliable in assessing the shock absorption capability and force dispersion effect of MG materials.

Keywords: Mouthguard material, Impact object, Load cell sensor, Film sensor

is applied by using a free-falling object, drop-, INTRODUCTION pendulum, piston, or impact rig11-16). Although these Be it contact or non-contact sports, play methods are useful in measuring the shock-absorbing an important role in preventing oral injuries and capability, they do not measure the changes in preserving the oral structures of athletes during actual intensity and distribution of stress under the sports1-4). The American Society for Testing and mouthguard. Three-dimensional finite element analysis Materials (ASTM) has classified mouthguards into (FEA) is another comprehensive method which uses a three basic types: stock, mouth-formed or boil-and-bite, virtual dynamic force to examine the shock absorption and custom-made5). Custom-made mouthguards are the capability of a mouthguard by visualizing the dispersion preferred choice for players and recommended by FDI of force three-dimensionally17-19). However, this method (Fédération Dentaire Internationale) World Dental has numerous limitations. The overall design is Federation6). Besides preventing orofacial injuries, rendered by a computer-aided design (CAD) software, custom-made mouthguards provide athletes with better and the cushioning or supporting effect of mouthguard comfort and wearability7) and can be processed for end- is analyzed with an explicit finite element code to predict use applications8). the impact force given the material properties and Ethylene vinyl acetate (EVA) is the most commonly impact velocity. Therefore, a “gold standard” test is yet used material for commercial and custom mouthguards to be available which could investigate shock absorption because of these attractive features: widespread capability accompanied with a full understanding of the availability, can be easily molded, and yet has adequate dispersion of force or energy. physical and mechanical properties8). Polyolefin is also To date, most impact tests for mouthguard materials another popular mouthguard material in terms of tensile was carried out by using impact objects that resemble a and tear strength, hardness, contact angle to water, ball or round object. In reality, sports-related trauma is shock-absorbing capability9), and good bonding durability not usually caused by ball-like or round objects, but by like EVA when stored in water under pressure10). angulated, sharp, thin, or pointed objects such as hockey In vitro studies on the shock absorption capability pucks, sticks used in , , shinty, of mouthguards quantify the direct impact on the latter bandy, or , or by players’ fingers. A previous by measuring force using load cells, accelerometers, study has investigated the influence of an impact fiber Bragg grating sensors and strain gages. Force object’s characteristics on impact force and the impact absorption ability of mouthguard materials20). However, this study20) had numerous limitations: only one type of

Color figures can be viewed in the online issue, which is avail- able at J-STAGE. Received Feb 26, 2014: Accepted May 14, 2014 doi:10.4012/dmj.2014-061 JOI JST.JSTAGE/dmj/2014-061 552 Dent Mater J 2014; 33(4): 551–556 mouthguard material was investigated and most impact absorption test: load cell sensor and film sensor (Figs. objects used were of a round shape. 2 and 3). Many orofacial injuries were sustained during team sports such ice hockey (2.2–5.0%) which involved 1. Load cell sensor the use of sticks, hockey pucks, or by collision21,22), and Three dynamic compression load cells (LMB-A-2KN, shinty (4.8%)23). In non-contact sports like field hockey, Kyowa Electronic Instruments Co., Tokyo, Japan) the second major cause of dentoalveolar injuries was by were placed 120° apart below a 10-mm-thick stainless hockey sticks (38.2%)24). In other words, many injuries were inflicted by impact objects which were not always spherical or round like a ball. Therefore, it is necessary to assess the shock absorption capability of a mouthguard material according to different types of impact objects. The purpose of the present study was to observe the changes in shock absorption capability and behaviour of two conventional mouthguard materials as a function of the impact object’s shape. Findings of the present study would also show that compared with data independently obtained using each measurement system, combined analysis using both load cell and film sensors provided superior reliability in impact analysis.

MATERIALS AND METHODS Specimen preparation Two commercially available mouthguard sheets were used in this study. One was 4-mm-thick EVA sheet (Erkoflex®, Erkodent Erich Kopp, Germany; Lot No. 1214004; coded as ERK), which was cut into 20 circular specimens of 5 cm diameter each. Another was 4-mm- thick Polyolefin sheet (MG21™, CGK Corp., Hiroshima, Japan; Lot No. 581240; coded as Ole), which was cut into 20 circular specimens of 5 cm diameter each.

Shock absorption test Specimen was placed on the stainless steel platform of an impact test machine (modified IM-201, Tester Fig. 2 Impact test machine. Sangyo Co. Ltd., Saitama, Japan). Free-fall drop test was carried out using a free-falling metal object (500 g) at a vertical height of 25 cm onto the top end of two different types of metal rods: one had a round flat-nosed (Fl) tip of 12.7 mm diameter, and the other had a bluntly pointed (Pn) tip of 3.2 mm diameter (Fig. 1). Two measuring systems were used for the shock

Fig. 1 Shapes of impact objects used in this study: (a) Fig. 3 Schematic illustration of shock absorption test and Round flat-nosed (Fl) rod tip; and (b) Bluntly two types of measuring systems used: load cell pointed (Pn) rod tip. sensor and film sensor. Dent Mater J 2014; 33(4): 551–556 553 steel platform of the impact testing machine. Changes mono-sheet type (MS) Prescale films were used to in load during and after impact load application, by measure peak stresses ranging from 2.5 to 10 MPa and dropping the free-falling object on both types of rods 10 to 50 MPa respectively. MS sheet (mono-sheet type) (with and without mouthguard material specimen), was composed of a polyester base coated with a color- were recorded via a sensor interface (EDX-100A, Kyowa developing material, with a layer of micro-encapsulated Electronic Instruments Co., Tokyo, Japan) in a personal color-forming material on top. LW sheet was composed computer. Sum of loads recorded by the three load cells of two polyester bases (A-film and C-film). A-film was was calculated, and the maximum sum of loads after coated with a layer of micro-encapsulated color-forming impact load application was registered as the first peak material, and C-film was coated with a layer of color- intensity. Data were interpreted and analyzed using developing material25). Film thicknesses are given as software (DCS-100A, Kyowa Electronics Instruments follows: MS sheet at 105±10 µm; LW sheet A-film at Co., Tokyo, Japan) at a sampling rate of 20 kHz. Load 95±10 µm; LW sheet C-film at 90±10 µm. applied without mouthguard material was taken as Impact load was applied on mouthguard material control to compare against the load applied with the specimen using two different rod tips. After load material. application, each induced red patch at load impact point (with and without specimen) was recorded by a 2. Film sensor camera (Data Shot FPD-100, Fujifilm Business Supply A precut piece of film sensor sheet (Prescale Film, Co., Tokyo, Japan). Peak stress and impressed stress Fujifilm Business Supply Co., Tokyo, Japan) was used distribution area (Da) were analyzed using software for the film sensor system. Seven types of Prescale films (Data Shot FPD-100S, Fujifilm Business Supply Co., were available to measure different pressure levels. Tokyo, Japan) (Fig. 4). Da was defined when stress level An appropriate film was selected for each pressure was at least 2.5 MPa. For each mouthguard material, range condition and placed under the specimen. Low five measurements were performed with each type of rod pressure two-sheet type (LW) and medium pressure tip at room temperature.

Statistical analysis Data were statistically analyzed using two-way analysis of variance (ANOVA), followed by post hoc analysis using Tukey’s multiple HSD test (p<0.05). Statistical software SPSS 10.0 J for Windows (SPSS Inc., Chicago, IL, USA) was used to perform all analyses.

RESULTS Without the mouthguard material, center of film sensor became perforated because of heavy force. As peak stress of control without mouthguard material exceeded 50 MPa, it could not be measured. Impressed stress Fig. 4 Film sensor system used in this study: (a) Red distribution areas (Da) of Fl and Pn controls were patch on Prescale sheet, which is induced at load successfully recorded. A schematic drawing of Da is impact point; (b) A record of pressure-impressed shown in Fig. 5. area from the software. Two-way ANOVA for peak intensity and peak

Fig. 5 Schematic drawings of impressed stress distribution areas (Da) for different combinations of mouthguard material and impact object shape. Da was defined when stress level was at least 2.5 MPa. 554 Dent Mater J 2014; 33(4): 551–556

Fig. 6 Peak intensities of controls and different Fig. 8 Impressed stress distribution areas (Da) of controls combinations of mouthguard material and impact and different combinations of mouthguard material object shape. and impact object shape. Bars (average+standard deviation bar) with the Da was defined when stress level was at least same letter are not significantly different (p>0.05). 2.5 MPa. Bars (average+ standard deviation bar) with the same letter are not significantly different (p>0.05).

DISCUSSION A wide range of materials are potentially suitable for making mouthguards: natural rubber, silicone rubber, urethane rubber, polyvinylacetate-polyethylene copolymer, EVA, and polyvinyl chloride15,26). EVA is commonly used as a mouthguard material because of its widespread availability coupled with good mechanical Fig. 7 Peak stresses of different combinations of 8) mouthguard material and impact object shape. and physical properties . Polyolefin is a recent-emerging Bars (average+standard deviation bar) with the mouthguard material which has been popularly same letter are not significantly different (p>0.05). used during the past decades. It exhibits good shock absorption capability with lower water absorption and higher adhesive strength than EVA9,10). Therefore, EVA and polyolefin were two mouthguard materials selected stress revealed significant interaction between for investigation in the present study. mouthguard material and impact object shape (p<0.001 In vitro studies which investigated the shock and p=0.026 respectively). absorption capabilities of mouthguard materials had For Da, significant difference existed between been carried out using different measurement methods mouthguard materials and impact object shapes such as load cells, accelerometers, and strain gages. (p<0.001 and p<0.001 respectively). However, it is not possible to observe the distribution of From the load cell sensor system, first peak absorbed force with these methods. intensities without any mouthguard material with flat- With CAD technology and FEA systems, it is nosed and pointed rods were 5,036±176 N (Ctrl-Fl) and possible to observe the distribution of absorbed force. 4,852±149 N (Ctrl-Pn) respectively. First peak intensities This phenomenon is gaining popularity with researchers decreased when mouthguard materials were inserted. in their quest to understand how and where the force First peak intensities with ERK-Fl and Ole-Fl were is distributed upon impact in response to different significantly greater than those of pointed rod on both impact objects. For example, in a study by Cummins and types of materials (ERK-Pn and Ole-Pn) (Fig. 6). Spears27) on the effect of mouthguard design on stresses Peak stress of Ole-Fl was 7.6±0.7 MPa, which was in the tooth-bone complex, FEA was used to observe significantly lower than ERK-Fl, ERK-Pn, and Ole-Pn the protective capabilities of mouthguards and stress (Fig. 7). distributions in the tooth-bone complex during collision. For controls without any mouthguard material, Da However, FEA investigations were not performed using values with flat-nosed and pointed rods were 17.9±3.4 real mouthguard materials; instead, simulations of mm2 (Ctrl-Fl) and 14.9±2.4 mm2 (Ctrl-Pn) respectively. CAD-generated structures were carried out in a virtual The Da value of Ctrl-Fl was significantly greater than world. This limitation can be overcome by scanning that of Ctrl-Pn. Da values of ERK-Fl and Ole-Fl were the real mouthguard or mouthguard material, and significantly greater than those of ERK-Pn and Ole-Pn then perform FEA to observe stress distribution in the (Fig. 8). material or tooth-bone complex. However, automatic Dent Mater J 2014; 33(4): 551–556 555

Table 1 Confirmation of the transmission of shock wave to absorption of impact load and an even distribution of the load cell sensor with or without film sensor that load. In term of stress distribution area, ERK-Fl and Ole-Fl presented significantly greater Da values ERK-Fl ERK-Pn than ERK-Pn and Ole-Pn, indicating that impact forces Load cell Load cell from flat objects were well distributed in both materials. + Load cell + Load cell A pointed impact object has a narrower or thinner Film Film surface area, causing a centric force distribution in the mouthguard materials, and hence a more concentrated 1198 ± 46 N 1212 ± 22 N 718 ± 11 N 715 ± 19 N stress distribution compared with a flat object with a The obtained data were statistically analyzed by unpaired-t- larger surface area. test (p<0.05). There were no significant differences between In a study by Takeda et al., it was reported that the first peak intensities with film sensor and without film while mouthguard materials were effective in reducing sensor (ERK-Fl; p=0.563, ERK-Pn; p=0.725) the impact force of any impact object, shock absorption ability depended on the object type20). In another study, it was mentioned that it is realistic and appropriate to select more than one sensor type and impact meshing of geometrically complex objects —such as the object to assess the effectiveness of a mouthguard mouthguard— could pose the problem of poor aspect material11). Results of the present study agreed with the ratios, which would significantly reduce the accuracy of aforementioned claims of Takeda et al.11,20). the solution. Sports that involve the use of sticks like ice hockey, Alongside the load cell sensor, a film sensor was also field hockey, shinty, bandy, and hurling are good used in this study to precisely record the impact area, examples where impact objects are different from round whose color was changed to red to indicate the impressed types such as the or ball. In this study, the stress distribution area28). The reliability and superiority rods were used to simulate different types of impact of such a combined measurement system was already objects and with a view to obtaining more realistic well documented in previous studies29,30). Low pressure insights into choosing the appropriate mouthguard two-sheet type (LW) and medium pressure mono-sheet material by the clinicians. For instance, a flat-nosed type (MS) Prescale films were used to measure peak rod resembled impact objects with a spherical end or stress levels ranging from 2.5 to 10 MPa and 10 to 50 surface like the , ball, , or bat. MPa respectively. A bluntly pointed rod resembled impact objects with Another reason of choosing the load cell sensor and a thin-edged, pointed end like the hockey stick, toe or film sensor for a free-falling object was because of their heel of the hurling stick, bandy stick blade and edge, or easy operation and data interpretation. A free-falling shinty stick. Results of this study showed that both EVA object of 500 g dropped from a height of 25 cm onto the and polyolefin materials exhibited shock absorption top of a rod without mouthguard material served as capability in different ways depending on the type of the control to generate a force more than the punching impact object. Further research should be carried out force of an Olympic boxer (3,427±811 N). It was then to improve the existing EVA and polyolefin materials compared against the force delivered on the mouthguard or develop new mouthguard materials which can show material to assess its shock absorption capability31). widely versatile shock absorption abilities in response to Round flat-nosed rod and bluntly pointed rod were used impact forces from different impact object shapes. in this study to simulate and compare the influence of impact object characteristics on impact force and CONCLUSIONS force dispersion by mouthguard materials. After each measurement, a new specimen was used to avoid Within the limitations of this in vitro study, it was inaccurate data from a deformed material. concluded that the shock absorption properties of To confirm shock wave energy transmission from mouthguard materials varied according to impact the film sensor to the load cell sensor, an additional objects’ shapes and that these changes varied depending test was carried out by keeping the film under the on the mouthguard material. It was also found that mouthguard material versus without inserting the film a combined use of load sensor system and film sensor (Table 1). There were no changes in shock wave energy system was a useful, reliable, and superior method in transmission to the load cell. The film sensor sheets understanding the shock absorption ability and force were very thin, which did not interfere with the dispersion effect of mouthguard materials. transmission of shock wave energy to the load cell. 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