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Sensors and Materials, Vol. 35, No. 2 (2023) 391–398
MYU Tokyo
S & M 3175

Design of Dental Mouth Prop
Hideyuki Yamamoto,1,2,3* Shintaro Hamasaki,4 Kaisei Tokieda,5 Ikuo Yamamoto,1,6,7
Keitaro Matsumoto,1,8 Takeshi Nagayasu,1,8 Seigo Ohba,2 and Yoshinori Sumita3**
1Medical-Engineering Hybrid Professional Development Center,
Nagasaki University Graduate School of Biomedical Sciences, 1-12-4 Sakamoto, Nagasaki 852-8523, Japan
2Department of Regenerative Oral Surgery,
Nagasaki University Graduate School of Biomedical Sciences, 1-7-1 Sakamoto, Nagasaki 852-8523, Japan
3Department of Medical Research and Development for Oral Disease,
Nagasaki University Graduate School of Biomedical Sciences, 1-7-1 Sakamoto, Nagasaki 852-8523, Japan
4Department of Integrated Engineering,Nagasaki University Graduate School of Engineering, 1-14 Bunkyo-machi, Nagasaki 852-8521, Japan
5Department of Mechanical Science,
Nagasaki University School of Engineering, 1-14 Bunkyo-machi, Nagasaki 852-8521, Japan
6Department of Mechanical Science,
Nagasaki University Graduate School of Engineering, 1-14 Bunkyo-machi, Nagasaki 852-8521, Japan
7Organization for Marine Science and Technology,
Nagasaki University, 1-14 Bunkyo-machi, Nagasaki 852-8521, Japan
8Department of Surgical Oncology,
Nagasaki University Graduate School of Biomedical Sciences, 1-12-4 Sakamoto, Nagasaki 852-8523, Japan(Received December 18, 2022; accepted January 30, 2023)

Keywords: mouth prop, oral health, cognitive dysfunction, dementia, elderly

Mouth props are widely used tools in clinical practice: from general dental practice to oral
care in residential elderly care facilities. Mouth props are particularly useful for keeping themouth open while administering oral care to persons in residential elderly care facilities who areunable to communicate with others. Currently, oral bites and such devices are used mainly as
mouth props in such cases. However, patients who have difficulty communicating resist theinsertion of the tools using their lips, which they often clench and defiantly refuse to open theirmouths. On such occasions, medical personnel and caregivers must wait until the range of mouth
opening is sufficient to insert a tool; this is a heavy burden for caregivers responsible foradministering dental and oral care during house calls. To resolve these difficulties, we developed
a mouth prop, which is easy to use by both medical personnel and caregivers, can be inserted
through a smaller range of mouth opening, can open the mouth by the application of force via thetool after insertion, and can prevent injury inside the oral cavity.

1.

Introduction

The world population has recently changed to a structure in which 20% of the population ismore than 60 years old.(1) The population aged 65 and over in Japan is 28.9% of the total, making
Japan the world’s most rapidly aging society.(2) Along with the aging population, approximately
80% of people aged 75 years or older in Japan have two or more concomitant chronic diseases:*Corresponding author: e-mail: yamamotohideyuki@nagasaki-u.ac.jp**Corresponding author: e-mail: y-sumita@nagasaki-u.ac.jp

https://doi.org/10.18494/SAM4284

ISSN 0914-4935 © MYU K.K.
https://myukk.org/

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about 60% of them have three or more.(3) Reportedly, about 20% of people aged 65 years or older
are frail elderly people who are assisted by their family members and friends.(4) The maintenanceof the oral hygiene of frail elderly people with multiple morbidities is a burden. The negative
cycle of deterioration in health because of poor oral hygiene has persisted as a significant anddifficult issue.(5,6)
In addition, the number of patients with dementia has doubled worldwide during the years
1990–2016. The number is estimated to have doubled every 20 years, presenting a major global
healthcare issue.(7,8)
The incidence of dementia is known to increase because of oral-related diseases resulting
from the deterioration of the oral environment. Many reports have described that the incidence
and severity of dementia are higher in proportion to the loss of teeth.(9–13) Others have explainedthat the incidence of dementia increases with the rate of periodontal disease.(12–15) The cognitive
ability also reportedly declines in proportion to the rate of increase of dental caries.(13) These
reports also indicate that elderly people with dementia might often be adversely affected by toothloss, periodontal disease, and dental caries when they require intervention in their oral
environment.(16,17) Self-care is difficult because of the declining cognitive ability. Care bycaregivers or family members as well as the education of caregivers in these matters are allneeded.(15,18)
Nevertheless, the priority of oral care is lowered by family caregivers and medical personnel
because of the resistance against care caused by a continued decline in cognitive ability, which
may manifest itself in a refusal to open the mouth.(19) Care-resistant behaviors (CRBs) areexperienced by most nurses during oral care at medical institutions. They pose major obstacles
to care, and addressing them requires much time and effort.(20,21) A survey questionnaire wasadministered in Japan to nurses in wards into which many patients who need help with oral care
had been admitted. About 30% reported difficulty in caring for patients who refused to open
their mouths. The nurses expressed the need for a tool to open the mouth for tasks ofadministering care.(22)Today there are mouth props such as versatile mouth gags and bite blocks for those who
follow directions to open their mouths. However, opening the mouth is difficult to do without
damaging the interior of the oral cavity when the patient cannot communicate or when thepatient resists mouth opening. These tools are not applicable to cases of multiple tooth loss,
which are often observed in dementia patients, or in cases involving a loosened tooth. Therefore,
in a clinical environment, an oral bite or other device may be used to hold the mouth open with atool made of material that does not damage the inside of the oral cavity when the mouth isopened. An important shortcoming of this method is that a physician must often wait a long time
for a patient to open the mouth. A long wait is not amenable to situations in which the mouth
must be opened immediately, such as when there is a risk of aspiration.
Although oral care becomes increasingly necessary as dementia advances, more CRBs canbe expected to occur as dementia progresses. The deterioration of the state of oral hygiene
because of heavier burdens of care is expected to engender further progression of dementia
symptoms. To resolve the various inherent difficulties, a novel device is needed so that oral care
may be carried out more simply and easily and to address needs for oral hygiene in elderlySensors and Materials, Vol. 35, No. 2 (2023)

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people with a declining cognitive ability. For that reason, we have developed a mouth prop that
does not damage the interior of the oral cavity. It does not damage gingival tissue and can beinserted easily. Moreover, it can open the mouth merely with the force exerted by a user.

2.

Design of Dental Mouth Prop

The device developed in this study was designed with the following objectives in mind: (1)The tool can be inserted through a small mouth opening gap. (2) There is no risk of the tool
being broken by the bite force of the patient or of fragments being swallowed. (3) The tool does
not damage the interior of the oral cavity, even when the molar area is edentulous. (4) The mouth
can be opened by a user applying reasonable force to the tool. (5) No excess force is applied
inside the oral cavity. (6) The mouth can be held open. (7) The tool does not come off easily. It
holds the molar area firmly even when the patient moves. The device consists mainly of twoparts: the main part and the tip.
Autodesk Fusion 360 was used to design the main part of the mouth prop. A tool that uses aclamp mechanism was made to respond to requirements 4, 5, and 6 listed in the previous
paragraph. The tip shape was designed to be thin when closed to respond to requirement 1(Fig. 1).
The shape of the tip of the mouth prop was also designed using Autodesk Fusion 360. The
mold was produced using a 3D printer. A silicone preparation was used to address requirement
2. The tip shape changes to enfold the gums when air is released so that the tool is useful when
there are no teeth. By holding its position firmly by deformation, requirements 3 and 7 aresatisfied (Fig. 2).
The main part of the tool consists of into three parts: the handle, the clamp mechanism, andthe opening. The handle was designed with a 77-mm-long fixed part and a 54-mm-long action
part. The clamp mechanism is at a maximum distance of 152 mm from the opening part. The
opening part is 85 mm long and 5 mm thick at the tip in a closed state. It can be opened to a
maximum distance of 61 mm. A circular motion of the handle is converted by the clamp
mechanism into a linear motion. The opening part is connected to the clamp mechanism so thatthe linear motion is converted into a circular motion at point A. A stopper is built into the handle.

Fig. 1.

(Color online) Prototype produced using a 3D printer.

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Sensors and Materials, Vol. 35, No. 2 (2023)(a)Fig. 2.

(b)(Color online) Silicone tip: (a) normal state and (b) under negative pressure.

The stopper is designed so that, when the handle is moved until it hits the stopper, the clamp
moves 2 mm, opening the tip of the mouth prop by 4 mm.
The tip is 24 mm thick in the normal state, 55 mm wide, and 14 mm tall. The thickness of the
top is 24 mm when the tip part is mounted on the main part, which is 27 mm less than the
maximum thickness of the versatile mouth gag.

3.

Manufacturing the Dental Mouth Prop

The handle of the main part of the prototype was prepared by Ultimaker Black Tough PLA,
using Ultimaker 5S as a 3D printer. The ring and slide rail of a quick bar clamp (LFX-20-277;
Kohnan Shoji Co., Ltd.) were moved to the clamp mechanism. The bracket was prepared by
Ultimaker 5S using Ultimaker Black Tough PLA. The opening was machined from pieces ofaluminum (Fig. 3).
For the tip of the prototype, a mold for casting silicone was prepared by Ultimaker 5S using
Ultimaker Black Tough PLA. The preparation protocol for silicone includes the following steps:(1) Design the mold.
(2) Print using a 3D printer.
(3) Pour silicone into the mold.
(4) Degas in the mold (10 min) (vacuum desiccator VW: AS One Corp.).
(5) Allow silicone to solidify at a normal temperature for 6 h.
(6) Remove the object from the mold.
The following silicone preparations were used: HTV-2000 (Engraving Japan Corp.), Reline II
ExtraExtraSoft (GC), EXAHIFLEX INJ (GC), and EXAHIFLEX RG (GC). Motions of the
preparations in response to negative and positive pressures were examined (Fig. 4).
The motion was greatest in the order of HTV-2000, Reline II ExtraExtraSoft, EXAHIFLEXINJ, and EXAHIFLEX RG. The motion was confirmed to be greater under negative pressure
than under positive pressure. EXAHIFLEX INJ was selected as the material for the model inthis study on the basis of its motion and ease of processing. We decided to use negative pressure
to induce motion.
A negative pressure device was prepared using a CHANCS385 diaphragm self-priming
pump (Fig. 5). A tube was connected to the silicone tip to move it (Fig. 6).

Sensors and Materials, Vol. 35, No. 2 (2023)

Fig. 3.

395(Color online) Main part of the prototype.

(a)

(b)

(c)

Fig. 4. (Color online) From the topdown: HTV-2000, Reline II ExtraExtraSoft, EXAHIFLEX INJ, and
EXAHIFLEX RG [(a) normal state and under (b) negative and (c) positive pressures].

Fig. 5.

(Color online) Negative pressure device.

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Sensors and Materials, Vol. 35, No. 2 (2023)

(a)

(b)

Fig. 6. (Color online) Silicone tip connected to the negative pressure device: (a) normal state and (b) undernegative pressure.

4.

Results and Discussion

An experiment was conducted using a dental model (ANA-3003-UL-JCP-D-28; Nissin).
Seven dentists with different lengths of clinical experience were asked to use the device on thedental model. They then evaluated the device via a questionnaire.
The items on the questionnaire were as follows.
Question 1: Was the handle easy to move?
Question 2: Was the force needed to move the handle appropriate?
Question 3: Were the time and number of pushes on the handle required for opening the mouth
appropriate?
Question 4: Was the force withstood by the locking function of the clamping mechanism
appropriate?
Question 5: W as the release of the locking mechanism easy to use?
Question 6: W as the size of the tip of the prototype appropriate for clinical practice?
Question 7: Was it possible to hold the tool stably while opening the mouth?
The seven items were evaluated on five levels from 1 to 5: 5 was chosen when the respondentjudged that it was sufficiently useful for practical application, 3 when it was acceptable for use,and 1 when it could not be used. Table 1 presents responses to the evaluation questionnaire.
The examination of the findings revealed the following information:
Question 1 (Was the handle easy to move?) showed that the shape of the handle was generally
good; however, a better shape is desired because a square part of the handle is uncomfortable.
Question 2 (Was the force needed to move the handle appropriate?) yielded good responses.
A respondent who answered that it was acceptable wanted the tool to move with less momentum.
Question 3 (Were the time and number of pushes on the handle required for opening themouth appropriate?) elicited good responses. A respondent who answered that they were
acceptable wanted more pushes with less maximum opening per push.
Question 4 (Was the force needed for the locking mechanism appropriate?) generally showed
good results. It was difficult to evaluate whether the tool could endure the strong bite force of an
actual patient because a model was used in this test.

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Table 1
Questionnaire for the mouth prop.
Evaluation item
Question 1
Question 2
Question 3
Question 4
Question 5
Question 6
Question 7

A
5
5
5
5
5
5
5

B
3
3
3
5
5
5
5

C
5
5
5
5
5
2
5

Evaluation
D
E
5
5
5
5
5
5
4
5
5
5
4
2
5
5

F
3
5
5
5
3
2
2

G
5
5
5
2
4
2
4

Mean
4.4
4.7
4.7
4.4
4.6
3.1
4.4

Question 5 (Was the release of the locking mechanism easy to use?) generally showed goodresults.
Question 6 (Was the size of the tip of the prototype appropriate for clinical practice?)
indicated that the size was in the acceptable range, but a smaller size would be needed for use
with some patients.
Question 7 (Was it possible to hold the tool held stably when opening the mouth?) generally
showed good results. One respondent wanted an increase in holding pressure.
These results show that the performance of the body of the prototype is sufficient for use. The
use of a smaller tip was suggested. Therefore, the shape of the tip needs to be developed further.

5.

Conclusions

We developed a device that can be inserted easily into and taken out of the oral cavity, andcan be used to open the mouth without damage. To date, only thick devices have had a suitable
shape to hold the teeth at the tip; other devices without a shape sufficient to hold teeth havedemonstrated a lower holding capacity. Our device is thin enough to be inserted into the oralcavity and can then change its shape to hold teeth by changing the shape of the silicone at the tip
using negatively pressurized air. Results of the evaluation of the mechanism of the main part andthe holding performance of silicone were good. However, the comments indicated that a smaller
silicone part was desirable. We are planning future improvements to yield a smaller size and ahigher holding capacity by changing the pattern of the silicone shape and the structure of the tip.
In the questionnaire completed by dentists, a concern was expressed about possible damage tothe instrument and strain on the temporomandibular joint (TMJ) due to high forces. Thissuggests that control is a necessary element to make the instrument safe for anyone to use. In the
future, we plan to improve the device by incorporating a pressure sensor so that it can only
operate below a certain pressure level, thereby avoiding damage to the device and strain on the
TMJ. We also plan to mechanize the mouth prop to control the amount of opening, the openingpressure, and the air pressure; with these improvements, the device could function as a new
training device for patients with opening disorders.

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Acknowledgments
This study was supported by the Medical–Engineering Hybrid Professional Development
Program, Nagasaki University Graduate School, and by Yamamoto Laboratory, Nagasaki
University School of Engineering. We extend our sincere gratitude for the support of allparticipants.

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