WO1993018709A1

WO1993018709A1 - Dental force transducer

Info

Publication number: WO1993018709A1
Application number: PCT/CA1993/000121
Authority: WO
Inventors: Jeffrey W. Osborn; Jian Mao
Original assignee: The Governors Of The University Of Alberta
Priority date: 1992-03-24
Filing date: 1993-03-24
Publication date: 1993-09-30
Also published as: AU3882593A; GB9206368D0

Abstract

A dental force transducer (7) for measuring human jaw muscle activity, comprising a hollow body (9, 11, 13, 15) having a pair of horizontal surfaces adapted to be clenched between upper (19) and lower teeth (21) for urging the pair of horizontal surfaces (9, 11) toward each other, a pair of vertical surfaces (13, 15) connected to respective opposite ends of the pair of horizontal surfaces such that the pair of vertical surfaces become deformed in response to the pair of horizontal surfaces being urged toward each other, and a plurality of strain gauges (25, 27, 29) mounted to the pair of vertical surfaces for exhibiting a change in resistance proportional to deformation of the pair of vertical surfaces, thereby indicating magnitude of bite force of the upper and lower teeth on the pair of horizontal surfaces from a location surrounding the teeth.

Description

DENTAL FORCE TRANSDUCER

Field of the Invention

This invention relates in general to force transducers, and more particularly to a novel dental force transducer for measuring human jaw muscle activity. Background of the Invention

The major function of human jaws is to break up food and pass the food to the remainder of the alimentary canal. To this end, teeth implanted in the maxilla and mandible are provided for breaking up the food. In order to move the mandibular teeth with sufficient force to break up food, human jaws are provided with far more muscle mass than is required to merely move the mandible. Furthermore, the human jaw is provided with many more differently oriented muscle elements than are required for producing bite forces in the directions needed to break up food. For example, each masseter muscle has differently oriented superficial, intermediate and deep elements.

It has been determined through research that, in order to create computer models of human jaw mμscle closing patterns, six parameters of bite force must be simultaneously measured. These six parameters include magnitude of bite force, direction of bite force in three dimensions (two angles) and point of application (three coordinates in three dimensions) .

It is known in the art to measure static bite forces while a subject clenches on a pair of teeth. Specifically, prior art techniques are known for measuring the magnitude of the bite force by placing a force transducer between the teeth. One such prior art technique uses strain gauge rosettes to measure the magnitude and direction of the bite force in the sagittal plane while a subject clenches on his/her incisors.

According to this prior art technique, it was assumed that the forces were symmetrical about the sagittal plane so that the magnitude and directions in the frontal plane were not measured. More recently, investigations undertaken at the Academic Centre for Dentistry in Amsterdam have used.a commercially available piezoelectric sensor to measure the magnitude and direction of a bite force in three dimensions. A description of this technique is provided in an article by T.M.G.J. van Eijden et al. , entitled "A Feedback Method to Determine the Three-Dimensional Bite- Force Capabilities of the Human Masticatory System", published in the Journal of Dental Research, February, 1988. According to the van Eijden technique, all six parameters of static bite force were measured while the EMG activity in selected jaw muscles was monitored. Brief Introduction to the Drawings

A description of the known prior art and preferred embodiment of the present invention is provided herein below with reference to the following drawings, in which:

Figures 1A, IB and 1C show the prior art van Eijden dental force transducer in side view; .frontal view; and enlarged cross-sectional frontal view, respectively.

Figures 2A, 2B and 2C show the dental force transducer of the present invention in side view; frontal view; and enlarged cross-sectional frontal view, respectively;

Figure 3 shows the dental force transducer of the preferred embodiment in perspective; and

Figure 4 shows a video screen display of signal output from the dental force transducer shown in Figure 2.

Detailed Description of the

Prior Art and the Preferred Embodiment

Turning to Figures 1A, IB and 1C, the prior art van Eijden force transducer is shown comprising a piezoelectric sensor 1 sandwiched between a pair of steel plates 3. In operation, the two stainless steel plates 3 (approximately 2 mm thick) cover the upper and lower dental arcades. As shown best in Figure 1C, the sensor 1 is positioned between the plates 3 at the point where the bite force is to be measured. The steel plates are stabilized on the teeth by quick-set acrylic 5. A significant disadvantage of the prior art van

Eijden transducer is that the dimensions are very large. Specifically, the full thickness of the transducer may be calculated as follows: at least 10 mm (thickness of the sensor 1) + 2 x 2 mm (thickness of the upper and lower stainless steel plates 3 of the housing) + 0 mm (assuming that the thickness of the acrylic 5 is negligible) resulting in a combined thickness of at least 14 mm.

Thus, although the prior art van Eijden transducer may be used to accurately measure the six parameters of the force exerted on the piezoelectric sensor 1, the upper and lower dental arcades must be separated by a large distance (e.g., at least 14 mm). In practice, it is very uncommon for human beings to apply bite forces via teeth which are separated by 14 mm or more.

Accordingly, with reference to the dental force transducer 7 of the present invention as shown in Figures 2A-2C, a system is provided for measuring bite forces between teeth which are separated by a realistic distance (e.g. 2 mm) .

The housing of the bite force transducer 7 of the present invention is preferably fabricated from a gauge 20 stainless steel. This suitable gauge of stainless steel may be obtained from Alex Alloys, of Edmonton, Alberta. The stainless steel housing is bent and welded into a hollow H-shaped cross-section as shown best with reference to Figures 2B, 2C and 3. The upper and lower surfaces 9 and 11 of the cross-bar of the H- shaped transducer 7 are spread preferably no more than 2 mm apart, as shown by dimension A in Figure 3. The spacing B between outer surfaces of the vertical plates 13 and 15 is approximately 25 mm. The width C of each vertical plate 13 and 15 is approximately 15 mm and the height D of each vertical plate 13 and 15. is approximately 17 mm.

Thin layers of quick set acrylic (such as Duralay™, manufactured by Dental Manufacturing Co., Worth, Illinois, U.S.A.), are placed in the upper and lower recesses of the H-shaped transducer 7, and the subject then bites into the acrylic layers until the upper and lower surfaces 9 and 11 of the cross bar are encountered. Once the acrylic is set, the transducer is removed from the patient's mouth and the acrylic 17 is trimmed so that it surrounds the occlusal surface of each tooth (Figure 2C) . The acrylic provides a stable base for the upper and lower occluding teeth 19 and 21 in the upper and lower recesses between the vertical arms 13 and 15 of the H-shaped transducer.

As shown in Figure 3, according to the present invention, a strain gauge rosette 23 is installed on the outer surface of each vertical plate 13 and 15 of the H- shaped transducer (the strain guage rosette which is mounted to surface 15 being obscured from view in Figure 3) . The strain gauge rosettes 23 are of well known construction, (e.g. WK-06-060WR-350 manufactured by Micro-Measurements Inc. , Raleigh, North Carolina, U.S.A.). After installation of the strain gauge rosettes 23 on each vertical plate of the transducer, a coating of moisture-proof glue is applied (e.g. M-Bond AE-10/15 manufactured by Micro-Measurements Inc.). Each rosette contains three independent strain gauges 25, 27 and 29, as shown in Figure 3. Each strain gauge is connected to a 1/4 Wheatstone bridge (not shown) , of standard construction (e.g. Model 84 Strain Gauge Conditioner) to which an excitation voltage of 3 volts is applied.

According to the preferred embodiment, the six outputs (three outputs from each of the two rosettes 23 per transducer) are converted to digital signals at a rate of 1Hz. The analog-to-digital conversion may be accomplished by any one of a number of well known circuits, such as a 12-bit A/D converter (e.g. Model 233) . The digital values for the detected force are then transmitted to an IBM interface card (Model 802) , which is installed in an IBM XT computer or compatible, by means of appropriate computer software (e.g. Quicklog™) .

As discussed above, and as shown specifically in Figure 3, each strain gauge rosette 23 contains three strain gauges 25, 27 and 29 oriented at 45° to each other. When a load is applied to the cross-bar 9, 11 of the H-shaped transducer, the vertical arms 13 and 15 of the transducer are deformed. Deformation of the cross¬ bar results in consequent deformation of the strain gauges 25, 27 and 29 cemented to the vertical surfaces, causing a change in resistance of the strain gauges.

Each change in resistance is proportional to the amount of the distortion. Therefore, by comparing the differences between the distortion of the two anterior gauges with the two posterior gauges, the direction of the force in the sagittal plane may be determined. By comparing the differences between the left and right gauges, the direction of the force in the frontal plane may also be determined.

According to a successful prototype of the invention, the transducers 7 were calibrated outside of the human mouth using standard loads of 10 N to 150 N at 10 N intervals. The directions of the loads were 0°, 15° and 30° away from the vertical axis at 45° intervals around a complete 360° in the horizontal plane. By comparing the magnitude of changes in the output from the six gauges per transducer, the equations were formulated for predicting the magnitude and the direction in the sagittal and frontal planes of an unknown load. Experimental results using the successful prototype have confirmed that by subjecting the transducer (outside of the human mouth) to a known load of up to 150 N in a known direction, the parameters predicted by the formulated equations are within ± 5 N, and ± 2° in an antero-posterior direction, and ± 4° edio-laterally of the true parameters. Since the strain values recorded by the strain gauge of the present invention are linearly related to the applied stress, the formulated calibration, equations result in a comparable accuracy for any loads within the range of those which can be applied by clenching the human teeth. Furthermore, since the strain guages are attached to a steel housing in the preferred embodiment, the effect of temperature variations for the gauges of the present invention is reduced to negligible values.

As discussed above, the results of dental force detection using the transducer of the present invention are converted to digital form and displayed via an IBM XT computer, although, other suitable computers or display means may be utilized without departing from the sphere and scope of this invention. A software program, written in BASIC, is used in the preferred embodiment to calculate the magnitude and direction of the bite forces from the received strain gauge force values. The results are then displayed graphically on a video monitor at 1 Hz so that a human subject can "see" the actual bite force and therefore produce the required bite force. A representative video display is shown in Figure 4.

For example, the dental force transducer of the present invention may be placed in position on the left first molar tooth, and the human subject may be asked to produce a bite force of 220 N at an angle of 8° forward from vertical in the sagittal plane and 15° to the right of vertical in the frontal plane on the first left molar tooth of interest. Three large concentric circles, divided into quadrants by a cross (see Figure 4) , then serve as a map for the direction of the bite force. The horizontal arms of the cross show medial, lateral, backward and forward directions. The concentric circles show angles of 10°, 20°, and 30° away from vertical. A yellow circle (small empty circle in Figure 4) is programmed so as to appear on the screen in the position that corresponds with the required direction of the. bite force (in this example 8° forward and 15° to the right) . The actual direction of the bite force being produced by . the subject is then displayed as a blinking red circle (small shaded circle in Figure 4) . The magnitude of the bite force is displayed on a vertical scale. The objective of the subject is to maintain a bite force of the required magnitude with the blinking red circle in the centre of the fixed yellow target circle.

The six columns of numbers appearing at the bottom of Figure 4 show the strain gauge values recorded by each of the respective strain gauges. These values are continually updated and scrolled across the display to serve as reassurance to the operator that the equipment is producing appropriate strain gauge values.

In summary, the invention of the present invention provides accurate detection of all six parameters of bite force (i.e. coordinates) required to analyze human masticatory function, utilizing a novel force transducer wherein the patient's teeth are separated by a reasonable distance.

Other modifications and variations are the possible. For example, the inventors have realized that the transducer of the present invention has potential scientific and industrial applications far beyond mere dental applications. For example, applications requiring micromeasurements of forces in three dimensions in small or confined areas may benefit from use of the novel force transducer according to the present invention.

Also, it is contemplated that the transducer may be made of generally U-shaped rather than being H-shaped, with the compressive force being applied across the bottom of the U-shape and strain guages being mounted on the outer surfaces of the vertical portions of the U-shape. All such modifications and variations are believed to be within the sphere and scope of the invention as defined by the claims appended hereto.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OF PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A force transducer, comprising a hollow body having a pair of horizontal surfaces to which a compressive load is applied for urging said pair of horizontal surfaces toward each other, a pair of vertical surfaces connected to respective opposite ends of said pair of horizontal surfaces such that said pair of vertical surfaces become deformed in response to said pair of horizontal surfaces being urged toward each other, and a plurality of strain gauges mounted to said pair of vertical surfaces for exhibiting a change in resistance proportional to deformation of said pair of vertical surfaces, thereby indicating said compressive load remote from application of said compressive force.

2. The force transducer of claim 1, wherein said hollow body is generally H-shaped, said pair of horizontal surfaces forming a cross-bar of said generally H-shaped body and said pair of vertical surfaces forming opposite vertical sides of said generally H-shaped body.

3. The force transducer of claim 1, further comprising a pair of layers of quick set acrylic placed on said pair of horizontal surfaces to provide a stable base for receiving said compressive load.

4. The force transducer of claim 1, wherein said plurality of strain gauges are arranged as a pair of strain gauge rosettes mounted on outer portions of respective ones of said pair of vertical surfaces.

5. The force transduce of claim 4, wherein each of said strain gauge rosettes comprises three strain gauges oriented at 45° to one another.

6. The force transducer of claim 5, wherein said strain gauge rosettes are mounted to said.pair of vertical surfaces using moisture-proof glue.

7. The force transducer of claim 1, wherein said hollow body is fabricated from stainless steel.

8. The force transducer of claim 1, wherein said pair of horizontal surfaces are disposed approximately 2 mm apart when no load is applied thereto.

9. The force transducer of claim 1, wherein said pair of vertical surfaces are disposed approximately 25 mm apart when no load is applied to said pair of horizontal members.

10. The force transducer of claim 1, wherein each of said pair of vertical surfaces is characterized by.a height of approximately 17 mm.

11. The force transducer of claim 1, wherein each of said pair of vertical surfaces is characterized by a width of approximately 15 mm.

12. A dental force transducer for measuring human jaw muscle activity, comprising a hollow body having a pair of horizontal surfaces adapted to be clenched between upper and lower teeth for urging said pair of horizontal surfaces toward each other, a pair of vertical surfaces connected to respective opposite ends of said pair of horizontal surfaces such that said pair of vertical surfaces become deformed in response to said pair of horizontal surfaces being urged toward each other, and a plurality of strain gauges mounted to said pair of vertical surfaces for exhibiting a change in resistance proportional to deformation of said pair of vertical surfaces, thereby indicating magnitude of bite force of said upper and lower teeth on said pair of horizontal surfaces from a location surrounding said teeth.

13. The force transducer of claim 12, wherein said hollow body is generally H-shaped, said pair of horizontal surfaces forming a cross-bar of said generally H-shaped body and said pair of vertical surfaces forming opposite vertical sides of said generally H-shaped body.

14. The force transducer of claim 12 , further comprising a pair of layers of quick set acrylic placed on said pair of horizontal surfaces to provide a stable base for receiving occlusal surfaces of said upper and lower teeth.

15. The force transducer of claim 12, wherein said plurality of strain gauges are arranged as a pair of strain gauge rosettes mounted on outer portions of respective ones of said pair of vertical surfaces.

16. The force transduce of claim 15, wherein each of said strain gauge rosettes comprises three strain gauges oriented at 45° to one another for measuring said bite force in sagittal and frontal planes.

17. The force transducer of claim 16, wherein said strain gauge rosettes are mounted to said pair of vertical surfaces using moisture-proof glue.

18. The force transducer of claim 12, wherein said hollow body is fabricated from stainless steel.

19. The force transducer of claim 12, wherein said pair of horizontal surfaces are disposed approximately 2 mm apart when no load is applied thereto.

20. The force transducer of claim 12, wherein said pair of vertical surfaces are disposed approximately 25 mm apart when no load is applied to said pair of horizontal members.

21. The force transducer of claim 12, wherein each of said pair of vertical surfaces is characterized by a height of approximately 17 mm.

22. The force transducer of claim 12, wherein each of said pair of vertical surfaces is characterized by a width of approximately 15 mm.