P.R. Wedendal and H. Bjelkhagen
Adapted from APPLIED OPTICS, Vol. 13, page 2481, November 1974
Scanned, filtered by optical character recognition. For details see original article. This resource is intended for engineering students at the University of Wisconsin.
Investigation in vivo of small deformation and mobility processes in the masticatory system of man has been until now a very intricate problem. Mechanical as well as noncontact methods have been utilized earlier in order to record the mobility pattern of teeth and prosthodontic appliances. In this paper holographic interferometry will be presented as a solution of some odontological measurement problems. The method was first tested in a simulator arrangement and then used in a number of clinical experiments. A special. totally reflecting paint was used for surface preparation prior to holography. A Q-switched double-pulsed ruby laser was combined with an electronic subminiature force sensor for pulse triggering, which was actuated by the masticatory force of the patient. Force increases and pulse positions were registered synchronously on the screen of an oscilloscope. The applied force exerted by the patient's masticatory muscles could thus be determined according to its point of application, direction, amplitude. and duration. The corresponding surface deformation was evaluated by means of a synchronized, double-exposed hologram. Conclusions could be drawn regarding the relative and absolute mobility of the teeth and related structures of the holographed jaw section.
Dynamics of Teeth in Function
The masticatory function of man actuates a three-dimensional intra-alveolar tooth mobility. Anatomical conditions, the magnitude and direction of the stress of mastication, the presence of pathological damage in the immediate vicinity of the tooth, and other factors act on the process. Tooth mobility has been studied by means of mechanical as well as noncontact methods. 1-4
There have been, however, considerable difficulties in adequately recording the complete dynamic pattern of teeth during function (i.e., depending on methodological errors and the disturbing effects of technical appliances). The presence of prosthodontic constructions in a patient's mouth has a certain influence on the type and degree of tooth mobility. Moreover, removable partial dentures have a rather great mobility tendency of their own. The mobility phases of parodontally anchored types of fixed bridgework are often very small and for this reason holographic interferometry has proved to be very suitable according to its measurement sensitivity.
Utilizing the ruby laser system described here, it is possible to obtain detailed information about functionally conditioned displacements and deformations of individual teeth and groups of teeth, as well as the functional dynamics of fixed bridgework in vivo.
P.R. Wedendal is with the Department of Stomatognathic Physiology, Faculty of Odontology, Royal Caroline Institute, Stockholm, Sweden, and H. 1. Bjelkhagen is with the Laser Re-search Group. Department of Production Engineering, Royal Institute of Technology, Stockholm, Sweden.
Received 4 .January 1974.
Ruby Laser System
Ruby laser Holobeam 651 was used for the investigation. The system consists of an oscillator and an amplifier and gives an output energy of 75 mJ per pulse during double-pulsed operation. Each pulse has a duration of 24 nsec.
Subminiature Pressure Sensor-Force Sensor
The subminiature pressure sensor used was a Kyowa, type PS-b KA. It consists of a metal cylinder closed at both ends with a 6-mm diameter and a 0.6-mm height. One of the flat end surfaces is very thin and easily deformed by pressure. Inside it are attached four extremely small strain gauges which are connected to a bridge circuit. The strain gauges are made of metal, which in combination with the full bridge arrangement provides the least possible influence of temperature on the measuring results.
In the present investigation a force sensor was required (Fig. 1). In order to transform the sensor
from a pressure sensor into a force sensor it was placed in a hollow cylinder made of steel. This was closed with a cover of about 0.2 mm thickness. A central steel rod of about 1.3-mm diameter and about 1-mm height projects perpendicular to the cover. The masticatory force is to be applied axially to the tip of the rod so that the cover is point loaded. The pressure sensor is located in the hollow cylinder so that the thin diaphragm of the sensor is deformed simultaneously with the cover of the cylinder. A soft cement serves as a medium for distributing the force. The diameter of the cylinder is about 7 mm and its height about 3 mm including the rod. It is provided with a cable shield of about 4 mm in length made of steel and having a rectangular section of about 2 mm )< 3 mm, through which is introduced a specially manufactured shielded four-wire conductor having a diameter of 2.5 mm. The subminiature pressure sensor is supplied with a stabilized voltage of 3 V
The output signal from the sensor is the unbalanced voltage produced in the measuring bridge by deformation. It has a maximum value of 1 mV per volt from the voltage supply. An amplifier made of Vibrometer is used as an amplifier for the output signal. In this system the output signal is connected to one channel of an oscilloscope. For reduction of noise from the sensor and the amplifier an integrating filter with a time constant of about 0.1 msec is used. Calibration of the sensor was made by loading with known weights and making suitable adjustment of the amplification. Calibration was established at 0.8 N/scale division on the vertical axis of the oscilloscope screen.
For registration of the laser pulses a photo detector of the silicon type (Siemens BPY-64) was used. It was placed so as to be illuminated by the laser beam. The signals were recorded on the second channel of the oscilloscope.
A two-channel storage oscilloscope (Tektronix model 549) was used. Since the flash duration of
the helical xenon arc-discharge lamp in the ruby laser was about 1 msec, a velocity of 200 Msec/cm in the horizontal axis of the oscilloscope was used.
Fig. 1. Subminiature force sensor. The masticatory force is concentrated on one point of application during the experiment by means of the vertical metal rod.
Method Development in Simulator Tests
As preparation for experiments in a biological environment, a test device was assembled with the intention to simulate the masticatory system of man. A pair of model sets of teeth (Frasaco) made of acrylic material and with the teeth individually screwed on to the base were fixed in an articulator of the type Dentatus ARL (Fig. 2). The degree of mobility of each tooth could be varied by tightening the axial fixing screw differently.
Acrylic material as well as living oral tissue has a certain degree of transmittance depending on the particular type of laser light used. For this reason it was necessary to invent a method for surface preparation prior to holography in order to obtain optimal visibility of the interference fringes in the hologram.
Several types of available plastic paint were tested. The best results were obtained, however, by using a special paint composed of gold dust, resin, CaOH2, and chloroform. This paint reflected the laser light totally.
The simulator arrangement was placed on a rubber plate on the holographic table in correct relation to the ruby laser, the reference mirror, and the hologram holder, according to the principle of the holodiagram .5 The resilience of the rubber caused a slight movement in the arrangement during the tests-corresponding to the patient's head movements. A number of tests were carried out in order to establish a technical routine for interference holography. In analogy with a planned patient test, the mobility of the artificial tooth 24 (see Table I) was examined as is described below.
The electronic force sensor was inserted between tooth 24 and its antagonists. The sensor was connected to the ruby laser via the oscilloscope and calibrated so that the first pulse of the ruby laser was to be released at a force of 2 N. This force level for pulse triggering seemed to be adequate with regard to biological conditions. The sweep rate of the oscilloscope was experimentally varied. A scale (horizontal) was chosen on the oscilloscope screen so that the laser pulses were distinctly positioned in the coordinate system. The force curve related to the pulses at the chosen sweep rate gave exact information about force increase between the pulses. Finger pressure was exerted on the upper movable part of the simulator. When the force level reached 2 N the first laser pulse was released. After 450 Msec, a second pulse of the same intensity as the first was released automatically.
By repeating the test under systematically varied basic conditions conclusions about simulator function could be drawn after comparisons were made.
Fig. 2. The simulator consists of an articulator-Dentatus Type ARL-frequently utilized within the dental laboratory technique. By means of this equipment it is possible to simulate jaw movements and tooth interrelations during the construction of prosthodontic appliances (crown- and bridgework, complete and partial dentures, etc.) The teeth of the acrylic models used in our experiments are individually screwed onto the base.
This system was accepted by resolution at the Federation Dentaire Internationale General assembly 1971. The first digit indicates the quadrant and the second digit the type of tooth within the quadrant.
Consideration was given to the utilization of ruby laser light in clinical experiments. The eyes were protected by means of black glasses, having no transmittance for light. A black wooden screen protected the face, leaving the jaw section exposed. Holography was performed with the same geometry arrangements in the holodiagram as in the simulator tests (Figs. 3~5). Salivation was inhibited prior to holography. For the development of clinical methods a female patient, 60 years old, was holographed.
All teeth were firmly attached to their sockets and no visible clinical mobility could be recorded. In analogy with the earlier simulator test, tooth No. 24 was chosen as the tooth to be examined holographically. The patient's lips and cheek were retracted by means of a translucent flat acrylic hook. The jaw section to be exposed was painted with gold paint, bright and smoothly adapted to the surface.
The cable of the subminiature force sensor was attached to an upper front tooth by means of a waxed silk ligature. The sensor was positioned in such a manner that the masticatory force was concentrated in the vertical rod, which was cemented to the occlusal surface of the tooth by means of acrylic cement. During the cementing the opposing teeth to be studied were kept simultaneously in contact with the lower metal surface, thus preventing bending and tilting of the sensor during the experiment. The hook as well as the cable to the sensor were located so as to not interfere with the masticatory process.
The patient was told to open her mouth widely and then bite together. When the masticatory force, thus applied to the sensor, reached a predetermined level of 2 N the first laser pulse was triggered. After a delay of 450 psec, the second pulse was automatically actuated. The double-exposed hologram was then developed and fixed. The force increase between the pulses could thus be compared with the hologram evaluation, and conclusions could thereby be drawn about deformation and tooth mobility. Owing to the experimental design used the force was fairly well defined with regard to point of application, direction, amplitude, and duration. The results of one registration are shown in Figs. 6 and 7.
Evaluation of Relative and Absolute Object Displacement
Object displacement is defined as the difference in the three-dimensional position of one particular point on the object surface, for example, a cusp of a tooth.
The muscle activity during mastication is estimated to cause two types of displacement. The absolute displacement is defined as the displacement of the cranium in relation to the holographic set up while the relative displacement is defined as the displacement of one tooth or a group of teeth in relation to the surrounding tissue. The main interest in this investigation was the relative displacement.
Fig. 4. Arrangement.
Fig. 5. Patient prepared for holography.
Static evaluation was used to determine motions in a direction that bisects the illumination and observation directions. The number of ellipses (in the holodiagram intersected during the delay between
the two pulses is used to determine the magnitude of the displacement. Either the hologram or a photographed reconstruction can be used for the static method. In this method the hologram is examined from only one position. The number of interference fringes on the tooth which is to be evaluated is corn-pared with the number of interference fringes on the surrounding teeth. The difference in the number of fringes reveals the amplitude of displacement with a resolution of h~(X/2). The direction (+ or -) cannot directly be interpreted from the fringe pattern. To solve this problem further experiments were made using the simulator.
Dynamic evaluation was used to determine the motion in a direction that is at right angles to the observation direction. The reconstructed double exposed hologram itself is needed for this evaluation.
The eye is moved around while the reconstructed object is observed through the holographic plate. Doing so, the observer sees the fringes move in respect to the virtual image of the object surface. A maximum motion of fringes is found for a certain direction of movement of the eye along the plate. This direction is identical with the motion of the object between the two exposures.
The dynamic evaluation is needed to extract an amount of the information stored in the holographic plate.
Results from Experiments in vivo
Evaluation of a Hologram from the Clinical Test
The mobility of the opposing teeth in the hologram was evaluated. These cusp displacements were inclined to a plane parallel to the holographic plate and were in the mesial direction.
The aim of this investigation was to develop a noncontact and nondestructive method for studies of the dynamics of human teeth and parodontally anchored prosthodontic appliances during function at low force levels.
The parameters of the experimental equipment were varied in an attempt to optimize the method. The subminiature force sensor could be calibrated for different pulse triggering levels and could be within wide limits during the experiment.
Fig. 9. Labial cusp displacement corresponding to 0.5 N force increase.
1. K. H. Kber, Zahnarztebtatt. 6, 605(1962).
2. Schweiz. Machr. Zahnk. lit, 1(1951).
3. Parfitt, J. Periodont. 32,102 (19611.
4. Arch.0ral Biol.9,565(l964).
5. N. Abramson, Appl. Opt. 8, 1235(1969).
6. N. Abramson, Appl. Opt. 9,97(19701.
7. N. Abramson, Appl. Opt. 11,1143(1972).