Project Number: P13061

Periodontal Measurement Test System

Raymond Boronczyk
Mechanical Engineering Student / Evan Lammertse
Mechanical Engineering Student
Samuel Remp
Electrical Engineering Student / Yokai Ro
Electrical Engineering Student
Ryan Shaw
Mechanical Engineering Student / Kristi Weaver
Mechanical Engineering Student

P13061

Abstract

The current method for performing the sulcus depth measurement is a painful, inconsistent and lengthy process. The Perio Alert system seeks to present a more effective, consistent and pain free measurement technique through the use of ultrasound. The design includes a mechanical fixture, a tooth phantom and electronic programming in Labview.The fixture allows an ultrasonic transducer to be moved through five axes to perform the sulcus depth measurement accurately and repeatedly without physically entering the sulcus. The final product can move the transducer through all axes while detecting material interfaces (and therefore changes in material) and storing the data for post-test analysis.

introduction

Periodontal disease affects over 50% of adults worldwide and is the leading cause of tooth loss. When periodontal disease has set in, the depth of the gap between the tooth and the gums (also known as the sulcus) has become greater than the baseline of one to three millimeters. Currently, the presence and extent of periodontal disease are determined using an invasive metal probe to measure the sulcus depth as seen in Figure 1 below. This process is time consuming, inconsistent and painful. Therefore, Perio Alert is in the development stages as an attempt to simplify the sulcus depth measurement process while improving its reliability and associated patient comfort. This concept uses an ultrasonic transducer to perform the sulcus depth measurement from outside the patient’s mouth. Implementing this method would allow for a repeatable measurement, thus reducing the amount of error associated with the process, all while creating a more pain-free, hands-off procedure.

Figure 1: Current sulcus depth measurement method

process and methodology

This project encompasses developing a method for testing a 10MHz ultrasonic transducer while performing the sulcus depth measurement. The main objectives of the project can be broken into three categories: the tooth phantom, the mechanical test fixture, and the electrical program.

A tooth phantom must be constructed to facilitate sulcus depth measurements throughout testing. The phantom should be representative of a human tooth with respect to ultrasonic properties. The phantom must allow for the sulcus depth to be varied in a reasonable amount of time.

The mechanical fixture must be capable of holding and maneuvering the transducer and the tooth phantom. All movementsmust be accurate and repeatable to ensure the measurements are representative of the sulcus and can be performed by multiple users. The test fixture must be compact and easily transported to allow for simple integration into an office setting. It should also allow for repairs and adjustments to be performed by untrained personnel, requiring the composition of an instruction manual.

Lastly, a system and/or program must be developed to serve as the user interface for the measurement process. This system should be simple yet effective, leaving little room for user error. It should also provide meaningful feedback so the sulcus depth measurement can be easily interpreted and explained. The system must also be capable of storing the data from each test in a designated location for post-test analysis and comparison to previous results.

The use of an ultrasonic transducer to perform the sulcus depth measurement is a relatively new concept. A pulser/receiver unit is used to transmit a signal from the transducer. When an ultrasonic wave is transmitted from the transducer it passes through materials at varying rates. The rate at which the wave passes through the material is based upon the ultrasonic properties of the material, such as acoustic impedance, density and speed of sound. When the wave encounters a new material, it will bounce back to the pulser/receiver and a voltage can be measured. Theoretically, this voltage could then be combined with distances traveled by the transducer to calculate the depth of the sulcus.

ULTRASONIC TRANSDUCER

Theproject readiness package (PRP) indicated that a 10 MHz ultrasonic transducer was available for use, as well as an oscilloscope and a pulser/receiver unit. However, some logistical issues arose and the team became responsible for purchasing a transducer. The team selected a 10 MHz contact probe offered by Olympus. Unfortunately, the team believes that 10 MHz is not a high enough frequency to detect the minute differences present in human tissue surrounding the sulcus. Furthermore, Olympus offers various styles of transducers (i.e. contact, immersion, etc.) that may have performed better for this particular application. However, due to limited time, budget, and knowledge the team was forced to make a selection to avoid losing schedule.

TOOTH PHANTOM

Initially, it was assumed that dentin, enamel, gums and bone must be considered for the tooth phantom to ensure that it closely resembled a human tooth. However, after multiple conversations with experts in the fields of periodontal and ultrasonic studies it was determined that the aforementioned four constituents of human teeth were not all necessary. The enamel need not be considered because when periodontal disease is present the enamel has been worn away. Furthermore, the use of a 10 MHz transducer and an oscilloscope introduce limitations on the data processing. Therefore, the tooth phantom design consists of materials representing dentin, gum tissue and bone.

Once the pertinent features of a human tooth were agreed upon baselines for the ultrasonic properties were determined. This process was difficult as little research has been performed regarding ultrasonic behavior within the mouth. However, after extensive digging, a subject study was found that included 42 adults (36 women and 6 men). The relevant ultrasonic properties include density, speed of sound within the material and acoustic impedance. A summary of the baseline properties used can be found in Table 1 below. Various non-biological materials were then researched in an attempt to find materials that are similar to the baseline human tooth properties, thus satisfying customer needs 8, 9 and 10 (specification 3.1). It was determined that concrete closely resembles the dentin, brick closely resembles mandibular bone and paraffin and polyurethane both closely resemble soft tissue (gums).

One main concern with the materials initially selected was the ability to manipulate them. Ultimately, when ultrasound attempts to pass through bone it is “blocked” or the signal fails to pass through the material and simply bounces back. That being said, to mimic bone the acoustic properties of the material chosen did not necessarily need to be similar to bone, rather a material must be found that will not allow ultrasonic waves to propagate through it. This opened up more options for the bone phantom material, allowing for easier selection and manipulation. Therefore, it was agreed that a square aluminum tube could be used to represent the bone. The gums were still to be modeled using paraffin and the dentin would be represented by concrete.

Table 1: Relevant ultrasonic properties for material selection

The tooth phantom design can be seen in Figure 4 below. Two options were considered for meeting customer needs 1 and 4 (specification 3.3),the ability to vary the sulcus depth. The first option would require that each phantom be able to vary its own sulcus depth, i.e. the gum phantom would be able to move relative to the dentin phantom, or vice versa. This option could create complications with regard to accuracy and repeatability. The second option was that separate tooth phantoms would be used, each with a different sulcus depth. This option would require some extra fixture design to incorporate multiple tooth phantoms. However, this option helps to satisfy customer needs 1, 3, 4 and 11 (specification 3.2) of being able to vary the sulcus depth in a reasonable amount of time as the fixture simply needs to orient itself to measure the next tooth phantom.

Figure 4: Final tooth phantom design

The final design consideration for the tooth phantom was its interface with the fixture. A plate was incorporated on the bottom of the tooth phantom. The addition of this plate allowed for the tooth phantom/turn table interface to be separate from the tooth phantom itself. This makes for an easy assembly/dis-assembly process that does not involve the tooth phantom materials. Each plate will remain mated to the tooth phantom it is initially paired with. A simple bolt pattern (eight holes) will be machined on the turn table to allow for two bolts to fasten each plate to the turn table. A few options were available for attaching the tooth phantom to the mounting plate; a combination of Scotch Extreme Mounting Tape and JB Weld proved to be the best option as it would not interfere with the turn table (as otheroptions might have).

Initially, the tooth phantom was designed to incorporate a representative sulcus. However, the project scope was redefined to simply require for the detection of material interfaces. Therefore, the presence of a sulcus was not necessary. The aluminum tube was cut into sections and a portion of a credit card was folded around the tube to allow for a flat surface as the concrete formed. The concrete was poured into the tube and leveled off with the credit card foundation. This assembly was then set aside to dry and the credit card was removed. The interface between concrete and aluminum was cleaned up and the gum material was applied.

The formation of the tooth phantom provided some unforeseen difficulties with regard to the soft tissue representation. Initially, paraffin wax was to be cut to the proper shape and size then slid over the square aluminum tube. However, upon attempting to cut the paraffin wax it became evident that the design was too thin for the paraffin to accommodate, as the paraffin simply shaved away. The team tested various materials that could be easily formed and would respond to the ultrasonic probe. Ultimately, two gum phantom materials were selected and tested, rubber and fabric paint.

MECHANICAL FIXTURE

The mechanical fixture portion of this project is responsible for positioning the ultrasonic transducer to make contact with the tooth phantom and perform a measurement. Based on customer need 5 (specifications 1.1, 2.1 and 4.1) the system must allow for five axes of movement so that the probe can reach the tooth and possibly perform measurements at various angles. The general design of the positioning system follows the logic of a basic computer numerically controlled (CNC) machine design and can be divided into the following three areas: three linear axes of movement, pitch, and yaw.

The three linear axes of movement involve the lateral, longitudinal, and vertical motions of the system. These axes of movement all utilize ACME lead screws to translate platforms, similar to a lathe, mill or simple CNC machine. Lead screws were integrated into the design since they are both accurate and relatively low priced. The lead screws chosen for this application are 3/8” diameter 303 stainless steel. The vertical application uses a 0.100” pitch screw while the lateral and longitudinal use 0.083” pitch screws. The screws are coupled with anti-backlash nuts made from a low friction lubricated plastic. These screw and nut assemblies mount via steel L brackets to a sliding platform made of Delrin plastic for each of the three axes. These platforms are supported by two ¼” diameter stainless steel guide rods each to remove some loading from the screw. Furthermore, as the name suggests, these rails also guide the platform along a straight line without allowing rotation. To support the small radial loads transmitted to the screw and provide a positive location to limit misalignment with the motor, two small steel ball bearings are used on the non-motor end of the screw. The radial ball bearings are more than capable of withstanding the small axial loads in the system. To keep the design somewhat flexible, simple setscrew collars are used to transmit the axial loads of the screw to the bearings. This allows for the system to be assembled loosely and then tightened down avoiding preloading the screw, housing, or the motor. The motor end of the screw is also turned down to accept a set screw style coupling that links the shaft to the mounted stepper motor. Flats were filed onto the screw shaft to ensure the coupling would transmit the motor torque.

Figure 2: Mechanical fixture

One of the major concerns surrounding the three linear axes of movement was friction. The lead screw pitch and the low friction material of the nut were chosen to amplify the torque of the system and minimize friction.Originally, the plastic sliding platforms were to be directly in contact with the guide rods. After initial assembly, the realistic misalignment of the guide rail bores inside the sliding platform was too great and caused a large amount of friction in the system. To combat this friction and stay within the torque specification of the stepper motors, the tables were fitted with oil impregnated bronze linear bearings. These bearings are pressed slightly into the end of the guide rail bores to limit the amount of surface area in contact with the guide rods and to keep the number of positive locations of components to a minimum.Locating the sliding platforms with the screw mount (L-bracket) and the guide rods can cause misalignment if components were not machined absolutely perfect. To overcome this, the platforms are consistently located using the guide rods, which are intended to carry the load, and then the screw is attached to the L-bracket in its neutral position.

The next component of movement is the pitch axis. This axis is responsible for mounting the transducer as well as controlling the angular pitch of the transducer using a servo motor. The entire pitch axis assembly is mounted to the vertical sliding platform. The pitch axis consists of three rectangular mounting blocks, a shaft, the probe mounting fixture and the servo motor, as seen inSection A of Figure 2.The probe mounting fixture is pinned to the shaft and then placed between the first two mounting blocks. The shaft continues to the third mounting block, which serves as a mount for the servo motor. Oil impregnated bronze bushings are used to reduce the friction of the system and support the shaft radially.

One important aspect of the pitch axis is the need for the transducer to be replaced with another if needed. Nearly all of the transducers available are very small and have a cable connected to them; therefore, theholding fixture must to be designed specifically for each transducer. A two piece design was selected to minimize the amount of machining required. This assembly can be seen in Section A of Figure 2. The larger probe mounting fixture which is pinned to the shaft is designed to have a channel which the probe holding fixture rides in. The probe holding fixture itself is two small pieces of aluminum that clamp the transducer between them.

A second important feature of the pitch axis is to ensure that contact with the tooth phantom is maintained throughout the test and that the system is not rigid, asstiff contact could damage the probe. To accomplish this, the transducer holding fixture is spring loaded. The channel in the mounting fixture was made longer, and a spring was placed between the back wall of the channel and the aluminum holding fixture. The transducer must have about 3/8” of give to allow for safe contact. One hurdle that was encountered was the need to have a stop built in to the travel so that the spring would always be engaged but would not force the aluminum holding fixture out of the channel. The solution was to increase the length of one of the bolts fastening the holding fixture together and mill a slot into the steel holding fixture where the bolt would ride.