This project looked to improve upon a Continuous Passive Motion device (CPM), which is commonly used to regain a normal range of motion (ROM) after a knee injury. The design addresses the current issue of patient compliance with the prescribed rehabilitation program.
An interactive patient monitoring system was designed that encourages usage and progression. The CPM device has been interfaced with a microcontroller to record data pertaining to the user’s rehabilitation session. This data is then extracted to a graphical user interface (GUI) to simultaneously display the patient’s progress in comparison to their baseline measure of their healthy knee on a laptop screen. Additionally, within this GUI the patient’s progress will be saved and transferred directly to their physical therapist via email in an easy to read report of the therapy session.
Introduction
A current commercial product used to induce knee motion is a continuous passive motion (CPM) machine. A CPM machine is a device often used on patients who have undergone knee surgery, such as a total knee arthroplasty (TKR) or an ACL reconstruction. The device passively flexes and extends the knee at slow speeds, which helps to reduce the swelling that may lead to joint stiffness.
There is great utility for these devices, but there are significant limitations which have motivated this project. The most prevailing limitation is the lack of patient compliance. According to physical therapists (PT) and orthopedic surgeons, current devices on the market show a common correlation between patient compliance and the overall success of their rehabilitation (Gebhadt, 2013). This device is generally sent home with the patient, where extensive periods of time, ranging between one to eight hours, are assigned as a part of their rehabilitation regimen. Currently, patients often do not abide to this lengthy process and struggle in their recovery process. Once the patient is permitted to begin the first stages of their rehabilitation process, long term or even permanent restrictions in their movement can result, if passive motion is not applied to the joint consistently.
Market Summary
Due to current methods of product distribution for this device, our device would be sold to medical device distributors that provide rentals services for patients. Our device would be seen as a necessity to physical therapists, because the current demands for these ROM rehabilitation devices are only increasing. There are two common knee surgeries which require the CPM device as a part of their regular rehabilitation process after surgery.
Additionally, insurance providers would be a strong proponent for the implementation of our device. Our device could be used as a means of financially justifying the rehabilitation expenses and insurance company pays for during a patient’s rehab process. It is likely an insurance company would like to see whether or not a patient is actually using the device, a service for which they pay, and whether or not the device is aiding in their overall recovery.
Knee surgeries are very common. In 2009, the NIH estimated that over 500,000 total knee replacements (TKR) were performed in the United States, and that number is expected to increase to 3.48 million by the year 2030.[1] Furthermore, the number of meniscus surgeries is also very high, with approximately 1 million performed every year in the United States alone. [2] Of these cases, over 600,000 experience severely impaired joint motion. In order to regain normal joint function the use of a CPM device is often required by the physician, especially for all total knee replacements patients.
Knee surgeries are very common. In 2009, the NIH estimated that over 500,000 total knee replacements (TKR) were performed in the United States, and that number is expected to increase to 3.48 million by the year 2030.[1] Furthermore, the number of meniscus surgeries is also very high, with approximately 1 million performed every year in the United States alone. [2] Of these cases, over 600,000 experience severely impaired joint motion. In order to regain normal joint function the use of a CPM device is often required by the physician, especially for all total knee replacements patients.
After searching extensively for competitive products on the market, several devices were found on the market, however, only one device was found to have similar desired features. A quantitative comparison of the competitive products can be seen in the House of Quality (reference Table 2).
The device found to be our closest competitor is the Otto Bock Knee CPM. This is the only device on the market that has capability of storing data from each therapy session. However, the only data it captures is the maximum angle of flexion and extension reached during that particular session and how long the device is used for. All of this data is stored on a SD memory card, which is small and can be lost very easily. The Otto Bock CPM is the lightest CPM on the market at only 24 pounds. The calf length is from 16.5” to 24” while the adjustable thigh length is from 12” to 17”. This device appears to accommodate larger patients due to the adjustable lengths. The range of motion of this device is -5° extension to 120° flexion. No price could be found for this knee CPM. Of these devices, there are no competitive products that provide any means of quantification for the stiffness or torque produced by the knee.
Customer Wants and Needs: Design House of Quality
In order to improve patient compliance and rehabilitation success, implementation of a monitoring device is needed. The monitoring device will record the progress of angle achieved, set a baseline value of the patient’s “healthy” knee’s stiffness, set a comparative stiffness value to the patient’s “unhealthy” knee, and compile a simple graphical analysis of data. The data collected will provide the PT with a more qualitative indicator of the patient’s progress and/or illustrate areas of the regimen (time, degree of flexion/extension, etc.) that present difficulty to the patient to achieve.
Interviews with physical therapist and patients led to a House of Quality analysis of the product to determine design specifications. The biggest concern of the physical therapist is to make the device more usable for patients, in order to increase compliance. Additionally, holding patient’s accountable by recording each therapy session will also increase compliance.
The wants of the Physical Therapist were the biggest concern because they will be the ones choosing to use the product for their patients. These wants were translated into our specifications by determining how we can link them together. For example, the confidentiality can be linked to the specification of having security. The resistance sampling rate can be tied to the physical therapists wants of having data representation and visual feedback of the patient’s progress.
Via serial communication between a microcontroller and MATLAB, our device will be able to save and record data in real time, as well as send a detailed email report to the Physical Therapist after each of a patient’s rehabilitation session. Using the GUI, the PT will have easy access to a simple report and graphical representation of the patient’s progress.
As a motivational tool to promote the continued use of the CPM device, the patient will have access to the GUI interface. This will allow them to see a visual representation of their progress and indicate the current quantification of their knee’s stiffness as a percent of the stiffness of their good leg.
Timestamps will be implemented into the display program to show the Physical Therapist when the patient uses the CPM device. It will also record the settings the patient was using and if they were implementing the regimen assigned by the Physical Therapist. These are an additional measure to help increase patient compliance with the rehabilitation protocol.
In making the device user-friendly, it was decided that the GUI and any necessary interactions that the user will have with the device will be intuitive and easy to read. The GUI will display the data sheet automatically and make it easy for the Physical Therapist to quickly look over the progress of the patient.
System Block Diagrams
Description of Subsystems
Knee Resistance
The knee resistance measurement subsystem measures the current through the CPM motor during a rehabilitation session. The current is proportional to the amount of stiffness in the patient’s knee joint. The current is measured using a Hall Effect sensor. The output of this current transducer is a voltage corresponding to the current through the motor. This voltage is between 0V and 5V. Amplification of these voltages is necessary to be able to detect subtle changes in current. An AD622 instrumentation amplifier is used for amplification. This difference amplifier is necessary because there is a large offset voltage produced by the Hall Effect sensor. The voltage subtracted from the output of the sensor before it is amplified comes from a digital to analog (D/A) circuit. The D/A circuit is a R-2R ladder network. The output of this circuit is controlled by the C code on the Arduino Uno microcontroller.
Microcontroller
The microcontroller subsystem consists of an Arduino Uno that is powered via a USB cable from a laptop, and code written in the C language. There are two main functions in the C code. They are data acquisition and offset voltage correction. The data acquisition function takes in the minimum and maximum angles of flexion as arguments and outputs values of current and angle if the CPM is within the angle range. This function is called over serial from the MATLAB graphical user interface and returns values of current, angle, flexion cycle number, and data points per cycle. The offset correction function is also called from MATLAB and samples the CPM motor current when the device is not in motion. This voltage is then sent through an optimization function that writes a voltage to the AD622 via the D/A circuit.
Graphical User Interface
The graphical user interface (GUI) subsystem is used to control the microcontroller subsystem, to display useful information during a rehabilitation session to the patient, to provide a rehabilitation progress summary to the physical therapist, and to send Email reports to the physical therapist as to keep track of patient compliance. The GUI password protects each program contained within it and provides a unique patient ID to maintain patient anonymity in case any data is intercepted. There are four stages in the GUI. The first is to create a patient ID. In this step, the patient enters their email address and creates a password. The physical therapist also enters his/her email address and a unique patient ID for the patient. The program sends an Email to the PT confirming that the ID was created. In the baseline step, the GUI will obtain an average current vs. angle waveform over a 15 minute period. This waveform is used to normalize the values of knee resistance of the patient’s bad leg. In the rehabilitation session stage, the GUI communicates with the microcontroller to quantify the patient progress thus far by means of numerical integration of the current vs. angle waveform compared to the integration under the baseline waveform. This quantitative comparison is in the form of a percentage of the baseline integration value. The final step is to view the patient’s overall progress. This is done on a graphical display that shows the lowest resistance value in each session thus far in the rehabilitation process.
Microcontroller Block Diagram
Knee Resistance Block Diagram
User Interface Block Diagram
Projected Electrical Power Budget & Power Source
Our main source of power consumption will come from the electrical motor running the CPM unit and all its controls. The energy source will be coming from the wall plug but the device itself will only require a 14V input voltage with a maximum current consumption of 180mA. These values were determined after our power supply for the device was changed to a new, functional unit following a failure of our original component. Using the equation P = IV, the individual power consumption for the CPM unit is approximately 2.5 W.
The second energy consuming component is the microcontroller and its accessory shields; Arduino Uno. We would be looking to supply our microcontroller with an input voltage from the 5V output given by a pin on the potentiometer located at the main joint of the CPM unit. The Arduino has 40 pins, 9 of which will either be connected to an analog input or involved in serial communication with another device. Each of the pins on the Arduino can sink up to 40mA of current.
The power consumption of the current transducer was determined to be negligible for consideration in the products overall power consumption. The power consumption of this unit will not alter our design with respect to the amount of input power available from the wall source or component pins on the CPM unit.
The second energy consuming component is the microcontroller and its accessory shields; Arduino Uno. We would be looking to supply our microcontroller with an input voltage from the 5V output given by a pin on the potentiometer located at the main joint of the CPM unit. The Arduino has 40 pins, 9 of which will either be connected to an analog input or involved in serial communication with another device. Each of the pins on the Arduino can sink up to 40mA of current.
The power consumption of the current transducer was determined to be negligible for consideration in the products overall power consumption. The power consumption of this unit will not alter our design with respect to the amount of input power available from the wall source or component pins on the CPM unit.
Experimental Results
Voltage vs. Current Relationship in Potentiometer Experiment
The first subsystem investigation we looked into was the output voltage on the potentiometer located at the main joint of the CPM unit. We sought to determine the relationship between the digital angle output on the control of the device and the voltage reading off of the red pin of the potentiometer. This would allow us to send an input to the microcontroller to determine the angle of the targeted knee joint at any time during a rehab session.
Our team set up a digital multi-meter to measure the voltage in real time on the pin. Then, we set the CPM unit to its maximum horizontal position until the digital angle reading on the controls read one degree. We then increased the bend in the main joint of the CPM unit by five degrees, taking a voltage reading at each increment.
There was a very strong negative correlation between the voltage output on the potentiometer and the angle reading on the controls. This will proved to be essential since we can now send an input relationship signal to the microcontroller that says for every 0.014V decrease in the signal, there is a one degree change in the knee joint.
Verification of Digital Angle Reading using Electro-Goniometer
Using an electro-goniometer we will confirm the overall calibration of the potentiometer, which is positioned at the joint of movement on the CPM, and compared against the angle displayed on the controller. Prior to using the electro-goniometer, it must be calibrated by using a 30-60-90 triangle. In doing so, we will set the base angles of 0, 30, 60, and 90 in Biopac. Biopac will then record the respective angle measurement from the electro-goniometer. The angle determined from the Biopac data will then be compared to the CPM controller display to determine accuracy.
Current vs. Load Relationship using FlukeView Technology
A test was done in order to determine whether or not the current through the CPM motor would increase with a resistance to movement. This would essentially allow us to determine if the motor current will decrease as a result of the patient regaining range of motion. To test this, a Fluke Power Quality Analyzer (PQA) was used along with the FlukeView Software which allowed us to see the motor current in real time. The Fluke PQA uses a Hall Effect in order to produce a voltage that is proportional to the magnetic field induced by the current running through the motor wire.
The test performed to determine if this relationship exists involved the following:
1. Put knee in CPM and apply no resistance to movement
2. Keep knee in CPM for at least 5 cycles
3. Now apply some resistance to movement to see how the current changes
4. Conclude whether the relationship is strong enough to use to measure patient progress
It can be seen that the motor current significantly increases (by about 75 mA) when the resistance to movement is applied. This should be a high enough sensitivity to be able to determine the amount of patient progress over time. There are some spikes and dips in the motor current when the CPM changes direction. This means that some signal sampling techniques will need to be applied to collect data only during a specific period of time during the movement cycle in order to avoid the current spikes showing up in the data analysis. An experiment in order to determine the optimal time to sample the signal will be done in the very near future. To quantify our knee stiffness measurement, the data will be presented as a ratio of the larger current from the stiffer knee over the current load from the healthy knee. With this method, the patient will be able to visually see their progress as their knee moves towards a more normal stiffness level.
Hall Effect Current Transducer
In choosing a Hall Effect Sensor, the team received a donated of Hall Effect sensors from Tech Support that was believed to meet our sensitivity needs. However, it was concluded that it was no sensitive enough to measure the small amount of current change that we needed to make a relationship. The team selected to get the LTS 6-NP Hall Effect Sensor from Digi-key. Our method for testing the sensitivity was again based on using the Fluke PQA of the motor. The sensitivity was determined to be 114.8 mV/A will be sufficient enough to measure the current change. Figure 9 below shows the plot of the current measured across a 1 ohm resistance versus the output voltage of the current sensor.
Verification of Current vs. Load Relationship using a Variable Resistance Knee Model
Our team developed a variable resistance knee model in order to test and verify the current to load relationship in the CPM. Through our previous investigations, our team has verified there is indeed a strong relationship between the motor current and the existence of a load on the CPM. However, in order for us to be able to make an accurate measure of difference between the “stiff” and “healthy” knees of the patient, it will be imperative we determine the specific algebraic relationship between load and current.
There is an increased voltage output from the current transducer as a greater load is applied to the CPM device. This increase output voltage was used to determine a relationship as know resistances were added to the model. When this relationship was determined, we were able to develop and algorithm within the data analysis coding. This allowed our device to assign an arbitrary baseline value to the “healthy” knee and a corresponding value to the “unhealthy” knee in order to monitor progress quantitatively.
Temperature Testing using BIOPAC Thermistor
In order to take a closer look at the environmental safety of our device’s design, we chose to conduct an internal temperature investigation. Because our device is now housed within the CPM’s existing motor housing, each of the three temperature transducers were placed in varying locations within the compartment.
The test was conducted using the BIOPAC hardware with 3 temperature transducers set up on the first three channels of the software. Then, the CPM was run on a repeated cycle between 25 and 75 degrees of range of motion for upwards of two and a half hours. We hypothesized that because there is such a low input current from the microcontroller that there would be a small and negligible change in temperature within the housing.
The ambient temperature was 78 degrees in room S366 of the Science Building. At the end of the trial, each of the three transducers recognized a slight change in temperature. However, the largest change in temperature was found in the 3 channel (located near a microcontroller) and was only an increase of only 17.68 degrees Fahrenheit. This is not nearly a large enough increase in temperature to cause any alarm on our system’s design and upon completion of this investigation we can reassure there is no potential danger.
Applicable Codes and Standards
The main references for our current standards that we plan to comply with are found in IEC 60601-1. Published by the International Electrotechnical Commission, a series of technical standards or the safety and effectiveness of medical electrical equipment were reviewed. Being in compliance with the IEC 60601-1 is seen as a requirement for most markets today.
To increase our potential influence on the ultimate marketability of our design, the following standards have been selected as applicable:
