UC San Diego: Medical Elbow Orthosis

Executive Summary

Background

Commercially-available prosthetic devices are continually evolving to become more accessible to both children and adults. The market for pediatric prosthetic devices is smaller than that of adults, but it is necessary to improve the lives of children hit by various diseases. Acute Flaccid Myelitis is one such affliction that attacks anterior horn cells of the spinal cord, which are the cell bodies of the motor nerves. Max (pictured top right) is a five year-old boy who suffered from this disease when he was only two years old. As a result his bilateral elbow flexors were severely impacted and could not support any force through his arms, leaving both of them to hang at his side. Luckily, he developed full functionality of both of his hands, and he retained sensation in both of his arms. He functions as any other five year-old would - he is robust, energetic, and learns quickly. Even without the ability to directly flex his elbows, he has cleverly figured out many different ways to move the position of his hands. The hope is to develop a device that will allow him to control actuation of both of his elbows and allow him to fully utilize both of his hands.

What is AFM?

Acute Flaccid Myelitis, known as AFM, is a rare but serious condition that affects the spinal cord and can weaken the body's muscles and motor neurons. AFM can affect the transmission of motor signals through neurons controlling the head and neck and can result in facial drooping or loss of muscle control. In the worst case, respiratory failure can occur. Many cases of AFM develop after contraction of a viral illness or fever-inducing disease, but the underlying cause is not yet known. It is unclear as to why a virus might trigger onset of AFM or why certain people are affected.

Requirements

To produce a satisfactory product, a list of general requirements was formed. Safety was the first priority, followed by function.

• The orthosis must be light enough for a five year-old to comfortably handle

• Orthosis components must not cause irritation or skin pinching

• Device must be portable for Max to wear and move around with

• Device must not hyperextend Max's arm or harm him in any way

• Orthosis weight must be distributed through a point other than the shoulder joint to avoid damaging connective tissue

• Control mechanism must be intuitive for Max to use

• Battery to power device must not cause any fires

Objectives

The goal of this project was to design a solution allowing Max to fully utilize his right hand by allowing him to control motion of his elbow as described in Figure 1. Due to the constant load on his shoulder joint, the weight of the device was minimized. The Wow design, defined as the design exceeding all expectations, ideally includes a system that can control the pectoral rotation of Max's arm to prevent drift. To reach this, the objectives were split into three goals:

1)     User-controlled actuation of the right arm

2)     User-controlled actuation of left arm

3)     Control of pectoral rotation

Objectives 1 and 2 were successfully met, as a system was developed which allowed Max to control flexion of both of his elbows. Pectoral rotation is a more complex problem that requires more degrees of freedom, and has been saved for a future consideration.

                     Figure 1: Elbow rotation

Human-Centered Design

One of the major challenges of this project was considering the human factor of the design. Prosthetic devices must account for more than the engineering functions they wish to restore to the user. To make them appealing to wear, they must adjust for human tendencies, factors and schedules. Comfort and ease-of-use have a large influence on a user's willingness to use the device. If the device is too complicated and/or too uncomfortable to operate, then the user will not be inclined to wear it. Aesthetics must also be considered as a user might not want to wear a prosthetic because of the way it looks. Figure 2 shows Max, who has a tendency to hyper-extend his elbows when resting. This led to the designing of a joint that eliminated the possibility of injuring Max by incorporating hard stops to joint motion.

             Figure 2: Max naturally hyper-extending his elbows

Final System Design

The final orthosis was composed of two major components. The first, pictured below in Figure 3, is a harness fabricated from Dacron, a polyester fiber known for its high resistance to stretching. This was necessary to support the weight of the rest of the orthosis, which would connect to the harness through straps on the cuffs of the arm. This distributes the weight of the devices on the user's arm downward through the torso and reduces the amount of pulling force through the shoulders. This harness was sized around a mold created from a scan of Max's torso, pictured in Figure 4. This mold was helpful in testing the fit of the harness without having to constantly test it on Max.

Figure 3: Device Harness                                                                              Figure 4: Harness fit over mold of Max's torso                     

The second component was the actuation device pictured below in Figure 5. This includes:

1) 2X Set of Brace cuffs

2) 2X Actuonix Linear Actuators

3) 2X Sets of custom-printed transferable joints

4) 1X Custom Printed Circuit Board from PCBExpress to eliminate wiring

5) 2X Custom sewn sensor gloves to house sensors

6) 1X Arduino Nano - Microcontroller

7) 1X Tenergy NiMH Cell Meter

8) 1X 6V 3,000 mAH Nickel-Metal Hydride battery

9) 2X Connectors for sensor wires

10) 4X Flex Sensors - two for each hand to pick up downward and upward bending

This assembly fits onto the user and is supported by the harness. The whole system is powered by one Venom NiMH 6V 3000 mAH battery, which connects to the PCB through a Tamiya connector. This supplies power to both linear actuators, the Arduino Nano and all four flex sensors. These flex sensors are concealed within the sensor gloves, which keep them pressed against the wrist to accurately pick up flexion. Once the wrist is flexed in a direction, the Arduino takes the flexion data and translates it into actuation of the linear servo, which opens or closes the user's arm. Note the green screw-in connectors on the PCB - these were added to simplify the sensor connections to the Arduino.

Figure 5: Joint and electrical component

Figure 6 below shows the entire device assembled onto the user.

Figure 6: User wearing harness + actuator assembly