Rehabilitation orthosis for arm flexion and extension

Context

Brachial plexus injuries are a significant medical problem, since they affect the mobility and sensation of the upper limb. These injuries are usually caused by accidents, mainly traffic accidents, and considerably reduce patients' quality of life because they can no longer carry out everyday tasks.

In Colombia, where motorcycles and bicycles are widely used, the incidence of these injuries is high. Although effective surgical treatments exist, limited access and long waiting times lead to additional complications such as muscle atrophy and permanent loss of mobility.

Orthoses and exoskeletons are a promising solution for the rehabilitation of these patients. This project focuses on the design and construction of a lightweight, adaptable orthosis that enables elbow flexion and extension, improving the effectiveness of rehabilitation and users' quality of life.

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Design

Conceptual design

In the initial phase, four concepts were proposed for power transmission:

1. Linear actuators: Use pistons to generate motion. Although precise, their weight and size make portability difficult.
2. Pulley systems: Provide significant mechanical advantage, allowing a compact and efficient design.
3. Direct-drive motors: Simple, but limited in terms of weight and cost.
4. Artificial muscles: Their softness and biomimicry are promising, but their high cost and difficult implementation make them less viable.

Using a decision matrix, the pulley system was selected as the most balanced option in terms of functionality, efficiency and adaptability.

Prototypes

Low-fidelity prototype

The first prototype used a system of cables and pulleys. Although functional, it had important limitations:

• Lack of mechanical advantage, which required more powerful actuators.
• Excessive bulk, making it hard to fit on the user's arm.
• A restricted range of motion due to mechanical interference.

Medium-fidelity prototypes

Inspired by the LIMS Ambidex system, the medium-fidelity prototypes included a block-and-tackle system to improve mechanical advantage. The main improvements were:

• Additional pulleys to guide the cables.
• Lighter and stronger materials, such as monofilament for the cables.
• Swivel mounts to reduce tangling.

The design was iterated using technologies such as 3D printing, resulting in a functional system that served as the basis for the final prototype.

High-fidelity prototype

The final prototype was built from aluminum and 3D-printed components. A backpack was designed to house the electronics and a MyActuator RMD-L-5010-35T distal motor controlled over CAN.

Visual debugging systems and a hyperbolic-tangent control curve were implemented to ensure safe and efficient operation.

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Results and conclusions

The project produced a functional orthosis, with some areas for improvement identified.

1. Positive results:

   • The mechanical design met the lightness and ergonomics requirements.
   • The electrical and drive systems worked as expected, enabling assisted motion.

2. Areas for improvement:

   • Under certain conditions the selected motor could not lift the weight of the arm, so a more robust sizing is needed.
   • Insufficient friction between components introduced an unwanted extra degree of freedom.

3. Potential impact:
   • This project lays a solid foundation for developing more effective rehabilitation devices, contributing significantly to patients' quality of life.

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Methods

The design followed an iterative approach that included:

1. Kinematic and dynamic analysis: To model and optimize the motion of the orthosis.
2. Rapid prototyping: Using 3D printing and CNC machining to evaluate the design iteratively.
3. Dynamic simulation: Data capture with the Tracker software to validate the range of motion and system parameters.

These methods made it possible to develop a prototype that meets the main objectives of the project and lays the groundwork for future research.