(L-449) Improving the Mechanical Design of an Elbow Joint Prosthetic While Simultaneously Optimizing Material Properties

Approximately 185,000 amputations occur in the United States annually. [1] Additionally, there is an increasing market demand for prostheses. The prosthetic and orthotic market was valued at 6.39 billion and is expected to expand at a growth rate of 4.3% by 2030. [2] This study aimed to evaluate and improve the TRS Fillauer Ener-joint prosthetic elbow. The TRS Ener-joint allows amputees to pursue high impact outdoor activities including. The device consists of a lever cam-lock to adjust the position of the elbow, and a polyurethane center that provides both spring and dampening characteristics, as shown in Figure 1. [3] The placement of the lever, and force required to engage it makes repositioning the device difficult.  

When redesigning the locking mechanism, there was an emphasis on evaluating the locking and elastic functions of the prosthetic. The locking function allows for selective retention of a specific position, without slipping or play. An ideal locking function requires convenient and minimal force to operate, ability to position at any angle, no deflection regardless of force applied when engaged, and no resistance to movement when disengaged. The elastic function allows for a reliable linear relationship between angular displacement and elbow torque in the flexion-extension direction. An Ideal elastic function requires adjustability of elbow stiffness, limited hysteresis, and high frequency vibration dampening. Therefore, the purpose of this study was to design multiple variants of the original prosthetic to compare ergonomics, locking functions, and elastic functions to determine the optimal locking mechanism for the TRS-Ener joint elbow.  

Different variations and locking mechanisms were first brainstormed between the researchers of the study. Concepts were modeled in SolidWorks (3D CAD), then printed in PLA on an Ultimaker 5S and assembled used standard hardware. Figure 2 shows a few models that were developed for testing.  

To evaluate the devices, the elbow assemblies were mounted into a single column Instron load frame, with rods to represent the user’s forearm and upper arm. Once the prosthetic was securely attached to the Instron, the extension was adjusted so that the elbow was placed at a 90-degree angle, verified using a long arm goniometer, as shown in Figure 3. Two cyclic test profiles were created with 30mm and 60mm of flexion and extension (total of 60mm and 120mm of displacement respectively), both profiles completed 5 cycles per test.  

The force displacement data was exported to excel. From here, the displacement vs force data was graphed. Then, the angle and moment were calculated and graphed. Goal seek was then utilized to determine the play and stiffness of each variant to allow for comparison between the designs. 

Presently, the full analysis has been completed on the original TRS-Ener joint elbow. Goal seek was set to use the average predicted error as an objective and several values of play and stiffness were calculated using the excel software. Goal seek settled on a mid-angle value of 89.931 degrees with a stiffness value of 1.43 N*m/deg and a play value of 1.36 deg. This same analysis will be replicated on the different designs and the values of stiffness and play will be compared to determine the optimal design. The ideal design will have a minimized average predicted error. 
While the full analysis and designs are not yet complete, multiple designs have been 3D printed and tested on the Instron. The first variant printed used a pin-locking mechanism. A quick release pin was used to disengage the lock by pulling on the ring and set to engage in the discrete holes installed inside the prosthetic when the ring was let go. While this device did appear to be more ergonomically feasible, it does not appear to match the functional quality of the original device. From a qualitative standpoint when the pin-lock was engaged, there was a lot of play when the device was manually pulled on. Additionally, once the Instron was set to a displacement of 120 mm, the hole the pin was locked into, broke from the increasing stress. The force-displacement curves were graphed together by the two designs at 120 mm. The pin mechanism curve demonstrates the amount of play in the device. There is a lot of jagged movements as opposed to the smooth curve of the original design. Due to these results different designs are continuing to be brainstormed. Presently, a gear locking mechanism is being designed on SolidWorks and will be further evaluated using the same methodology.  

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