Validation of a marker-based motion capture method for measuring the coupling between the ulna and a bypass socket attached to the forearm

Pickard, Nicolaas; Harthikote Nagaraja, Vikranth; Kenney, Laurence

Abstract

Background
Poor comfort and function are consistently linked to prosthetic non-use and abandonment [1]. Current prosthetic devices are considered to “not offer sufficient functional advantage to compensate for any inconvenience or discomfort involved in its use” [2]. Socket fit is often referred to in prosthetics research but can be difficult to quantify objectively [3]. Prince [4] proposed socket fit as a measure of the balance between mechanical function and user comfort. Socket mechanical function can be measured as the stiffness of the socket-limb interface or mechanical coupling. Previous marker-based motion capture methods to measure socket-residuum coupling such as by Wernke [5] as well as Tang [6] and Cullen [7], are limited by their inability to track the socket and residuum independently due to the socket occluding the residuum. Prince [4] independently tracked the intact forearm and an open-frame bypass socket through the use of 3D scanning technology, but his method was limited due to its complexity.
Methodology
Previous (currently unpublished) work proposed an ulna tracking frame suitable for use with a trans-radial residuum (conventional ISB frames utilise wrist styloids [8]) (Figure 1).

Figure 1: Ulna tracking frame, with the origin situated at the midpoint of the epicondyle markers (E), the y-axis is defined as the line through E and U, the x-axis is perpendicular to the plane through EL (lateral humeral epicondyle), EM (medical humeral epicondyle) and U (ulna), and the z-axis is the cross-product of the x and y axes.
This study used said ulna tracking frame and markers on a bypass socket to investigate ulna-socket coupling. By measuring change in ulna-socket pose during a change in applied load the ulna-socket interface stiffness could be measured for certain degrees of freedom. To validate this method the interface stiffness was calculated for 10 participants with intact upper-limb anatomy. We would expect increases in socket tightness to be associated with increases in measured ulna-socket interface stiffness.
Following ethical approval (ref 6737) and informed consent, participants donned the test socket and seated themselves with their forearm, resting on a table, pointing upwards (Figure 2).

Figure 2: Reference pose for data collection, upper-arm parallel to ground (or table) and elbow in 90° of flexion with the forearm pointing vertically upwards) (Image created using Kinebody Pro [9])
Loads were applied through a pulley-and-string system (Figure 3); loads were applied axially, along the long axis of the arm (Y-axis) (from -8N to +8N in 1N intervals) and horizontally, offset from the long axis to create a moment about the Y-axis (-0.4Nm to +0.4Nm in 0.05Nm intervals). These loading conditions were chosen to mimic pistoning and twisting between the forearm and socket, commonly reported as problematic when wearing a prosthesis [10]. Each loading condition and magnitude was repeated three times, with the protocol repeated for three socket tightness settings (loose, medium, and tight).

Figure 3: Set-up for -Z transverse loading (left) and +Y axial loading (right) of the bypass socket. Load was applied to an outer strut of the bypass socket, pulling away from the socket, with weights attached to the other end of the string via a pulley system.

Results
Data collection will occur throughout November and December 2024. Preliminary results from piloting (Figure 4) show correlations between stiffness and tightness setting, and participants reported a noticeable increase in relative socket-limb movement with looser tightness settings.

Figure 4: Pilot results showing change in load vs change in displacement in the y-axis during y-axial loading (top) and change in moment vs change in angle around the y-axis during y-rotational loading (bottom), for three different socket tightness settings.
Conclusion
Proposed follow up work will assess how varying levels of socket coupling impact socket function, as well as verifying this method with a trans-radial prosthesis user. Further proposals include the use of video fluoroscopy to provide a “gold standard” measure of the ulna position and improve the accuracy of this method.

• Presentation Conference Type: Poster
• Conference Name: Trent International Prosthetics Symposium
• Start Date: Mar 19, 2025
• Deposit Date: Apr 1, 2025
• Peer Reviewed: Not Peer Reviewed