Discussion

This study aimed to explore the feasibility of using B-mode ultrasound to assess residual bone movement within a transtibial prosthetic socket during dynamic tasks. The hypothesis was that ultrasound imaging could provide meaningful measurements of bone displacement, thereby improving prosthetic fitting through optimization of probe positioning, socket design, and imaging techniques.

To address this hypothesis, we had three primary objectives. First, we sought to design and 3D print a prosthetic socket that could be used safely and comfortably on healthy individuals. Second, we aimed to determine whether ultrasound recordings could be successfully acquired during dynamic movement using a hands-free setup. Third, we focused on identifying the optimal position for the ultrasound probe within the socket, taking into account limitations such as imaging depth and the anatomical features of the limb.

To achieve these goals, a combination of advanced scanning, modeling, and fabrication techniques was used. For the forearm prototype, we employed a Structure Sensor 3 connected to an iPhone 13, using the Structure Sensor Calibrator and Structure App to capture the 3D model of the limb. For the leg prototype, a HandySCAN 3D scanner was used to achieve higher accuracy, with assistance from an experienced M2 intern. The scans were cleaned and processed in Autodesk Meshmixer, where they were converted into solid bodies and exported as STL files. These files were then imported into Autodesk Fusion 360, where further modifications were made to the socket, including splitting the model, adding screw holders, creating M6-compatible holes, and designing nut recesses.

A custom case for the ultrasound probe was developed by integrating the probe’s CAD file into the socket design, as attempts to scan the probe proved difficult. The case was merged with the socket in Fusion 360, and its positioning was iteratively adjusted to ensure optimal imaging conditions. Following guidance from the literature (Jonkergouw et al., 2024), we ensured the probe remained perpendicular to the residual bone and was positioned as distally as possible without compromising image quality. The final models were verified in Ultimaker Cura and fabricated using an Ultimaker S5 3D printer.

Ultrasound recordings were acquired using the Vscan Air Wireless Ultrasound system, connected via a mobile app. The probe was loaded into the custom case and secured within the socket, after which dynamic movements were performed and screen-recorded. Videos were post-processed using open-source software to remove interface icons and converted from .mp4 to .avi format. Data analysis was conducted in MATLAB using a script developed by a lab collaborator. The script identified and quantified bone displacement relative to the probe by analyzing intensity changes between skin, fat/muscle, and bone layers.

Bone movement was detected by identifying signal peaks relative to a baseline established while the subject stood at rest. These peaks represented displacement events during motion. Ultrasound images were captured at a fixed depth of 3 cm, which imposed limitations on the probe’s positioning and image clarity, particularly in regions with greater soft tissue coverage. It is anticipated that increasing the probe depth or using a higher-resolution system would improve data quality and probe placement flexibility.

To verify consistency, the procedure was repeated across four trials with the same subject. The resulting data showed waveform patterns similar to those reported in previous studies, suggesting that the setup was effective. As expected, bone displacement was minimal due to the healthy condition of the subject. Nonetheless, the detection of consistent motion patterns validates the feasibility of the technique and supports its potential application in amputee populations.

Some limitations remain. The most significant constraint was the limited imaging depth of the ultrasound probe, which restricted visibility and required estimated placement of the probe relative to underlying anatomical structures. Additionally, the study was conducted on a single subject, which limits the generalizability of the findings. If provided with additional time, future work would involve testing the socket on a larger number of participants to evaluate reproducibility. Furthermore, synchronizing the ultrasound footage with external video of the limb’s motion would allow for correlation with specific gait phases, improving interpretation accuracy. Overall, this proof-of-concept study offers a promising foundation for future research on bone displacement assessment within transtibial prosthetic sockets.