This exploratory interventional study investigates dynamic prosthetic fit in unilateral transtibial prosthesis users by objectively measuring socket-leg interface movements (SLMs) during functional activities. During one single study visit, participants complete standardized laboratory tasks using three different suspension systems (pin-lock, passive vacuum, and active vacuum) applied to a transparent replica of their current prosthetic socket. Motion capture, force measurements, and patient-reported outcomes are used to evaluate how suspension systems influence interface motion, rotation resistance, gait characteristics, and user comfort. The study aims to improve clinical understanding of prosthetic fit and support more informed prosthetic decision-making in routine care.

Transtibial prosthetic fit strongly influences user comfort, gait quality, and long term functional outcomes. Current clinical assessment methods rely primarily on subjective clinical observation and user feedback. Dynamic socket-leg interface motion (SLM) offers an objective biomechanical indicator of prosthetic fit, but existing studies are limited in sample size, measurement directions, and evaluation across different suspension systems.

This study evaluates SLMs using 3D motion capture, force sensors, and a dynamometer while participants perform three standardized task categories:

isometric rotation tasks to determine rotation resistance and torque thresholds for interface rotation; turning/rotational/pivoting walking tasks to assess functional performance and transverse plane dynamics; and treadmill walking at multiple speeds to quantify proximal-distal pistoning and gait kinematics.

Each participant is tested using three suspension system conditions (pin-lock, passive vacuum, active vacuum), starting with their preferred suspension system followed by a counterbalanced order to minimize bias. A transparent "test socket" replicating the participant's own socket allows interface visualization and marker tracking without altering the intended use of the prosthesis. The investigational device consists only of CE marked components used within their intended purpose.

Participants attend one study visit with a duration of approximately 4 to 5 hours, including prosthesis adjustment and placement, marker setup, data collection across all three task categories and with all three suspension system conditions.

The primary aim is to quantify the effects of suspension system and load conditions on SLMs and gait performance. Secondary aims assess relationships between objective SLM measures and patient reported comfort, functional ability with the prosthesis, and prosthesis satisfaction. Findings will support improved understanding of dynamic fit behavior, help characterize suspension system performance, and inform future development of objective prosthetic fitting assessment methods.

Inclusion Criteria:

• Age ≥16 years
• Unilateral transtibial amputation
• Activity level K2-K4
• Actively using a prosthesis for ≥12 months
• Ability to understand and communicate in Norwegian (oral and written)
• Digital file of current socket model available (required for test socket fabrication)

Exclusion Criteria:

• Use of a thigh corset attached to the prosthesis
• Requirement of a walker for ambulation
• Open wounds, skin ulcers, or blisters on the residual limb
• No liner
• Missing digital socket model file
• Medical advice to avoid physical activity similar to study tasks
• Other medical conditions that may worsen with activity and have not been cleared by a healthcare professional

Individual participant data (IPD) from this study cannot be shared because the data are not anonymous. According to REK regulations and GDPR requirements, the research team must retain the identification key for as long as the data are used for research purposes. As long as this key exists, the dataset is considered identifiable, and sharing identifiable or potentially re-identifiable data externally is not permitted. Therefore, IPD cannot be shared.