# Medical Imaging-Based Kinematic Modeling for Biomimetic Finger Joints and Hand Exoskeleton Validation

## TL;DR

This paper introduces a new method to design hand exoskeletons that better mimic natural finger movements, improving comfort and effectiveness for rehabilitation.

## Contribution

The study introduces the Instant Radius Method (IRM) for modeling finger joint kinematics using medical imaging, enabling biomimetic exoskeleton design.

## Key findings

- Exoskeleton fingertip trajectories with biomimetic joints matched physiological movements more closely, reducing RMSE significantly.
- Pressure at MCP and PIP joints decreased by 15–25% in mean and 20–30% in peak values, with more uniform distribution.
- The IRM-based framework improved kinematic alignment and human–machine compatibility in multi-DOF exoskeletons.

## Abstract

Hand rehabilitation exoskeletons play a critical role in restoring motor function in patients with stroke or hand injuries. However, most existing designs rely on fixed-axis assumptions, neglecting the rolling–sliding coupling of finger joints that causes instantaneous center of rotation (ICOR) drift, leading to kinematic misalignment and localized pressure concentrations. This study proposes the Instant Radius Method (IRM) based on medical imaging to continuously model ICOR trajectories of the MCP, PIP, and DIP joints, followed by the construction of an equivalent ICOR through curve fitting. Crossing-type biomimetic kinematic pairs were designed according to the equivalent ICOR and integrated into a three-loop ten-linkage exoskeleton capable of dual DOFs per finger (flexion–extension and abduction–adduction, 10 DOFs in total). Kinematic validation was performed using IMU sensors (Delsys) to capture joint angles, and interface pressure distribution at MCP and PIP was measured using thin-film pressure sensors. Experimental results demonstrated that with biomimetic kinematic pairs, the exoskeleton’s fingertip trajectories matched physiological trajectories more closely, with significantly reduced RMSE. Pressure measurements showed a reduction of approximately 15–25% in mean pressure and 20–30% in peak pressure at MCP and PIP, with more uniform distributions. The integrated framework of IRM-based modeling–equivalent ICOR–biomimetic kinematic pairs–multi-DOF exoskeleton design effectively enhanced kinematic alignment and human–machine compatibility. This work highlights the importance and feasibility of ICOR alignment in rehabilitation robotics and provides a promising pathway toward personalized rehabilitation and clinical translation.