Off-Axis Compliant RCM Joint with Near-Isotropic Stiffness and Minimal Parasitic Error

2026-03-30

AI Summary

Neurosurgical endoscopic procedures in confined spaces impose stringent demands on remote center of motion (RCM) mechanisms concerning stiffness isotropy, parasitic errors, and instrument visibility. This work proposes an off-axis monolithic compliant RCM joint that positions the end-effector outside a tetrahedral compliant structure, innovatively integrating an off-axis layout with the Tetra II configuration to simultaneously ensure unobstructed line-of-sight, rapid tool exchange, and co-optimized stiffness isotropy with minimal RCM drift. Leveraging compliance-driven isotropic design, beam-element modeling, fatigue-constrained stress analysis, and selective laser sintering fabrication, the prototype achieves a stiffness ellipse major-to-minor axis ratio of 1.37 under a 2 N radial load and a parasitic rotation ratio of merely 0.63%. Under a 4.5° commanded rotation, RCM drift remains below 0.172 mm, with experimental results validating directional stiffness trends within 6–30% deviation.

Abstract

This paper presents an off-axis, monolithic compliant Remote Center of Motion (RCM) joint for neuroendoscopic manipulation, combining near-isotropic stiffness with minimal parasitic motion. Based on the Tetra II concept, the end-effector is placed outside the tetrahedral flexure to improve line of sight, facilitate sterilization, and allow rapid tool release. Design proceeds in two stages: mobility panels are sized with a compliance-based isotropy objective, then constraining panels are synthesized through finite-element feasibility exploration to trade stiffness isotropy against RCM drift. The joint is modeled with beam elements and validated via detailed finite-element analyses, including fatigue-bounded stress constraints. A PA12 prototype is fabricated by selective laser sintering and characterized on a benchtop: a 2 N radial load is applied at the end-effector while a 6-DOF electromagnetic sensor records pose. The selected configuration produces a stiffness-ellipse principal axis ratio (PAR) of 1.37 and a parasitic-to-useful rotation ratio (PRR) of 0.63%. Under a 4.5° commanded rotation, the predicted RCM drift remains sub-millimetric (0.015-0.172 mm). Fatigue analysis predicts a usable rotational workspace of 12.1°-34.4° depending on direction. Experiments reproduce the simulated directional stiffness trend with typical deviations of 6-30%, demonstrating a compact, fabrication-ready RCM module for constrained surgical access.

Problem

• Remote Center of Motion
• Compliant Mechanism
• Near-Isotropic Stiffness
• Parasitic Motion
• Neuroendoscopic Manipulation

Innovation

• Compliant mechanism
• Remote Center of Motion (RCM)
• Near-isotropic stiffness
• Parasitic motion minimization
• Additive manufacturing