Experimental Characterization of Multi-Axis Tissue Resistance Forces for Robot-Assisted Minimally Invasive Spine Surgery

Robot-assisted minimally invasive spinal procedures encounter substantial obstacles due to uncharacterized tissue resistance forces that compromise pedicle screw placement precision. This study developed force-torque characterisation methodologies to enhance robotic surgical control algorithms through biomechanical tissue modelling. The experimental apparatus integrated a custom-designed probe incorporating a 6-axis force/torque sensor with cylindrical guidance components, positioned to align with the spinal instrumentation trajectories. Testing utilized a multi-layered lumbar phantom constructed from biomimetic silicone materials engineered to replicate the mechanical properties of anatomical muscles and soft tissues. The sensor measurements underwent gravitational compensation and filtering protocols to ensure measurement fidelity. Experimental validation revealed vertebral level-dependent force escalation patterns, with a maximum tissue resistance of 27.6 N observed at the L4 level during pitch angular motion. Progressive force amplification from superficial anatomical positions (L1:15.1 N) to deeper insertion sites (L4:27.6 N) demonstrated an 83% resistance increase across vertebral levels. Linear force-depth relationships exhibited gradients of 0.55 N/mm for pitch motions and 0.30 N/mm for roll motions, confirming that angular trajectory deviations at increased insertion depths generated proportionally elevated tissue resistance. Experimental results provide critical parameters for developing adaptive robotic control systems that enhance surgical precision and safety.