Analysis of physical properties of the ulnar nerve during puncture using regional block needles. Measuring station proposal and preliminary test results.

BACKGROUND: Safe robotic or manual regional nerve blocks require quantitative data on needle-nerve interaction forces, yet such data are scarce. NEW METHOD: We designed a purpose-built measurement station to characterize how puncture mechanics vary along the human ulnar nerve. Eight fresh-frozen cadavers provided paired proximal upper-arm and distal forearm ulnar-nerve segments. A custom system built around a 3-axis 3-D-printer chassis, dual 50N load cells (0.1N resolution), and an ATmega328P controller advanced 21G block needles at 10mmmin⁻¹ in 0.2mm steps. Puncture-force profiles were recorded at 20 locations per segment at 5mm intervals. At each nominal depth, the force value used for curve analysis was calculated as the median of fifty technical force readings. Maximum puncture force was analyzed using a hierarchical linear mixed-effects model accounting for repeated measurements within anatomical segments and pairing within donors. COMPARISON WITH EXISTING METHODS: One similar system was found, but with different methodology of measurements and different statistical analysis. RESULTS: At the 0mm reference position, predicted peak puncture force was approximately 33.3% lower in upper-arm than forearm segments (β=-0.174, p<0.001). The interaction between anatomical region and distance was significant (β=0.00139, p<0.001), indicating different spatial puncture-force patterns between anatomical regions. Paired donor-level comparisons of Slope Coefficient, Penetration Work, Roughness Index, and Withdrawal-to-Insertion Work Ratio showed no statistically significant differences after correction for multiple comparisons. CONCLUSIONS: The measurement system delivered reproducible force-displacement curves and minimized artifacts from sample fixation. These preliminary ex vivo findings indicate that ulnar-nerve puncture mechanics vary with anatomical region and position along the nerve and warrant confirmation in larger donor cohorts.