Ex vivo breach detection using electrical conductivity during robotic pedicle drilling in the spine

• Pedicle preparation is hazardous because surgeons lack real-time, in-depth vertebral visualization, particularly when scoliosis makes screw entry points challenging.Source: §1 · p. 2
• Prior approaches include patient-specific guides and nonvisual sensors, while robotic systems commonly use stereotactic navigation, including SpineAssist, ROSA Spine, and ExcelsiusGPS.Source: §1 · pp. 2–3
• The robotic platform integrates a clinically used SpineGuard bioelectrical conductivity sensor, called DSG, with a motorized drilling system.Source: §1 · p. 3
• Navigation and robotic systems face usability, cost, accuracy, and registration-error limits, and trajectory planning alone does not fully prevent breaches.Source: §1 · p. 3
• An adaptive-threshold, dual-alert conductivity algorithm stopped drilling before breaches in all 51 ex vivo trials.Source: §1 · p. 3
• The mean error remained within [-1, +1.4] mm, and 100% of drillings were graded A or B.Source: §1 · p. 4

2.1 Experimental Setup

• Tissue-dependent conductivity differences and signal changes near cortical micro-fractures provide cues for detecting lateral or medial breaches before the drill fully enters the spinal canal.Conductivity is lowest in cortical bone, higher in cancellous bone, and highest in blood and soft tissue.Source: §2.1 · pp. 4–5

2.2 Drilling Workflow

• The workflow used fresh pig vertebrae immersed in saline with approximately 120–130 mmol/L NaCl to mimic the ionic environment of cerebrospinal fluid.The solution was selected to reproduce CSF conductivity, particularly its sodium and chloride composition.Source: §2.2 · p. 5

2.3 Vertebrae Collection and Preparation

• Fresh pig vertebrae were drilled toward the spinal canal along a trajectory that avoided growth cartilage, with drilling performed at 10 N and 30 rpm.The vertebrae were clamped for continuous camera inspection; a surgeon created the entry point and hand-guided the robot into position.Source: §2.3 · pp. 5–6

2.4 Methodology

• Breach results were checked by ball-tip palpation, post-drilling visual inspection of the spinal canal, and CT scans combined with video analysis.These checks were used to assess cortical integrity and confirm the absence of false positives and false negatives.Source: §2.4 · p. 6

2.5 Breach Detection Algorithm and Dataset

• Offline tuning moved the measurement window to begin at 2 mm and allowed alerts from 3 mm because breaches in thinner pedicles could precede the original 5 mm window endpoint.The adaptive reference is based on initial drilling measurements, with upper and lower bounds intended to limit false negatives and false positives, respectively.Source: §2.5 · pp. 6–7
• The detector uses conductivity and drilling depth to identify changes associated with transitions from bone to fluid-filled regions, then triggers an alert when either of two criteria is met.One criterion compares current conductivity with an adaptive threshold; the other detects a conductivity variation above a calibrated 230 mV gradient threshold within the search window.Source: §2.5 · pp. 7–8

3 Results

• The surgeon manually positioned the drill at the pedicle entry point before automatic drilling, generally using 10 N of force and up to 100 N as needed.Force and rotational speed were adjusted according to vertebral bone quality.Source: §3 · p. 8
• All 51 drillings stopped immediately before perforation, with 6 grade A and 45 grade B outcomes and none near the 2 mm grade C threshold.The vertebrae were fresh lumbar pig specimens; grade A and B placements are deemed clinically acceptable.Source: §3 · p. 9
• A sharp conductivity peak marked cortical micro-breach onset across cases, after which drilling stopped.Conductivity decreased on entering less-conductive cortical bone in the described example.Source: §3 · p. 9
• Stopping errors averaged 0.7 mm (SD 0.45 mm) and ranged from –0.9 mm to +1.4 mm.The error measure was canal deformation length, with positive values indicating deformation and negative values indicating stopping before deformation.Source: §3 · p. 9

4 Discussions

• The earlier lamb-vertebrae evaluation used favorable, vertical spinous-process trajectories to test robot controls, drill-bit design, and conductivity signal quality.The initial algorithm was also evaluated in those experiments.Source: §4 · pp. 9, 11
• Figures 5–8 document conductivity and detection timing, canal-boundary deformation, before-and-after canal appearance, and deformation distances from the canal.Figure 8 identifies 0.5–1 mm as the most frequent deformation range.Source: Figure 5 · p. 10, Figure 6 · p. 11, Figure 7 · p. 11
• The later evaluation drilled fresh pig pedicles along trajectories toward the spinal canal, excluding clinically unacceptable deviations to isolate breach detection.The discussion describes pig vertebral morphology as similar to humans.Source: §4 · pp. 11–12
• Growth cartilage in porcine specimens may confound detection through higher conductivity and false positives, particularly on pedicle-to-vertebral-body trajectories.The discussion proposes characterizing its conductivity for the decision logic or using mature specimens without growth cartilage.Source: §4 · p. 12

5 Conclusions

• Across 51 porcine vertebrae, continuous conductivity monitoring stopped drilling before spinal-canal perforation, with a 100% detection rate and all cases graded A or B.The conclusion describes the approach as an automatic breach-detection algorithm based on real-time electrical conductivity measurements.Source: §5 · p. 12
• Electrical conductivity offers an X-ray-free approach that can complement navigation, reduce intraoperative imaging, and limit radiation exposure.Source: §5 · p. 12
• The online detection method could be integrated into standalone surgical tools or added as a safety feature to existing drilling systems.Source: §5 · p. 12
• Further research may support more advanced detection and deployment from assisted manual through semi-autonomous drilling to fully autonomous systems.Source: §5 · pp. 12–13