Comparative Analysis of Autonomous Robotic and Manual Techniques for Ultrasonic Sacral Osteotomy: A Preliminary Study

In this paper, we introduce an autonomous Ultrasonic Sacral Osteotomy (USO) robotic system that integrates an ultrasonic osteotome with a seven-degree-of-freedom (DoF) robotic manipulator guided by an optical tracking system. To assess multi-directional control along both the surface trajectory and cutting depth of this system, we conducted quantitative comparisons between manual USO (MUSO) and robotic USO (RUSO) in Sawbones phantoms under identical osteotomy conditions. The RUSO system achieved sub-millimeter trajectory accuracy (0.11 mm RMSE), an order of magnitude improvement over MUSO (1.10 mm RMSE). Moreover, MUSO trials showed substantial over-penetration (16.0 mm achieved vs. 8.0 mm target), whereas the RUSO system maintained precise depth control (8.1 mm). These results demonstrate that robotic procedures can effectively overcome the critical limitations of manual osteotomy, establishing a foundation for safer and more precise sacral resections.

💡 Research Summary

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This paper presents an autonomous robotic system for ultrasonic sacral osteotomy (RUSO) that integrates a Misonix BoneScalpel ultrasonic osteotome with a seven‑degree‑of‑freedom KUKA LBR Med 14 robot arm, all coordinated through an NDI Polaris Vega optical tracking system. The authors motivate the work by highlighting the clinical challenges of en‑bloc sacrectomy for chordoma and chondrosarcoma, where millimetric deviations from the planned osteotomy path can damage the S1‑S4 nerve roots and compromise oncologic margins. Existing computer‑assisted navigation (CAN) improves accuracy but still relies on manual tool handling, resulting in typical execution errors of 2–4 mm and occasional outliers up to 20 mm.

To overcome these limitations, the authors develop a fully calibrated workflow. First, a hand‑eye calibration using the AX = XB formulation determines the rigid transformation between the robot base and the optical tracker. Second, a pivot calibration with the osteotome tip fixed in a divot yields the tip‑to‑tool transformation, enabling precise tip tracking. A third tip‑calibration step aligns the tip frame to the robot end‑effector using an optical digitizer. These calibrations establish a common reference frame {S} for all subsequent measurements.

Experimental validation is performed on PCF 15 Sawbones blocks that mimic sacral bone density. A standardized osteotomy task is defined: a straight 100 mm cut at two target depths, 4 mm (cortical transection) and 8 mm (cortical + cancellous). For manual ultrasonic osteotomy (MUSO), an experienced surgeon uses the same BoneScalpel instrument equipped with a 3‑D‑printed rigid body holder; four repetitions are recorded for each depth. For the robotic approach (RUSO), the robot autonomously executes an insertion‑cut‑retraction cycle after the entry point is digitized, with three repetitions per depth. The optical tracker records the 3‑D trajectory of the tip throughout each trial.

Four quantitative metrics are analyzed: (1) trajectory accuracy (RMSE from the planned line), (2) executed length, (3) procedure time, and (4) achieved depth. Results show that RUSO attains a mean trajectory RMSE of 0.11 mm, an order of magnitude better than MUSO’s 1.10 mm. Depth control is dramatically improved: MUSO overshoots the 8 mm target, reaching 16 mm, whereas RUSO consistently achieves 8.1 mm, staying within 0.1 mm of the plan. Executed lengths are comparable, and RUSO reduces overall procedure time by roughly 15 % due to the elimination of multiple manual passes.

The discussion emphasizes that the robotic system’s ability to simultaneously regulate surface trajectory and cutting depth eliminates the primary source of error in manual sacral osteotomies—operator‑dependent over‑penetration. The authors acknowledge limitations: experiments are confined to synthetic phantoms, lacking the variability of real sacral anatomy, soft‑tissue interaction, bleeding, and intra‑operative constraints. The current workflow still requires a human to register the entry point and supervise safety, so it is not fully autonomous. Future work will involve cadaveric or animal studies, integration of force/impedance feedback, and refinement of the system for operating‑room ergonomics.

In conclusion, this preliminary study demonstrates that an autonomous ultrasonic osteotome robot can achieve sub‑millimeter trajectory fidelity and precise depth control, outperforming the current manual standard. The work establishes a technical foundation for safer, more accurate sacral resections and suggests a clear path toward clinical translation.