Identification of milling parameters for manual cutting of bicortical bone structures

MINARO HD: control and evaluation of a handheld, highly dynamic surgical robot

Purpose

Current surgical robotic systems are either large serial arms, resulting in higher risks due to their high inertia and no inherent limitations of the working space, or they are bone-mounted, adding substantial additional task steps to the surgical workflow. The robot presented in this paper has a handy and lightweight design and can be easily held by the surgeon. No rigid fixation to the bone or a cart is necessary. A high-speed tracking camera together with a fast control system ensures the accurate positioning of a burring tool.

Methods

The capabilities of the robotic system to dynamically compensate for unintended motion, either of the robot itself or the patient, was evaluated. Therefore, the step response was analyzed as well as the capability to follow a moving target.

Results

The step response show that the robot can compensate for undesired motions up to 12 Hz in any direction. While following a moving target, a maximum positioning error of 0.5 mm can be obtained with a target motion of up to 18 mm/s.

Conclusion

The requirements regarding dynamic motion compensation, accuracy, and machining speed of unicompartmental knee arthroplasties, for which the robot was optimized, are achieved with the presented robotic system. In particular, the step response results show that the robot is able to compensate for human tremor.

Usability of cooperative surgical telemanipulation for bone milling tasks

Purpose

Cooperative surgical systems enable humans and machines to combine their individual strengths and collaborate to improve the surgical outcome. Cooperative telemanipulated systems offer the widest spectrum of cooperative functionalities, because motion scaling is possible. Haptic guidance can be used to assist surgeons and haptic feedback makes acting forces at the slave side transparent to the operator, however, overlapping and masking of forces needs to be avoided. This study evaluates the usability of a cooperative surgical telemanipulator in a laboratory setting.

Methods

Three experiments were designed and conducted for characteristic surgical task scenarios derived from field studies in orthopedics and neurosurgery to address bone tissue differentiation, guided milling and depth sensitive milling. Interaction modes were designed to ensure that no overlapping or masking of haptic guidance and haptic feedback occurs when allocating information to the haptic channel. Twenty participants were recruited to compare teleoperated modes, direct manual execution and an exemplary automated milling with respect to usability.

Results

Participants were able to differentiate compact and cancellous bone, both directly manually and teleoperatively. Both telemanipulated modes increased effectiveness measured by the mean absolute depth and contour error for guided and depth sensitive millings. Efficiency is decreased if solely a boundary constraint is used in hard material, while a trajectory guidance and manual milling perform similarly. With respect to subjective user satisfaction trajectory guidance is rated best for guided millings followed by boundary constraints and the direct manual interaction. Haptic feedback only improved subjective user satisfaction.

Conclusion

A cooperative surgical telemanipulator can improve effectiveness and efficiency close to an automated execution and enhance user satisfaction compared to direct manual interaction. At the same time, the surgeon remains part of the control loop and is able to adjust the surgical plan according to the intraoperative situation and his/her expertise at any time.

Estimation of Penetrated Bone Layers During Craniotomy via Bioimpedance Measurement

OBJECTIVE

Craniotomy is the removal of a bone flap from the skull and is a first step in many neurosurgical interventions. During craniotomy, an efficient cut of the bone without injuring adjoining soft tissues is very critical. The aim of this study is to investigate the feasibility of estimating the currently penetrated cranial bone layer by means of bioimpedance measurement.

METHODS

A finite-element model was developed and a simulation study conducted. Simulations were performed at different positions along an elliptical cutting path and at three different operation areas. Finally, the validity of the simulation was demonstrated by an ex vivo experiment based on use of a bovine shoulder blade bone and a commercially available impedance meter.

RESULTS

The curve of the absolute impedance and phase exhibits characteristic changes at the transition from one bone layer to the next, which can be used to determine the bone layer last penetrated by the cutting tool. The bipolar electrode configuration is superior to the monopolar measurement. A horizontal electrode arrangement at the tip of the cutting tool produces the best results.

CONCLUSION

This study successfully demonstrates the feasibility to detect the transition between cranial bone layers during craniotomy by bioimpedance measurements using electrodes located on the cutting tool.

SIGNIFICANCE

Based on the results of this study, bioimpedance measurement seems to be a promising option for intra operative ad hoc information about the bone layer currently penetrated and could contribute to patient safety during neurosurgery.

An experimental evaluation of the force requirements for robotic mastoidectomy

HYPOTHESIS

During robotic milling of the temporal bone, forces on the cutting burr may be lowered by choice of cutting parameters.

BACKGROUND

Robotic bone removal systems are used in orthopedic procedures, but they are currently not accurate enough for safe use in otologic surgery. We propose the use of a bone-attached milling robot to achieve the required accuracy and speed. To design such a robot and plan its milling trajectories, it is necessary to predict the forces that the robot must exert and withstand under likely cutting conditions.

MATERIALS AND METHODS

We measured forces during bone removal for several surgical burr types, drill angles, depths of cut, cutting velocities, and bone types (cortical/surface bone and mastoid) on human temporal bone specimens.

RESULTS

Lower forces were observed for 5-mm diameter burrs compared with 3-mm burrs for a given bone removal rate. Higher linear cutting velocities and greater cutting depths independently resulted in higher forces. For combinations of velocities and depths that resulted in the same overall bone removal rate, lower forces were observed in parameter sets that combined higher cutting velocities and shallower depths. Lower mean forces and higher variability were observed in the mastoid compared with cortical/surface bone.

CONCLUSION

Forces during robotic milling of the temporal bone can be predicted from the parameter sets tested in this study. This information can be used to guide the design of a sufficiently rigid and powerful bone-attached milling robot and to plan efficient milling trajectories. To reduce the time of the surgical intervention without creating very large forces, high linear cutting velocities may be combined with shallow depths of cut. Faster and deeper cuts may be used in mastoid bone compared with the cortical bone for a chosen force threshold.

Force-based control of a compact spinal milling robot

BACKGROUND

Spine-milling operation during laminectomy surgery requires steady manipulation and intraoperative monitoring. A spinal milling robot with force-based control is introduced to improve the operation safety.

METHOD

The robot is designed with compact structure and simple configuration. Real-time thrust force is measured and three stages corresponding to the anatomical structures of the vertebra are identified, based on the analysis of typical characteristic parameters of the force profiles. The cross-correlation to the standard profiles are adopted to judge the milling status. A 1 mm margin is prescribed to stop the procedure before the lamina is thoroughly milled through.

RESULTS

Automatic robot-milling experiments on porcine vertebrae are conducted, based on the force-based control method, and the procedure is stopped when the critical condition is met. The average thickness of the milled part is 1.1 mm, and no penetration occurs.

CONCLUSION

The spinal milling robot could provide steady manipulation, facilitate the surgeon's labour with an automatic feeding process and improve the safety of the operation with enhanced monitoring.

Robot- and computer-assisted craniotomy (CRANIO): From active systems to synergistic man—machine interaction

Computer and robot assistance in craniotomy/craniectomy procedures is intended to increase precision and efficiency of the removal of calvarial tumours, enabling the preoperative design and manufacturing of the corresponding implant. In the framework of the CRANIO project, an active robotic system was developed to automate the milling processes based on a predefined resection planning. This approach allows for a very efficient milling process, but lacks feedback of the intra-operative process to the surgeon. To better integrate the surgeon into the process, a new teleoperated synergistic architecture was designed. This enables the surgeon to realize changes during the procedure and use their human cognitive capabilities. The preoperative planning information is used as guidance for the user interacting with the system through a master—slave architecture. In this article, the CRANIO system is presented together with this new synergistic approach. Experiments have been performed to evaluate the accuracy of the system in active and synergistic modes for the bone milling procedure. The laboratory studies showed the general feasibility of the new concept for the selected medical procedure and determined the accuracy of the system. Although the integration of the surgeon partially reduces the efficiency of the milling process compared with a purely active (automatic) milling, it provides more feedback and flexibility to the user during the intra-operative procedure.

Fast and accurate registration of cranial CT images with A-mode ultrasound

PURPOSE

Within the CRANIO project, a navigation module based on preoperative computed tomography (CT) data was developed for Computer and Robot Assisted Neurosurgery. The approach followed for non-invasive user-interactive registration of cranial CT images with the physical operating space consists of surface-based registration following pre-registration based on anatomical landmarks. Surface-based registration relies on bone surface points digitized transcutaneously by means of an optically tracked A-mode ultrasound (US) probe. As probe alignment and thus bone surface point digitization may be time-consuming, we investigated how to obtain high registration accuracy despite inaccurate pre-registration and a limited number of digitized bone surface points. Furthermore, we aimed at efficient man-machine-interaction during the probe alignment process. Finally, we addressed the problem of registration plausibility estimation in our approach.

METHOD

We modified the Iterative Closest Point (ICP) algorithm, presented by Besl and McKay and frequently used for surface-based registration, such that it can escape from local minima of the cost function to be iteratively minimized. The random-based ICP (R-ICP) we developed is less influenced by the quality of the pre-registration as it can escape from local minima close to the starting point for iterative optimization in the 6D domain of rigid transformations. The R-ICP is also better suited to approximate the global minimum as it can escape from local minima in the vicinity of the global minimum, too. Furthermore, we developed both CT-less and CT-based probe alignment tools along with appropriate man-machine strategies for a more time-efficient palpation process. To improve registration reliability, we developed a simple plausibility test based on data readily available after registration.

RESULTS

In a cadaver study, where we evaluated the R-ICP algorithm, the probe alignment tools, and the plausibility test, the R-ICP algorithm consistently outperformed the ICP algorithm. Almost no influence of the pre-registration on the final R-ICP registration accuracy could be observed. The probe alignment tools were judged to be useful and allowed for the digitization of 18 bone surface points within 2 min on average. The plausibility test was helpful to detect poor registration accuracy.

CONCLUSION

The R-ICP algorithm can provide high registration accuracy despite inaccurate pre-registration and a very limited number of data points. R-ICP registration was shown to be practical and robust versus the quality of the pre-registration. Time-efficiency of the cranial palpation process may be greatly increased and should encourage clinical acceptance.

Thermal osteonecrosis and bone drilling parameters revisited

INTRODUCTION

During the drilling of the bone, the temperature could increase above 47 degrees C and cause irreversible osteonecrosis. The result is weakened contact of implants with bone and possible loss of rigid fixation. The aim of this study was to find an optimal condition where the increase in bone temperature during bone drilling process would be minimal.

MATERIALS AND METHODS

Influence of different drill parameters was evaluated on the increase of bone temperature. Drill diameters were 2.5, 3.2 and 4.5 mm; drill speed 188, 462, 1,140 and 1,820 rpm; feed-rate 24, 56, 84 and 196 mm/min; drill point angle 80 degrees , 100 degrees and 120 degrees and external irrigation with water of 26 degrees C.

RESULTS

Combinations of drill speed and drill diameter with the use of external irrigation produced temperatures far below critical. Without external irrigation, temperature values for the same combination of parameters ranged 31.4-55.5 degrees C. Temperatures above critical were recorded using 4.5 mm drill with higher drill speeds (1,140 and 1,820 rpm). There was no statistical significance of different drill point angles on the increase or decrease of bone temperature. The higher the feed-rate the lower the increase of bone temperature.

CONCLUSIONS

The external irrigation is the most important cooling factor. With all combinations of parameters used, external irrigation maintained the bone temperature below 47 degrees C. The increase in drill diameter and drill speed caused increase in bone temperature. The changes in drill point angle did not show significant influence in the increase of the bone temperature. With the increase in feed-rate, increase in bone temperature is lower.

Robot- and computer-assisted craniotomy: resection planning, implant modelling and robot safety

BACKGROUND

In cases of cranial tumour, manual resection of the cancerous tissue can be very stressful and time-consuming, due to the adhesion of the subjacent dura mater. Computer-assisted planning, navigation and robotic craniotomy, with optional skull reconstruction using customized implants, are of increasing clinical interest in craniofacial and neurosurgery.

METHODS

Using preoperative computed tomography (CT) images, an automatic segmentation of the tumour is performed, followed by resection planning. The skull reconstruction is performed using computer-assisted implant modeling and manufacturing. Risk analysis of the robot-guided intervention led to the development of a new hexapod robot system.

RESULTS

Results from registration and robot accuracy on plastic and Anatomical skull are shown. The concept of a stand-alone safety system is presented to supervise the robot during the intervention. The entire process from preoperative CT scan to intraoperative robot assisted removal of tumourous bone is shown in laboratory and anatomical trials.

CONCLUSION

The laboratory and anatomy studies conducted so far provided a substantial basis for further improvement of the system's integration in the surgical workflow and the final approval of the system for initial clinical studies.