Mixed reality temporal bone surgical dissector: mechanical design
© Hochman et al. 2014
Received: 28 January 2014
Accepted: 27 June 2014
Published: 8 August 2014
The Development of a Novel Mixed Reality (MR) Simulation.
An evolving training environment emphasizes the importance of simulation. Current haptic temporal bone simulators have difficulty representing realistic contact forces and while 3D printed models convincingly represent vibrational properties of bone, they cannot reproduce soft tissue. This paper introduces a mixed reality model, where the effective elements of both simulations are combined; haptic rendering of soft tissue directly interacts with a printed bone model.
This paper addresses one aspect in a series of challenges, specifically the mechanical merger of a haptic device with an otic drill. This further necessitates gravity cancelation of the work assembly gripper mechanism. In this system, the haptic end-effector is replaced by a high-speed drill and the virtual contact forces need to be repositioned to the drill tip from the mid wand.
Previous publications detail generation of both the requisite printed and haptic simulations.
Custom software was developed to reposition the haptic interaction point to the drill tip. A custom fitting, to hold the otic drill, was developed and its weight was offset using the haptic device. The robustness of the system to disturbances and its stable performance during drilling were tested. The experiments were performed on a mixed reality model consisting of two drillable rapid-prototyped layers separated by a free-space. Within the free-space, a linear virtual force model is applied to simulate drill contact with soft tissue.
Testing illustrated the effectiveness of gravity cancellation. Additionally, the system exhibited excellent performance given random inputs and during the drill’s passage between real and virtual components of the model. No issues with registration at model boundaries were encountered.
These tests provide a proof of concept for the initial stages in the development of a novel mixed-reality temporal bone simulator.
KeywordsHaptic 3D Virtual Reality Temporal Bone Surgery Training Education
Concern for patient safety and outcomes underscore the importance of the ever-increasing pace of development in medical simulation.
Printed models provide another approach to temporal bone simulation and can be created from virtual models using a variety of techniques. A plaster based printer (Z-Corporation, Rock Hill, SC) uses digitized data and places successive layers of material, building a physical model (additive manufacturing). The printed models provide a realistic sense of hardness, but are ill-equipped to present soft tissues .
Each of the former simulations have been generated in our Lab , however, there is an opportunity to address the limitations in both with an entirely new mixed-reality paradigm. Mixed reality (MR) techniques fuse digital data with the human perception of the real environment. The MR simulation will combine a physical printed bonemodel with a virtual soft-tissue model. The virtual model will employ a haptic device (HD2, Quansar Corporation, Markham, Ontario) that provides real-time contact force representation. This device is a 6 Degree of Freedom (DOF), 10 newton per axis manipulandum. The haptic device will generate forces encountered during interaction with virtual soft tissues, such as decompression of a sinus or dural plate, as well as permit metric assessments. The resulting MR model should provide a platform that is capable of both the realistic force-feedback and visualization surgical trainees require.
Segment DICOM data
Mechanical merger of a six DOF haptic device with otic drill
Generate a virtual haptic model
Generate a 3D printed temporal bone model
Develop registration techniques to co-locate printed and virtual models
Develop control algorithms for virtual force-feedback interaction
The challenges in creation of novel MR Temporal Bone Simulation are considerable. This paper addresses just one aspect, specifically the mechanical merger of a six DOF haptic device with an otic drill system (Medtronic, Minneapolis MN). This is accomplished with a custom gripper mechanism. In this structure, the drill becomes the haptic end-effector.
In the MR simulation, a user must be able to employ the drill as they would in surgery. The merged haptic drill device needs to successfully navigate three different contact conditions during device operation: movement in free space, contact with rapid-prototyped bone, and contact with virtual soft tissue. Throughout these conditions and in transitions between them, the user should feel only the weight of the drill. Further, real and virtual forces need be displaced to the drill tip from the normative mid-shaft of the haptic end-effector.
Software was also developed to offset gravitational forces permitting the user to feel only the weight of the drill and not the custom fitting. The software guides the haptic device motors to dynamically generate an upwards force, equal to that of the drill fittings, compensating for their additional weight.
A series of tests were performed to ensure the operability and stability of the re-designed system under normative conditions. The first set of tests determined the ability of the system to generate sufficient power to offset the weight of the custom end effector.
A second set of tests determined the effectiveness of the system in position control mode during disturbances from the user.
Stability of the system during the drill’s passage through a MR model was monitored [Figure 4]. No issues with registration or instability at model boundaries were encountered.
The integration of printed and virtual simulation provides a platform which can represent otic drill forces as well as contact of the drill (or potentially other instruments) with embedded soft tissue structures and facilitate metric assessments. A MR system can complement existing simulations in creating a realistic and reproducible surgical training platform, which can be used to teach/assess trainees. The system permits mistakes and explorations of technique and represents an immense opportunity to improve patient safety.
Several significant challenges remain to be addressed. Highly accurate co-location of the virtual and printed simulations is requisite and will be complicated by the need to change the position of the simulation during dissection, as in real surgery. The virtual force algorithms need to be modified for the HD2 haptic as they were initially developed for a 3 DOF device. Further, investigations of construct and concurrent validity are necessary.
These tests provide evidence for the effective mechanical and software design of a novel system, integrating an otic drill with an existing haptic device. The new system provides gravity compensation and operational stability during interactions with a simple mixed-reality model.
These tests provide a proof of concept for the initial stages in the development of a novel mixed-reality simulator.
Medtronic: Donation of Otic Drill System
NSERC-Engage Grant (Industry Collaborator: Quanser)
Health Sciences Centre Foundation
University of Manitoba Faculty of Medicine
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