PatientSim [bibcite key=Fuerst2012b,Fuerst2014a] is our pilot project where we are developing a hybrid surgical simulator for cement augmentation techniques (see Figure 1).
Vertebral compression fractures (VCF's) are mainly due to osteoporosis affecting primarily the vertebral bodies of the thoracolumbar region. The incidence rate in Europe was 1.4 million in 2002 and 700.000 in the U.S. in 1997 respectively [bibcite key=Melton1997,Epos2002]. Three compression fracture types with different degrees of severity can be classified. In nearly all types of VCF's, the endplates (cranial and caudal) of the vertebral body are more or less disrupted [bibcite key=Zhao2009]. Therefore, a suitable treatment method (conservative or surgical) should be selected only after a detailed assessment of the fracture [bibcite key=Peh2008,Hardouin2002].
|Figure 1: PatientSim – a hybrid surgical simulator for cement augmentation techniques.|
In case of a surgical intervention, cement augmentation techniques like vertebroplasty or kyphoplasty are very common. Both interventions are characterized through the application of artificial bone cement (mostly PMMA – Polymethylmethacrylate) into the affected vertebra(e). In case of kyphoplasty, an inflatable balloon is inserted prior restoring the initial height of the vertebral body. Thus, small cavities develop why the applied cement is more viscous and injected with less pressure compared to vertebroplasty. This decreases the risk of cement leakage. There are different ways of learning these procedures.
- First, a mentor teaches novice surgeons directly on patients leading to poor patient safety.
- Second, novice surgeons attend an intervention-specific course, where they can perform on human specimen under expert control. Although these courses are close to reality, some disadvantages should be mentioned. Besides high costs, the availability and especially ethical aspects limit the usage of human specimen. Furthermore, fluoroscopy is used for instrument guidance leading to radiation exposure and limited training periods.
- Third, and addressed by this project, the use of a surgical simulator simulator allows to overcome the latter mentioned drawbacks.
Augmented Reality Simulator
Due to the advantages in comparison to VR simulators or box trainers, an augmented reality simulator is going to be developed. The whole simulation environment is designed in a three-layered structure (see Research, Figure 1). The patient phantom with artificial, physical vertebrae and soft tissue is the key element of the hardware layer. The surgeon is able to work on these structures using real surgical instruments supported by a detailed visualization. As a consequence the image guidance is achieved by a fluoroscopy simulation, which relies only on 3D models of anatomy and instruments and therefore requires no X-ray radiation. The rigid tools necessary for transpedicular bone access were equipped with sensor coils providing position and orientation in 3D using an electromagnetic tracking system.
Central to the hardware abstraction layer is a C++ console application; the device server. Connected clients can easily send scripting commands to communicate with certain hardware (e.g. the tracking system) via User Datagram Protocol (UDP) or Transmission Control Protocol (TCP). On the application layer, a graphical user interface (GUI) is responsible for a detailed visualization of the anatomical data. The GUI is programmed in C++ using various open source applications like Qt or Kitware's visualization toolkit (VTK). Each desired anatomical structure can be loaded as a CAD model and registered using the characterized insertion instruments. The area of surgery is visualized in two-dimensional projections to improve the surgeon's spatial perception. During each insertion trial, task-specific parameters (duration, instrument path length, etc.) are recorded for post-processing.
Artificial vertebrae as well as artificial soft tissue are the main components of the patient phantom. Three artificial vertebrae are embedded and clamped into a torso, which is covered by artificial soft tissue. A suitable polyurethane foam recipe with comparable haptic feedback was developed imitating the inner cancellous core of vertebrae. Additionally, different resin materials were tested covering the spongy structure as a layer of cortical bone. The resulting, artificial imitations were validated in reference to human specimen. Similar to the artificial vertebrae, the soft tissue imitations were developed and validated (see research area "Artificial Bones and Soft Tissue" for more details). After training of the bone access the penetrated vertebrae are removed from the clamping device and replaced by new vertebrae. Both, task-specific parameters as well as the accessed artificial vertebrae, which can be opened by using a band-saw, can be used for a final discussion with a medical expert.