Prime goal of the researchers was to improve specialist physician training, as Lorenz underlines: “Today, specialist physicians in training practice on phantoms, i.e. artificial bones. Material behaviour, however, does not mirror real material behaviour closely enough, which in fact also holds true for donor bone material – a rare and extremely valuable resource in medical training and research.”
Specialist physicians in training have to assist in a number of surgeries before they are asked to perform the milling step themselves. “Wouldn’t it be much better if the physicians in training had performed several simulations that are as close as possible to the real thing before that crucial moment arrives? Wouldn’t it be better to give them an opportunity to develop the necessary feel for the procedure, build self-confidence and allow them to make errors from which they can learn?“, Lorenz asks. In minimally invasive surgery such as keyhole interventions, VR training has been around for about 20 years, Lorenz points out and adds that “For procedures that require high forces such simulation products are not available, neither commercially nor for research purposes.”
The HIPS users see the virtual patient’s hips through VR glasses while they operate a surgical milling instrument which in turn is connected to a lightweight robot, the BR iiwa. The user has to mill the hip joint’s acetabulum – something that is difficult to teach, train and learn. The robot delivers direct haptic feedback by simulating the resistance of a real bone. Thus the user acquires a better feel for the forces at work and the area that can be worked.
The simulation is based on real data. In order to compute the forces and torques when milling the hip the researchers conducted biomechanical tests with anatomical specimens. “We found out that we are dealing with up to 200 Newton. The force is needed when the surgeon applies pressure on the acetabulum. The forces applied on the surgeon’s wrists when the machine stops is between 20 and 50 Newton depending on the size of the wrist,” Lorenz reports.
With these measurement data researchers at the University of Bremen, Germany, designed a material model which allows the surgeon to compute within a millisecond where the surgical milling instrument meets the acetabulum and which forces are present.
In a next step a software module was developed which features a user interface integrated by FAKT Software GmbH. The result is an interactive application based on an anatomical model by CAT Production GmbH which had created the anatomical 3D models and the virtual OR.
The team at the Technical University Chemnitz developed the interface with the robot arm to “tell” the robot the force it has to simulate. “Originally, the robot was certified for collaborative human-machine tasks on the shop floor. We managed to use the existing interface to provide the user with haptic feedback while maintaining the certification,” Lorenz explains.
Dynamic HIPS - The next steps
Scientists are developing a dynamic hip implant simulator in the "Dynamic HIPS" project, which officially started on 1 May 2020.
The project pursues several goals that will make it possible to develop the necessary training devices. The researchers want to determine the forces, torques and speeds that occur during the three surgical steps to be simulated. On this basis, they want to further develop a robotic arm and existing haptic devices – devices that can convey realistic sensory perceptions. Equally important is the creation of a mathematical model that simulates the resistances and material removal at the bone. This information must be transmitted to the robot within a millisecond to give the surgeons a realistic feeling.
A second set of objectives deals with VR technology, which enables joint training over long distances ("remote training"). With the help of a multi-user system, experienced surgeons can pass on their medical expertise to trainee surgeons without being on-site themselves. This functionality not only facilitates the transfer of knowledge to emerging and developing countries but also serves the exchange of knowledge between experienced surgeons.
Researchers face the challenge of minimizing the time delays in synchronizing VR scenes despite the large spatial distance between users so that both surgeons are in an identical situation. At the same time, the scientists want to strengthen the interaction between users in the VR environment by providing ways to communicate without words, for example by pointing to virtual 3D drawings. In the future, it should also be possible to record sequences and add audio commentary to create training videos.
In 2011, after having completed his informatics program at Westsächsische Hochschule Zwickau, Germany, in the previous year, Mario Lorenz was appointed assistant at the Chair of Machine Tools at the Technical University Chemnitz in the field of VR and Augmented Reality (AR). Since 2016, he has also been guest scientist at the Clinic for Orthopaedics, Trauma und Plastic Surgery at University Hospital Leipzig. His research focuses on the use of VR and AR technologies in production and medical training.