Engineering aspects of pre-surgery planning using virtual reality.

Dreucean, Mircea ; Sticlaru, Carmen ; Hoigne, Dominik 等

1. INTRODUCTION

The benefit of virtual reality technology has touched the medical world and added a very useful training method in surgery. One of the first applications for virtual systems in medicine was the stent surgery. Another direction is the development of training software for orthopedic surgery based on virtual reality. The simulator has to include a number of virtual objects like surgery table, bones, tissues, muscles, blood vessels as well as surgical instruments like scalpel, holders, drills and many others. There are a few challenges in this process of virtualization:

* All the components have to be created as CAD parts and the human bones should be customized for every patient. On this purpose a spiral CT in the Imagistics laboratory and the MIMICS software for the 3D reconstruction based on the CT slices were used. The members of the team use ProE to create the surgical instruments. The parts created in this way are always too large to be used in the virtual scene. Therefore the number of triangular facets must be reduced and the parts should be transformed.

* In order to obtain a virtual reality model of the surgery room as well as the patient's anatomy, one or more haptic devices must be used. For small systems the simple use of a phantom device and a normal stereoscopic screen could satisfy, but in the case of larger models a CAVE system is more relevant.

The research team is oriented to the 3D reconstruction of the human skeleton parts based on CT scans using MIMICS and to the design and rapid prototyping of the implants of various types, both osteosynthesis implants and complete joint replacements and their interaction with the bone. Many FEA studies have been developed for the bone-implant interaction. (Krauze, et al., 2008)

2. CAD MODELS OF THE FEMUR

The study was oriented to the combination bone-implant at the level of the proximal femur. The first problem to solve was the achievement of a CAD model of the femur as close as possible to the real bone. In the first step of the procedure the femur was 3D rebuilt from a series of CT slices. The reconstruction was developed in MIMICS and the result was a realistic replica of the bone. The bones were virtually fractured in the trochanteric region in a manner the most frequent fractures of the proximal femur occurs. Over this model a set of implants were inserted and contact conditions were defined between the implants and bone with the aim of taking this models into FEA and virtual environment.

2.1 Types of fractures

An exact analysis of the fracture is important for choosing the optimal treatment strategy. For proximal femur fractures, the classification system from the AO (Sadowski, 2002), (***, 2009) was used. The trochanteric fractures are subdivided in three types: pertrochanteric simple (Type A1) (fig. 2a), pertrochanteric multifragmentary (Type A2) (fig. 2b) and intertrochanteric fracture (Type A3) (fig. 2c).

The femur with fractures was designed in ProE and the CAD models are ready to be taken in the virtual reality application. The models of different types of implants will be used for training the surgeons. As used for planning a real osteosynthesis of a patient, the models of the bones can be generated using the MIMICS software, as mentioned before, which can produce 3D models based on CT scans. Recently it was developed the method for the generation of the 3D models starting from normal 2D X-ray. (Matthews, 2007).

In some cases the intertrochanteric fracture is multifragmentary and reversed and separates the greater trochanter completely from the femur. In this case the forces at the fracture line are different from the normal fracture type A1 and special measures have to be taken in the process of reduction of the fracture and implantation of the implant (fig. 2d).

In the pertrochanteric multifragmentary fracture (type A2) and in the multifragmentary intertrochanteric fracture (type A3) the trochanter minor is broken as well. So, the forces acting on this dorsomedial column have to be compensated by the implant. The abductor muscles stabilize the pelvis especially during walking in one leg and standing. If the trochanter major is fractured completely, the abductor muscles are disconnected and cannot affect the leg any more. Major limping is the consequence. That's why one of the main goals of the operation is to fix the trochanter major.

The different types of implants are intended to keep the fragments in position during the healing time. The reposition of the fragments in correct position has to be achieved priory. The reposition success in proximal femur fractures is assured mostly by indirect traction of the fragments, usually on a traction table and without cutting the skin.

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2.2 Types of fixation implants

Four different types of implants are designed and documented in this study:

* Fixed implant, designed as a bended blade with cutting edges, (also called "blade plate") which is driven from lateral side through the trochanter region into the center of the femoral head. The other end is in good contact with the external femur fixed with screws (fig. 3a).

* Sliding screw-plate. The screw splints the fracture inside while the plate is fixed extramedullary. The dynamic connection between the plate and the screw allows the screw sliding along this determined angle allowing impaction of the fracture in the axis but no other dislocation. For bed-ridden patients this is not applicable and the dynamic implant is not the implant of choice because there is no compression in the fracture (fig. 3b).

* Intramedullary dynamic implant enables also a dynamic connection of the femoral head and the stem. In contrast to the sliding screw plate, the fixation on the stem in the femur occurs in the intramedullary space. It is assumed that it supports better the dorsomedial forces in cases of fracture of the trochanter minor (A2 and A3). The insertion of this nail is performed from proximal lateral side through abductor muscles. There is a discussion if this circumstance could compromise the function of the abductor muscles and cause limping. Another risk of the nail is that during the insertion, the fractured trochanteric region blasts. The head screw is inserted through a small incision from lateral side (Fig. 3 c). (Koumoutsakos, 2003).

* Ender nails are flexible stabs integrated in the core of the bone, in marrow, from distal end up to the femoral head. They are usually used for fractures at children (fig. 3d).

Fig. 3 represents the CAD models of the four types of implants for trochanteric femoral fractures, based on the types of implants normally available on the international market.

3. INTEGRATION OF THE FRACTURE IN THE VIRTUAL REALITY MODEL

In order to make the manipulation of the separate objects possible in the virtual reality environment, using a haptic device like a glove or phantom joy stick, the parts must have draggers and manipulators attached (Krauze, et al., 2008). This can be achieved with special software like Inventor which has a proprietary file format for 3D models including the draggers and handlers for objects. The objects used in this kind of simulation with virtual reality are rigid. In this way, the collision detection problem is easier and can be solved using bounding boxes and distance fields. In the case of flexible objects (e.g. internal organs, lever, lungs, etc) the collision detection problem is more complicated.

The Layered Depth Images algorithm can be used in such cases, with good results both for rigid and deformable bodies (Krauze, et al., 2008).

Better immersion is achieved by using large stereoscopic screen or CAVE systems, letting the surgeon study and manipulate 3D geometric representations of the fracture elements on a virtual environment with a higher degree of realism. The system can be adapted to a variety of surgical procedures using both common surgical tools and custom instrumentation. The implemented tools let the surgeons learn how to fix fractured bones and perform preoperative planning (Talaba et al., 2006).

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The virtual environment used for pre-surgery planning in Orthopedics runs on the basis of a complex software structure. The structure is organized around the holo-cave and comprises a few intercommunicating modules: an interaction devices module, a Multi User Server module that performs the administration of the 3D model and of the users, a Virtual Environment Server which coordinates local projections and navigation devices, a remote network data management module, a 3D geometric identification and modeling of the fractures based on reconstructed 3D CT images. System also facilitates the remote collaboration in a virtual surgery room.

4. CONCLUSIONS

Planning the osteosynthesis on a virtual reality-based model allows comparing different therapeutic concepts as well as testing the stability in walking. The forces acting on the hip are complex and an additional tool like Finite Element Analyze could be useful for the validation of a certain solution. The model can also be used as a teaching tool for young surgeons and the components of the model can be integrated in a medical CAD library.

The next step in the development of the research theme is the completion of the set of surgery room equipments in cooperation with the surgery team in Basel. Another step is the addition of new models of body elements in the collection of virtual objects used in the training of orthopedic surgeons in the virtual environment.

5. REFERENCES

Koumoutsakos, P., (2003), A Virtual Surgery Environment, Semester thesis SS 2003, Institute of Computational Science ETH Zurich, Switzerland

Krauze, A., Kaczmarek, M., Marciniak, J., (2008), Numerical analysis of femur in living and death phase, Journal of Achievements in Materials and Manufacturing Engineering, Vol 26, Issue 2, February 2008

Matthews, F., et al., (2007), Patient-specific three-dimensional composite bone models for teaching and operation planning, Journal of Digital Imaging, ISSN 0897-1889 September 21

Sadowski, C., et al., (2002), Treatment of Reverse Oblique and Transverse Intertrochanteric Fractures Screw-Plate, Journal of Bone & Joint Surgery, Vol 84-A(3), March 2002

Talaba, D., Mogan, Gh., et al.,(2006), Virtual Reality in Product Design and Robotics, Workshop on Virtual Reality In Product Engineering And Robotics, Bulletin Of The Transilvania University Of Brasov ISSN 1221-5872, 2006.

*** (2009) http://www.aosurgery.org--AO Surgery Reference, Accessed on: 30.05.2009