Institute for
Robotics and Process Control

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Reposition of Femoral Shaft Fractures

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Project Description

Problem Formulation

In this research project methods for robot assisted fixation of femoral (the human thigh bone) shaft fractures are developed and evaluated in cooperation with the Department of Trauma Surgery of the Hannover Medical School(External). Those fractures are often a result of high energy traumas like traffic accidents and are nowadays usually fixed with a so-called intramedullary nail. The x-ray images below illustrate such a fracture, which is stabilized with a nail.

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X-ray image of a broken femur [Source: AO Principles of Fracture Management]

In order to insert the nail into the bone's medullar cavity, a hole has to be drilled into the bone at the hip end side of the leg. The intramedullary nail is inserted through this hole until it reaches the fracture region. Subsequently the bone fragments are aligned by the surgeon and the nail is further inserted. Finally the nail is interlocked with the bone by means of screws at both sides of the fracture.

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The course of an intramedullary nailing operation

The quality of the operation result can be measured by two parameters: The lenght of the leg and the rotation about the leg axis. Depending on the type of fracture, the precise re-establishment of these parameters can be very difficult to achieve by the surgeon. If these parameters deviate too much from the physiologically correct values, a second correction operation might become necessary.

Project Goals

The primary goal of this research project is the development and evaluation of computer and robot assisted methods in order to support this challenging surgical procedure. With the combination of image analysis, force/torque guided robot control, and preoperative planning and simulation, the achievable reduction accuracies should be increased.
For the patient a precisely repositioned fracture has the advantage of reestablishing the physiological conditions of the leg and in particular of the joint axes in the knee and the hip. Furthermore, the probability of complications and possible re-operations is reduced.
Besides the increased quality of the operation outcome, the simplification of the surgical procedure will be a further advantage on the surgeons' side. With conventional methods, the repositioning is a very challenging task. Achieving a correctly repositioned fracture requires high efforts to move the bone fragments against counter acting muscle forces and comes along with a conspicuous exposure to x-ray radiation. Reducing the x-ray exposure to the OR staff is a second very import goal of this project.

Further Questions

This project not only focuses on the repositioning of fracture segments, but also covers relevant questions in the context of the whole surgical procedure. An important aspect is for example the precise hip side opening of the medullar cavity by means of a surgical drill. Here vision based algorithms lead to an automated robotized drill guidance tool, which can conspicuously improve the accuracy and reliability of such surgical tasks.

Pose Estimation of Cylindrical Objects for a Semi-Automated Fracture Reduction

Summary

Below we present the results of our method for computing the relative target transformations between broken cylindrical objects in 3d space.
We first compute the positions and orientations of the axes of every cylindrical object. This is achieved by a specially adapted Hough transform. These axes are the most important attributes for the segmentation of fractured bones and can also be used as an initial pose estimation (constraining 4 of the overall 6 degrees of freedom of the reduction problem).
After these preprocessing steps, the relative transformation between corresponding fracture segments can be computed using well-known surface registration algorithms. Here we are using a special 2D depth image correlation and a variant of the ICP (Iterative Closest Point) algorithm. A project goal is using these methods for computing the target poses of bone fragments in order to allow for a computer assisted semi-automated fracture reduction by means of a robot.

Matching Results

Further information about the automated computation of target poses of fractured bones can be found here:
3D Puzzle

Fracture Reduction using a Telemanipulator with Haptical Feedback

Summary

We developed a complex system, which allowed to use a robot as telemanipulator for supporting the fracture reduction process. Our robot is a standard industrial Säubli RX 90 robot. The robot is controlled by the surgeon by means of a Joystick with haptical feedback. Intraoperative 3D imaging of the fracture is the base information for the surgeon during reduction. These 3D volume images are automatically segmented by the PC resulting in highly detailed surface models of the fracture segments (cp. the figure below), which can be used by the surgeon to precisely move the fragments to the desired target poses. An optical navigation system ensures that the 3D scene presented on the PC display is always in accordance with the real surgical situation; the virtual 3D models always move in the same way as the real bone fragments, which are moved by the robot.
All forces and torques acting in the operation site can be measured by means of a force/torque sensor mounted at to robots hand. These forces are fed back to the joystick. This way, the surgeon is able to feel the forces acting on the patient because of distracted muscles or contacts between the fracture segments.

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The interaction principle of a 3D telemanipulated fracture reduction

Results

In a first test series, the telemanipulator system was evaluated in our anatomy lab using broken human bones (without surrounding soft tissue). It could be shown that reduction accuracies with mean values of about 2° and 2mm can be achieved for simple fractures. Even for complex fractures the achievable accuracy stays below 4°. From a clinical point of view, these values are more than acceptable.
Furthermore, the telemanipulator system was also tested on human cadavers; complete specimens with intact soft tissues around the broken bone. The results have been similar to those outlined above. In addition we could show that to telemanipulated reductions achieve significantly higher reduction accuracies than manual reductions, which have been performed by an experienced surgeon on the same fractures.

Conclusion

The presented form of visualization and interaction with a telemanipulator system for fracture reduction in the femur turned out to be efficient and intuitive. All test persons have been able to perform reliable reductions with high reduction accuracies after only a short time of learning. These results clearly show the potential of robotized fracture reduction, which will ensure high quality outcomes of such operations in the future.

Further Information


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