With the assistance of image based computer navigation, a robot is to be used to position and maintain bone fracture components. Based on initial three dimensional computer tomographic x-ray data of the fracture, the navigation system will operate in real-time. This will enable planning and simulation before the operation commences and obviate the need for continuous x-ray exposure. Additional x-ray exposure will also be significantly reduced with only occasional confirmational pictures being necessary thoughout the operation. As in conventional minimally invasive surgery, the bones will be secured to a fixture by means of pins. The fixture will then be prehended impactively by the robot and the ends of the bone fracture precisely mated without the danger of excessive muscle and tissue strain due to positional overshoot. Forces applied to the patient will be continuously monitored by means of force and torque sensors mounted at the robot wrist. A further advantage of the robot is that the bone may be held indefinately allowing surgery to proceed without undue operator fatigue.

First tests with Reporobo in the laboratory

Reporobo demonstrated at the Bavarian forum of mechatronics 2003

Initial Phase - choice of appropriate robot

The first phase of the project was to determine the most suitable robot. This was not so straight forward as one may assume. Ignorance of basic robotic technology has led many medical research groups to the false choice at the initial stages. Examples include:

Robots lacking the necessary articulation or large enough work envelope to reach the objective without first having to uneccessarily remove large portions of bone or tissue simply in order to gain access (e.g. RoboDoc).

Inadequate strength for the task. Most robots hitherto type approved for surgery are intended for minimally invasive surgery or bone milling and fulfill these tasks well (e.g. CASPAR) but are unable to handle forces up to 300N.

Programmability. Many robot programming languages resemble PLC systems and are simply too clumsy. Ok in the automobile manufacturing where an expert will write a program which will run without pause for six months but not in surgery where each program must be written specifically for each individual patient!

After much consideration and comparrison between systems the first phase was completed with the choice of a Stäubli RX130 with Adept V+ programming language.

The repositioning of bone segments (for example in a femur fracture) is done by the robot by means of an external fixture. Using a fully articulated 6 axis robot allows complete movement of the bone fragments within a spherical work envelope. After investigating many possible prehension strategies it was decided to make the securing point between the carbon fibre connecting rods and the stainless steel pins the prehension point. This means that only a two finger gripper is necessary. All important is the correct finger form. This prevents any possible post-prehension movement of the fixture in any of the 6 degrees of freedom.

	mechanical 3 finger gripper	pneumatic 3 finger gripper	2 finger gripper 1st concept	2 finger gripper 2nd concept

	mechanical 3 finger gripper	pneumatic 3 finger gripper	2 finger gripper 1st concept	2 finger gripper 2nd concept

The basic gripping mechanism chose is a pneumatically driven device from the firm Schunk. Exhaustive tests on all forms of gripping system and startegy have led to the conclusion that the only viable way to reliably prehend and retain the fixture is with this specially designed gripper system.

In order to control the movement of the bone fragment non-visually, a suitable interface must be developed. This facilitates data transfer between the robot controller and existing surgical equipment such as C-bow and the navigational equipment. One example is the infra-red positional identification and navigation system from the firm Brainlab.

After initial tests on femur fracture modells, the different control methods must be tested on the cadavers under professional surgical supervision. Only after successeful operation procedures on cadavers have been determined can type approval for the robotic system for use on live patients be considered.

The first paper for this project was made for the Internationalen Telemedizinkonferenz in Regensburg. Soon there will be further informations.

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