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Computer Assisted Total Hip Replacement

Computer-Assisted Total Hip Replacement: used only with posterior mini-incision

Computer navigation is a new technique for total hip replacement. This research has been developed at the Dorr Arthritis Institute with funding through the Dorr Institute for Research and Education. The reason for using this computer program in surgery is to give more precision to the placement of the acetabular and femoral components, the mating of the femoral head into the cup, and providing accuracy for the determination of leg length and the offset of the hip (relationship of the femur to the pelvis). The computer program works by registering the anatomy of the patient onto the computer software so that the computer can then determine the position of the implants that are placed into the hip. The anatomy of the pelvis and of the femur for the individual patient is determined by registering the pelvis and the femur of that patient. Once this anatomy is known to the computer software, when the cup and femoral stem are placed into the bone, the computer will be able to define the precise position of these components in relationship to the bone. Results have proven the accuracy of the computer for component positions and perfecting the mating of the components. The accuracy of the computer methods have been validated by CT scans.

Technical Aspects of Computer Assisted Total Hip Replacement

All the information is registered into the computer by a camera with two "eyes" that identifies three silver balls that are present on a holding device. The camera emits an infrared light that reflects off the balls. The balls are attached to each holding device and each guide that is used so that the information can be registered from each of these tools. To register the pelvis a holding device for the pelvis tracker is attached to the pelvic rim by three pins. Three bony sites are touched on the pelvis (the two anterosuperior iliac spines and the pubis). By registering these three bony sites the computer can determine the anteroposterior plane of the pelvis. Once this plane of the pelvis is known then the correct cup position into the acetabulum can be defined by the computer.

The position of the femur is determined by a holding device attached to the femur bone just superior to the knee. The tracking device is held in this position and the anatomy of the femur is determined by touching the femur bone on the trochanter and the two epicondyles of the femur as well as the two malleoli of the tibia. This allows the computer to determine the plane of the femur bone. This is registered by the infrared light locating the tracker signals from the tools to send the position to the computer. The signals are obtained from the silver reflecting balls which are attached to tools and are located by the dual-eye camera.

When the acetabulum is reamed in preparation for implantation of the femoral component, the position of the reamer in the acetabulum is shown by the computer. The acetabulum has been outlined by using the guide to make several points within the acetabulum to outline the shape and bony walls of the acetabulum which are displayed on the computer screen. The position of the reamer within the acetabulum then can be seen within the acetabulum on the computer screen. When the cup is placed into the acetabulum there is a tracker on the insertion tool so that the computer can recognize the position of the cup within the bony acetabulum. The computer screen shows an outline of the structure of the acetabulum and the cup and gives numerical values of the position of the implant and the relationship of the implant to the acetabulum. This position of the acetabulum is adjusted for the tilt of the pelvis that is present from the position of the patient on the operating table. This adjustment provides near absolute precision for the position of the acetabular implant.

The position of the femoral stem in the femur can also be defined on the computer screen during implantation of the femoral stem. The level of cut of the femoral neck can be measured so that the relationship of this to the stem for leg length and offset is known. As the femur is prepared by broaches, and as the stem is inserted into the femur, the computer screen shows the level of the stem compared to the level of the neck cut. This allows the surgeon to understand how much further the stem needs to be inserted to give correct leg length and offset. The computer also shows the center of the femoral bones so that the position of the implant in relationship to the center of the bone is seen on the computer screen. The numerical values of the position of the implants gives the relationship of the implants to the body so that the leg length is known and the relationship of the femur to the pelvis (offset) is known. This precision optimizes the stability of implants which can protect against dislocation and can reduce the wear of the bearing surface by decreasing impingement of the femur bone against the pelvis or the femoral stem against the cup. This stability and prevention of impingement is done by improving the mating of the femoral head into the cup by knowing the numerical values of the cup position and the femoral position.

The only cost for the patient with the use of the computer is the scars from the pins in the lower thigh and on the skin above the pelvis. To date, we have had no other complications such as infection of the pin sites or fracture of the femur bone or breakage of the pins. Theoretically, it is possible that these complications could occur.

The advantage of the computer is the precision of placing the implants to provide a more stable construct and one which has less risk of impingement which is known cause of failure. The precision of the computer and the knowledge for the surgeon of implant position by use of the computer, improves the total hip replacements that are performed with small incisions which necessarily limit the direct vision of the surgeon to know the implant positions as compared to what can be seen by the traditional incision exposure.

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