Computer-guided Total Knee Replacement is an exciting new technique which promises several advantages for patients.
Total Knee Replacement (TKR) was first described in the 1940s, but it was not until the 1970s that the biomechanics of the knee were better understood. Early TKR implants varied tremendously in design and function, but most modern knee prostheses look remarkably similar. Today there are only minor differences in design and philosophy.
During a knee replacement the bottom of the femur and the top of the tibia are removed and replaced with metal surfaces. A plastic insert is then attached to the tibial component, creating a metal on plastic articulating surface. The back of the knee-cap may also be replaced.
One of the main problems with modern implants is ensuring that they are straight all the time. Achieving the ideal implant position in 3 dimensions is crucial to the long-term function and longevity of the replacement. To be even a few degrees out may have significant consequences for the survival of the replaced knee.
Conventional instrumentation involves driving long rods up the femur and often down the tibia in order to aid surgeons achieve anatomical alignment. However, even in experienced hands the technique is not infallible.
Earlier versions of computer guided techniques were developed in order to improve the alignment. These involve specialised 3-D registering cameras in the theatres. They are often clumsy, and in most hands the surgical time is longer than with conventional alignment techniques. There are also several inherent inaccurate steps.
The Latest Generation
The latest generation of computer guided techniques remove several of these problems and have been shown to improve accuracy.
Patients are sent for a quick pre-operative MRI (or CT) scan. This is not a full diagnostic scan but simply builds a 3-d image of the knee. The planning is then done at the surgeon’s convenience. Once the exact optimal position of the implants is selected, the images are sent to the factory where a unique set of small plastic jigs are made for that patient.
Intra-operatively these tailor-made jigs fit snugly on the femur and tibia. They are then used to accurately position the cutting blocks, which in turn ensure that the implants are positioned accurately. In essence, the “computer guided” part of the operation is done before the operation. Unique advantages of this new system include:
- Improved accuracy of implant positioning, thereby optimising function and implant survival
- Decreased theatre time, decreasing wound exposure time and cost
- Smaller surgical exposures, reducing bleeding and enhancing rehabilitation
- Fewer instruments are needed in theatre.
While all patients may benefit from these new techniques, the advantages are significant in the “difficult knee”. Such cases would include patients with significant varus or valgus knees, and overweight patients.