A model knee joint.
With SatNav, the location of the car is identified by the signal received from the different GPS satellites by a receiver in the car. Once the car location is identified, the route can be planned.
During computer guided joint replacements, special infra-red trackers fixed to the bones act like satellites. The surgeon maps the hip or knee joint by special instruments. The infra-red signals are picked up by a camera and relayed to a computer which creates a three-dimensional model of the joint which aids surgeon to plan optimal bone re-section and alignment (like a route map).
Most of the conventional hip and knee replacements are done with the aid of manual jigs requiring an element of "eye balling".
Many studies have shown that with manual jigs, there will be some patients in whom the implants are placed in less than ideal position (outliers).
This can lead to early wear and loosening of the artificial joint (like the car tyres wearing faster if not properly aligned).
The main advantage of computer guided surgery is that it is more accurate and helps to decrease the "outliers" which is particularly important in active persons and certain type of bearings like metal on metal hip resurfacings. Studies have also shown that accurate alignment during knee replacement is associated with decreased length of stay and improved early function.
Although this technology has become more user friendly in the recent years, it still requires special training and equipment. Hence it is used only by a relatively small number of surgeons.
A recent innovation in knee replacement is "patient specific instrumentation" - this will be explained in next week's column.