Fitting the Perfect Implant for an Artificial Hip

A new measurement method could allow orthopedic surgeons to precisely calibrate leg length after a hip replacement, so that it matches its original length.

Developed by researchers at the Fraunhofer Institute for Machine Tools and Forming Technology (IWU; Chemnitz, Germany) the new measuring system is based on a small plastic box containing two light emitting diodes (LEDs) that is affixed to the patient’s shin. The doctor then takes hold of one of the patient’s heels and lifts it upward. With that motion, the two lights trace an arc that is recorded by a camera positioned about 1.5 meters to the side of the patient. The measurement is taken twice, once right before the operation and once after the implant has been temporarily inserted.


The box remains on the leg during the operation. If the distance between the two lights changes in the second measurement, i.e., if the leg becomes shorter or longer, the arc traced by the LEDs will change. A software program compares both arcs to determine if the leg is the same length it was before the procedure. If necessary, adjustments to the artificial hip can be made to correct the arc. Initial testing of the measuring system prototype has already met with success at Leipzig University Hospital (Germany).

“The margin of error in our process is less than one centimeter,” said Doctor of Scientific Medicine Ronny Grunert, a researcher at IWU. “Eventually we’d like to get that down to five millimeters. Currently, the usual procedure calls for the doctors to determine leg length with a tape measure, which can lead to errors of up to two centimeters.”

The principle of measurement used is similar to that of a compass; the hip joint serves as the center point of the circle of the compass, while the LEDs act as the “pencil” tracing the arc.

Related Links:
Fraunhofer Institute for Machine Tools and Forming Technology
Leipzig University Hospital