Optical scan improves fit of artificial hips

08 Apr 2015

LED and camera system could cut limping and discomfort; Fraunhofer-led team shows system at Medtec expo.

When a patient receives a new hip, it is usually adjusted only approximately to leg length. Greater accuracy requires a more precise measuring process as well as adjustable implants.

Now, a new type of measurement method coupled with a modular implant should allow orthopedic surgeons to precisely calibrate leg length after the operation so it matches its original length. The researchers will be presenting their development at the Medtec expo in Stuttgart, Germany, between April 21-23.

Currently, orthopedic surgeons have no suitable method for precisely measuring leg length before the operation or for adjusting the implants accordingly. The result is that after the operation, the leg can actually be longer or shorter than it used to be. This leads to problems with the spine, which have to be resolved using shoe inserts.

A new measuring technique developed by the Fraunhofer Institute for Machine Tools and Forming Technology IWU, will allow orthopedic surgeons to measure their patients’ leg lengths much more precisely.

The Fraunhofer researchers collaborated on the new system with several project partners: the Clinic for Orthopedics, Trauma Surgery and Plastic Surgery at the Leipzig University Hospital; University of Applied Sciences Zwickau and its Research and Transfer Centre; AQ Implants; and MSB-Orthopädie-Technik.

“The margin of error in our process is less than one centimeter,” said Dr. Ronny Grunert, a researcher at IWU. “Eventually we’d like to get that down to 5mm.” 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.

How it works
With the patient in a prone position, the doctor affixes a small plastic box containing two LEDs to the patient’s shin. The doctor lifts the patient’s heel. With that motion, the two lights trace an arc that is recorded by a camera positioned about 1.5m to one side of the patient. The principle is similar to that of a draughtsman’s compass; the hip joint, from which the leg hangs, is essentially the point of the compass, while the LEDs act as the pencil.

If the distance between the two changes, i.e. if the leg becomes shorter or longer, that will change the arc traced by the LEDs. The doctor takes this measurement twice – once immediately before the operation and once after the implant has been temporarily inserted. The LED box remains on the leg during the operation.

A software program compares both arcs to determine if the leg is the same length it was before the procedure. If necessary, the doctor can make adjustments to the artificial hip. Initial testing of a measuring system prototype has already met with success at the Leipzig University Hospital. There are plans for a clinical trial later this year, and the new system could be on the market in about two years.

Unbreakable, adjustable implants

The Fraunhofer researchers also optimized the hip implants, again working together with partners from industry, medicine and research. “We’ve developed an implant that can be adjusted to each individual patient,” said Grunert. The new system was developed within the “artificial joints” cooperative network, which is funded by Germany's Federal Ministry for Economic Affairs and Energy (BMWi) and coordinated by Fraunhofer IWU.

The trick was to do away with prefabricated implants in various sizes and use a modular system instead. In this method, the doctor can select the right hip stem as well as the right neck for each patient. Special screw connections are used to attach the individual parts to each other and the combined unit is implanted in the hip for testing. The doctor now measures the leg length, and, if necessary, can easily separate the implant’s various components to exchange them for better-fitting parts or adjust them as required.

About the Author

Matthew Peach is a contributing editor to optics.org.