Active patellar tracking measurement: a novel device using ultrasound.

BACKGROUND

Many patients suffer patellar instability that may relate to transient patellar tracking abnormalities.

OBJECTIVE

To develop and test a technique to measure dynamic patellar tracking.

STUDY DESIGN

Controlled laboratory and in vivo study.

METHOD

A functional knee brace was modified to allow an ultrasound transducer to be mounted laterally to the femur, following the path of the patella during knee movement. An ultrasound system was used to measure patellar mediolateral position parallel to the femoral transepicondylar axis. Ten subjects with no patellar instability were studied to obtain patellar tracking and accuracy data.

RESULTS

The interobserver and intraobserver reproducibility ranged from 0.2 +/- 0.1 mm to 1.0 +/- 0.5 mm. The accuracy of the ultrasound measurement was checked against magnetic resonance imaging and was 0.6 +/- 1.9 mm. The patella moved medially then laterally from extension to flexion when sitting. Squatting and stepping produced a more lateral path, without the initial medial translation. The patella was more lateral during knee extension than during flexion.

CONCLUSIONS

This novel method for measurement of dynamic patellar mediolateral tracking was found to have good intraobserver and interobserver reproducibility, and the measurements matched closely with those obtained from magnetic resonance imaging reconstructions of static patellar positions. Some preliminary data for tracking in 3 activities were obtained from 10 normal knees.