[Measurement precision of sonography to evaluate movable meniscal bearings]

After resection of the ACL, the knee exhibits a paradoxical movement but the effect of this movement on function,wear, and implant survival in a meniscal bearing total knee replacement (TKR) is not known. Up to now, only radiological methods are described to measure the meniscal bearing's movement. We developed a method using ultrasound for analyzing MB motion. In an in vitro study we compared ultrasound of 5 MHz with 8 and 12 MHz, we studied the effect of malrotation on the projection of plain and digital radiographs, and compared ultrasound of 8 MHz with plain and digital radiographs. In all three methods the distance was measured between the anterior border of the tibial component and the anterior rim of the meniscal bearing. The accuracy and precision of 8 and 12 MHz were identical and statistically significantly more accurate than 5 MHz (p<0.01). Malrotation had a significant impact on the radiological projection. Neutral rotated radiographs and ultrasound of 8 MHz did not differ in accuracy and precision, but ultrasound was significantly more accurate than any radiograph with a malrotation of 1 degrees or more (p<0.001). We conclude that ultrasound of 8 MHz is an alternative to radiological methods for the measurement of meniscal bearings' motion.