A new method for accurate measurement of displacement of the knee menisci

It is difficult to measure the displacement of the menisci under external forces by the radiographic method using markers because the movement may be smaller than the detecting limit. A device with a flexible needle attached to a journal of a miniature linear bearing is developed to investigate the meniscal functions which depend on the mobility. The needle tip is fixed to the target point in the meniscus and the axial displacement is measured with a laser sensor. According to the test, the lower limit for detection was 10 microns, the error due to non-linearity was less than 5 per cent and the reactive force was less than 0.15 N in the operating range of +/- 3 mm. Displacement vectors were determined at four points in a pair of menisci in a porcine knee subjected to external forces. This accurate method will lead to further investigation into the mechanical function of the menisci.