A biocompatible medical implant has at least one part thereof composed of ceramic material with the formula NaxKyNbO3, wherein 0≦x≦0.8, 0.2≦y≦1, and x+y=1. The ceramic material can be polarized to give the implant spatially varying piezoelectric properties. Signals can be measured from the respective piezoelectric regions of the selectively polarized ceramic material to provide an indication of the degree of fixation of the implant to surrounding tissue.