A below-knee prosthesis in which an adapter is displaceable and fixable in relation to the lower socket end on a circular segment surface whose circular arc lies in the sagittal and frontal plane and whose midpoint is the center of the knee. ...http://www.google.co.uk/patents/US5755812?utm_source=gb-gplus-sharePatent US5755812 - Below-knee prosthesis having an adapter displaceable on a spherical segment surface 
