The present invention provides an improved reference source. The condyle surface is defined by one or more radii of curvatures, which are controlled to reduce or delay the onset of anterior translation of the femoral component relative to a tibial bearing. In one exemplary embodiment, the capacitive element is a capacitor. The insulating layer coats the conductive wire. The measurement result is compared with a reference distance or distances to discriminate between normal and abnormal rotational run-out of the tool holder.