Abstract: In a total hip replacement, a stem of a femoral component is disposed in a patient's thigh bone with a ball extending in such femoral component above the stem. The ball is disposed in a hemispherical cup positioned in the patient's pelvis and enveloping the ball for frontal movement of the ball relative to the cup. A low-friction liner (e.g., plastic or ceramic) is disposed at the bottom of the cup. To determine whether the ball is properly positioned in the cup, a first member (e.g., a coil) is disposed on the bottom of the liner. A second member (e.g., a patch defining a loop) is disposed on the patient's thigh in operatively coupled (e.g. magnetic) relationship to the first member. The operative coupling between the members is variable in accordance with variations in the disposition of the ball in the cup. When the first and second members are respectively a coil and a patch, the coil and a capacitance may be fixed in the patient's body in a circuit resonant at a particular frequency.