Apparatus and methods are disclosed for simultaneously tracking multiple finger and palm contacts as hands approach, touch, and slide across a proximity-sensing, multi-touch surface. A centrifuge rotor encloses a closed separation space, which has a radially outer part for the heavy phase, a radially inner part for the light phase and a central gas-filled space. The prediction direction may be at least one of horizontal, vertical or diagonal. The bone fusion device is preferably inserted by using an arthroscopic surgical procedure.