Abstract: A device and methods for use in determining the trajectory of a patella of a patient during a navigated knee replacement surgical procedure are described. The device includes a first component including a mounting for attachment to a posterior surface of the patient's patella and a second component having an outer surface presenting a formation adapted to engage with a femoral part of the patient's knee. A slider mechanism allows the second component to translate relative to the first component in a substantially medial-lateral direction of the patient when the first component is attached to the patient's patella. The device can be used in a method to help determine the medial-lateral position for a patella implant.