An acetabular cup guide comprising: a distal end 11 shaped to allow removable connection to a surgical instrument； a proximal end comprising one or more contact surfaces shaped to conform to an acetabulum； and a central proximal-distal axis extending through the acetabular cup guide. One or more of the contact surfaces is deflectable towards the central proximal-distal axis to reduce an axial cross section of the acetabular-cup-guide. Distal end 11 may be shaped to allow removable connection to an impaction shaft. The contact surfaces may comprise a fixed contact surface 4 and three deflectable contact surfaces 5. The fixed contact surface 4 may be provided with one or more cone shaped protrusions 9 that taper as they extend away fixed contact surface 4 for engaging cartilage in the acetabulum. The deflectable contact surfaces 5, may comprise at a proximal end, a resiliently deflectable hinge section 6. The acetabular-cup-guide may be a patient specific instrument, where the contact surfaces comprise a fixed contact surface 4 with one or more protrusions 9 on the fixed contact surface 4, and with the acetabular cup guide shaped so protrusions 9 engage a specific patient’s acetabular notch and transverse ligament.
