Orthopaedic instrument

A patient specific instrument for use in a ball and socket joint arthroplasty procedure, method of manufacture and method of use are described. The instrument comprises an attachment mechanism for attaching the instrument to the socket of the patient, which has at least first and second parts which can engage the rim of the socket of the patient. Each part is shaped, or can transform to a shape, to match the shape of a respective portion of the rim of the socket. A body bears at least one guide formation for guiding a component to be used in the arthroplasty procedure. The instrument includes a locking mechanism by which the body and attachment mechanism can be assembled in a unique configuration into the patient specific instrument. The patient specific instrument is attachable to the socket of the patient only at a single pre-planned position.