1. A hip-joint device comprising: a patient-specific acetabular resection guide including: a dome-shaped surface bounded by a periphery, wherein the dome-shaped surface is patient-specific three-dimensional surface designed to anatomically match and be received into an acetabulum of the patient in one position, based on a three-dimensional image of a hip joint of the patient reconstructed from a medical scan of the hip joint; and a plurality of guiding formations defined on the resection guide, the guiding formations having patient-specific shapes and orientations for guiding corresponding osteotomies relative to the acetabulum for correcting hip dysplasia