Traditional small head THRs sometimes have this as a lifetime restriction to avoid dislocation. I have seen people dislocate small head THRs through internal rotation combined with flexion doing something as simple as buckling their ski boots, long after surgery. One of the theoretical advantages of resurfacing is that the dislocation risk in extremely low, so there is usually no long term restriction on internal rotation. However, while the muscles and joint capsule are healing, many surgeons put on short term restrictions, often around six weeks, to avoid any chance of dislocation while things are healing and tightening up. You should check with your surgeon to confirm how long he would like the restriction, and when it is safe to start working on internal rotation exercises.