It is my understanding that hip replacements can be done from either the front or the back of the hip joint which would determine whether the ROM is limited in flexion or extension.  Depending on the incision site I stay away from extreme flexion of the joint (teaser) or weight bearing extension of the joint (leg pull back). The client should be able to tell us what the physical therapist has advised them to do prior to being released from their care.  Hope this helps. 

Hip replacements are not a one size fits all issue to work with.  As Pam indicated, there are two main techniques used, posterior and anterior.  Depending on the approach the surgeon used, there are deifinite postions and exercises to either omit or use extreme caution.  Find out the approach or find out what the clients restricitons were post op and we can be of better assistnace.  The more you know as a teacher, the better the questions you ask; the better questons you ask, the more helpful you can be.