My Husband Recently Had A Bone Marrow Biopsy. The Attending

my husband recently had a bone marrow biopsy. the attending tried 3 different locations on his hips and buttocks to to get a successful procedure. it was not successful as the dr said there was a lot of scar tissue "on" the bone and he could not get what he needed. what causes scar tissue to form on the hip or buttock? we are trying to find out why he is anemic.