A lumbar selective nerve root block is a target specific epidural steroid injection for diagnosing and treating low back and leg pain. The dura is a protective covering of the spinal cord and its nerves. The space surrounding the dura is called the epidural space. In the lower back it is called the lumbar epidural space. If the needle is positioned next to an individual nerve root within the epidural space, it is called a selective nerve root block. It puts the medication directly along an inflamed nerve root. A selective nerve root block can determine if a specific nerve is causing your pain.