Every study has certain criteria each subject must meet to be a part of the study. Sometimes this inludes age, gender, type of symptoms, or failure to get better with nonoperative care.
When patients with disc problems are studied, researchers often look for patients who don't have any other problems or who don't have any other symptoms. Anyone with back pain that goes down the leg may be excluded. In some cases, subjects end up needing surgery and must drop out. Others injure themselves again or even die before the study is over.
You may wish to call the department sponsoring the study and find out more of the details.