People often visit primary care physicians and chiropractors for low back pain (LBP). It is a very common problem, and one that usually resolves on its own. Research has repeatedly shown that X rays are rarely useful in the evaluation and treatment of simple back pain. Outside of certain "red flags" (fever, history of cancer, weight loss, and a few others), there is rarely any reason to get an X ray of someone's spine when they come to see the doctor for low back pain.
Both anecdotal and quantitative studies have shown that chiropractors are very likely to order spine X rays. Given the inutility of these studies, and the radiation exposure, what reason could there be (aside from financial incentive) for chiropractors to order X rays?
Diagnosis of bone and joint disease
X rays can be very effective at diagnosing traumatic bone and joint injuries such fractures and dislocations. These films can be very tricky to interpret, and nearly all doctors have their films over-read by a radiologist. Given that chiropractic is not useful in the treatment of an acute fracture or dislocation, and that chiropractors are not qualified to read these films, this seems a poor excuse for ordering films.
X rays are not very sensitive or specific for the diagnosis of other important bone diseases such as osteoporosis, and can be very tricky when cancer is a consideration, as some bony cancer lesions show up on X rays and some do not. X rays done and interpreted by a chiropractor or any other unqualified individual can lead to a false sense of security.
Diagnosis of "subluxation"
Subluxation is a word with two meanings. In orthopedics, it refers to a specific kind of dislocation of a joint, one that can be clearly identified on an X ray. In chiropractic, it refers to an often-invisible displacement of the vertebrae that can cause back pain and even systemic disease. Subluxation in the chiropractic sense has never been shown to exist. Vertebral subluxations in the orthopedic (i.e., real) sense are rarely clinically significant and are outside the purview of chiropractors.
X rays are a significant intervention. Anytime ionizing radiation is applied to a human being, there better be a good reason for it. If there is no evidence that the X ray will help in a meaningful way with diagnosis or treatment, then no X ray should be done. There is no clear reason any chiropractor should ever order an X ray.