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After so many tests, why is it so difficult to diagnose FAI?

Q: I've been monkeying around with a hip problem for months now. I've seen the doctor four or five times. Each time, there's a different test ordered. First, there was an examination with poking and prodding. Then it was X-rays, and then an MRI. So far, they think I may have something called FAI. Now they want to inject a numbing agent (like Novacaine) into the hip to see if the pain is really coming from the hip joint. Why is something like this so difficult to figure out?

A: Femoroacetabular impingement, otherwise known as FAI, does not have one symptom or one clinical test that tells the physician, "Yes, this person has femoroacetabular impingement FAI)". Instead, the diagnosis is one of exclusion based on patient history, clinical symptoms, physical examination, and the results of imaging studies (X-rays, MRIs).

We say the diagnosis is one of exclusion because there are other hip problems with similar presentation. The examining physician formulates the diagnosis by excluding other potential causes in order to make the final determination that the condition responsible for the patient's symptoms is, indeed, FAI.

Usually the physician begins with the patient history that might tip him or her off that this could be FAI. Here's where the "exclusion" piece comes in. Because patients with hip pain have a variety of symptoms, questions must be asked to help determine the cause of those symptoms. Was there an accident or injury affecting the hip? Did the person have any hip problems as a child (e.g., slipped capital epiphysis or Legg-Calve-Perthes disease)? Were there any previous surgeries on the hip?

Activity level (especially activities that involve repetitive hip motion) is a key risk factor for FAI, whereas a history of alcohol and steroid use might point more toward something like osteonecrosis (death of bone cells). Labral tears (the labrum is a fibrous rim of cartilage around the hip socket) can cause similar symptoms to FAI (e.g., painful clicking, popping, snapping with hip motion) but is usually associated with a specific injury (often sports-related).

During the physical assessment, the physician uses different tests to determine whether the pain (or other symptoms) is intra-articular (coming from inside the hip joint) or extra-articular (structures around the joint but not inside the joint). Observing how you sit, stand, and walk might provide some additional helpful diagnostic clues. For example, there is a tendency among patients with FAI to sit with a slouched posture to take pressure off the hip.

Imaging studies are also helpful and an important part of the diagnostic process. Change in certain angles of the hip (as measured on X-rays) with no sign of hip dysplasia is diagnostic of deformity associated with FAI. On the other hand, MRI-evidence of labral or other cartilage damage helps rule out FAI as the potential source of symptoms.

Limited hip internal rotation is a red flag for FAI. But most other positive findings only point to the hip as the source of the symptoms, not the actual cause. A positive response to injection of an anesthetic (numbing) agent into the hip can confirm the source of pain. Relief of pain with injection directly into the joint confirms the hip (not lumbar spine or groin) as the true origin of pain. These additional areas where pain can occur with FAI cause some diagnostic confusion.

Since there is no one test, symptom, or clinical finding that confirms a diagnosis of femoroacetabular impingement (FAI), a thorough evaluation is required. With patience and persistence, the orthopedic surgeon can sort through important points in the patient history. Combining that information with findings from the physical examination and imaging studies will be necessary to make the final diagnosis. As we mentioned, the differential diagnosis is often one of exclusion through a process or "ruling out" other hip conditions, one at a time.

It sounds like your physician is following the best practice standard for making an accurate diagnosis of your problem. You may have to be equally patient until all the pieces are put together to solve the puzzle.

Reference: Jeffrey J. Nepple, MD, et al. Clinical Diagnosis of Femoroacetabular Impingement. In Journal of the American Academy of Orthopaedic Surgeons. July 2013. Vol. 21. Supplement. Pp. S16-S19.