A magnetic resonance imaging (MRI) is very useful for detecting subtle abnormalities of the hip joint that may not be readily apparent on plain xray. In the past 10 years, MRI scans have allowed us to appreciate the subtleties of cartilage and labral degeneration that cause severe hip pain well before obvious osteoarthritis of the hip develops.
For example, these are the xrays of a young woman who came to see me complaining of bilateral hip pain. She was referred to my office after non-weight bearing films were obtained at the hospital. She has a very low body mass index and she has no history of hip trauma but she was a dancer, cheerleader, and runner when she was in high school.
Her plain xrays demonstrate the surprising finding of a tiny cyst in the acetabulum in the area of the acetabulum where most of the stress of weight bearing is concentrated. This may correlate with the fact that her symptoms are worse after she has been standing for most of the day.
An MRI arthrogram of the hip was obtained. An arthrogram involved injecting the joint with a contrast material that expands the capsule surrounding the hip joint and allows us to see the anatomic detail inside the hip much more clearly. The following two images show that the radiologist has successfully injected the contrast material into the joint because the contrast material encircles the femoral head and outlines the inside of the joint. If the injection was not successful, we would see a smudge of contrast in the tissues outside of the hip joint.
After the arthrogram was performed, a series of different MRI sequences were used to evaluate the inside of the hip joint.
These images show that their are tiny, symmetric cysts in in the weight bearing portion of the acetabulum on both sides. However, they do not show any evidence of labral pathology, avsacular necrosis, or classic osteoarthritis of the hip. The cause of the subchondral cysts is a bit of a mystery. There is a recent publication in the journal hip that suggests that these cysts may be a normal anatomic variant. I concluded that because the anesthetic portion of the intra-articular injection did not alleviate her pain it was not possible to attribute her pain to the findings on the MRI scan. I reassured her that she did not need treatment at the present time, but if her pain persists, we will plan on obtaining a follow up MRI scan in 1 year to make sure that the cysts have not enlarged.
Cyst-like lesion of the acetabular roof – an abnormal finding or an anatomical variant? Hip Int. 2010 Apr-Jun;20(2):258-60. Tzaveas AP, Villar RN. The Wellington Hospital, London, UK.
Abstract: Cyst-like lesions are frequently found in the area of the acetabulum on MRI scans. However, their presence is not always abnormal. We report four patients with such lesions found on MRI where, during hip arthroscopy, an area resembling a horseshoe-like extension of the cotyloid fossa was found. Clinicians must be aware that not all cystic acetabular lesions are pathological and may simply represent a normal anatomical or developmental variant of the hip joint. They do not always represent an indication for hip arthroscopic surgery.