Using X-rays to Diagnose Hip ImpingementX-rays are an excellent diagnostic tool for assessing hip problems. X-rays can be taken from the front (anterior view) or from the side (lateral view). An oblique view is taken at an angle between the front and side or back and side.
In this study, researchers use the frog-leg lateral view to look for hip impingement. The frog-leg view is taken with the patient lying down. The hip and knee are both bent or flexed. The hip is rotated outward. The upper leg and knee are moved out to the side, away from the body. The foot stays close to the body along the inside of the opposite knee.
Impingement means some structure is getting pinched or pressed up against the bone. One particular type of impingement called cam impingement is the focus of this study.
With a cam impingement, the round head of the femur (thigh bone) presses up against the front rim of the acetabulum (hip socket). There is a thin layer of cartilage called the labrum along the rim of the acetabulum. The pinched labrum causes painful symptoms.
Cam impingement occurs if the femoral head is flattened or not quite spherical or round-shaped. This is described as aspheric. A change in the shape of the femoral head also alters the femoral head-neck angle. These factors contibute to the development of impingement.
Standard anterior-posterior (AP) views don't show the decreased head-neck angle. For this reason, lateral views are used most often to diagnose cam impingement. But according to the results of this study, the frog-leg view gives the most accurate visualization of the aspherical femoral head. Frog-leg views also show the change in the head-neck angle that occurs in patients with cam impingement.
Two groups of patients were included in this study. The first group had back or leg pain. There was no evidence of a hip problem. This was the control group. The second group had groin pain and signs of impingement.
AP, lateral, and frog-leg lateral X-rays were taken for all patients in both groups. The frog-leg view was most likely to show an aspheric femoral head in patients who had impingement. The AP view was a better way to show aspheric heads in the control group.
The authors point out that all three views are helpful in diagnosing hip impingement. But the frog-leg lateral view readily shows hip impingement. It is a reliable and inexpensive diagnostic tool.
John C. Clohisy, MD, et al. The Frog-Leg Lateral Radiograph Accurately Visualized Hip Cam Impingement Abnormalities. In Clinical Orthopaedics and Related Research. September 2007. Vol. 462. Pp. 115-121.
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