September 2000

Subtle hip findings elude x-ray diagnosis

By Karen Sandrick

Hip injuries in athletes usually are few and far between, so radiologists don’t often schedule MR, CT, or a bone scan in an initial workup. But when a patient has nagging hip, groin, or low back pain, imaging can find small and subtle stress fractures in the hip before they become full breaks. If missed, these can lead to avascular necrosis, nail-and-screw hip arthroplasty, or even a hip replacement that may have to be revised in about 15 years.

This is why Dr. Nancy Major, an assistant professor of radiology at Duke University, favors a quick MRI referral for endurance athletes with vague hip or back problems.

“As soon as you suspect an athlete may have a hip stress fracture, you need to get them off their weight-bearing immediately and image them with MRI. You have a diagnosis right away, and the patient can get the needed weight-bearing protection,” she said.

MRI studies of the hip are uncommon in athletes, said Dr. J. Bruce Kneeland, a professor of radiology at the University of Pennsylvania. MR imaging of the hip typically is done to rule out atraumatic osteonecrosis when there is no reason to think a person has the disease—in someone who isn’t taking steroids or suffering from alcoholism, for example—or to spot a radiographically occult fracture in elderly women with severe osteoporosis.

Athletes do, of course, sustain fractures of the hip or pelvic ring, and MRI is an excellent modality for evaluating them. But MRI is not a frontline imaging tool; it is performed when plain radiographs cannot identify a cause of persistent pain.

According to Major, orthopedic surgeons and sports medicine physicians shouldn’t be satisfied with radiography, since gases in the bowel and the configuration of the sacrum can obscure hip stress fractures. Physicians should go to MRI whenever an elite athlete has groin or back pain because early diagnosis of a stress fracture will dramatically affect the athlete’s training.

“Athletes who have a big track meet coming up over the weekend or who are training for the Olympics will want to run through pain. What we need to do is make sure they don’t have a stress fracture. Then they can go ahead and participate in their sport,” Major said.

She bases her opinion on a study of four marathoners who complained of low back and vague buttock pain after running more than 50 miles a week. Although each of the patients was treated for disk disease, none of their symptoms improved. So Major performed further imaging investigations and found clear signs of hip stress fractures in every patient.

CT showed unilateral vertical disruption of cortical bone through the sacrum and sclerosis in the sacral foramina in two patients, and bilateral cortical disruption and sclerosis along the ala sacralis in a third patient. T1-weighted MR produced a low-intensity signal in the lateral aspect of the sacrum, which became high intensity on T2-weighted images in two patients. And radionuclide bone scanning revealed linear uptake in the ala sacralis along the sacroiliac joint in another patient. Results from the study were reported at the annual meeting of the American Roentgen Ray Society in May 1999 and published in the March 2000 issue of the American Journal of Roentgenology.

As Major pointed out in an interview, CT provides a detailed view of bone, particularly cortical disruption on outside bony surfaces, and it detects differences associated with stress fractures, such as lucency and thickening of cortical bone and focal sclerosis in cancellous bone.

Bone scintigraphy and single-photon emission computed tomography (SPECT) find stress fractures that are so small they are overlooked by plain films and MRI. Such oversights include stress fractures of the proximal femur or in the interior rim of the pelvis or pubis, said Dr. Mariano Fernandez, a professor of radiology at the University of Cincinnati.

“It takes only a small injury to attract enough deposition of the radioisotope-labeled bone scanning agent to be detected with our gamma camera. That determines the high sensitivity of the bone scanning technique and why it can be used in instances where other x-ray or MRI techniques do not yield any abnormalities,” he said.

SPECT, which provides sections of the skeleton at various levels and in different directions, increases the sensitivity of imaging the pelvis and enables the abnormality to be traced to a given area in the pelvis, he said.

“Because SPECT images are able to provide enhanced contrast between normal bone and abnormal bone or lesions, something that is not obvious on regular planar bone scan images may show up on SPECT images,” Fernandez added.

MRI nevertheless is the preferred method of scanning for stress hip fractures, Major said. She images the entire pelvis to determine whether the source of pain resides in the hip or is referred from another part of the pelvis, such as the pubic sympysis or the sacrum. She begins by obtaining T1-weighted images, which should visualize any fracture lines or fractures, and follows with fat-suppressed T2-weighted images to look for adjacent bone marrow edema.

Because MRI is multiplanar, it avoids some of the technical problems associated with CT, such as motion or other artifacts that affect the reformatting of axial slices into coronal or sagittal images and, as a result, may camouflage a stress fracture. CT imaging in the axial plane also may completely miss a fracture in the same plane, she said.

Although Major found small stress fractures in only a small number of long-distance runners, she believes that hip problems may be occurring in other endurance athletes, such as sprinters who train extensively.

“We’re seeing more stress-like injuries than we ever did before. The reality is that stress fractures around the hip might be more common than we thought,” she said. “With our sophisticated imaging techniques and with the increase in the number of orthopedic surgeons who specialize in treating athletic injuries, we can diagnose these fractures before they become real problems.”

CT and MRI identify avulsion injuries

Acute avulsion injuries of the pelvis may be obvious on plain films. But subacute, chronic, or healing injuries can have a mixture of lytic and sclerotic changes and may easily be confused with a neoplasm or infection. Because such findings can be ominous, especially in children, avulsions need more than plain radiography; they need CT or MRI.

So said Dr. Georges El-Khoury, a professor in the musculoskeletal division of the University of Iowa, Iowa City. El-Khoury prepared a scientific exhibit on the imaging features of avulsion injuries for the 1999 meeting of the RSNA. Images and commentary from that exhibit were published in Radiographics (May 1999).

Avulsions of the hip are not unusual in children who engage in competitive team sports. The injuries occur from an eccentric, extreme, or unbalanced contraction of muscle, and the muscle pulls violently on the epiphyseal center or physis, which is the weakest link in the musculotendinous apparatus.

An acute avulsion, which produces sudden pain and loss of muscular function, is often clearly visible on plain radiography. An anteroposterior radiograph may reveal acute avulsion of the ischial tuberosity, a common injury in runners, hurdlers, and football players, with sharp definition of the bone fragment and displacement. Yet sometimes they are difficult to spot on plain films.

“Avulsion of the sartoris muscle is very common in runners and hurdlers, and their pain is suggestive of an abnormality in the region of the anterior inferior iliac spine. However, you may not see anything on plain films if there is no displacement. For this kind of bony injury, I like to do CT,” El-Khoury said.

El-Khoury typically takes 3 to 5-mm slices of the hip. Because most of the CT units in his department are helical, he can obtain volumetric data for viewing sagittal sections on an independent imaging station. More often, however, he uses sagittal or coronal reconstructions to afford better visualization of the injury.

Subacute or chronic avulsions, which emerge slowly and eventually prevent an athlete from performing, may cause pain on exertion or even without strenuous activity. Images of these injuries show the effects of recurrent, repetitive stresses on the musculoskeletal connection.

While anteroposterior radiographs may depict a subacute avulsion of the anterior inferior iliac spine and ischial tuberosity, these films may not clearly outline the margins of the injury and it may be mistaken for a more fulminate disease. Healing avulsions, in particular, produce a mass of callous bony repair, which can be misinterpreted as tumor, El-Khoury said.

A history of persistent groin pain, radiographic findings of chronic avulsion of the right symphysis pubis, and long-term follow-up may suffice to diagnose chronic avulsion. Coronal T2-weighted MR images, however, may be indicated to find evidence of avulsion, such as bone marrow edema and reactive changes in the soft tissues around the site of the injury.

“You get a history that helps you focus on the hip in a specific way, then you start the diagnostic process with a plain film. If you decide it’s a bony injury, CT is the way to go. MRI is best suited for evaluating muscles, tendons, and ligaments,” El-Khoury said.

Radiologists need to move beyond radiography, whether that means going to CT or MRI, he said.

“Tumors in children are quite common, and for the untrained eye, a subacute, chronic, or healing avulsion could look like a tumor,” El-Khoury said. “CT or MRI is needed so you can put the mind of the treating physician, the parents, and the patient at ease if you’re not dealing with a tumor.”