Applied Radiology

Dr. Daffner is a Professor of Radiologic Sciences, Drexel University School of Medicine, Philadelphia, PA; and the Director of the Division of Musculoskeletal, Trauma, and Emergency Radiology, Allegheny General Hospital, Pittsburgh, PA.

The symptomatic patient with a suspected fracture and normal radiographs challenges the diagnostic skills of clinicians and radiologists alike. Magnetic resonance imaging (MRI) is the most sensitive diagnostic tool for identifying fractures in their earliest stages. This paper will review the utility of MRI for identifying two types of fracture: the truly "occult" injury and stress fractures. MRI should be used in those situations in which a management decision must be made, when the patient's employment depends on his or her ability to function, and in the "elite athlete."

Occult fractures

The patient with a suspected fracture poses a challenge to the clinician and to the radiologist alike. In most instances, there are two common threads to this situation. First, there is a symptomatic patient who has a history that suggests an injury and physical findings that reinforce that assessment. Secondly, however, any radio-graphs that are obtained are "normal." Under most circumstances, prudent medical care dictates that the patient be treated as if he or she had sustained a fracture. Follow-up radiographs should be performed in 7 to 10 days, when they most likely would show the fracture.

There is a subset, however, of patients in whom there is a "need to know now" whether or not a fracture is present. Patients in this category include those in whom a management decision needs to be made immediately, those for whom their employment will depend on their ability to perform their job, and finally, the "elite" athlete. The first group is the most commonly encountered. In the typical scenario, an elderly resident of a nursing home falls, complains of hip pain, and cannot bear weight on the affected side. Radiographs are obtained and are reported as showing no evidence of fracture (Figure 1A). The dilemma facing the emergency department physician is whether to admit the patient or to send him or her back to the nursing home.

The second group includes the patient who sustains an injury to an extremity that is used extensively in his job. Consider the case of the right-handed heavy machinery operator who fell and complains of wrist pain. His radiographs (Figure 2A) are reported as showing no fracture. Should he be allowed to continue to use his right hand at work?

The final group, the "elite" athletes, are either professional athletes or key members of competitive college (or even high school) teams. There is tremendous pressure from the coaches, owners, parents, and players themselves for them to be allowed to continue to play (Figure 3).

The solution for each of these clinical dilemmas may be found in performing magnetic resonance imaging (MRI) on these patients. It is well known that MRI is the most sensitive diagnostic tool for identifying fractures in their earliest stages ^1-6 (Figures 1B, 2B, and 3). In the ideal world, all patients with suspected fractures would be studied emergently. However, in today's environment of concerns for the costs of healthcare and imaging studies, it is prudent to be highly selective in deciding which patients are to be sent for MRI.

There are three areas for which MRI is most useful for the early diagnosis of occult injuries: the hip, the wrist, and the sacrum. Hip fractures are most likely to be suffered in an elderly, osteopenic patient. The typical clinical scenario is as given above and is one that is repeated on a daily basis in virtually all emergency departments. In these instances, MRI may be the most cost-effective study, since it can not only diagnose a fracture in its earliest stage (Figures 1 and 4) but also can be used to rule out a fracture or to find another cause of the patient's discomfort (Figure 5). To facilitate evaluation of these patients, we have developed a protocol for a limited MRI examination in the emergency situation. Furthermore, we charge for a limited examination. We typically perform coronal T1-weighted and short tau inversion recovery (STIR) sequences. If these are normal, no further images are obtained, and the patient is sent home. Some have advocated using the coronal T1-weighted sequence alone, since it has a high degree of sensitivity and specificity for finding an occult fracture. ^4,5 On T1-weighted images, fractures appear as linear areas of low signal; on STIR images these same areas will be of high signal intensity, frequently surrounded by a zone of edema (Figure 4). Similarly, occult fractures of other body parts may be studied in a similar fashion, in the clinical setting where there is a "need to know now" (Figure 3).

Stress fractures

Stress fractures are common, but frequently overlooked, injuries that are specific not only to the activities that produce them, but also to location. ⁷ Two types of stress fracture are recognized: fatigue fractures, which occur in normal bone placed under the stress of a new or abnormal activity; and insufficiency fractures, which are the result of normal activities on bones of abnormal or deficient bone mineral. The clinical picture of a stress fracture should suggest the diagnosis, if carefully analyzed. Typically, stress fractures produce pain with activity that is relieved by rest. All other bone lesions typically produce pain at rest or with activity and are worse at night. Both lesions often respond to analgesics, and this criterion cannot be used to confidently differentiate between the two.

The mechanics of stress fractures depend on muscle pull, particularly from fatigue of opposing muscle groups. Since the majority of most fatigue fractures occur in the lower limbs, poor posture potentiates the effects of muscle pull. Furthermore, the conditions of the activity, such as terrain and equipment, also contribute to the development of stress fractures. For example, a runner who typically runs on a gravel or grass track is more likely to develop a stress fracture of one of the bones of the lower limbs if he or she changes venue to pavement. In many athletes, a change in brand of footwear will often result in stress fractures. ⁷

In most instances, the individual with a fatigue fracture will cease the activity or seek medical advice, soon after the pain begins. The elite athlete, however, faces both external and internal pressure to continue in his or her endeavors for fear of losing his or her position to another in the highly competitive world in which they participate. There are definite hazards of continued activity on bones that are undergoing stress fracture, and radiologists should alert their clinical colleagues, trainers, and coaches (if they see them) to these hazards. These risks include: completing the fracture, distraction of the fragments, fracture of another bone in the same limb, fracture of the same bone in the opposite limb, progressive disability, and longer healing times. ⁷

Insufficiency fractures, on the other hand, typically occur in older individuals, who don't fit the stereotype of the young athlete. It is for this reason that clinicians frequently neglect to ask their patients about any form of physical activity. Physical fitness is now generally recognized as beneficial at any age, and many older people are engaged in it. Mall walking is a popular form of recreation, in which some shopping malls open their doors before regular business hours to provide a safe, all-weather environment in which to walk. Some of the larger malls may be one-half mile from end to end. It is common on any given day to see large numbers of people, mostly seniors, strolling at a brisk pace through the mall.

Although osteopenia is the common denominator for the occurrence of insufficiency fractures, previous irradiation is also a contributing factor. Many patients with insufficiency fractures of the sacrum and pelvis have a history of previous pelvic malignancy, for which they received radiation therapy. ^8-12 Unfortunately, such a history is likely to steer the clinician to believing that the source for the pelvic or sacral pain is metastatic disease, a decision that could have a deleterious effect on the patient.

Imaging of stress fractures typically includes radiography, radionuclide scanning, and MRI. ^8-14 The radiographic findings will depend on the stage of the injury. In the earliest stages, radio-graphs are likely to be normal. After 1 to 2 weeks, vague cortical lucencies and periosteal reaction will be encountered. In later stages, callus and endosteal thickening will occur. Stress fractures in elite athletes tend to be diagnosed when the injury is fairly advanced, since they typically continue in their endeavors despite the pain. However, patients with insufficiency fractures often present to their primary care physician or to the emergency department when they begin experiencing pain. In these individuals, radiographs are typically normal. Sacral insufficiency fractures are notorious for not producing any radiographic changes. For these patients, radionuclide imaging or MRI is performed. In most instances, a positive bone scan is likely to result in an MRI being performed. For this reason, I recommend MRI as the imaging procedure of choice (assuming the radiographs are normal) (Figure 6).

The typical insufficiency fracture produces linear abnormalities on both the radionuclide bone scan and MRI. In sacral insufficiency fractures, the abnormalities are vertical as well as linear ⁸ (Figure 7). If bilateral sacral fractures are present, the radionuclide pattern resembles the stylized "H" of the Honda logo (Figure 8). The MRI signal changes due to any stress fracture are the same as with any fracture, that is, decreased signal intensity on T1-weighted images, increased signal intensity on T2-weighted and STIR images, and enhancement with gadolinium (Figure 9). Confirmation of a sacral insufficiency fracture should be with computed tomography (CT).

It is important for the radiologist who suspects a stress fracture to communicate this to the referring physician. More importantly, the radiologist should emphasize the need to avoid performing a biopsy on these lesions, since healing fractures can be mistaken for osteosarcoma histologically.

Conclusion

MRI is an extremely useful diagnostic tool for identifying suspected fractures in symptomatic patients with normal radiographs. Two groups of injuries for which this modality is best used are the "occult" fracture and stress fractures. In either situation, a tailored, limited examination, consisting of T1-weighted and STIR images can rapidly diagnose the fractures. In addition, MRI is useful for identifying causes of the patient's symptoms that are not due to fractures.