Applied Radiology

Dr. Neitzschman is Associate Professor of Radiology and Nuclear Medicine, Dr. Monu is Associate Professor of Radiology, and Dr. Mena is a Clinical Instructor at Louisiana State University Medical Center in New Orleans, LA.

W hen patients present with extreme pain following trauma to the lower extremity and radiographs fail to demonstrate a specific abnormality, MRI has been found to often be useful in establishing the diagnosis. This article addresses specifically the use of MRI in diagnosing intracapsular hip fractures, muscular injuries, tibial plateau fractures, and injuries about the knee, as well as stress fractures of the lower extremity.

Intracapsular hip fractures

MRI is an important modality in the detection of early hip fractures when radiographs are inconclusive. The ability to demonstrate intracapsular hip fractures at an early stage, prior to displacement, is important because of the low association of avascular necrosis with non-displaced fractures (figure 1). For example, if a stage 1 garden fracture, or non-displaced fracture, is allowed to progress to a stage 3 or 4 displaced fracture, it has a significant chance of developing avascular necrosis of the femoral head (figure 2). ^1-3

Hip fractures are associated with incapacitation in more than 50% of patients. There is up to a 20% rate of mortality within one year of such a fracture. In addition to considerable medical costs, there also is tremendous psychological impact associated with these fractures. A possible hip fracture should be suspected in patients who present with severe hip pain and are unable to bear weight, even when radiography fails to demonstrate an abnormality. ⁴ This can occur in young adults who develop stress fractures due to overuse. More commonly, though, it occurs in the osteoporotic postmenopausal woman due to insufficiency fractures. Our objective, as radiologists, is to make the diagnosis soon after these fractures occur.

Limited sequence MRI is preferable to radionuclide bone scintigraphy in diagnosing these fractures. By limiting the MR sequences to long-axis coronal T1 images and a coronal STIR sequence, the examination can be performed in under 10 minutes and can be priced comparably to a radionuclide bone scan. Bone scintigraphy may not demonstrate all fractures in healthy young adults until 72 hours after the insult, and in elderly females it may take as long as 5 to 7 days for fractures to be demonstrated. However, uitlizing MRI, the marrow edema can be detected almost immediately after the fracture occurs.

Muscular injuries and abnormalities

MR imaging is very sensitive to differences in water distribution and can be useful in the detection of a variety of muscle abnormalities secondary to muscle overuse and injury. ⁵

Acute muscle injury --Muscular injuries which begin during activity and persist during exercise are called strains and are secondary to violent muscular contractions during forceful stretching. These injuries often are seen in sports which require rapid acceleration. Strains most commonly occur in muscles which cross two joints and are, therefore, subject to stretching at more than one joint. These injures are most often found at the musculotendinous junction and most commonly occur during eccentric muscular contractions. Strains are muscular tears, and are subdivided by grades which are dependent upon the degree of muscular injury. ^5,6

A grade I strain is a minor degree of tearing of the muscular fibers. Patients with this type of strain recover without sequelae. MR imaging will demonstrate an increased T2 signal, usually with a feathery appearance and a slight increase in muscle size. Grade II strains are partial tears of the muscle which result in loss of strength. It may be difficult to differentiate grade I and grade II strains on the basis of imaging alone. However, an identifiable mass is more indicative of a grade II strain. Grade III strains are full thickness muscle tears. ^5,6,7 MRI detection of discontinuity of the muscle fibers or detection of a gap with retraction of the edges and blood in the gap is consistent with a grade III strain.

Acute direct muscle injury may occur secondary to penetrating or non-penetrating trauma. A penetrating wound can result in lacerations which may heal by scar formation.

Non-penetrating injuries cause contusion at the site of the insult (figure 3). These injuries are characterized by tenderness, diffuse swelling and, usually, a discrete hematoma. ^5,7 The patient often presents with limitation of strength and motion. These injuries are graded by the degree of severity as determined by the extent of motion loss. For patients with mild injuries, recovery usually takes less than one week. With moderate contusions, motion is limited by one-third to
two-thirds of normal, with the disability lasting an average of 8 weeks. The quadriceps and the gastroc-nemious muscles are the most commonly involved muscles.

It is important to differentiate an intermuscular from an intramuscular hematoma as intramuscular hematomas are associated with more complications and a longer recovery time. Additionally, myositis ossificans may occur as a complication of muscular contusions. If surgical biopsy or needle biopsy is performed early, and if the biopsy is taken from the center of the lesion, pathology could be confused with that of an osteosarcoma or a fibroscarcoma. When radiographs are normal, an unusual cause of pain may be acute myonecrosis of muscle secondary to cholesterol embolic infarction (figure 4).

Delayed onset muscle soreness (DOMS) --Some individuals may experience severe muscular pain, swelling, and stiffness following unaccustomed vigorous exercise or following the resumption of training after a layoff. This is known as delayed onset muscle soreness (DOMS). This pain is different from fatigue which occurs during or immediately following exercise. Symptoms are commonly localized at the musculotendinous junction and gradually increase during the first 24 to 36 hours following the exercise. Such symptoms are most severe in 2 to 5 days, and gradually decrease in severity until they disappear, usually 7 to 10 days following the initial event. ^8,9

DOMS is similar to muscular strains in that muscles involved in eccentric muscular action and those which cross two joints are most commonly affected. Strains differ from DOMS in that strain patient's symptoms develop immediately following the muscular contractions during exercise, whereas DOMS has a quiescent period of several hours before symptoms occur (figure 5). The injuries are associated with prolonged T1 and T2 relaxation times of the involved muscles. ⁸

Chronic muscle disorders --Patients with chronic muscular disorders can be differentiated from those with normal muscle by the detection of fatty infiltration or fatty replacement using standard imaging techniques.

Denervation is a known cause of muscle pain that may follow trauma and manifest T2 prolongation on MR evaluation. If allowed to progress, the muscle will atrophy and will be replaced by fat (figure 6).

MRI In the evaluation of knee fractures

Tibial plateau fracture --Limited sequence long axis MR imaging of
the knee may detect fractures when the
radiographs fail to reveal any abnormality (figure 7).

In 1979, Schatzker ¹⁰ described six types of fractures of the tibial plateau; his classification is the most commonly used among orthopedists and radiologists today. Types of fractures in Schatzker's classification include: Type 1, cleavage or wedge type fracture of the lateral tibial plateau; Type 2, lateral wedge fracture with adjacent depression; Type 3, pure central depression; Type 4, wedge depression fracture of the medial plateau; Type 5, bicondylar fracture of the tibial plateau; and Type 6, tibial plateau fracture with disassociation of the metaphysis from the diaphysis.

Computerized tomography has for many years been the gold standard in the diagnosis of these fractures. ¹¹ To date, two studies have been performed to compare computerized tomography to magnetic resonance imaging in this clinical scenario. In 1994, a study by Kode et al for assessment of tibial plateau fractures demonstrated that MR imaging was equivalent to CT in most patients (specifically for fracture evaluation) and superior in five patients. A total of 21 patients were evaluated under this protocol. While CT was able to demonstrate and infer certain soft-tissue injuries, MRI demonstrated meniscal injuries in 55%, ACL tears in 27%, and complete collateral ligament tears in 53%. ¹²

In a study by Holt et al, 22 patients were evaluated, and magnetic resonance imaging demonstrated a similar rate of soft-tissue injury as CT imaging did. Additionally, MR was shown to characterize fracture fragments and displacement, and to show occult fractures, bone bruises, and internal derangements. These injuries are not addressed specifically by the Schatzker classification. ¹³

A common inference from these two independently-performed studies is that MRI plays a significant role in assessment and treatment planning of tibial plateau fractures, perhaps making it the imaging modality of choice for suspected tibial plateau fractures (figure 8). For some patients MRI will lead to significant changes in their medical management and may reveal reasons for treatment failure. ^14,15

Bone contusions --A very common injury seen by MRI evaluation in patients with knee trauma is a bone contusion or trabecular injury. This injury is identified on MRI as a reticulated area of marrow exhibiting T1 and T2 prolongation. ^14,16,17 It is important to recognize that the bone bruise itself is usually painful and may be the cause of the patient's symptoms. In most instances the pain subsides on its own, usually in 6 to 8 weeks. ¹⁸

In patients who have degenerative osteoarthrosis of the knee, a horizontal cleavage tear is frequently seen, though it may be an incidental finding. In patients with a horizontal cleavage tear along with a bone bruise, the bone contusion is often the cause of the patient's symptoms, and surgery is usually unnecessary in this setting.

ACL injuries --MRI has an established role in diagnosing ACL injuries. ¹⁶ Deepening of the sulcus terminalis greater than 1.5 mm suggests a torn ACL (figure 9). The presence of a SEGOND fracture, as well as avulsion of the anterior tibia at the insertion of the ACL, also may suggest the presence of an ACL tear.

Stress fractures of the lower extremity

Stress fractures occur as a result of repetitive prolonged muscular action on bone and do not occur in non-weight-bearing bones. The insufficiency fracture may occur with normal or physiologic activity in a patient whose bones are deficient in mineral or elastic resistance. ¹⁹

MR imaging has been valuable in defining the stages of a stress fracture (figure 10). The earliest manifestation of a stress fracture is the "stress response" which is due to bone marrow edema. ^19-21 If the causative action of the stress fracture is discontinued, prevention of the cortical insult producing a fracture and the proliferative periostitis may be prevented. The usual stress fracture occurs perpendicular to the long axis of the bone and occurs over a relatively short segment.

Longitudinal stress fractures are a much less common type of stress fracture (figure 11). Patients with longitudinal stress fractures of the tibia usually present with posterior medial tibial pain. These fractures occur over a relatively long segment, and a vertical fracture line may be seen on either MRI or CT. ^20,21 As reported by Anderson recently, longitudinal stress fractures may follow shin splints as a continuum of fatigue damage to bone. ²² AR