Osseous Trauma

Anatomy

A familiarity with basic skeletal anatomy helps in understanding the MRI findings in osseous trauma. Cortical (compact) bone is found along the periphery of flat and tubular bones. The subchondral plate, the layer of compact bone at the articular end of a long bone, plays an important role in providing support for the overlying articular cartilage.

Trabecular (cancellous) bone is composed of a meshwork of osseous struts that support the overlying cortex. Trabecular bone is found primarily in the axial skeleton and near the ends of long bones, where it plays an important role in absorbing dynamic stresses. Because of its elastic compliance, cancellous bone has a load-bearing capacity 10 times that of compact, cortical bone. This shock-absorbing capability is especially important around joints, where the trabecular bone dissipates axial forces away from the subchondral plate and overlying articular cartilage.

In a developing long bone, the physis (growth plate) allows for lengthening of the bone through enchondral ossification. Before closure, the growth plate constitutes the weak link in the muscle-tendon-bone and bone-ligament-bone units. As such, it is particularly vulnerable to traumatic injuries at epiphyses (at the ends of long bones) and apophyses (growth centers not involved in longitudinal bone growth, such as the trochanters of the hip).

Overview of Osseous Trauma

Osseous injury may result from a single traumatic event, or may occur over time as a result of repetitive stresses that lead to the gradual breakdown and ultimate mechanical failure of the bone. Acute trauma may result in impaction injuries (ranging from bone contusions to complete fractures) or avulsion fractures in which a tendon or ligament pulls off a piece of bone, cartilage, or both. Chronic injuries, resulting from less intense, repetitive trauma, range from a focal stress reaction to a frank fatigue or insufficiency fracture.

Imaging Options

Conventional radiographs should be the first modality used for evaluating osseous trauma, but many acute and chronic osseous injuries are not detectable on radiographs. Most of these radiographically occult injuries can be detected with radionuclide bone scanning, but this technique has several limitations. The examination requires 4 to 6 hours, delaying diagnosis in these patients, who often are awaiting triage in the emergency department, and the scan may be falsely negative for 24 to 72 hours after the injury, especially in elderly, osteoporotic patients. Finally, a positive scan is nonspecific because the images are of extremely low spatial resolution, and a wide variety of osseous pathology can result in abnormal uptake. This can be especially problematic in older patients, in whom insufficiency fractures and neoplasms are common.

MRI is exquisitely sensitive for detecting osseous injuries that result from either acute or chronic repetitive trauma. By using streamlined protocols, MRI can be cost-competitive with a bone scan, while providing a more rapid and specific diagnosis. As a result, we use MRI as the next study in a patient who has normal radiographs and a suspected osseous injury. A normal MR image essentially excludes the presence of an osseous injury. An additional strength of MRI is its ability to show soft tissue injuries that may mimic a fracture clinically in cases in which no osseous injury is present ( Fig. 8.1 ).

Impaction Injuries

Contusion

Trabecular injuries that result from impaction forces are known as bone contusions, bone bruises, or microtrabecular fractures . Pathologic studies of these injuries reveal trabecular fractures and edema and hemorrhage in the adjacent marrow fat. MRI findings become evident within hours of the injury, and their detection is important for several reasons. An isolated bone contusion on an MR image may explain a patient’s symptoms and avoid an unnecessary arthroscopic procedure. The sites of contusion also may provide clues about the mechanism of injury and associated soft tissue abnormalities that may be present.

Additionally, although most contusions resolve without complications, there is evidence that focal contusions involving the subchondral bone are associated with damage to the overlying cartilage. Some investigators believe that this may place the patient at increased risk for collapse of the articular surface in the short term or for the development of osteoarthritis later. Detection of a subchondral contusion may result in a more conservative treatment plan, including a longer delay before returning to a normal activity level, to allow for trabecular healing and to minimize the potential for collapse of the overlying articular surface.

Bone contusions are most conspicuous on STIR or fat-saturated T2W images, where they appear as focal areas of increased signal intensity, usually with poorly defined margins, presumably secondary to the hemorrhage and edema related to trabecular fractures ( Fig. 8.2 ). Contusions are typically of intermediate signal intensity on T1W images (lower than fat, higher than muscle) because marrow fat is intermixed with the hemorrhage and edema. Contusions are easily missed on non–fat-saturated fast spin echo–T2W images because the trauma-related edema and surrounding marrow fat display similar signal intensities.

Contusion Patterns

By analyzing the locations of osseous contusions, the mechanism of injury can often be inferred, and associated soft tissue abnormalities can be predicted and careful inspection should follow. Two commonly encountered patterns are discussed here ( Box 8.1 ).

Box 8.1

Contusion Patterns in the Knee

Anterior Cruciate Ligament Tear

Lateral Compartment

• Mid anterior lateral femoral condyle

• Posterior lateral tibial plateau

Medial Compartment

• Mid anterior medial femoral condyle

• Posterior medial tibial plateau

Patellar Dislocation

• Medial facet of the patella

• Anterior lateral femoral condyle

Anterior Cruciate Ligament Tear

Osseous contusions are evident on MRI studies in approximately 80% of patients with acute anterior cruciate ligament (ACL) tears. The most common mechanism of injury that results in a complete tear of the ACL involves a twisting force combined with a valgus stress. The impaction involves the mid to anterior aspect of the lateral femoral condyle against the posterolateral tibial plateau, resulting in typical contusions at these sites ( Fig. 8.3 ). Another contusion pattern involving the mid to anterior medial femoral condyle and posteromedial tibial plateau may also be seen after an ACL tear. These contusions probably arise from a medial impaction force that results from the unstable knee rebounding after the ACL tear and lateral injuries have occurred (see Fig. 8.3 ).

Lateral Patellar Dislocation

The typical contusions that result from patellar dislocation involve the medial patellar facet and the lateral femoral condyle (usually in a more anterior location compared with the contusion seen with an ACL tear) and are explained by the mechanism of injury. The patella dislocates in a lateral direction. As it relocates, the medial facet of the patella impacts against the anterolateral margin of the lateral femoral condyle, resulting in contusions at these sites ( Fig. 8.4 ). Common associated findings include a tear of the medial patellofemoral ligament (a portion of the medial retinaculum at the level of the patella), a large joint effusion, and a chondral or osteochondral injury to the medial patella or lateral trochlear groove. If the knee was markedly flexed at the time of the dislocation, the osteochondral injury may involve the weight-bearing surface of the lateral femoral condyle. Recognition of the osteochondral injury is important for appropriate treatment and surgical intervention.

Radiographically Occult Fracture

In the case of a radiographically occult fracture, in addition to marrow edema, a fracture line is present. The fracture line occasionally may be obscured by adjacent marrow edema, but is usually identified as a linear or curvilinear focus of low signal intensity on T1W images that may show either low or high signal intensity on STIR images ( Figs. 8.5-8.7 ). Marrow signal intensity returns to normal with healing.

It has been well established that MRI is able to show radiographically occult fractures in a rapid, cost-effective manner, allowing for optimal patient management. This is possible because the associated marrow edema and hemorrhage are present on MR images from the time of injury. Even if a fracture is evident on conventional radiographs, MRI may provide additional useful information regarding the true extent of the fracture, the presence of additional fractures or contusions, and associated soft tissue injuries.