Occult Scaphoid Fracture

Scaphoid fractures can present a diagnostic dilemma in the setting of posttraumatic wrist pain and normal radiographs. Up to 65% of scaphoid fractures are radiographically occult immediately following injury. In general, wrist splinting with follow-up radiographs assessing for bony remodeling is the management of choice at most institutions for suspected occult scaphoid fractures. MR imaging has excellent sensitivity for fractures and can be used in the early posttraumatic setting to confidently exclude fracture and avoid unnecessary immobilization.

MR imaging has been shown to be both more specific and more sensitive for radiographically occult scaphoid fractures compared with both CT and plain films, with sensitivity and specificity around 100%. Small field of view acquisition using both FS T1-weighted and fat suppressed T2-weighted images of the wrist are sufficient for diagnosis. The coronal plane is the easiest plane in which to detect scaphoid injury because most fractures are oriented transverse or oblique to the long axis of the scaphoid. Coronal planes are preferred using STIR for screening or rapid evaluation. Trabecular disruption in the scaphoid manifests as linear T1 hypointensities and T2 hyperintensities without cortical disruption in nondisplaced fractures. Otherwise, bone edema, trabecular disruption, and cortical disruption may be present on MR imaging in radiographically occult scaphoid fractures ( Fig. 1 ).

Occult scaphoid fracture. ( A ) Coronal T1-weighted image shows linear hypointense fracture lines in the scaphoid waist. ( B ) In this case the fracture line is better seen on sagittal T1 images. ( C ) Sagittal T2 FS images show diffuse increased signal in the scaphoid consistent with bone edema and surrounding soft tissue edema.

Occult Hip Fracture

Occult hip fractures in the setting of trauma or falls can be readily appreciated using MR imaging. Up to 46% to 54% of fractures of the hip and/or pelvis are occult on initial radiographs. Hip fractures have significant morbidity and prompt diagnosis is necessary to ensure the best outcomes.

In addition to MR imaging having superior sensitivity for detection of hip fractures, it is also helpful in detecting unexpected fractures of the pelvis and soft tissue injury, which clinically can mimic hip fractures after trauma.

Similar to other sites of fracture, pelvic and hip fractures show linear T1 hypointensities and edema on T2-weighted images. STIR imaging provides increased sensitivity for detection of marrow and soft tissue edema. A small field of view used for hip imaging may exclude the sacrum, depending on the institution’s protocol. In our opinion, a wider field of view that includes the ipsilateral sacrum provides both sufficient resolution and sufficient sensitivity for detection of ipsilateral bony abnormalities and significant soft tissue injuries in the setting of trauma ( Fig. 2 ).

Occult hip fracture. ( A ) Narrow field of view coronal T1-weighted image shows linear hypointensities. ( B ) Wide field of view coronal T2 FS image shows significant bone edema focused in the femoral neck. Edema is most commonly seen in the medial femoral neck in occult hip fractures.

Traumatic Chondral and Osteochondral Injuries

Traumatic chondral injuries are most often occult on radiographs, but are superbly shown on MR imaging. The Outerbridge grading system is preferred by our surgeons:

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  Grade 0: normal

  
  
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  Grade 1: softening, mildly increased T2-weighted signal without any significant loss of thickness

  
  
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  Grade 2: defect less than 50% of cartilage surface

  
  
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  Grade 3: defect greater than 50% of cartilage surface

  
  
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  Grade 4: full-thickness defect, often with underlying bone edema

It is important to note the size, location, cartilage surface integrity (such as fraying), and grade of the lesion in the report. Multiplanar imaging is needed for all joints. Most joint surfaces extend along multiple planes and evaluation in the axial, sagittal, and coronal planes may be necessary to see the full extent of the articular surface. Also, careful evaluation of the joint space for chondral and osteochondral bodies (free-floating avulsed cartilage) is needed because these can lead to locking in the acute setting and eventually early onset osteoarthritis. In the presence of bone edema, careful inspection of the cartilage is needed ( Fig. 3 ).

Chondral injury. ( A ) Coronal PD FS image shows a full-thickness cartilage defect (grade 4) with mild underlying bone edema following trauma in the weight-bearing surface of the medial femoral condyle. ( B ) Sagittal PD FS in the same patient shows an osteochondral body along the anterior articular surface.

Osteochondral defects (OCDs) can be seen on radiographs during the initial trauma work-up, but surgeons prefer further characterization with MR imaging before repair. In particular, detail of the cartilage is not possible with radiography. It is important to identify lesions that have full-thickness cartilage injuries with fracture extending into the underlying bone as well. The most important information to report is the stability of the OCD, because this guides treatment. Findings on T2-weighted images suggestive of an unstable fracture are as follows ( Fig. 4 ):

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  Increased signal tracking around the fragment

  
  
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  A 5-mm focus of cystic change between the OCD and adjacent normal bone

  
  
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  High-signal-intensity linear defect in overlying cartilage and/or 5-mm focal cartilage defect

Acute traumatic OCD. Coronal ( A ) and sagittal ( B ) T2 FS images of the ankle joint show a 6-mm cystic focus between a chondral lesion and adjacent bone, suggestive of unstable OCD. Also, note the extensive marrow edema about the lesion.

Lisfranc Injury

Lisfranc injuries are associated with grave outcomes when missed. Approximately 50% of untreated Lisfranc ligamentous complex injuries go on to develop severe degenerative arthritis, planovalgus deformity, and instability of the midfoot.

Injury to the Lisfranc ligamentous complex commonly occurs with an associated fracture dislocation in high-velocity injuries, and can be readily discerned using plain film radiography. In low-velocity injury, subtle disruption of the ligamentous complex can be missed with weight-bearing radiography alone. Also, in the setting of polytrauma, injury to the Lisfranc ligamentous complex may be overlooked or the patient may not be able to participate in weight-bearing views, resulting in late diagnosis. The exquisite anatomic detail of this complex ligament can be readily appreciated using MR imaging, which is a helpful adjunct in the assessment and treatment planning of the foot.

The Lisfranc ligament complex consists of 3 ligament bundles connecting the medial cuneiform and the base of the second metatarsal. The complex consists of the dorsal, interosseous, and plantar bundles. Stability of the midfoot is primarily derived from the plantar and interosseous ligament bundles, and careful assessment of these structures is prudent. Small-field-of-view, multiplanar MR images using T1-weighted non–fat-suppressed images and fluid-sensitive fat-suppressed images should be obtained. MR imaging shows T2 signal prolongation in an injured ligament with partial tearing or frank disruption of the ligament ( Fig. 5 ). T1-weighted imaging helps detect occult fractures and pathologic widening of the space between the second metatarsal base and the medial cuneiform and first metatarsal. Also, MR imaging can help show subtle dorsal displacement of the second metatarsal base caused by dorsal Lisfranc ligament disruption. Marrow edema can help direct attention to locate subtle ligament injury.

Lisfranc injury occult on radiographs. ( A ) Axial PD FS image shows increased signal within partially torn fibers in the plantar Lisfranc ligament. ( B ) Axial PD FS image in the same patient shows increased signal and disruption of the interosseous Lisfranc ligament. ( C ) Axial PD FS image through the dorsum of the foot shows low-grade tear of the dorsal Lisfranc ligament.

Patellar Dislocation, Transient Patellar Dislocation

Patellar dislocation can generally be established on physical examination and plain film radiography. However, some investigators have found that greater than 50% of patellar dislocations are not diagnosed correctly initially. Significant swelling and spontaneous reduction of the patella make clinical and radiographic evaluation difficult. In the presence of hemarthrosis and suspected patellar dislocation, MR imaging is indicated to look for OCDs, fractures, and disruption of stabilizing soft tissue structures.

The evaluation of patellar dislocation with MR imaging can be used to assess for osteochondral lesions, which can be missed or underestimated using radiography alone. Cartilage injury occurs in up to 95% of first-time patellar dislocations and has important implications for management. The size of the osteochondral injury helps the surgeon determine whether conservative or surgical management is appropriate. Although no strict criteria exist, lesions with subchondral bone greater than 9 mm are often fixated surgically and hence the size of the osteochondral lesion should always be reported. The integrity of the medial patellar retinaculum and medial patellofemoral ligament should be carefully assessed in patellar dislocation. In a normally aligned patella on axial images, increased marrow signal on T2-weighted images in the medial aspect of the patella and lateral femoral condyle represent bone bruises and indicate lateral dislocation of the patella ( Fig. 6 ).

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