Exercise: Joint Trauma

EXERCISE 7-2. JOINT TRAUMA

7-4. In Case 7-4 (Figure 7-13), the most likely diagnosis is

A.   lunate dislocation.

B.   perilunate dislocation.

C.   transcaphoid fracture-dislocation.

D.   distal radius fracture.

7-5. The basketball player in Case 7-5 (Figure 7-14) shows which of the following injuries?

A.         Fracture

B.         Dislocation

C.         Osteoarthritis

D.         None of the above

7-6. The basketball player in Case 7-6 (Figure 7-15) has which type of abnormality on the MR image?

A.         Tendon injury

B.         Muscle injury

C.         Ligament injury

D.         Cartilage injury

7-7. The MR image in Case 7-7 (Figure 7-16) shows which abnormality?

A.   Muscle injury

B.   Ligament injury

C.   Tendon injury

D.   Cartilage injury

Radiologic Findings

7-4. In the frontal projection (Figure 7-13 A), there is dis-organization of the carpal arcs. The capitate is no longer articulating with the lunate and partly over-laps the scaphoid (arrow). The scaphoid is elongated on this view but not fractured. On the lateral projec-tion (Figure 7-13 B) the lunate is still in line with the distal radius, but the capitate has been dislocated dorsally (open arrows). Therefore, the patient has a dorsal perilunate dislocation (B is the correct answer to Question 7-4).

7-5. The AP radiograph of the shoulder of a basketball player (Figure 7-14 A) shows inferior displacement of the humeral head out of its normal position within the glenoid. The scapular-Y view of the shoulder (Figure 7-14 B) shows that the humeral head (H) is dislocated anteriorly in relation to the glenoid (G), thus representing an anterior shoulder dislocation. S, scapula; C, coracoid. This is the classic appearance of an anterior dislocation of the shoulder. (B is the cor-rect answer to Question 7-5.)

7-6. The sagittal MR image shows the advantage of MR imaging in this clinical setting. Figure 7-15 shows a tear of the anterior cruciate ligament (ACL) (arrow) (C is the correct answer to Question 7-6).

7-7. Figure 7-16 shows the ends of a torn supraspinatus tendon (arrows) in the squash player (C is the correct answer to Question 7-7).

Discussion

Dislocation or subluxation: The terms subluxation and dis-location are often used interchangeably. However, subluxa-tion refers to partial loss of congruity between the articulating ends of bones, whereas dislocation denotes com-plete loss of congruity. Disruption or loss of the integrity ofthe restraining ligaments around the joint leads to insta-bility and thus permits dislocation to occur. Severe hyper-flexion or hyperextension forces often cause traumatic dislocations. Fractures are frequently associated with traumatic dislocations.

Carpal Dislocation

The normal arrangement of the carpal bones of the wrist is seen on the AP view of the wrist (Figure 7-17 A). Note the three smooth, parallel arcs in the proximal and mid-carpal rows (arcs of Gilula). The lateral view of the wrist (Figure 7-17 B) shows that the radius, lunate, and capitate are in an almost straight line. There are two major types of wrist carpal dislocation: perilunate and lunate dislocations. In a perilunate dislocation, the lateral film shows that the lu-nate maintains its normal articulation with the radius and the capitate is displaced dorsally. In a lunate dislocation, the lunate has a triangular shape on the frontal projection (Figure 7-18 A) and is displaced from its normal articula-tion and the radius and capitate maintain a linear relation-ship (Figure 7-18 B). Carpal dislocations are usually produced by a fall on the outstretched hand (foot) and are more common common mechanism of injury to the ACL is the “clipping” injury with valgus stress and internal rotation of the knee. On MR, the injured ACL is diagnosed by high signal intensity within the substance of the ligament (the so-called pseudo-mass). There may also be other associated abnormalities such as bone contusions (usually on the posterolateral aspect of the tibia and the anterolateral aspect of the femur that result from the transient dislocation that occurs at the time of in-jury, the so-called kissing contusions) and medial collateral ligament injury from the valgus stress (Figure 7-21). There may also be associated meniscal tears, usually vertical tears in the acute setting (Figure 7-22).

The posterior cruciate ligament (PCL) serves to limit the posterior translation of the tibia in relation to the femur. The PCL is commonly injured in kicking sports such as soccer and is also injured in automobile accidents if the tibia im-pacts on the dashboard and is translated posteriorly in rela-tion to the femur in a flexed knee.

The normal PCL on MR is a homogeneous structure that originates from the inner aspect of the medial femoral condyle and extends far posteriorly to insert onto the poste-rior aspect of the tibia (Figure 7-23). The PCL should easily be seen on all knee MR studies. Tears of the PCL are diagnosed using MR imaging. Partial tears are identified by increased T2in young adults. The diagnosis is usuallymade by radiographic examination, although CT may be used after reduction to evaluate the wrist for joint con-gruity and for the presence of intraarticular fracture frag-ments (“loose bodies”).

Shoulder Dislocation

The two main directions in which the proximal and humerus dislocates are anterior and posterior. Anterior dislocation, usually caused by falls, is most common and is seen in about 95% of cases. In an anterior dislocation, the humeral head is displaced anteriorly and inferiorly to the scapular glenoid fossa. There are various subtypes of anterior dislocation: sub-glenoid, subcoracoid, and medial. These subtypes are based on the location of the humeral head relative to the glenoid fossa and coracoid process.

Posterior dislocation is relatively uncommon. It is most commonly associated with severe contraction of the muscles of the shoulder girdle, which may occur in electric shock or convulsions. A diagnosis of posterior dislocation in one shoulder should prompt investigation of the other shoulder, because this injury is often bilateral.

If the postreduction radiographs are normal after a single instance of dislocation, there is usually no need for anotherimaging study in the acute setting. However, if there is a re-currence of dislocation or if the patient remains chronically symptomatic, MR imaging or CT arthrography of the shoul-der should be obtained to search for the cause of the disloca-tions and any associated shoulder abnormalities resulting from the dislocation.

CT arthrography and MR imaging are used to investigate the shoulder for cartilage and soft-tissue injuries resulting from shoulder dislocation. After an anterior dislocation, there is frequently associated injury of the anterior glenoid labrum. This is produced by impaction of the posterior and lateral aspect of the humeral head against the anterior and in-ferior portion of the glenoid. There may also be an accompa-nying compression fracture of the humeral head, referred to as a Hill-Sachs deformity.

Hip Dislocation

The hip is a relatively stable joint because of the surrounding strong muscles and joint capsule, and significant trauma common mechanism of injury to the ACL is the “clipping” injury with valgus stress and internal rotation of the knee. On MR, the injured ACL is diagnosed by high signal intensity within the substance of the ligament (the so-called pseudo-mass). There may also be other associated abnormalities such as bone contusions (usually on the posterolateral aspect of the tibia and the anterolateral aspect of the femur that result from the transient dislocation that occurs at the time of in-jury, the so-called kissing contusions) and medial collateral ligament injury from the valgus stress (Figure 7-21). There may also be associated meniscal tears, usually vertical tears in the acute setting (Figure 7-22).

The posterior cruciate ligament (PCL) serves to limit the posterior translation of the tibia in relation to the femur. The PCL is commonly injured in kicking sports such as soccer and is also injured in automobile accidents if the tibia im-pacts on the dashboard and is translated posteriorly in rela-tion to the femur in a flexed knee.

The normal PCL on MR is a homogeneous structure that originates from the inner aspect of the medial femoral condyle and extends far posteriorly to insert onto the poste-rior aspect of the tibia (Figure 7-23). The PCL should easily be seen on all knee MR studies. Tears of the PCL are diagnosed using MR imaging. Partial tears are identified by increased T2 signal and swelling within the ligament. Complete tears of the ligament are diagnosed by discontinuity of the ligament fibers at some point along its course (Figure 7-24). MR imag-ing is extremely important in the evaluation of the knee of the injured athlete and is used frequently in this setting.

Supraspinatus Tendon Tears

The supraspinatus, infraspinatus, teres minor, and subscapu-laris muscles (SITS muscles) comprise the rotator cuff. De-spite being the most unstable joint in the body, the rotator cuff muscles help to stabilize the joint. The most commonly torn tendon in the shoulder is the supraspinatus, and it usu-ally tears approximately 1 centimeter proximal to its inser-tion onto the anterior aspect of the greater tuberosity of the humeral head. The supraspinatus tendon is easily seen on MR imaging as a low-signal-intensity structure, and tears of the supraspinatus tendon are well demonstrated on MR. The most common causes of supraspinatus tendon tears are aging and impingement. Acute rotator cuff tears are unusual. MR imaging is vital in the preoperative evaluation of the patient suspected of having a rotator cuff tear. The morphology of the tendons, the size of the tear, and other associated abnor-malities of the joint, including pathology of the glenoid labrum, can be diagnosed with this technique. Importantly, the degree of muscle atrophy associated with chronic tears can be assessed, thereby suggesting the probability of a suc-cessful postoperative recovery and rehabilitation.

Achilles Tendon Rupture

The injury of Achilles tendon rupture occurs most frequently in patients in the fourth and fifth decades of life. Althoughthe injury may occur in any person, individuals who do not exercise regularly (“weekend warriors”) are more susceptible to this tear.

The clinical history and physical findings are often enough to make a diagnosis of Achilles tendon rupture. Ra-diographic stress views should not be performed in the set-ting of a suspected Achilles tendon rupture because the stress may actually make the tear worse. Any question of whether the tear is partial or complete should be resolved, as the treatment for each of these is different. Moreover, the clinician needs to know the level of injury and how far the tendon fragments are separated. MR imaging is currently the imaging technique of choice to evaluate the Achilles tendon, although ultrasound is an excellent alternative and is used more commonly in Europe for this injury. The whole length of the tendon, including its insertion on the calcaneus, and any associated injuries can be shown in de-tail. In cases of acute complete rupture of the tendon, the MR images show discontinuity of the normally low-signal-intensity fibers of the Achilles tendon, which are replaced by edema and hemorrhage. MRI helps to quantitate the amount of distraction between the ends of the torn tendon. The Achilles tendon may also avulse a small portion of bone from its calcaneous attachment (Figure 7-25). In partial tears, areas of intermediate to high signal, representing re-gions of partial disruption, are seen within the normally low-signal-intensity tendon, and some of the fibers of the tendon remain intact. Ultrasound may also be used to eval-uate the Achilles tendon, and color Doppler examination may be used to follow the process of revascularization and healing of a partially torn tendon.