Книги по МРТ КТ на английском языке / MRI for Orthopaedic Surgeons Khanna ed 2010

MCL injuries comprise part of the “terrible triad”; the other two components are ACL tears and medial meniscus tears. However, it has been found that lateral meniscus tears, rather than medial meniscus tears, are more common with acute ACL tears.36

posterolateral corner injuries. Coronal oblique images o er the best imaging for evaluating and diagnosing the posterolateral structures (Fig. 8.26). Specific attention should be given to including imaging through the entire fibular head. A high-energy varus injury to the knee can place tension on, and even rupture, the peroneal nerve after the postero-

Posterolateral Corner

The posterolateral corner of the knee is complex from an anatomic and, therefore, imaging standpoint. The variability of the posterolateral structures makes the images of this area of the knee even more di cult to interpret. The posterolateral corner structures resist varus and external rotation forces. The structures that comprise the posterolateral corner are the following:

•LCL

•Arcuate ligament

•Popliteal tendon

•Lateral head of the gastrocnemius

•Posterolateral capsule

•Biceps tendon

•Meniscofibular ligament

•Popliteofibular ligament

•Fabellofibular ligament (when the fabella is present)

Although the biceps tendon and LCL usually are identifiable, identifying the popliteofibular ligament is more di cult.37 This di culty could contribute to the incidence of missed

Fig. 8.26 This coronal fat-suppressed T2-weighted image of the right knee shows tears of the fibular collateral ligament (arrow) and biceps femoris tendon at the fibular head. Note the corresponding impaction injury of the medial femoral condyle.

lateral knee structures are torn. Careful attention should also be given to noting discontinuity or edema of the peroneal nerve.

The arcuate complex is composed of the LCL, arcuate liga-

Fig. 8.28 This coronal fat-suppressed T2-weighted image of a patient with an MCL sprain in the right knee shows the deep and superficial (arrow) components of the MCL complex, separated by fluid and edema.

ment, and popliteus tendon. It provides posterolateral stability of the knee to varus and rotatory stress. The arcuate ligament is an inconsistent structure and is occasionally absent in individuals with large fabellae. Injury to this complex area of the knee is seen as increased signal intensity on T2-weighted images because of the underlying edema (Fig. 8.27).

Posteromedial Corner

The posteromedial corner is composed of the deep and superficial MCLs, the posterior oblique ligament, and the oblique popliteal ligament. Damage to the posteromedial corner can cause valgus instability of the knee and increased anteromedial subluxation. The posterior oblique ligament is attached to the posterior horn of the medial meniscus. Some authors have advocated surgical repair of damage to this complex.38 Coronal images (Fig. 8.28) o er the best views of

Table 8.5 Collateral Ligament Tears

Fig. 8.31 This axial fat-suppressed T2-weighted image of the left knee shows a tear (arrow) of the medial patellar retinaculum at its patellar attachment.

baseball) and through contact mechanisms (e.g., laterally directed force to the medial side of the patella). Patients can be first-time dislocators, which implies acute trauma, or recurrent dislocators, implying chronicity and instability. The MRI findings in the recurrent dislocator without a history of trauma may not be as definitive. The patella has usually already reduced by the time the MRI study has been obtained, but it is frequently subluxated laterally. Images of acute patellar dislocations commonly show bone bruises of the lateral trochlea and the medial patella (Fig. 8.30). When the patella displaces over the lateral femoral condyle, varying sizes of osteochondral fragments may be fractured from the patella or trochlea, or both. On MRI, these loose bodies may be found in the joint, and fluid–fluid levels may be seen. MRI typically reveals a rupture of the medial patellar retinaculum, including the medial patellofemoral ligament (Fig. 8.31). Noting the location of the medial patellofemoral ligament is important because it can guide decisions about repair or reconstruction and may help identify the optimal location for the incision. Edema may also be seen in the region of the vastus medialis oblique. The only clue for chronic dislocators may be an attenuation or laxity of the medial retinaculum and medial patellofemoral ligament. Large e usions are commonly seen on MRI of these injuries. MRI also can help the clinician determine the degree of predisposition to patella dislocation. For instance, a hypoplastic lateral trochlea seen on axial images suggests instability secondary to insu cient osseous constraint. Identification of these predisposing factors assists with surgical planning and determination of prognosis.

Quadriceps and Patellar Tendon Ruptures

The quadriceps tendon has a laminated appearance on MRI because it is formed as a conjoined structure from four muscles. Understanding this concept allows the orthopaedic surgeon to avoid misreading the normal appearance of the quadriceps tendon as an intrasubstance injury or strain of the quadriceps tendon. Partial quadriceps tendon tears often are associated with increased T2-weighted signal, especially on fat-suppressed images, just anterior to the quadriceps tendon.

Patellar tendon ruptures (Fig. 8.32) usually occur in young patients (<50 years old), whereas quadriceps tendon ruptures (Fig. 8.33) occur in older patients, in patients with comorbidities such as diabetes mellitus and obesity, and in patients on steroids. Sagittal images are ideal for visualizing both types of ruptures. Tendon discontinuity with patella alta is seen on T1-weighted sagittal images with patellar tendon rupture. Discontinuity of the tendon and patella baja can be seen in patients with quadriceps tendon rupture.42 Both types of rupture show edema on T2-weighted images. It is helpful to di erentiate an avulsion from an intratendinous failure because bone-to-bone healing allows for a better prognosis. Although the diagnosis of quadriceps and patellar tendon rupture can frequently be made by history and physical examination (including palpation of a suprapatellar or

infrapatellar defect), MRI may still provide valuable information regarding the presence or absence of other intraarticular injuries, such as cruciate ligament or meniscal tears, that may be treated at the time of ligament repair or reconstruction.

Muscle Strain

Muscle strains about the knee are common.43 The biceps femoris (Fig. 8.34) is by far the most commonly strained muscle at the knee, followed by the semimembranosus and semitendinosus muscles.44 A muscle strain is identified on MRI as a region of increased T2-weighted signal within the substance of the muscle without frank discontinuity of the muscle fibers. One third of all strains involve multiple muscles.43 The length of abnormal signal in the strained muscle is predictive of the athlete’s ability to return to play.45 Care must be taken to di erentiate an avulsion from a strain, given that avulsions often require surgical intervention.

Loose Bodies

Loose bodies are an indication for arthroscopic surgery. There are two typical sources for loose bodies:

•Osteophytes and degenerative cartilage debris in the osteoarthritic knee

Fig. 8.34 This coronal fat-suppressed T2-weighted image of the right knee shows intramuscular edema (arrow), compatible with strain, involving the biceps femoris muscle.

•Osteochondral fragments from acute trauma, such as when the patella dislocates

The source of the loose body sometimes is recognizable; when it is, it o ers valuable information. MRI is useful for locating chondral or osseous fragments in the knee, especially purely chondral and unmineralized lesions, which are not visible on conventional radiographs. Estimating the size of osteochondral fragments is important because the size dictates what treatment options are available. Loose bodies are found most frequently in the suprapatellar pouch, the medial and lateral gutters, and the posterior compartments46 (Fig. 8.35). A good clue that allows for diagnosis of a loose body is identification of the osteochondral donor site. Multiple loose bodies seen on MRI without evidence of donor sites may suggest synovial osteochondromatosis, a benign proliferative disorder of the synovium.

MRI and MR arthrography can be used to evaluate loose bodies, which typically have low signal on T1-weighted and T2-weighted pulse sequences. Gradient-echo images are also frequently used, especially to help di erentiate the diagnosis from PVNS. As with other diagnoses, it is important to correlate the MRI findings with those seen on conventional radiographs.

Osteochondroses

Osgood-Schlatter Disease

Osgood-Schlatter disease is an osteochondrosis of the immature apophysis of the patellar insertion found commonly in adolescent males. The patella tendon insertion is tender to the touch. The adolescent can have severe pain with activity, especially with forceful extension of the flexed knee. MRI shows focal fragmentation of the patellar tendon insertion onto the tibia with associated edema (Fig. 8.38). Patients are treated with activity modification, rest, and occasionally casting. Very rarely, surgery is necessary to remove recalcitrant, painful fragments.

Sinding-Larsen-Johansson Disease

This entity, which is similar to but less common than OsgoodSchlatter disease, occurs at the inferior pole of the patella.

Discoid Meniscus

This entity occurs in approximately 5% of the population.50–52 The lateral meniscus is much more commonly involved than is the medial meniscus.53 Discoid menisci were first classified by Wantanabe et al54 as follows:

Fig. 8.38 This sagittal fat-suppressed proton-density image shows Osgood-Schlatter disease: thickening and abnormal signal intensity involving the distal patellar tendon (arrow); bone marrow edema and slight distraction of the tibial tubercle apophysis; and fluid accumulation and edema involving the deep infrapatellar bursa, posterior to the distal patellar tendon.