Knee Patterns

In the setting of acute trauma, identifying the pattern of marrow abnormality is helpful in directing the search for potential sites of cartilage injury. From these patterns it is possible to infer instability patterns related to the principal mechanisms of injury [107]. Sanders and colleagues [108] have described five basic patterns in the knee that are associated with soft-tissue injury: (1) the pivot shift injury, (2) the dashboard injury, (3) the hyperextension injury, (4) the clip injury, and (5) the lateral patellar dislocation.

The pivot shift injury pattern is characterized by marrow hyperintensity in the posterior lateral tibial plateau, and the central portion of the lateral femoral condyle near the terminal sulcus. This injury pattern is a result of a valgus load applied to the flexed knee combined with internal rotation of the femur or external rotation of the tibia, and is associated with disruption of the anterior cruciate ligament. Impaction of the lateral femoral condyle on the tibial plateau can result in osteochondral fractures or delamination injuries of cartilage in the terminal sulcus of the lateral femoral condyle (deep sulcus sign), or posterior lateral tibial plateau.

The term dashboard injury reflects the common mechanism of injury in which the flexed knee of a front seat passenger strikes against the dashboard in a head-on motor vehicle collision. When the force is applied to the superior tibia, this can produce posterior translation of the tibia, leading to disruption of the posterior cruciate ligament (PCL) and posterior joint capsule. Blunt trauma applied more superiorly to the patella of a flexed knee may result in osteochon-dral fractures of the patellofemoral joint [109]. Lower velocity loading of the patellofemoral joint, such as that from a fall on a flexed knee, can produce acute chondral injuries of the patella and trochlea in a ''kissing'' pattern reflecting the contact points at the time of injury (see Fig. 8). Patellar chondral injuries are generally best seen on axial views, and consist of fissures often associated with high signal in the deep radial zone of cartilage. Delamination injuries are also encountered with this mechanism, and can be recognized by linear signal at the bone cartilage interface on fat-suppressed proton density or T2-weighted images. With this mechanism of injury, the femoral cartilage is frequently injured at the inferior margin of the trochlear groove and is best visualized on sagittal images.

Hyperextension injuries resulting in marrow contusions in the anterior femoral condyle, and tibial plateau can produce anterior cruciate ligament (ACL), PCL and meniscal injuries. When combined with a valgus force, the marrow contusions are located medially, and are associated with injuries of the postero-lateral corner. A hyperextension mechanism can be associated with shear injuries or osteochondral fractures of the anterior femoral condyle, best seen on coronal or sagittal images overlying the site of marrow contusion (Fig. 9).

Clip injuries of the knee result from a direct valgus force applied to a partially flexed or extended knee, producing compressive loading of the lateral compartment and tensile stress of the medial stabilizers. Soft-tissue injuries consist of sprain injuries of the medial collateral and anterior cruciate ligaments, as well as meniscal tears. Compressive loading of the lateral compartment can result in impaction injuries of the central femoral and tibial surfaces [110]. As illustrated in Fig. 10, avulsion osteochondral fractures can result medially.

Transient lateral dislocation of the patella can be recognized by focal marrow edema of the inferomedial patella and anterolateral margin of the lateral femoral condyle. In athletes, transient patella dislocation typically occurs in adolescents and young adults as a result of a twisting motion while the knee is in 10° to 20° of flexion with the foot planted. Patellar dislocations are associated with injuries of the medial retinaculum. These include sprain injuries and avulsion fractures of the medial patellofemoral ligament, either from the adductor tubercle or medial patella, or strain injuries of the vastus medialis obliquus muscle. Suspicion of a transient patellar dislocation should lead to careful evaluation of the patellofemoral articular surfaces for evidence of cartilage injury or

Avm Vastus Lateralis

Fig. 9. Hyperextension injury: 20-year-old professional hockey player 2 weeks following knee injury. (A) Sagittal fat-suppressed, T2-weighted FSE image demonstrates focal marrow edema in the anterior margin of the lateral tibial plateau and femoral condyle, consistent with a hyperextension mechanism of injury. (B) Coronal fat-suppressed, PD-weighted FSE T2-weighted image demonstrates a small osteochondral fracture overlying the femoral site of marrow edema (arrow).

Fig. 9. Hyperextension injury: 20-year-old professional hockey player 2 weeks following knee injury. (A) Sagittal fat-suppressed, T2-weighted FSE image demonstrates focal marrow edema in the anterior margin of the lateral tibial plateau and femoral condyle, consistent with a hyperextension mechanism of injury. (B) Coronal fat-suppressed, PD-weighted FSE T2-weighted image demonstrates a small osteochondral fracture overlying the femoral site of marrow edema (arrow).

Fig. 10. Clip injury: 31-year-old female catcher on a softball team 1 week following a valgus stress injury of the knee, resulting in tears of the MCL and ACL. Note the large marrow contusion in the lateral femoral condyle from direct impaction. An avulsion osteochondral fracture is present in the medial margin of the medial tibial plateau (arrow).

osteochondral fracture, which at arthroscopy are present in more than 66% of cases [111]. The prevalence of osteochondral injury identified on MRI studies obtained after patellar dislocation is similar ranging from 58% to 78% [40,112114]. Osteochondral injuries are particularly prevalent in the inferomedial patella and adjacent to the median ridge. In a review of 81 patients who had lateral patellar dislocation, Elias and colleagues [112] observed osteochondral injuries on MRI in the inferomedial patella of 70% of subjects, and the antero-lateral femoral condyle in 5%. This study demonstrated a concave deformity of the inferomedial patellar surface consistent with an osteochondral impaction fracture in 44% of subjects. Shear forces applied to the cartilage of the median ridge can result in oblique flap tears or delamination injuries that appear as linear hyperintensities on fat-suppressed, PD-weighted FSE images. The rate and magnitude of loading of the shear force influences the location of cartilage injury. When shear force is applied at high speed but with low energy, cracks are produced along the articular cartilage surface. At low speed and low energy, splits initially occur in the deeper layers [115].

Long-term, many young athletes have persistent retropatellar pain or evidence of patellofemoral instability [116]. It appears that individuals who have a single episode of patellar dislocation are at greater risk of developing patello-femoral osteoarthritis than individuals who have a recurrent pattern of dislocation [117]. This may be a result of greater force needed to dislocate a stable patella, and higher potential for osteochondral injury.

The knee is the most common joint for OCD; however; the pattern of involvement differs from that observed following acute trauma. The lateral margin of the medial femoral condyle is the most common site of involvement (85%) followed by the inferocentral portion of the lateral femoral condyle (13%) and the anterior lateral femoral condyle (2%) [118]. Less frequently,

OCD can involve the patellofemoral joint [119,120], and can be a source of persistent anterior knee pain in the young athlete [121,122]. Radiographs are relatively insensitive to osteochondral lesions of the patella, and these are likely under-recognized as a cause of anterior knee pain. Osteochondritis dissecans usually occurs in the distal half of the patella, and is frequently associated with a history of anterior knee trauma [123]. On the femoral trochlea, the lesion is most frequently observed on the anterior lateral femoral condyle close to midline [124] in adolescent athletes who have repetitive flexion extension related to running or jumping activities [125].

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