ABSTRACT FOR THE 2008 ISMRM MEETING – Toronto, ON, Canada MR IMAGING OF THE PEDIATRIC KNEE Lecturer:

FOR THE 2008 ISMRM MEETING – Toronto, ON, Canada MR IMAGING OF THE PEDIATRIC KNEE Lecturer: Andrea S. Doria, M.D., Ph.D., M.Sc. Date of Submission: December 7, 2007 A variety of imaging techniques is now available to investigate the growing knee. MRI, however, is playing an increasing role in the evaluation of bone, cartilage, joint space, tendon, soft tissue inflammation and other abnormalities in children’s and adolescents’ knees. MRI has become especially useful in the assessment of disorders of growing knee joints with the recent publications on normal age-related variation in MR imaging signal intensity within the cartilaginous epiphysis of the distal femur [1] and proximal tibia and fibula [2, 3]. The spectrum of pediatric knee disorders ranges from traumatic derangement, osteochondritis / osteonecrosis, rheumatic diseases, infection / inflammation to tumors. We discuss the ethopathogenesis and MRI findings that characterize these various diseases affecting the knee region of children and adolescent and summarize recent MRI techniques developed for early detection of morphologic and functional disorders of the knee. TRAUMATIC DERANGEMENT / OSTEOCHONDRITIS / OSTEONECROSIS Conventional MRI (T1-weighted, T2-weighted spin-echo/fastspin-echo and gradientecho sequences) has proven to be valuable in evaluating fractures of the distal femur and proximal tibia in children and adolescents [4, 5]. It can detect abnormalities in the cartilage that are associated with subsequent growth disturbances and provides accurate mapping of physeal bridging and associated growth abnormalities that have already occurred. Whereas providing improved delineation of non-displaced physeal fractures of the knee compared to conventional radiography, it simultaneously allow for evaluation of soft tissue structures [5]. With regard to injuries to the extensor mechanism of the knee, MRI is able to clearly demonstrate imaging characteristics of quadriceps muscle injury, patellar sleeve fracture, patellar dislocation, and patellar tendon injury with development of osteochondritis (either at the proximal region of the tendon, Sinding-Larsen-Johansson syndrome, or at the insertion of the patellar tendon at the tibial tuberosity, Osgood-Schlatter disease) [6]. The mechanism of injury of the quadriceps muscle involves placing stress on the quadriceps group with the knee flexed. In young healthy patients this injury occurs by strong deceleration as when someone is running and his/her leading foot is planted [6]. This musculotendinous unit is most often injured within 2 cm of the upper margin of the patella [7]. Patellar sleeve fracture is an acute cartilaginous avulsion from the lower pole of the patella occurring during forceful contraction of the quadriceps muscle against a partially flexed knee [8]. Acute patellar dislocations are common injuries in children, accounting for approximately 9-16% of acute trauma in young athletes with hemarthrosis [9]. In acute dislocation, there is complete, lateral displacement of the patella from the femoral trochlear groove. MR imaging can detect occult traumatic patellar dislocation by the identification of a constellation of indirect signs of injury: hemarthrosis, osteochondral injury, bone bruising of the medial patellar facet and lateral femoral condyle, and soft tissue damage to the medial retinaculum and medial patellofemoral ligament [10]. Sinding-Larsen-Johansson syndrome (children’s equivalent of patellar tendinitis or “jumper’s knee”) refers to a chronic insertional tendinopathy seen in skeletally mature athletes in Doria AS 2008 ISMRM 2 which a traction apophysitis is noted in the inferior patellar pole. This injury is due to repeated stress or vigorous exercise, and is more prevalent in boys [11]. Osgood-Schlatter disease is a chronic traction apophysitis of the tibial tubercle caused by repetitive traction trauma to the apophysis with a resulting tender prominence of the tibial tubercle [11]. Typically, both Sinding-Larsen-Johansson syndrome and Osgood-Schlatter disease adequately resolve by skeletal maturity without any need for surgery. Patellofemoral pain syndrome (chondromalacia patellae, “runner’s knee”) is a condition characterized by softening, fraying, and ulceration of patellar articular cartilage, that results from poor alignment of the patella as it slides over the distal femur [12, 13]. Patients with chondromalacia patellae frequently have abnormal patellar "tracking" toward the lateral aspect of the femur. This slightly-off-kilter pathway allows the undersurface of the patella to grate along the femur causing chronic inflammation and pain. Certain individuals are predisposed to develop chondromalacia patellae: females, knock-kneed or flat-footed runners, and those with an unusually shaped patella undersurface [14]. Previous studies demonstrated that spectral presaturation with inversion recovery (SPIR) sequences are superior to magnetization transfer contrast (MTC) sequences in the identification of low grade lesions in the patellar cartilage [15]. Osteochondritis dissecans (OCD) is the most common cause of a loose body in the joint space in adolescent patients [16]. In spite of the better prognosis of juvenile osteochondritis dissecans in comparison to the adult type [17], this entity may potentially lead growing joints to collapse and may predispose them to secondary arthritis. Note is made that grade I OCD (subchondral bone defect without articular cartilage interruption) can be confused with variants of ossification during normal development of the knee [17]. Osteonecrosis of the knee is a frequent complication of treatment of leukemia and lymphoma in children [18]. The most typical feature of the early osteonecrosis lesion is the characteristic interface between living and dead bone at the lesion’s periphery which is readly visible without the use of a contrast agent. In early MR-evident osteonecrosis, radiographs are often normal. In the immature skeleton, the medial meniscus is injured more frequently than the lateral meniscus or the anterior cruciate ligament (ACL) [19]. Initial studies on the overall diagnostic performance of conventional MR imaging (proton-density weighted, T2-weighted and T1weighted spin-echo imaging) for assessment of meniscal tears reported values of sensitivity and specificity of 83% and 95%, respectively, in children [19], and of 74% and 95%, respectively, in adults [20]. The majority of meniscal tears in children under the age of 10 years are due to a lateral discoid meniscus. Discoid meniscus is an abnormality of the fibrocartilaginous meniscus of the knee in which the meniscus is discoid rather than semilunar in shape [21]. With relation to anterior cruciate ligament (ACL) injuries, the sensitivity and specificity of conventional MR imaging for evaluation of ACL tears in pediatric knees are 95% and 88%, respectively [22]. Tibial avulsion fractures and partial tears are more common in younger, less rigid skeletons that may absorb the forces of trauma. As children’s joints mature, complete ACL tears and associated injuries occur in frequencies approaching those patterns seen in adults [23].