Clinical history: An 8 year old gymnast who injured the knee 5 days ago presents with posterior knee pain. (1a) T1 and (1b) fat suppressed proton density-weighted sagittal images are provided.
What is the finding? What is your diagnosis?
Normal developmental irregular ossification of the lateral femoral condyle.
Irregular ossification of the distal femoral epiphysis is a common observation on radiographs in children and is frequently bilateral, but not always symmetrical 1. The normal variation in the ossification of the femoral condyle can mimic osteochondritis dissecans (OCD), and several MR imaging features are helpful in distinguishing this normal variant from OCD.
The secondary center of ossification forms at an early stage of skeletal maturation. It is spherical and smooth in contour and is located in the central portion of the cartilaginous precursor. As the bony epiphysis continues to enlarge by means of enchondral ossification, the margins of the epiphysis may appear irregular or fragmented, simulating OCD or an acute fracture. Common locations for this imaging pitfall are the distal femoral epiphysis, the trochlea of the elbow and the tarsal navicular. In these regions, several small foci of ossification will eventually merge and appear solid.
Normal irregular ossification of the femoral condyles was present in 66% of the boys and 44 % of the girls 1 in a review of knee radiographs of 147 healthy, asymptomatic children between the ages of 3 and 13 years. The lateral condyle was involved in 44 % and the medial condyle in 12 %. The changes were frequently bilateral, but were not symmetrical. The regions of irregularity on radiographs have been found to decrease in size or disappear spontaneously within a mean observation period of 3.5 months 2. In a study utilizing knee MRI of 4 asymptomatic boys between the ages of 8 and 11 years with irregularity of the lateral femoral condyle it was shown that the radiolucent zones on the radiographs represented uncalcified cartilage 2. Location in the inferocentral posterior femoral condyles with intact overlying articular cartilage, accessory ossification centers, spiculations, residual cartilaginous model, and lack of bone marrow edema are features of normal variant ossification in this region 2,3.
Additional examples of normal irregular ossification are provided.
Care should be taken not to misdiagnose the normal irregular ossification as stage I OCD, because the accepted treatment of stage I OCD is restriction of activity, which may have a negative effect on patient lifestyle.
75% of cases of OCD of the knee are located in the posterolateral aspect of the medial femoral condyle, often extending into the intercondylar region. The lateral condyle is involved in 20% of patients and both knees are affected in one third of patients. The peak incidence is around 12 to 13 years of age 4,5.
It has been postulated that the rather favorable prognosis of juvenile OCD in the knee could be partially explained by the erroneous diagnosis of developmental variants of ossification as stage-I OCD 3 (articular cartilage thickening with abnormal signal).
In addition to normal irregular ossification of the femoral condyles, there are a number of additional variants related to skeletal maturation that have been recognized. Several of these variants will be demonstrated in the following cases.
Posterior metaphyseal stripe
In children and young adults there is subperiosteal fibrovascular tissue which is most prominent at the metaphyseal levels of the long bones. On MRI the normal subperiosteal tissue is best appreciated in the posterior aspect of the distal femur and appears as a thin stripe of bright signal on water-sensitive images. It enhances intensely after the administration of contrast material because of its rich vascularization. A similar metaphyseal stripe is frequently seen in the proximal tibial metaphysis. After skeletal maturity, the metaphyseal stripe disappears 6,7.
Avulsive cortical irregularity
Also known as distal femoral cortical irregularity, distal femoral metaphyseal irregularity and cortical desmoid, this variant represents a fairly common incidental imaging finding along the posterior aspect of the medial distal femur, 1-2 cm proximal to the physis. Some investigators have implicated a traumatic cause, related to an avulsive injury, while others consider this entity developmental in origin. Cystic and proliferative variants have been described. The cystic type is considered a fibrous cortical defect and appears as a cortical lucency or excavation on radiographs lateral to the medial supracondylar ridge of the femur. The proliferative type occurs along the medial supracondylar ridge of the femur and has regions of speculation or irregularity. T1-weighted images show low signal intensity and T2-weighted images show variable signal intensity within the lesions. Occasionally, an MRI is ordered when the lesion mimics an aggressive neoplasm, such as an osteosarcoma, on radiographs 8,9,10.
Residual red marrow is frequently present in the metaphyseal and adjacent diaphyseal portions of the femur and tibia in children, and may simulate abnormalities such as neoplastic infiltration, stress reaction and contusion. The regions of normal red marrow are longitudinally oriented with straight margins and have a base at or adjacent to the physis. On T1-weighted images they should always be of increased signal relative to muscle 7. On T2-weighted images the hematopoietic marrow is hyperintense due to its high water content with signal that approximates that of muscle. After the administration of contrast material, red marrow enhances to a higher degree than fatty marrow because it is more vascularized 11.
A bipartite patella is a result of failure of fusion of a secondary ossification center and should not be mistaken for an acute fracture. It is frequently bilateral in distribution and can be a cause of pain if there is disruption of its synchondrosis. A classification system according to Saupe 12 is based on location. Type I is at the inferior pole of the patella and is seen in 1% of cases. Type II is at the lateral margin of the patella and constitutes 20% of cases. Type III is in the superolateral patella and is the most common, found in 75% of cases. MR findings associated with a symptomatic bipartite patella include edema in the bipartite fragment (usually type III), abnormal signal across the synchondrosis, and cartilage discontinuity 13,14,15.
Dorsal defect of the patella
A dorsal defect of the patella is a variant related to normal ossification, present in 0.3 to 1 % of the population, located in the superolateral aspect of the articular surface of the patella. Histologically, it consists of fibrosis with or without bone necrosis. The cartilage over a dorsal defect is usually, but not always, intact. It may be a source of patellofemoral symptoms and knee pain. On T1-weighted images the signal of a dorsal defect is inhomogeneous and on gradient echo images the signal is equal to or greater than that of the cartilage. Healing may occur spontaneously 16.
Normal variant accessory ossification center at the inferior pole of the patella
A focus of bone at the lower pole of the patella may represent a normal variant, a patellar sleeve fracture (acute osteocartilaginous avulsion of the lower pole of the patella), Sinding-Larsen-Johansson disease or patellar tendinopathy (jumper’s knee). The normal accessory center may be single or multiple and may or may not be separated from the patella. MR imaging is important for the correct diagnosis in the child with anterior knee pain 8.
In order to properly evaluate findings in pediatric knee MR imaging, it is essential to be aware of the variant changes of the osseous structures that occur with age. Imaging pitfalls in children are often related to normal variations in ossification patterns of the femoral condyles, tibia and patella. It is also important to keep in mind that some developmental variants may become symptomatic. MRI is an excellent modality in the pediatric setting because of its ability to provide accurate assessment of bone marrow and cartilage without the use of ionizing radiation.
Additional normal variants of the knee have been discussed by Dr Carroll in the December 2007 Radsource MRI Web Clinic.
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