Magnetic Resonance Imaging of Normal Variants of Bone Marrow of the Pediatric Knee

Purpose or Learning Objective: To describe normal knee bone marrow findings in pediatric patients on magnetic resonance imaging (MRI) and discuss variations of bone that may be mistaken for pathology and those that may become symptomatic.

Methods or Background: A total of 72 patients 5 to 18 years of age with knee pain and 26 asymptomatic patients 5 to 12 years of age were examined on 1.5-T MRI. A scanning protocol was performed with coronal, sagittal, and axial tomograms in the proton-density fat-suppressed and T1-weighted sequences.

Results or Findings: Hematopoietic marrow usually appears in the metaphyseal and adjacent diaphyseal areas and may mimic neoplastic infiltration, stress reaction, and contusion. A focal periphyseal edema zone was found in seven children with knee pain and without other abnormalities. Secondary ossification center of the femur is found in most children ages 5 to 12 years and must be differentiated from osteochondritis dissecans, which was diagnosed in 26 children. A posterior metaphyseal stripe is frequently visible in the posterior proximal tibia and disappears after reaching skeletal maturity. Distal femur cortical irregularity and nonossifying fibroma should not be misinterpreted as an aggressive periosteal reaction or parosteal osteosarcoma. Patella bipartite is a result of a failure of fusing accessory centers with the dominant ossification center and should not be misinterpreted as a fracture. MRI that shows a normal variant accessory ossification center is frequently associated with marrow edema and/or an abnormal surrounding fluid signal. This suggests the finding is related to avulsion injury or repetitive stress, patellar sleeve avulsion, or Sinding-Larsen-Johansson disease.

Conclusion: Awareness of normal variants, common incidental findings, and the normal evolution of bone marrow will aid in the interpretation and differentiation of abnormal conditions.

Publication History

Article published online:
22 May 2024