A 14-year-old boy with no significant past medical history presented to the pediatric orthopedic clinic with left knee pain that began four months prior. The patient localized the pain to underneath/inside his kneecap and denied any history of trauma. He reported the pain to be worse with running and jumping activities. There was mild pain with patellar compression. All other focused physical examination findings of the left lower extremity, past medical history, and review of systems were unremarkable.
Radiographs of the left knee were initially obtained (Figure 1) followed by MRI of the left knee without and with intravenous contrast (Figure 2). CT-guided biopsy was performed for definitive diagnosis.
The patella is a sesamoid bone, embedded within the quadriceps tendon superiorly and patellar tendon inferiorly. The patella demonstrates similar architecture to the epiphyses of long bones and is, therefore, termed an “epiphyseal equivalent.” Examples of other epiphyseal equivalents include the greater and lesser trochanters of the femur, the carpal bones, calcaneus, as well as most apophyses. Knowledge of this information helps narrow the differential diagnostic considerations for lesions typically encountered in the epiphysis.
Chondroblastoma is a benign cartilaginous neoplasm typically seen in the epiphyses of long bones or epiphyseal equivalents. Although benign, the tumor is locally aggressive and typically produces surrounding edema of the bone marrow. On radiographs, typical features are of a well-defined osteolytic lesion with thin peripheral sclerotic border. They typically arise eccentrically and may extend to the metaphysis, although breach of the cortex is rare. Internal calcifications may be present, and a joint effusion may be seen. Periosteal reaction distal to the lesion (along the diaphysis when the lesion is in the epiphysis) distinguishes chondroblastoma from other epiphyseal lesions.1 CT shows similar findings to radiographs, with better delineation of extent, periosteal reaction, and internal matrix. MRI is optimal for evaluating transphyseal extent and degree of surrounding marrow edema. Chondroblastoma typically demonstrates isointense T1 signal, variable hyperintense T2 signal within the lesion, hyperintense surrounding STIR signal due to edema, and moderate heterogeneous enhancement with enhancement of the surrounding bone and soft-tissue edema.2 A thin hypointense rim with lobulated margins is characteristic. Fluid-fluid levels can be seen due to association with secondary aneurysmal bone cyst (ABC), which have been reported to occur with chondroblastoma.
The skeletal system is the most common site for Langerhans cell histiocytosis (LCH). LCH in the skeletal system is often referred to as eosinophilic granuloma. LCH primarily occurs in older children and young adults and may occur anywhere in the skeletal system with variable appearance. The imaging features on radiographs depend on the phase of disease. Features in long bones range from permeative appearance to focal lytic lesions with endosteal scalloping, cortical thinning, and periosteal reaction. An associated soft-tissue mass is often present. On MRI, LCH typically demonstrates hypointense to isointense T1 signal, hyperintense T2/STIR signal, and avid enhancement with or without enhancing soft-tissue component. Diaphyseal location is most common in long bones.4
Pyogenic osteomyelitis with a Brodie abscess may mimic certain bone lesions. On radiographs, the typical appearance is of a round lucent lesion with or without peripheral sclerosis depending on the time at which the bone is imaged in the course of infection. On MRI, the abscess demonstrates variable T1 signal and central increased T2 signal. On postcontrast images, enhancement is usually peripheral with enhancement of the surrounding marrow, and nonenhancement of the purulent core. Diffusion-weighted sequences, if performed, would demonstrate central diffusion restriction due to pus. Location is often an important distinguishing feature, as hematogenous osteomyelitis in older children tends to affect metaphysis/metaphyseal equivalents. 6 In younger patients (infants and toddlers), epiphyseal location of infection is more common due to transphyseal vascular supply.
Chondromyxoid fibroma is an extremely rare, benign cartilaginous neoplasm that most commonly occurs in the second and third decades of life. It is typically located in the metaphysis of long bones and may extend to the epiphysis but is almost never exclusively epiphyseal. On radiographs, they are typically eccentrically located lobular lesions with a well-defined sclerotic margin. They are often expansile with geographic bone destruction. MRI features are nonspecific and generally include low T1 signal, intermediate to high T2 signal, and peripheral nodular enhancement on postcontrast images.5
Giant cell tumor (GCT) is usually a benign tumor that typically arises in the metaphyses of skeletally mature patients. Epiphyseal location of the tumor is most often due to extension along the fused physis, helping distinguish this entity from other epiphyseal-based lesions. Four characteristic features of GCT in a long bone include occurrence in skeletally mature patients, abutment of the articular surface, being well-defined without sclerotic margins, and eccentric location. The overlying cortex is often thinned, and periosteal reaction is rare. Association with ABC formation is documented.3 On MRI, GCT demonstrates low to intermediate T1 signal, heterogeneously hyperintense T2 signal, and enhancement of the solid components. 3 Perilesional edema is typically much less than that seen in chondroblastoma. It is a rare tumor, comprising 1% to 2% of all primary bone tumors. However, it is the most common neoplasm of the epiphysis in skeletally mature patients. The typical presentation is localized pain, although due to its epiphyseal location, it may trigger synovial reaction.
Malignant bone lesions, both primary and metastatic, are rare in the epiphyses. Osteosarcoma usually occurs in the metaphyseal-diaphyseal region of long bones and may extend into the epiphysis. Osteoid matrix, permeative growth, and cortical destruction are typical features.7 Epiphyseal clear cell chondrosarcoma is a subtype of chondrosarcoma that may be found in the epiphysis. This tumor, however, usually affects an older population (third to fifth decades) and generally produces little to no surrounding marrow edema.8 While bony metastatic lesions can have a variable appearance depending on the primary tumor, epiphyseal hematogenous metastasis is rare and, therefore, would be considered unlikely when encountering a solitary epiphyseal lesion.
Chondroblastoma
When encountering lesions of the patella, the differential diagnosis is that of epiphyseal lesions. While tissue sampling is necessary for definitive diagnosis, it is important to account for the patient’s age as well as specific imaging features to help narrow the differential. Chondroblastoma is the most common epiphyseal lesion in a skeletally immature patient. Both chondroblastoma and GTC may involve the epiphysis and have an associated ABC component, but GCT classically enters the epiphysis by crossing a fused physis. Primary malignant tumors and metastases of the epiphysis/patella are rare. Langerhans cell histiocytosis may cause cortical breach and soft-tissue component, distinguishing it from chondroblastoma. Presence of surrounding marrow edema is a hallmark of chondroblastoma. Infection with Brodie’s abscess may have a similar appearance, in which case clinical findings are helpful in differentiating the two. Additional considerations for patellar abnormalities in the pediatric population include osteochondral lesions and dorsal defect of the patella.
Treatment for chondroblastoma typically involves surgical curettage and packing. Radiofrequency ablation may also be used.9 Complete eradication of lesions may be difficult due to proximity to the growth plate; thus, lesions may recur and cause growth arrest with resultant limb-length discrepancy.
Nahl D, Sung A. Patellar Bone Lesion. J Am Osteopath Coll Radiol. 2021;10(3):24-27.
