AP and lateral radiographs of the right knee
AP and lateral radiographs of the right knee
Axial CT and Sagittal MR (T1-weighted) images of the right knee
The list of entities that involve the epiphysis or apophysis is relatively short, and includes:
Giant cell tumor is almost never seen until the plates have closed. ABC and enchondroma may be seen in the metaphysis or diaphysis prior to closure of the growth plates, but generally do not involve the epiphysis until after the physis is closed.
A very helpful clue in this case is the presence of a fluid-fluid level on the MR exam. Remember that the patient was lying supine in the scanner, and the vertical line seen on the image is really a horizontal line. Fluid-fluid levels indicate that the entity under consideration contains fluid. They can be seen in a number of entities, including aneurysmal bone cyst (at least 50 % of the time on CT -- probably more often on MR), giant cell tumor, osteosarcoma, and other entities.
Sagittal MR (T1-weighted) image showing fluid-fluid level (arrows)
Putting this all together, the most likely entities remaining on the differential diagnosis list are ABC and giant cell tumor, probably in that order.
Aneurysmal bone cyst (ABC) is associated with a preexisting lesion (such as giant cell tumor, chondroblastoma, fibrous dysplasia, etc.) in 30 - 50 % of cases. There is also commonly an associated history of trauma. These two facts have led to the theory that ABC is a vascular anomaly which arises due to trauma or due to some preexisting lesion. These facts also have led some investigators (notably at the AFIP) to suggest that there is really no such thing as an ABC -- rather, that all lesions called ABC are a reactive lesion due to something else. Regardless of this controversy, most musculoskeletal physicians probably still find this term useful, and continue to use it.
ABC's are usually seen in the 1st through 3rd decades of life, and usually present with a geographic lesion in the metaphysis of a long bone, the pelvis, or the posterior elements of the spine. By the time they are diagnosed, these lesions are usually fairly expansile, which has prompted the term "aneurysmal" in their name. Histologically, they are usually filled with blood.
These lesions are, in themselves, benign. However, they may occasionally present with a quite aggressive appearance radiographically, and may be mistaken for a malignant tumor. A caveat to consider is that although an ABC is benign, the precursor lesion from which is arises may actually be a sarcoma.
The usually treatment is curettage, and recurrence following curettage is a common occurrence (about 50 %).