Case 10
A 26-year-old male presented with right anterior knee pain. Physical examination was entirely normal.
A: Presenting AP radiograph of the knee.There is a well marginated radiolucent lesion occupying the medial half of the patella that is partially obscurred by the overlapping of the femoral condyle.
B: Lateral view at presentation. The lesion appears as a radiolucency entirely within the patella that has resulted in thickening of the patella.
C: Axial CT image. The lesion appears contained although there is an impression fracture of the medial aspect of the patella.
The patient failed to appear for admission to the hospital. Two years later he was seen again with increasing discomfort.
D: Lateral radiograph two years after initial presentation. The lesion now occupies the entire patella, appears divided by septae, but is still contained by a thin shell of reactive bone. The patella is almost twice its' normal thickness.
E: Isotope scan of the knee region. There is moderate focal increased istope uptake indicating continuing activity of the lesion.
F: Axial CT image. The lesion is clearly divided into several compartments by thick septae of mature bone. The contour of the bone is deformed and there is significant irregularity of the subchondral plate.
G: Axial T-1 weighted MRI. The material within the lesion has a moderately bright signal indicating a high water content.
H: Sagittal T-2 weighted MRI. Again the image shows both the compartmentalization and the high water content of the lesion.
A diagnostic arthroscopy was done.
I: View of patella through the arthroscope. The articulating surface of the patella shows marked degenerative changes with a remarkable bluish discolororation suggesting a highly vascular process.
In view of the changes in the articular cartilage, a patellectomy was done.
J: The articular cartilage of the excised patella. The degenerative changes are randomly distibuted over the articular surface. The discoloration is not as vivid as it was in vivo.
K: The cut surfaces of the surgical specimen. The cancellous bone within the thickened patella has been replaced by fleshy vascular tissue with two large cavities containg old blood.
L: A closer look at the specimen. The cavities are lined with a distinct membrane that sperates them from the surrounding vascular tissue.
M: A panoramic view of the periphery of one of the blood filled cavities. The cavity contains red blood cells and is lined by a thin but distinct membrane.
N: A low power view of the edge of a cavity. Red cells fill the cavity. A membrane seperates the cavity from the bone.
O: Higher power of the edge of the cavity. The membrane is made up of spindle shaped mesenchymal cells and contains several several small multinucleatd giant cells.
Diagnosis: Aneurysmal Bone Cyst
The clinical course, radiographic features and pathologic features point to the diagnosis of aneurysmal bone cyst. The only unusual features are the absence of a fluid/fluid level on the CT and MRI (explained by the predominance ofthe solid fleshy component) and the unusual location in the patella. The lesion is a primary ABC, with no evidence of an associated process.
Although curettage would have a negligible risk of recurrence, in view of the degenerative changes in the articular cartilage, patellectomy appears prudent.
