Sunday, September 19, 2010

Shadows in the Knee

Photo by ambrown

In the early days of medical imaging, radiologists used only x-rays to image the body. These pioneers saw shadowy representations of the tissue that is inside all of us. With the advent of cross-sectional imaging techniques such as CT, ultrasound, and MRI, we are now able to visualize anatomic structures directly, and no longer have to deduce the anatomy from a shadow-like representative object.

Occasionally, however, events conspire to create ghost-like shadows even on cross-sectional imaging such as MRI. In this case, a 31 year old male presented with knee pain, and was referred for an MRI:

(A) Sagittal proton-density image reveals an unusual appearance to the bones, with visualization of the ghostly outline of the femoral secondary epiphysis (blue arrows) and the tibial secondary epiphysis (red arrows). (B) Sagittal T2 fatsat image is less striking, with areas of nonspecific increased signal visualized in the femur and tibia.

A sagittal proton-density image through the patella reveals the ghostly outline of the patellar epiphysis:

A manifestation of growth arrest was suspected. Further questioning of the patient revealed that he had a serious ski accident in childhood, resulting in a femur fracture requiring prolonged immobilization, confirming the diagnosis. Epiphyseal growth arrest lines have been described (Yao L, Seeger LL, Clin Imaging. 1997 Jul-Aug;21(4):237-40) and can lead to a "bone-in-bone" appearance, as in this case. Causes of a "bone-in-bone" appearance on conventional radiographs include bone infarction, osteopetrosis, and Paget's disease. On MRI, however, this appearance is highly suggestive of an epiphyseal growth arrest event in the patient's past.

Vic David MD