Femoroacetabular impingement mimicking avascular osteonecrosis on bone scintigraphy

A 42-year-old man presented with one year history of progressive left hip pain resulting in restricted mobility in flexion and internal rotation. There was no history of trauma, and a radiograph of the left hip demonstrated a cortical bump in the femoral neck and a slight narrowing of the joint space. Tc-99m hydroxymethylenediphosphonate bone scan [Figure 1] anterior and posterior planar images (a) demonstrated an incomplete “donut” of increased activity surrounding a relatively “cold” left femoral head suggesting the presence of avascular necrosis. Single photon emission computed tomography (SPECT) of the pelvis was performed to further evaluate this area (b). Correlation between scintigraphic uptakes and radiological findings on pelvic CT scan is shown at selected coronal slices of CT (c) and SPECT study (d) and radiograph (e) of left hip. The increased tracer uptakes were located at the lateral bump on the left head-neck junction (thick arrows), in the superolateral and inferointernal aspect of the acetabulum related to degeneration (thin arrows) and in an osteophyte in the inferior aspect of the femoral head (heads of arrows). CT axial slices of left femoral head (f) did not show typical findings of avascular necrosis (low-density areas or clumping and distortion of trabeculae), so all findings suggested the diagnosis of femoroacetabular impingement (FAI) of the cam type.

FAI is characterized by a pathologic contact during hip joint motion between skeletal prominences of the acetabulum and the femur.[1] Two types of impingement are distinguished [Figure 2a]: Cam, which is associated with reduced femoral head-neck offset (more common in young men) and pincer, present with acetabular retroversion (more common in middle-aged women), with the majority of the patients having both (mixed type).[2]

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Figure 1 Anterior and posterior (a) planar images with Tc-99m hydroxymethylene diphosphonate (HMDP) demonstrated abnormal uptake in the left femoral head. Axial, coronal and sagittal slices of SPECT of the pelvis (b) allowed a better anatomical location. Correlation between scintigraphic uptakes and radiological findings is shown at selected coronal slices of CT (c) and SPECT study (d), and radiograph (e) of left hip. CT axial slices of left femoral head (f) did not suggest the presence of avascular necrosis

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Figure 2 (a) Graphical illustration of the types of femoro-acetabular impingement. The shaded areas indicate the sites of abnormality. (b) Volumetric imaging of pelvis showing pathological uptake around left head-neck junction (arrows)

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Although bone scintigraphy is not included in the diagnostic protocol of FAI,[2] it should be included on the differential diagnosis when evaluating patients with hip-related symptoms. The scintigraphic features reflect the pattern of remodeling that occurs at the sites of impingement[3] (increased focal uptake involving the anterosuperior aspect of the acetabulum and/or increased tracer uptake along the anterosuperior aspect of the femoral neck), with normal blood flow images.[45] Positive bone scintigraphy can be an early indicator of intraarticular cartilage damage that may present before the onset of symptoms.[4] These findings can be misdiagnosed as degenerative changes[5] to avascular necrosis,[3] so it is very important to add tomographic images, preferably with SPECT-CT for anatomic information because early surgical correction can reduce the progression of osteoarthritis.[6] Surgery eliminates the pathomechanics of structural deformities: Cam impingement can be treated with anterolateral osteoplasty, often arthroscopically whereas pincer-type impingement can be treated with periacetabularosteotomy, so these treatments will ultimately reduce the necessity of hip replacements.[5]

We would like to remark that volumetric imaging of our case [Figure 2b] showed pathological uptake around left head-neck junction (arrows). This finding could suggest that real structure of FAI anatomic abnormality is not restricted to a focal defect, but reach the whole femoral head-neck junction although this finding has not been described in the literature to date.