Pelvis & Hip Imaging: Systematic Interpretation

On MRI, avascular necrosis shows characteristic findings that vary with stage. The pathognomonic finding is the 'double-line sign' on T2-weighted images - an inner bright line representing granulation tissue and an outer dark line representing reactive sclerosis at the interface between dead and living bone. On T1-weighted images, there is a low signal band in the subchondral region that follows the contour of the femoral head - this represents the reactive interface. As disease progresses, I look for the crescent sign which indicates subchondral fracture, and eventually femoral head collapse and flattening. Staging guides treatment: Stage I shows only marrow edema and may benefit from core decompression; Stage II shows the characteristic changes without collapse, where core decompression with or without biologics may help; Stage III shows subchondral fracture (crescent sign), where joint-preserving options are limited; Stage IV shows femoral head collapse and flattening, typically requiring arthroplasty. The extent of involvement (percentage of head affected) and location also influence prognosis - larger lesions and weight-bearing zone involvement have worse outcomes.

I assess acetabular fractures using the AP pelvis and Judet oblique views initially. On the AP pelvis, I identify six key lines: the iliopectineal line (anterior column), ilioischial line (posterior column), acetabular roof, anterior wall, posterior wall, and the teardrop. The Judet 45-degree oblique views are essential: the obturator oblique (injured side up) shows the anterior column and posterior wall, while the iliac oblique (injured side down) shows the posterior column and anterior wall. CT with 2D reconstructions and 3D rendering is essential for complete assessment. CT allows accurate classification using the Judet-Letournel system into elementary patterns (anterior wall, posterior wall, anterior column, posterior column, transverse) or associated patterns (such as both-column, T-shaped, transverse with posterior wall). CT also shows features critical for surgical planning: marginal impaction of the weight-bearing dome, intra-articular fragments, femoral head injury, and the relationship of fracture lines to the sciatic notch. 3D reconstruction helps visualize the overall fracture pattern and plan surgical approaches.

In an elderly patient with a fall, hip pain, and inability to weight-bear but normal X-rays, I have high clinical suspicion for an occult hip fracture and cannot discharge this patient based on normal X-rays alone. My next step is MRI of the hip, which is the gold standard for detecting occult hip fractures. MRI will show the fracture as a low signal line on T1-weighted images with surrounding bone marrow edema that appears bright on T2/STIR sequences. MRI has sensitivity approaching 100% for hip fractures and can also differentiate between non-displaced intracapsular and extracapsular fractures, which is critical for surgical planning. If MRI is unavailable or contraindicated, CT is an alternative that can detect cortical fractures, though it may miss some purely trabecular injuries. Bone scan is another option but takes 24-72 hours to become positive in elderly patients and is less specific. Given the significant morbidity of missed hip fractures - including displacement of a non-displaced fracture with worse outcomes - I would not discharge this patient without further imaging.