The Journal of Bone and Joint Surgery

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IMAGE QUIZ ARCHIVE

Image Quiz
Acute Onset of Hip Pain in a Sixty-four-Year-Old Woman¹ (continued)
Answer: Subchondral insufficiency fracture of the femoral head and acetabulum.
Fig. 1	Fig. 1 An anteroposterior radiograph made five weeks after the onset of pain in the right hip shows a crescent sign (arrows) at the superolateral portion of the femoral head. Joint space narrowing is seen and is associated with developmental dysplasia of the acetabulum. For larger view, click on image
Fig. 2	Fig. 2 An anteroposterior radiograph made eight weeks after the onset of hip pain shows progressive joint space narrowing and partial collapse of the femoral and acetabular subchondral bone. For larger view, click on image
Fig. 3-A	Fig. 3-B For larger view, click on image
Fig. 3-A A T1-weighted magnetic resonance image shows a subchondral linear pattern of very low signal intensity with a convex shape to the articular surface (arrows). Fig. 3-B A fat-suppressed magnetic resonance image shows a subchondral linear pattern of low signal intensity with a convex shape to the articular surface. An area of slightly high signal intensity is noted focally on the lateral side of the acetabulum (arrows).
A bone mineral density measurement in the calcaneus was 0.593 g/cm² (88.5% of the mean in young adults, which was within normal limits). The Singh index, as assessed on a radiograph of both hips made one month after the onset of the pain, was grade 4. Because of the intractable severity of the pain, the patient underwent a right total hip replacement after a one-month observation period (two months after the onset of the pain).
On gross examination, the superolateral portion of the removed femoral head was seen to be slightly flattened. The cut section revealed a fracture line beneath the articular cartilage in the superior portion of the femoral head. Below the fracture line, there was a whitish area, but no wedge-shaped opaque yellow region, such as is normally found in osteonecrosis, was observed (Fig. 4). A specimen obtained from the weight-bearing portion of the acetabulum, measuring 15 × 15 mm in diameter, showed partial detachment of the acetabular cartilage with underlying subchondral bone (about 4 × 10 mm) (Fig. 5). This region was considered a subchondral fracture. The remaining cartilage had a normal appearance.
Fig. 4	Fig. 4 A section of the femoral head shows a fracture line beneath the articular cartilage in the superior portion of the femoral head. Below the fracture line, a whitish area is observed. However, no wedge-shaped opaque yellow region is seen. For larger view, click on image
Fig. 5	Fig. 5 A section of the acetabulum shows partial detachment of the acetabular cartilage with underlying subchondral bone (arrows). For larger view, click on image
For larger view, click on image
Fig. 6-A	Fig. 6-B
Fig. 6-A An osteochondral fracture is seen in the midportion of the specimen obtained from the weight-bearing area of the acetabulum (×40). Note that articular cartilage along with attached subchondral bone is detached from the specimen. Fig. 6-B Higher-powered view (×200) of the area of the acetabulum identified by the arrow in Fig. 6-A. The superficial portion of the fractured area is covered by fracture callus (arrows) and associated granulation tissue.
Discussion
Subchondral insufficiency fracture of the femoral head generally occurs in elderly patients, who tend to have osteopenic bone and/or to be overweight. It characteristically presents as acute pain in the hip without obvious antecedent trauma.
Some subchondral insufficiency fractures of the femoral head resolve spontaneously, but several have shown progressive collapse requiring surgical intervention. Findings on magnetic resonance imaging are characterized by a subchondral linear or serpiginous pattern of very low signal intensity on T1-weighted images with an associated bone-marrow edema pattern. This subchondral low-intensity band has been shown histologically to be a fracture line.
We believe that our patient sustained a subchondral insufficiency fracture of the femoral head because the findings were consistent with previously published criteria; that is, she was an elderly woman, she was obese, she had an acute onset of hip pain, magnetic resonance imaging showed a bone-marrow edema pattern with an associated subchondral low-intensity band on the T1-weighted image, and the histological findings were consistent with that diagnosis. The differential diagnosis includes osteonecrosis and transient osteoporosis of the femoral head. Osteonecrosis of the femoral head could be ruled out because there was no histological evidence of osteonecrosis. Because magnetic resonance imaging also demonstrates a low-intensity band in osteonecrosis, it is sometimes difficult to distinguish between that lesion and a subchondral insufficiency fracture of the femoral head. However, it is possible to do so in two ways. First, the shape of the low-intensity band in osteonecrosis is usually concave to the articular surface, whereas in a subchondral insufficiency fracture the low-intensity band on T1-weighted images often parallels the articular surface and has a serpiginous shape. Second, in osteonecrosis the subchondral bone segment proximal to the low-intensity band does not show high signal intensity on fat-suppressed images, whereas it usually does in a subchondral insufficiency fracture. It should be noted, however, that when the necrotic region has undergone repair by granulation tissue and appositional bone formation, the proximal segment may show a high signal intensity in osteonecrosis. Radiographic evidence of focal bone loss has been reported to be a characteristic finding of transient osteoporosis of the femoral head; it was not seen in our patient.
In our patient, acute joint space narrowing and progressive collapse were seen within two months after the onset of hip pain. Considering the pathophysiology of the rapid disappearance of the joint space, it is interesting to note that this patient had a subchondral insufficiency fracture on the acetabular side as well as in the femoral head. An incongruent joint at these apposed fracture sites may produce excessive loads on the cartilage that lead to destruction of the joint.
Reference
1. Motomura G, Yamamoto T, Miyanishi K, Shirasawa K, Noguchi Y, Iwamoto Y. Subchondral insufficiency fracture of the femoral head and acetabulum: a case report. J Bone Joint Surg Am. 2002;84:1205-9.