We present a case of a 53-year-old woman with subchondral insufficiency fracture (SIF) of the femoral head without history of severe osteoporosis or overexertion. Plain radiographs showed acetabular overcoverage with excessive lateralization of the acetabular rim. A diagnosis of SIF was made by typical MRI findings of SIF. The lesion occurred at the antipodes of the extended rim. Increased mechanical stress over the femoral head due to impingement against the excess bone was suspected as a cause of SIF. The distinct femoral head deformity is consistent with this hypothesis. This is the first report of SIF associated with acetabular overcoverage.
Subchondral insufficiency fracture (SIF) of the femoral head is a rare injury with an uncertain etiology. It primarily occurs in elderly women with poor bone quality and occasionally in young active patients as a fatigue fracture [
A healthy 53-year-old Japanese woman visited our hospital with acute onset of left hip pain. She was an elementary school teacher with no history of trauma, overexertion, corticosteroid intake, or alcohol abuse. She just had a physical education class as routine work a day before the onset of left hip pain. Physical examination revealed pain-induced limitation of the range of motion (ROM) of her left hip, with 90° flexion, 30° abduction, 20° adduction, 5° external rotation, and 15° internal rotation. The initial plain radiographs and computed tomography (CT) images of the left hip revealed a lateralized acetabular margin that extended laterally and distally from the posterolateral side of the acetabular rim (Figure
Initial radiograph and magnetic resonance imaging (MRI) of the left hip. (a) Anteroposterior view of the left hip shows acetabular overcoverage due to extended lateralization of the acetabular margin (white arrow). (b) T1-weighted image shows bone marrow edema with diffuse low intensity expanding from the lateral aspect of the femoral head to the intertrochanteric area. An irregular, serpentine, low intensity band (white dotted arrow) is present in the subchondral area. (c) Short-tau inversion recovery (STIR) image shows high signal intensity corresponding to the area of low intensity in (b). Black asterisks indicate the bone marrow edema of the femoral head and intertrochanteric area. (d) No apparent labral tear at the extended rim was observed but the labrum was relatively small and thin on the T1-weighted image (white arrow).
SIF was diagnosed based on the typical MRI findings and she received conservative non-weight bearing treatment for 6 weeks using crutches. Subsequently, we allowed gradual partial weight bearing, one-third of her weight per week. Plain radiographs and CT images taken after 4 weeks of therapy showed partial collapse of the subchondral bone (Figures
Partial collapse of the femoral head and serial MRI images. (a) Coronal computed tomography (CT) image after 4 weeks of therapy shows subchondral collapse (white dotted arrow) at the antipodes of the lateralized rim (white arrow). (b) A subchondral insufficiency fracture (SIF) is evident at the posterolateral aspect of the femoral head in the oblique three-dimensional CT view. STIR images show that the high intensity lesion (c) is smaller after 10 weeks of therapy and (d) has almost disappeared after 4 months of therapy.
Plain radiographs at the last follow-up. (a) Anteroposterior X-ray obtained at the 2-year follow-up and (b) its enlarged image. Progression of the subchondral collapse was not observed but the acetabular overcoverage in both hips still remained (a dotted arrow indicates right and a white arrow indicates left side of the acetabular overcoverage).
SIF is a rare injury with an unknown etiology. By definition, SIF involves bone fragility secondary to osteoporosis or osteopenia leading to subchondral fractures in the femoral head with no evidence of osteonecrosis [
Abnormalities are not generally evident on plain radiographs or CT in the early phase of SIF. Several months after onset, sclerotic changes due to fracture healing are sometimes observed in the subchondral area. MRI is the most useful modality for making a precise diagnosis. The characteristic MRI findings of SIF are diffuse bone marrow edema and a low intensity band on T1-weighted images. The shape of the low intensity band is typically discontinuous, irregular, serpentine, and parallel to the articular surface [
Although our patients showed typical characteristics of SIF on MRI, there was a clear difference from conventional SIF with respect to the location of the main lesion. In conventional cases of SIF, the main lesion tends to be observed at the anterosuperior portion of the femoral head [
Recently, femoroacetabular impingement (FAI) has attracted attention as a cause of hip disorder. FAI involves abnormal abutment of the femur and acetabulum. It can be divided into two subtypes based on morphological abnormalities: cam and pincer. Asphericity of the femoral head causes cam impingement, whereas pincer impingement is a pathological condition caused by acetabular overcoverage [
Although the inverted acetabular labrum was proposed to be the trigger of SIF [
In the present case, early diagnosis of SIF was possible because MRI was performed at the initial consultation. Subchondral collapse was observed during treatment; however, we could prevent further progression of the collapse by an early and appropriate load relief on the femoral head. Although conservative therapy appeared to be successful in our case, the patient should be closely followed up to assess further pathological changes because the morphological abnormality such as acetabular overcoverage in both hips still remained (Figures
The authors declare that there is no conflict of interests in preparing this paper. The authors, their immediate families, and any research foundation with which they are affiliated did not receive any financial payments or other benefits from any commercial entity related to the subject of this paper.