Date Published: June 4, 2012
Publisher: Informa Healthcare
Author(s): Yasushi Akamatsu, Naoto Mitsugi, Takeshi Hayashi, Hideo Kobayashi, Tomoyuki Saito.
The primary event preceding the onset of symptoms in spontaneous osteonecrosis in the medial femoral condyle (SONK) may be a subchondral insufficiency fracture, which may be associated with underlying low bone mineral density (BMD). However, the pathogenesis of SONK is considered to be multifactorial. Women over 60 years of age tend to have higher incidence of SONK and low BMD. We investigated whether there may be an association between low BMD and SONK in women who are more than 60 years old.
We compared the BMD of 26 women with SONK within 3 months after the onset of symptoms to that of 26 control women with medial knee osteoarthritis (OA). All the SONK patients had typical clinical presentations and met specified criteria on MRI. The BMDs measured at the lumbar spine, ipsilateral femoral neck, and knee condyles and the ratios of medial condyle BMD to lateral condyle BMD (medial-lateral ratios) in the femur and tibia were compared between the two groups. The medial-lateral ratios were used as parameters for comparisons of the BMDs at both condyles.
The mean femoral neck, lateral femoral condyle, and lateral tibial condyle BMDs were between x% and y% lower in the SONK patients than in the OA patients (p < 0.001). The mean femoral and tibial medial-lateral ratios were statistically significantly higher in the SONK patients than in the OA patients. A proportion of women over 60 years of age have low BMD that progresses rapidly after menopause and can precipitate a microfracture. These findings support the subchondral insufficiency fracture theory for the onset of SONK based on low BMD.
2 of the authors (YA, NM) measured the femorotibial angle on 135 knee radiographs and the interobserver interclass correlation coefficient was 0.996. In addition, the same authors classified the SONK lesions, and the kappa coefficient to determine the interobserver agreement of the radiographic staging was 0.88.
We found that BMD of the femoral neck, lateral femoral condyle, and lateral tibial condyle were significantly lower in the SONK patients than in the OA patients. Also, the femoral and tibial medial-lateral ratios were significantly higher in the SONK patients than in the OA patients. Our findings support the subchondral insufficiency fracture theory for the onset of SONK from low BMD in women > 60 years of age.