Low bone mineral density is associated with the onset of spontaneous osteonecrosis of the knee

Abstract

Background and purpose: 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.

Methods: 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.

Results: 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.

Interpretation: 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.

Figure 1.

Flow diagram for identifying patients with SONK who were eligible.

Figure 2.

A. An AP radiograph from a 74-year-old woman, who had had sudden onset of right knee pain 7 weeks previously, showing a radiolucent oval lesion in the medial femoral condyle. The patient was classified as being at stage 2 of SONK and Kellgren-Lawrence grade 3. B. A coronal T2-weighted MRI showed an area of low signal intensity. C. A sagittal T2-weighted MRI with fat suppression showed subchondral changes and extensive bone marrow edema

Figure 3.

A. An AP radiograph from a 65-year-old woman, who had had sudden onset of left knee pain 10 weeks previously, showing no lesions in the medial femoral condyle. The patient was classified as being at stage 1 of SONK and Kellgren-Lawrence Grade 1. B. A coronal T2-weighted MRI showed an area of low signal intensity. C. A sagittal T2-weighted MRI with fat suppression showed subchondral changes and bone marrow edema

Figure 4.

The necrotic angle (Mont et al. 2000) was measure in the sagittal plane (A) and the coronal plane (B). The 2 angles were summed to give the combined necrotic angle. In this case, the combined necrotic angle was 108° (40° + 68°).

Figure 5.

An AP dual X-ray absorptiometry image of the right knee of a 74-year-old woman 7 weeks after the onset of pain (the same patient as in Figure 2) showing a necrotic lesion surrounded by a sclerotic area in the medial femoral condyle. In the tibial condyles, five square regions of interest were marked on the frontal view. A line extending to the lateral and medial edges of the proximal tibia was divided into 5 equal lengths and 5 square regions of interest were marked underneath it. The medial tibial condyle BMDs in the 2 medial square regions of interest and the lateral tibial condyle BMDs in the 2 lateral square regions of interest were calculated for the tibia. In addition, the lateral and medial femoral condyle BMDs were calculated in square regions of interest of the same size as those on the tibial condyles located on a line passing through the tips of the medial and lateral condyles, with the midpoints of their distal sides at the points of contact.