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Letters to the Editor |
1 Department of Radiology, Beth Israel Deaconess Medical Center and Harvard Medical School
330 Brookline Avenue, Boston, MA 02215
e-mail: fhall{at}bidmc.harvard.edu
Editor:
I enjoyed the comprehensive review of magnetic resonance (MR) imaging of the postoperative knee by Dr McCauley in the January 2005 issue of Radiology (1). I would like to comment on the section he devotes to osteonecrosis, which is described as "a potentially debilitating complication of knee arthroscopy." The diagnosis of this entity with MR imaging was popularized by a classic article published in 1991 (2), which Dr McCauley references three times.
Current opinion holds that many and probably most of the lesions described previously as stress-related subchondral osteonecrosis of the knee, usually seen after meniscal tear or meniscectomy, actually represent subchondral insufficiency or stress fractures. This was first suggested by Yamamoto and colleagues in the hip (3) and later in the knee (4), on the basis of careful histologic evaluations. In the knee, subchondral loading is accentuated by meniscal surgery, particularly for large radial or root tears, which prevent the meniscus from maintaining its hoop strength, which in turn is responsible for helping to convert axial loads to more horizontal tensile forces (5).
In keeping with this new theory, Yao et al (5) found in their series of 25 such lesions, which they termed "presumptive stress reactions of the knee," that MR imaging never showed the more specific MR signs of osteonecrosis, such as a demarcated area of normal fatty marrow signal intensity and serpentine low-signal-intensity lines circumscribing a zone of devitalization. Rather, the open-ended lines of low signal intensity, when present, were more reminiscent of stress fractures. These authors hypothesized that these subchondral lesions "likely exist along a spectrum from stress responses/reactions to frank fractures" (5). This is in keeping with the MR images of postmeniscal repair "osteonecrosis" that are shown by Dr McCauley in figure 12 (1).
The exact relationship between subchondral stress reaction and/or fracture and spontaneous osteonecrosis of the knee, or SONK, is speculative (5). The latter entity was first described and popularized (6) before the use of MR imaging, which revolutionized our understanding of osseous stress reactions and stress fractures (7). Obviously, these subchondral lesions, when seen on radiographs, and often referred to as osteonecrosis dissecans, are generally more advanced than the corresponding abnormalities detected only with MR imaging.
References
43 Orcutt Drive, Guilford, CT 06437
e-mail: troycemccauley{at}comcast.net
I thank Dr Hall for his letter. I did not deal with the debate over the nomenclature used for what is commonly called osteonecrosis in the knee because the nomenclature is in a state of change, and no uniformly accepted replacement for the term osteonecrosis has yet emerged.
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