SONK in Postmeniscectomy Knees
Abstract & Commentary
Synopsis: Spontaneous osteonecrosis of the knee (SONK) is a distinctly different entity than "classic" osteonecrosis and is common in postmeniscectomy knees.
Source: Kobayashi Y, et al. Juxta-articular bone marrow signal changes on magnetic resonance imaging following arthroscopic meniscectomy. Arthroscopy. 2002;18(3):238-245.
Spontaneous osteonecrosis of the knee (sonk) was first described in 1968, but only recently has it been routinely recognized. Several case reports have described this phenomenon in patients following meniscectomy, but the present article reports a high rate of SONK in these patients. Kobayashi and colleagues identified 93 patients who had a partial or total arthroscopic meniscectomy. MRI was performed in all patients between 1 and 24 months postoperatively. The incidence and rate of bone marrow changes and the relation to the extent of meniscectomy were evaluated. An alarming 34% incidence of bone marrow signal changes was detected. The changes were more commonly seen in MRIs obtained within 8 months of the index procedure. The changes most frequently involved both the femoral condyle and the tibial plateau, followed by the tibia alone. Changes to the femoral condyle alone were less common. Extent of meniscectomy was directly related to the development of these changes.
Comment by Mark D. Miller, MD
There are 2 types of osteonecrosis involving the knee. The most commonly recognized type is "classic" osteonecrosis associated with risk factors such as steroid use and alcoholism, similar to that seen in the hip. This presents with a wedge-shaped pattern of involvement on MRI and may respond to core decompression. Spontaneous osteonecrosis, on the other hand, is more commonly focal, involving the subchondral bone and self-limited. Spontaneous osteonecrosis, as this study points out, has been increasingly recognized in postmeniscectomy knees, and may account for the occasional patient who just doesn’t get better following simple arthroscopic meniscectomy.
Although this is an extremely interesting study, there are several problems with the study design. First, the study appears to be at least partially retrospective. Next, not all of the patients had a preoperative MRI. In fact, it is not clear in the article exactly how many patients had preoperative MRIs. Although all patients did have a postoperative MRI, the timing of these studies was highly variable, and the MRIs were done with a .3 Telsa magnet. These images have not been as accurate as 1.5 Telsa studies. Finally, as Kobayashi et al point out, it is impossible to draw clinical conclusions or information regarding prognosis of these patients from this study. What does appear to be clear, at least from my clinical practice, is that SONK may be the cause of postmeniscectomy pain in many patients, and the treatment plan should not include repeat arthroscopy.
Dr. Miller, Associate Professor, UVA Health System, Department of Orthopaedic Surgery, Charlottesville, VA, is Associate Editor of Sports Medicine Reports.
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