Our data also reinforce the importance of radiologist–orthopedic surgeon communication regarding the presence of SONK. We emphasize the importance of communicating the MRI findings clearly, whether the lesion is called SONK, SPONK, or insufficiency fracture. The orthopedic surgeons in our series may have been unaware of the presence of these lesions before arthroscopic meniscectomy, given the wide variety of terms being used in radiologic reports.
The natural history of spontaneous osteonecrosis of the medial tibial plateau has also been studied.18 There were 3 outcome patterns—acute extensive collapse of the medial tibial plateau, rapid progression to varying degrees of osteoarthritis, and complete resolution. It has been shown that resolution of SONK can occur in the early stages of the disease, within several months, but often the changes progress to bone destruction and articular cartilage collapse.19
In our series of patients, there was a female predominance, and mean age was 64 years. We investigated cartilage loss, meniscal tear, and meniscal extrusion to see if we could predict outcomes in patients who had the lesion before arthroscopy and if we could predict who might be at risk for developing the lesion after arthroscopy. Type of surgical procedure was also reviewed. For the sake of simplicity, we divided the follow-up patients into 2 groups: those managed with conservative treatment, which we deemed a reasonable outcome, and those who subsequently required knee joint replacement, which we deemed a poor outcome. As seen from our representative cases, both groups had patients with cartilage loss, meniscal tear, and meniscal extrusion to varying degrees. There were no risk factors pointing to a reasonable or poor outcome. In the group of patients with prearthroscopy lesions, we found the same problem. We were unable to identify a risk factor that might suggest a poor rather than a reasonable outcome. We must also emphasize that, in our review of patient charts, we could find no other causes for osteonecrosis. In particular, arthroscopic causes of acute chondral loss (eg, thermal wash, laser, bupivacaine pain pumps, epinephrine in irrigant) were not identified.
This study consisted of a series of cases managed at our institution over the past 8 years. Our data and this study had several limitations:
We may have been unable to identify other SONK cases that belonged in the group from our institution. In addition, we had only 11 patients for comparison with patients without SONK. Likewise, there were only 6 knees each in the prearthroscopy and postarthroscopy SONK groups. We also used images obtained from 1-T, 1.5-T, and 3-T closed MRI devices and one 0.7-T open device. These were, however, at the same institution.
Timing of our imaging was not uniform. In particular, in 3 of the patients who developed SONK after arthroscopy, preoperative MRI studies were performed quite some time before surgery. However, in these patients, more recent preoperative radiographs did not show any evidence of lesions. It can also be seen that postarthroscopy follow-up of patients varied. It is possible that, on longer follow-up, some of the cases we classified as having a reasonable outcome may have gone on to require total knee arthroplasty. One could argue that, in the patient who developed SONK within 1 year after surgery (Figure 4), the lesion was not related to the surgery. However, this patient’s radiographs 3 months after surgery did not show the SONK lesion but clearly showed prominent medial joint space narrowing—a new finding.
Only 1 musculoskeletal radiologist evaluated the radiographs, MRIs, and tomosynthesis (similar to computed tomography) studies for this investigation.
This lesion is not common, thus giving us a small group to analyze.
Despite our data limitations and the retrospective nature of this study, we compiled a reasonably representative sample of surgical SONK patients that matches other samples reported in the literature. Unfortunately, we could not identify any risk factors pointing to the likelihood of developing SONK or any risk factors pointing to either a reasonable or a poor prognosis in these patients. The etiology of the lesion remains an enigma. Our finding 6 cases of prearthroscopy lesions that did not necessarily result in a poor outcome, combined with our inability to identify any risk factors for SONK, points to the lack of a causal relationship with arthroscopy.