Histopathological evaluation of spontaneous osteonecrosis of the knee: time to reconsider history and nomenclature–a scoping review

Spontaneous osteonecrosis of the knee (SONK) was first described by Ahlbäck et al. in 1968. However, subsequent studies revealed that subchondral fractures, rather than necrosis, are the most common histopathological finding in bone samples from patients diagnosed with SONK. This has led to ongoing debate regarding the accuracy of the term "SONK." Therefore, SONK is an inappropriate definition for this condition since the absence of necrosis in most histopathological samples of patients with such diagnosis. A PRISMA-compliant scoping review was conducted using PubMed, Scopus, and Cochrane databases. All original research studies reporting histological analyses of bone samples from patients who underwent surgery following a diagnosis of SONK were considered for inclusion. Extracted data included general study characteristics, radiographic and magnetic resonance imaging (MRI) findings, time elapsed between arthroscopy and histological sampling, and the results of histological examinations of bone samples. Eight articles met the inclusion criteria, comprising 90 patients (91 knees), of which 87 bone samples were analyzed histologically. Of these, 7 showed undetermined outcomes. Among the remaining 80 samples, necrosis was identified in 35 cases and absent in 45. Subchondral insufficiency fractures (SIFs) were detected in 41 cases, with 19 of these also showing necrosis. The histopathological evidence suggests that the term "SONK" is inappropriate, as SIFs, rather than necrosis, are the predominant finding. We therefore recommend adopting "SIF" as a more accurate descriptor for this condition.