Spontaneous Osteonecrosis of the Knee: a Systematic Review of Conservative Treatment Approaches

Background: Spontaneous osteonecrosis of the knee (SONK) is an insufficiently defined disease. Although recent research suggests that SONK may be caused by subchondral insufficiency fractures, its precise etiology remains unclear. Currently, no therapeutic guidelines exist. Purpose: This systematic review aimed to collate the scientific literature on conservative treatment approaches for SONK and to derive respective recommendations. Methods: : The PUBMED database was searched for relevant articles published until November 2020. All articles in German or English language dealing with the conservative therapy of SONK were included. Animal research studies and reviews were not considered. Studies that used specific treatment outcomes as inclusion criterion and those that lacked a conservative treatment group, magnetic resonance imaging-based diagnosis, reported outcomes of conservatively managed patients or basic descriptions of the treatment modalities were excluded. Results: : Twenty-two articles comprising a total sample size of 521 patients were included. Eighteen of the included studies (82%) tested the effects of some form of restriction of weight-bearing, bisphosphonates or a combination of both. Sixteen studies (73%) reported clinical and/or radiological improvements. However, outcomes varied considerably in dependency of the radiologic presentation, demography and further prognostic factors. Conclusion: Many conservative therapy methods appear effective in the treatment of orthopedic syndromes classified as SONK, with the most robust evidence existing for restriction of weight-bearing and bisphosphonates. The heterogeneity of radiologic presentations and treatment outcomes suggests that the generic term SONK might obscure the presence of different underlying pathologies. Indeed, many lesions may represent subchondral fractures, which should be referred to as subchondral fractures of the knee (SFK). A clear delineation of the disorders and standardized MRI-based diagnostic criteria are required.