Spontaneous osteonecrosis of the knee, also known as Ahlback disease, SONK or even SPONK, has similar appearances to osteochondritis dissecans of the knee but is found in an older age group.
SONK is seen more frequently in women (M:F 1:3) and affects older patients, typically over the age of 55.
Osteonecrosis in SONK has no predisposing factors. However, by definition, secondary osteonecrosis of the knee occurs secondary to an insult. SONK is not thought to be caused by bone death but may be caused by osteoporosis and insufficiency fractures 6. Some authors suggest that the primary event leading to spontaneous osteonecrosis of the knee is a subchondral insufficiency fracture.
It is almost always unilateral, usually affects the medial femoral condyle (but can occasionally involve the tibial plateau 9) and is often associated with a meniscal tear.
In the later stages features seen include:
- flattening of the medial femoral condyle
- subchondral radiolucent focus
- complicating subchondral fracture with periosteal reaction
Features can vary dependant on the stage:
- ill-defined bone marrow oedema and a lack of peripheral low signal intensity rim as seen in avascular necrosis and bone infarcts
- focal subchondral area of low signal intensity adjacent to the subchondral bone plate representing local ischaemia (considered as a specific MRI finding 12); this area shows no enhancement on post-contrast
- deformity of the subchondral bone plate (flattening or focal depression) in the weight-bearing area of the involved condyle
Treatment and prognosis
Prognosis varies from complete recovery to total joint collapse 2. Treatment can either be operative or nonoperative.
Possible considerations include
History and etymology
It was first systematically described by Ahlback in 1968 2
- 1. Takeda M, Higuchi H, Kimura M et-al. Spontaneous osteonecrosis of the knee: histopathological differences between early and progressive cases. J Bone Joint Surg Br. 2008;90 (3): 324-9. doi:10.1302/0301-620X.90B3.18629 - Pubmed citation
- 2. Björkengren AG, Alrowaih A, Lindstrand A et-al. Spontaneous osteonecrosis of the knee: value of MR imaging in determining prognosis. AJR Am J Roentgenol. 1990;154 (2): 331-6. AJR Am J Roentgenol (abstract) - Pubmed citation
- 3. Fotiadou A, Karantanas A. Acute nontraumatic adult knee pain: the role of MR imaging. Radiol Med. 2009;114 (3): 437-47. doi:10.1007/s11547-009-0380-z - Pubmed citation
- 4. Ramnath RR, Kattapuram SV. MR appearance of SONK-like subchondral abnormalities in the adult knee: SONK redefined. Skeletal Radiol. 2004;33 (10): 575-81. doi:10.1007/s00256-004-0777-7 - Pubmed citation
- 5. Zywiel MG, Mcgrath MS, Seyler TM et-al. Osteonecrosis of the knee: a review of three disorders. Orthop. Clin. North Am. 2009;40 (2): 193-211. doi:10.1016/j.ocl.2008.10.010 - Pubmed citation
- 6. Mears SC, McCarthy EF, Jones LC et-al. Characterization and pathological characteristics of spontaneous osteonecrosis of the knee. Iowa Orthop J. 2010;29: 38-42. Free text at pubmed - Pubmed citation
- 7. Yamamoto T, Bullough PG. Spontaneous osteonecrosis of the knee: the result of subchondral insufficiency fracture. J Bone Joint Surg Am. 2000;82 (6): 858-66. Pubmed citation
- 8. Narváez JA, Narváez J, De Lama E et-al. Spontaneous osteonecrosis of the knee associated with tibial plateau and femoral condyle insufficiency stress fracture. Eur Radiol. 2003;13 (8): 1843-8. doi:10.1007/s00330-002-1775-6 - Pubmed citation
- 9. Lotke PA, Nelson CL, Lonner JH. Spontaneous osteonecrosis of the knee: tibial plateaus. Orthop. Clin. North Am. 2004;35 (3): 365-70, x. doi:10.1016/j.ocl.2004.02.009 - Pubmed citation
- 10. Yates PJ, Calder JD, Stranks GJ et-al. Early MRI diagnosis and non-surgical management of spontaneous osteonecrosis of the knee. Knee. 2007;14 (2): 112-6. doi:10.1016/j.knee.2006.10.012 - Pubmed citation
- 11. Breer S, Oheim R, Krause M et-al. Spontaneous osteonecrosis of the knee (SONK). Knee Surg Sports Traumatol Arthrosc. 2013;21 (2): 340-5. doi:10.1007/s00167-012-2017-3 - Pubmed citation
- 11. Pape D, Seil R, Kohn D et-al. Imaging of early stages of osteonecrosis of the knee. Orthop. Clin. North Am. 2004;35 (3): 293-303, viii. doi:10.1016/j.ocl.2004.04.001 - Pubmed citation
- 12. Roemer FW, Frobell R, Hunter DJ et-al. MRI-detected subchondral bone marrow signal alterations of the knee joint: terminology, imaging appearance, relevance and radiological differential diagnosis. Osteoarthr. Cartil. 2009;17 (9): 1115-31. doi:10.1016/j.joca.2009.03.012 - Pubmed citation