Subchondral Insufficiency Fracture of the Knee
Spontaneous osteonecrosis of the knee (SONK), also called subchondral insufficiency fractures of the knee (SIFK), was first described by Ahlback in 1968 and is a diagnosis that is sometimes overlooked and attributed to knee osteoarthritis . Knee osteoarthritis (OA) is a chronic degenerative disease that leads to loss of articular cartilage, whereas subchondral insufficiency fractures of the knee is an acute disease of the subarticular bone. In the knee joint, impact forces are indirectly transmitted to the bone through the cartilage and menisci. SIFK, which may be caused by mechanical overloading of the subchondral bone, is probably related to the weakness of the subchondral bone compared with that of the articular cartilage and is often secondary to meniscus extrusion or bone fragility . There is also secondary osteonecrosis of the knee that may be related to other factors such as corticosteroid treatment or sickle cell anemia and there is also a third category called postarthroscopic osteonecrosis. It can lead to subchondral collapse, secondary osteoarthritis, and the need for surgical management.
A 65 year old active female is referred to physical therapy by his primary care provider for insidious onset left medial knee pain. The patient described pain upon awakening one morning around 4 weeks ago. Complaints include progressive knee pain that is worse with weight bearing activities and worse at night. There is slightly limited ROM due to a small effusion. Her past medical history is remarkable for mild osteopenia that is treated with supplemental calcium and vitamin D. She feels that her symptoms are getting worse and she is starting to limp and do less weight bearing activities due to the pain. X-rays of the knee showed preserved joint space with no definitive lesions or signs of osteoarthritis. There was a questionable sclerotic rim lesion over the medial femoral condyle. What is the most likely diagnosis?
- A. Osteoarthritis of the knee
- B. Lateral meniscus tear
- C. Subchondral insufficiency fracture of the knee
- D. Transient osteoporosis of the knee
C is the correct answer. Subchondral insufficiency fracture of the knee typically affects individuals that are 55 or older and presents with sudden non traumatic unilateral knee pain. The pain typically gets worse with weight bearing and at night. Osteoarthritis of the knee would likely have changes present on the radiograph and no sclerotic lesion would be present. Meniscus tears are typically traumatic or have an inciting event and would not present with a sclerotic rim lesion on the x-ray. Transient osteoporosis of the knee can present in a similar manner but more commonly affects the hip and would not have any sclerotic rim lesions over the medial femoral condyle but would have diffuse bone edema on MRI and decreased bone density on a DEXA scan.
Sibilska, Aleksandra, et al. “Spontaneous osteonecrosis of the knee: what do we know so far? A literature review.” International Orthopaedics (2020): 1-7.