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.
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
There is no definitive consensus on the etiology of SIFK and earlier theories proposed a vascular etiology. It is proposed that the primary etiology for SIFK is a subchondral insufficiency fracture resulting in localized osteonecrosis based on both histopathologic analysis and marked similarities to subchondral insufficiency fractures of the femoral head. The insufficiency fracture leads to an accumulation of fluid in the bone marrow and results in focal ischemia and subsequent necrosis .
SONK or SIFK most commonly occurs in ages fifty or older. There are certain risk factors that have been found that include female sex, cartilage degeneration, low bone mineral density and medial meniscus posterior root tears . Risk factors for progression include lesion size on radiographs, anatomical angle (femorotibial angle) on sagittal MRI and depth on MRI. The anteroposterior knee radiograph can be used to determine the stage of SIFK progression in the medial femoral condyle, which was classified into four stages. Stage one has a normal radiographic appearance but is found on MRI. Stage 2 has the presence of a radiolucent subchondral oval lesion, flattening of the convexity of the condyle or both. Stage 3 shows an expanded radiolucent area surrounded by a sclerotic halo and Stage 4 has apparent secondary OA changes .
Figure 1. Subchondral insufficiency fracture of the knee (SIFK) with crescent sign
No algorithm or definite consensus exists for treatment, but most recommendations are based or guided on the extent or severity of the disease or lesion. Nonoperative management is an option for smaller lesions (less than 3.5 cm) or stage 1 or 2 lesions . Non-operative management consists of NSAIDs, protected weight bearing and bisphosphonates. The latest research published in 2018 by Bhatnagar et al. showed excellent results over a period of one year using a combined therapy with NSAIDs and bisphosphonates in a group of ten patients, mainly with stage 1 SONK . Mostly small studies exist for bisphosphonates with both IV and oral bisphosphonates. One study with 17 patients given 70 mg alendronate once a week showed less progression to osteoarthritis compared to a prior study at the same hospital without pharmacologic agents being used .
Table 1. Treatment algorithm used in  based on Koshing Staging and BML Grade
Table 2. Summary of surgical treatments for SIFK (adopted from )
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.