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What is Spontaneous Osteonecrosis of the Knee (SONK)?

Spontaneous osteonecrosis of the knee (SONK) is a painful knee condition of idiopathic or unknown cause that occurs spontaneously. It is a distinct clinical entity with no consensus regarding the etiology of the condition. SONK is the most common form of osteonecrosis of the knee.

The condition most commonly affects the medial femoral condyle; although it can also affect the patella, tibial plateau, proximal tibia, and lateral femoral condyle. Ninety-nine percent of patients have only one joint involved. SONK is seen more frequently in middle-aged and elderly patients. Women over the age of 55 are 3 times more likely to develop this condition than men.

Diagnosis and treatment are quite challenging given the insidious and nonspecific onset of the condition. If left untreated, SONK can ultimately lead to secondary osteoarthritis, subchondral collapse, and the need for surgical intervention.

Causes of Spontaneous Osteonecrosis of the Knee (SONK)

Even though SONK is deemed to be an idiopathic condition, several investigators have proposed that a subchondral insufficiency fracture resulting in localized osteonecrosis is the primary cause of SONK. The investigators believe that insufficiency fractures result in the collection of fluid in the bone marrow, leading to focal ischemia and subsequent bone necrosis (death of bone tissue) in the affected region. The condition is also believed to be caused by a meniscal root tear. A classic SONK patient may be active, exercises regularly, and may have a history of osteopenia or osteoporosis. Further evidence is required to pinpoint the precise pathogenesis of the condition, as it remains a source of dispute.

Signs and Symptoms of Spontaneous Osteonecrosis of the Knee (SONK)

Some of the signs and symptoms of spontaneous osteonecrosis of the knee (SONK) include:

  • Sudden and spontaneous onset of severe knee pain
  • Limited range of motion secondary to pain
  • Swelling
  • Effusion
  • Tenderness

Diagnosis of Spontaneous Osteonecrosis of the Knee (SONK)

The diagnosis of SONK includes a medical history and a physical examination along with imaging tests such as an X-ray and MRI scan. Physical examination determines the cause of pain and other related problems while X-rays and MRI scans aid in confirming the condition by providing visualization of the internal structures. Anteroposterior, oblique, and lateral plain films of the knee help to demonstrate flattening of the involved condyles in later stages of the disease. MRI scans are useful in confirming early stages of the condition by detecting the presence of bone edema. MRI scans are also helpful to detect concurrent meniscal tears.

Treatment of Spontaneous Osteonecrosis of the Knee (SONK)

Treatment of spontaneous osteonecrosis of the knee (SONK) is planned based on the severity and extent of the disease and to measure this the condition is classified into 4 stages:

  • Stage 1: A patient with a symptomatic knee but normal findings on X-ray
  • Stage 2: Patients with flattening and subchondral radiolucencies without collapse
  • Stage 3: Extension of these radiolucencies with subchondral collapse
  • Stage 4: Further degenerative changes with osteophyte and osteosclerosis formation

In the early stages of the SONK, your physician will employ non-surgical treatment options such as:

  • Anti-inflammatory medications
  • Multimodal analgesia
  • Protected weight-bearing
  • Strengthening exercises for femoral muscles
  • Bisphosphonates (a class of drugs that prevent bone loss)
  • Activity modifications

Surgical treatment is considered in advanced stages and when symptoms fail to respond to non-surgical treatment. Surgical treatment options include:

  • Arthroscopic debridement or cleansing of the joint
  • Drilling the area of osteonecrosis of the knee to decrease pressure on the bone surface
  • Procedures to lessen the weight from the affected area
  • Unicompartmental or total knee replacement
  • High-tibial osteotomy, a procedure to realign the knee joint

Your doctor will discuss all the surgical options and recommend the one most appropriate for you.

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