Spontaneous Osteonecrosis of the Knee (SONK)
Spontaneous Osteonecrosis of the Knee (SONK) is a clinical condition causing sudden onset specific severe knee pain and swelling. The pain is usually constant and unremitting being present day and night.  The discomfort is usually worse on taking weight on the leg.  

Who gets SONK?
SONK affects more women than men (3:1) and is seen most often in patients over the age of 55. SONK is also more common in people who have had meniscal tears. Usually, SONK affects only one knee, not both.  

Why does SONK occur?
The exact cause of SONK is not known, but it is thought to be damage to the bone termed ‘subchondral insufficiency fracture’ which results in build-up of fluid in the bone marrow resulting in focal ischemia and subsequent necrosis – or collapse of parts of the bone. The problem is that If this condition is not treated, the cartilage or bone in the knee joint may collapse, as the supporting bone weakens, resulting in arthritic pain.

Anatomy of the knee joint
The knee joint is made up of three bones; the femur (thigh bone), the tibia (shin bone) and the patella (knee cap). The femur and tibia move together like a hinge, allowing bending (flexing) and straightening (extending). The knee joint carries the whole weight of the upper body, so to protect the femur and tibia and prevent them grinding together there are two cushions made of cartilage called menisci, which sit between the bones. The knee joint is held together by ligaments and tendons. A capsule surrounds the whole joint, and has a lubricating fluid inside which helps the hinge moving smoothly.

Risk factors for SONK
SONK is most commonly spontaneous with no obvious underlying cause. The following factors are known to increase the risk of developing SONK:
  • Old age
  • Being female
  • History of smoking
  • High blood pressure
  • High cholesterol
  • Diabetes
  • Osteoporosis
  • Meniscal tears (“footballer’s injury”)


Symptoms of SONK
Patients usually describe a severe pain in 

the knee,which came on suddenly, and is worse at night and when putting weight on that leg (including when climbing stairs). The pain usually comes on “out of the blue”, rather than after an injury.  Some swelling of the knee can occur. Swelling and pain may stop the knee being able to bend or straighten the leg.

MRI image of SONK affecting the medial femoral condyle – shown as the white area on the left of the picture compared to the normal darker bone signal in the remainder of the knee

Diagnosis

Diagnosis is usually made on the symptoms but is confirmed on MRI scan looking for changes in the appearance of the knee joint. Blood tests and bone density scans (DEXA scans) can also be performed to look for the underlying cause of the SONK, which may for example be high cholesterol or osteoporosis. Early investigation and diagnosis is important to ensure quick recovery.

Treatment
SONK is usually a progressive condition that, if left untreated, results in collapsing of the bone that supports the articulating surface and therefore arthritis. The three main treatment options are:
  • Non-operative conservative management
  • Core Decompression
  • Joint surface replacement

Immediate treatment
As soon as the diagnosis is suspected it is important to take the weight off the leg and to use crutches. This immediately takes the pressure off the bone and should reduce the discomfort.

Conservative treatment
By keeping weight off the leg the bone can spontaneously recover and heal. This involves being touch weight-bearing on crutches, meaning only the toes can touch the floor and body weight is taken through crutches rather than the knee. Pain-killers and physiotherapy are also important for relieving pain and helping movement. The regime must be followed strictly for a minimum of six weeks to be effective. Pain may need minimal during this period but it is important to persevere with the touch weight bearing to give the knee time to strengthen.

Core Decompression Surgery
Core decompression is an operation to decompress the damaged bone. 2-3mm drill holes are made into the damaged area of bone through a small incision made on the thigh under x-ray image intensifier. The drilling immediately reduces the pressure in the bone and blood supply to the joint is improved, thereby preventing further damage and collapse. Following the procedure strict touch weight-bearing on the affected knee is required for 4-6 weeks, after which full weight-bearing is usually allowed.

Joint replacement
In more severe cases where part of the joint has started to collapse, uni-compartmental joint replacement surgery may be an option.

Outlook
The outlook with SONK varies depending on the level of pain, the size of the affected area, and the time of diagnosis. The outcome ranges from complete recovery to progression of symptoms, which may include development of secondary arthritis or collapse of the affected area. However, if caught early and if the symptoms are managed effectively, the outlook for SONK can be very good.  

For more detail please see separate section written for the healthcare professional. Written by Katie Bonner and Leila Makhani, Medical Students, For the knee team at University Hospital Coventry and Warwickshire NHS Trust: Tim Spalding, Pete Thompson and Mark Taylor, Consultant Orthopaedic Surgeons. January 2012. Updated 2021