Spontaneous OsteoNecrosis of Knee (SONK)

Spontaneous OsteoNecrosis of

Knee (SONK)(Ahlback's disease)

Dr.Avik SarkarKB Bhabha Municipal General Hospital, Bandra (West), Mumbai

• Spontaneous Osteonecrosis of the Knee, also known as Ahlback's disease is the result of vascular arterial insufficiency to the medial femoral condyle of the knee resulting in necrosis and destruction of bone

INCIDENCE

• Elderly Women are generally affected

[seen more frequently in women (M:F = 1:3) and typically over the age of 55]

RISK FACTORS (POSSIBLE)

•  Corticosteroid use

•  Systemic Lupus Erythematosus (SLE) 

• Alcoholism

• Pancreatitis

• Sickle Cell disease

• Rheumatoid Arthritis

• Caisson disease

• Gaucher disease

SITE / LOCATION

• 99% of patients have only one joint involved

• usually epiphysis of medial femoral condyle is involved

PATHOPHYSIOLOGY

• Osteonecrosis in SONK has no predisposing factors.

• Speculations include

-subchondral insufficiency fracture

-may be caused by a meniscal root tear

PRESENTATION / SYMPTOMS

• sudden onset of severe knee pain, increased at night and with activity

• effusion• limited range of motion secondary to pain• tenderness over medial femoral condyle

IMAGING

• X-Ray

• Technetium-99m scan

• MRI

X-Ray• Standing AP view• Standing Lateral view• Tunnel view (long axis of femur at 600 to long axis of

tibia)

• Initially, no abnormalities may be seen,

but as the disease progresses,

flattening of the weight-bearing portion

may occur.

• A radiolucent area forms in the subchondral

bone, surrounded by a rim of sclerosis.

Later in the disease course, the subchondral

bone collapses, leading to secondary arthritic

change and varus deformity.

Technetium-99m scans• Show a localized area of radioisotope uptake in

the medial femoral condyle.• Bone Scans provide a correct diagnosis in only

40-70% of cases• Less effective for diagnosing secondary

osteonecrosis than for diagnosing SONK

MRI• On T1-weighted images, osteonecrosis is seen as a discrete area of low

signal intensity, replacing the high-intensity signal normally produced by marrow fat.

• The T2-weighted image shows an area of low signal intensity surrounded by a high-intensity signal caused by edema.

• Specificity and Sensitivity of MRI are 98% in SONK, hence MRI is the diagnostic study of choice.

STAGING• Aglietti devised the following classification system which is a

modification of an earlier classification by Koshino  :

• Stage I: Plain radiograph findings are normal. Diagnosis must be made from MRI or bone scan.

• Stage II: Radiographs show flattening of the weightbearing portion of the condyle

• Stage III: Radiographs show a radiolucent area surrounded by sclerosis

• Stage IV: Radiographs show a more defined ring of sclerosis and subchondral bone collapse forming a calcified plate, sequestrum, or fragment

• Stage V: Narrowing of the joint space, osteophyte formation, and/or femoral and tibial subchondral sclerosis is shown

TREATMENT NON-SURGICAL / CONSERVATIVE

• In the early stages of the disease, treatment is not surgical. Treatment include-

• Medications to reduce the pain (NSAIDS)• A brace to relieve pressure on the joint surface• A conditioning program with exercises to strengthen

quadriceps and hamstring muscles• Lifestyle & Activity modifications to reduce knee pain

TREATMENTSURGICAL

• If more than half of the bone surface is affected, surgical treatment may be considered. Options are

• Arthroscopic debridement• Osteochondral grafts• High Tibial Osteotomy (HTO)• Core decompression• Unicondylar Knee Arthroplasty• Total Knee Arthroplasty

Arthroscopy

• For debridement of degenerative tears in the menisci.

• Mixed results as it may cause further degeneration of the knee joint and possibility of increased interosseous pressure.

Osteochondral Graft

• Both allograft and autografts have been used with discouraging results

High Tibial Osteotomy (HTO)

• High tibial osteotomy (HTO) has been used in patients of SONK, with encouraging results.

Core Decompression

• The principle is to reduce interosseous pressure, thereby restoring adequate circulation.

• Some success have been observed in the earlier stages of osteonecrosis.

Unicondylar Arthroplasty

• Unicondylar arthroplasty

 has been used with success,

as the disease usually is

confined to one condyle 

Total Knee Arthroplasty

• Knee arthroplasty is indicated in the late stages of the disease. Indications are -

• degenerative changes

• severe pain

• functional disability

PROGNOSIS

• Aglietti et al reported that lesions greater than 5 cm2 had a worse prognosis than lesions with areas less than 3.5 cm2. Hence, prognosis of SONK is directly related to the size of the lesion.

DIFFERENTIAL DIAGNOSIS• Osteochondritis Dissecans

– more common on lateral aspect of medial femoral condyle in adolescent males

• Transient Osteoporosis– more common in young to middle age men

• Bone Bruises and Occult Fractures– associated trauma, bone fragility or overuse

• Idiopathic Osteonecrosis of the Knee– lesion is not crescent shaped