Spontaneous OsteoNecrosis of Knee (SONK)

1. Spontaneous
OsteoNecrosis of
Knee (SONK)
(Ahlback's disease)
Dr.Avik Sarkar
KB Bhabha Municipal General Hospital, Bandra (West), Mumbai

2. • 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

3. INCIDENCE
• Elderly Women are generally affected
[seen more frequently in women (M:F = 1:3)
and typically over the age of 55]

4. RISK FACTORS (POSSIBLE)
• Corticosteroid use
• Systemic Lupus Erythematosus (SLE)
• Alcoholism
• Pancreatitis
• Sickle Cell disease
• Rheumatoid Arthritis
• Caisson disease
• Gaucher disease

5. SITE / LOCATION
• 99% of patients have only one joint
involved
• usually epiphysis of medial femoral
condyle is involved

6. PATHOPHYSIOLOGY
• Osteonecrosis in SONK has no
predisposing factors.
• Speculations include
-subchondral insufficiency fracture
-may be caused by a meniscal root tear

7. 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

8. IMAGING
• X-Ray
• Technetium-99m scan
• MRI

9. 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.

10. 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

11. 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.

12. 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

13. 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

14. TREATMENT
SURGICAL
• 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

15. 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.

16. Osteochondral Graft
• Both allograft and autografts have been
used with discouraging results

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

18. Core Decompression
• The principle is to reduce interosseous
pressure, thereby restoring adequate
circulation.
• Some success have been observed in the
earlier stages of osteonecrosis.

19. Unicondylar Arthroplasty
• Unicondylar arthroplasty
has been used with success,
as the disease usually is
confined to one condyle

20. Total Knee Arthroplasty
• Knee arthroplasty is indicated in the late
stages of the disease. Indications are -
• degenerative changes
• severe pain
• functional disability

21. 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.

22. 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