Age / Sex : 38 / F

Age / Sex :  38/ Female

Chief complaint : Spontaneously developed sudden knee joint pain, left

Spontaneous Osteonecrosis of knee (SONK)

Discussion

Findings:.

Knee plain radiograph shows a focal crescentic area
surrounded by band of sclerosis in medial femoral condyle, with mild bony collapse.

RI bone scan of both knees shows a focal hot uptake in the left medial
femoral condyle.

MRI shows a focal area of subchondral collapse with separation of
osteochondral fragment and adjacent bone marrow edema and complex tear of
medial meniscus.

Differential Diagnosis:

Osteochondritis Dissecans (OCD)

Subchondral Fracture

Stress Response/Fracture

Osteoarthritis

Diagnosis:
 

Spontaneous
Osteonecrosis of knee (SONK)

Discussion:

SONK =
Spontaeneous OsteoNecrosis of Knee = Ahlbäck disease

-Necrosis of
weight-bearing portion of femur or tibia with associated subchondral fracture
and collapse

-Middle-aged
and elderly patients, F>M

-can be
associated with meniscal tears

-Etiology:
(controversial)

Traumatic theory:
osteoporotic patient suffers insufficiency fracture following trivial trauma,
bone subsequently becomes necrotic

Vascular theory:
thrombotic venous occlusion leading to venous hypertension and hypoxic death;
recently less accepted in the knee

-Radiographic
Findings:

(+/-) Subchondral sclerosis

(+/-) Articular surface collapse

Effusion

-MR Findings:

T1WI: Necrosis:
discrete area of subchondral decreased SI

Surrounded by intermediate signal
(combination of fat and edema)

T2WI: Focus of low SI
ischemia or fracture surrounded by diffuse increased SI edema

(+/-) Linear often serpiginous low
signal abnormality adjacent to edema (probable fracture)

(+/-) Articular surface collapse

References:

Björkengren
AG, AlRowaih A, Lindstrand A, Wingstrand H, Thorngren KG, Pettersson H.
Spontaneous osteonecrosis of the knee:value of MR imaging in determining
prognosis. AJR Am
J Roentgenol. 1990 Feb;154(2):331-6