Fact-sheet: Spontaneous osteonecrosis of knee
Updated on 09/09/2019 at 12:00 PM
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Definition
Spontaneous osteonecrosis of knee, also called mechanical osteonecrosis.
Decompensated subchondral bone fracture
(accumulation of impacts in a weight-bearing zone on weakened bone, impaired bone repair followed by subchondral ischemia).
Clinical features
Most common form of osteonecrosis of the Knee.
Sex ratio: 3 women to 1 man.
Prevalence increases with age (up to 10% after age 65).
Risk factors: low bone mineral density/meniscal tear/meniscectomy/Osteoarthritis.
Sudden onset of symptoms in 75% of cases, pain worsened by weight bearing, focal tenderness on palpation, joint effusion.
Initial painful phase lasting approximately 6 to 8 weeks, followed by resolution or progression to disabling chronic pain.
Laboratory findings
No value
Radiography
Location: 90% of cases occur at the medial femoral condyle, in the weight-bearing zone.
Initial radiograph often normal.
- Focal flattening of the articular surface of the medial femoral condyle.
- Focal epiphyseal collapse with subchondral lucency.
- Sclerotic halo surrounding the mechanical osteonecrosis.
- Secondary subchondral bone collapse.
- Fragmentation or even release of the necrotic zone, giving rise to intra-articular loose bodies.
- Later, possible periosteal new bone formation.
Ultrasound
No practical value
CT
The radiographic findings are seen more clearly.
Not indicated in the acute phase, as it is poorly sensitive.
MRI
The most sensitive examination in the initial phase.
Findings include:
-A hypointense subchondral band without enhancement after injection.
-Adjacent bone marrow edema.
-Subchondral bone impaction.
-Subchondral dissection.
-Cystic changes of the subchondral bone.
-Cartilage abnormalities due to subchondral collapse.
Nuclear medicine
Bone scan showing intense uptake at the medial femoral condyle with central photopenia of the necrotic zone.
Management
Necrotic bone does not heal.
Prognosis depends on the extent of the affected surface.
Medical management for small lesions (unloading).
Surgical treatment.