Compiled by: Elizabeth Sobieraj, M.D.
History: An 82-year-old woman who recently injured her left knee; she heard a “pop” on the medial side of the knee.
MRI Findings: Subchondral fracture without subchondral collapse located on the weight-bearing portion of the medial femoral condyle with diffuse osteoedema in the medial condyle and in the overlying medial tibial plateau without macro fracture.
Associated findings: Complex trizonal predominantly horizontal tear of the medial meniscus extending from the body to the posterior horn. Moderate (grade 2) MCL sprain. Moderate to severe (grade 3-4) chondromalacia of the medial and patellofemoral compartments, mild to moderate chondromalacia (grade 2-3) of the lateral compartment.
Diagnosis: Spontaneous osteonecrosis of the knee (SONK), synonymous with Ahlbaeck disease. Associated findings discussed above.
Discussion:
Spontaneous osteonecrosis of the knee is defined as: “Necrosis of the weight-bearing portion of the femur or tibia with associated subchondral fracture or collapse” (3). It often presents with the acute onset of pain in the knee of an elderly patient -- usually female, with pain in the medial compartment of the knee, often after minor trauma. Pain is worse at night.
Typical MRI findings: Bone marrow edema of the subchondral bone with linear or crescentic subchondral fracture in the weight-bearing portion of the medial femoral condyle or plateau (may involve lateral femoral condyle or plateau). Very often there is an associated medial meniscal tear.
Differential diagnosis (3): Subchondral fracture (younger patients; may be indistinguishable), osteoarthritis, osteochondritis dissecans OCD (young patient) and stress response/fracture.
Etiology is unknown, but the possibilities are the following (2, 3, 5):
o Traumatic insult causing microfracture of the subchondral plate and collapse of overlying bone and cartilage,
o Post-traumatic condition initiated by a subchondral insufficiency fracture,
o Vascular insufficiency resulting in infarction of the bone, or
o Meniscal injury and impact of the articular surface against a fragmented meniscus, which could result in local ischemia.
Staging, grading, or classification criteria based on radiography (femoral condyle) (3, 4):
Stage I: Normal x-ray. Lesion may resolve; may be diagnosed at MRI with diffuse edema, serpiginous fracture line.
Stage II: Subtle flattening of condyle.
Stage III: Lucent subchondral area with sclerotic halo.
Stage IV: Sclerotic halo enlarges, collapse ensues.
Stage V: Stage IV and degenerative changes.
Treatment depends on staging, conservative or surgical (core decompression, osteochondral allograft, proximal tibial osteotomy, knee replacement).
