MRI is highly sensitive, specific, and accurate in the detection of AVN.

T1-weighted images: AVN most often presents with a crescentic, ring-like or well defined band of low signal within the superior portion of the subchondral femoral head bone marrow. This band is thought to represent the reactive interface between the necrotic and reparative zones, and typically extends to the subchondral plate.

http://radsource.us/avn-of-the-hip/

According to UW, alcohol abuse is a risk factor for avascular necrosis. The most common site is the femoral head, which presents with pain that is exarcebated by weight bearing without inflammatory signs on physical examination.

FA 2020: 463

Causes of avascular necrosis: CASTS Bend LEGS. +Corticosteroids +Alcoholism +Sickle cell disease +Trauma +SLE

+“the Bends” (caisson/decompression disease) +LEgg-Calvé- Perthes disease (idiopathic) +Gaucher disease +Slipped capital femoral epiphysis

FA 2019 pg 455 on avascular necrosis of bone: Infarction of bone and marrow, usually very painful. Most common site is femoral head (watershed zone) (due to insufficiency of medial circumflex femoral artery). Causes include Corticosteroids, Alcoholism, Sickle cell disease, Trauma, SLE, "the Bends" (caisson/decompression disease), LEgg-Calve- Perthes disease (idiopathic), Gaucher disease, Slipped capital femoral epiphysis- CASTS Bend LEGS.

(A) Risk factors for developing avascular necrosis include:

1) Trauma = Injuries, such as hip dislocation or fracture, can damage nearby blood vessels and reduce blood flow to bones.

2) Steroid use= Use of high-dose corticosteroids, such as prednisone, is a common cause of avascular necrosis. The reason is unknown, but one hypothesis is that corticosteroids can increase lipid levels in your blood, reducing blood flow.

3) Excessive alcohol use = Consuming several alcoholic drinks a day for several years also can cause fatty deposits to form in your blood vessels.

4) Bisphosphonate use = Long-term use of medications to increase bone density might contribute to developing osteonecrosis of the jaw. This rare complication has occurred in some people treated with high doses of these medications for cancers, such as multiple myeloma and metastatic breast cancer.

5) Certain medical treatments = Radiation therapy for cancer can weaken bone. Organ transplantation, especially kidney transplant, also is associated with avascular necrosis.

(B) Medical conditions associated with avascular necrosis include:

Pancreatitis Diabetes Gaucher's disease HIV/AIDS Systemic lupus erythematosus Sickle cell anemia

I thought it was septic arthritis bc pain with weight bearing + mri, but 2 months with septic arthritis come on, it was an easy question and I missed it.

I think the small dark area on the left head of femur and the darkened neck are the avascular sites.

Neck: http://img.medscapestatic.com/pi/meds/ckb/15/19515tn.jpg

Head: (obvious lesion on the RT femur, but similar discrete lesion on the left as seen on the practice NBME) http://radsource.us/wp-content/uploads/2005/11/1a.jpg

maybe someone can explain why this is avascular necrosis and not sepsis. It doesn't mention fever or absence of fever. The MRI has a small amount of hypodensity but to get avascular necrosis seems odd/

Causes

1) Avascular necrosis occurs when blood flow to a bone is interrupted or reduced. Reduced blood supply can be caused by:

2) Joint or bone trauma. An injury, such as a dislocated joint, might damage nearby blood vessels. Cancer treatments involving radiation also can weaken bone and harm blood vessels.

3) Fatty deposits in blood vessels. The fat (lipids) can block small blood vessels, reducing the blood flow that feeds bones.

4) Certain diseases. Medical conditions, such as sickle cell anemia and Gaucher's disease, also can cause diminished blood flow to bone.

For about 25 percent of people with avascular necrosis, the cause of interrupted blood flow is unknown.