AVN (Avascular Necrosis), also called Osteonecrosis, is a disease that results from the temporary or permanent loss of blood supply to bone. Without a supply of blood, the afflicted areas of bone die, leaving behind what are referred to as “lesions”. From there, the body's natural healing process attempts to breakdown the dead bone and regrow new bone in its place. However, more often than not, the lack of blood supply that caused the onset of the disease has the secondary effect of also inhibiting healing and impeding or preventing new bone growth. If the bone breaks down faster than it regenerates, the end result is often critical deformation or complete collapse.  The risk of bone collapse is much higher in load bearing joints such as hips, knees, shoulders, and ankles.

The sad truth is that very little is really known about the disease, AVN. Medical researchers and doctors have been able to draw some correlations to possible risk factors though. The top causes of AVN are thought to be steroid medications, injury to bone, alcohol use, and some other related diseases.

Steroid Medications
Oral steroids, such as Prednisone, and injected corticosteriods both have been strongly linked as likely causes of AVN. The assertion is that steroid use can interfere with the body's ability to break down fatty substances in the blood stream. These fatty deposits may then cause blockages in bone blood vessels resulting in constriction and death of the affected bone site. The degree of risk present in the use of steroid medications is a hotly debated topic. Some sides claim the risk of steroid medication use to be very low.  On the other hand, other groups assert that over 70% of all AVN patients took steroid medications before onset of the disease.

Injury
Another known correlation to AVN is trauma-related injury to the bone. Significant blunt force trauma, stress, fracture, or even compression are thought to significantly increase the risk of developing AVN. Certain types of car accidents, for example, can inflict enough blunt force trauma to damage the blood vessles inside of bone, cutting off the blood supply and resulting in AVN. Also, studies show that up to 20% of people that dislocate a hip go on to develop AVN.

Alcohol Use
Excessive alcohol use is thought to be another very significant cause of AVN. The underlying theory is that excessive alcohol use disrupts the body's natural ability to breakdown fatty substances in the blood. These fatty substances then lodge in narrowed blood vessles in bone restricting blood supply and causing AVN.

Other Potential Factors
Other medical conditions and medical treatments are known to have correlations to AVN as well. Some of the top associated risks are Gaucher's disease, radiation therapy, chemotherapy, decompression disease, and other blood disorders.

References
[1] "Osteonecrosis", National Institute of Arthritis and Musculoskeletal and Skin Diseases. Retrieved on 2009-04-10.
[2] Jeanne K Tofferi and William Gilliland. "Avascular Necrosis", emedicine. Retrieved on 2009-04-10.
[3] Ninomiya S. An epidemiological survey of idiopathic avascular necrosis of the femoral head in Japan. Annual Report of Japanese Investigation Committee for Intractable Disease. 1989.
[4] Lawson-Ayayi S, Bonnet F, Bernardin E, et al. Avascular necrosis in HIV-infected patients: a case-control study from the Aquitaine Cohort, 1997-2002, France. Clin Infect Dis. Apr 15 2005;40(8):1188-93. [Medline].

AVN is considered one of the most dreaded and sinister diseases because it can cause excruciating levels of pain over long durations. Many factors determine how pain will present with AVN, including the stage of AVN advancement, location and size of lesions, and physical fitness of the patient.

At the low end, some patients with early onset AVN only feel pain on par with Arthritis. In fact, many AVN patients are misdiagnosed as having other diseases at first.

Though, in more advanced or severe AVN cases, the common and notable trait of AVN is that it can be one of the most excruciating and debilitating diseases known to modern medicine. In fact AVN pain severity is often ranked very near to the severity of bone cancer. Not only is severe AVN pain extremely unpleasant, but left improperly treated, severe AVN pain can cause permanent nerve and neurological damage.

The key difference between a bone fracture and Avascular Necrosis (or Osteonecrosis) is that a fractured bone still has an intact network of cells and blood vessels necessary for regeneration where as Avascular Necrosis is the condition where blood supply to the bone cells is blocked and the regenerative bone cells die. 

• Osteoblasts are primarily located on the surface of bone tissue and hold the important responsibility of forming new bone.[2] 
• Osteoclasts are the cells which breakdown and resorb bone by secreting acid and enzymes.[3]
• Osteocytes are simply mature osteoblasts that become trapped in the bone matrix. Osteocytes make up 90% of cells in mature bone.[4]

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