Avascular necrosis or Osteonecrosis is a fairly common hip condition.
Young patients are affected. Some of the causes leading to osteonecrosis are steroid intake, alcoholism, connective tissue disorders like SLE, Sickle cell disease, hypertension, decompression sickness, thrombosis, In a large number of individuals the cause is unknown or Idiopathic. Young and middle aged patients are affected. Involvement of both hips is fairly common.
Hip AVN

Hip showing advanced changes of avascular necrosis
Diagnosis- It takes a long time for the bony changes to be visible on plain x rays.
MRI scans and nuclear bone scans reveal the diagnosis early on when the condition is suspected. Both hips need to be imaged simultaneously. CT scans help to identify the extent of the bony involvement.
Treatment
The treatment depends on the stage of the disease. Most often conservative or non operative measures fail. Surgical treatment ranges from core drilling to total hip replacements. In the early stages core drilling can lead to a positive result in a good proportion of cases.
When this has failed, or the stage is more advanced, hip resurfacing is advised. However the surgeon should do so after a diligent assessment of the extent of disease. Hip resurfacing can be done when the extent of the femoral head involvement is less than 30 percent. In early cases partial hip resurfacing can be done in a few selected centers

Hip resurfacing operation
The presence of cysts makes hip resurfacing more difficult to perform with confidence. In these cases there are two options.
One is the Proxima hip replacement

where the bone is amputated at the base of the head similar to the BMHR. Then a short stem is impacted into the top portion or Proximal part of the neck and metaphysis of the femur. This has a large ball of the same diameter as the natural femoral head. The socket is also lined with a metal cup. Thus a trunnion of large diameter metal on metal results. The advantages are stability and less bone loss. The Proxima hip has a ten year follow up in the hands of its designer surgeon.
The other option is the Mid head resection Birmingham hip or BMHR.

Birmingham Mid head resection prosthesis
This operation involves amputating almost the entire head of the femur containing the cysts, impacting a conical stem into the neck portion of the femur and attaching a Birmingham hip like prosthesis. The cup also gets a metal liner.
All the options mentioned are bone sparing options.
There is not much difference in the extent of the bony resection in the Proxima hip replacement and the Birmingham Mid head resection prosthesis. The former has a longer follow up.
A total hip replacements with a variety of non metallic bearings is the only advisable replacement in patients with blood dyscrasias like sickle cell disease, thallesemia etc. Ceramic on Ceramic, Ceramic on polyetheylene, metal on poly are the available choices.


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