What could be the cause of sudden osteonecrosis in knee?

Dear Dr. Roach: Iam a 69-year-old Caucasian woman with newly found necrosis of the knee bone, along with a fracture. I do not have diabetes, but I received a month of steroids last November when I had COVID. I also use Adair daily for asthma. I am scheduled for total knee replacement this coming October. My primary care doctor, my orthopedist and my knee surgeon do not know how this occurred. I would like to know so that I might be able to prevent it from happening in the future. I felt fine until after a long walk three months ago. I’ve had to use a cane for walking ever since, and am unable to walk very far, due to pain. — S.R.

Dear S.R.: Osteonecrosis, also called avascular necrosis, literally means “death of bone.” The top of the femur in the hip is the most common bone affected, but many other bones can be affected.

I can’t say for sure what caused your knee bone osteonecrosis, but there are three possibilities I’d consider. The first is the steroids you took when you had COVID19. Moderately strong evidence shows that steroids, such as dexamethasone, reduce the risk of dying in hospitalized people with COVID who have low oxygen levels (94% or lower). Unfortunately, the dose of steroids used, although it may have been lifesaving, does put people at higher risk for developing osteonecrosis. In my opinion, this is the most likely cause.

A second possibility is a rare condition called “spontaneous osteonecrosis of the knee.” This essentially “unknown cause” tends to affect a specific part of the knee (the medial femoral condyle).

The third possibility I’d consider would be the COVID-19 itself. I found a case series of people with osteonecrosis that occurred in people with COVID-19 infection. Their doses of steroids were not high. However, all these cases were in the hip, not the knee.

This is not a complete list. Trauma to the knee, excess alcohol use, smoking, radiation treatment and osteoporosis medications (when used for a very long time) all can predispose to osteonecrosis.

Dear Dr. Roach: Can COVID-19 be transmitted by mosquitoes? My wife is constantly being bit by them in the morning and evening, and this has become a concern of hers. — R.J.T.

Dear R.J.T.: There has been no evidence of mosquito transmission of COVID-19, and it is very unlikely to do so. Some viruses can be transmitted by mosquitoes, but COVID19 is typically transmitted by respiratory droplets. If an infected person coughs or sneezes, a nearby person can inhale the virus.

A side note about wearing a face mask: Masks help in two ways. If the person who is infected and may not know it is wearing a mask, they will spread far less virus through breathing, coughing or sneezing. That’s why it’s also important NOT to take your mask off to cough or sneeze, even though it’s uncomfortable and against instinct. Any mask can reduce the risk of spreading COVID-19.

Second, wearing a mask filters out the large respiratory particles that are most likely to transmit infection. N95 or KN95 masks are most effective at doing this, but a cloth mask with a surgical mask on top is also effective.