Osteonecrosis of the femur.
Osteonecrosis (also known as avascular necrosis) is the death of a bone (or, more correctly, segments of a bone) due to the absence of blood supply to the tissue. Despite being hard and sturdy, bones are just as alive as other tissue in your body - if they lack blood, they die. As the bone dies and begins to collapse, it can cause anything from mild/moderate arthritic pain in the nearest joint, to intense and debilitating pain at the exact site of collapse.
The exact cause of avascular necrosis isn’t known, but it can be triggered by many things: gout, alcoholism, long-term steroid treatment, decompression sickness, and radiation (such as with the Radium Girls and the people who used Radithor water), are a few of the many conditions and risk factors that can lead to it.
The femur is the most commonly affected bone, especially at the trochanter articulating with the hip. When that’s the case, full hip replacement is generally the only treatment with an overall positive outcome. The humerus, knees, and jaws are also affected at times.
I find it interesting that this text mentions that the disease is by far the most commonly seen in children ages 9-13, with a bias towards males. These days, it’s most prevalent in 30-60-year-old adults. Legg-Calve-Perthe’s syndrome causes the death of the femur head in children, and the cause is still unknown. Though it’s self-limiting (only affecting the femur head), the severe degenerative arthritis that it can cause can be debilitating to anyone, and for a kid that wants to run around with their friends, can be especially devastating.
Legg thought the cause was trauma, Calve thought it was rickets, and Perthe thought the necrosis was caused by infection. Over 100 years and thousands of hypotheses tested, the only thing we know for sure is that it’s caused by a reduction of blood-flow to the femur head and that so far we don’t know how to cease progression to the point of complete hip degeneration. 1 in 1200 kids in the United States will develop this syndrome before the age of 15.
Practical Observations on Necrosis of the Tibia. Thomas Whately, 1815.