What is osteonecrosis (also known as “avascular necrosis” or “AVN”)?How does hip dislocation lead to avascular necrosis?How does avascular necrosislead to end stage arthritis?
Avascular necrosis is the death of bone, secondary to loss of blood supply and resultant ischemia as seen on the right.
Hip dislocation can cause disruption of the blood supply to the head and thus lead to AVN
Recall that bone is alive. Hence:
- If bone (like all living tissue) is deprived of its blood supply, ie gets ischemic, it will die. (A principle illustrated below)
- If bone dies, it does not remodel (you remember this…).
- If bone does not remodel, micro-damage does not get repaired just as a ship that does not get painted with rust.
- If enough damage accumulates, the bone loses its normal material properties (strength and compliance). It will break, not bend
- Specifically, the sub-chondral bone will collapse, as shown here.The bone is literally not supporting the surface, which then too will collapse
- If the sub-chondral bone collapses, the joint surface of course becomes irregular and no longer smooth
- If the joint surface is not smooth it will damage its rival surface.
This is not a histology slide–it’s a picture of sandpaper. The effect of a rough surface on a smoother one is to make the smooth rough
Note: although you can detect muscle fairly quickly (within hours after an infarct, certainly), acutely dead bone (within hours after an infarct, say) would look normal if examined histologically. That is because for bone, the blood supply primarily serves maintenance, not sustenance. By (poor) analogy: if you can’t get water to drink (sustenance) in three days, you die; whereas if you can’t get water to shower (maintenance) in three days, you don’t start stinking immediately, but stink you will eventually.
