"His system apparently does not like joint invasion!"

My initial flare of reactive arthritis (ReA) was probably an enteric infection. There is a theory that remnants of whatever organism causes ReA may "hide" in the tissues surrounding large joints like the knees and the joints of the spine. Anything that "stirs up" the joints can cause the remnants of the organism to be exposed. The immune system senses the infection is still present and starts to attack the joints again. The joints are victims of collateral damage as the immune system attempts to rid the body of the recurring infection.

Another theory is the structure of the offending organism is chemical similar to the tissues surrounding the joints. The immune system has a hard time distinguishing the organism from the body's own tissues and attacks both.

I don't know if either theory is true but they were interesting to me.

Both of my knees were replaced and I had an "aberrant healing process" after my knee replacements. The orthopedic surgeon said I shouldn't ever have surgery again because of "heterotopic ossification."

Heterotopic ossification (HO) is a condition where bone grows in soft tissue or muscles, instead of in the skeleton. It can occur after an injury, such as a spinal cord injury or musculoskeletal trauma, or for no known reason.

HO is bone growth where bone doesn't belong. The surgeon doesn't want to remove the extra bone because the "invasive nature of the procedure" could cause more abnormal bone growth and make the problem even worse.