Hi!
Adenomyosis is a form of endometriosis. Pathologically it is endometrial glands that are abnormally located within the muscle wall of the uterus. MANY women have some degree of adenomyosis, and we're not sure why. Some studies suggest that it could be placental scarring that causes adenomyosis. So, in your case, I think it is more likely that the abnormally implanted placenta caused the adenomyosis than vice versa.
It is difficult to see adenomyosis on ultrasound, and the only definitive way to diagnose it is with a hysterectomy. Likewise that is the only cure, although some of the treatments for endometriosis ( OCs, Lupron) will help alleviate symptoms in some cases.
In fact, if I am contemplating hysterectomy and want to be sure that it will help solve the problem I will often pre-treat with GnRH agonists (lupron). If those work, then surgery will probably be helpful, if not--it won't.
I hope this is helpful!
Dr B