Well the SI joint is tight enough and built for conformity that it does "regrow" it's ligments at times, meaning that if you bring the two sides together with a belt it stands a chance that the two ends of broken ligs will "find each other". You will see this done with ankles, when there is no break, "just" a sprain or tearing of ligs, they'll pop the person in a cast boot to try to get the ends to meet again. But yes, if the ligs are completely shot, nowhere to be found, we can't regrow them. the prolo causes a reaction that I'm not an expert on, but mimicks ligaments I believe by making everything stiff.