Complex sentence: The cause of NICO is allegedly avascular osteonecrosis (AO) (also known as ischemic osteonecrosis) due to chronic low-grade infection, or, in more modern versions of the theory, susceptibility to thrombosis.
Split into simpler sentences: The cause of NICO is allegedly avascular osteonecrosis (AO) (also known as ischemic osteonecrosis). This bone ischemia would result from a chronic low-grade infection, or, in more modern versions of the theory, susceptibility to thrombosis.