[Pathophysiology of aseptic femoral head necrosis: Pathogenesis and histopathological differential diagnosis].

backgroundAseptic osteonecrosis is characterized by a complete death of the tissue (necrosis), more specifically, an ischemic necrosis of the lamellar bone tissue. The denotation aseptic refers to causal pathogenesis; therefore, it is not a matter of an infectious, septic-induced bone necrosis as in the case of acute infectious osteomyelitis. Formal pathogenesis leads to either (1) a hypoperfusion of the lamellar bone in the sense of an ischemic necrosis or (2) to directly induced damage of osteocytes and osteoblasts, which causes aseptic osteonecrosis. CAUSE: The causes of hypoperfusion/ischemia are manifold and entail vascular malformations, coagulopathies, haemoglobinopathies, thrombotic embolisms, myeloproliferative illnesses, air embolisms, decompression-caused illnesses, macro as well as micro traumata and, finally, vasculitis with necrosis, which complete the vascular-induced spectrum. Direct toxic damage to the osteocytes and osteoblasts is primarily caused by alcohol abuse, medical drug therapies (i. e. chemotherapeutic substances or cortisone) and disorders of the lipid embolism. Contemporary molecular and cellular models of pathogenesis assume a so-called dysbalance of the catabolic and anabolic osseous metabolism in osteocytes and osteoblasts. The RANKL-RANK system, the ROS system and PPAR-gamma signal transduction are involved in the molecular pathogenesis. DIFFERENTIAL DIAGNOSIS: The most relevant histopathologic differential diagnosis entails the complete spectrum of focal osseous changes among which osteonecrosis can occur to a diverse extent: infectious osteomyelitis, chronic immunologic induced osteomyelitis, pseudoarthrosis, infected pseudoarthrosis, bone fractures and malignant metastatic intraosseous diseases and non-metastatic intraosseous malignant diseases.