Philippe Orcel

Hip pain is likely to be due to a local or regional bone disease when the hip joint is painless and of normal mobility by physical examination. Targeting the right diagnosis is easy in a patient affected by bone disease (osteoporosis, Paget's disease) or when the background of the patient is suggestive (malignancy, pregnancy). However it could be more tricky in most cases: transient osteoporosis of the hip, bone fissures, Paget's coxopathy, and tumoral cell osteolysis of bones surrounding the hip are the main causes, in addition to femoral head osteonecrosis, which is detailed in another chapter.

Osteoclast activation is a critical cellular process for pathological bone resorption, such as erosions in rheumatoid arthritis (RA) or generalized bone loss. Among many factors triggering excessive osteoclast activity, cytokines such as IL-1 or tumour necrosis factor (TNF)-α play a central role. New members of the TNF receptor ligand family (namely receptor activator of nuclear factor-κB [RANK] and RANK ligand [RANKL]) have been discovered whose cross-interaction is mandatory for the differentiation of osteoclasts from hemopoietic precursors, in both physiological and pathological situations. Osteoprotegerin, a decoy receptor which blocks this interaction, decreases osteoclast activity and could have a fascinating therapeutic potential in conditions associated with upregulated bone resorption.