HYPOTHESIS 3 - TRAUMA INDUCED MYOSITIS

Damage to muscles could be due to many factors.  There could be contusions (bruise to muscle with varying degrees of hemorrhage and fiber disruption); a strain (longitudinal stretching/tearing of muscle fibers); and lacerations to muscle.  Contusions and strains result in disruption of the normal architecture secondary to edema, hemorrhage or overstretching.  Lacerations result in muscle damage by actually tearing the muscle itself.  Muscles are highly vascularized, and the blow from the car caused a disruption of fibril and vasculature continuity leading to subsequent leakage of fluid, cells and blood into the subcutaneous space in the legs.  The hit from the car could have led to overstretching of muscles again disrupting fibrils and causing hemorrhage and edema.  There seems to be a fracture involved as well.  A fractured piece of bone could have directly, internally lacerated some muscle and surrounding vasculature.  This disrupted the muscle fibers and led to leakage of blood constituents into the surrounding area.  
	The leakage of blood constituents into the surrounding area led to further damage.  Neutrophils and macrophages come in to clean up the surrounding hemorrhage and along with the already damaged cells, inflammatory mediators are released.  Cytokines have a chemotactic as well as pain mediated action.  The cause vasodilation and even more cells infiltrate the area.  Prostaglandins are also released causing even more pain.  The damage to the muscles and ensuing pain are adding to the reluctance of the dog to want to walk.  The edema and inflammation could also be causing pressure damage to surrounding nerves increasing the pain and inability to walk, this is compounded by the fact that the edema and inflammation are present in both the right and left hind legs.