Patterns of histological and immunoistochemical alterations in the constriction of the fetlock annular ligament (FAL) of the horse

Constriction of the fetlock annular ligament is a fairly diffuse condition in athletic horses, which suffer from trauma of tendons and ligaments. A consequence of this constriction is an inflammatory process, mostly chronic, characterised by fibrous thickening resulting in pressure and pain of the contents of the fetlock canal. Special attention was paid to the redistribution of the components of the fetlock annular ligament, the presence/absence of inflammatory cells, and in particular to the capillaries of the ligament itself in 36 exercised horses suffering from constriction of this ligament plus 3 Standardbreds taken as controls. The methods used were Hematoxylin-eosin, Gomori for reticular fibres, and Orcein-Van Gieson for elastic fibres, and the immunohistochemical reaction for FVIII-RAG, as marker for the endothelium of blood vessels. The cases were divided into groups A, B, and C on the basis of absence (group A) or presence of inflammatory cells. In this case group B indicates the presence of chronic inflammation and group C the presence of acute reaction. The FVIII-RAG capillary positivity was found especially in the cases with chronic inflammation, so it is presumable that cytokines produced by the inflammatory cells, besides their role in vasculogenesis and angiogenesis, might play a part as promoters for FVIII-RAG in endothelial cells.