Constriction of the fetlock annular ligament: Relationship between clinical and histopathological findings

Abstract

Stenosis or constriction of the fetlock annular ligament (FAL) has been diagnosed in 92 limbs of 85 horses, which have been subsequently submitted to FAL desmotomy. A previous clinical examination, including radiographic and ultrasonographic imaging, as well as synovial fluid analysis when needed, allowed a new classification of the constriction based on clinical findings: Type 1 (primary stenosis of FAL usually associated with synovial effusion of the tendon sheath); Type 2a (stenosis of the FAL secondary to tendinitis of the superficial or deep digital flexor tendon); Type 2b (stenosis of the FAL following a previously healed tendinitis with secondary thickening of the synovial membrane of the tendon sheath and subcutaneous tissue); Type 3 (stenosis of the FAL secondary to septic tenosynovitis). Type 1 included 40 cases (42 limbs); Type 2a 21 cases (26 limbs); Type 2b 17 cases (17 limbs) and Type 3 7 cases (7 limbs). In 29 cases a sample of the FAL was taken during surgery and submitted for histological examination. A further classification was made, based on histological findings, and samples were divided into 3 different groups. In group A the FAL, the synovial membrane and subcutaneous tissue appeared thickened, with irregular distribution of collagen fibres, increased number of desmocytes, lack of reticular fibres, and moderate angiogenesis. Most of the cases that were clinically classified as Type 1, Type 2a and Type 2b were included in group A. Group B was characterised by similar findings, but by the additional presence of inflammatory cells like lymphocytes and granulocytes. A small number of cases clinically classified as Type 1 and Type 2a were included into group B. In group C, a marked infiltration of inflammatory cells (lymphocytes and neutrophils) and, in more recent cases, the presence of fibrin were the dominant feature. All cases classified in Group C were clinically assigned to Type 3. The histologic examination supported the diagnosis of "desmitis" in cases belonging to the groups B and C, wheras cases included in group A showed only thickening of the FAL without signs of inflammation.