Pathology of the Vertebral Column of Horses with Cervical Static Stenosis

Specimens of ligamentum flavum, joint capsule, and dorsal lamina were collected at surgery or necropsy from 25 horses with cervical static stenosis. All horses had myelographic evidence of dorsal compression of the spinal cord caused by soft tissue and/or bone in the caudal cervical area, primarily at C6-7. Most horses also had radiographic evidence of degenerative joint disease of articular facets. Histologically 19 horses had osteosclerosis and cartilage retention in the dorsal lamina, and 24 horses had increased fibrocartilage at the ligamentum flavum attachment to dorsal lamina. The ligamentum flavum and joint capsule had fibrovascular tissue in 20 horses. Fibrocartilaginous tissue, old hemorrhage, and fat necrosis were not unusual. One horse each had a synovial cyst, eosinophilic granulomas in the joint capsule, and osteochondrosis of articular facets. These findings indicate that abnormal biomechanical forces or instability of articulations result in stretching and tearing of the ligamentum flavum and joint capsule with subsequent fibrovascular and fibrocartilaginous proliferation, osteosclerosis of the dorsal lamina, and osteophyte formation on the articular facets. Cervical vertebral malformation, a leading cause of ataxia (wobbler syndrome) in horses, is subdivided into two syndromes; cervical vertebral instability and cervical static stenosis. Cervical vertebral instability most commonly affects horses 8 to 18 months of age and is characterized by narrowing of the sagittal di- ameter of the spinal canal during neck flexion, pri- marily involving cervical vertebrae C3 to C5. Cervical static stenosis most commonly affects horses 1 to 4 years of age and is characterized by dorsal or dorso- lateral compression of the spinal cord regardless of neck position, primarily involving cervical vertebrae C5 to C7.4,26,27,32 The clinical result of cervical static stenosis is predominately a hindlimb ataxia although forelimb signs also may be present.4J0.26.27.32,39 Clinical improvement has been reported following subtotal larninect~my.~~, ~~.~~ One study of horses with cervical vertebral malformation described "osteopetrosis" of the dorsal lamina and vertebral bodies, retention of cartilage cores in thickened trabeculae of the vertebrae, and variable degenerative joint disease of articular fac- et~.~~ Additional gross findings reported in horses with cervical static stenosis include hypertrophy of the liga- mentum flavum between the dorsal laminae and fi- brous thickening of the joint capsule of articular facets causing dorsal or dorsolateral spinal cord compres- ~i~n.~~,~~,~~ Synovial cysts compressing the spinal cord dorsolaterally also have been rep~rted.~.~ Cervical vertebral malformation resulting in spinal canal stenosis and ataxia also occurs in dogs, especially Great Danes and Doberman pin~~her~.~,~J~, ~~,~~,~~ Ra- diographic and gross findings in dogs include hyper- ostosis of articular facets, elongation of the vertebral arch, craniodorsal displacement of cervical verte- brae,3,4J5,28,33,35,39 hypertrophy of the ligamentum fla- vum or dorsal longitudinal ligament,17,35 intervertebral disc degenerati~n,~J~ ,~~