Canine Intervertebral Disk Disease: Hansen Type II Herniation
Veterinary Diseases II
Intervertebral disk disease, commonly referred to as IVDD, is one of the most common diseases causing paralysis of the rear legs in dogs. In 1952, a scientist named Hansen classified the cause of intervertebral disk disease into two different types of disk herniation, Hansen Type I and Hansen Type II. This paper will focus on Hansen Type II IVDD, and specifically on herniations of the thoracolumbar region of the spine, the most common area in the canine for Type II herniations to occur. (Sharp & Wheeler, 2005).
Intervertebral disks exist in between adjacent spinal vertebrae. Their main functions are to provide necessary cushioning between these boney structures and give flexibility to the vertebral column. (Coates, 2000). In order to understand the etiology of IVDD Hansen Type II, it is essential to first comprehend the components that make up the intervertebral disk. Each disk is surrounded by rings of strong fibrocartilagenous material called the outer annulus fibrosus, which connect firmly to the vertebral column. Inside the center of the annulus fibrosus exists a gelatinous-like substance called the nucleus pulposus. (McGavin & Zachary, 2007).
In non-chondrodystrophic (long-legged) dog breeds, degeneration of the intervertebral disks is an age related change of the nucleus pulposus. As the dog ages, the center of the disk undergoes fibroid metaplasia, a condition in which the center becomes more fibrous and progressively thicker. Fibroid degeneration causes the nucleus pulposus to lose water and proteoglycans as well as elasticity. This gradual loss of elasticity places an undue amount of mechanical stress on the annulus fibrosus. Over time, the annulus fibrosus becomes damaged and begins to slowly bulge outward toward the spinal cord. Eventually, this causes a dorsal protrusion of...