Chapter 54 Diseases of the Musculoskeletal System
Disorders of the musculoskeletal system are of great importance to the equine industry. The main purpose of a horse is to work, and if it has lameness problems it will not be able to do much work. One of the first things that must be understood is the difference between a blemish and an unsoundness. A blemish is an alteration in the appearance that does not affect the horse’s serviceability. An unsoundness, in contrast, does affect the horse’s ability to do its job. Most unsoundnesses occur in the horse’s front limbs, distal to the knee.
Hoof abscesses are the most common cause of acute, severe lameness in horses. Horses with a history of foreign body penetration into the hoof capsule and those with chronic laminitis are more likely to experience development of hoof abscesses. The condition occurs when bacteria gain access to hoof structures and form abscesses. Pain occurs because abscesses cannot expand due to the hoof’s rigid structure. Pain is relieved when abscesses rupture.
Navicular syndrome is most common in large-bodied horses, especially of quarter horse, Thoroughbred, and Warmblood breeding. Onset is most often seen in 6- to 8-year-old horses. The condition can occur when excessive stress and strain on flexor tendons puts pressure on navicular bursa or excessive concussion on coffin joint leads to inflammation of navicular bursa. This leads to inflammation and erosion of navicular bone. Heavily muscled horses with small feet, horses with short, upright pasterns, and horses with improperly trimmed hooves (long toe, low heel) are more likely to acquire the condition.
Thrush is a bacterial infection of sulci of frog and is usually found in horses that are kept in wet conditions. The most common bacterium isolated from horses with thrush is Fusarium necrophorum. A horse is more likely to contract thrush if it is kept in wet, dirty conditions, especially if the feet are not properly maintained.
Sidebone is usually found in the front feet of older horses. Draft breeds are overrepresented in the population of affected horses. Sidebones usually start as an unsoundness, then regress to a blemish. The inciting cause for a horse to experience development of sidebones is direct trauma to the foot.
Chip fractures of the pastern generally involve the long pastern bone. Repetitive stress and overflexing of the hock can cause these fractures. Horses that have long, weak pasterns are more likely to experience development of these fractures.
Sesamoid bones are part of the suspensory apparatus. Wear and tear occurs at the abaxial surfaces where suspensory ligaments pass. Sesamoid bone fractures are most commonly seen in athletic horses. Stress and strain on the fetlock joint, often from repeated overextension of the joint, puts force on the suspensory ligaments, which, in turn, pull on the sesamoid bones, causing them to fracture. Performance horses with long, weak pasterns are more likely to fracture their proximal sesamoids than are other horses.
Sesmoiditis is an osteitis of the proximal sesamoid bones, and it is most often encountered in racehorses. These horses often present with a history of gait restriction during training. Extreme stress and strain and repetitive loading lead to tearing of suspensory ligament attachments. This, in turn, leads to inflammation of the proximal sesamoid bones. Horses with long, weak pasterns are more likely to be affected by sesamoiditis.