Age

The age, sex, breed, and use of the horse are basic vital facts (Box 3-1). Flexural deformities, physitis, other manifestations of osteochondrosis, and angular limb deformities are age-related problems. Infectious arthritis (hematological origin), lateral luxation of the patella, and rupture of the common digital extensor tendon are conditions usually unique to foals. Emphasis on training skeletally immature, 2- and 3-year-old racehorses causes predictable soft tissue and bone changes, often resulting in stress-related cortical or subchondral bone injury. Liautard observed more than 100 years ago: “When an undeveloped colt, whose stamina is not yet established and constitution not yet confirmed, with tendons and ligaments relatively tender and weak, and bones scarcely out of the gristle, is unwisely condemned to hard labor, it is irrational to expect any other results than lesions of one or another portion of the abused apparatus of locomotion. They will be fortunate if they escape a fate still worse, and become sufferers from nothing worse than mere lameness.”¹ This statement aptly summarizes the situation then and now. The high value of races for 2- and 3-year-olds results in high-intensity training for early 2-year-olds, which may result in injury such as maladaptive or nonadaptive remodeling of the third carpal bone (C3), precluding racing at a young age.

Some problems are unique to older horses (Box 3-2). Overall, osteoarthritis (OA) and other degenerative conditions such as navicular disease are most common but certainly are not unique to the geriatric horse. Some horses have a remarkably early onset of navicular disease or OA despite little physical work, suggesting a genetic predisposition to the condition. These problems worsen with advancing age, particularly if several limbs are involved. In former racehorses, progressive OA is of particular concern; this condition most commonly affects the carpal and metacarpophalangeal joints (Figure 3-1). Occasionally in older horses, severe, progressive OA of the carpometacarpal joint occurs without any history of carpal lameness (Figure 3-2). In some horses, angular deformities (the most common is carpus varus) develop at the carpometacarpal joint. Inexplicably severe OA of the carpometacarpal and middle carpal joints is most commonly seen in Arabian horses (see Chapter 38). Primary OA of this joint is rare in young horses, even in racehorses with middle carpal joint abnormalities, unless C3 slab fracture or infectious arthritis occurs. OA of the coxofemoral joint is rare in horses with the exception of young horses with osteochondrosis, but it does occur in older horses.

Fig. 3-1 An aged Thoroughbred broodmare with severe forelimb deformity caused by primary severe osteoarthritis of the right metacarpophalangeal joint and secondary or compensatory (chronic overload) carpus varus in the left forelimb.

Fig. 3-2 Dorsopalmar digital radiographic image of the right carpus in a 13-year-old Arabian mare with pronounced lameness as a result of severe, progressive osteoarthritis of the carpometacarpal and middle carpal joints (medial is to the right). There is substantial varus limb deformity present. Comfort improved after partial carpal arthrodesis.

(Courtesy Dr. Dean Richardson.)

An unusual group of soft tissue injuries of unknown origin occurs in older horses. Superficial digital flexor tendonitis and suspensory desmitis generally are considered overuse injuries and usually occur in upper-level performance horses or racehorses. However, severe tendonitis and desmitis do occur, often suddenly and without provocation, in older (teenage) horses. Horses usually are turned out at pasture when initial lameness is observed. In some horses, superficial digital flexor tendonitis is severe and progressive, later leading to flexural deformity because of adhesions. Suspensory desmitis may be unilateral or bilateral, may involve the forelimbs or hindlimbs but is more common in the hindlimbs, and is most common in the older broodmares. The name degenerative suspensory (ligament) desmitis (DSD) was given to a syndrome, often seen in older horses and most common in Peruvian Pasos, in which severe, often bilateral suspensory desmitis occurred^2-4

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