Conformation and musculoskeletal problems in racehorses
Conformation is the physical manifestation of the muscles, bones, tendons and ligaments. These components combine to absorb stress and protect joints when limbs are loaded or in extension or flexion. In horses working at full speed, improper alignment or malformation can place any one of these pieces under abnormal stress predisposing the horse to injury. The majority of racehorses do exhibit conformational flaws to some degree but when these faults become exaggerated the likelihood of poor performance or injury is increased. When looking at young stock in particular (e.g., weanlings, yearlings), one should keep in mind that these flaws may become a bigger risk with time. A compressive list of conformation flaws is far beyond the scope of this entry but the following are generally considered to be undesirable. As a general rule however, anything which deviates from the proper alignment of the bony column of the leg, front or hind, can be perceived as a conformation defect.
Carpal varus bowed knees): Horses with this conformation place abnormal stress on the inside or medial aspect of their knees. At speed, this can result in an increased likelihood of carpal fractures. Overtime, it can lead to arthritis and/or collapsing of the joint. A young, growing animal with this conformation is likely to worsen until the growth plate closes at approximately 2 years. Surgical intervention in the young horse can arrest this trend.
Offset knees in a young horse can predispose to a carpal varus conformation.
Fetlock varus (toed-in): This conformation puts strain not only on the outside branch of the suspensory ligament but also potentially on the inside of the knees. Foals or yearlings may subsequently develop a carpal varus conformation secondary to this conformation as the fetlock is not properly aligned underneath the horse resulting in a ‘drift’ of the knee. Horses with severe fetlock varus conformation may interfere (i.e., hit the other leg) at speed and may develop degenerative changes due to abnormal stresses on the inside of the fetlock joint.
Carpal valgus (knock-kneed): Unlike horses with carpal varus conformation, the majority of horses are born with this conformation but improve during the first two years of life. Those with a marked valgus deviation in one or both legs that do not improve have an inefficient gait and will stress the outside of the knee.
Carpus deviated behind the vertical (back at the knee). Here, the bony column of the leg is not supported properly predisposing the horse to hyperextension of the knee and fetlock making carpal fractures and tendon injuries more likely.
Club foot: Most often found in the front feet, this is a result of contracture of the deep digital flexor tendon. These horses place abnormal concussion on the tip of their coffin bones leading to chronic inflammation. A club foot may be present from birth or develop secondarily.
Tarsal valgus/varus: Post or straight legged behind: Horses with this conformation are less able to absorb the impact of concussion at speed potentially leading to bone, ligament and tendon injuries. In general though, conformation abnormalities behind are less concerning than those in the front as most of the weight of the horse in borne in front.
Again, these are only a few of the many aspects of conformation. It should be noted also that many successful racehorses exhibit one or more of these traits to some degree. A good horse can often overcome slight flaws. However, any of the above characteristics taken to extreme are probably best avoided.