Lameness is defined as an abnormal stance or gait caused by either a structural or a functional disorder of the locomotor system. The horse is either unwilling or unable to stand or move normally. Lameness is the most common cause of loss of use in horses. It can be caused by trauma, congenital or acquired disorders, infection, metabolic disorders, or nervous and circulatory system disease.
Lameness is not a disease per se but a clinical sign. It is a manifestation of pain, mechanical restrictions causing alteration of stance or gait, or neuromuscular disease. Pain is the most common cause of lameness in all horses. Mechanical lameness is best typified by complete upward fixation of the patella with its characteristic gait abnormality but can also be the result of fibrotic myopathy of the semitendinosus muscle or of restrictions caused by annular ligaments, adhesions, or severe fibrosis.
It is critical to correctly determine the cause of the lameness, because treatment varies greatly depending on the cause. For example, the mechanical lameness of complete upward fixation of the patella will not respond to analgesics, whereas lameness caused by pain often responds to systemic or local analgesics and anti-inflammatory drugs. Some causes of lameness produce very characteristic and classically described gaits. In fibrotic myopathy, a mechanical lameness, the affected limb is pulled back and down quickly before the end of the protraction phase, giving the impression that the foot “slaps down” on the ground. The signs are most obvious at the walk. In stringhalt, a neuromuscular disorder, the affected limb is hyperflexed during the cranial or swing phase, while the stepwise caudal jerking movement before foot contact does not occur. Unfortunately, many causes of lameness do not produce a characteristic gait abnormality, making diagnosis a challenge.
Pain-related lameness can be classified as weight bearing (supporting leg) or nonweight bearing (swinging leg) lameness. Although lameness is most often observed as a weight-bearing deficit, it may be composed of both. A supporting leg lameness is seen when the horse reduces the amount of time or reduces the amount of force applied to the weight-bearing limb. The most consistent and easily recognized clinical signs of lameness are the head nod associated with forelimb lameness and the sacral rise, also called a pelvic rise or hip hike, associated with hindlimb lameness. Hindlimb lameness should be assessed from the side as well as from behind, because this provides an opportunity to assess arc of foot flight, duration of protraction and retraction phases, length of weight-bearing phase, and the presence or absence of a sacral rise. Forelimb lameness should be observed from the front and side. Hindlimb and forelimb lameness in many horses will be accentuated when the horse is worked in a circle with the affected limb on the inside.
Factors that predispose horses to lameness include physical immaturity, which may occur in premature or dysmature foals, and training older foals before maturity. Other factors include preexisting developmental orthopedic disease (eg, osteochondrosis, flexural limb and angular limb deformities); poor conformation; improper hoof balance or shoeing; failure to adequately condition performance horses; monotonous repetitive stresses on bones, tendons, ligaments, and joints in performance horses; hard, slippery, or rocky surfaces upon which horses work; and extremely athletic activities. Inciting factors in lameness include direct or indirect trauma, fatigue resulting in incoordination of muscles (which often occurs in racehorses at the end of races), inflammation, infection, and failure to recognize early disease before it creates significant pain.
Lameness in one part of a limb often results in secondary soreness in another area of the same limb and may result in lameness of the contralateral forelimb or hindlimb from overuse due to compensation. The entire horse should be evaluated for secondary lameness even when the cause of the primary problem is obvious. Secondary lamenesses are very common in performance horses but may occur in all types of horses. A dramatic example of a secondary lameness occurs when biomechanical laminitis develops in the normal contralateral limb of a horse with limited weight bearing from a severe orthopedic problem causing shifting of weight from the injured limb to the normal limb.