Lameness is an abnormal gait or stance of an animal that is the result of dysfunction of the locomotor system. In the horse, it is most commonly caused by pain, but can be due to neurologic or mechanical dysfunction. Lameness is a common veterinary problem in racehorses, sport horses, and pleasure horses. It is one of the most costly health problems for the equine industry, both monetarily for the cost of diagnosis and treatment, and for the cost of time off resulting in loss-of-use.Seitzinger AH, Traub-Dargatz JL, Kane AJ, Kopral CA, Morley PS, Garber LP, Losinger WC, & Hill GW. "Comparison of the Economic Costs of Equine Lameness, Colic, and Equine Protozoal Myeloencephalitis (EPM)". Proc. ISVEE, 2000, Vol. 9.
Causes of lameness Lameness is most commonly caused by pain, but may also be the result of neuromuscular disease or mechanical restriction. Lameness itself is a clinical sign, and not a diagnosis.
Pain Pain is the most common cause of lameness in the horse. It is usually the result of trauma or orthopedic disease, but other causes such as metabolic dysfunction, circulatory disease, and infection can also cause pain and subsequent lameness.
Orthopedic causes of lameness are very common and may be the result of damage to the hoof, bone, joints, or soft tissue. Horses are predisposed to orthopedic lameness by conformational flaws, poor hoof balance, working on poor footing, repetitive movements, poor conditioning for a given activity, and competing at a very high athletic level.
Metabolic causes of lameness include hyperkalemic periodic paralysis (HYPP) and polysaccharide storage myopathy, which directly affect muscular function.
Circulatory causes of lameness occur when blood flow to an area is compromised. This may be due to abnormal blood clotting, as in the case of aortic-iliac thrombosis, or decreased blood flow (ischemia) to an area, such as is sometimes seen in laminitis.
Infectious causes of lameness are the result of inflammation and damage to tissue. These include problems such as cellulitis, hoof abscesses, and septic arthritis.
Mechanical lameness Mechanical lameness is caused by a physical abnormality, such as scar tissue, that prevents normal motion of a limb. Mechanical lameness does not cause pain. Classic examples of mechanical lameness include upward fixation of the patella and fibrotic myopathy, but any type of adhesion (often secondary to scarring during healing post-injury) or fibrosis can cause mechanical lameness. Upward fixation of the patella occurs when the normal locking mechanism of the stifle, which allows the horse to "lock" its hind legs into a fixed position so it may stand with minimal effort, engages at inappropriate times such as when the horse is walking. It results in an inability to flex the stifle, so the horse must walk with an extended hind leg. It is associated with straight hind limb conformation (post leg), poor muscling of the hind limb, stifle trauma, and genetics. Fibrotic myopathy is caused by damage to the hamstring muscles, usually from trauma or intramuscular injection. It results in the formation of scar tissue and produces a characteristic gait where the horse prematurely "slaps" the ground with its hind foot, shortening the stride length in the damaged leg.
Neurological lameness Neurologic lameness may be the result of infection, trauma, toxicities, or congenital disease. Neurological evaluation of a lame horse may be warranted if the cause of the lameness is not obvious. Signs more commonly associated with a neurologic cause include unilateral muscle atrophy, paresis, paralysis, or dysmetria. Neurologic causes of lameness include:
Cervical vertebral stenotic myopathy (Wobbler disease): compression of the spinal cord in the cervical (neck) region results in lameness, ataxia, and change in gait, especially in the hind legs, and neck stiffness or pain. Stringhalt: caused by damage to the long digital extensor muscle, or from eating the weed Hypochaeris radicata; it produces a gait characterized by rapid, spasmodic hyperflexion of the hind limbs. Shivers: a rare neuromuscular disorder characterized by muscle tremors, difficulty picking up the hind feet when asked to lift for farriery work, hyperflexion or hyperextension of the hind limbs, and abduction of the hind limbs. Normal athletic function is often maintained, at least initially. Cerebellar abiotrophy: a rare genetic disorder that occurs in Arabian horses, producing ataxia, a base-wide stance, proprioceptive deficits, and high-stepping gait. Damage to individual nerves will affect the muscles they innervate and subsequently alter gait. Radial nerve paralysis will cause a dropped elbow and make it difficult to extend the affected limb. Suprascapular nerve damage will lead to atrophy of the main muscles of the shoulder (sweeny). Femoral nerve paralysis causes hyperflexion of the stifles, hocks, and fetlocks and the horse usually walks on the toe of the hind foot. Infectious causes affecting the nervous system may be bacterial, viral, protozoal, or rickettsial. Those that commonly alter gait include tetanus, botulism, Lyme disease, equine protozoal myeloencephalitis (EPM), rabies, West Nile virus, equine encephalitis virus, and equine herpesvirus 1. Neoplasic (cancerous) changes in the brain or around the spinal cord may also cause alterations in gait.
Signs of lameness Manifestations of lameness include any alteration in gait from what is normal for the horse. In general, it is harder to detect hind limb lameness when compared to lameness in a front limb when using visual cues.
Identifying a front limb lameness The classic sign of lameness in a front leg is a prominent "head bob." Viewed from the side, the horse raises its head and neck when the lame leg hits the ground, which helps to unload the lame leg. This is sometimes remembered by the adage "down on sound." A head bob is usually easy to see when one leg is lame, but can be subtle in very mild unilateral lameness, or in the case of bilateral front limb lameness. A horse may also try to reduce impact on a lame front leg by tensing of the muscles of the shoulder. In this case, it will stiffen the limb just before it hits the ground, a sign that may be noticed by an astute observer.
Identifying a hind limb lameness Evaluation of hind limb lameness can be difficult. Generally, the viewer watches the hip, sacrum, gluteal muscles, or hemi-pelvis (pelvis of one side of the body) when examining lameness in the hind end. Examination should be performed both watching the horse from behind while trotting away from the examiner, and from the side as the horse passes.
When watching from behind, the viewer often looks for a "hip hike" or "pelvic hike." This occurs when the horse raises the pelvis on the lame side as it is bearing weight, trying to shift weight off of the painful leg in a manner similar to raising of the head in front limb lameness. This is a sudden, short upward motion of the hemi-pelvis or gluteal muscles. The hip of the lame leg does not always rise above the level of the hip of the sound leg, which can be confusing to those looking for a "hip hike." Instead, it is an exaggerated upward movement that is watched for during weight bearing. Additionally, horses with a hind limb lameness will tend to reduce the degree of leg use. To do so, some horses will reduce the contraction time of the gluteals on the side of the lame leg, leading to a "hip roll" or "hip dip" and appearance that the hip drops a greater degree on the side of the lame leg.
These apparently contradictory statements ("hip hike" vs "hip drop" on the lame leg) can be explained by when the horse perceives pain during the stride. If the pain is perceived during the early stance phase of the stride, the horse will try to unweight the leg and produce a "hip hike." If the pain is perceived during the pushoff phase of the stride, the horse does not push with as much force, subsequently reducing the height the pelvis is raised, and leading to a "dipping" motion during the swing phase of the stride as the horse brings the limb forward. In some cases, both a hip hike and a hip drop can be seen in the same horse.Keegan KG. Head Nods and Hip Hikes. Western Veterinary Conference 2013. Some practitioners simply look for the side with the greater overall deviation, which occurs in the lame leg
Another method to detecting hind limb lameness requires watching the pelvis from the side. The entire pelvis is evaluated, and its relative position vertically (relative to the ground) is compared at various points in the stride. The pelvis normally rises and falls with each step. Horses with pain during the early stance phase of the trot will not allow the pelvis to drop as much when the lame leg is weighted, trying to avoid extra weight on the painful leg. Subsequently, the pelvis will not fall as much downward when the lame leg is bearing weight as when the sound leg is bearing weight. When viewed from behind, this is seen as a "hip hike." If the horse is more painful as it is pushing off a limb, it will not push off with as much force, and the pelvis will rise less on the lame side relative to the same point of the stride when the sound leg is on the ground. This is seen as the "hip roll" when viewed from behind.
Other signs that indicate hind limb lameness include "plaiting" with the hind feet, which can be seen in cases of sacro-iliac pain, carrying the hindquarters to one side or drifting in one direction,Dyson SJ. "Lameness Associated with the Stifle and Pelvic Regions. Proc. AAEP, 2002; Vol. 48; 387-411. decreased impulsion, and the saddle slip to one side.
Other signs of lameness Several other signs are applicable to both front and hind limb lameness. One method is to look at the relative time a leg spends in the cranial (forward) phase of the stride. For a front leg, this is when the lower leg is in front of the horse, i.e. angled forward, while the caudal phase is when the leg is underneath the horse, i.e. angled backward. For a hind leg, the cranial phase occurs when the lower leg is under the body of the horse, and the caudal phase is when the limb is out behind the body of the horse.
A normal horse with have a cranial phase and a caudal phase of equal length: the horse will bring the leg as far forward as it does backward. In a lame horse, the cranial phase will be shorter when compared to the caudal phase, so it appears to spend more time with the leg backward than it does forward. A shortened cranial phase is most commonly seen in cases of bilateral lameness, lameness of the upper limb (such as shoulder or hip pain), and osteoarthritis of the hocks.
Decreased fetlock drop during the stance phase of the stride may be seen in cases of lameness, with the lamer leg producing less drop than the sound leg as the horse tries to relieve weight on the painful limb. Decreased height to the stride (flight arc), or dragging of the toes, also indicates lameness, as the horse avoids bending its joints.Clayton, Hillary. Cinematographic analysis of the gait of lame horses IV: Degenerative joint disease of the distal intertarsal joint. J Equin Vet Sci. 1986; 7(5): 274–278. In the front limb, decreased flight arc is usually seen in cases of shoulder, knee, or fetlock joint pain, and is often associated with reduced cranial phase and lengthened caudal phase of the stride. At times, lameness may be heard. Usually the horse has a stronger, louder sound on the beat where the sound hooves hits the ground, but a softer, less resonate sound occurring on the beat where the lame leg is hitting the ground. Again, this is because the horse is landing with less force as it tries to avoid weighting the painful leg.
Lastly, behavioral changes and decreased performance may indicate pain, even if obvious clinical lameness is not evident.
The lameness evaluation