Physical Exam of the Horse Hoof
Abnormal hoof conformation has become so very common that many horse owners and veterinarians have become “numb” to it. So says Debra Taylor, DVM, MS, Dipl. ACVIM, of Auburn University’s College of Veterinary Medicine. But taking some time to become familiar with the healthy hoof can sharpen their ability to readily identify problems that could be contributing to lameness.
During a presentation at the 2013 Western Veterinary Conference, held earlier this year in Las Vegas, Nev., Taylor reviewed healthy hoof conformation characteristics and described some common and potentially function-affecting abnormalities practitioners should watch for during a hoof examination.
Hoof Asymmetry—”Although it is not always associated with lameness, asymmetry of the equine hoof should not be overlooked as a possible indication of previous, impending, or chronic lameness,” Taylor said.
Taylor said it’s important to look for asymmetry in the hoof itself and differences between paired limbs. A horse can develop hoof asymmetry as a result of uneven weight bearing caused by a variety of issues, including asymmetrical movement, stance, and tendon tension, along with pain.
Coronary Band—The coronary band is dynamic, Taylor said, and asymmetric weight bearing can influence its shape. She described a healthy coronary band (as viewed from the side) as nearly straight or with a slight upward arch. Often, horses develop one-sided coronary band asymmetry in the heel that is referred to as sheared heels. Horses with sheared heels frequently experience displacement of one hoof quarter along with the heel bulb, and they commonly develop painful conditions such as quarter cracks or thrush.
Veterinarians can use the angle the coronary band forms with the ground to estimate the position of the coffin bone within the hoof capsule. A normal coronary band angle is considered to be about 20° to 25°, Taylor said; if the coronary band angle is greater than 30°, the horse probably has an extremely low or negative palmar/plantar angle (the angle bottom of the coffin bone make with the ground; hooves with an angle of greater than 45° undoubtedly have a negative palmar/plantar angle, she added). Simply put, negative palmar/plantar angles mean the horse’s heel is being crushed.
“At the other extreme, a coronary band parallel to the ground as viewed from the side is indicative for a high palmar angle,” she said, noting this is often seen in laminitic horses or those with a club foot.
The coronary band angle could also indicate other problems within the hoof capsule. Taylor explained that the coronary band angle correlates with the normal forces the coffin joint encounters and the force the deep digital flexor tendon exerts on the navicular bone. For instance, she said, as the coronary band angle increases, the palmar/plantar angle decreases, and both the torque on the coffin joint and the force on the navicular bone increase. These biomechanical forces could initiate or aggravate heel pain, deep digital flexor tendon strain, and/or coffin joint disease.
Additionally, Taylor said, in a healthy hoof, the hair along the coronary band should lie flat on the hoof wall; consider hair pointing in an outward direction an abnormality because it could indicate excessive ground reaction forces on the hoof wall.
Hoof Walls—Healthy hoof walls are smooth; have a light sheen; are free of prominent growth rings; and lack flares, cracks, and bruising, Taylor said.
The presence of prominent growth rings can indicate a number of problems, including reduced blood perfusion in the corium (the hoof’s dermis, or the middle soft tissue layer that connects the coffin bone to the rigid hoof capsule and contains the hoof’s blood supply). resulting from abnormal hoof loading, diet changes, exercise intensity, or systemic disease. Growth rings can also indicate a negative palmar/plantar angle; Taylor said growth rings in these horses are “often wider in the toe region and narrower in the heel region due to uneven blood flow caused by overloading of the heel.”
Chronic and excessive overloading of the hoof wall also can cause flares or cracks, Taylor said.
Hoof wall bruising is indicative of trauma, she said, and bruises can form near the coronary band when ground reaction force pushes the hoof capsule against the coronary region’s vascular (blood vessel-rich) tissue. While some isolated bruises can be caused caused by a single acute event, others—typically seen as a wide band of bruising—are an indication of chronic trauma, such as laminitis, Taylor explained.
Frog—The frog is a “highly dynamic” structure that changes in response to terrain and other hoof demands, Taylor said. A frog’s width should be approximately 50 to 60% of its length, and the portion closest to its apex (point) should be substantial enough to touch the ground when the horse is bearing weight.
“If this portion of the frog does not engage the ground, fibrocartilage in the (rear) of the foot is hypothesized to develop poorly or atrophy, contributing to a weak heel,” she said.
A healthy frog has a shallow central sulcus, wide enough for a ring or index finger to fit, Taylor said. A common frog defect is a contracted (too narrow) central sulcus, which creates an anaerobic (lacking oxygen) environment ideal for thrush development. The central sulcus will remain contracted until the thrush resolves.
“When thrush gets in there, the horse may try to avoid using the back of his foot,” she said, creating a mechanical situation that can predispose the foot to lameness.
Collateral Sulci— When seeking information about potential internal problems, Taylor said, a horse’s collateral sulci (the grooves located adjacent to the frog) can be very telling. The sulci run parallel to and remain a fixed distance from the bottom of the coffin bone in the front half of the hoof and the collateral cartilages in the rear half of the hoof. The sulci should be relatively linear
and shouldn’t undulate in depth. She explained that unlike many other hoof structures, the depth and contour of the collateral sulci aren’t typically altered by hoof care efforts.
In a healthy foot, the distance between the ground and the collateral sulci, where they converge at the apex of the frog, is 10 to 20 millimeters, Taylor said. The coffin bone’s concave solar surface (located on the bottom of the bone, just above the sole of the foot) sits about 10 to 11 millimeters above this point.
There could be problems when the collateral sulci develop a stair-step or undulating shape and become significantly deeper in the heel, Taylor said: This shape, often found on horses with long toes and “underrun” heels, is likely indicative of poor heel development. She recommended veterinarians perform radiographs on hooves with this conformation to evaluate the coffin bone’s position; the palmar/plantar processes of these coffin bones might be dangerously close to the ground, she said. Some veterinarians hypothesize that the negative palmar/plantar angle is associated with lameness in not only the foot but also proximal (higher) portions of the limb.
“This conformation in hind feet may be associated with pain in the hocks, suspensory ligaments, gluteal and lumbar regions,” Taylor said.
Heel Base—Next, Taylor described the heel base, which includes the hoof wall, buttress (the back the part of the hoof that makes initial contact with the ground from a heel-first landing), sole angle (the degree between the coffin bone and a straight horizontal line), and bars. In a healthy hoof, she said, the heel bulbs shouldn’t touch the ground, and the heel tubules should be straight and nearly parallel (within 5°) to the tubules in the toe region. The heel tubules’ most palmar (furthest to the rear) weight bearing surface should be at the base of the frog.
Underrun heels—which can be caused by a variety of issues ranging from a horse’s conformation to improper trimming aand/or shoeing—can lead to an array of problems. “Underrun heels that grow forward towards the widest part of the foot often collapse under the weight of the horse, causing heel tubules to run nearly ground parallel,” Taylor explained. “The bars and the angle of the sole may be crushed, deformed, or injured as a consequence of the severely underrun heel.”
Additionally, Taylor said, researchers have recently postulated that inadequate fibrocartilage development in the digital cushion (a soft tissue structure in the hoof capsule, above the frog) is a precursor to tissue injury and lameness. She recommended practitioners become familiar with what a healthy heel looks and feels like. “A sense of normal can be learned by palpating the digital cushions of sound horses with good feet and comparing those findings with those of horses with poorly conformed feet,” she said.
In a healthy hoof, the combined tissues of the frog and digital cushion should measure about two inches, she said; hooves with combined tissues measuring less that that will likely be predisposed to injury. Underrun or collapsed heels with underdeveloped digital cushions deform easily when you apply thumb pressure, while healthy heels will not give way as readily. Horses with suboptimal digital cushion volume and fibrocartilage usually have either narrow, contracted or wide, thin underrun heels, and they likely are at risk for lameness, she said.
Hoof Sole—The sole surface should be concave, calloused, and about as wide as it is long, measuring 12 to 15 millimeters thick under the distal rim of the coffin bone, she said. A sole with this thickness can effectively protect the coffin bone from trauma. A simple way to predict sole thickness is by placing a ruler (calibrated in millimeters) in the collateral sulci at the apex of the frog, measuring the distance between the deepest part of the sulci and the bottom of the hoof, Taylor explained.
She explained that thin-soled horses have very little or no depth of the collateral sulci at the frog apex. Horses with zero collateral groove depth at the apex of the frog generally have a sole depth of less than 7 millimeters, she said, which can predispose them to sole brusing, subsolar infection, and coffin bone remodeling or rim fractures.
While not all abnormally formed hooves contribute to unsoundness, some hoof abnormalities can have potentially function-altering consequences. Taylor relayed that differentiating normal and abnormal is crucial when performing a physical exam of the hoof for both horse owners and veterinarians. “The key is to know what a healthy hoof looks like so when you see an unhealthy one, it’s recognizable,” she concluded.
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