Hoof Capsule Distortion And Its Relationship To Foot Lameness
Scott Morrison DVM
Rood and riddle Equine Hospital
The foot is well recognized as a common source of lameness. It has been estimated in multiple retrospective studies that the heel region alone accounts for more than one third of all chronic lameness in the horse. It shouldn’t be a surprise that the only region of the horse’s body in constant contact with the ground is most susceptible to trauma and injury. The foot receives the initial forces generated during ground impact. A healthy foot significantly dampens the vibrations generated during ground impact.1 The heel region in particular is designed for this purpose and houses the structures most responsible for absorbing shock such as the frog, digital cushion, collateral cartilages, bars and an extensive vascular system. All these structures work together in harmony to absorb and dissipate shock. Therefore for the heel to accommodate this task it needs to have healthy and fully functional structures.
At higher speeds, when shock absorption is most needed, the heel should contact the ground first. The phases of the stride and foot loading sequence should be a coordinated pattern of heel contact, loading, breakover etc.. Those which land toe first, don’t utilize the heel to absorb shock and as a result commonly present with lameness issues further up the limb. Horses with low heels and long toes appear to be prone to toe first landing. These cases typically have a negative palmer/plantar angle of the distal phalanx relative to the ground. Repetitive toe first landing is an inefficient and potentially damaging landing pattern. The normal foot loading pattern is out of sequence. It can be seen in horses that are fatigued or lazy and don’t fully extend the shoulder, it can also be seen in the long toe, low heel syndrome or in cases which have preexisting heel pain and land on the toe in an effort to protect the heel. It is however, normal for horses to land toe first when landing from a jump.2 The height and arc of the jump affect the degree of toe first landing. Besides bypassing the heel’s shock absorbing function, toe first landing has been shown to actually put more strain on the deep digital flexor tendon and navicular apparatus.3 The point of initial ground contact not only affects the foot, but also all the structures of the limb. A normal heel first landing puts a gradual increase in strain on the deep digital flexor tendon, whereas toe first landing puts abrupt high peak strains on the tendon, almost like a snapping action. Visualize a normal horse galloping, as the limb is fully extended the heel impacts the ground, as the horse’s body passes over the limb (and the limb is now in the vertical position) the toe region sinks into the ground while the fetlock displaces downward (full extension of fetlock joint). This pattern slowly increases tension on the tendon; in fact as the limb is fully loaded the toe is sinking into the ground creating a heel wedge effect, which helps take some strain off of the tendon during this position. In a toe first landing pattern the toe would impact the ground, then as the body passes over the limb the heel rocks backward as the fetlock descends downward. This "out of phase rotation”2 of the distal interphalangeal and fetlock joints have been shown to put high strains on the deep digital flexor tendon3 and navicular apparatus. Horses which are used for jumping tend to be predisposed to "navicular region pain”. In fact MRI studies of horses with navicular region pain often show the damage and pain is to the deep digital flexor tendon4. It is also fairly common to see lameness in areas further up the limb in cases with chronic heel pain. Suspensory ligament desmitis is a common sequel in these cases. In our clinic it is fairly common for horses with chronic heel pain to also present with suspensory ligament desmitis. A possible explanation for this is that toe first landing increases stress to the suspensory ligament by the "snapping action” of the out of phase rotation, combined with the decreased shock absorption provided by the heel region.
To minimize the occurrence of such injuries, the foot needs to be properly balanced and shod. When the foot is allowed to get out of balance causing abnormal weight distribution, the hoof capsule becomes distorted and susceptible to injury. A healthy foot is recognized as having adequate sole depth, good bar structures, robust collateral cartilages and digital cushion. Having strong well developed anatomical components is essential for normal foot function. The spatial arrangement of these structures is important as well. The buttress of the heel (base of support should be located at the widest part of the frog, the center of the coffin joint which correlates to the widest region of solar surface should be in the center of the foot’s weight bearing surface. The breakover point and base of support should be located an equal distance from the center of the coffin joint.
Structures of the foot must have proper form and spatial arrangement to function properly. Ideally the coffin joint should be in the center of the foot’s weight bearing surface. Many feet develop low heels and a longer toe. This puts the foot out of balance, the coffin joint is placed more towards the back of the foot, increasing load on the heel region and creating a lengthy toe lever, putting more strain on the deep digital flexor tendon and potentially contributing to or exaggerating a toe first landing. Some feet naturally have a low heel and longer toe and can’t be made normal, but they can be managed and helped with trimming and shoeing. Rolled toes, rockered toes, fitting the heels with adequate support and maintaining the center of articulation of the distal interphalangeal joint in the center of the foot’s weight bearing surface are ways to compensate.
Besides good farriery and foot balance, the ground conditions or footing is also an important factor. Footing that is too soft or too hard can interfere with the dynamics of limb loading. Ideally the footing should be soft enough to allow the toe to sink in, look at a foot print in a grass field, this is probably ideal. Footing that is to soft will put more stress on soft tissue structures and footing that is too firm can cause foot bruising and generally causes more wear and tear on the boney column. Management is also important in prevention. Keep the feet on strict shoeing schedule and don’t let the toes get too long. In the shod horse the hoof angle decreases by a mean of 3.5 degrees in an 8 week period.5,6 Therefore working a horse that is overdue for trimming/shoeing increases the risk of injury. And finally keeping the horse fit and well conditioned for his/her job can eliminate fatigue as a source of abnormal foot landing patterns.
The horse’s foot is capable of handling huge impact forces without structurally collapsing. This is due to the fact that when a horse is traveling, the moving foot fills with blood during the swing phase. Probably from centrifugal force filling the non-weight bearing tissue maximally with blood, creating turgor pressure. This fluid in closed spaces may help support the architecture of the foot during ground impact. It is with the help of this mechanism that the foot is able to withstand huge impact forces. Most hoof capsule deformities (underrun, collapsed heels) slowly develop over time. I believe most of these distortions occur while the foot is semi-static (while the horse is just standing around). It is during this period that the foot is mostly dependent on the architecture of the foot tissues for support. Long-term low magnitude loading is more likely to create distortion rather than short-term high magnitude force.7,8 Horses standing in a stall with little arch/sole support slowly fatigue the integrity of the capsule and propagate distortions. The analogy would be a steel beam being able to lift a big truck without bending or bowing, but take that same beam and suspend a lighter load from it for many years and it begins to sag and bow. This is probably very similar to the etiology of hoof capsule distortions. The heel region tends to show hoof capsule distortion first since it is generally made up of softer more elastic structures than the toe. Distortions are portrayed as flares, flattened sole arch, underrun heels, collapsed heels and gross assymetries. Mild hoof capsule asymmetries are normal. Hoof capsule shape changes in response to loading and conformation of the limb above can best be appreciated by observing the foot of a foal as it conforms to the changing limb conformation.
In the growing horse, trimming and shoeing can have a huge influence on the growth and development of the boney column. However in the adult horse, the limb conformation cannot be changed only managed. Several trimming and shoeing mechanisms can be implemented to prevent these feet from becoming severely distorted and dysfunctional. Additionally shoeing modifications can influence the point of ground contact, offer additional support to the limb and help optimize the efficiency and loading characteristics of the limb. Care must be taken when implementing shoeing techniques as often there is a boundary between offering support and applying too much leverage or force. Simply stated there is a line between under shoeing, over shoeing, and getting it just right for each individual horse or limb. The concept is particularly important in athletes.
The heels should be fit to the widest part of the frog. The break over point is positioned at a point equidistant in front of the coffin joint (equal distant in front of the widest part of the foot as the heel fit is behind) Fitting the foot symmetrical to the foot generally means fitting the shoe equidistant to the frog. Severely compromised or distorted feet usually require some form of axial support to redistribute weight off of compromised areas of wall. Axial support refers to recruiting structures within the margins of the wall for support. Shoe modifications such as heartbars, broadened branches, onion heels, heel plates, and sole support materials are various options. A good evaluation of the foot, footing, environment and discipline is required to properly utilize each of these.
The club foot is recognized as having strong heel structures and plenty of heel mass; however this foot type can also be affected with chronic heel pain. Since the club foot overloads the toe and bone column, arthritis, sidebone, pedal osteitis of the apex of the P-3, navicular bone sclerosis, osteoarthritis, and contracted heels are common pathologies seen. The compressive forces on the navicular bone are increased as the DDFT is pulled taught against the flexor surface of the navicular bone. It has been shown that upright or club feet have thinner fibro cartilage layer compared to normal feet. Most likely a result of the increased compressive forces of the DDFT against the flexor surface of the navicular bone. Club feet often have increased wall growth in the heels and slow wall growth at the toe. This is the foot’s attempt to raise the heels and unload or accommodate the contracted deep digital flexor tendon. The foot remodels to accommodate all phases of the stride. Since the deep digital flexor tendon is under the most tension just before heel lift off (break over), it is this phase of the stride, which must be addressed when re-balancing the clubfoot. It has been my experience that significantly enhancing/easing break over can allow these feet to return to a normal appearance.
Most clubfeet can achieve equal toe/heel growth and resolve the anterior dish with these simple mechanics. It is important to realize that we are not "fixing” or resolving the contracture, we are merely accommodating it with simple shoeing mechanics and allowing the foot to return to a more normal shape, with even wall growth, no dish, good anterior sole depth, and therefore be a stronger, healthier foot.
High speed video and gait analysis studies are needed to better understand how these club feet respond to this shoeing but it appears to me from observing these horses and talking to owners, these horses stride out and load the heels more. It is my hypothesis that these don’t have to take a "short step” to accommodate the tendon in the caudal phase of the stride. Since the break over is eased they lengthen the caudal phase and thus take longer strides. The other possibility is that they are just more comfortable in this style shoe and change their gait for the better. So when presented with any lower limb lameness which may be secondary to the tendon contracture, my first approach is to re-balance the club foot by trimming the heels down and moving the breakover point back beneath the anterior coronary band. This makes the foot behave or function more like a normal foot and often times the secondary lameness resolves or greatly improves. As these feet load the heels, the heel contracture tends to improve with time and use, however, in cases with atrophied frogs and robust bars, I encourage heel spreading by thinning out the bars, unhooking the point of the heels, and loading the frog, sulci, and bars with elastic impression material These feet often benefit from building the material up slightly above the ground surface of the shoe. Almost creating a "bumper” or artificial frog. This technique helps engage the foot’s shock absorbing structures.
Contracted heels can occur as a result of a chronic heel pain condition in which the heels have not been loaded normally. Heel contracture can also be seen in club feet from mechanical under loading of the heel region. Contracted heels should be differentiated from collapsed heels; some heel collapse cases can also look contracted. Collapsed and under run heels occur from overloading and bending of the horn tubules, whereas heel contracture results from under loading, the horn tubules are usually straight and strong and aren’t bent or folding over. Heel contracture can become a secondary source of lameness, worsening the lameness grade, and contributing to the cycle of abnormal landing, foot dysfunction and lameness.
Most heel contractures and collapse improve as the foot is rebalanced and supported. Severe cases of heel contracture may require a spring shoe application for 1-2 shoeings to achieve significant correction, then maintained as previously described.
Sheared heels are an interesting combination of focal overloading, hoof capsule distortion and possible lameness. Sheared heels are when one heel bulb and heel quarter is displaced proximally compared to the adjacent heel bulb. Sheared heels are generally seen medially on front feet. Overloading of the medial heel is the cause and is usually a result of conformation or poor hoof care resulting in imbalance. In many cases a conformation fault such as outward rotation or a base wide stance can be appreciated as the culprit. However I have also seen sheared heels in cases with good conformation. Potential contributing factors in these cases can be trimming, shoeing or long term overloading of the foot/heel secondary to a problem in another limb.
As with all hoof capsule distortions the portion of the foot under the most load becomes more vertical and the region under less vertical load develops a flare. Histological studies by Dr. Bowker’s lab have shown that there is a greater concentration of laminae on the flared side of the hoof.9 The stimulus for laminae to be produced may therefore be from a tearing (horizontal type force) rather than vertical force. This scenario leaves the hoof capsule under the most vertical load vulnerable to distortion (sheared heels, displaced coronary bands, cracks etc.)
In the sheared heel case, as the heel becomes overloaded the heel quarter becomes vertical and contracts inward toward the base of the frog. In doing so it becomes more under the limb or vertical force vector, thus increasing the load further on the sheared heel, displacing it upward. Quarter cracks, bruising, inflammation of the wing of the distal phalanx and wall separations commonly accompany the condition.
Trimming and shoeing techniques to treat sheared heels have varied in practice and in the literature.10 Some advocate trimming the sheared heel lower other advocate trimming the opposite heel lower. Since the sheared heel quarter is displaced upward, the wall length becomes longer on that side. However if you take a radiograph of the foot the coffin bone is lower (closer to the ground on the sheared side. So we have a condition in which the wall length is longer, the sole depth is thinner and the coffin bone is tipped to that side. Is it more important to match the wall length and try to make the coronary band parallel to the ground? Or should the foot be trimmed to make the solar border of the distal phalanx parallel to the ground and achieve even sole thickness lateral to medial? This is often a subject of debate. Observing how limb position and digital loading patterns respond to alterations in medial-lateral balance can help decide how to trim and shoe for this condition.
If the lateral heel is trimmed lower the limb postures with a base narrow stance, on the other hand if the medial heel is trimmed lower the limb will respond with a base wide posture. If the lateral toe is trimmed lower the limb will take a toe in posture and if the medial toe is trimmed lower the limb will toe out. Basically there are four quadrants of the foot that can be manipulated, thus altering posture. The quadrant most directly beneath the bone column receives the most vertical force. Therefore lowering the medial toe and heel in a case with a medial sheared heel will only exacerbate the condition. The foot should be first trimmed to achieve normal proportion and spatial arrangement of structures: heel positions trimmed back to widest part of frog, widest point of foot (DIP joint) centered in the middle of the weight bearing surface. Then the height of the heel can be adjusted to achieve distal phalanx alignment in the frontal plane. The addition of a bar, heart bar or pad can help redistribute the force to the frog and relieve the displaced heel. Removing an area of wall, shoe or pad to create a space between the hoof and the shoe or "Floating the quarter” is also helpful in allowing the heel to drop back downward.
Interestingly the most effective way I have found to correct a sheared heel is trimming for medial to lateral distal phalanx alignment and using a spring shoe to open the contracted half of the foot. The sheared heel can be considered a unilateral contracture. The contracted heel then becomes displaced upward as it is more in line with the vertical line of force.
Therefore "pushing” that heel out gets it away from the vertical force line and allows it to drop down. This technique can be effective without any flotation methods. Horses can continue their normal riding/exercise routine while wearing the shoe. Once the sheared heel is corrected the foot will need to remain in a shoe with frog support to prevent the heel from displacing again.
The manner in which the foot contacts the ground and loads affects the health of the foot as well as the structures above it.
The viscolelastic nature of the hoof capsule allows it to change shape in response to loading under normal circumstances feet reveal uneven loading as mild asymmetries in hoof shape. More severe hoof capsule shape changes are referred to as distortions and are the result of long term abnormal loading patterns. Examples of hoof capsule distortions are flares, displacement of coronary bands, dishing of dorsal hoof wall, cracks, sheared heels, contracted heels, under run heels and collapsed heels.
Abnormal loading can be over or underloading of one region of the foot. Abnormal loading patterns can be the result of conformation, trimming/shoeing practices and lameness.
Trimming and shoeing can help redistribute load and improve or resolve hoof capsule distortions. Although the adult conformation cannot be changed, trimming and shoeing techniques can alter point of ground contact, weight distribution during the loading phase and decrease the influence of the deep digital flexor tendon on hoof capsule distortion in the case of club foot syndrome.
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Common Ailments of the Foot