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LOW PALMAR OR PLANTAR ANGLE
(Click on any image to see a larger version)
What is it?
The triangular bone inside the hoof is known by several different terms: distal phalanx, P3, coffin bone, pedal bone. In the front feet, the two corners of the coffin bone that are closest to the heels are known as the palmar processes. In the back feet these are known as the plantar processes.
Ideally, when viewed from the side on radiograph, the lower perimeter of the coffin bone should be positioned so that the heel end is slightly higher than the toe end of the bone, up to about 5º. This allows room for the back of the bone to descend within the hoof as the heels of the hoof capsule expand during movement.
In some horses the coffin bone has repositioned so that the palmar or plantar processes are lower than ideal, and may even lie below the horizontal plane. This is known as ‘low palmar angle’ in the front feet and ‘low plantar angle’ in the back feet, and may occur in either the fore or hind feet, or sometimes in all four feet.
How does low palmar/plantar angle affect the horse?
This radiographic image shows a hind foot with a negative plantar angle of about -1.5º.
The red line drawn through the centre of the coffin bone and two pastern bones shows how the first pastern bone (P1) is positioned at a sharper angle relative to the two lower bones, resulting in the joint being kept in an extended position.
Extension of the joint between P1 and P2 generally only occurs briefly in a moving horse as the hoof rolls over to lift off the ground. The three bones should be in alignment in a stationary, resting horse. In this extended position, increased tension on the deep digital flexor tendon is continuous, which then puts constant pressure on all structures in the navicular region, possibly causing a considerable degree of pain or discomfort to the horse.
A low palmar/plantar angle also affects all the joints higher up the leg as each joint is forced into a more open, extended, position than is normal. This in turn can stress those joints and associated soft tissues, resulting in discomfort or pain in any or all of the joints above the hoof. It is the equivalent of we humans walking on our heels while our toes are elevated, which is virtually guaranteed to cause aches and pains in the ankles, knees, hips and lower back.
The horse compensates for this chronic discomfort by adjusting overall body posture to take pressure off the extended joints of the limbs. The horse may appear to be overly straight in the hind legs despite good conformation, and may have stifle or hock problems. Some horses become tightly braced through their shoulders, back or loins, showing an uneven spinal profile over the lumbar and croup region with virtually no pelvic flexion in any direction. Other horses adopt a camped-out stance, or may stand in a saw-horse posture even when the feet themselves are not uncomfortable or laminitic. A short step length and toe-first landing with the front feet can be a sign of a low palmar angle in one or both forefeet. In the hind feet low plantar angle may manifest as a short step length with the toes being stabbed into the ground, especially noticeable at trot.
The stay apparatus of both the front and hind limbs, which enables a horse to rest or sleep whilst standing with no muscular effort, depends on an even balance between the extensor and flexor systems of each limb. That even balance is disrupted when any or all of the four coffin bones are not sited properly within the hoof; the horse then needs to put some energy into the muscular effort required to maintain his balance and posture. Consequently, the horse may appear to tire easily, showing low energy or stamina despite adequate nutrition and conditioning.
What does low palmar/plantar angle look like?
The external appearance of the hoof is not a reliable indicator of how the bone inside is placed. Low palmar or plantar angle is often associated with crushed or under-run heels and a long toe, or wide quarter flaring with hoof tubules that converge at ground level.
It would not have been surprising to find the left hind foot (medial view) shown here had a negative plane orientation to the coffin bone within; the subsequent radiograph (lateral view) showed a positive angle of about 3°.
Conversely, some horses with an upright, boxy hoof shape and deep collateral grooves have been found on radiograph to have low palmar or plantar angles to the coffin bones. The images below show an example of a horse with a small, upright hind hoof form that was concealing a coffin bone with a negative angle of approximately -1.5°.
Radiographic imaging is the only way to accurately determine the orientation of the bone inside the hoof, but the horse may display some clues in body posture and movement as indicated previously. Both horses in the two examples above had significant body issues that were not initially associated with the internal structure of their hind feet.
Some, but not all, horses with a low palmar or plantar angle to the coffin bone may show distortions in the external sole of the hoof as depicted in the photo below, which should not be confused with healthy sole concavity.
Photo courtesy T. Bauer
This foot has a dip, or ‘cup’, between the tip of the frog and the toe. There is no clear junction between the sole and the hoof wall at the toe; the white line has migrated forwards along with the curved extension to the sole.
For comparison, this photo shows a normal, healthy foot where there is a distinct junction between the sole and hoof wall.
Testing for low palmar/plantar angle
Whilst radiographs will give an accurate picture of the siting of the bone relative to the hoof capsule, an indication may sometimes be derived by fixing experimental temporary soft pads below the horse’s heels. After compression by the weight of the horse, the wedged pads elevate the heels by about one centimeter or so. This will also change the angulation of all the joints above the coffin bone. Even when radiographs are available showing a palmar or plantar angle of 4°or 5°, it should not be assumed that is sufficient angle for that particular horse, as experimenting with raising the heel area may still produce significant improvement to the way the horse moves.
If the horse does have a low palmar/plantar angle to the coffin bone, elevating the heel area will provide instant relief from the chronic strain of over-extended joints, and can often be seen within the first few steps after the pads have been attached.
Boots with inserted wedge pads are ideal for this experiment, but unlikely to be readily available for most horses where low palmar or plantar angle is initially suspected. The photos below show how to make a simple experimental soft pad to place under the heels of each foot, about 2cm - 3cm thick to allow for some compression under the weight of the horse, so the finished wedge elevates the heels by about 1 cm. Pads should be fixed under both front feet, or both hind feet, or all four feet if low bone angle is suspected in all four feet.
Rolled up towelling works well, but anything soft can be used. Use duct tape to secure the pad to the sole, taking care to keep the tape below the hairline.
Allow the horse to stand in the pads for about 5 minutes before walking out. This will compress the excess thickness of the pad and allow the horse time to adjust to the different feel. Walk the horse on firm ground, not a soft arena surface, as that will clarify any differences in gait. Ensure the horse walks with some energy, a ‘marching’ pace, as low energy in itself may result in a short step and toe-strike that will mask any changes from the raised pads. It is helpful to walk and trot the horse, before attaching pads, on the same surface so that comparisons are easy. Videoing both ‘before’ and ‘after’ sequences are especially useful for assessing any changes.
Observe any changes in step length, willingness to walk out, general demeanour etc as the horse walks in the pads. If the horse remains calm, it may be possible to try a short distance of trotting. If a low palmar/plantar angle is present, changes to step length can often be seen immediately, but the horse should be walked for 5 to 10 minutes to assess if there is any meaningful difference. With some horses, little difference can be seen at a walk but it becomes obvious when trotting. Without pads, many horses with a low plantar angle will stab the hind toes into the ground; when pads are attached the horse starts to place the heels down first almost immediately, as shown in the photos below.
Walk BEFORE raising heels
Trot BEFORE raising heels
The photos on the left above show a horse in walk and trot with bare feet, landing toe first. His body is stiff and braced with a short step length.
Walk AFTER raising heels
Trot AFTER raising heels
The photos on the right above show the same horse a few minutes later with raised, soft pads taped beneath the hind heel area – landing heel first. His body is less braced, moving with a longer step.
If no difference can be easily seen in the horse, remove the pads as it can be assumed that low palmar/plantar angle is not relevant for that particular horse. If there has been a clear positive change and the horse appears to be comfortable with the pads, they can be left in place for a few hours provided the horse is on a safe non-slip surface.
A number of horses have struggled to be comfortable barefoot on gravelly surfaces despite several months or even a couple of years of diet change and magnesium supplementation. They may have developed rock-hard soles even when living on wet ground, but move with an exaggerated action (hyper-extension) in the front limbs that can be mistaken for a good heel-strike, but have a short, stabbing toe-strike in the hind limbs. These horses are extremely reluctant to move at all on stones or gravel, but are instantly able to walk out confidently on gravel with bare front feet when their hind feet are fitted with soft wedge pads. This would appear to indicate that in some horses at least, what is perceived to be forehoof tenderness, is in fact originating in the hind feet. The problem may be pain around the navicular structures and coffin joint, not necessarily laminar or sole weakness as is easily assumed.
More research work is needed in this area to determine exactly which joints are being affected and what other factors may be involved.
This horse was initially unable to walk on the gravel surface, yet moved confidently over the sharp stones within 10 minutes of fixing soft pads under her hind heels; her front feet remained bare.
What causes low palmar/plantar angle?
The precise pathway by which any individual horse has acquired a low angle to the coffin bones is usually unknown. There are likely to be several factors involved but a weak laminar connection and/or soft, thin digital cushion is generally present. The hoof wall is held tightly to the bone inside by strong, velcro-like lamina that cover the inside of the hoof wall and the outer surface of the bone. At the back of the foot, strong, thick, lateral cartilages and digital cushion provide support to the rear of the coffin bone, limiting any repositioning of the bone even when the lamina are temporarily compromised. If the lamina connection is weak or stretched, and there is inadequate support within the back of the foot, the positioning of the bone inside the hoof can change in a number of undesirable ways.
It is possible for the bone to tip downwards, as in founder, or to be become displaced to one side, or to tip backwards as shown in the example below.
On this foot, the distance between the bone and the exterior hoof wall is wider at the coronet than at the toe, yet still has a positive 1° palmar angle.
Images courtesy of A. Till
As horses naturally bear more of their weight on their front feet than their hind feet, hoof problems including laminitis, ringbone and sidebone are more commonly seen in the front feet. Any chronic discomfort or pain in the front feet may induce the horse to transfer extra weight to the back feet to relieve the discomfort, taking more weight on the hind feet than nature intended. This may possibly cause the bone inside to sink downwards at the heels if the digital cushion and lateral cartilages are insubstantial. The horse has effectively transferred the discomfort from the front feet to the back feet, but is no closer to being completely pain-free and sound.
Some horses with a healthy, tight lamina connection may still acquire a low palmar or plantar angle, as a result of chronic body bracing, possibly due to magnesium deficiency. Lack of magnesium can cause a shortening and tightening of the major muscle groups, and a consequent shortening of the step length that prevents the horse from moving normally with a heel-first landing. Other problems in the front feet, such as ringbone and sidebone, may also cause the horse to move with a short step length in all four feet, which in itself may prevent the horse from being able to land heel-first. Without the stimulation of ground contact for the frog that occurs in normal heel-strike movement, it is likely the lateral cartilages and digital cushion will not have developed the fibrocartilage necessary for their role in supporting the back of the foot, and thereby preventing displacement of the coffin bone.
How to fix the problem
Along with appropriate trimming, dietary changes (including magnesium supplementation) are essential to facilitate the growth of a strong, tight lamella connection between the hoof wall and the bone. The stress of ongoing pain caused by low coffin bone angle will in itself increase loss of magnesium and therefore increase daily need. Once the feet and body are pain-free, the horse can begin using his body and feet in a normal manner, including loading the heels with each step. This stimulates development of fibrocartilage in the digital cushion and lateral cartilages, resulting in a stronger and more functional back-of-the-foot.
The images below demonstrate the positive changes that can occur when the horse’s body has everything needed to build a strong foot. Despite a history of laminitis, Cushings and weak feet, improvements in this aged TB gelding occurred solely through diet; the horse was not in work and did not wear boots and pads.
May 2003, age 11 (right hind)
The -1.5° negative plantar angle has misaligned the three bones of the pastern and hoof.
October 2012, age 21 (right hind)
Improved digital cushion depth and strength has increased plantar angle to positive 4°, bringing the bones of the pastern and hoof into closer alignment.
Structural changes in the feet from dietary adjustments can take many months or longer to be visible. During that period, the horse can gain considerable relief by wearing boots with soft, wedged pads for a few hours each day. In some horses, the comfort of raised pads has accelerated the rehabilitation process so that significant changes in digital cushion integrity have occurred in just a few weeks. In building a thicker and stronger digital cushion, the horse is, in effect, growing his own internal ‘wedge pad’.
It is very important that the whole back of the foot is raised, not just the heels. It is not unusual for metal shoes with wedged heels to be used in an attempt to address this problem. This approach does initially change the angulation of the pastern and hoof bones, even giving some temporary relief to the horse, but it does not give the necessary support or soft-touch stimulation to the frog that is needed to promote digital cushion development. Ultimately the unsupported frog can sink, or prolapse, down between the wedged heels, which only exacerbates the underlying structural weakness.
It cannot be assumed that all feet on the same horse need the same degree of wedging; it is quite possible for every foot to have different needs during the rehabilitation process. An effective way to assess each horse is to simply experiment with different size wedges – most will require between 2° and 4°. Even a 1° difference can be significant to the horse on just one foot. Start with a 2° wedge inside boots and then watch or film the horse at walk and trot, looking for any subtle differences in the way the horse moves, eg step length, movement in the pelvis, spinal profile, toe-strike or heel-strike, facial expression. Then try a 3° wedge and repeat the exercise to see if that produces further improvement, or conversely note if the horse is not moving so freely as with the 2°, and so on until an appropriate combination of wedge sizes has been determined for all feet. A 4° wedge can be made by using two 2° pads in the one boot. It is essential that a thin layer of foam or other soft material is placed between the wedge and the sole of the hoof if hard plastic wedges are used; the nerve endings in the frog need the tactile stimulation provided by the touch of soft material.
The aim is for the horse to wear the boots and pads for as long as possible each day, and certainly during any exercise the horse may be performing. Some horses can tolerate wearing boots for the majority of the day, for others half the day or less is about the limit. Carefully check the heel bulbs every day for early signs of rubbing or bruising. Sprinkling hydrated lime into the pads may help to reduce the chance of fungal or bacterial infections of the frog if the horse is living in humid or wet weather conditions.
What type of wedge pad?
Ready-made 2° and 3° wedge pads made of hard plastic, eg True Flite brand, are available online from farrier supply stores such as:
These pads are inexpensive and long-lasting although a little difficult to cut to shape. They can be used individually or in combinations to make 4° and 5° wedges if needed. It is then easy to progressively reduce the degree of wedging as the horse gradually builds his own internal wedge, ie his digital cushion.
A soft, flat pad to insert between the wedge pad and sole of the hoof can be cut from high-density foam or the type of squiggle mat found in most hardware stores (see photo left). The squiggle mats are cheap, easy to cut, and allow for some airflow around the sole of the hoof, but like foam, will have to be replaced frequently.
Boots with a soft or elasticated heel area are ideal as the raised wedge pads will place the heel bulbs higher up within the boot. Boots that fitted the foot when flat, may not fit when wedge pads are inserted.
How long does it take?
Using this protocol, along with a diet that optimises magnesium intake, has produced spectacular changes very quickly in some horses. The photos below are of the same stockhorse shown ealier in this article. Rehabilitation work and magnesium supplementation did not commence until some months after the initial experiment to assess the effect of raising the heel region of the hind feet; no improvements in the feet occurred during that time. Changes in digital cushion thickness and texture in the hind feet could be seen and felt after just one month of wearing boots with wedge pads for no more than 12 hours per day, and about six weeks after commencing magnesium supplementation.
This is the photo showing the horse in trot immediately after taping experimental soft wedge pads to his hind heels.
This photo shows the horse in trot without boots or wedge pads, just four weeks after commencing rehabilitation.
Subsequent radiographs revealed this horse had a 3° plantar angle on the left hind and a 3.5° palmar angle on the right fore. Boots with wedge pads were fitted to all four feet to see if that produced any difference in movement compared to just hind wedge pads.
With wedged inserts in boots on all four feet, the horse trotted with a greater degree of suspension and softness in his body than had been seen previously, indicating that the horse needed to develop more digital cushion in the front feet also.
Every horse is different, but generally, larger and heavier horses will need more time to develop robust digital cushions that are strong enough for their role in giving support to the back of the foot.
Principles of Conformation Analysis
D Bennett, PhD
Active Interest Media, 2012
Physical Therapy and Massage
for the Horse
Jean-Marie Denoix, Jean-Pierre Pailloux