Written and presented July 2006 by R.F. (Ric) Redden, DVM
The present day full rocker motion shoe has a convex ground surface and is designed to offer a self-adjusting palmar angle (PA). For optimum results the most prominent point of the convex surface is placed directly beneath the center of articulation of the coffin joint. The design and degree of rocker dictates the range of PA adjustment. The ability to adjust the PA while in the static state sets the mechanical action of this shoe well above those that do not influence the static PA. Square toes, rocker toes, rolled toes and flat shoes set back under the toe to decrease breakover effort can offer advantageous mechanical action when the horse is moving, but fail to allow self-adjustment when resting. The rocker action has a profound effect on solar fimbrae reconstruction and subsequent sole growth, apparently via the altered Deep Digital Flexor (DDF) tension.
Flat shoe enthusiasts consider the shoe radical or extreme as it differs greatly from the more recent traditional shoe. However, blacksmiths in the 14th century were forging a very similar full rocker shoe throughout most of Europe for many problems similar to what we encounter today. Unfortunately, the shoe was not described at the time of popularity, simply because there were no authors, editors or printers publishing books during that era. Today, it has once again become an extremely valuable tool for treating foot ailments that are influenced by the DDF.
Sport and speed horses are frequently diagnosed with caudal heel pain and long toe, underrun heels. Clinical and radiographic examination reveals that a broken back digital axis is a common finding in horses with crushed heel syndrome. It is common knowledge that horses suffering from the long toe, lower heel syndrome require less breakover to combat the effects of long toe. Backing the toes up has been considered a reliable means for aiding breakover. Unfortunately, pulling the toes back has very limited mechanical advantage. Removing the face of the wall back to the stratum medium gives the illusion that the toe angle has improved, though the PA and heel angle have remained unchanged.
Wedge shoes and pads are frequently used to correct the low toe angle hoof as they are an effective tool for correcting the PA to some degree. The positive effects of the wedge can often be seen immediately. Unfortunately, the downside of raising the PA with a wedge too gradually from toe to heel is that it invariably crushes the digital cushion. Use of wedge rim pads or wedge shoes allows the frog and cushion to fall through the shoe, further compromising the heel tubules. Despite the long term ill effects of this method, the concept remains popular simply because the technique of backing the toe up and using a wedge system greatly enhances the appearance of the foot. The thought that if it looks good it must be healthy is a common misconception.
The full rocker motion shoe has numerous mechanical advantages that allow self-correcting digital alignment. The rocker action greatly enhances the static stance phase that does not occur with flat shoes, square toes, rolled toes and shoes set back under the toe. The advantage this shoe has over all other flat shoes is the ability to quickly and easily alter tension in the DDF. Load forces are constantly being transferred from areas of inflammation to less painful, healthier areas of the foot. Therefore, from the clinical as well as radiographic follow up, the shoe has the capability to enhance the healing environment for a variety of foot problems that are influenced by the DDF.
The majority of foot pain is illustrated by the force of tension or compression. Most problems occur in the toe or heel. Apparently, as the PA rocks forward, this shoe relieves tension forces on the laminae and therefore the sole compression. During the same action the heel tubules are compressed and tensions over the navicular bursae, navicular bone and associated ligament attachments are reduced.
Thin soles, white line disease, full thickness toe cracks and laminitis are syndromes that affect the toe area. Reducing tension forces on the wall, laminae and compressive forces on the sole corium aids healing by enhancing perfusion to vital growth centers of the sole and horn wall. Venogram studies suggest that direct correlation exists between increased perfusion and growth center stimulation. The crushed and underrun heels and inflammation of the heel apparatus in general is a major cause for chronic heel pain found in most all breeds. The shoe offers a means to shorten the stance phase while moving and resting, apparently reducing compressive as well as tension forces within the heel area, which subsequently reduces pain.
Specific Uses of the Shoe
Thin hoof walls and thin soles are frequently encountered on performance or speed horses. Farriers are often accused of removing too much foot. Perhaps some do, but the competent farrier knows when there is no foot to be taken off. In these cases, close observation will reveal 3-4 sets of nail holes in a foot, confirming that no foot has been removed and little growth has been present for the past several resets. Farriers routinely brush the dirt and broken horn tubules off the ground surface and either try to find a place to secure a rail or use composites to attach the shoe.
Foot mass diminishes for many other reasons than a visit from the farrier. Lack of demand on the foot, excessive moisture, nutrition, speed and other man-induced demands on the foot have cumulative effects on the strength of feet. Inherited and congenital weakness compounds the problem, as the reserve offered by a strong, upstanding hoof capsule is simply not there. Using the full motion rocker shoe as a tool to aid quick mass recovery is one of its most promising assets.
Venogram studies by Redden1 suggest that healthy, vascular supply to the sole corium and heel apertures requires 10-12mm of space below the palmar plantar surface of PIII. A PA of 3-5 degrees is required to sustain the solar papillae that are responsible for sole and other sensitive structures of the heel apertures. Sole proper and horn wall structure play a major role in protecting those vital growth centers. The minimum depth of sole required to adequately protect and support the vascular supply is breed and use dependant. A minimum of 5-8mm of sole (non-sensitive) is considered adequate in most light breeds. Therefore a healthy foot on a light breed horse should have a minimum sole depth of 15-18mm measured radiographically from the ventral side of the apex to the ground or shoe surface. This measurement does not include a 3-5mm natural cup of the sole that can also be measured radiographically.
Most all horses lose hoof mass once in training, as sole depth diminishes. Farriers often feel obligated to cup the sole to reduce sole pressure on the shoe. However, cupping the sole that is losing mass soon becomes counterproductive. Cupping the foot that doesn’t have a cup is detrimental to the foot as the protective, supportive function of the sole is greatly reduced, setting off a cascading series of events that further weaken the capsule. As the sole thins the wall follows pursuit. Stimulating accelerated sole growth likewise stimulates rigid horn wall growth. Further studies are needed to better understand the mode of action.
Anatomy of the Shoe
Any shoe can be forged with a convex ground surface. The depth of the shoe branch and the design, e.g. flat, wedge, rail and the degree of convexity, determine the mechanical potential of the shoe. To help simplify mechanical action, the author has developed a scoring system. For every 2º the PA is altered in the static state the shoe gets a score of one. For example, the shoe required to raise a pre-shoeing PA of negative 4º to a post-shoeing PA of positive 6º would be a 5 point shoe.
The shoe is designed to set solidly on the anterior quarter of the foot referred to as pillars by Duckett and the posterior quarter, which is located on either side of the foot at the widest part of the frog. The degree of convexity or rocker will depend on the predetermined goals, the problem you wish to solve, the mass of hoof present (sole depth and horn quality), and the PA, The breed, gait and intended use must also be considered. Informative soft tissue detail lateral and AP radiographs are required to accurately design and fit the shoe to meet the mechanical goals in mind. Post shoeing radiographs are required to confirm whether the mechanical goals were met and to set the new base line, which is needed to accurately record progress.
Several methods of attachment are available depending on the severity of the problem, and rehab vs. continued training. When adequate foot mass is present the nail pattern is placed in the center and either side of the most convex part of the shoe. This attachment site assures that the shoe moves at the same speed as the horse and is not trapped on the ground as the foot breaks forward. Nailing in front of the widest part of the foot has long been advocated for flat shoeing. This concept is thought to prevent entrapment and contraction of the heels. Further studies are needed to confirm the validity of this concept. Flat shoes nailed anterior to the widest part of the foot are often stationary as the heel comes up in an attempt to start breakover. This “loose shoe” appearance will invariably create large oval nail holes in a matter of days and weaken the wall adjacent to the nails. The full rocker shoe attached at the center of the convex surface shows little or no nail hole wear from reset to reset, indicating it is not resisting the movement of the foot.
Adhesives - The shallow foot (less than 15mm of sole with a zero or negative PA) cannot be trimmed to meet the convex shape of the shoe. Therefore to apply the desired mechanics the shoe can be set into a bed of composite and held off the ground until it has cured.
Nail and Adhesive – A combination of adhesive and a couple of nails also works well for the shallow foot. Caution is due. Driving nails through the cured composite can create problems by deflecting the nails. Cleaning up the nail holes before the composite cures is recommended.
Trimming the Foot
The depth of sole and the PA are basic deterring factors for all trims regardless of the goals for the shoe. For example, there is very little to trim on a thin soled horse (less than 15mm and with a zero or negative PA). Determining how to fit a rocker rail to this foot requires rasping the toe (ground surface) at a very low angle (10-15 degrees) in a plane perpendicular to the long apex of the foot. The goal is to produce a flat, smooth plane across the sole well in front of the location of the apex of PIII. Removing sole directly over the apex of this foot is not conducive to better soundness.
The heels are then backed up from just behind the widest part of the foot to very close to the widest part of the frog. Removing the section of horn tubules that are bent forward and folded towards the midline of the foot offers a more solid end tubule loading and significantly increases shoe/horn contact. Caution is advised when fitting a full rocker shoe to this type of foot.
The frog should not be touched. Fortunately, most thin soled, underrun heeled feet will have a wide, thick frog that protrudes well below the bearing surface of the heel tubules. To effectively push the heels back with a prominent frog, the rasp must be worked along the sides of the frog. Removing frog to get to the heels is a common error and often the cause of post-shoeing lameness.
Fitting the Shoe
Deciding on whether to rocker a flat shoe, rail, wedge or a full rocker shoe depends on the complaint (ailment), whether the horse is rested or trained, and what degree of mechanical action is needed to adjust load and/or internal tension from areas of influence to sounder parts of the foot. Once the shoe is selected it is fitted to the shape of the foot allowing approximately 1/8 inch of extra width on either quarter. Using a rocker jig or the step of the anvil the ground surface is forged, creating a smooth, convex ground surface.
When forging, start the rocker at the heel and work forward to assure the most optimum point of convexity is at or slightly behind the widest point of the foot. Once the desired amount of rocker has been applied to the shoe lay the heels of the shoe (ground surface) on the face of the anvil and soften the last 1-1 1/2 inch of the branch. This is a vital step. Allowing the convex surface to continue into the bulb area will invariably cause post shoeing pain. This is a common error that many farriers fail to detect.
When the shoe is fitted properly there will be an air space between the toe of the foot and the toe of the shoe, and the shoe will sit solidly on the four points of contact. The shoe will not sit on the quarters. When a large air space is present (4-5mm) the shoe will require a bed of composite to offer adequate foot/shoe contact. Nailing with a small 2-3mm air space under the quarter does not pose a problem.
How to Determine Radius of Convex Surface
The principal design of the shoe offers a variety of mechanical influences on the PA. One of the goals of the shoe is to re-establish healthy digital alignment. The majority of horses do not have matching feet, PA, bone angle or toe angle, therefore the mechanical demand will be different from foot to foot. Normally when used as a rehabilitation shoe on a case that is not in training, the higher score shoe would be used with the goal of promoting accelerated sole and horn growth while the horse is out of training. For example, the performance or speed horse that has a sole depth of less than 7mm or a zero to negative 5 degree PA and very poor quality, thin walls can benefit greatly with a couple of 4-6 week periods using a 5-7 score shoe. This would be considered a moderate rocker. Many times hunters, jumpers, cutting horses, reining horses and other slow sport horses can go back into training with the same degree of rocker once the foot mass has been restored. Some compete well with new foot mass and the same mechanics, where others need slightly less mechanics.
Tendon surface angle (TSA), heel and toe angle may differ greatly between two feet on the same horse. Therefore, slightly different mechanical action is indicated for each respective foot to correct digital alignment. For example, the club foot with thin sole and a zero PA will need only slight to moderate rocker to achieve desirable PA. The opposite foot with a negative PA will require a higher score shoe to offer the same self-adjusting healing alignment.
The score of the shoe is influenced by the shoe design, how much rocker is in the shoe and where the peak of the rocker is located. Moving the rocker from the widest point towards the heel increases mechanical score.
Speed horses – When race horses are diagnosed with sore feet, pedalostitis, heel pain and often quarter cracks, the trainers are looking for a quick fix. Re-establishing sole depth and rejuvenating crushed digital cushions with horses in full training is a much slower process than in the horse that is laid up for a few weeks. A much lower profile shoe (less mechanics) is used on speed horses that remain in full training. Using a very low mechanical shoe on a horse in training will yield small improvements in sole depth, heel mass and horn growth; therefore the speed of progress is limited.
The low profile rocker is placed on the foot in the same fashion. Normally it is glued on with composites for the first few shoeings, and the peak of the convex surface is positioned in a fashion that offers center to toe loading when static. Most low score shoes can change the PA 4-6 degrees without taking any foot off. The author recommends all speed horses be shod a minimum of 4-6 weeks before going at race speed. This offers healing time (though limited by training speed) and gait adjustment. Once reset, the foot mass, digital alignment and muscle soreness associated with extremely low PA has normally been improved and the horse has adjusted to the shoe.
Informative venograms are very useful as a tracking tool as they clearly reveal the speed and degree of solar fimbrae reconstruction, which appears to play a major role in healthy sole growth.
The full rocker motion concept shoe has been used frequently over the past several years by the author in his exclusive podiatry practice.