LOAD SHIFTING IN THE EQUINE HOOF
INTRODUCTION
The abaxial location of the equine forelimb in relation to the horses’ center of mass insures that the distal limb will be subjected to asymmetrical loading even in well conformed individuals. Poor conformation, non-physiologic farriery and hoof capsule pathology can exacerbate this load asymmetry to the point of medial or lateral capsular collapse. This state of imbalance is characterized by a host of readily observable clinical signs. Grossly, the overloaded wall will have lost its conical nature and be base narrow to the coronet. The coronet band will be jammed proximally through the quarter region and may be accompanied by heel shear. The hoof often lands and loads unevenly as the horse is moved across a flat surface. The horn tubules of the overloaded wall may have lost their normal ground orientation and appear folded over or crushed, most obviously at the heel and bar regions. Sub mural bruising is common and weak hoofed individuals will often develop quarter cracks. The gross asymmetry of these hooves is best visualized from behind the horse being stood on a hard level surface. This condition, despite being well recognized for decades often remains overlooked prior to overt lameness and the significance of lateral/medial imbalance under diagnosed in clinical practice. Treatment of these individuals can be a source of frustration and meaningful correction can prove illusive. Improved radiographic techniques can aid veterinarians and farriers in the diagnosis of individuals in need of correction.
Radiology is an invaluable tool when sorting out individuals exhibiting a normal capsular load asymmetry compared to those that have exceeded the hooves physiologic parameters and are in a state of capsular collapse and structural imbalance. In order for this differentiation to be made, a specific radiographic landmark that indicates misalignment of the boney column must be identified. Radiographic projections must also be free from rotational artifact with the subject standing in a neutral posture on well prepared hooves. The author considers the orthoclastic potential of the distal phalanx too excessive to be relied upon for a consistent symmetry landmark on the standing AP projection. The landmarks used here to assess boney column alignment with the ground plane are two points on the most distal aspects of the articular surface of the middle phalanx (figure 1). An assumption was made that a line produced by these two points should be parallel or not exceeding 2.75 degrees to the ground plane when the hoof is in a state of physiologic lateral/medial balance. The angle between this line and the ground plane is termed P2/P3 joint tilt. Joint tilts greater then 4.0 degrees are considered non physiologic and in need of lateral load shifting due to medial hoof wall collapse. Joint tilts less then -4.0 degrees are conversely considered in need of medial load shifting with the lateral wall in a state of collapse. Joint tilts between +/- 3.5 and +/-4.0 degrees are considered border line and active load lateralizing techniques may be foregone for more conservative adjustments. Load is effectively shifted away from the collapsing wall or lateralized by elevating the collapsed wall up from the ground plane compared to the contra lateral or normal side.
Materials and Methods:
Hoof preparation is a key component to this process and all attempts should be made to coincide the initial radiographic study with the attending farrier present. Hooves that tend toward lateral/medial imbalance are often more upright and prone to rapid heel growth. For this reason, care should be taken to establish the live sole plane and not leave excess hoof at the heel and bar regions. Aside from calling attention to the suspicion of lateral/medial imbalance, the author prefers to allow the farrier the space to balance the hoof as they see fit. Provided there are not any glaring problems with the hoof preparation, veterinary input is best saved until all the data has been collected. It is important keep in mind that the correction and maintenance of lateral/medial imbalance is ultimately accomplished by the farrier. A generally competent farrier should not feel as though they are to blame for the current problem. Rather, as veterinarians we are there to provide the necessary skeletal information to correct a pathological problem. Radiographs can also be obtained any time during the first two weeks of the shoeing interval and shoes do not necessarily need to be removed to obtain an accurate measurement of coffin joint tilt.
The adage “garbage in garbage out” will certainly hold true while obtaining the necessary radiographic projections to identify an accurate representation of lateral/medial balance. A light plane of sedation makes the acquisition of good quality structural views much easier in all but the best patients. The working surface must be firm and level and the patient in a neutral stance. The hoof blocks must have markers that will clearly identify a level cross plane underneath the central third of the frog. The blocks should be checked for level using a six inch carpentry bubble prior to shooting the x-rays. Rotational artifact is minimized by insuring that the cross plane, which simulates the ground plane, runs parallel to the line created by the heel bulbs. Position of the block must therefore be adjusted to individuals that toe in or out and may not always appear true dorsally. It should be noted that palmar aspect of the hoof, especially the heels and frog, more grossly define the bone structure within the hoof while the dorsal wall and toe quarters can be very misleading. The x-ray beam is centered just below the coronet band, perpendicular to the cross plane.
Analysis of the radiograph can be accomplished with any computerized system capable of calculating the angle between the cross plane and the line described in figure 1. Most digital x-ray systems are capable of performing this calculation. All views in this study were analyzed using the epona tech metron soft wear on digital pictures of standard film radiographs. Use of this software is well documented by the company and their protocols were followed with one exception. The lateral and medial points used to define the P2/P3 joint tilt were selected as the most distal aspect of the articular surface of the second condoyle which is often more axial than described by epona tech.
Through out the course of this study many different farriery techniques were employed to correct for joint tilts beyond 3.75 degrees with one basic principle adhered to. One millimeter of lift per degree of tilt was applied across the shoe to effectively elevate the low side of the joint. Twice this amount of lift is required to put theses angles into a congruency. How this was achieved was largely dependent upon the subtleties of the hoof capsule, the farrirers’ preferences and the available supplies and equipment. A caliper to measure each branch of the shoe is necessary to calculate the differential across the shoe. Generally speaking, the author prefers a cut out leather rim pad which is attached to the shoe with leather cement and riveted at the heels. This pad can then be reduced on a belt sander to create the appropriate differential. Sideways positioned wedge pads also work well. Some farriers prefer to thin one branch of the shoe using a hammer and the forge. Quite often a leather rim shim has simply been added to one branch of the shoe and blended into the toe region as smoothly as possible. Removal of excess hoof wall from the contra lateral side can be used to supplement this differential but should not be relied on exclusively. A follow-up radiograph should be used to confirm that the joint tilt has been normalized by trimming alone before a decision to forego active load shifting is made. Owners and farriers are instructed to reset these shoes one or two more times prior to the recommended follow-up radiograph. Most horses are kept at some level of training or exercise.
Results
From March 2001 through February 2005 there were 549 standing AP radiographs taken and analyzed in this fashion during the context of clinical lameness examinations. The majority of these cases responded to diagnostic anesthesia at or below the base of the sesmoids. The distribution of P2/P3 joint tilts is summarized in graph 1. This graph represents a mixed population of light breed pleasure and performance horses. Approximately thirty farriers are represented by these cases . None of these horses were engaged in flat track race training at the time of exam.
Eighty four percent (462/549) of this population is considered to be in physiologic lateral/medial balance when a P2/P3 joint tilt less than 3.75 to the medial and greater than -3.75 degrees to the lateral are used as the benchmarks. As would be expected, there were small number of cases overlapping this range, demonstrating the necessity of a holistic approach in all podiatry cases. When the graph is truncated ( graph 2) to show the individuals in a state of collapse there are 4.5 % (25/549) failing laterally and 11.3 % (62/549) failing medially. Because a relatively well conformed horse will load the medial aspect of the hoof more than the lateral aspect, a larger percentage of medial collapse is expected, 71% (62/87) in this population.
All individuals that underwent this type of load lateralizing treatment showed a significant improvement in hoof quality and growth during the approximately 12-18 weeks that these shoes were kept on. The majority of individuals showed a return of physiologic P2/P3 tilts with no more than two re-sets. A small percentage of individuals have remained in a load lateralizing shoe for six to eight months before the lift was removed. As mentioned previously, most of these horses were exhibiting some degree of lameness. In February 2005, eighty five percent (60/87) of the owners whose horses underwent load lateralizing treatment were contacted and responded to a follow-up survey in which they were asked if their horses were currently sound, improved or lame. Eight-five percent (51/60) were said to be sound, six percent (4/60) improved and eight percent (5/51) still lame.
Discussion
The lack of both a control population and an exclusive treatment protocol would preclude this method of diagnosis and load lateralizing treatment from taking statistical credit for an estimated 85% return to soundness. However, the consistent improvements in hoof quality, the repeatable return to physiologic P2/P3 joint tilts, and the absence of both short and long term complications are reasons to consider this a safe and effective means of sorting out and correcting lateral/medial balances problems. Several cautionary points should be made at this time. On four occasions individuals were reported to be lame after one of the resets and found to have had the wrong side of the hoof elevated. These shoes were removed and the horses kept unshod for several days. When the correct side of the hoof was elevated the lameness resolved. During the initial application of these shoes, it is important to have a meeting of the mind, so to speak, with the farrier. Incorrect application of the lift can then only be due to a human error. Also, during the application process, it is important to check the fit of the shoe both before and after nailing. Many farriers are not accustomed to applying shoes with an unleveled solar surface. The shoe should make even contact before and after nailing and must be worked until this is the case.
Balance of the digit must be considered in three dimensional space. Redundant toe, especially of the contra lateral toe-quarter will push load back toward the failing heel and quarters. Failure to normalize these forces will result in a less satisfactory resolution of the lateral/medial problems. The large majority of these horses were kept at toe percentages of 55-60% using methods beyond the scope of this discussion. However, because a failure to account for a hoof that has run forward might negate any load shifting therapy, a brief discussion of anterior/posterior balance must follow.
The last decade has produced a veritable cacophony of information on the ill effects of redundant toe and it would seem a cruel irony that so many hoof care professionals still fail to recognize the negative impact this imbalance has on the equine digit. Thankfully, there are a great many more farriers, as there have been for hundreds of years, who understand what it looks like to set a horse up with his hooves underneath the boney column. In simple terms, a state of anterior/posterior balance suggests that the percentage of hoof/shoe in front of the center of rotation compared with the percentage of hoof/shoe behind the center of rotation not exceed some yet to be precisely defined limit. There, is however, an aesthetic quality about a hoof balanced in this plane that a well trained eye can easily recognize. The difficulty is getting that hopelessly run forward foot back under the horse wear it belongs. This can not be achieved unless the heels are lowered and the break over reduced simultaneously.
Finally, the powerful effect of time must not be overlooked both during and after the shims have been removed. Many of these horses have collapsed because growth of the contra lateral side of the hoof has not been kept in check. The shoeing interval must be adjusted for the individuals growth rate. The effect that growth rates exerts on the hoof and our shoeing treatments should be considered a cornerstone in physiologic farriery. Science and technology have made great strides since Albert Einstein first noted the inseparable nature of space and time in our universe. For the horse, failure to correct lateral/medial imbalance will ultimately result in progressive osteoarthritis of the digit, the rate of this progression is largely determined by the reserve capacities of the individuals hoof conformation, concurrent pathologies and the level of work at which they perform.
