Hoof Biomechanics: The Relationship between Form & Function

 

Honor Ame Walesby, DVM, MS, DACVS

Assistant Professor, Equine Surgery

Equine Health Studies Program

Louisiana State University

School of Veterinary Medicine

Baton Rouge, LA  70803

 

Functional Anatomy

            The navicular bone is located between the tendon of insertion of the deep digital flexor tendon and the distal interphalangeal joint (DIPJ).1  It has its own supporting ligaments: proximally, the collateral sesamoidean ligament (CSL) which originates on the distal lateral and medial aspects of the first phalanx and inserts on the lateral and medial wings of the proximal border of the navicular bone; and distally, the distal sesamoidean impar ligament (DSIL) which originates at the distal border of the navicular bone and inserts onto the flexor surface of the distal phalanx.1  The CSL are elastic while the DSIL is relatively inelastic; as a result, the navicular bone moves as a single unit with the distal phalanx to deflect load.2  The dorsal and proximal borders of the navicular bone are covered with hyaline cartilage at the points where they articulate the distal and middle phalanges.1, 2  The subchondral bone plate and intervening medullary cavity of the navicular bone consist of trabeculae arranged parallel to the tensile forces placed on the limb.1, 2  This parallel arrangement suggests that the major force experienced by the navicular bone is compression; and that the role of the navicular bone is to provide a constant angle of insertion for the deep digital flexor tendon (DDFT). 1, 2

            The fibrous joint capsule of the DIPJ blends with the DSIL palmarly and the fibrous portion of the digital synovial sheath (T-ligament) more proximally.1, 2  The T-ligament is a fibrous connection between the DDFT and the middle of the palmar surface of the middle phalanx.1, 2  The palmar pouch of the synovial lining of the joint capsule is divided into proximal and distal pouches.1, 2  The distal pouch extends in a palmar direction along the distal margin of the navicular bone.1, 2  Invaginations from this distal pouch occupy synovial fossa at the distal margin of the navicular bone.1, 2

Extension of the DIPJ is caused by the action of the common digital extensor muscle through its insertion onto the extensor process of the third phalanx.  Flexion of the DIPJ is caused by the DDF muscle as its tendon of insertion glides over the palmar aspect of the navicular bone to insert on the semilunar line of the distal phalanx.3  Friction is reduced by the interposition of the navicular bursa between the DDFT and the navicular bone.3  The position of the DDFT against the navicular bursa is deep to the middle third of the frog and level with the coronary band at the heel quarters.3

            The forces acting upon the distal limb during impact include: 1) downward tension upon the lamina of the hoof wall and DDFT; 2) downward compression from the middle phalanx; 3) upward compression from the sole; 4) upward compression of the navicular bone secondary to tension placed on the DDFT; and 5) tension on the extensor process of the third phalanx exerted by the common digital extensor tendon and the extensor branches of the suspensory ligament.3  Downward axial compression is transferred from the limb to the third phalanx through the middle phalanx.3  As the DIPJ descends, the navicular bone exerts pressure against the navicular bursa and DDFT secondary to tension on the CSL. 3

 

Foot Balance

            Foot balance is the establishment of correct anatomical relationships in the distal limb, and should be considered as an essential prerequisite of normal physiology.2

The pastern axis is an imaginary line passing through the center of the proximal and middle phalanges. 4  The foot axis is continuous with the pastern axis, and runs parallel to the dorsal aspect of the third phalanx. 4  These lines are measured from a lateromedial radiographic projection of the distal limb taken while the horse is standing with both forelimbs square underneath the body.  If radiographic projections are not available, then the dorsal hoof wall should be parallel to the dorsal aspect of the pastern and the heel walls, when the horse is viewed from the side. 4  In the ideal situation, these axes should form a straight unbroken line (foot-pastern axis) that is parallel to the hoof wall at both the toe and the heel. 4  The foot-pastern axis becomes broken forward when the pastern angle is less upright than the hoof wall angle (e.g., club foot). 2, 4-6  The foot-pastern axis becomes broken back when the pastern angle is more upright than the hoof angle (e.g., underrun heel). 2, 4-6  A strong correlation exists between the angle of the dorsal hoof wall and the angle of the DIP.  As a result, deviations in the foot-pastern axis usually originate at the level of the DIP. 2, 4-6

Foot balance is crucial to the equal distribution of weight and force over the foot during impact and propulsion. 6  This distribution is more precisely defined in terms of equal medial to lateral distribution of weight, because more weight is normally placed on the caudal half of the foot. 6  Medial-to-lateral balance is assessed both with the foot held off the ground and in the weight-bearing position. 5, 6  When viewed from the dorsal aspect in a weight bearing position, the vertical distance between the ground and any two comparable points on the medial and lateral aspects of the coronary band should be equal. 5  It is important to note that the slopes of the medial and lateral wall will often differ with the medial wall being slightly steeper in angle than the lateral wall. 5  When viewed from the solar surface with the foot held off the ground, the foot should be symmetric about the frog. 4, 5 An imaginary line which bisects the heels should bisect the frog, emerge at its apex and then bisect the sole before reaching the center of the toe. 3  The width of the frog should equal half its length and the distance from the center point of the frog to the medial and lateral walls should be equal. 4  Hoof symmetry is dynamic, therefore the way in which the foot strikes the ground should also be evaluated.  Ideal medial-to-lateral balance may actually be disadvantageous in some animals affected with angular limb deformities. 2  Instead of trimming and shoeing to achieve static medial-to lateral balance, it is of greater importance to allow for level foot-ground contact and even distribution of the forces associated with weight bearing. 2

The support dimensions of the foot, the length and position of the bearing surface, should be considered.  The support dimensions are evaluated from the side with the horse standing on a flat surface such that the feet are squarely underneath the body.  Ideally, a vertical line bisecting the metacarpal region should brush the heel bulbs positioning the ground reaction forces dorsal to the axis of the third metacarpal bone. 5  An imaginary line is dropped from the palmar/plantar aspect of the flexor tendons to the ground surface.  The distance, from where this line contacts the ground bearing surface to apex of the toe, is referred to as the load distance. 4  The distance from the toe to the point where the heel contacts the ground is termed the base distance. 4  It is thought that the base distance should be at least 60% of the load distance.  This base-to-load ratio usually places the heel directly beneath the center of the large metacarpal/metatarsal bone. 4  In a stationary horse, these distances and this ratio are static.  In the moving horse, these values change to reflect the dynamic strain and force placed on the phalanges, flexor apparatus, and hoof wall. 5

In summary, the following principles apply to a balanced foot: 1) the frog is the most reliable landmark on the solar surface of the foot; 2) break-over occurs 1 to 1.5 inches anterior to the apex of the frog; 3) the heel should be trimmed back to the widest portion of the frog; 4) the ratio of toe-to-heel length should be 3:1; 5) hoof mass should be distributed one third anterior and two thirds posterior to the widest portion of the foot; and 6)the frog, bars, and hoof wall contribute to the support of the caudal aspect of the foot. 7  If these principles are maintained then both the skeletal and soft tissue elements of the distal limb are in the optimal position to accommodate the forces of weight bearing and movement.

 

Biomechanical Research Studies

The effect of toe angle on ligament and hoof wall strain, is such that tension in the superficial digital flexor tendon (SDFT) and suspensory ligament (SL) are not influenced by toe angle.  This is due to the fact that large changes in toe angle (up to 10°) are required before affecting a small shift (approximately 1°) in fetlock angle.  Because the SDFT and SL insert proximal to the DIPJ, they are not influenced by angular changes of the DIPJ.  The DDFT and extensor branches of the suspensory ligament (EB), however, are directly affected by toe angle.  Strain in the DDFT decreased from 2.5% to 1.4% when toe angle increased from 55° to 78°, while strain in the EB increased.  This change in strain is secondary to change in the moment of forces at the DIPJ initiated by toe angle change.  Increasing the toe angle increases the dorsal angle of the DIPJ, which reduces tension in the DDFT, decreases compressive forces exerted by the DDFT on the navicular bone and navicular bursa.  Compressive forces measured at the dorsal hoof wall were found to decrease, while compressive forces at the heel quarters were found to increase in response to an increase in toe angle.  Thus the use of heel wedges, in the form of pads or shoes, will decrease tension in the DDFT and thus secondary compression of the navicular bone by the DDFT. 8

            The effect of medial to lateral imbalance, is such that the sagittal plane of the foot is no longer continuous with the sagittal plane of the pastern, the distal phalanx is no longer parallel to the ground, the long wall bears the full force of initial weight bearing retarding growth, and short wall smacks the ground as the DIPJ rotates and the limb bears the full weight of the horse resulting a secondary dorsal displacement of the coronary band and heel bulb on the short side (sheared heel).  The result is unequal loading of the foot with unequal weight distribution to the supporting structures.  Research using foot force sensors has shown the distribution of force to the long side of the foot to be increased four-fold at the time of impact. 9  Lameness ensues as DIPJ effusion, sheared heels, and hoof wall cracks develop. 5

            The effect of toe length, is such that toe length determines the length of the lever arm over which the limb rotates, as well as the timing of the hoof lift.  A long toe will delay break-over by allowing the horse’s body to move farther forward relative to the limb before heel lift.  The subsequent swing phase is abbreviated, and may be insufficient to prepare the hoof for impact. 10  The delay in break-over increases compressive forces exerted upon the navicular bone by the DDFT, tension in the CSL, and dorsal rim pressure on the joints of the affected limb. 6  The longer the toe becomes, the more flexion of the DIP, pastern, and fetlock joints is required to advance the limb as the projecting distal edge moves away from the point of rotation within the DIPJ. 10  Toe length has traditionally been left long in racehorses to increase stride length.  Research has definitively shown that toe length does not increase stride length or influence the height of the stride, and a direct correlation exists between toe length and catastrophic musculoskeletal injury (CMI). 11-13

            The effect of toe and heel angle, is such that low toe angles have been shown to correlate with lameness that prevented racing or training, increased risk for suspensory apparatus failure, and CMI in racehorses. 12 Investigators concluded: that a positive correlation exists between the magnitude of discrepancy between the toe and heel angle (underrun heel exists when the toe angle is 5° greater than the heel angle) and that correcting this discrepancy should help prevent CMI. 12  Another similar study concluded that lower hoof angles (49° to 52°): adversely affect the break-over pattern of the foot; displace the vertical limb axis caudal to the ground bearing surface; and that these adverse biomechanical effects contribute to injury. 13  Researchers investigating the effect of hoof angle on the kinematics of the proximal interphalangeal joint (PIPJ) found: that raising the toe 6° induces extension in the PIPJ; and that the application of heel wedges induced DIPJ flexion at a rate of 0.3° per 1° of wedge, DDFT relaxation, and PIPJ flexion.  These investigators concluded that the application of heel wedges may have a profound effect on the PIPJ, which should be considered when selecting a shoe for horses affected by PIPJ osteoarthritis. 14

            The effect of horseshoes on the biomechanics of the equine foot, a study comparing the effect of different horseshoes on ground reaction force on the DIPJ, peak DIPJ moment, and compressive force on the navicular bone concluded: the DIPJ moment arm during break-over was reduced by Natural Balance Shoes® (NBS) and quarter-clip shoes; break-over duration (heel off to toe off) was not significantly reduced by NBS, quarter-clip, or toe-clip shoes; and that peak force on the navicular bone was not significantly reduced by NBS, quarter-clip, or toe-clip shoes. 15  A study evaluating the biomechanical effects of rocker-toed shoes concluded: that there was no difference between rocker-toed shoes and standard flat shoes with respect to the duration or ease of break-over or the proximity of break-over to the center of the toe; and that rocker-toed shoes did not influence stride characteristics in sound horses, thus there is no justification for the use of rocker-toed shoes in sound horses. 16

A group of researchers investigating the effect of normal shoes, egg-bar shoes, and shoes with heel wedges on the kinematics of the distal forelimb of the horse during the stance phase of walking and trotting on hard and soft track surfaces concluded: that soft tracks, especially sand, allow a natural forward rotation of the hoof and thus relieve pressure in the navicular area; the extra forward rotation of the hoof induced by heel wedges on hard tracks was almost the same for soft tracks, thus heel wedges and sand tracks seem to have the most effect in unloading the distal forelimb; and the egg-bar shoe has only intermediate effects on unloading of the distal forelimb. 17  Another study evaluating the effect of heel wedges and of egg-bar shoes on the forces exerted by the DDFT on the navicular bone concluded: that heel wedges do and egg bar shoes do not reduce the force exerted by the DDFT; heel wedges lead to a small increase in load of the SDFT and SL; egg-bar shoes reduce the animation of the trot whereas heel wedges do not influence the quality of the trot. 18

Fatal musculoskeletal injury is the most common cause (83%) of death in horses at California racetracks.  Shoe characteristic affect gait kinematics, kinetics of the lower limb and hoof, and hypotheses link imbalances of these components of locomotion to the pathogenesis of limb fracture in racehorses.  A group of investigators examined the relationship between CMI, suspensory apparatus failure (SAF), and cannon bone condylar fractures (CDY) and horse shoe characteristics.  These investigators concluded: that toe grabs and rim shoes were associated with CMI and SAF; toe grabs were associated with CDY; toe grabs were positively associated with CMI, SAF, and CDY when all cases were included and the magnitude of the association increased with increasing height of the toe grab; and rim shoes were identified as possible protective factor for CMI and SAF because the odds for horses with rim shoes to be affected by CMI, SAF or CDY were 30% less than the odds for horses shod without rim shoes.  By raising the toe, toe grabs decrease the functional angle of the shod hoof, delay hoof break-over, and increase the length of the lever arm of the ground reaction force on the fetlock  As a result, strain within the suspensory apparatus is increased and predisposes the horse to injury.  Lower hoof angles (< 50°) have been reported for injured horses.  The incidence of injury and the odds ratio for being injured increases in magnitude with increasing toe grab height; this correlation indicates the relationship between toe grabs and SAF is causal.  The effect of toe grabs on CDY may be attributed to concurrent CDY and SAF among fatally injured horses, thus decreasing the use of toe grabs and replacing them with traction devices more consistent with natural hoof morphology such a rim shoes is recommended.19

 

References

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  15. Eliashar E, McGuigan MP, Rogers KA, et al.  A comparison of three horseshoeing styles on the kinetics of break-over in sound horses.  Equine Vet J 2002; 34: 184-190.
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  17. Scheffer CJW, Back W.  Effects of navicular shoeing on equine distal forelimb kinematics on different track surface.  Vet Quart 2001; 23: 191-195.
  18. Willemen MA, Savelberg HHCM, Barneveld A.  The effect of orthopaedic shoeing on the force exerted by the deep digital flexor tendon on the navicular bone.  Equine Vet J 1999; 31: 25-30.
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