In this part, we will look into the shape of feet, foot asymmetry, the effect of imbalance and importance of proper trimming in both the front and back feet.
The feet
Looking at the shape of a horse’s hooves, and the conformation of their feet, can also play a key role in maintaining soundness and preventing orthopedic issues. Orthopedic problems can happen due to repetitive force application in an abnormal direction (1). Noting asymmetry can help to guide the horse’s owner to acquire foot balance radiographs, or discuss remedial shoeing with their farrier. It is also important to understand that environmental factors (2) and exercise (3, 4) also affect hoof growth.
A 2009 study (5) showed that foot conformation, including asymmetric or uneven forefeet, can be influenced by genetics. Uneven feet, particularly at elite level showjumping, had a detrimental effect on the length of the sporthorse’s career (6).
A ‘functional foot’ consists of a parallel hoof-pastern axis, appropriate sole depth and hoof wall thickness, uniform growth rings below the coronary band and a solid heel base (7).
A normal foot should be well balanced, with two key features:
The sole of the foot should be symmetric (medial to lateral aspects), if a line was drawn through the centre of the foot from the toe to the heel.
The bottom of the foot should be perpendicular to the metacarpus or metatarsus. A foot with a long toe has increased tension through the deep digital flexor
tendon and related structures. This does not take into account the shoeing interval, with an increase in dorsal hoof wall of 1.4 cm in the forelimbs and 1.5 cm in the hindlimbs over an 8 week showing interval (8). The hoof pastern axis (HPA) is assessed from the side, with the horse standing squarely on all four feet. The HPA takes into account whether three entities are parallel:
The dorsal hoof wall.
Heel angle.
3. A line drawn through the three phalanges, transecting through the centre of the
fetlock, pastern and coffin joints.
As the toe elongates over an 8 week showing interval, the hoof angle decreases by about 3.3° (8).
Horses are able to compensate for these morphological changes and maintain their neuromuscular pattern of movement through decreased extension through the fetlock (9).
Imbalance may be present in both a mediolateral or dorso-palmar/plantar orientation.
Image 1: Radiographs can be a great tool in tracking hoof pastern axis and sole depth. Note how thin the sole is at the caudal aspect of the pedal bone in the radiograph on the left, and how mediolateral imbalance is present in the foot, with differing heel lengths between the medial and lateral aspects.
Horses with mediolateral imbalance can have a relatively longer medial heel compared to the lateral heel, or vice versa. This can happen due to the handedness (left- or right-handed) of the farrier (10) or conformational disorders which lead to uneven loading of weight during landing, resulting in more breakdown on one side. This condition is commonly known as sheared heels, and although many horses with this imbalance can still be sound, the horse may be at increased risk of developing issues, including collateral ligament damage (7).
A broken back HPA is the most commonly encountered dorsopalmar/ plantar imbalance. Horses with this condition generally have long toes and underrun heels, and this can lead to navicular syndrome, chronic heel bruising, coffin joint inflammation and heel cracks (7).
Although less common, a broken forward HPA can result in an increase in risk of injuries to the coffin joint, sole bruising and the navicular bone’s suspensory ligament (7). Again, remedial farriery under radiographic guidance is an essential part of treatment.
The back feet
Image 2: Note the negative angle S at -4.81°. This horse was diagnosed with sacroiliac disease, however still exhibited signs of weight shifting after the joint was treated by its regular vet. A particular interest has been given to the angulation of the distal phalanx in relation to the sole of the foot, commonly termed angle S. The plantar aspect of the pedal bone should be higher up than the tip. A 2020 study (11) showed that the mean S angle in nonlame horses was 1.6°. A negative S angle was linked to increased risk of bone injury in the cannon bone (12),
hock/ suspensory pain (13) and stifle pain (12). It is not known whether the foot conformation predisposes lameness or vice versa. Although the S angle can only be precisely calculated after obtaining a lateromedial radiograph, analysis of digital photography (14) and assessment of the external characteristics of the hoof capsule (15) can be extremely useful tools in both the tracking of progress between treatments and being able to recognize a lengthening toe and the risks associated with it.
Another indicator of toe length is the correlation between gluteal pain on palpation and excessive hindlimb toe length (16). Shortening the toe, and therefore the breakover distance, results in a decrease in gluteal pain within a few days or weeks.
Finally, it is normal for horses hind feet to be toe out (17), and this conformation is key in allowing for maximal extension through the hindlegs through increased engagement of the hocks.
Splints
The splint bones are palpable, and become more fine distally. The majority of splint bone fractures are due to external trauma, and almost a third are undiagnosed for up to 3 weeks, with lameness or discomfort on palpation not always present (18). Early recognition can ensure that the appropriate treatment is undertaken, especially as callus formation may lead to suspensory ligament injury (18) and suspensory branch desmitis (19).
References
Johnston, C. and Back, W., 2006. Hoof ground interaction: when biomechanical stimuli challenge the tissues of the distal limb. Equine Veterinary Journal, 38(7), pp.634-641.
Hampson,B.,deLaat,M.,Mills,P.andPollitt,C.,2013.Theferalhorsefoot.Part A: observational study of the effect of environment on the morphometrics of the feet of 100 Australian feral horses. Australian Veterinary Journal, 91, pp.14-22.
Peel, J., Peel, M. and Davies, H., 2006. The effect of gallop training on hoof angle in Thoroughbred racehorses. Equine Veterinary Journal, 38(S36), pp.431-434.
Faramarzi,B.,Thomason,J.andSears,W.,2009.Changesingrowthofthehoof wall and hoof morphology in response to regular periods of trotting exercise in Standardbreds. American Journal of Veterinary Research, 70, pp.1354-1364.
Ducro, B., Bovenhuis, H. and Back, W., 2009a. Heritability of foot conformation and its relationship to sports performance in a Dutch Warmblood horse population. Equine Veterinary Journal, 41(2), pp.139-143.
Ducro, B., Gorissen, B., Eldik, P. and Back, W., 2009b. Influence of foot conformation on duration of competitive life in a Dutch Warmblood horse population. Equine Veterinary Journal, 41(2), pp.144-148.
O'Grady, S. and Poupard, D., 2003. Proper physiologic horseshoeing. Veterinary Clinics of North America: Equine Practice, 19(2), pp.333-351.
van Heel, M., Moleman, M., Barneveld, A., Weeren, P. and Back, W., 2005. Changes in location of centre of pressure and hoof-unrollment pattern in relation to an 8-week shoeing interval in the horse. Equine Veterinary Journal, 37(6), pp.536-540.
van Heel, M., van Weeren, R. and Back, W., 2006. Compensation for changes in hoof conformation between shoeing sessions through the adaptation of angular kinematics of the distal segments of the limbs of horses. American Journal of Veterinary Research, 67(7), pp.1199-1203.
10.Ronchetti, A., Day, P. and Weller, R., 2011. Mediolateral hoof balance in relation to the handedness of apprentice farriers. Veterinary Record, 168(2), p.48. 11. Clements, P., Handel, I., McKane, S. and Coomer, R., 2020. An investigation into the association between plantar distal phalanx angle and hindlimb lameness in a UK population of horses. Equine Veterinary Education, 32(S10), pp.52-59. 12.Walmsley, E., Jackson, M., Wells‐Smith, L. and Whitton, R., 2019. Solar angle of the distal phalanx is associated with scintigraphic evidence of subchondral bone injury in the palmar/plantar aspect of the third metacarpal/tarsal condyles in Thoroughbred racehorses. Equine Veterinary Journal, 51, pp.720-726.
13.Pezzanite, L., Bass, L., Kawcak, C., Goodrich, L. and Moorman, V., 2019. The relationship between sagittal hoof conformation and hindlimb lameness in the horse. Equine Veterinary Journal, 51, pp.464-469. 14.White, J., Mellor, D., Duz, M., Lischer, C. and Voute, L., 2008. Diagnostic accuracy of digital photography and image analysis for the measurement of foot conformation in the horse. Equine Veterinary Journal, 40(7), pp.623-628. 15.Kalka, K., Pollard, D. and Dyson, S., 2020. An investigation of the shape of the hoof capsule in hindlimbs, its relationship with the orientation of the distal phalanx and comparison with forelimb hoof capsule conformation. Equine Veterinary Education, 33(8), pp.422-429. 16.Mansmann, R., James, S., Blikslager, A. and vom Orde, K., 2010. Long Toes in the Hind Feet and Pain in the Gluteal Region: An Observational Study of 77 Horses. Journal of Equine Veterinary Science, 30(12), pp.720-726. 17.Mawdsley, A., Kelly, E., Smith, F. and Brophy, P., 1996. Linear assessment of the Thoroughbred horse: an approach to conformation evaluation. Equine Veterinary Journal, 28(6), pp.461-467. 18.Jackson, M., Fürst, A., Hässig, M. and Auer, J., 2007. Splint bone fractures in the horse: a retrospective study 1992–2001. Equine Veterinary Education, 19(6), pp.329-335. 19.Dyson, S., 2007. Diagnosis and Management of Common Suspensory Lesions in the Forelimbs and Hindlimbs of Sport Horses. Clinical Techniques in Equine Practice, 6, pp.179-188.