There is a complexed relationship between the hoof, its conformation and the forces applied to it which affects the entire musculoskeletal system, but deeper complexities are realised when there is asymmetry between pairs of hooves, stated as an uneven pair when there is an angle difference of >1.5 degrees (Hobbs et al 2018).
Skeletal asymmetry is a natural part of biological variation and sidedness has been recognised certainly in man and has been recorded historically in the horse with anecdotal evidence of them being harder to train on the right and stiffer turning to the left. Watson et al (2003) showed 76% of thoroughbred racehorses to have a larger right third metacarpal and hinted at asymmetries effecting coordination and balance, Pearce et al (2005) questioned whether common femoral asymmetries were congenital or acquired through workload. Merkies et al (2019) studied the thoracic symmetry of ridden horses finding that as a rule they were larger on the left side, the mechanics for this is unknown but could be congenital or acquired and uneven hooves have been documented as a possible compensatory mechanism to limb length disparities (Wilson et al 2010).
Other Studies of the hoof found that uneven hooves could be attributed to foal grazing habits producing laterality, Van Heel et al (2006) found 50% of foals to have a preferred side to load while feeding which directly caused them to develop uneven feet and loading patterns, this was shown to develop into sidedness in trot-canter transitions (van Heel et al 2010) and this conformational defect has been documented as predisposing horses to early retirement from elite level competition more markedly in show jumpers (Wiggers et al 2015).
Hoof growth has been shown to exacerbate the asymmetry as the changes in angle of a flat foot compared to an upright foot were shown to be disproportionate, the different load bearings of the internal structures is something that has to be taken into consideration when setting shoeing intervals (Moleman et al 2006). The biomechanical effects on locomotion have also been studied, Clayton et al (1990) showed that a flatter hoof had a longer breakover duration and Buchner (2013) discussed how the extension of the metacarpophalangeal joint had a direct relationship with peak vertical force, which when combined with the study Scheffer and back (2001) showing the reduction in fetlock extension with heel wedges, one can see that uneven feet will experience different biomechanical forces through the stride.
Wiggers et al (2015) studied the differences in locomotive function of high-low hooves and echoed the finding of Clayton et al (1990) finding that the more acute angled hoof had a longer transition period between breaking and propulsion and added that the higher heeled hoof was “stiffer” and had a reduced vertical displacement, also having a reduced braking and an earlier transition from braking to propulsion, assuming the absence of pathology this can be attributed to a difference in loading pattern and therefore the presence of a “mechanical lameness” too subtle to the human eye. Wiggers et al (2015) concluded that the difference in the angle of the pair of hooves is more relevant then the absolute measurements, the stance times and peak forces were found to be unchanged by hoof conformation which suggests these are controlled by the neuromuscular system.
We can take from this that high-low hooves are functionally the same as having a sub-clinical lameness, importantly we must remember that horses are quadrupeds and these asymmetries in propulsion have to be compensated for in order to maintain intended velocity. Weishaupt (2008) outlined the compensatory adaptations in locomotion of lame horses, in forelimb lameness the vertical impulse decreases in the lame forelimb and ipsilateral hindlimb, while increasing in the contralateral forelimb and diagonal hindlimb during trotting. Hobbs et al (2018) built on the studies of Wiggers et al (2015) adding 3 dimensional vector readings to all four hooves, finding again that the difference in hoof angle between pairs was directly related to the difference in vector forces (fig.1).
Fig.1 Authors illustration of the forelimb findings of Wiggers et al (2015) and Hobbs et al (2018).
Hobbs et al (2018) found that the lower hoof in the pair had increased breaking forces as found in Wiggers et al (2015) and an increased vertical force, mirroring the pattern of Weishaupt (2008) for lameness, with asymmetric propulsive forces the animal adapted hind limb locomotion in order to maintain the same speed.
“In this study, for some horses the bigger the difference in hoof angle, the more propulsive the diagonal hind becomes. For some horses the ipsilateral hind produced more medial force, but for all horses the left hind produced more lateral force. You also see that the side of the high hoof (fore and hind) produces less vertical force, although this is quite subtle. All of these adjustments are needed to maintain straight line motion at a steady state.” Hobbs (2019)
Fig.2 The asymmetrical propulsive forces of the front limbs create spinal stiffening as a functional connection to contralateral hind limb compensations, this creates a full body kinetic chain compensatory pattern.
Wiggers et al (2015) questions whether the difference in loading patterns were due to mechanical function or actually a sign of sub-clinical lameness, the author questioned whether solely the angles of the hoof are responsible for the biomechanical function and how much does the elasticity of the flexor units involved in the contrasting conformations play a role? Those two parameters are closely linked, Wiggers et al (2015) discussed the differences in fetlock mechanics, the flatter foot showed a larger vertical fetlock displacement and a suppler fetlock spring. Scheffer and Back (2001) found that 5 degree wedges decreased the maximal fetlock displacement, so it could be argued that wedging the flatter foot could artificially create more evenness, however wedging should always be done cautiously, weighed up against the potential consequences and applied to the correct individual conformation, this applies to a low foot with a broken back hoof pastern axis more readily than an ideal hoof pastern axis (that is the lower hoof because the opposing foot is boxy) as we know that the application of wedges affects most markedly the distal interphalangeal joint.
further research would need to be undertaken to confirm the effect of wedges on the measurements of Hobbs et al (2018) as studies on wedges have been contradictory, Chateau et al (2006) found no significant effect on maximal extension for example. Although the hoof angle has an effect, a percentage of the difference in vertical displacement could be attributed to the difference in elastic function of the distal limb tissues in particular the suspensory ligament, and the deep and superficial digital flexor muscles and tendons, which would not necessarily be immediately changed by a change in angle, however shortening of the flexor unit over time of a wedged foot could create more “stiffness”, all of this is theoretical and would need evidence based research to quantify and as stated by Wiggers et al (2015)
“Radiological, ultrasonographic or MR scan, and biochemical evaluation of the distal limb tissues of uneven footed horses, with special attention to the suspensory apparatus and the superficial and deep digital flexor tendons will add to the understanding of the etiology.”
We can see again, as in my article on “disordered pathology and hoof morphology”, that high-low hoof conformation can be created by physiological disorder and has a direct effect on the wider musculoskeletal system (fig.3), predisposing the animal to orthopaedic issues, these animals should possibly be more regularly monitored and treated for physiological dysfunction, they will be more prone to performance issues and these considerations are more pronounced the more asymmetric the feet. The title image also shows the common effects on the asymmetrical appearance and development of the shoulders and the anecdotal twist in the spine.
The job of the team involved with the horse is to assess the severity and recognize where asymmetric tolerance ceases and pathologic change and damage may begin. Appropriate trimming and/or shoeing combined with appropriate bodywork and riding is the key to keeping these horses sound. Dr Ridgeway (2016) discussed in some detail the physiological effects on the musculoskeletal system and highlighted the benefits of interventions that increase symmetry and balance, expressing the “functional limb length disparity” of high-low hooves and described how the difference in joint angles affects the muscular development. Ridgway (2016) also talked about vertebral function touched on by Hobbs et al (2018) who speculated that vertebral stiffening may be required to apply the locomotive adaptations required. Ridgway described the animals response to the imbalanced propulsive forces,
“The horse has to twist his head and neck to keep the eyes level. Horses therefore, often exhibit muscle pain, stiffness and spasm at the base of the neck. Moreover, because of dural torque (the tube in which the spinal cord is suspended and anchored), vertebral dysfunction and fixation occurs at the base of the neck. This, then, accounts for the muscle tension and pain around the sixth and seventh cervical vertebrae. It also causes dural torque (twisting) at the level of the poll and at the lumbo-sacral connection. Chiropractic issues are therefore common at all three levels as a result of the High Heel/Low Heel” Ridgway (2016)
Fig.3 All of the possible compensatory and pathological implications of high-low hooves. Clearly showing horses with high-low may be in need of more regular body work intervention.
These issues can then be exacerbated when rider and tack are brought into the equation, so again, in the authors opinion these horses require closer monitoring by all the practitioners involved, gait and rider analysis could also go a long way in keeping the team working optimally (See my article on objective gait analysis).
Farriery for this conformation is debated, research found considerations outlined by O'grady (2019) which should involve assessment of the individual conformation, assessing which hoof is closest to ideal and then shoeing each foot for optimal biomechanical function, remembering that form follows function, even if that means different shoeing styles on the same horse, for instance in the Authors experience, horses with the low hoof having a broken back hoof pastern axis could benefit from a shortened breakover and a wedge on that foot, compared to a “boxy” foot which may need the heels trimmed down and the toe fitted full, a club foot will need mechanics to compensate for an already strained deep flexor unit. O'Grady (2019) outlines protocols for more extreme cases. Treat each hoof individually and look at the whole horse, foot mapping can play a helpful role in restoring proportions. farriers will very rarely be able to create a matching pair but can do their part in creating more balanced functionality, both in the trimmed and bare foot. More research is needed to better understand other, further, possible applications to create more evenness and the author looks forward to learning practical farriery applications.
References
Hobbs. S, Nauwelaerts. S, Sinclair. J, Clayton. H, Back. W, 2018, Sagittal plane fore uneveness is associated with fore and hindlimb asymmetrical force vectors in the sagittal and frontal planes, Plos one, https://doi.org/10.1371/journal.pone.0203134
Watson KM, Stitson DJ, Davies HMS. Third metacarpal bone length and skeletal asymmetry in the Thoroughbred racehorse. Equine Vet J. 2003; 35: 712–714. pmid:14649365
Pearce GP, May-Davis S, Greaves D. Femoral asymmetry in the Thoroughbred racehorse. Aust Vet J. 2005; 83: 367–370. pmid:15986917
van Heel MCV, Kroekenstoel AM, van Dierendonck MC, van Weeren PR, Back W. Uneven feet in a foal may develop as a consequence of lateral grazing behaviour induced by conformational traits. Equine Vet J. 2006; 38: 646–651. pmid:17228580
Van Heel MCV, Van Dierendonck MC, Kroekenstoel AM, Back W (2010) Lateralised motor behaviour leads to increased unevenness in front feet and asymmetry in athletic performance in young mature Warmblood horses. Equine Vet J 42: 444–450. pmid:20636782
Wiggers N, Nauwelaerts SLP, Hobbs SJ, Bool S, Wolschrijn CF, Back W. Functional locomotor consequences of uneven forefeet for trot symmetry in individual riding horses. PLoS One. 2015; 10: e0114836. pmid:25646752
Moleman M, Van Heel MCV, Van Weeren PR, Back W (2006) Hoof growth between two shoeing sessions leads to a substantial increase of the moment about the distal, but not the proximal, interphalangeal joint. Equine Vet J 38: 170–174. pmid:16536388
Clayton HM (1990) The effect of an acute hoof wall angulation on the stride kinematics of trotting horses. Equine Vet J Suppl 9: 86–90. pmid:9259814
Buchner HHF (2013) Chapter 9: Gait adaptation in lameness. In: Back W and Clayton HM editors. Equine Locomotion, 2nd edition. Elsevier Health Sciences. Pp. 175–197.
Scheffer CJ, Back W (2001) Effects of ‘navicular’ shoeing on equine distal forelimb kinematics on different track surface. Vet Q 23: 191–195. pmid:11765238
Weishaupt MA. Adaptation strategies of horses with lameness. Vet Clin N Am Equine Pract. 2008; 24: 79–100.
Investigation into thoracic asymmetry in ridden horses
K. Merkies , J. Alebrand Related information, B. Harwood Related information, K. LaBarge Related information, L. Scott Related information
, 2019
. Chateau H, Degueurce C, Denoix JM (2006) Three-dimensional kinematics of the distal forelimb in horses trotting on a treadmill and effects of elevation of heel and toe. Equine Vet J 38: 164–169. pmid:16536387
(G.H. Wilson; K. McDonald; M.J. O’Connell, University of West England, Gloucestershire. Equine Veterinary Journal, 2009, (3) 238-241.
Farriery for Mismatched Feet
Reprinted with permission from the Farrier Products Distribution. Originally printed in The Natural Angle, Volume 16: Issue 2
Stephen E. O'Grady, DVM, MRCVS