„Since the turn of the century, we have known that the hoof is not a constant, unchanging body part. Depending on whether the horse’s hoof is either unloaded or carrying the entire horse’s weight, not only do the anatomical parts contained within the horse’s hoof change but also the form and shape of the hoof itself.“ (Lungwitz 1883: 4)
1 Historical overview[1]
The first historical reference to the elasticity of the horse hoof was made in 1754. The Frenchman Etienne Guillaume LAFOSSE, deemed one of the pioneers of the modern veteran anatomy, was the first person to describe the modification of the hoof under the load of the horse’s weight. LAFOSSE attributed his observations to the elasticity of the horn.
More than half a century later, the Englishman Bracy CLARK (1810) revived this theme, finding prompt access into the already blossoming sciences of veterinary medicine and hoof know-how. CLARK was of the opinion that the hoof’s elastic quality was due to it’s functional structure made up of the hoof wall, the sole and the frog. Unlike LAFOSSE, he attached great importance to the flattening of the sole arch and the expansion of the heel wall as a direct consequence of the frog yielding to ground counter pressure. He formulated trimming guidelines for the hoof, which included the thorough trimming of the sole.
The first practical experiments and measurements aimed at clarifying the issue of movement in the horse shoe took place some 40 years later. GLOAG (1849) was the first researcher to study hoof modification on both living and dead hooves. Unlike CLARK, he arrived at the conclusion that the hoof under load, did not cause the heels to widen and the sole to sink but merely caused a slight sinking of the heel bulb and the frog. Less than a year later, REEVE’s (1850) measurements proved the contrary. He discovered a sinking of the sole and a spreading of the heels. For his research, he used living horses shod with horseshoes, featuring horseshoes with a cross bar equipped with spikes aimed at the sole. During trotting and cantering, the spikes left an impression on the hoof sole, proving that the sole sank under load. In similar fashion, he proceeded to prove movement in the hoof wall. These findings were substantiated in the following year by Henri Marie BOULEY (1851) and William MILES (1852).
A decade later, August Gottlob Theodor LEISERING and Heinrich Moritz HARTMANN began their own research on living and dead hooves. They used „calipers“ which they positioned at specific points of the hoof wall and the sole. Whilst the measurements were somewhat contradictory, overall they spoke for a widening of the heel wall in the coronary region (2-4mm) and weight-bearing edge (2-3mm), as well as a contraction of the side wall in the coronary region (1-2mm) and a similarly sized expansion of the weight-bearing edge (1-2mm). Their measurements demonstrated a flattening of the sole, primarily in the heel region of the sole 1,5mm), which flattened more than the rest of the sole.
As a result, LEISERING distinguished between a constricting coffin bone mechanism and a widening navicular bone mechanism, combining to make up the movement of the hoof capsule. These views on hoof mechanism became established and subsequently enjoyed widespread, unrestricted approval.
Nobody dared „to doubt the accuracy of these proceedings. When suddenly, like a bolt out of the blue, a new teaching on the hoof mechanism arrived on the scene.“ (LUNGWITZ 1883: 5). In 1881, the viennese professor Jakob LECHNER challenged the existing views on hoof mechanism with a new theory. Through his own experimental research on hoof preparations, LECHNER concluded that the existing hoof widening theories („Erweiterungstheorien“) were fundamentally incorrect. In his view, the existing erroneous assumptions stood in the way of improving horseshoeing practices. LECHNER pitched his hoof rotation theory („Rotationstheorie“) against the recognised hoof widening theory. The rotation theory states that whilst the hoof expands in the region of the coronet at the moment of maximum loading, the bearing edge of the heel and angle of the wall/bar contract instead of expanding. Upon load removal, the hoof (inversely) contracts at the coronet, whilst the bearing edge rotates outwards up to it’s resting point. Furthermore, LECHNER contradicted the theory of the sole sinking under load, and suggested instead that during loading, the „Sohlenäste“ (transition from bar to sole) were raised due to the pull of the deep flexor tendon. LECHNER’s new theory of hoof mechanism served as catalyst, kicking off numerous new investigations. Whilst some of the research supported the hoof rotation theory (GIERTH 1882 and DOMINIK1889/90), the majority of the results refuted it (LUNGWITZ, A. and SCHAAF 1882, LUNGWITZ, A. 1882 and 1890, BAYER 1882, MARTINAK 1882, STEGLICH 1883, FÖHRINGER 1889, SCHWENTZKY 1890).
There waged a lively discussion on the nature of hoof mechanism over the next few decades. Many new studies were carried out in an attempt to cast light on the secrets of the hoof mechanism, often presenting results which contradicted predecessing research. Numerous papers were published, with various hoof mechanism or hoof mechanics themes filling the pages of the newly established journals. There was no consensus as to which hoof modifications took place during loading and unloading. Everyone agreed however, that special consideration should be given to the principles of the hoof mechanism and its critical importance to shoeing practice and maintaining soundness of health in horses.
„The key issue of horseshoeing is avoiding anything that could weaken or cancel out the hoof mechanism.“ (LEISERING; HARTMANN 1893: 157)
„A precise knowledge of the hoof mechanism is of paramount importance for the horseshoeing practice; deformations, hoof ailments, etc. can be addressed effectively, allowing the usability of the horse to be maintained over time.“ (WALTHER 1906: 481)
Whilst LEISERING and HARTMANN, together with numerous veterinarians of their time, shared the view that the undisturbed hoof mechanism was defined by a widening of the heels and that this widening should not be hindered by the horseshoe, WALTHER alongside LECHNER, GIERTH, DOMINIK and EBERLEIN represented the opposing view that the heels narrowed under load. Consequently, WALTHER blamed an excessive rather than an impaired hoof mechanism as the origin of contracted hooves. (WALTHER 1906: 490)
The following table shows an overview of the various hoof mechanism studies carried out during the 19th and 20th centuries.
author/year | assessment method | hooves | results: hoof modification during loading |
Gloag 1849 | pressure testing: wax insole | living and dead | · no widening of heels · no lowering of sole |
Reeve 1850 | pressure testing: Eggen-horseshoe (spikes on bar) | living, all gaits | · widening of bearing edge · lowering of sole |
Bouley 1851 | widening measurement (wm) | living and dead | · widening of bearing edge · lowering of sole |
Miles 1852 | wm: hoof wall circumference drawn on paper; weight-bearing vs. non-weight bearing | living, while standing | · widening of complete bearing edge |
Leisering; Hartmann 1870 | isolated wm: calipers | living and dead, standing | · widening of bearing edge (heels and side wall) · widening of coronet above heels and contraction above side wall · lowering of sole in the region of the transition from the bars to the sole („Sohlenäste“) |
Lechner 1881 | sections of hoof preparations, fixation with pins | dead | · narrowing of bearing edge and widening of coronet band (Rotationstheorie/rotation theory) |
A. Lungwitz; Schaaf 1882 | wm: measurement of hoof widening | living, standing, all gaits | · widening of bearing edge and coronet in heel region · widening increases with faster gaits · front hooves widen more than hind hooves |
A. Lungwitz 1882 | wm: calipers, slide calipers and strips of paper stuck to the bulbs | living | · confirmation of the above results · widening of the bearing edge is better in bare hooves, frog and flexible sole make ground contact · also widening of contracted heels, apart from last segment which collapses |
Bayer 1882 | acoustic wm: electrical bell | living, standing, all gaits | · widening of bearing edge and coronet · severely contracted hooves show narrowing of bearing edge · whilst widening of the coronet · increased hoof mechanism after 24 hours with a moist bandage |
Gierth 1882 | isolated wm: own measuring tool for assessment of hoof rotation | living | · considerable widening in the coronet, no movement in the bearing edge generally, except strong narrowing of bearing edge when measured at rear |
Martinak 1882 | wm: calipers, assessment of levers | living | · widening of heel bulb and bearing edge section |
Steglich 1883 | own measuring instrument, imitating the biomechanics of the whole limb | dead | · strong widening of the coronet in the heel section (due to sinking of coffin bone between hoof cartilage) and milder widening of the bearing edge (due to spreading of the digital cushion as well as sinking of the sole). |
Peters 1883 | sections of hoof preparations, fixation with pins | dead | · elasticity in the laminar horn allows the coffin bone to rotate around it’s tip, to sink (depression theory) · hoof wall is being deformed by the pull of the coffin bone resulting in a widening of the bearing edge in the heel section and sole sinking · strongest deformation during toeing-off |
Föhringer 1889 | acoustic wm: electrical bell | living | · mild widening of the coronet in the heel region and strong widening of the bearing edge |
A. Lungwitz 1890 | acoustic wm: electrical bell, further advanced | living | · expansion of heels and sinking of the sole · narrowing of the front half of the hoof and decrease of hoof height in shod hooves narrowing of the bearing edge at the heels possible |
Schwentzky 1890 | acoustic wm: electrical bell, further advanced | living, standing | · widening of bearing edge (apart from 4 abnormal hooves from a total of 22 hooves). · sinking of the sole |
Dominik 1889 | measuring apparatus for assessment of lever pressure | dead | · narrowing of bearing edge and widening of coronet band in the heel area |
Dominik 1890 | horseshoe with nails pointing at the hoof wall | living, trot | · constant narrowing of bearing edge in heel region |
Kösters 1903 | acoustic wm: electrical bell, further advanced | living, standing and walking | · constant widening of bearing edge and strong widening of the coronet in heel region · heel angle moves outwards and back · yielding upper toe wall · widening up to the „indifferent curve“ (parallel to the tubular horn and transition to quarter wall) no movement above. · no difference between front and rear hooves |
Eberlein 1903 | marks bar circumference, sharp-edged heel buttresses (Trachtenkappen) | living | · rub-grooves on horseshoes always within the unloaded heel bearing edge, this proves exclusively inwards movement of the heels and Lechner’s rotation theory (Lechners Rotationstheorie) |
Richter 1905 | wm: calipers, different measuring apparatuses, i.a. advanced electrical bell | living, standing | · narrowing of the front and widening of the rear hoof sections, sinking of the sole, decrease in hoof height · strongest modification takes place whilst sagging of limb at fetlock joint · modification is strongest with bare hooves, even when horseshoes support the frog · strongest modification in front hooves |
M. Lungwitz 1907/1909 | wm: measuring of lever pressure and electrical bell according to Bayer | living and dead, walking | · lowering of the sole and receding of the upper toe wall · strong widening of the coronet and mild widening of bearing edge in the heel region · shodden hooves (frog without ground contact) show milder widening, with occasional contraction of the heels on horseshoe, a sturdy frog always promotes widening · strongest modification whilst toeing-off |
Rudert 1921 | isolated wm: measuring of lever pressure for lowering of sole | living, lifting of partner limb | · lowering of sole, especially in the rear section of the hoof and most of all at the ridge of the frog · lowering of the sole is more pronounced on front feet than rear feet |
Habacher 1923 | wm: bell modified with optical signal | living, all gaits, jumping | · strong widening of the coronet and mild widening of the bearing edge in heel section · lowering of the sole and receding of the upper toe wall in all but last centimeter · indifferent line, not parallel to horn tubule · modification increases with load and speed |
Akerblom 1930 | wm: measuring of lever pressure (precursor of Mechano-Ungulograph) | living, all gaits, different terrain | · first continuous recording movement/gaits · there is no consistent form of hoof modification, it changes individually to a high degree · healthy bare feet demonstrate a stronger coronet and heel modification · differentiates between internal and external hoof movements |
Brunke 1931 | wm: measuring of lever pressure with scissor arm and curve plotter | living, all gaits, different terrains, different horseshoes | · distinctive widening of the bearing edge in the heel section as compared to the coronet · modification on hard ground more pronounced than on softer ground · horseshoes with insoles do not obstruct hoof mechanism (Hufmechanismus) |
Scholz 1952 | wm: mechano-ungulograph | living, during walk, different horseshoes | · horseshoe obstruct movement of the bearing edge and amplify movement of the coronet (over flexibility /„Überfederung“) · studs amplify heel movement, quarter clips obstruct widening of the heels · pointed hooves demonstrate more movement in the coronet in comparison to blunt hooves |
Weber 1957 | wm: mechano-ungulograph | living, during walk, different hoof shapes and horseshoes | · shoeing using various horse shoes influences movement of bearing edge and coronet (extent and balance) · effect differs depending of hoof form (wide, diagonal, etc.). recommends suitable standard horseshoe for each hoof type |
Knezevich 1962 | wm: hoof loading device, dial indicator, strain gauges | living and dead, all gaits | · overall similar results with both living and dead hooves · constant contraction of the toe coronet · bearing edge of the toe narrow (14x) or widen (15x) · indifferent curve (indiff. Linie) documented, parallel to tubular horn · more pronounced widening of the bearing edge (heel section) in comparison to the coronet · modifications stronger in warmblood than in draft horses |
Mair 1973 | strain measurement: strain gauges | living and dead, all gaits, different terrains, ridden and unridden | · lowering of the sole, more pronounced in living horses · increased loading leads to stronger widening of the hoof capsule in sections of the bearing edge · bearing edge of the heels widens more than dorsal section · hard ground amplifies movement |
Nieschalk- Meier 1979 | strain measurement: strain gauges and hydraulic press | living and dead, all gaits, ridden | · stance phase is characterised by compression, further amplified by pushing off · no differences between bare or shoed feet or between back and fore feet · no increase in movement with increased loading |
Preuschoft 1981 | strain measurement: strain gauges and hydraulic press | living and dead, all gaits, different terrains | · often without proportionality between loading and modification · constant widening of he heels · modification also present in hanging limb phase (narrowing) · no difference between back and fore feet |
Fischer-leitner 1974 | hoof loading device, dial indicator and radiology | dead | · lowering of the coffin bone and negative rotation during loading · causing recoiling/yielding of the toe wall, lowering of the sole and widening of the heels |
Harders 1985 | crackle finish measurement | living and dead | · stronger crack formation on fore feet and bare feet (independent of frog ground contact) · crack formation dependent on hoof form · lessening of crack formation on soft ground · no indifferent curve (indiff. Linie) detectable |
Source: Table generated using HARDERS tabular summary (1985: 26ff.) and supplemented with data from DOMINIK (1890), AKERBLOM (1930), WEBER (1957), FISCHERLEITNER (1975) and HENKE (1997).
During the history of the flourishing sciences of veterinary medicine and hoof knowledge, there was rarely a subject so intensively researched and passionately debated as the hoof mechanism. The form modification of the hoof under loading was the subject of great dispute. The often contradicting research results served to fuel discussion time and time again.
Eventually, the widening/expansion theory was commonly used to describe the modification of the hoof capsule and remains, until today largely unquestioned.
The expansion theory - valid to-date. The narrowing theory - previously contested and today outdated.
2 Current status
Even today, the elastic modification of the horn capsule during loading and unloading is recognised as hoof mechanism. The arising changes in modification are currently described in terms of the expansion theory:
„During the stance phase, the downward pull of the coffin bone causes the suspensory apparatus to pull the corresponding section of hoof wall downwards. As the wall is only moderately compressible in direction of the horn tubules, it is prone to bending around the tip of the coffin bone, moving in a downward and rearward direction corresponding to it’s angle to the ground. This „inward“ movement is greatest at the toe section of the coronet, reaching up to 1.5mm. The amplitude of movement decreases in size from the top down to the bearing edge approaching zero. … Upon lowering of the short pastern, the hoof capsule expands, just behind the broadest section of the coronet, resulting in the heel section of the coronet and the heel bulb to move outwards. … Due to the downwards-rearwards rotation of the coffin bone as well as the downward motion of the short pastern, the sole and the frog are brought against the ground causing the solar arch to flatten. This flattening of the sole brings about a widening of the bearing edge behind the broadest section of the hoof, which together with the heel section of the coronet are moved outwards.“ (RUTHE et al. 2012: 143f.)
The inherent subject hoof movement continues to be researched. Nowadays, this is carried out using mostly new methods and to a certain extent, under new premises. Some of the techniques used more recently include photoelastic synthetics (DAVIES 1996, DEJARDIN 2001), frozen sections and histological methods (HENKE 1997), the finite element analysis (HINTERHOFER et al. 1997, THOMASON et al. 2002), computer tomography (APPELBAUM; BRAGULLA 1996, APPELBAUM 2001), kinematic measurement system and high frequency infrared cameras (WEISSBACHER 2001) as well as digital photography (WOERGETTER 2003, HINTERHOFER et al. 2006).
These modern techniques allow the historical research and methodology of hoof mechanism in their full inconsistency to be reproduced. However, this new research is unable to explain the occurrence of these variations and inconsistencies. The literature describes on the one hand the essence of hoof mechanism, the outwards movement of the heels (COLLES 1989 cit by HENKE (1997: 86), VERSCHOOTEN et al. 1996, HENKE 1997), but as in the previous century, research showing the inwards movement of the heels is also to be found (WEISSBACHER 2001, WOERGETTER 2003, HINTERHOFER et al 2006, WOERGETTER et al. 2006 cit by HINTERHOFER et al. 2006). Using a loading device on the hooves of slaughter horses, HINTERHOFER et al. arrived at the conclusion that approximately 50% of the hooves moved outwards at the heels section, with the remainder moving inwards (2006: 314). There are also contradicting research findings with regards to the behaviour of the sole under load. With the help of the finite element model, HINTERHOFER (1997) calculated a slight elevation of the sole in the palmar region. Research by HENKE (1997), WEISSBACHER (2001) and HINTERHOFER et al. (2006), proves a constant sinking of the sole in the rear section of the hoof.
All the authors note that the hooves used in their investigation are entirely regular and healthy. Despite attempts to explain the contradicting results with the methodology of measurement (APPELBAUM 2001), they remain ultimately unexplained.
DAVIES et al. (2009) states that the hoof mechanism has not been sufficiently explored. There are currently three theories that offer an explanation as why the heel must widen under loading. (DAVIES 2009: 4) Both historically inherited theories explain expansion of the heels (a) with the pressure of the ground on the frog and the digital cushion, which leads to the „material“ within the hoof to be displaced, in-turn pushing the wall outwards or (b) with the opposing perspective based on the depression theory („Depressionstheorie“). The latter states that the rotation of the phalanx bones around the tip of the coffin bone (negative rotation of the coffin bone) and the sinking of the short pastern bone (the „queen of hoof mechanism“ PETER 1883: 5) between the collateral cartilages pushing apart the wall in the rear section of the hoof. A newly added theory, established on findings hoof mechanism research, uses the finite element analysis in order to visualise hoof movement in the hoof capsule model. Within the scope of this research work, it was repeatedly found that the hoof capsule, even when deprived of its’ inner workings (calculations only carried out on horn capsule) and also when no pressure was applied to the frog, an appropriate hoof mechanism with a sinking of the upper section of the toe wall and a widening of the heel section took place. A third theory based on this premise has recently been established which (c) states that hoof form and its’ geometry leads to hoof mechanism (DAVIES et al. 2009: 4). SMEDEGAARD; VINDRIIS (1995) also come to the conclusion during their testing of horses whilst trotting, that the hoof mechanism is a reaction of the hoof capsule (leaf spring system) to the impact force of the ground. HENKE points out that the plasticity of the horn capsule as the foundation of hoof mechanism had been described by NICKEL in the previous century:
„According to NICKEL (1938), the varying structure of the horn tubules in various sections of the hoof lend horn the capacity to react to pressure, flexing or traction in different ways, enabling the hoof capsule to deform as a whole.“ (HENKE 1997: 92)
If this discovery of plasticity of the hoof capsule is taken seriously, it is possible to explain why the long history on the hoof mechanism has produced ever contradicting results. If pressure, flexion, and traction actually do affect the horn tubules in different ways, it is clearly evident how the mechanics of hooves can vary as much as the form of hooves themselves.
Despite this, both veterinary medicine and hoof literature continue to uphold the two significant form modifications which characterise hoof mechanism; „the horizontal expansion of the heels, which move apart from each other…, and a concavity or vaulting of the dorsal hoof wall“. (DAVIES et al. 2009: 4)
Even today, the hoof mechanism is attributed a major role in maintaining a healthy hoof. The benefit of hoof mechanism stands as clearly proven and is summarised as follows:
„The benefits of the hoof mechanism are manifold. 1. the impact is dampened and the torso is protected against the usual effects of contusion and vibration. 2. The hoof mechanism promotes the elasticity of the whole leg and contributes to the light and elegant gait of the horse. 3. The hoof mechanism actively supports the blood circulation in the hoof dermis, which in turn encourages a vigorous hoof growth.“ (LUNGWITZ 1883: 5)
and accordingly in the modern version:
„the hoof mechanism or the modification of the hoof are significant for various aspects of horse health. As an elastic component between the ground and the coffin bone, the hoof plays a role in dampening the limb during footfall. The vibrations caused in the horn capsule and the suspensory apparatus during landing are dampened, with only a small amount reaching the coffin bone and phalanx joints. This safeguards against overloading of the articulate cartilage and therefore the formation of arthrosis. Furthermore, the cyclic compression and decompression of the inner hoof parts as induced by hoof mechanism supports the blood circulation which has positive effect on nutrient and oxygen supply.“ (RUTHE et al. 2012: 143f.)
Due to these health benefits for hoof and horse, special consideration for the hoof mechanism during hoof treatment and shoeing is promoted today, as it used to be.
3 Consequences for hoof treatment
Hoof mechanism theories have always had consequences on hoof treatment. Upon discovering the elasticity of the horn capsule in the 18th century and closer investigation in the 19th century, one began to implement the newly gained findings in the hoof treatment and shoeing practice. Since then, there has been plethora of measures, which are expressly undertaken, with the purpose of either strengthening or restoring hoof mechanism, in case „it has been weakened“. (HANSLIAN 1932: 175). Ever since discovering the elasticity of the hoof capsule, there has been a search for types of shoeing that respect movement within the hoof and hamper it’s function as little as possible.
In 1836, CLARK, one of the first theoreticians and advocates of the widening theory of hoof mechanism, built a steel tray shoe which was designed in such a way that the toe section was kept flexible by using nail rivets. This hinged design allowed the bars of the horseshoe to follow the outwards movement of the hoof wall. Much to the amusement of his opponents, the rivets at he front became loose and the horseshoe proved to be a failure. The hoof wall suffered to such an extent due to the instability of the horseshoes, that the hooves frequently cracked under the strain. (LEISERING/HARTMANN 1893: 17f.; LINGENS 2008: 27) CLARK actively promoted his view that the lowering of the sole bared great importance for the proper function of the hoof mechanism. He demanded that the sole be trimmed until it is elastic. As a benchmark, „as to how much should be removed, he recommends ‚the fresh appearance of the sole‘ or the ‚feeling that the sole gives to pressure from thumb‘ “. (KRAUSE 1926: 14)
MILES (1852) also demands „the central point of his method of preparation should be the sinking and the rising hoof sole full with the aim of achieving full mobility“ and trimming the horn „until the sole yields to strong thumb pressure“. (KRAUSE 1926: 19) The following years saw followers of the Clark theory surpassing themselves with various measures aimed at promoting hoof mechanism. With „reduction of the heels, cutting of sole shanks, thinning out the bars“ they proceeded to „achieve the aim‚ ‘that the hoof should bend like a branch of a willow tree‘ “. (PETERS 1883: 46) Moreover, a whole series of measures were developed over time aimed at resurrecting the weakened hoof mechanism of „sickly“ hooves. The heels were thus trimmed until the underdeveloped frogs reached the ground, or one tried cutting grooves into the heels or into the bars as well as thinning out/rasping the heel wall in order to reactivate narrow hooves. Special horseshoes for heel widening as well as horseshoes with bar-clips were produced designed to force the rear section of the hoof apart.
- For the historical overview I refer to the works of LEISERING; HARTMANN (1893: 145ff.), AKERBLOM (1930: 4ff.) and HARDERS (1985: 6ff.)
- compare also PELLMANN (1995: 91f.) and HENKE (1997: 81f.)
- According to investigations of HIRSCHBERG/BRAGULLA and against statements in the literature, bicuspid valves can be detected in the veins up to the border between deep and superficial layer of the dermis. The valves enhance the blood flow out of the hoof. (2007: 34)