Sole depth and weight-bearing characteristics of the palmar surface of the feet of feral horses and domestic Thoroughbreds

Brian A. Hampson Australian Brumby Research Unit, School of Veterinary Science, The University of Queensland, St Lucia, Brisbane, QLD 4072, Australia.

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Alexandra D. Connelley Australian Brumby Research Unit, School of Veterinary Science, The University of Queensland, St Lucia, Brisbane, QLD 4072, Australia.

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Melody A. de Laat Australian Brumby Research Unit, School of Veterinary Science, The University of Queensland, St Lucia, Brisbane, QLD 4072, Australia.

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Paul C. Mills Australian Brumby Research Unit, School of Veterinary Science, The University of Queensland, St Lucia, Brisbane, QLD 4072, Australia.

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Chris C. Pollitt Australian Brumby Research Unit, School of Veterinary Science, The University of Queensland, St Lucia, Brisbane, QLD 4072, Australia.

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Abstract

Objective—To determine solar load-bearing structures in the feet of feral horses and investigate morphological characteristics of the sole in feral horses and domestic Thoroughbreds.

Sample—Forelimbs from cadavers of 70 feral horses and 20 domestic Thoroughbreds in Australia.

Procedures—Left forefeet were obtained from 3 feral horse populations from habitats of soft substrate (SS [n = 10 horses]), hard substrate (HS [10]), and a combination of SS and HS (10) and loaded in vitro. Pressure distribution was measured with a pressure plate. Sole depth was measured at 12 points across the solar plane in feet obtained from feral horses from SS (n = 20 horses) and HS (20) habitats and domestic Thoroughbreds (20).

Results—Feet of feral horses from HS habitats loaded the periphery of the sole and hoof wall on a flat surface. Feral horses from HS or SS habitats had greater mean sole depth than did domestic Thoroughbreds. Sole depth was greatest peripherally and was correlated with the loading pattern.

Conclusions and Clinical Relevance—The peripheral aspect of the sole in the feet of feral horses had a load-bearing function. Because of the robust nature of the tissue architecture, the hoof capsule of feral horses may be less flexible than that of typical domestic horses. The application of narrow-web horseshoes may not take full advantage of the load-bearing and force-dissipating properties of the peripheral aspect of the sole. Further studies are required to understand the effects of biomechanical stimulation on the adaptive responses of equine feet.

Abstract

Objective—To determine solar load-bearing structures in the feet of feral horses and investigate morphological characteristics of the sole in feral horses and domestic Thoroughbreds.

Sample—Forelimbs from cadavers of 70 feral horses and 20 domestic Thoroughbreds in Australia.

Procedures—Left forefeet were obtained from 3 feral horse populations from habitats of soft substrate (SS [n = 10 horses]), hard substrate (HS [10]), and a combination of SS and HS (10) and loaded in vitro. Pressure distribution was measured with a pressure plate. Sole depth was measured at 12 points across the solar plane in feet obtained from feral horses from SS (n = 20 horses) and HS (20) habitats and domestic Thoroughbreds (20).

Results—Feet of feral horses from HS habitats loaded the periphery of the sole and hoof wall on a flat surface. Feral horses from HS or SS habitats had greater mean sole depth than did domestic Thoroughbreds. Sole depth was greatest peripherally and was correlated with the loading pattern.

Conclusions and Clinical Relevance—The peripheral aspect of the sole in the feet of feral horses had a load-bearing function. Because of the robust nature of the tissue architecture, the hoof capsule of feral horses may be less flexible than that of typical domestic horses. The application of narrow-web horseshoes may not take full advantage of the load-bearing and force-dissipating properties of the peripheral aspect of the sole. Further studies are required to understand the effects of biomechanical stimulation on the adaptive responses of equine feet.

The equine hoof capsule is the tough epidermal structure that encases the vulnerable dermal and skeletal structures within. The structure and function of the wall component of the equine hoof capsule have been investigated.1–3 However, the contribution of the solar surface of the hoof capsule as a weight-bearing and force-dissipating structure has received limited attention, and its morphological details are poorly defined. The importance of the sole to the mechanical integrity of the equine foot may have been underestimated.

The sole of the equine hoof makes up the distal casing of the hoof capsule and occupies the region between the wall and cuneus ungulae (ie, frog) on the palmar surface of the hoof. It has generally been considered a protective layer for the structures immediately subjacent to it, including the DP, solar corium, and digital cushion. In the past, it was considered that a horse should bear its weight on the external hoof wall. Traditional and modern farriery practices use this model. A horseshoe is applied to the external hoof wall, and the central solar surface of the foot is allowed to bear weight only when that horse spends most of its time on an SS.4 However, the sole, which is firmly attached to the external hoof capsule through its attachment to the white line, may also be an important load-bearing structure.5–7 The recent popularity of the wild horse foot model5,7 has challenged traditional farriery practices. The proponents of this model suggest that the study of the hooves of wild horses may improve knowledge of the equine hoof, which may benefit hoof-management practices.

Feral horses in Australia have been tracked with global positioning system technology, and they can travel up to 28 km/d over rocky HS.8 The ability of these horses to travel long distances over rough terrain with apparently no adverse effects may be attributable to the unique morphology of their hooves, which have adapted in response to the challenging environment.

The high volume of biomechanical events may induce adaptive responses of the weight-bearing structures in contact with the substrate.3 The relationship between biomechanical loading and tissue responses is not fully understood. It has not been determined whether optimal foot health is obtained by increases in mechanical loading.

It is widely accepted that a thick sole contributes to good foot health.9 Thin soles have been incriminated as a risk factor for sole bruising, abscesses, corns, laminar tearing, and a lack of confidence of movement.9 Despite the perception that the sole is an important functional structure in equine feet, minimal empirical research has been conducted to investigate sole depth measurements in horses. Sole depth is commonly measured as the distance from the distal tip of the DP to the ground surface on a lateromedial radiograph.6,10 This single reference point apparently has been selected for convenience, and there is no evidence to suggest that the measurement actually represents the extent of the solar structure. Greater understanding of sole morphology, including similarities and differences between population groups, may assist in determining whether sole thickness relates to foot health and the manner by which this can be tested clinically.

The load-bearing pattern of equine feet has been investigated in managed domestic horses.4,11–15 However, 1 study5 was conducted to investigate load-bearing structures of wild horses. The author of that report5 used the feet of equine cadavers to make assumptions regarding load-bearing structures by evaluating the marks made by painted feet pressed against a paper surface. In contrast to results of studies in domestic horses, the peripheral aspect of the sole of the foot of wild horses appeared to contribute markedly to the weight-bearing surface, possibly because of the short length of the hoof wall in wild horses. The link between the role of the sole in load bearing and sole morphology has not been investigated. The purpose of the study reported here was to determine the load-bearing structures of the feet obtained from fresh cadavers of feral horses under various vertical load magnitudes and for horses that lived on various substrate types. Additionally, sole morphology of feral horses and domestic Thoroughbreds was investigated to assess the link between form and function in various substrate environments.

Materials and Methods

Sample—Forelimbs were obtained from the cadavers of 70 feral horses and 20 domestic Thoroughbreds in Australia. All feral horses were obtained from standard feral horse-culling operations unrelated to the study. Age of feral horses was determined on the basis of dentition; horses were assessed as mature (estimated age > 4 years). Age was the only selection factor for feral horses. Samples from managed domestic Thoroughbreds were obtained from an abattoir; these horses were identified as Thoroughbred on the basis of type and brand. The project was approved by the University of Queensland Animal Ethics Committee, which monitored compliance with the Animal Welfare Act (2001) and The Code of Practice for the care and use of animals for scientific purposes.

Assessment of load-bearing structures—Left fore-limbs of mature feral horses from each of 3 populations were used. Feral horse populations from SS, HS, and a combination of SS and HS habitats (10 horses/habitat) were selected on the basis of the hardness of substrate on which they spent most of their time. The area of each horse population ranged from 4,000 to 200,000 hectares. Limbs were disarticulated at the radiocarpal joint, immediately packed in ice, and frozen within 8 hours after horses were euthanized.

Feet were selected as representative of the typical foot type for each population (Figure 1). Feet obtained from feral horses that lived in sandy SS habitats (SS group) had long overgrown walls that were flared and had large fragments that had broken off. The quarters eroded with a large arch from the toe-quarter to a point just dorsal to the heel. Feet obtained from horses that lived in a combination of SS and HS habitats (combination group) had shorter walls and some flaring of the walls, but length appeared to have been maintained by abrasion rather than by fragmentation of the hoof wall. Feet obtained from horses that lived in HS habitats (HS group) had short walls that were worn to the level of the peripheral aspect of the sole, had minimal flaring of the wall, and had smooth beveled distal aspects of the walls. The quarter arch was minimal or nonexistent. The distal aspect of the dorsal wall occasionally was worn excessively so that the dorsal wall was square and concave.

Figure 1—
Figure 1—

Photographs of typical foot conformations of feral horses in Australia that lived in SS habitats (SS group; A), a combination of SS and HS habitats (combination group; B), or HS habitats (HS group; C). The SS group had long overgrown walls that were flared and had large fragments that had broken off. The combination group had shorter walls and some flaring of the walls, but length appeared to have been maintained by abrasion rather than by fragmentation of the hoof wall. The HS group had short walls that were worn to the level of the peripheral aspect of the sole, had minimal flaring of the wall, and had smooth beveled distal aspects of the walls. The scale in the upper left is in centimeters.

Citation: American Journal of Veterinary Research 72, 6; 10.2460/ajvr.72.6.727

Limbs were thawed at 25°C on the day of the experiment, which was ≤ 2 weeks after collection. Limbs were loaded on a custom-built hydraulic loading device designed to apply a designated downward vertical force of 0 to 7,000 N via a hydraulic ram moving at a rate of 10 cm/s. The limbs were mounted with the third metacarpal bone positioned vertically and the loading arm positioned over the center of the foot. The load was applied through the proximal aspect of the carpus by use of a cup attached to the distal end of the hydraulic ram. During loading, the position of the third metacarpal bone changed with respect to the vertical axis, depending on the load and tissue compliance. A pressure platea was mounted on the base of the hydraulic loading device (Figure 2). The pressure plate used in the study was a pressure measurement plate that contained pressure-sensitive polymer sensors (surface area, 0.39 cm2). The threshold value for the pressure plate was set at 5 N/cm2 to enable us to discard noise-related data. All pressures greater than this threshold were distributed over 256 available colors between the threshold (blue) and the maximum value (red). A frequency of 253 Hz was used for data collection. Loading data were analyzed with the pressure plate software to provide the sum of the vertical forces and area of the load-bearing surface.

Figure 2—
Figure 2—

Photograph of the left forelimb of a feral horse placed in a hydraulic apparatus used for loading the distal aspect of the limb vertically though the proximal portion of the carpus (a pressure plate is positioned on the base of the hydraulic loading apparatus; A), and the left foot of a feral horse sectioned through the DP tip, indicating locations for epidermal and dermal depth measurements (B). FLE = Far left epidermis. FRE = Far right epidermis. MD = Mid dermis. ME = Mid epidermis. MLD = Mid left dermis. MLE = Mid left epidermis. MRD = Mid right dermis. MRE = Mid right epidermis.

Citation: American Journal of Veterinary Research 72, 6; 10.2460/ajvr.72.6.727

Load-bearing surface response among foot types—Highly repeatable results (< 10% variation) in load magnitude and load-bearing surface response of 3 consecutive loads for 3 representative limbs were determined in a preliminary study. On the basis of the results of that preliminary study, all 30 limbs from the feral horses were loaded once over the pressure plate with a vertical force of 2,500 N. The feet were loaded over a 3-mm-thick firm rubber mat to protect the surface of the pressure plate. This firm rubber mat presumably added some compliance to the testing system and was kept constant for all trials. The loading pattern and area of load-bearing surface were determined by use of the pressure plate software. Data were analyzed to determine differences in load-bearing surface areas among the 3 sample populations. Colored pressure distribution was analyzed subjectively to identify differences in loading patterns among groups.

Effect of substrate on load-bearing pattern—The 10 limbs from the HS group were loaded over the pressure plate at a load of 2,500 N. Three substrates were placed between a foot and the pressure plate to simulate loading at the mid stance position on various substrates. Substrate 1 was the aforementioned 3-mm-thick firm rubber mat. Substrate 2 was a 20-mm-deep layer of 10 × 30-mm gravel placed over the same rubber mat. Substrate 3 was a 20-mm-deep layer of coarse sand placed over the same rubber mat. Substrates were placed in a plastic container from which the bottom had been removed; the plastic container maintained the gravel and sand substrates within the foot area. The inside perimeter of the plastic container had a mark (depth of 20 mm) to assist in maintenance of a consistent depth of substrates 2 and 3. Substrate was mixed and the surface was smoothed between subsequent trials to remove the previous footprint and minimize compacting with successive trials. Each limb was loaded once on each of the 3 substrates. The area of the load-bearing surface was determined by use of pressure plate software. Data were analyzed to determine differences in load-bearing surface areas among the 3 substrates. Colored pressure distribution was analyzed subjectively to identify differences in loading patterns among substrates.

Effect of load magnitude on the load-bearing surface response—The 10 limbs from the combination group were incrementally loaded over a 3-mm-thick firm rubber mat on the pressure plate at loads of approximately 800, 1,300, 2,500, 3,500, 4,500, and 6,000 N. The load-bearing surface (area in square centimeters) was determined by use of pressure plate software. Data were analyzed to determine differences in load-bearing surface areas among the 6 incremental loads. Visual examination of the color-coded pressure pattern on the pressure plate was used to subjectively analyze loading patterns and to determine changes in palmar foot structures that contacted the ground surface and exceeded the threshold value during loading. Additionally, the minimum vertical force above the threshold value required to initiate contact of the frog was recorded.

Assessment of sole morphology—Feet of the left forelimbs of feral horses from each of the SS and HS groups (n = 20 feet/habitat) and from 20 managed domestic Thoroughbreds were evaluated. The 20 mature feral horses in the SS group were from a population of > 10,000 feral horses from an unfenced 100,000-hectare area of Northern Queensland and the Northern Territory. The area was coastal tea tree plain, and habitat was entirely sand. Horses were light Thoroughbred-type animals. The 20 mature feral horses in the HS group were from a population of > 20,000 feral horses from an unfenced 300,000-hectare area of hard rocky desert in central Australia. The 20 Thoroughbreds were selected from a group of 40 Thoroughbreds on the basis of gross observation of foot health. Any feet with gross hoof lesions and obvious signs of poor foot health were excluded from the study. The state of hoof care (trimming and shoeing) was not a selection factor, and hooves ranged from having received no recent hoof care to having been recently trimmed and shod. The racing history of the Thoroughbreds was not known, although several horses appeared to have been recently shod and were in race condition. Age of Thoroughbreds was estimated by use of a combination of results of dental examination and identification of brand year.

The left forelimb of each of the 60 horses was disarticulated at the radiocarpal joint immediately after the horses were euthanized. Limbs were immediately chilled on ice and stored at −20°C within 8 hours after sample collection. The feet remained frozen throughout data collection to minimize the effects of tissue shrinkage attributable to moisture loss.

The method for basic assessment of hoof morphology has been described in detail.16 Hoof morphology of the sample population was determined in an effort to categorize the type and variety of feet in the sample population.

The hoof was removed from each forelimb by use of a band saw via a single transverse cut 2 cm proximal to the coronary band through the middle phalanx; all cuts with the band saw were made by the same investigator (BAH). The hoof was placed on a flat surface, and the most palmar weight-bearing point was marked on the external hoof wall with an ink marker. Four sectional cuts were then made in succession with the band saw. The hoof was positioned on its dorsal wall, and a sagittal section bisecting the hoof was made through the midline of the frog. All additional cuts were made on medial and lateral sagittal half sections at the same location. One cut was made perpendicular to the sagittal plane of each half section at the level of the tip of the DP; this cut was made perpendicular to the sole-bearing surface. Another cut was made at the junction of the sole and the tip of the frog; this cut also was made perpendicular to the sole-bearing surface. The palmar heel cut was made in the plane perpendicular to the sagittal section through the most palmar weight-bearing point of the heel (previously marked with ink); this cut also was made perpendicular to the sole-bearing surface.

Each of the cut sections (sagittal, tip of the DP, tip of the frog, and palmar heel) was positioned separately in normal anatomic alignment so that a photograph could be obtained for use in determining the full sole depth perpendicular to the sole-bearing surface. A digital camerab fitted with a 55-mm lensc and high-performance flash unitd was screw mounted to the proximal end of a custom-built jig and positioned 700 mm from each section. The camera was aligned to the center of each coronal section, and a 100-mm ruler with the hoof number and section identification was placed in the section plane to provide image calibration and identification. Each section was placed in the same position in relation to the camera and photographed by the same investigator (BAH).

One investigator (ADC) performed all measurements. Prior to performance of measurements, an internal validity study was conducted. The full set of depth measurements for the dermal and epidermal sole (n = 12) was performed 5 times by 1 author on 6 randomly selected hooves. The investigator was unaware of the group assignments for all hooves, and the order of measurements was randomized for the internal validity assessment and main evaluation of sole depth.

Digital image measurements were obtained from 5 points across sections of the tip of the DP and tip of the frog (Figure 2) by use of computer measuring software.e Measurements at 2 locations were of only epidermal depth; the epidermal depth was measured from immediately at the end of the last dermal lamellae to the solar surface of the epidermis perpendicular to the ground surface on medial and lateral segments of the hoof. A few specimens had loose epidermis adhered to the sole, and this was measured as part of the epidermis. The third measurement was the dermal and epidermal depth from the cut surface of the medial sagittal section. The DP bone-dermis junction to the solar surface of the epidermis was measured, which represented the central axis of the hoof. The fourth and fifth dermal and epidermal measurements were from the DP bone—dermis junction to the solar surface of the epidermis on medial and lateral sides of the hoof at a point halfway between the first epidermal measurement and the middle bisecting measurement.

A palmar heel measurement was obtained from the most distal dermal-epidermal junction to the palmar extent of the epidermis perpendicular to the ground surface. It measured the vertical epidermal support in the most palmar weight-bearing surface of the heels.

Statistical analysis—Statistical analyses were performed by use of a statistical program.f Sole depth measurements were compared among groups by use of an ANOVA. A Welch t test was used when comparing measurements between 2 groups. Tukey multiple comparison of means was used to perform pairwise comparisons between 3 groups. Sole depth measurements were correlated by use of a Pearson product moment correlation. All measurements were reported as mean ± SE. Values of P < 0.05 were considered significant.

Results

Load-bearing surface response among foot types—Load-bearing surface response to vertical loading differed significantly (P = 0.01) among the 3 groups of feral horses. The load-bearing surface response was significantly greater in the HS group than in the SS and combination groups. However, the load-bearing surface response between horses from the SS and combination groups did not differ significantly (Figure 3).

Figure 3—
Figure 3—

Mean ± SE load-bearing surface response of the left forefeet of feral horses (n = 10 horses/group) to vertical loading of 2,500 N. The load-bearing surface response for feral horses from 3 types of habitat is indicated in the first group, and the in vitro response of feet of HS horses to 3 types of surface substrate (substrate 1, 3-mm-thick firm rubber mat; substrate 2, 20-mm-deep layer of 10 × 30-mm gravel placed over the same rubber mat; and substrate 3, 20-mm-deep layer of coarse sand placed over the same rubber mat) is indicated in the second group. *Value differs significantly (P = 0.01) from the values for the SS and combination groups. †Value differs significantly (P = 0.01) from the value for substrate 1. See Figure 1 for remainder of key.

Citation: American Journal of Veterinary Research 72, 6; 10.2460/ajvr.72.6.727

Feet from the SS group had long overgrown flared walls with the quarters broken away to create a large arch from the toe-quarter to a point just dorsal to the heel. Feet from the SS group had the lowest pressure contact areas when loaded on substrate 2 (gravel). The pattern of pressure was typically a 3-point loading pattern with pressure on either side of the hoof wall mid-line and on both heels (Figure 4). There was a lack of loading of the sole and frog in the HS group.

Figure 4—
Figure 4—

Photographs of typical load-bearing surface patterns produced by use of the pressure plate for feet of feral horses from the SS group (A), combination group (B), and HS group (C). Feet were placed on a 3-mm-thick firm rubber mat and loaded with a force of 2,500 N.

Citation: American Journal of Veterinary Research 72, 6; 10.2460/ajvr.72.6.727

Feet from the combination group had shorter walls and some flaring, but wall length was maintained by abrasion rather than by fragmentation. When loaded on substrate 2 (gravel), feet from the combination group typically had a 3-point load-bearing surface pattern on the hoof wall periphery, which was larger in the heel and dorsal areas, compared with results for the SS group (Figure 4). The mean load-bearing surface for this group was 48% greater than the mean for the SS group. When the limbs from the combination group were under load, 2 of the 10 feet had contact of the frog; however, there was no contact of the central aspect of the sole.

Feet from the HS group had the shortest walls, which were often worn to the level of the peripheral aspect of the sole. Feet from this group also had minimal wall flaring, and the distal aspects had smooth beveled walls. The quarter arch was minimal or nonexistent. The distal aspect of the dorsal wall was occasionally worn excessively so that the dorsal wall was squared and concave. When loaded on substrate 2 (gravel), feet from the HS group typically had a load-bearing pattern with a full hoof wall and peripheral aspect of the sole, with the higher pressures concentrated at the dorsal wall and heels (Figure 4). Contact was evident at the quarters, but it was reduced in magnitude. The mean load-bearing surface for the HS group was 184% greater than that for the SS group.

Effect of substrate on load-bearing surface pattern—Load-bearing surface response was significantly (P = 0.01) different among the various substrates tested. Load-bearing surface response for substrate 1 differed significantly (P = 0.01) from the response for substrates 2 and 3, whereas load-bearing surface responses did not differ significantly between substrates 2 and 3 (Figure 3).

Effect of load magnitude on the load-bearing surface response of the equine foot—The load-bearing surface increased in proportion with the vertical force applied (Figure 5). Overall, vertical force had a significant (P = 0.01) effect on the load-bearing surface area.

Figure 5—
Figure 5—

Mean ± SE load-bearing surface response of the left forefeet of feral horses in the SS and HS groups (n = 10 horses/group) to incremental increases in the vertical loading force applied by use of a hydraulic apparatus.

Citation: American Journal of Veterinary Research 72, 6; 10.2460/ajvr.72.6.727

Assessment of sole morphology—Gross hoof morphometric measurements were summarized (Table 1). The 20 feral horses in the HS group comprised 5 females and 15 males, whereas there were 7 females and 13 males in the SS group and 9 females and 11 males in the Thoroughbred group. Mean ± SD age for the HS group, SS group, and Thoroughbred group was 10.5 ± 4.3 years, 13.3 ± 5.6 years, and 11.9 ± 5.4 years, respectively. Height at the highest point of the shoulders (ie, withers) with the horse positioned in lateral recumbency was 1,598.5 ± 39.8 mm and 1,579 ± 52.9 mm, respectively, for the HS and SS groups.

Table 1—

Mean ± SD morphometric measurements for feral horses (HS and SS groups) and managed domestic Thoroughbreds (n = 20 horses/group).

VariableHS groupSS groupThoroughbreds
MWA82.4 ± 4.182.8 ± 2.777.7 ± 4.6
MFA5.5 ± 4.09.7 ± 5.511.8 ± 7.4
LWA80.7 ± 3.576.5 ± 5.275.9 ± 4.1
LFA4.3 ± 5.38.5 ± 16.97.9 ± 5.2
FL98.1 ± 8.195.2 ± 9.4112.2 ±7.7
FW126.7 ±9.9119.2 ±8.3134.7 ± 7.9
Frog-ground5.1 ± 0.93.3 ± 2.81.0 ±2.9
DWA56.0 ± 2.356.8 ± 4.457.5 ± 4.8
HA43.7 ± 2.841.3 ±6.738.0 ± 6.2
DFA0.4 ± 1.04.9 ± 4.17.9 ± 4.8
DWL79.2 ± 4.787.0 ± 6.790.9 ± 6.2
QR3.7 ± 1.16.6 ± 4.54.9 ± 3.2
Wall-sole−0.8 ± 0.81.5 ±2.61.1 ± 2.3
Solar cup3.0 ± 1.35.5 ± 2.93.9 ± 1.0

DFA = Dorsal flare angle. DWA = Dorsal wall angle. DWL = Dorsal wall length (lateral view). FL = Foot length (sole view). Frog-ground = Height of frog above surface of ground. FW = Maximum foot width (sole view). HA = Heel angle. LFA = Lateral flare angle. LWA= Lateral wall angle. MFA= Medial flare angle. MWA= Medial wall angle. QR = Quarter relief height. Solar cup = Depth of cup on sagittal section. Wall-sole = Length of distal aspect of dorsal wall beyond sole.

Tip of the DP—All 12 measurements were performed 5 times each during a preliminary validation study in each of 6 feet. The mean variance was 3.9% of the mean value. The mean epidermal depth at all 5 measurement locations was greatest in the HS group, followed by the SS group and the Thoroughbred group (Figure 6). Epidermal depth differed significantly (P = 0.01) among the 3 groups at each of the 5 measurement locations. Medial and lateral epidermal depth measurements were significantly (P = 0.01) correlated at the extreme periphery (r = 0.74) and mid periphery (r = 0.82) measurement sites, irrespective of group. All measurements differed significantly (P = 0.01) between feral horses (HS and SS groups) and managed domestic Thoroughbreds.

Figure 6—
Figure 6—

Mean ± SE epidermal sole depth measured at 5 locations at the tip of the DP in feral horses from HS (black bars) and SS (light-gray bars) habitats and managed domestic Thoroughbreds (dark-gray bars). There were 20 horses/group. *Within a location, the value differs significantly (P = 0.01) from the values for the HS and SS groups. †Within a location, value differs significantly (P = 0.01) from the value for the HS group. See Figure 2 for remainder of key.

Citation: American Journal of Veterinary Research 72, 6; 10.2460/ajvr.72.6.727

Within feral horses, there was a significant (P = 0.01) difference in middle epidermal and mid right epidermal depth between the SS and HS groups. However, there were no significant differences between the SS and HS group for all other measured variables (Figure 6).

Tip of the frog—All epidermal and dermal depth measurements at the tip of the frog were greatest in the HS group and reduced in the SS and Thoroughbred groups (Table 2). An overall comparison of sole depth measurements revealed significant differences among the groups. A comparison between HS and SS groups revealed that the far left epidermis was the only location for which measurements did not differ significantly. Values for the mid dermis did not differ significantly between the HS and Thoroughbred groups. Comparisons between the SS and Thoroughbred groups revealed significant differences for all measurements, except for mid left dermis, mid dermis, middle epidermis, mid left total, and mid right dermis. Dermal depth at the tip of the frog had the same pattern as did the epidermal depth among the 3 groups.

Table 2—

Mean ± SE sole depth measured at the tip of the frog and palmar heel for feral horses that lived in HS habitats (HS group; n = 20 horses) or SS habitats (SS group; 20) and for managed domestic Thoroughbreds (20).

LocationHS groupSS groupThoroughbreds
FLE11.7 ±0.3510.8 ± 0.608.5 ± 0.34
MLD3.9 ±0.123.0 ± 0.173.3 ±0.13
MLE*13.2 ± 0.398.8 ± 0.407.7 ± 0.32
MLT*17.1 ± 0.3512.3 ±0.3611.0 ±0.32
MD3.0 ±0.162.4 ± 0.202.8 ±0.13
ME12.6 ± 0.779.6 ± 0.5910.6 ± 0.52
MT15.6 ± 0.7912.0 ±0.5913.2 ± 0.45
MRD4.1 ±0.153.0 ±0.123.2 ±0.17
MRE*12.1 ±0.528.7 ± 0.437.1 ± 0.31
MRT*16.2 ± 0.4711.7 ±0.4510.3 ± 0.30
FRE*12.3 ± 0.3810.8 ± 0.417.9 ± 0.30
MPH*15.6 ± 0.5520.4 ± 0.8913.3 ± 1.00
LPH*16.0 ± 0.7118.9 ± 0.8312.5 ± 0.68

Sole depth at the tip of the frog is significantly (P < 0.05) greatest in the HS group, followed by the SS group and the Thoroughbreds. Medial and lateral sole depth at the palmer heel was significantly (P = 0.01) greatest in the SS group, followed by the HS group and the Thoroughbreds.

Within a row, values differ significantly (P < 0.05; pairwise multiple comparisons) among all 3 groups.

FLE = Far left epidermis. FRE = Far right epidermis. LPH = Lateral palmer heel. MD = Mid dermis. ME = Mid epidermis. MLD = Mid left dermis. MLE = Mid left epidermis. MPH = Medial palmer heel. MLT = Mid left total. MRD = Mid right dermis. MRE = Mid right epidermis. MRT= Mid right total. MT= Mid total.

Palmar heel—A significant (P = 0.01) difference was detected in medial and lateral vertical epidermal support depth of the palmar heel measurement among the 3 groups. The SS group had the greatest epidermal depth, followed by the HS group and the managed domestic Thoroughbred group (Table 2).

Discussion

In the study reported here, we investigated the ground surface load-bearing properties of the feet of feral horses from HS, SS, and a combination of HS and SS habitats when loaded in vitro. Predictably, different patterns of loading measured subjectively and on the basis of load-bearing surface area were observed among feet of horses from habitats with differing substrate types. Feet of horses from HS habitats had the greatest area of loading on the distal aspect of the hoof wall and peripheral aspect of the sole. Load-bearing surface area increased with incremental increases in load. Surprisingly, the frog and central aspect of the sole did not load until high vertical forces were applied. When loaded on substrate 2 (gravel), feet of horses from the SS habitat had a lower load-bearing surface area, with load directed through the distal aspect of the hoof wall and a lack of loading on the sole. As expected, the type of substrate covering the loading plate affected the load-bearing surface. Deep sand (substrate 3) and gravel (substrate 2) resulted in a large load-bearing surface that included the central aspect of the sole and the frog.

Additionally, we evaluated detailed morphology of the dermal and epidermal sole of the feet of feral horses from SS and HS environments and domestic Thoroughbreds. We found a significant difference in sole depth among the 3 groups of horses. Feral horses from HS habitats had greater mean sole depth than did feral horses from SS habitats. Furthermore, both feral horse populations had greater mean sole depth than did managed Thoroughbreds. This implied that a hoof may adapt its solar depth to match its environment. Sole depth was greater peripherally near the hoof wall than it was centrally. This is consistent with results of the loading study that indicated that increased sole depth correlated with areas of greatest load bearing and thus greatest rate of wear. Analysis of the data implied that there may be a differential growth rate of sole horn across the solar plane in response to biomechanical feedback mechanisms. The increased sole depth in the horses from the HS habitat may have provided a greater dampening structure to cope with the lack of compliance of the environment. On the other hand, feet of horses from the SS habitat may have benefited from the viscoelastic properties of the entire hoof capsule because of a greater distribution of solar loading for the SS environment; thus, they may have required less dampening by the peripherally located sole tissue.

Conclusions that can be drawn from this study are limited in part by the use of cadaver limbs. However, limbs were frozen soon after the horses were euthanized to ensure that moisture loss from tissues was minimal. The limbs remained frozen when morphometric measurements were performed to avoid tissue changes during thawing. Loading was performed on a different set of limbs to allow the evaluation to be performed with limbs thawed and warmed to ambient temperature.

Many biomechanical studies have been performed in vitro. Investigators in some of those studies17,18 have found a good correlation between in vitro and in vivo measurements.

The peripheral aspect of the sole may be underestimated as an important load-bearing and force-dissipating structure. The increased load-bearing surface area generated by contribution of the peripheral aspect of the sole would reduce force per area, thus reducing stress on individual hoof wall tubules and structures within the foot as well as reducing the rate of wear in that area. Increasing the load-bearing surface area can dampen forces transmitted proximally up the limb.3 The application of a narrow-web horseshoe to the hoof wall may not maximize the load-bearing and force-dissipating properties of the solar structures of equine feet. However, wide-web horseshoes have characteristics similar to those for the hooves of horses from HS habitats and distribute the load-bearing surface area in a similar manner. Sole depth may be an important feature in the consideration of health of equine feet. Feet of horses from HS habitats had a pattern of loading on the peripheral aspect of the sole, which may be subject to a high rate of wear. However, as the study reported here indicated, the epidermal sole is thickest at the perimeter of the sole. There may be less wear in the peripheral aspect of the sole because of the increased surface area with ground contact or a differential wear pattern. There may be a differential growth rate for epidermal sole across the solar surface. We found that horses from HS habitats had a greater mean sole depth than did horses from a combination of HS and SS habitats and Thoroughbreds. However, the epidermal sole of horses with thinner soles, because of the softness of the substrate on which they moved, was likely to be subject to a lower rate of wear. Modulation of the growth rate of the epidermal sole was more likely to be a function of rate of wear, which is determined by biomechanical loading, rather than by a regional anatomic pattern. To our knowledge, the concept of a differential growth rate for the epidermal sole has not been proposed. A controlled study of regional growth rate of the sole horn would confirm or refute this assumption.

On a hard, flat surface, horses from the HS habitat bore weight directly on the perimeter of the sole, the moist tubules of the inner hoof wall, and the white line. Because of the bevel in the distal aspect of the hoof wall in all feral horses from HS habitats, the tubules in the outer hoof wall did not directly bear weight on the hard, flat surface. This architecture has been interpreted by some authors5,19 as suggesting that the forces on the sole are transmitted vertically to the solar surface of the DP. We suggest that the forces on the sole are directed through the strongly attached white line to the hoof wall and finally to the DP via the lamellae. This model is supported by the results of other studies.20,21 Although there seldom may be hard, flat surfaces in the environment of feral horses, consideration of the functional anatomy is worthwhile. On a hard but uneven surface, which feral horses are likely to encounter frequently, the most distal structures on the palmar surface of the feet of those horses will receive a greater load than will the adjacent slightly relieved structures.

Assumptions on the role of the load-bearing structures of the solar surface of the foot have been made from evaluation of the imprint of the unshod feet of horses on impressionable surfaces such as sand and granite.5,7,22 It appears from imprints that the entire solar surface of a foot bears weight, including the frog, bars, and central aspect of the sole. A fault with this subjective interpretation is that an imprint provides no detail of differential load magnitude among solar regions. According to our data, the role of the frog, bars, and central aspect of the sole in foot loading, at least on substrate 2 (gravel), may have been overestimated. In the present study, these structures did not receive substantial amounts of loading unless the vertical load was high or the foot was placed in deep sand (substrate 3). In the present study, 5 of 10 feet from feral horses from an HS habitat required a vertical force > 5,000 N to compress the frog over a 3-mm-thick firm rubber surface (substrate 1). This is equivalent to the maximum vertical load produced in a 400-kg horse during a fast trot or slow canter.18 Assessment of the foot morphology in the study reported here indicated that the palmar surface of the frog in this group of horses was a mean ± SE of 5.1 ± 0.9 mm above the ground surface. Investigators in another study17 reported frog-ground contact in Standardbred horses shortly after horseshoe removal at lower loads (1,800 N) than we detected in the present study. Investigators in a third study23 reported frog-ground contact for domestic ponies and horses wearing horseshoes at trotting loads. Hoof morphometric measurements were not reported in either of those studies17,23; thus, it is not possible to draw any conclusions concerning the relative flexibility of feet of feral versus domesticated horses from those studies. However, the presence of the large solar cup (mean ± SE, 3.0 ± 1.3 mm in feet of horses from the HS habitat and 5.5 ± 2.0 mm in feet of horses from the SS habitat) and the overall larger mean sole depth in feral horses, compared with that in domestic horses, indicated that the solar surface of the feet of domestic horses may be more flexible and more likely to flatten than is that of feral horses.

In the present study, we described the morphology of the sole of equine feet in more detail than in other reports. The depth of the sole varied among populations of horses, but the shape and distribution of sole material were similar for all horses. The epidermal sole is a dome-shaped structure that follows the contour of the DP adjacent to it. The mean depth of the sole cup varied from 3.0 to 5.5 mm, depending on the population. The thickest solar epidermal support region was in the sole periphery near the junction with the white line. The sole epidermis narrowed from the periphery to the center. Mean sole epidermal depth increased from the dorsal to the palmar aspect of the feet in each population, again following the contour of the palmar surface of the DP. The sole dermis depth had a pattern similar to that of the epidermal depth. The mean thickness of the sole dermis for all horses was greatest in the periphery (3.4 mm) and thinner toward the center (2.7 mm).

To our knowledge, sole depth for various populations of horses has not previously been compared. Investigators in 1 study24 used lateromedial radiographs of the feet of 103 Thoroughbred racehorses to measure sole depth at a single location between the most cranial aspect of the DP and a radiopaque marker placed on the sole; a mean ± SE sole depth of 11.1 ± 1.6 mm was found. In addition, investigators in that study24 reported that radiographic assessment of soft tissue thickness was significantly (P < 0.001) correlated with postmortem measurements in 26 horses. In the present study, mean ± SE sole depth corresponding to that of the tip of the DP in radiographs was 14.7 ± 0.76 mm for feral horses from the HS habitat, 12.6 ± 0.56 mm for feral horses from the SS habitat, and 9.5 ± 0.57 mm for managed domestic Thoroughbreds. Investigators in another study10 also measured the sole depth at a single location in 40 large warmblood horses before and after the feet were trimmed. The mean ± SE sole depth after trimming was 13 ± 2.2 mm in that study.10 It is intuitive to expect that sole depth should increase with size of the horse. In comparison to horses of large breeds, our groups of feral horses from the HS habitats were smaller in stature, but the sole depth (14.7 ± 0.76 mm) was larger. Comparison of the sole depth of small feral horses to that of larger domestic horses further emphasizes the effect of the environment on the morphology of equine feet.

The SS and HS groups had significantly greater mean sole depth than did the Thoroughbreds. Because the SS habitat was entirely soft sand, it is reasonable to assume that the Thoroughbreds were kept on a harder substrate than were the horses in the SS group. Feral horses travel greater distances than do managed domestic horses.8 Therefore, the distance traveled may account for some of the differences. However, feral horses in the SS and HS environments traveled long distances in search of feed and water. There was a significant difference in sole depth between the 2 feral horse groups, which indicated that sole depth may have responded to distance traveled and substrate hardness. The bovine hoof has a similar sole structure to that of the equine hoof. A report25 of observations of dairy cows concluded that growth of the sole horn was accelerated by load bearing on hard surfaces. Further research is required to determine mechanisms for sole growth, stimulus, and response.

The morphology of the equine foot may be a product of several factors, including genetics, life history, intensity of loading, environmental substrate, and human intervention. The functional importance of sole depth has not yet been established. Further studies are required to determine the correlation between sole depth and foot health in domestic horses. The thin sole of racing Thoroughbreds may be a result of selection to increase speed while galloping or a direct result of inadequate biomechanical stimulation. However, the resultant lightweight foot may be more vulnerable to injury and lameness. Hence, this foot type should not be used as the template for horses that do not require optimal speed. The more robust foot of feral horses inhabiting an HS environment may be a better model for managed domestic horses.

ABBREVIATIONS

DP

Distal phalanx

HS

Hard substrate

SS

Soft substrate

a.

RSfootscan Scientific Version, RSscan International, Olen, Belgium.

b.

Nikon D100 digital camera, Nikon Corp, Tokyo, Japan.

c.

Micro Nikkor lens, Nikon Corp, Tokyo, Japan.

d.

Nikon SB-800DX Speedlight flash, Nikon Corp, Tokyo, Japan.

e.

Image J, version 1.4.3, National Institutes of Health, Bethesda, Md. Available at: rsbweb.nih.gov/ij/index.html. Accessed Oct 30, 2009.

f.

R, version 2.7.2, R Foundation for Statistical Computing, Vienna, Austria. Available at: www.r-project.org/. Accessed Jan 25, 2010.

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