ARCHIVE - ARTICLES 2010

June 10 - Assessment of hoof size to body weight in a group of six thoroughbred horses

 

Archive - Forge Magazine - June 2010
Assessment of hoof size to body weight in a group of six thoroughbred horses

M. N. Caldwell FWCF and R. Sayers DipWCF
The Farriery Department, Myerscough College, Myerscough Hall, Billsborrow, Preston, Lancashire PR3 0RY
Telephone 01956 642000 - email [email protected]

Aim
To assess whether a relationship exists between body weight and hoof size in horses using a method previously devised by Turner and Raymond (2006).
Introduction
Horse’s hooves are required to deal with large forces. With an average horse weighing 500kg, each foot has to dissipate up to 5000kg (1250 Newtons [N]) of force while simply standing still. In motion, at a gait where only one foot is in contact with the ground at any one time this force can exceed 9000N (Turner 2006).
The principle factors that can affect the distribution of forces within the foot while stationary, are the weight of the horse and the contact area of the foot. It would be logical to assume that an overweight horse standing on small feet would give rise to relatively high loads being born by each foot, and may lead to the hard hoof horn being overloaded. Most biological materials are optimised for stress and have an inbuilt tolerance. Overloading the hard horn structures is believed to subject internal structures to excess pressures, which may compromise arterial and venous flow to some components of the foot, for example ischaemia associated with navicular syndrome.
Bowker (1998) stated that horses with poor blood supply through the lateral cartilages had poor heel growth, height and angle, whereas horses with prolific blood supply had good heel growth, height and angle. Could it be possible that large horses on small feet will be predisposed to erratic hoof growth associated with some hoof abnormalities as a direct result of a hoof size to body weight ratio?
Turner and Raymond (2006) claimed that there may be a relationship between horses’ body weight and hoof size, which would indicate whether certain horses were prone to excessive internal hoof pressures, and thus to these abnormalities or not. They had given some thought to this notion, and used a method to determine hoof weight to body size ratio that took the circumference of the foot, distal to the coronary band as an indicator of hoof size. They determined that the horse's weight multiplied by a factor of 12.56 kilonewtons (kN) divided by the hoof’s circumference squared, would be an acceptable weight to hoof ratio.
Weight (pounds) x 12.56, divided by the circumference (squared) = hoof size to body weight ratio
Turner and Raymond (2006) state that the maximum acceptable hoof size to body weight ratio is 78lb per square inch for healthy feet. They hypothesised that although hoof size could not be altered, weight should be decreased in those horses with a maximum hoof size to body weight ratio greater than 78lb per square inch. They quote the average range of the circumference of the foot at the coronary band as 13 inches to 14.5 inches, from which they produce an ideal weight chart, although they do not confirm the breed type of horses within their study group.
Turner also used hoof size to determine the upper limit of optimum body weight. His calculation states that:
Weight (pounds) = 78 x circumference (squared) divided by 12.56
Turner and Stork (1998) hypothesised that the mass of the horse was an important factor in hoof capsule pathology and has attempted to classify common hoof morphologies to variations in perceived ideal hoof metrics (Balch, White and Butler 1991).

Fig 1. After Duckett (1990) Verification of external measurements for the geometric proportions trim. The linear length of the toe (B) should correspond to the distance from the dorsodistal border of the toe to the widest point of the foot (C). The measurements (C and B) should also correspond to the distance from the position referred to as Duckett’s dot, to the widest point of the frog at its juncture with the heel bulbs (D) (Chapman 1987 and Duckett 1990). The foot is said to be proportionate if all three distances are of a similar value ± 6mm

Table 1. Descriptive data of the six thoroughbred event horses used for the trial
Materials and methods
The front and hind feet of six thoroughbred event horses of similar age, size, and weight were measured using a calibrated set of scales for accuracy (Table 1). All horses were professionally managed and trained at the same stables, and had a similar diet and exercise regime.
All feet were trimmed by an experienced farrier, following a geometric proportions protocol (Caldwell, Reilly and Savoldi 2010). Dorsopalmar, lateral medial and proximal distal proportions were verified by comparison of the dorsal hoof wall linear length measurements from the dorsodistal border to the coronary hair line taken with engineers calliper/dividers to key external anatomical land marks along the bearing border.
1. The linear length of the toe should then correspond to the distance from the dorsodistal border of the toe to the widest point of the foot (Fig 1). This is said to represent the centre of rotation of the distal interphalangeal joint (Duckett. 1990).
2. This measurement should also correspond to the distance from the position referred to as Duckett’s dot or COP to the widest point of the frog at its juncture with the heel bulbs (Duckett 1990).
The foot is said to be proportionate if all three distances are of a similar value ± 6mm (Fig 1).
External hoof measurements were taken of the:
1. Hoof’s circumference at the coronary border (Fig 1), using tailors’ tape, placing the tape just below the coronary band and measuring a complete circumference on each individual hoof through biaxial heel bulbs.
2. The dorsal hoof wall linear length from the dorsodistal border to the coronary hairline using an engineer’s digital Vernier calliper.
3. The bearing border width at the hoof’s widest point using an engineer’s digital Vernier calliper.
4. The bearing border length (BBL) along the longitudinal axis of the frog from the dorsodistal aspect of the wall to a point parallel to the heel buttress using an engineer’s digital Vernier calliper.
Validity of measurements tested by a coworker taking external hoof measurements (dorsal hoof wall, bearing border width and bearing border length) for comparison.

(Fig. 2) Distribution curve of hoof circumference plotted across all feet n = 7.4

Table 2. After Turner (2006), data collected for calculation of tabulate horse body weight and hoof circumferences (hoof size metrics) included body weight (W), coronary circumference (C) toe length (DHW) bearing border width (BBW) and bearing border length (BBL). Calculated hoof size to body weight ratio (HWR) is shown in both pounds (HWR 1"²) and kilo’s (HWR cm²). Optimum body weight to hoof size is shown in pounds (OW lbs) and kilos (OW kg)

Fig. 3 Scatter graph demonstrating that no correlation exists between bearing border length and hoof size to body weight ratio
p> -0.29, R² = 0.055
Data collection and analysis
Data were recorded on a spreadsheet. Calculations of hoof/body weight ratio, conversions from metric to imperial and basic statistical analyses of were performed using the spreadsheet.
Turner and Raymond (2006) formula incorporates imperial measurements of body weight and coronary band circumference. Our calculations incorporate metric/imperial conversions.
As previously stated, Turner’s calculation for hoof size to body weight ratio was:
Weight (pounds) x 12.56 (KN) divided by circumference (squared) = hoof size to body weight ratio
This was modified for our study to convert external hoof metric measurements to US imperial, in line with Turner and Raymond (2006) our formula for hoof size to body weight ratio was:
Weight (kg) (or multiply by 2.2 to find weight in pounds) x 12.56 divided by circumference (cm) [multiply by 2.5 to convert to inches) = N, divided by circumference (squared) = hoof size to body weight ratio (pounds) (or divide by 2.2 to find hoof size to body weight ratio inkg)
We used the following formulae to calculate the upper limit of optimum bodyweight converting back to metric by a factor of 2.2lb perkg.
Optimum bodyweight (pounds) equals hoof size to body weight ratio multiplied by coronary band circumference squared, divided by 12.56
Mean averages of each matrix for each subject (Table 2) within the group were used to calculate the population average body weight, coronary circumference, toe length, border bearing weight, bearing border length, circumference squared, hoof size to body weight ratio and optimum body weight relationships and distribution of the group (Table 3).
Variations were explored examining data for front feet only, comparisons of left front (LF) and right front (RF), hind feet only, comparisons of left hind (LH) and (RH) and comparisons between front and hind.
Relationship data analysis between limbs were performed using Pearson’s and a two-tailed paired ‘T’ test where appropriate (table 5) using the spreadsheet. Regression analysis was generated automatically by the system when scatter graphs were used to plot relationships.

Fig. 4 Distribution curve of hoof size to body weight ratio plotted across the whole sample n = 8.3

Fig. 5 Scatter graph demonstrating the strong negative relationship of hoof size to body weight ratio p> 0.98, (average 74lb per square inch ± 6 [n = 7]) and hoof wall circumference (average 15.14 inches ± 0.6 [n =7])
Results
Whole population
Results of calculated hoof size to body weight ratio following the methodology of Turner (2006) on a cohort of six thoroughbred event horses are shown in Table 2.
Weight ranged between 1311 and 1382lbs (average 1342 ± 23 [n = 7] imperial (596 and 628kg, average 610 ± 10.4 [n = 7]).
Hoof coronary circumference ranged between 14 inches and 16.5 inches (average 15.14 inches ± 0.6 [n =7] imperial, between 35 cm and 41 cm (average 38 ± 1 [n =7]) (Fig 2).
The external hoof measurements for the whole group for dorsal hoof wall length ranged between 83 and 108 mm (average 98 ± 6 [n = 9]). Bearing border width ranged between 122 and 145 mm (average 135 ± 7 [n =8]). Bearing border length ranged between 120 and 140 mm (average 134 ± 5 [n = 6])
There appeared to be no significant correlation between dorsal hoof wall and hoof size to body weight ratio p>0.06. There was a negative correlation between bearing border width and hoof size to body weight ratio and bearing border length and hoof size to body weight ratio p> -0.29 (Fig 3).
Turner (2006) stated that the maximum acceptable hoof size to body weight ratio is 78lb per square inch. Our data calculates hoof size to body weight ratio with a range of
62 and 85 pounds per square inch, average 74 ± SD 6.5 (n = 7) imperial, and between 25 and 34kg per square centimetre (average 29 ± SD 2.6 [n = 8]) metric (Fig 4). There was strong negative correlation between coronary circumference and hoof size to body weight ratio p>0.98 (Fig 5).
Results, mean averages per horse
Each matrix for all four limbs (Table 2) was combined to produce an average data set from each individual horse (Table 3).
The population mean body weight was 1342lb ± 22 or 610kg ± 10. The average coronary band circumference measured 15.14 inches ± 0.6 or 38cm, and the average external hoof measurements were dorsal hoof wall length (98mm ± 4), bearing border width (135 mm ± 4) and bearing border length (134 mm ± 4). Population average hoof size to body weight ratio was 74lb per square inch ± 4, or 30kg per square centimetre ± 2 (Table 3).
Results, optimum weight to hoof size
Using circumference (squared) x Turners 2006 calculation as a predictor of optimum body weight, the range was between 1295 and 1543lbs (average 1425lbs ± 78 or 589kg and 701kg; average 648kg ± 35 [n = 8]). The average actual bodyweight for our group at 1342lbs or 610kg was in line with the optimum bodyweight prediction± SD (Table 3).
Based on Turner’s estimate of hoof size to body weight ratio at 78lb per square inch, the average optimum weight for our population was calculated at 1425lb ± 78lb or 648kg ± 35 (n = 8) (Table 3).
Results show a direct relationship between hoof circumference squared and optimum body weight p> 0.1 in all horses in our sample (Fig 6, Table 4).

Table 3. Mean averages of each matrix for each individual subject (Table 2) within the group were used to calculate the population average body weight, coronary circumference, dorsal hoof wall, bearing border width, bearing border length, circumference squared, hoof size to body weight ratio and optimum bodyweight relationships and distribution of the group. Note: Optimum bodyweights were calculated for each horse based on a hoof size to body weight ratio of 78lb per square inch (Turner 2006)

Table 4. Pearson’s correlations between actual weight, predicted weight and hoof metrics

Table 5 Range, Mean, S.D., & distribution data for front feet only including calculations for hoof size to body weight ratio and optimum body weight
Results, front feet
Table 5 below shows the data and results of calculations for front feet only.
Coronary circumference ranged between 14 inches and 16.5 inches (average 15.3 inches) ± SD 0.7 (n =7) imperial, and between 35.3 and 41.25 cm mean 38.3 ± S.D 1.7 (n =7) (Fig 7).
The external hoof measurements for front feet were: dorsal hoof wall length ranged between 83 and 102 mm (average 94.5 ± SD 5.6 [n = 9]). Bearing border width ranged between 127 and 140 mm (average 133.6 ± 5.7 [n =8]). Bearing border length ranged between 120 and 140 mm (average 134 ± SD5.4 [n = 9])
Hoof size to body weight ratio calculated a range of 62 and 85lb per square inch, (average 72 ± SD 6.5 [n = 8] (Fig 8) imperial, and between 24.7 and 33.9kg per circumference squared (average 28.9 ± SD 2.6 [n = 8] metric). There was strong negative correlation between coronary band circumference and hoof size to body weight ratio p>0.97 (Table 4).
Table 6 shows the difference in range and average data when front feet only were used to calculate hoof size to body weight ratio and optimum body weight.
Reduction in the mean foot size coronary circumference of 0.44cm (5/16th), between the whole group and the front feet only, increased hoof size to body weight ratio by 0.64kg (1.6lbs) per square inch.

Fig. 6 Scatter graph demonstrating the direct circumference squared to optimum bodyweight p>0.1

Fig. 7 Distribution curve of coronary circumference plotted across front feet only n = 7.4.

Fig. 8 The correlations between hoof metrics bearing border length p>0.05, bearing border width p> -0.5, dorsal hoof wall length p> -0.3 metric and C p> -0.3 with hoof size to body weight ratio (pounds)

Fig. 9 Distribution curve of hoof size to body weight ratio plotted across front feet only n = 8.5. Optimum body weight prediction ranged between 1217lb and 1691lb (average 1459lb ± S.D. 130 or between 553kg and 769kg average 663kg ± S.D. 59KG [n = 7]).
Results, differences left fore and right fore foot
Table 7 shows the comparative average data range of left front and right front.
The difference between left front and right front coronary band circumference ranged between right front -0.25 inches and -0.5 inches (average -0.29 inches imperial), and between -0.63 and -1.25 cm (average -0.73cm p> 0.92).
The difference between external hoof measurements for left front and right front where right front dorsal hoof wall ranged between -2.0 and -5.0mm (average -0.7mm p>0.82). Bearing border width right front ranged between 0 and -3.0mm mean 0.2mm p>0.98. Bearing border length right front ranged between 0 and -10.0mm mean -1.2mm p>0.78.
The difference in calculated hoof size to body weight ratio left front and right front ranged right front between 1.9 and 5.75lb per square inch (average 2.85lb per square inch p>0.92, and between 0.8 and 2.3kg per square centimetre (average1.14kg per square centimetre p>0.92). There was strong negative correlation between coronary band circumference and hoof size to body weight ratio p>0.98.
Optimum body weight predictions based on left front and right front differences in hoof size to body weight ratio right front ranged between -51lb and -89lb (average -55lb or between -23kg and -40kg mean -25kg p>0.92).
Results, hind feet
Table 8 lists the results of hind feet hoof metrics data and calculated hoof size to body weight ratios and optimum body weights. Included in Table 8 are front feet descriptive statistics for comparison and the differences between front and hind feet.
Hind foot coronary band measurements ranged between 14.5 inches and 15.5 inches (average 14.96 inches) ± SD 0.4 inches, and between 36.3 cm 38.9 cm, (average 37.4 cm ± SD 0.9cm [n = 8]). (Fig 10).
The hind feet external hoof measurements where dorsal hoof wall length ranged between 90 and 108 mm (average 101 mm ± SD 5mm [n = 7]). Bearing border width ranged between 122 mm and 138 mm (average 130 mm ± SD 4.3mm [n = 8]). Bearing border length ranged between 128 mm and 140 mm (mean 134 mm ± SD 3.7 [N = 6].
The hind feet calculated hoof size to body weight ratio ranged between 68.6 and 81lb per square inch, mean 75.5lb per square inch and between 27.4kg and 32.4kg per square centimetre (average 30.2kg per square centimetre ± SD 1.6kg per cm² [n = 5]).
(Fig 11).
Hind feet optimum body weight predictions ranged between 1306lb and 1492lb (average 1390lb ± SD or between 594kg and 678kg, average 632kg ± SD 29.7kg per square centimetre [n = 4]).

Table 6. Differences in mean averages between the front feet only data and the whole group data

Table 7. The range of differences between left front and right front and shows a strong correlation between left front and right front data

Table 8 calculates hoof size to body weight ratios and optimum body weights. Also included are front feet descriptive statistics for comparison and the differences between front and hind feet.
Results, differences front and hind feet
Table 8 shows the statistical differences between front and hind feet.
The difference between front and hind coronary band circumference ranged between front right -0.5 inches and 1 inch (average 0.35 inch) and between -1.25 and 2.50 cm (average 0.89 cm p> 0.98).
The difference between external hoof measurements for front and hind were front right dorsal hoof wall ranged between -6.0 and -7.0mm (average -6.4mm p>1). Bearing border weight front right ranged between 5 and 7mm (average 9.3 mm p>0.98). Bearing border length front right ranged between 0 and -8.0mm average -1.2mm p>0.97.
The difference in calculated hoof size to body weight ratio front and hind ranged front right between -6.6 and 3.9lb per square inch, (average -3.2lb per square inch p>0.92, and between -2.7 and 1.56kg per centimetre squared (average -1.3kg per square centimetre [c p>0.98]). There was strong negative correlation between coronary band circumference and hoof size to body weight ratio p>0.98.
Optimum body weight predictions based on front right and HD differences in hoof size to body weight ratio front right ranged between -89lb and 199lb average 69lb or between -40kg and 90kg average 31.2kg p>0.98.
Results, differences left and right hind feet
Table 9 below shows the comparative mean data range of left hind and right hand. The difference between left hind and right hind coronary band circumference was left hind 0 inches p> 0.55. The difference between external hoof measurements for left hind and right hind were, left hind dorsal hoof wall ranged between 3.0 and 8.0mm (average 3.2mm [p>0.82[). Bearing border weight left hind ranged between -2.0 and -3.0mm (average -0.5mm [p>0.88]). Bearing border length left hind ranged between 0 and -2.0mm (average 0.5 mm [p>0.78]).
The difference in calculated hoof weight ratio left hind and right hind ranged left hind between 0 and -1.05lb per 1 square inch, (average -0.05lb per 1 square inch [p>0.62] and between 0 and -.42kg per cm², average -0.26kg per square centimetre c p>0.62..
Optimum bodyweight predictions based on left hind and right hind differences in hoof size to body weight ratio left hind were 0 mean -0.26lbs or -0.12kg p>0.56.

Fig. 10 Hind foot size circumference distribution (n = 8).

Fig. 11 Hind foot distribution hoof size to body weight ratio (n = 5).

Discussion
In nature, the shape and form of a structure relate directly to function. The hoof capsule is such an example of these design concepts. Although there is some debate over the position of exact centre of body mass (Butler 2005, Back and Clayton 2001, Williams and Deacon 1999), in the horse it is generally accepted that the horses’ mass is distributed through the limbs with shape and size of the hoof capsule directly influenced by the sum of that mass.
In describing hoof abnormalities such broken hoof axis, under run heels, contracted heels, sheared heels, mismatched hoof angles and small feet, it has been well documented that increased load may be a major contributory factor in both hoof morphology and lameness. Savoldi (2006) goes as far to suggest a link to lower limb pathologies and solar arch morphology, which he links to increased stress from excessive weight bearing.
Turner (2006) states that the maximum hoof size to body weight ratio acceptable is 78 pounds per square inch (34.5kg per cm²) and hypothesises that increased body mass will have a direct influence on hoof form and function. Turner makes no mention of the breed and type used for his study. Our population was fully fit New Zealand thoroughbred event horses of (average 16.2 hh and weighing 610kg).
The data from our sample calculates a mean hoof size to body weight ratio of 74 pounds per square inch SD ± 6 (30kg per cm² ± SD 2) (n = 8), confirming Turner’s estimate for optimum weight distribution (Turner 2006). There was an inverse relationship between circumference (squared) and hoof size to body weight ratio p>0.98 R² = 0.95.
Arabian et al (2001) suggests centre of mass of the digit are dependent upon hoof metrics coronary band circumference, bearing border length, bearing border width and dorsal hoof wall among others. Using Turner’s formula to calculate optimum body weight, our study found a positive relationship between the square circumference and body weight p>0.998, R² = 1. The formula accurately predicted W ± S.D for all horses in our study group. Given this link it is easy to postulate how excess body weight might be linked to certain hoof capsule morphologies and pathologies within the foot (Turner 1998, 2006; Savoldi 2006). The fact that optimum weight might be predictable adds to the understanding of how dynamic loading might also influence the shape of the bearing border (Wilson and others 1998; Viitanen and others 2003; van Heel and others 2004, 2006).
Throughout the literature, there is a prevailing assumption that an increase in bodyweight influences bearing border width and bearing border length. There appeared to be a no relationship the whole sample average bodyweight and hoof metrics of dorsal hoof wall (p>0.06). When the front feet only were analysed, the probability of a relationship between dorsal hoof wall and body weight increased to p0.46. There was a strong relationship between body weight and bearing border width (p>0.71) and a modest relationship with bearing border length (p>0.16). It is reasonable to assume that this can be attributed to form and function of the front feet.
In the horse it is generally accepted that the horses mass is distributed through the limbs with 60% being supported by the fore limbs and 40% through the hind limbs. The front feet are generally larger and rounder with less concavity to the soles to provide a greater surface to dissipate weight (Butler, 2005; Back and Clayton, 2001, Williams and Deacon 1999).
However, even modest reductions in circumference of the order of 0.4 cm, increased the hoof size to body weight ratio by mean 0.6kg per square centimetre, suggesting some validity to hoof size to body weight ratio as a measure of hoof health. Unnatural increases in circumference are usually associated with undesirable decreases in dorsal hoof wall angle either at the toe or heel or both and are thought to be as a result of increased body weight (Turner 1992). Dorsal hoof wall angle is a key hoof wall shape factor in affecting the strain distribution around the hoof wall (Zhang et al; 2008). Hoof Angle has a small effect on strain seen at the heel. Increasing the dorsal hoof wall angle reduces heel strain but increasing dorsal hoof wall increases heel strain (Fig. 12). The indications are that the relationship between hoof size to body weight ratio is more important in hoof management than the relationship between hoof metrics alone.
Standard farriery text (Butler 2007) advocates bilateral symmetry of front foot metrics. Somewhat surprisingly, the data from our sample suggests a tendency towards mismatched front feet. The right front of four out of the six horses in our study was larger across every metric - 3% (circumference (p>0.92), bearing border length (p>0.98) and bearing border length (p>0.78 [n = 7]). Mean hoof wall ratio was calculated -25kg (p>0.92) for the right front. The margin of difference suggests this could be attributed to tolerance error bearing border width and bearing border length ± 6mm allowed for in the foot trimming protocol or the collation of measurements. All trimming was conducted by experienced farriers familiar with the trimming protocol. And collection of hoof metrics data was checked by a coworker, which should eliminate most of the variance.
Hoof metrics for hind feet were not significantly different from those of the front feet and calculations for hoof size to body weight ratio and optimum bodyweight exhibited only modest differences in hoof size to body weight ratio and individual hoof metrics with those of the front feet.
Conclusion
Turner’s calculations for hoof size to body weight ratio and optimum bodyweight are valid and could be a significant indicator of potential and impending foot pathology. The strong statistical relationship between coronary band length and optimum body weight suggests that considerable more attention could be afforded foot to body weight proportions during clinical evaluations.
Clinical relevance
Understanding the effects of load on the morphology of the hoof capsule will help in the design and application of quantifiable farriery interventions for the treatment of lower limb pathologies and common hoof morphologies linked to poor dynamic foot balance.