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RADIOLOGICAL MEASUREMENTS FROM THE FEET OF NORMAL HORSES WITH RELEVANCE TO LAMINITIS.

P.J. CRIPPS B.V.Sc. B.Sc. M.Sc. PhD. MRCVS.

University of Liverpool, Department of Veterinary Clinical Science and Animal Husbandry, Leahurst, Neston, South Wirral, L64 7TE.

ROBERT A EUSTACE B.V.Sc. Cert.E.O. Cert.E.P. FRCVS.

The Laminitis Clinic, Mead House Farm, Dauntsey, Chippenham, Wiltshire. SN15 4JA.

Keywords: radiological measurements; laminitis; founder distance

Summary.

Latero-medial radiographic projections were made from the feet of 25 normal horses. Three angles and three distances, were measured from the radiographs. From these, normal ranges of calculated variables were obtained. The width and length of each foot and the height of the horse at the withers was also measured. In view of its possible importance in clinical laminitis particular attention was paid to founder distance (D). This is the vertical distance between the proximal limit of the dorsal hoof wall and the proximal limit of the extensor process of the distal phalanx. There were significant differences between breeds in D (P < 0.05) and in Wall Thickness (P < 0.01). Further studies investigated the effects of time, differing radiographic projections and different operators on D. D did not alter significantly (P> 0.05) over a six week study period. It was concluded that errors due to radiographic beam obliquity were not of practical importance. Errors in measuring D between operators can be minimised by use of a standard radiographic technique. Although there was some variation in the value of D measured over time and with different operators which could make the interpretation of early or subtle changes difficult, measurement variation is not likely to interfere with the assessment of large changes associated with severe laminitis.

Introduction.

The normal radiographic anatomy of the digit has been described (Quick and Rendano 1977; Shively 1977; Rendano and Grant 1978; Smallwood and Holladay 1987). Linford (1987) recorded a radiological and morphometric study of the feet of a group of normal Thoroughbred racehorses and of a group of animals exhibiting marginal distal phalangeal fractures. The radiological measurement of wall thickness was described and compared between the two groups of horses. Mean wall thickness was 14.6 mm in the front feet of sound Thoroughbred racehorses. and measurements greater than 16.6 mm were regarded as indicative of laminitis. Linford also found that wall thickness was proportional to foot size. Linford, O'Brien and Trout (1993) reported on the normal appearance of digital tissues in racing Thoroughbreds and related changes in the radiological appearance of the distal phalanx to racing performance. The relationship between hoof angle and lower limb joint angles in a group of normal horses has been described by Bushe et al (1987). These authors found that the mean Angle R (Figure 1) of the front feet of 10 horses was 5.5 degrees when the hoof angle (Angle S) was 50 degrees. They found a strong correlation between coffin joint angle and hoof angle; a change of 1o in hoof angle would be expected to cause a 0.91o change in coffin joint angle.

Clinical experience in the treatment of severe laminitis led the second author to believe that the distance of vertical displacement of the distal phalanx may be related to outcome. This study measured this distance in order to establish a database of normal values. In addition angles and distances from the feet of normal horses were measured to compare with the results of previous reports.

Materials and Methods.

Measurements were taken from 25 clinically normal horses and ponies whose feet showed no clinical abnormality or distortion. Radiographs were taken with the horse shod and standing on flat ground. Protruding nail heads have the effect of raising the toe of the foot thereby altering the angles we wished to measure. We therefore ensured that none of the feet had been so recently shod that nail heads were proud of the ground surface of the shoes. The sole and wall were cleaned using a hoof pick and wire brush. The horn on the dorsal part of the hoof just below the coronary band was lightly rasped to remove rough perioplic horn. The proximo-dorsal hoof wall was palpated just below the coronary band and the point at which the wall horn began to yield to moderate digital pressure was marked with an indelible pen. A straight stiff wire marker, of known length, was taped to the dorsal hoof wall with the proximal end at the indelible pen line. The horses' feet were radiographed standing on a flat wooden block 71 mm thick incorporating a metal ground line. All feet were radiographed whilst bearing weight and with the limb in as vertical a position as possible. Radiographs were made using a fixed 20 mAmp ; 80 kVolt X-ray generator with variable timer. The machine was used on the ground so that the central radiographic beam was 98 mm above the ground. The radiographic beam was aligned to be parallel to the ground and the long axis of the navicular bone. The beam was aimed at the centre of the distal phalanx (Figure 1).

Two measurements of foot size were made; the distance across the foot from quarter to quarter and from the toe, up the centre of the foot to the base of the frog. The height of the horse, to the withers, was measured using a measuring stick.

In order to study the repeatability of the measurement of founder distance, the same technique was used to measure the feet of two horses at weekly intervals over a period of six weeks.

The effect of proximo-distal radiographic beam obliquity was examined by taking radiographs of a foot with the centre of the beam at three heights (27, 54 and 87 mm) above the foot surface of the block. In the last projection the centre of the beam was at the same height as the top of the wire marker.

The effect of dorso-palmar beam obliquity was investigated with radiographs which were obtained with the central beam at 5, 13 and 20 degrees from true lateral. Three such projections were made dorsal and palmar to true lateral. All projections were made on the same occasion and consecutively.

In order to measure any variation in founder distance when three operators, two of whom were inexperienced in the technique, placed the dorsal wall wire marker, the feet of two horses were radiographed sequentially. Each operator placed the wire, the radiograph was then taken and measured by the second author, who had taken all the radiographs.

Radiological Measurements.

Radiographs were fixed onto a horizontal viewing box, a sheet of clear film was overlaid and lines were drawn with a fine tipped pen. Six measurements were made from each latero-medial radiograph (see Figure 1).

1. Angle S, the angle between the dorsal hoof wall and the ground.

2. Angle T, the angle between the dorsal cortex of the distal phalanx and the ground.

3. Angle U, the angle between a line connecting the centres of curvature of the proximal and distal interphalangeal joints and the ground. The curvature of these joints is not uniform but a `best fit' line can be drawn.

4. The vertical distance, d, between the top of the dorsal wall wire marker and the proximal limit of the extensor process of the distal phalanx.

5. The length of the dorsal wall wire marker.

6. The distance between the dorsal wall marker and the dorsal cortex of the distal phalanx. A point mid-way along the dorsal cortex of the distal phalanx was chosen for measurements and the line of measurement was perpendicular to the dorsal cortex.

From these measurements four calculations were made:

a. Angle H: Angle T minus Angle S, as described by Stick et al (1982).

b. Angle R: Angle T minus Angle U.

c. D, the true distance between the top of the dorsal wall wire marker and the proximal limit of the extensor process of the distal phalanx after correcting for magnification. This is calculated using the following formula:

Actual length = length measured radiographically x actual length of marker / radiographic length of marker.

d. WT: the actual thickness of tissue between the dorsal hoof wall and the dorsal cortex of the distal phalanx after correction for magnification according to the above formula.

WToH, the relationship between wall thickness and height, was calculated by dividing a horse's mean WT by its height in cm.

Horses were also classified according to breed as Pony (Adult Height less than 147.3 cm), Thoroughbred (Thoroughbred or Thoroughbred Cross excluding Hanoverian), Hanoverian (Hanoverian or Hanoverian Cross) and Other.

Statistical Analyses.

The data were analysed using the Minitab statistical package (Minitab Inc; State College, PA, USA.) and Genstat 5 (NAG Ltd, Oxford, UK). Elementary statistical description used standard measures. Differences in WToH between horse breeds were examined using the Mann-Whitney or Kruskal-Wallis test. Further analysis of the founder distance (D) used regression analysis and Analysis of Variance and when appropriate due regard was given to split-plot design. Correlations quoted are those of Pearson. A critical probability of P = 0.05 (5%) was assumed and was based on a 2-sided null hypothesis of no difference.

Results.

The physical details of the animals are given in Table 1. A basic statistical description of the data obtained from the front and hind feet is given in Tables 2 and 3 respectively. Tables 2a and 3a further divide the results according to breed.

Comparison of the founder distances found in ipsilateral and contralateral pairs of feet is shown in Table 4. The differences between left and right feet, were not statistically significant (P>0.05). Neither were the differences between fore and hind feet for the 18 animals where readings on both front and hind limbs were available. There was however a significant difference in founder distance (D) between individual horses (P < 0.001).

Table 5 shows the results from repeated measurements of D obtained from two horses over a six week period. There were significant differences between the two horses (P < 0.01) and between feet within a horse (P < 0.01), but not between weeks (P > 0.1). In both these horses the forefeet had significantly smaller founder distances than did the hind feet (P < 0.001) but there was no significant difference between corresponding left and right feet (P > 0.1). For individual feet of the same animal the standard deviations of the measurements over the six week period varied between 0.62 and 1.14 mm: the maximum difference in the founder distance was 3.2 mm and the minimum difference was 1.4 mm. The maximum difference between the front feet of the same animal was 3.6 mm and between the hind feet 4.4 mm.

Table 6 shows the effect on founder distance of varying the heights of the X ray beam relative to the foot. When the differences between weeks had been corrected for, the effect of beam height was statistically significant (P = 0.04). The regression coefficient was b = 0.0147 with 95% confidence limits being 0.001 - 0.028: in other words, the estimated effect of altering beam height by 100 mm is to alter D by approximately 1.5 mm.

The effect of varying dorso-palmar beam obliquity is shown in Table 7. There was no significant linear relationship between the angle of obliquity and D: The regression coefficient b = 0.0013 (P = 0.49).

In order to assess how much variation in D there was between the forefeet and hind feet of the same animal, the greatest difference between any front foot and any hind foot was calculated for 18 of the normal horses. The average value was 3.52 mm with a standard deviation of 2.02 mm and a median of 2.9 mm. The minimum was 0.0 mm and the maximum 7.8 mm. Eleven of the animals (60%) had a value of < 4 mm and seventeen (94%) were <6 mm.

Table 8 shows the correlations between the average values of foot length, foot width and wall thickness and the horses' height. Horse height, foot length, foot width and wall thickness were all significantly correlated with each other (P < 0.01). Founder distance was not significantly correlated with any of these variables (P > 0.10).

Table 9 illustrates the effect of different operators on the measurement of founder distance D. For each horse there were significant differences between feet and between operators (P < 0.01) but operators were consistent relative to each other for each foot: Operator 3 scored the lowest and Operator 2 the highest values of D. For measurements on a single foot the maximum difference between operators was 4.9 mm.

The ratio of wall thickness to height, WToH, varied between 0.083 and 0.124 with a median of 0.107 and a mean of 0.105. The median values for fore feet were 0.103, 0.108, 0.099 and 0.117 respectively for Ponies, Hanoverians, Thoroughbred and other breeds. Similar results were obtained for hind feet. When individual feet were used as the unit of measurement Breed medians were significantly different for the fore feet (P < 0.01) but not for the hind feet (P > 0.05).

Discussion.

The measurement of coffin joint angle in this study is in close agreement with that of Bushe et al (1987). These authors found that the mean Angle R of the front feet of 10 horses was 5.5 degrees when the hoof angle (Angle S) was 50 degrees. In this study Angle R was found to be 4.8 degrees at a hoof angle of 50.5 degrees. The close agreement between these two separate studies supports the reproducibility of Angle R despite it being a `best fit' measurement (Angle U). Table 10 shows a morphometric comparison between the results of this study and that of Linford (1987). Both sets of data are from the front feet and are corrected for magnification.

These results suggest that the varied breeds and types of animals used in this study tended to have more upright feet with more strongly developed heels than the group of Californian Thoroughbreds investigated by Linford. Although the angle between the solar margin of the distal phalanx and the ground is not reported in either study, Linford states that the average solar angle in his horses was approximately 2 degrees. The solar angle in this study of normal horses was rarely less than 8 degrees.

This study found smaller correlations between horse height, foot length, foot width and wall thickness than those reported by Linford (1987). Our data suggest that wall thickness may be related to the breed or type as well as to the size of the horse. The mean wall thickness measurements from front feet of normal Thoroughbred and Thoroughbred cross horses in this study is 16.3 mm; that for hind feet being 16.0 mm. However Linford (1987) found a mean wall thickness measurement of 14.6 mm in the front feet of sound racing Thoroughbreds; measurements greater than 16.6 mm were regarded as indicative of laminitis. It is not possible to be sure of the reason for this discrepancy but it may be due to differences between the two Thoroughbred populations with respect to horse size, or amount of bone build-up on the distal phalanx resulting from dissimilar athletic activities. Differences in measurement techniques may also have contributed. From our results we suggest that investigators who are using our protocol on thoroughbreds should exercise caution when choosing a cut-off between acceptable and abnormal radiographic values for thoroughbreds. We also suggest that it would be valuable to consider the type of horse when diagnosing laminitis abnormalities from wall thickness measurements.

From histological specimens of chronic foundered feet, Kameya et al (1980) have recorded a horn layer thickness of 22.2 (SE +/- 8.3 mm). They also described an increase in laminar horn thickness in "non-laminitic" feet from the same animals of 6.6 +/- 4.1 mm. A normal thickness of 3.7 +/- 0.9 mm was recorded form control feet of unaffected animals.

This study demonstrated that while dorso-palmar obliquity did not produce errors in the measurement of D and this contrasts with the situation that has been reported for Angle H (Koblik et al (1988)). There was a real effect of proximo-distal obliquity on the founder distance. However the estimated effect, 1.5 mm change in D for every 100 mm change in beam height, is not likely to be of clinical importance assuming conventional radiographic technique. Nevertheless it does emphasise the need for a standardised technique and the importance of keeping the central beam constant during a sequence of radiographs.

The method of measurement of angles and distances, drawing over the film on a lightbox, was not found to be a significant source of errors even between observers (unpublished data). The largest error found when different observers measured founder distance from radiographs was 1 mm. This error is independent of the founder distance measured.

There was variation between measurements of D taken of the same foot on different occasions. We do not know the reason for this but the results of our experiments on measurement technique (see below) suggest that they may be of biological origin and not due to radiographic technique. The maximum change in D recorded on a single foot was 3.2 mm and we feel that this is small compared to the D measurements in clinical cases (Eustace & Cripps 1999). Since our results do not suggest that the variation in D increases with its mean, the greater the mean value the smaller the coefficient of variation will be. We believe that measurements of D may be of limited usefulness in the early stages of laminitis. However, further studies indicate that values of D outside the normal range may be of great significance for prognosis (Eustace & Cripps 1999).

When we examined D in left and right feet on the same horse when there was only one measurement available, the difference in D was similar to the difference in D between the same foot on repeated measurements. The maximum difference in D between left and right feet is 4.4 mm.

It is interesting to find note that for the two horses examined repeatedly the mean distance D was significantly different for front and hind feet, the latter distances being greater. This could be attributed to the smaller hoof angle (S) for hind feet than front feet. An increase in angle S could theoretically result in smaller distance D measurement although the difference in hoof angle between front and hind feet in this study makes this factor insignificant in practice. These results suggest that for individual horses the difference in founder distance between fore and hind feet tends to be consistent, however, the differences are small and likely to be of little clinical significance.

This study demonstrates that different operators may not agree on the placement of the dorsal wall wire marker. This can lead to differences in the measurement of D between observers. These differences are due to different operators feeling that the dorsal limit of the dorsal wall horn ends at differing positions. Relevant factors in making such decisions include the pressure applied to the wall, the amount of overlying perioplic horn and the degree of maturity of the wall horn. However in this study the same feet were examined consecutively so that the only variable responsible for the results is the individual's appreciation of the dorsal limit of the dorsal wall as determined by their palpation. We therefore recommend that an indelible mark be placed on the foot of clinical cases if there is any likelihood of sequential radiographs being taken on the same foot.

Conclusion.

This study found that in this group of normal animals the mean values in front feet were as follows; Angle S 50.5o (SD 5.03o), Angle T 49.4o (SD 4.67o), Angle U 44.6 o (SD 5.24o), Angle H -0.9o (SD 2.37o), Angle R 4.8o (SD 6.14o), WT 16.3 mm (SD 2.40), D 4.1 mm (SD 2.17).

Corresponding values for hind feet were as follows; Angle S 49.1o (SD 3.18o), Angle T 48.9o (SD 3.06o), Angle U 42.3o (SD 5.31o), Angle H -0.1o (SD 1.59o), Angle R 6.6o (SD 5.64o), WT 16.6 mm (SD 2.01), D 4.6 mm (SD 2.47).

Founder distance was found to be easy to measure and reliable if inter-operator error was controlled. This study of normal horses should provide a database against which to measure founder distance in horses with laminitis.

Acknowledgements.

The authors acknowledge the work of Linda Belton and Caroline Green who assisted in this project. They were awarded the fourth year prize at Bristol Veterinary School for their part in the study.

References.

Bushe, T., Turner, T.A., Poulos, P.W. and Harwell, N.M. (1987)
The effect of hoof angle on coffin, pastern and fetlock joint angles.
Proc. 33rd Ann. Conv. Am. Ass. equine Pract. 729-737.

Eustace & Cripps (1999)
Factors involved in the prognosis of laminitis in the UK.
Equine Veterinary Journal, 31. 5. 433-443

Kameya, T., Kiryu, K. and Kaneko, M. (1980).
Histopathogenesis of thickening of the hoof wall laminae in equine laminitis.
Japan J. Vet. Sci. 42. 361-371.

Koblik, P.D., O'Brien, T.R. and Coyne, C.P. (1988).
Effect of dorsopalmar projection obliquity on radiographic measurement of distal phalangeal rotation angle in horses with laminitis.
J. Am. Vet. Med. Ass. 192. (3). 346-349.

Linford, R.L. (1987).
A radiographic, morphometric, histological, and ultrastructural investigation of lamellar function, abnormality and the associated radiographic findings for sound and footsore Thoroughbreds, and horses with experimentally induced traumatic and alimentary laminitis.
PhD Thesis. University of California. USA.  

Linford, R.L., O'Brien, T.R., and Trout, D.R. (1993).
Qualitative and morphometric radiographic findings in the distal phalanx and digital soft tissues of sound Thoroughbred racehorses.
Am. J. Vet. Res. 54. 1. 38-51.

Quick, C.B. and Rendano, V.T. (1977).
Equine radiology; the pastern and foot.
Mod. Vet. Pract. 1022-1027.

Rendano, V.T. and Grant, B. (1978).
The equine third phalanx; its radiographic appearance.
J. Am. Vet. Rad. Soc. 19. 125-135.

Shively, M.J. (1977).
Normal radiographic anatomy of the equine digit.
Southwest Vet. 30. 193-199. 

Smallwood, J.E. and Holladay, S.D. (1987).
Xeroradiographic anatomy of the equine digit and metacarpophalangeal region.
Vet. Rad. 28. 5. 166-173. 

Stick J.A., Jann H.W., Scott E.A. and Robinson N.E. (1982).
Pedal bone rotation as a prognostic sign in laminitis of horses.
Am. Vet. Med. Ass. 180 (3) p.251-253.