Relationship between the Distal Phalanx Angle and Radiographic Changes in the Navicular Bone of Horses: A Radiological Study

Table of contents

1. Introduction

oot pain is described as the most common cause of forelimb lameness in sport horses(Dyson2011 a ) and has been associated with poor hoof balance and conformation (Turner 1986). Some authors have been suggested that changes in foot conformation increase the load on the palmar aspect of the foot and so the navicular bone is overstressed predisposing to foot pain and lameness. Unfortunately, there is limited information about the relation between the distal phalanx (P3) orientation within the hoof and the radiological changes in the navicular bone.

A condition that causes foot pain is the one called "Negative palmar angle syndrome (NPAS)". This is a term in which the solar or palmar and/or plantar margin of P3 has a negative angle in relation to the ground surface, and sole depth under the dorsal-distal processes when viewed on a lateral radiographs (Floyd 2010). The term "long toe, low heel" has been used to describe this condition and has been accepted as being abnormal among veterinarians and farriers. Conversely, a recent study in horses with foot pain indicated the variations in shape of the distal phalanx were not accurately predicted by external characteristics of the hoof capsule . On the other hand, a marked correlation between hoof conformation and forces applied to the equine foot has been also described (Eliashar et al 2004). According to the aforementioned, in many cases it is very difficult to predict an abnormal condition in the structures inside the hooves when only the external hoof capsule is seen.

In normal conditions, the distal phalanx solar angle or palmar angle with the surface range between 2º -10º (Parks 2003). As the border of the distal phalanx is the insertion point of the deep digital flexor tendon (DDFT), a change in its orientation, increases the DDFT tensile force and subsequently the force it exerts on the navicular bone during the different phases of a stride (Wilson et

2. Materials and Methods

3. a) Horse Selection

One hundred and forty-six feet (146) from horses were used for the study (54.79% from Chilean Criollo horses and 45.21% from Warmbloods horses) (385 -590 kg bwt) in routine work and shoeing status examined consecutively between July 2015 and December 2016. All horses were assessed for lameness. Sound horses were immediately enrolled for the study. Lame horses selected for this study had lameness abolished after a palmar digital nerve block was performed, using 1.5 mL mepivacaine 2% (Vetacaine TM ) a injected just proximal to the lateral cartilages of the distal phalanx. Horses presenting with laminitis or lameness located anywhere else were excluded from the study. Horses selected should have been trimmed within 5 weeks. Additionally, age, gender and breed were recorded.

To establish the normal reference values for the hoof radiographic measurements of the Chilean Criollo horses and to investigate the differences of hoof radiographic measurements between Chilean Horses and Warmblood horses, horses were categorized by breed.

4. b) Radiographic Image Adcquisition

Eighty forelimbs from Chilean horses (left front n=40; right front n=40) and sixty-six forelimbs from Warmblood horses (left front n=33; right front n=33) were radiographed obtaining a total sample of one hundred and forty-six feet (n=146). All radiographic examinations were performed after standard foot c) Image Analysis Radiographs were analyzed using an image analysis software (Metron-DVM 7.05 for windows) c . Following the instructions of the program, 10 parameters on the LM view were measured. The following measurements were obtained: Palmar angle, descent of the distal phalanx, distance of the distal phalanx to ground, hoof angle, proximal HL zone, distal HL zone, percentage of the weight-bearing surface of the toe, coffin joint angle, pastern joint angle, length of the middle phalanx (Figure 1 and Figure 2).To determine the radiographic condition of the navicular bone ("navicular score") a standardized classification was used as described by Dyson (2011 b ) (Table 1).

5. d) Data Analysis

Statistical analyses were run on a specialized statistical software (SPSS Inc, version 19 for windows) d . A Kolmogorov-smirnov test was performed to assess whether the data were normally distributed. A t-student test for independent variables was used to compare the data between breeds. All measurements were compared to determine whether they were significantly different between groups. A Spearman correlation test was run to determine the association between the radiographic hoof values and the radiographic score of the navicular bone. The significance level was set at p<0.05.

6. III.

7. Results

One hundred and forty-six feet (146) from horses were used for the study (47.83% geldings; 42.39% mares; 9.78% stallions). Eighty feet were from Chilean Criollo horses and sixty-six feet were from Warmblood horses (29.53% Holsteiner; 10.16% Selle Frances; 5.52% Warmblood cross). The mean ±standard deviations (s.d.)and t-student test of the data obtained for radiological hoof values for Chilean and Warmblood horses are summarized in table 2.

There was significant difference between groups for palmar angle, toe/support %, third phalanx distance to the ground, and length of the middle phalanx determined radiologically (table 2). Warmblood horses have a smaller palmar angle (3.39± 3.37) than Chilean Criollo horses (6.46± 3.88)(p= 0.000) as well as the toe/support % (65.12± 5.48 and 67.35± 5.78 respectively, p= 0.033). Additionally, there was a significant difference in the length of the middle phalanx in which the Chilean Criollo horses have a shorter middle phalanx bone (3.99± 0.53, p value 0.000). The other measurements determined radiologically showed no difference between breeds (table 2).

Table 3 summarizes means ±s.d. and t-student test results when horses were assessed by limb, showing no statistical differences when right and left legs were compared between each other. This situation was seen in both breeds. ). Lateromedial, 60º dorso proximal oblique navicular (upright pedal) and palmaro proximal -palmaro distal (Navicular Skyline) radiographic views were obtained. For lateromedial view, the foot to be examined was placed on a block 6 cm high and the x-ray beam was centered approximately 1 cm distal to the coronary band, midway between the dorsal and palmar aspects of the hoof. The x-ray generator was set at 76 Kvp and 1.2 mAs. For the 60º dorsoproximal navicular view, the hoof was placed over the x-ray tunnel in a square stance and the x-ray beam was centered in the coronary band and the x-ray generator was set at 78 Kvp and 1.6 mAs. The last radiographic view was obtained with the limb over the tunnel and placed backwards and the x-ray beam was centered between the heel bulbs following the pastern angle and the x-ray generator was set at 80 Kvp and 2.0 mAs.Radiographs were obtained using a digital x-ray machine (Envision G2 DR panel) b and a Poskom PXP-20HF x-ray generator.

Additionally, the mean ±s.d. for the navicular score for each breed was analyzed. Chilean Criollo horses (0.95 ± 0.80) showed a lesser value when compared with Warmblood horses (1.23 ± 0.83). These results were statistically different (p= 0.038).

Each measurement determined radiologically was correlated with its respective navicular score. The palmar angle and hoof angle (rho= -0.190, p= 0.024) showed a weak negative correlation with the navicular score (rho= -0.173, p= 0.041) (table 4). The other parameters measured did not show significant association with the navicular score.

IV.

8. Discussion

This study was performed in order to establish the relationship between the distal phalanx angle within the hoof capsule and the radiological condition score of the navicular bone.Additionally,a database of reference values of the radiographic hoof values from the Chilean Criollo horses were obtained and compared with the values obtained from Warmblood horses. Hoof trimming has shown a remarkable influence on hoof conformation and in some measurements that describe the position of the third phalanx within the hoof capsule (Kummeret al2006) so in our study,horses were excluded when the feet had not been trimmed within 5 weeks.

The selection and use of Metron software for this study was based on the previous results obtained where it was determined that Metron software can be used to objectively measure most of the parameters predefined by the software (Vargas Rochaet al 2004).

Chilean Criollo horses showed a larger palmar angle when compared with Warmblood horses, finding somehow expecteddue to the described lower palmar angle of Warmbloods compared to other breeds (Kummeret al 2006). Toe/support % was larger in Chilean horses and thus they should have a better capacity to dissipate the ground reaction forces within the hoof capsule compared with Warmbloods. Nonetheless, one study showed no differences when the presentation of catastrophic pathologies and toe/support % were compared (Kane et al 1998). According to the FédérationEquestreInernationale (2017), a Pony is a small horsewhose height at the withersdoes not exceed 148 cms. Chilean horsesare considered as Ponies due to their height (<145 cms),so a shorter middle phalanx compared to Warmblood horses was expected. Chilean horses tend to have narrow, upright, and small feet relative to their body size (Reckmann 1999 According to the results of our study, there was a significant negative correlation between the navicular score and the palmar angle. There was also a significant negative correlation between the navicular score and the hoof angle. The aforementioned results, were in accordance to our expectations and these may be the reflection of the increased force exerted by the DDFT due to a highest moment arm force (Wilsonet al 2001) to the navicular bone when the hoof presents a low palmar angle (neutral to negative) (Floyd 2010).This situation has also being documented by Weaver et al (2009)were they topographically map pressure distribution across the palmar surface of the navicular bone in response to forces applied by the deep digital flexor tendon (DDFT). This study showed and evaluated the effect of raising the heels in vitro showing the relationship between the DDFT and navicular disease.Moreover, Eliasharet al (2004) concluded that an increase in the palmar angle by 1° would decrease the force of the DDFT on the navicular bone by 4%, supporting the biomechanical overload suffered by the navicular bone when an abnormally low palmar angle is present. Additionally, no significant correlation has been found between heel collapse and the palmar angle (Floyd 2010) thus the radiographic evaluation to determine the hoof inner structures measurements is mandatory.Considering biomechanical and risk factors for development of navicular disease, the palmar angle of the distal phalanx should play an important role in the presentation of the disease. According to Dik and van den Broek (1995) and Diket al (2001), horses presenting with different palmar angles should present different shapes of the navicular bone based on a shape-dependent distribution of the forces exerted on the navicular bone. For example, navicular bone shape 1 and 2 are associated with overloading of the distal interphalangeal joint, and navicular bone shape 3 is related with strain of the collateral ligament of the navicular bone (Dik and van den Broek1995).

As this study did not evaluated the correlation between the palmar anglewith the presentation of clinical navicular disease,further investigation is required in this matter. Nonetheless, recent studies has shown very interesting data regarding correlations between radiographic measurements of the foot and abnormalities of specific structures found with magnetic resonance imaging (MRI) (de Zaniet al 2016). Moreover, it has been documented that the larger the palmar angle, the smaller the likelihood of a DDFT or navicular bone lesion (Holroyd et al 2013).

In conclusion, this study contributes to the informationalready available in the literature helping to have a better understanding of changes suffered by inner structures of the hoof capsule. We have documented the reference hoof values from the Chilean Criollo horses and at the same time we have shown a few difference between this breed and Warmblood horses. Additionally, we have demonstrated that there is a significant statistical correlation between the radiographic navicular score and the palmar angle. Given these results, a radiological evaluation of horse's feet before and after shoeing is always recommended. To fully understand the implication of the changes suffered on the palmar angle in horses presenting navicular disease, further investigation is needed.

Figure 1.
al 2001; Weaver et al 2009; de Zani et al 2016). Furthermore, biomechanical overloadexerted over the navicular bone has shown to be harmful and may result from aged related accumulation of workload (Dik et al 2001). Accordingly, in theory the more tensile forces exerted to the navicular bone (biomechanical overload) the moreradiographic changes should be found (Dik et al 2001; Wilson et al 2001; Weaver et al 2009). The changes seen in the navicular bone in horses presenting navicular disease are well documented (Verschooten et al 1989; Wright 1993; Buttler et al 2000; Dyson 2011 b ; Komosa et al 2013). In this study, we hypothesized that the compressive force exerted by the DDFT over the navicular bone due to an abnormal distal phalanx angle,as described by different authors (Wilson et al 2001, Eliashar et al 2004; Weaver et al 2009; Holroyd et al 2013), has a correlation with the radiographic changes observed in the navicular bone. Additionally, we provided a database of normal values of the hoof capsule in Chilean horses that can be used by veterinarians and farriers as a guideline for routine and orthopedic shoeing.
Figure 2. G
Relationship between the Distal Phalanx Angle and Radiographic Changes in the Navicular BoneofHorses: A Radiological Study preparation as describe by Buttler et al(2000).Radiographic views selected to evaluate horses`feet were based on previous studies performed in different breeds (Dyson et al 2011; Dyson 2011 b ; Thieme et al 2015; Wright 1993
Figure 3.
; Vergara 2012), hence the larger palmar angle in Chilean horses is mostly due to the hoof conformation and its relation with the inner structures of the hoof capsule (Dyson et al 2011). When the radiographic navicular score was obtained, Chilean horses showed a lower mean score than Warmblood horses used in this study. The above situation was suspected according to the hoof and palmar angle obtained, where larger palmar angle has been associated with a smaller probability to present navicular bone or DDFT lesions (Holroyd et al 2013). This situation is most likely related to the forces exerted by the DDFT to the navicular bone (Wilson et al 2001, Eliasharet al 2004, Weaver et al 2009).
Figure 4. Table 2 :
2
a Significant difference P< 0.05
Note: G
Figure 5. Table 3 :
3
Chilean Horses Warmblood Horses
LF RF LF RF
Parameters Mean ± s.d. Mean ± s.d. P value Mean ± s.d. Mean ± s.d. P
value
Palmar angle 6.69 ± 4.02 6.19 ± 3.94 0.474 3.59 ± 3.28 3.05 ± 3.48 0.768
P3 descent 0.45 ± 0.48 0.32 ± 0.51 0.321 0.53 ± 0.65 0.56 ± 0.56 0.588
P3 dist. to ground 1.83 ± 0.56 1.87 ± 0.52 0.754 2.08 ± 0.56 1.97 ± 0.58 0.500
Hoof angle 51.99 ± 2.37 50.64 ± 8.41 0.360 51.17 ± 4.08 51.24 ± 4.75 0.713
Prox. HL zone 1.52 ± 0.33 1.53 ± 0.34 0.991 1.57 ± 0.23 1.61 ± 0.27 0.823
Dist. HL zone 1.47 ± 0.32 1.50 ± 0.51 0.865 1.42 ± 0.25 1.45 ± 0.23 0.710
Toe/Support % 67.00 ± 6.34 67.49 ± 5.53 0.736 65.41 ± 5.27 64.84 ± 5.85 0.949
Coffin joint angle 9.20 ± 6.44 9.91 ± 6.92 0.846 8.98 ± 7.19 9.51 ± 7.28 0.751
Pastern joint angle 3.28 ± 4.46 1.96 ± 4.39 0.320 3.66 ± 4.81 1.98 ± 5.41 0.147
Length of P2 4.00 ± 0.53 3.98 ± 0.55 0.988 4.63 ± 0.33 4.71 ± 0.33 0.892
P3, distal phalanx;
Figure 6. Table 4 :
4
Parameters Rho Spearman P value
Palmar angle & Navicular score -0.173 0.041 a
P3 descent & Navicular score -0.136 0.108
P3 dist. to ground & Navicular score 0.096 0.259
Hoof angle & Navicular score -0.190 0.024 a
Prox. HL zone & Navicular score -0.045 0.595
Dist. HL zone & Navicular score -0.058 0.498
Toe/Support % & Navicular score 0.118 0.165
Coffin joint angle & Navicular score 0.074 0.386
Pastern joint angle & Navicular score 0.021 0.801
Length of P2 & Navicular score 0.113 0.182
P3, distal phalanx; HL, hoof-lamella; P2, middle phalanx.
Note: a Significant difference P< 0.05
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Appendix A

Appendix A.1 Acknowledgments

The authors would like to acknowledge Dr. T. Herthel and for the critical review of the manuscript.

Appendix B

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Notes
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Date: 2017-01-15