Changes in hoof balance are associated with increased strains in the soft tissue structures of the equine digit and injury. These links express the importance of regular assessment of hoof proportions to maintain soundness in horses, as a result, monitoring hoof proportions has become common practice in the farriery profession. This study tested the repeatability and reliability of an application for measuring hoof balance and found statistically excellent inter and intra-observer reliability, a Friedman two-way ANOVA was used to test for differences between the 9 observers. There was no significant difference between the measurements of each observer (F(8) = 6.779; p=0.561). A Wilcoxon rank test was used to test the intra-observer reliability of the three observers who repeated measurements and showed no significant difference (p 1.000).
However, measurements between observers had a range that would be clinically significant. HoofmApp is an accurate tool for measuring hoof proportions with a high intra-rater reliability. However, due to a lack of established anatomical datum points within research there remains subjectivity in the placement of the templates affecting inter-observer reliability. Further research should look to test reliable methods for establishing datum points to measure from when using applications to assess external hoof balance from lateral photographs.
Introduction
Changes in equine hoof proportions occur as a natural effect of hoof growth, affecting the alignment of the horse’s digit (van Heel et al. 2004, 2005, Moleman et al. 2006), or by changes in the direction and magnitude of forces acting upon it (Caldwell 2017), or anecdotally being a result of the restriction of natural function of the hoof caused by the fitting of shoes (Roepstorff et al. 1999). Hoof proportions are also influenced by farriery intervention (Kummer et al. 2006), which has been shown to be inconsistent between individuals (Kummer et al. 2009). These changes in hoof proportions can alter intra hoof biomechanical function, affecting strains within the capsule (Thomason et al 1992, Dyson 2011) and biomechanics (Wilson et al. 2016, Colborne et al 2016, Hobbs et al. 2018), explaining a link between hoof proportions and lameness (Dyson 2011), catastrophic injury (Kane et al. 1998), and navicular syndrome in the front feet (Waguespack et al. 2010, 2011, Ruff et al. 2016, Osbourne et al. 2021).
The association between poor hoof proportions creating a “long toe, low heel” conformation, and navicular syndrome, is caused by an increased strain in the deep digital flexor tendon, which is transferred onto the distal sesamoid (Waguespack et al. 2010, 2011, Eliashar et al. 2012, Ruff et al. 2016, Osbourne et al. 2021). In the hind limb, the same long toe, low heel hoof conformation affects limb posture (Sharp and Tabor 2022) and is linked to pathologies above the digit into the limb (Mannsman et al. 2010, Pezzanite et al. 2018, Clements et al. 2020). These links express the importance of regular assessment of hoof proportions to maintain soundness in horses.
Ideal hoof proportions have been outlined to optimize biomechanical function and minimize the risk of lameness (Caldwell et al. 2016). These include a straight hoof pastern axis (Turner 1992, O’Grady and Poupard 2003, O’Grady 2006,2013, Logie 2017), which is affected by heel to toe height ratios, which ideally should not exceed 3:1 (Dyson 2011), and heel to toe angle difference which is suggested as ideally below five degrees (Dyson 2011). Ideal proportions of equal, 50/50, heel support to toe support of the hoof around the center of rotation of the distal interphalangeal joint, is widely accepted as optimum for biomechanical efficiency and reduced strain on the deep digital flexor tendon during breakover (Russel 1903, Ferrie 2007, Caldwell 2017, O’Grady and Ovnicek 2020).
With ideal hoof proportions and the associations between hoof proportions, lameness and injury being outlined, the importance of documenting hoof balance is suggested as good practice within the farriery industry to quantify changes in shape during training, and to assess the effects of farriery intervention (Sellka et al. 2020). However, although it has been recognized that digital photography of the hoof is a reliable tool for assessing hoof balance (White et al. 2008), assessment in general practice remains largely a subjective visual evaluation, possibly limiting the ability to recognize insidious changes in hoof proportions until they become pathological.
Objective measurements from photographs of hooves are used extensively in equine research. Studies have used digital cameras with the photos transferred into a computer, with image software then utilized to take linear and angular measurements. While this methodology has been shown to be accurate and repeatable for research purposes (Hampson et al. 2010, Dyson et al. 2011, Sellka et al. 2023), this technique is time consuming and costly, possibly leading to a reduction of utilization in everyday practice. This could explain why research has shown recognition of poor hoof proportions has historically been low (Mannsman et al. 2000), with long toe, low heel conformation still being suggested as the most common (Clements 2021).
Recently, a template for locating the center of rotation of the distal interphalangeal joint has been tested within HoofmApp (2.0). [Mobile app]. [01/01/2021], a smartphone application specifically designed for hoof balance assessment from a photograph or radiograph. The templates within the application are positioned according to certain reference points and to each other to tell the user the angles and proportions of the digit. Sharp (2021) reported the template position of the center of rotation of the distal interphalangeal joint from a lateral photo, showed statistical equivalence of the HoofmApp assessment with the radiographic anatomical location (Figure 1). However, this study only tested the intra-observer repeatability of one of the templates. To evaluate this application as a reliable tool for hoof balance assessment, the inter-observer reliability of all its templates is needed.
Fig.1 Lateral digital photo of a hoof positioned for accurate assessment (White et al. 2008) with the templates for external hoof proportion measurements from HoofmApp. A is center of rotation template and coronet angle. B shows toe versus heel base of support template added. C shows Heel and toe angles added. C shows Heel to toe height ratios added. D shows pastern angle added.
Methods
Five lateral photographs of equine cadaver feet were selected to show different hoof conformations. The five photos were sent to 10 volunteer observers, a random selection of farriers, owners, and practitioners, over the age of 18, invited through the authors Facebook page, who already had the application on their phones. Sample size was calculated using G* Power, one way analysis of variance test with a large effect size due to the high intra-observer reliability from the pilot study (Sharp 2021).
One observer did not complete the measurements. Three observers were asked to measure twice. Participants received a link to a 10-minute video recording of basic training on using the templates. They were given a link to a shared folder to access the five photographs and asked to add the templates (Figure 1) to a folder individually accessible to them when measuring external proportions had been completed.
Data taken from the HoofmApp templates, placed by the observers were, coronet angle (CA), heel support percentage (HS) and heel (HA) and dorsal wall angles (DW). This data was put into SPSS to test for inter and intra-observer reliability.
Results
The data were assumed to be of a non-parametric distribution due to sample size, therefore a Friedman two-way ANOVA was used to test for differences between the 9 observers. There was no significant difference between the measurements of each observer (F(8) = 6.779; p=0.561).
The intra-class correlation coefficient was above .9 for both the upper and lower bound, showing statistically excellent inter-observer reliability.
The minimum and maximum ranges for the individual measurements were Heel angle 7/16, Coronet angle 4/7, Dorsal wall angle 2/6, heel support percentage 3/9.
A Wilcoxon rank test was used to test the intra-observer reliability of the three observers who repeated measurements and showed no significant difference (p 1.000).
Discussion
Studies have shown us that even small changes in hoof proportions, affecting palmar angle, can have significant effect on moment arms and strains on internal structures of the hoof capsule (Moleman et al. 2006), as little as 1° difference can equate to 6% less strain on the navicular (Eliashar 2012). Therefore, when measuring hoof capsule proportions, the importance of precision in record keeping is highlighted. The results of this study showed that statistically there was excellent reliability in measuring hoof proportions using HoofmApp, however, when looking at the range of measurements, clinically these differences would be significant. With a p value of 1 for intra-rater reliability the results show that if the same observer uses HoofmApp, their measurements are completely reliable and repeatable except by chance, however, there is a wide range of measurements with random untrained observers. The smallest range was present when measuring dorsal wall angle, with the largest range being heel angle measurements. This could be accounted for by the fact it was noted that the discrepancies in heel angle measurements increased with more acute heel angles especially if the frog and bulbs of the hoof were more visible, and there was minimal contrast between the wall and bulb color, meaning the beginning and end of the heel became more difficult to visualize, whereas the dorsal wall angle is much more clearly defined (Figure 2).
Figure 2. A showing a lower heel angle with visible frog and bulb very similar in color to the shadowed heel wall. B showing a more upright heel with a larger area of heel to measure and minimal soft tissue structures visible to obstruct view. Yellow area shows an example of variability in template positioning due to a lack of standardized anatomical datum points for coronet angle. Image courtesy of John Stewart.
The differences in coronet angle could be due to differences in curvature and an ill-defined difference between the end of the hairline and beginning of the heel bulb (Figure 2). This measurement would in turn affect heel support percentages, as well as differences of opinion on where to measure the end of the toe when it is rounded.
This contrasts with the findings of White et al. (2008) which found a standard deviation of <0.1 for all measurements. This difference could be a result of the methods used to obtain the heel angles, White et al. (2008) attached ball bearings with double sided adhesive tape to the palmar aspect of the lateral heel at the level of the coronary band and at the palmar most weightbearing surface, clearly demarcating a line to take the angle measurement from. In practice this would be impractical considering these points were determined using a scalpel blade pushed under the lateral wall in a dorsal direction between the lateral wall and the shoe, whilst the horse was fully weightbearing.
This study suggests that when the observer has established their own datum points for taking angle measurements from, this is repeatable using HoofmApp, and is therefore a useful tool for a single observer to monitor and document changes in hoof proportions. However, without standardized anatomical datum points established, and taught within the equestrian community, this repeatability is lost between observers, affecting the ability of the application to be more useful between professionals. Further research could help to establish these datum points and test their reliability on lateral photographs without invasive marking of the heel, with dissemination of those findings increasing the inter professional usability of HoofmApp.
Conclusion
HoofmApp is an accurate tool for measuring hoof proportions with a high intra-rater reliability. However, due to a lack of established anatomical datum points within research there remains subjectivity in the placement of the templates affecting inter-observer reliability. Further research should look to test reliable methods for establishing datum points to measure from when using applications to assess external hoof balance from lateral photographs.
References
Caldwell, N, M. (2017) An investigation into the use of hoof balance metrics to test the reliability of a commonly used foot trimming protocol and their association with biomechanics and pathologies of the equine digit.
Clements, P. E. et al. (2020) ‘An investigation into the association between plantar distal phalanx angle and hindlimb lameness in a UK population of horses’, Equine Veterinary Education, 32(S10), pp. 52–59. doi: 10.1111/eve.13186.
Clements, P. (2021) ‘Therapeutic farriery of the hind feet for horses with hindlimb orthopaedic injuries’, UK-Vet Equine, 5(1), pp. 6–11. doi: 10.12968/ukve.2021.5.1.6.
Colborne, G. R. et al. (2016) ‘Associations between hoof shape and the position of the frontal plane ground reaction force vector in walking horses’, New Zealand Veterinary Journal, 64(2), pp. 76–81. doi: 10.1080/00480169.2015.1068138.
Eliashar, E. (2012) ‘The Biomechanics of the Equine Foot as it Pertains to Farriery’, Veterinary Clinics of North America - Equine Practice, 28(2), pp. 283–291. doi: 10.1016/j.cveq.2012.06.001.
FERRIE, J., (2007) Shoeing around the coffin joint. [Online] Available at: http://www.forgeandfarrier.co.uk/shoeingaroundcoffinjoint.htm
Hobbs, S. J. et al. (2018) ‘Sagittal plane fore hoof unevenness is associated with fore and hindlimb asymmetrical force vectors in the sagittal and frontal planes’, PLoS ONE, 13(8). doi: 10.1371/journal.pone.0203134.
Kummer, M. et al. (2006) ‘The effect of hoof trimming on radiographic measurements of the front feet of normal Warmblood horses’, Veterinary Journal, 172(1), pp. 58–66. doi: 10.1016/j.tvjl.2005.03.008.
Kummer, M. et al. (2009) ‘Comparison of the trimming procedure of six different farriers by quantitative evaluation of hoof radiographs’, Veterinary Journal, 179(3), pp. 401–406. doi: 10.1016/j.tvjl.2007.10.029.
Logie, S. (2017) ‘The hoof pastern axis and its relevance to soundness’, Equine Health, 2017(34), pp. 18–20. doi: 10.12968/eqhe.2017.34.18.
Moleman, M. et al. (2006) ‘Hoof growth between two shoeing sessions leads to a substantial increase of the moment about the distal, but not the proximal, interphalangeal joint’, Handbook of Environmental Chemistry, Volume 5: Water Pollution, 38(2), pp. 170–174.
O'GRADY, S. E., (2006) Strategies for shoeing the horse with palmar foot pain. Proceedings 52nd annual convention of the American Association of Equine Practitioners 209
O'GRADY. S. E. (2013) How to evaluate the equine hoof capsule, American Association of Equine Practitioners Convention Proceedings
O’GRADY, S. E. & POUPARD, D. A. (2003) Proper Physiologic Horseshoeing, Veterinary Clinics of North America Equine Practice 19 333-351
O'Grady, S.E. and Ovnicek, G. (2020), Foot Care and Farriery: BASIC FOOT CARE. Adams and Stashak's lameness in horses, pp.1091-1142.
Pezzanite, L. et al. (2019) ‘The relationship between sagittal hoof conformation and hindlimb lameness in the horse’, Equine Veterinary Journal, 51(4), pp. 464–469. doi: 10.1111/evj.13050.
POWELL, C. (2006) Difference in toe and heel angles, FWCF thesis, https://www.wcf.org.uk/fwcf-thesis?tid=186c76fef2fa64f520746499bc2d e115bc40b340adf6362381686d4204dc46b3 Accessed 25/08/2021
Roepstorff, L., Johnston, C. and Drevemo, S. (1999) ‘The effect of shoeing on kinetics and kinematics during the stance phase.’, Equine veterinary journal. Supplement, 30, pp. 279–285. doi: 10.1111/j.2042-3306.1999.tb05235.x.
Russell, W.I.L.L.I.A.M., 1897. Scientific horseshoeing.
Sharp, Y. (2021) ‘Pilot study into the accuracy of an external template for the assessment of dorsopalmar balance according to internal structures’, Forge Knowledge, Forge-September-2021-HR.pdf (forgeandfarrier.co.uk) pp 22-26
Sharp, Y. and Tabor, G. (2022) ‘An Investigation into the Effects of Changing Dorso-Plantar Hoof Balance on Equine Hind Limb Posture’, pp. 6–8.
THOMASON, J. J., BIEWENER, A. A. and BERTRAM, J. E. (1992) ‘ Surface Strain on the Equine Hoof Wall In Vivo : Implications for the Material Design and Functional Morphology of the Wall ’, Journal of Experimental Biology, 166(1), pp. 145–168. doi: 10.1242/jeb.166.1.145.
TURNER, T. (1992) The use of hoof measurements for the objective assessment of hoof balance. Proceedings of the American Association of Equine Practitioners, 38 389-395
Van Heel, M. C. V. et al. (2004) ‘Dynamic pressure measurements for the detailed study of hoof balance: The effect of trimming’, in Equine Veterinary Journal, pp. 778–782. doi: 10.2746/0425164044847993.
Van Heel, M. C. V. et al. (2005) ‘Changes in location of centre of pressure and hoof-unrollment pattern in relation to an 8-week shoeing interval in the horse’, Equine Veterinary Journal, 37(6), pp. 536–540. doi: 10.2746/042516405775314925.
White, J. M. et al. (2008) ‘Diagnostic accuracy of digital photography and image analysis for the measurement of foot conformation in the horse’, Equine Veterinary Journal, 40(7), pp. 623–628. doi: 10.2746/042516408X313625.
Wilson, A. et al. (2016) ‘Foot placement of the equine forelimb: Relationship between foot conformation, foot placement and movement asymmetry’, Equine Veterinary Journal, 48(1), pp. 90–96. doi: 10.1111/evj.12378.