Can we Locate the Centre of Rotation from a Lateral Photo?
Pilot study into the accuracy of an external template for the assessment of dorso - palmar hoof balance according to internal structures. – Yogi Sharp DipWCF, BSc (Hons)
Hoof balance has been shown to be an important factor in lameness and catastrophic injury (Kane et al. 1998, Dyson 2011). However, there is a low recognition, both by owners and professionals, of the external reference points that can be used to assess hoof balance in the absence of radiographs. Mannsman et al. (2000) quantified the lack of recognition of poor external hoof proportions. The study found only 10% of 50 randomly radiographed horses presented with radiographs that were considered normal. 40% of the horses were in a category that included inadequate caudal support and/or poor palmar angle. Mannsman et al. (2000) discussed how, to enable preventative hoof care, owners need to be made aware of any potential or existing foot problems. Providing the farrier with a tool to show and assess balance from, can aid shoeing decisions and encourage positive interaction among the owner–farrier–veterinarian team quickly and practically. An easily accessible and costeffective source of external templates that can be used to quickly assess hoof balance, from a lateral photograph, could help recognition of poor dorso-palmar balance earlier before radiographs become necessary due to lameness. This could educate the owner, train the farriers eye and enable more objective documentation and aid intervention decisions.
This study sought to test a template within an app, used as a tool, to provide hoof balance information. Predispositions of Poor Hoof Balance Dyson (2011) stated that the ratio of dorsal to palmar coronary band heights and the shape of the coronary band were significantly different between lame and non-lame horses and this proportional relationship is a measure of dorso-palmar balance. Hoof proportions are directly linked to digit proportions. Poor dorso-palmar balance, leading to a broken back HPA (BBHPA), has been shown to predispose the horse to navicular syndrome. Waguespack and Hanson (2010) outlined the biomechanical considerations of a BBHPA and stated that the primary source of pressure on the navicular bone (NB) is compression from the deep digital flexor tendon (DDFT), Ruff et al. (2016) expanded on this, expressing the increased compressive force on the NB from the DDFT in conformations exhibiting increased dorsiflexion, this was supported by Uhl et al (2018) which stated conformations described by Ruff et al. (2016) as being mechanically predisposed to navicular and that DDFT lesions corresponded with areas of increased load. Many other papers have expressed the importance of a good HPA, (Witte (2013), Zani et al. (2015), Logie (2018), Brown (2020)), outlining its importance in optimal performance and the predispositions of a BBHPA.
These studies show the importance of hoof proportions, dorso-palmar balance and HPA and the need for its regular assessment for the prevention and treatment of injury. In the authors work a very high percentage of consultations present with a BBHPA and poor dorso-palmar balance, showing that owners and often their farriers have some recognition that their horses feet do not appear ideal but lack something to quantify their suspicions. The proportions of the hoof directly affect the HPA and have a direct influence on flexor structure strain. Page and Hagan (2002) discussed the distance between the distal phalanx and the toe and how it directly affected HPA, Redden (2003) took this measurement and expressed how it created toe leverage when it was too great, causing toe flares. Dr Weller discussed the biomechanical implications on the flexor structures from a long toe lever arm.
Fig.1 The increase in extending moment caused by poor hoof proportions and the forward migration of the centre of pressure. Image reproduced by permission of R.Weller.
Fig.1 shows how the centre of pressure is subject to hoof proportions, it moves dorsally and away from the centres of rotation as the toe lever arm increases. This increases the extending moment and collapsing force on the limb which has to be counteracted by the strain within the flexor structures. This shows the importance of recognition of hoof proportions around the centre of rotation.
Hoof Proportions and Landmarks
Snow and Birdsall (1990) and Dyson et al (2011) discussed how a difference in angle between the hoof wall and heel greater than 5 degrees suggested a collapsed heel, affecting HPA, however this is questioned by anecdotal evidence, Powell (2006) and Caldwell et al. (2016) which suggest a wider range of Heel:toe angle differences within a range of normality. Turner (1992) and Dyson et al. (2011) mention the established ideal of there being a 3:1 toe:heel height ratio and that this ratio affected the angle of the hoof, inevitably this will affect HPA. However, the authors own experiential opinion and anecdotal evidence, supported by Caldwell et al. (2016) would question the 3:1 ratio in light of natural variation. These studies suggest, however, that measuring external proportions can play a role in determining hoof balance, and Dyson et al. (2011) showed how external assessment of the hoof can give good indication of its predisposition to pathology. While heel:toe proportions require further research to establish ranges of normality, other proportional measurements are more established.
There have been studies into external reference points for recognising internal landmarks that are important for creating efficient biomechanics around, to reduce the risk of injury. Ducket was the first to establish external reference points in relation to internal anatomical points of interest. Caldwell (2016) discussed, summarised and tested the importance and accuracy of these external reference points such as Duckets dot and the external centre of rotation, achieved by foot mapping. Caldwell (2016) discussed Duckets theory which stated that balance was theoretically achieved by way of proportional measurements and Ducket encouraged proportional optimization around the internal anatomy. Caldwell (2016) found that the external reference point centre of rotation, was related to the position of the internal location of centre of rotation of the distal interphalangeal joint (DIPJ) and Caldwell (2018) stated that creating proportional dimensions around this point, on an individual basis, was a good model for creating biomechanical efficiency. This supported O’Grady (2006) which expressed how shoeing to place the COR in the middle of the base or shoe was biomechanically optimal and this practice is widely accepted as an ideal for farriery intervention. Shoeing to create equal distance from heel to breakover from COR is widely advocated by leading podiatrists such as Dr Myers and EPC solutions and Dr Redden amongst others. Although the optimum proportions of the foot differ between shod and barefoot and different authors, the common denominator is the use of COR as the datum point. Caldwell (2018) discussed how the widely accepted 50:50 geometric post-trim proportions of the bearing border around CoR advocated by Colles et al., (1983; 1989) was contrasted by the theories of Jackson, 1992 and Ovnicek 2003, whose trimming methods consistently created proportions of 60:40 around COR in the barefoot. Trimming on an individual basis, accounting for phalangeal alignment as well as proportional hoof measurements has become the authors preferred method and is supported by Caldwell et al. (2016), this could create a range of barefoot proportional measurements around COR, however many practitioners, as stated, and papers, going back to the theories of Ducket point toward creating 50/50 around COR when shod. The external COR is found by different foot mapping methods, some of which require certain external anatomy to be visible, for example, the inner edge of the white line to be visible (Caldwell 2018), this is no longer possible when shod and can not be assessed from a lateral view. Ferrie (2007) discussed a method for establishing COR from a lateral view, again highlighting the importance of balance around this point. It stated that when divided into thirds, the transition point from the first to second third from the toe correlated with the COR. Berger (2017) discussed balance around the COR as a method that optimizes the biomechanics of the hoof and tested the reference point of Ferrie (2007). The study found that the coronet band marker had the smallest mean difference from mark to COR, however also had the largest standard deviation. Further research could prove it to be an accurate reference point in finding the COR within a working tolerance.
In order to use this reference point a true lateral photo of the hoof is needed. Dyson (2011) cited earlier studies, stating that Digital photography provides an accurate method of measurement of hoof conformation and has been shown to be superior to hoof angle measuring devices (Moleman et al., 2005). White et al. (2008) tested the practicability, precision and accuracy of the process of obtaining measurements of horses’ feet using photography. Excellent precision was identified within and between operators regardless of image origin. High levels of accuracy were also identified, especially for heel height/toe height ratios and coronary band angle, indicating that photography and radiography may be used interchangeably. The photographic technique included visually aligning the heel bulbs as is done when obtaining lateromedial radiographs (Kummer et al. 2004) and supporting the camera at the same level as the foot. This shows that subject to protocols in image acquisition, quantified templates overlaid on lateral photographs could provide accurate assessment of hoof balance. The external reference points that have been outlined by studies to suggest ideal geometric proportions for optimal dorso-palmar balance, from a lateral external view are as follows.
Fig.2 External reference markers from existing literature.