1.0 Introduction
The hoofs interaction with the ground should provide the horse with mechanical and functional efficiency while considering the skeletal structures of the limb (Ovnicek et al. 2003, Eliashar 2012, Caldwell et al. 2016, O’Grady and Ovnicek 2020). Hoof balance is generally discussed in two planes, dorso-palmer/plantar (DP) balance and medio-lateral balance. DP balance relates to the external characteristics of the lateral aspect of the hoof. The ideal relationship between the heels and toe of the hoof and their effect on hoof pastern axis (HPA) have been suggested (O’Grady and Poupard 2003, Dyson et al. 2011, Logie 2017, Brown 2020, Turner 2020, Ferrie 2007, Berger 2017) (Figure 1).
Figure 1. The parameters used in the farriery and veterinary industries to assess DP hoof balance from lateral external characteristics. A. Line drawn through the pastern and hoof, (following parallel to their dorsal surface) is the HPA. A vertical line that bisects the third metacarpal bone should intersect the ground at the palmer aspect of the heels. Connect these two lines to form a triangle (O’Grady and Poupard 2003). B. HPA should be straight (Logie 2017, Brown 2020, Turner 2020). Ca/b. The base of the shoe should be bisected (Ca) by a line dropped from 1/3 of the most dorsal and most caudal aspects of the hairline (Cb) (Ferrie 2007, Berger 2017). Da/b. The heel height (Da) to toe height (Db) ratio should be no greater than 3:1 (Dyson et al. 2011). Ea/b. The heel angle (Ea) should be within 5° of the dorsal wall angle (Eb) (Dyson et al. 2011). Authors own image.
Research has outlined the links between poor DP hoof balance, lameness, and catastrophic injury (Snow and Birdsall 1990, Turner 1992, Balch et al. 1993, Kane et al. 1998, Dyson et al. 2011) with poor DP balance having direct links to navicular syndrome (Waguespack and Hanson 2010, 2011, Ruff et al. 2016, Osbourne et al. 2021) in the front limbs and pathologies positioned higher within the hind limbs (Mansmann et al. 2010, Pezzanite et al. 2019, Clements et al. 2020). Therefore, objectively quantifying what hoof balance is, is vital to create not only functional efficiency but to reduce the incidence of poor balance induced injury for equine welfare.
Farriery can manipulate hoof proportions and induce change in toe length, in heel or toe height and angles, and changes in ground contact area (Eliashar 2012). These changes can be achieved by trimming, use of a special shoe, incorporation of prosthetics such as a wedge, or any combination. Herein lies the issue, what constitutes correct balance of the hoof capsule, shoe placement, and the application of any prosthetics, remains debated within the industry on social platforms. The debate is driven by the influence of certain studies that argue against creating an ideal, due to what has been measured as common (Craig 2020) and others arguing the existence of natural variation meaning the commonly suggested theory of geometric proportions for hoof balance do not hold true (Caldwell et al. 2016). The confusion created within the industry because of the contradictory views affects the reliability of practice and ultimately can affect equine welfare.
This review will critically appraise the existing literature on two DP hoof balance parameters that are widely debated within the farriery industry, base proportions around the centre of rotation (CoR) and hoof pastern axis (HPA).
2.0 Methods
Relevant papers were sourced using google scholar and the terms “equine hoof balance” (11,900 results) “long toe low heel horse” (63,300 results) “hoof balance and navicular” (1,620 results) “hoof balance and pathology” (19,400 results) and “equine hoof pastern axis” (1,980 results). The first ten pages of results was scanned for abstracts and titles that related to defining hoof balance and/or its relationship to injury or pathology. Papers were also sourced from authors quoted in relation to the debate within the industry and from the worshipful company of farriers FWCF thesis list.
3.0 Reference Points for Establishing Base Proportions
Russel (1988) suggested guidelines for the perfect placement of the shoe, equality of opposite points around the center of gravity (center of the sole surface) (Figure 2).
Figure 2. Authors representation of Russel (1988) perfect shoe placement. A=A, B=B=B=B, C=C, D=D=D=D to create equality of opposing points around the solar surface center (Russel’s center of gravity). Authors own image.
Ducket (1990) was the first study to correlate external reference points with internal anatomy and established two main points of reference, Ducket’s dot and bridge (Figure 3), the dot correlated with the extensor process of the distal phalanx, the action line of the extensor and flexor tendons, and the bridge correlated with the widest part of the sole. Ducket (1990) described the dot as the “balance point of the distal phalanx” and center of pressure of the hoof but suggested that equivalence of proportions around the bridge, which corresponded with Russell (1988) center of gravity, created mechanical balance. Caldwell et al. (2016) discussed Ducket’s theory, agreeing that balance was theoretically achieved by way of proportional measurements and outlined a mapping protocol to find the external reference point centre of rotation (eCoR) and found it to relate to the position of the internal center of rotation (CoR) and Ducket’s bridge (Figure 3). Creating proportional dimensions around this point, has been supported as a good model for creating biomechanical efficiency (Turner 1993, O’Grady 2006, Ferrie 2007, Caldwell et al. 2016, Berger 2017).
Figure 3. Hoof mapping systems. A. Hoof mapping system to locate eCoR established by Caldwell et al. (2016) (Red lines). Ducket’s dot (Green dot) and Ducket’s bridge (Orange line). B. The external points of reference on a radiograph to show their relationship to internal anatomy. C. Ferrie (2007) CoR location method using HoofmApp template showing base split proportions. Authors own image.
Much of the literature after Ducket (1991) has suggested that biomechanical efficiency is created by placing the CoR in the middle of the base or shoe; a practice that is now widely accepted as an ‘ideal’ for farriery intervention. Ferrie (2007) discussed and tested a method for establishing CoR from the lateral view (Figure 3) agreeing with balance around the CoR as a method that optimizes the biomechanics of the hoof due to it being the point at which the hoof rotates around. Moon (2019) argues that equivalence of base proportions around CoR are impractical, suggesting shoeing around the centre of articulation surface of the distal phalanx (COAS) (Figure 4) as more practically achievable, and the most important consideration for functional mechanics. However, the study offers no objective quantification of this statement, only stating that it is a more practically achievable parameter. The studies above agree that proportions of the hoof are important for biomechanics and that hoof balance is achieved when specific proportions of the bearing border length are equivalent. However, no research to date quantifies their theories of proportional balance around their recommended points, or discuss the physics involved in creating optimal biomechanics of the hoof.
Figure 4. Radiographs showing the different points of reference used for establishing base proportions around. Blue shows the CoR. Green shows the COAS. Red shows the PoB. The line from the dorsal to caudal aspect of the capsule through CoR is a template from HoofmApp showing CoR and PoB. Authors own image.
More recently some research, Yxklinten and Sharp used mathematical models to define a balanced shod hoof as when there is equal ground reaction force on the base of the shoe. The study suggested an alternative point for creating equal proportions around, the point of balance (PoB)(1/4 of the hairline from the toe) (Figure 4).
The PoB is a mathematical rather than anatomical point, equivalence of base proportions around this point create equilibrium at midstance when neither the toe area nor the heel area sinks into the ground more than the other as the horses’ body weight moves over the hoof on soft ground.
Hoof growth throughout a shoeing cycle should be taken into consideration when considering appropriate base proportions, as it can affect the position of the hoof’s solar surface in relation to the bone column, thereby changing the base proportions around the points of reference and altering the orientation of the distal phalanx (van heel et al. 2004; 2005; Moleman et al. 2006). This hoof growth and changes to phalangeal alignment will affect the position of COAS relative to the base as can be seen in figure 2. This suggests COAS as a poor reference point for establishing ideal proportions as the resultant base proportions, although easier to achieve, will be dictated by the health of the capsule and its existing proportions. Using the centre of articular surface of the middle phalanx (CoA) is widely used in the farriery industry and creates an anatomical point that doesn’t change with changes in hoof morphology (Figure 5).
Figure 5. Radiographs showing the different points of reference used for establishing base proportions around with the COAS changes to the CoA. Blue shows the CoR. Green shows the CoA. Red shows the PoB.
The rotation of the distal phalanx around its axis highlights the importance of considering hoof growth, and changes in hoof pastern axis and phalangeal alignment when choosing which point of reference to shoe around. Yxklinten and Sharp discuss this in reference to shoeing around CoR versus PoB leading them to suggest a “zone” where a working tolerance of balance is achieved during the shoeing cycle.
4.0 Hoof Pastern Axis
Craig (2019) measured the palmar Angles (PA), coffin Joint angles, and pastern joint angles of over 2600 forefeet. The study found on average the pedal bone was 10 degrees broken back from the middle phalanx, while the middle phalanx was 5 degrees broken back from the proximal phalanx. This resulted in what the study called a combined 15-degree “uprightness”, creating a broken-back bony column (Figure 6).
Figure 6. A radiograph mark up using the joint angle protocol from Craig (2019). The pedal bone is 5 degrees broken back from the middle phalanx, the middle phalanx is 13 degrees broken back from the proximal phalanx, creating an 18 degree “uprightness” and broken back bone column. Measured using HoofmApp. Authors own image.
The study challenged the notion of a straight Hoof-Pastern Axis (HPA) as ideal, suggesting instead that a broken-back-hoof-pastern-axis (BBHPA) is normal. However, the study had some limitations. The only common factor among the horses included in the study was that they were predominantly from veterinary clinics. Without any case history, the interpretation of the results becomes questionable. It is reasonable to assume that many of these cases were admitted for radiographic investigation due to lameness or for diagnostic purposes. Additionally, a smaller subset may have been admitted for pre-purchase exams, and it is unclear which of these horses were deemed sound. Considering these limitations, if case history, pathology, and lameness diagnosis had been included alongside the 10 radiographed cases (which presented with common broken HPA), the interpretation of the data could have been significantly impacted.
Although Craig (2019) was not published in a peer reviewed journal, it has become important in the industrial debate due to it being consistently referenced on social media platforms and presents the rhetoric that drives the argument against an ideal of a straight HPA (Logie 2017, O’Grady 2018, Turner 2020, Brown 2020). Reviewing this study in the context of peer reviewed research on this same subject matter may help to better define what can be gleaned from the results.
The debate about aiming for a straight HPA is not about simple aesthetics but whether there are pathological implications of certain hoof balance presentations and their link to lameness. Dyson et al. (2011) measured morphometric parameters of the hoof and identified a link between heel to toe height ratios (HTHR) and lameness, and between a higher coronet angle (CA) and the lame foot in unilateral lameness. These morphometric parameters can be affected by farriery intervention (Figure 7).
Figure 7. Before and after photos of authors farriery intervention to change HTHR effecting HPA and CA. Before image (Left) HTHR 3.92, CA 29°, HPA 10° broken back. After image HTHR 2.09, CA 21°, HPA 0° broken back. Measured using HoofmApp. Authors own image.
Dyson et al. (2011) identified a 3:1 HTHR as maximum for an ideal hoof conformation. Changes to HTHR has been shown to affect HPA and CA (Sharp and Tabor 2022), However, no research has established an ideal HTHR and anecdotally this would be dependent on the individual horse. Although Dyson (2011) did not measure HPA, a logical suggestion could be that the higher the ratio between the two, the less likely a straight HPA would be possible. This would be supported by Balch et al. (1993) which suggested that the actual angle of the dorsal wall was less significant than the HPA in relation to lameness and Sharp and Tabor (2022) which found that plantar angle was less significant than HPA in the relationship between hind hoof balance and hind limb posture.
There is a significant body of research agreeing on the consequences of poor phalangeal alignment or BBHPA, most notedly its resultant increase in flexor tendon strain (figure 8).
Figure 8. PoF changes with digit alignment. Adapted from Wilson and Weller (2011) with point of force trajectory according to Yxklinten and Sharp (in press). The calculations for moment arms affecting the joints of the distal limb. Authors own image.
The mathematical model proposed by Weller and Wilson (2011) shows how both a long toe and BBHPA increases strain in the flexor structures due to the CoP and point of force (PoF) trajectory moving away from CoR. This movement of the PoF away from the centres of rotation increases the moment arms around the joints and must be counteracted by increase strain in the flexors to stop the limb from collapsing. This increase in strain is transferred onto the podotrochlea apparatus; the resultant strain increase is known to predispose to navicular syndrome (Waguaspack and Hanson 2010; 2011; Eliashar 2012; Ruff et al. 2016; Osbourne et al. 2021). These studies suggest that a straight HPA is something to aim for, which can be created by the application of certain prosthetics, such as wedges. However, other research has been used to question the implications of applying wedge prosthetics to the hoof. Viitanen et al. (2003) measured a huge increase in intra-articular pressure (IAP) in the distal interphalangeal joint with elevating the heels, suggesting the increased pressure in the joint may directly cause pain and lead to further pathology. However, the limitation of this study is that there was no baseline of optimum IAP, nor was there a clear outline of what the HPA was when the elevation was applied. Therefore, it is unclear as to whether the increase in IAP was within physiologically normal limits or whether the increase in heel height created a broken forward hoof pastern axis which would take the digit beyond the ideal. The study however noted that the “balanced foot” was ideal, the study made no definition of the ideal only discussed the increased flexor strain associated with a BBHPA. The research could therefore have an alternative interpretation that agrees with the practice of raising the heels to restore straight phalangeal alignment.
Lawson et al. (2007) used mathematical models to measure an increase in superficial digital flexor tendon (SDFT) and suspensory ligament (SL) strain with heel elevation, suggesting implications for these structures with the application of wedges. This suggestion has been called into question by recent studies using ultrasound to measure cross section area of the SDFT under load, showing that the strain in the SDFT decreased with heel elevation similarly to the DDFT (Hagen et al. 2018), however SL strain did increase with elevation. These studies have been used to interpret that heel elevation creates risk of injury to the SL. The issue with this interpretation is that while both studies measure an increase in SL strain, neither study establish an ideal strain of the SL. It is, therefore, unclear as to whether the measured increase in SL strain is within its physiological norm or heading toward pathological. Considering the increase in flexor strain because of a BBHPA it could be suggested that creating a straight HPA therefore distributes excess strain in the flexors back onto the SL restoring normal load share between these structures (Figure 9). This concept of appropriate strain share between the DDFT, SDFT and SL is the same principle behind Yxklinten and Sharp suggesting the importance of equilibrium at midstance as the toe or heel sinking in during mid-stance would increase strain in the SL and DDFT, SDFT respectively (Figure 9).
Figure 9. The strain share between the DDFT, SDFT and SL in different digit alignments. Bold lines suggest increased strain. In the aligned position (equilibrium at midstance) there is appropriate strain share between all 3 structures. When broken back we have an increased strain in the DDFT and SDFT. When broken forward we have an increased strain in the SL. Authors own image.
5.0 The relationship between hoof pastern axis and base proportions
Yxklinten and Sharp discuss the importance of digit alignment in the ability to align the pressure point of the surface of the hoof (PPSH), which would correspond with Russel (1988) centre of gravity, with the PoB to create equilibrium. The more broken back the alignment the more dorsal the PPSH, following a similar pattern to the CoP (figure 8). Ovnicek (2009) tested the accuracy of Ducket’s bridge in determining internal CoR and found that it corresponded with a point anterior of the CoR and showed the distance between the two points got larger as the digit alignment got more broken back. This finding supports digit alignment being important in establishing equivalence of base proportions around internal anatomy. Interestingly, when appraising the images from Caldwell et al. (2016), although the study claimed that eCoR correlated with CoR, their image would agree with Ovnicek (2009) that eCoR often correlated with a point anterior of CoR toward CoA (Figure 10).
Figure 10. Authors representation of image from Caldwell et al. (2016) showing the external markers correlation to internal anatomy.
Caldwell et al. (2016) did not outline the effects of digit alignment on the relationship between eCoR and CoR, therefore it is unclear as to whether this would have affected the understanding of the relationship between the two points. If eCoR was consistently found to be anterior of CoR it could be suggested that Ducket’s bridge and eCoR measure near to the PoB. This would imply that equivalence of base proportions around eCoR and Ducket’s bridge could prove to be points at which hoof balance as defined by Yxklinten and Sharp (in press), where we have equal distribution of GRF’s on the base of the shoe, is achieved. However, it could be shown that this is subject to phalangeal alignment, further research is needed to test the different mapping methods, their datum points relationship with internal anatomy and whether the relationship is dependent on phalangeal alignment.
6.0 Conclusion
It is widely accepted that creating equivalence of base proportions around CoR achieves biomechanical efficiency, while a new balance point has been defined for creating equilibrium at mid-stance. The efficacy of base proportions in creating optimal biomechanics through appropriate positioning of PoF in relation to CoR, creating equilibrium at mid-stance, and maintaining an appropriate lever arm of the toe, is affected by HPA. Furthermore, as HPA becomes more broken back, the base proportions of the hoof, and established external reference points, move dorsally of both the CoR and PoB meaning it becomes less and less practical to create equivalence of base proportions around these points. Base proportions and HPA should therefore be considered simultaneously in practice when attempting to create ideal DP hoof balance. DP hoof balance theories should create biomechanical efficiency, there is limited research providing mathematical models outlining and defining the physics of proportional hoof balance (Yxklinten and Sharp) and further research is needed to test the theoretical model. While Yxklinten and Sharp present a balance point for the moment of shoeing, the effects of hoof growth bring into question the application of this balance point in practice, rather an area of anatomical points between the CoR and PoB is suggested for further research. The large body of research outlining a relationship between poor alignment and pathology suggest that a straight HPA is something to aim for within daily practice to optimise biomechanics, create appropriate load share between the flexor structures and the SL and to aid in the ability to create appropriate base proportions.
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