The Truth about Hoof Pastern Axis
There is conflicting rhetoric traversing the social universe about the importance and ideals of phalangeal alignment and stance angles, concepts being used interchangeably with hoof pastern axis (HPA). Firstly lets establish that these are not the same thing.
Fig.1 Hoof Pastern axis is an external reference to the relationship between the pastern and the hoof wall. Phalangeal alignment is a reference line through the centre of rotation of each joint of the digit. There is some correlation but HPA is not a reliable reference for phalangeal alignment, this expresses the importance of radiographs.
For the purpose of this discussion phalangeal alignment and HPA will be used interchangeably, referring to the orientation of the bony column.
With the advent of increased and improved diagnostic modalities, body sensors, digital fluoroscopy and various pressure plate systems over the last few decades, we have moved towards a better understanding of both dynamic and static biomechanical efficiency and optimized load sharing of structures of the equine digit. Recently, a small group of professionals known to the farrier community have questioned these findings.
John Craig of Eponamind recently presented a study entitled "Stance Statistics" where out of 17,000 lateral radiographs, 2600+ of forefeet were selected on metron blocks and measured for statistical significance recording Palmar Angles (PA), and Coffin and Pastern Joint angles. Over 2000 of these radiographs were obtained from Veterinary clinics worldwide, while a smaller subset of 250 radiographs came from Epona's client database. The findings showed that the average pedal bone was 10 degrees broken back from the middle phalanx and the middle phalanx was an average of 5 degrees broken back from the proximal phalanx, resulting in what the study called a 15 degree uprightness (uprightness of P1 and P2 creating a broken-back bony column).
Fig.2 Representation of the median phalangeal alignment from Craig (2020).
Note the pathological changes in the distal limb, specifically spurring of the navicular bone, joint space pinching cranially at P2/P3 and caudally at P1/P2. Note the slight concave curvature to the dorsal face of P3 indicating periosteum inflammatory changes from excessive deep flexor pull on the parietal face of P3, lipping and bone loss at the distal tip P3 and exostosis at the extensor process. In order to accurately assess phalangeal alignment the cannon bone should be vertical as a constant to compare joint angles. In this case, the horse is standing camped under which breaks the bony column back more than in reality, compared to if the horse was standing square with cannon bone vertical to the ground. In a square stance, the coffin and pastern angles would be smaller, closer towards bony alignment, yet still deviating away from alignment enough for pathology to ensue.
Results from the Craig (2020) study concluded that the mean average of the lateral radiographs showed broken back bony alignment aka uprightness of P1 and P2 with a combined 15 degrees, in other words, broken-back PIP and DIP joint angles combined.
The interpretation of these results suggested that the established ideal of a straight HPA is in fact pathological and farriery intervention looking to establish ideal HPA is contraindicated. It was suggested instead, that a broken-back-hoof-pastern-angle (BBHPA) is normal.
The population used in the study introduced many uncontrolled variables which influenced the results producing questionable data for scientific study. The single common denominator was that the majority of horses represented in the study came from vet clinics. Without any case history the interpretation of the results is suspect: it would be prudent to assume the majority of these cases were admitted for radiographic investigation for lameness and diagnosis, with a smaller subset perhaps admitted for pre-purchase exam, and of those we do not know which were deemed sound. It is the opinion of the author that if case history, pathology and lameness diagnosis had been included alongside the radiographed cases (presenting with common broken HPA), the interpretation of that data would be impacted significantly.
Fig.3 Re-created with permission of EPC solutions. Data sets of PA (Palmar/Plantar Angles) cannot offer meaningful data unless a) the bone angle of the coffin bone is considered in combination with the PA and b) the pastern conformation (ie. upright, sloping) is considered. Hoof angle is a summation of PA and Coffin bone angle combined.
Therefore casually citing an average PA as normal without regard to a wide variety of bone angles and pastern conformation is not scientific.
Critical thinking would beg the question: what is the study actually showing?
If one collects numbers from a random pool of stats without any controls, does "common" constitute "normal" or "correct"? Reviewing this study in context of other studies on this same subject matter may help to better define what can be gleaned from the results and critically applied interpretation.
Bones of the distal limb are suspended in the hoof capsule, sharing with space-occupying soft tissue, tendons, ligaments, vasculature, nerves, and connective tissue- all playing a role in bony orientation of the phalanges. Additionally, peer reviewed studies have shown us that phalangeal alignment or lack there-of, directly affects the biomechanical efficiency of the hoof; changes occur across a multitude of planes including stress and loading centers across the hoof, strain on soft tissue, joint space compression and articular pressure to name a few.
Fig.4 Even the changes in phalangeal alignment from hoof growth affect these parameters. Moleman et al (2006) showed that hoof growth broke the HPA and increased the moment arm around the distal interphalangeal joint (DIPJ) surmising that the effects of a Broken back HPA (BBHPA) translate into increased load onto the DDFT, distal phalanx and DIPJ structures. Earlier studies also showed similar effects of hoof growth in breaking the HPA and the corresponding negative correlation biomechanically. Van Heel et al (2004,2005) and Moleman et al (2006) findings highlighted the effects of hoof growth on both biomechanics and soft tissue load.
Fig.5 Van heel et al (2004) discovered a common trace of the centre of pressure (COP). The study found that hoof growth affected this trace, the COP moved caudally with hoof growth creating higher loads on the heel and increased time to mid-stance load bearing, meaning longer load times of the heels. This was exacerbated in a lower angled hoof, which commonly presents as a BBHPA, also showing that weaker heeled horses will suffer greater deformation from the same length of shoeing cycle.
Fig.6 Hoof growth affects the proportions of the foot and therefore the forces acting upon it, the distance from the centre of rotation (COR) to breakover increases, creating a larger lever arm for the deep digital flexor (DDFT) tendon to overcome and the COP moves back towards the heels predisposing them to exceeding their elastic modulus and exposing them to greater creep forces. This is the same for a BBHPA.
Fig.7 Adapted from van Heel et al (2005), this shows the changes in hoof wall length and angle over a shoeing cycle with the subsequent backward movement of the COP. Although the horse has compensatory mechanisms which counteracted some of the effects of growth the increased load on the DDFT and navicular is pronounced.
The findings of Van Heel, markedly the movement of the COP caudally is debated, with some studies and experiential opinion stating the forward movement of the COP, however the clear increased load on the DDFT, distal phalanx and DIPJ structures are relevant to this discussion. Clayton (1990a, 1990b) showed the increase in breakover time of the fronts and hinds with a BBHPA and an increase in toe first landings in the forefeet. This could be attributed to painful heel first landings as the forward migration of the hoof means point of impact is now located under the wings of the distal phalanx rather than under the digital cushion and anti-concussive haemodynamic structures. This also helps explain why BBHPA’s commonly suffer with collapsed heels as their elastic modulus is exceeded by cyclic impact overloads.
Fig.8 Broken Phalangeal alignment, BBHPA, long toe/low heel conformations are often comorbidities and commonly have the heels based under the pedal bone rather than the digital cushion.
Although figures 4-7 are primarily discussing the effects of hoof growth, fig 4 shows how this effect is also a result of the change in phalangeal alignment and a foot that has become long due to hoof growth exhibits the same predispositions as a BBHPA.
Outside of hoof growth a straight HPA has been described as important for optimum biomechanical functionality (O’Grady 2018, Brown 2020) but more importantly a BBHPA has been linked to pathology and inefficient biomechanics by repeatable peer reviewed studies.
Waguespack and Hanson (2010, 2011, 2014) outlined the biomechanical considerations and stated that the primary source of pressure on the navicular bone (NB) is compression from the deep digital flexor tendon (DDFT) also stating that creating a straight HPA was an effective treatment for navicular. 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 echoed by Uhl et al (2018) which stated conformations described by Craig (2020) as being “normal” were found to be mechanically predisposed to navicular and that DDFT lesions corresponded with areas of increased load. The distal limb bony conformation these papers were referring to is the conformation that Craig (2020) would like to suggest as normal, leading one to hypothesize that only the more severe deviations towards negative PAs (NPA) are pathological.
Fig.9 A typical presentation of a BBHPA predisposed to navicular syndrome. In the top left corner a more ideal HPA is presented. The BBHPA has certain features that directly affect biomechanics and pressure on the NB. A long lever arm from the centre of rotation (COR) (which is the centre of the distal condyle of the middle phalanx (P2)) to the toe means increased loading forces generated by the DDFT/Muscle belly; it has to overcome this toe lever arm to initiate breakover unrollment. This, in addition to the fact that the DDFT already has increased load statically on the navicular bone and the tendon itself is “stretched” closer to its elastic limit predisposes the tendon to injury/breakdown.
Fig.10 Radiographs of 2 fore hooves from the same horse. The bottom radiograph exhibits a BBHPA bordering on NPA, increased dorsiflexion (P1,P2 uprightness) in the foot with a BBHPA has created increased load on the navicular apparatus and clinically presents with significant pathology; this supports the findings of the papers referenced, showing that a BBHPA conformation was the contributary factor in the development of navicular changes. These findings correlated with a retrospective study by the author which found the majority of randomly selected navicular cases as presenting with a BBHPA, with DDFT pathology as the defining factor in the presentation of unilateral lameness in bilateral navicular changes.
Negative palmer angles, the extreme of a BBHPA, were outlined by Floyd (2010), the main defining point being that the wings (palmer process) of the distal phalanx has a negative angle in relation to the ground surface. Floyd (2010) expressed that there were different grades of NPA, lower grades can be addressed with trimming and routine shoeing while higher grades required mechanical intervention. Like NPA, BBHPA’s exhibit the same variances in severity; some can be addressed with trimming and shoe placement while others require more involved farriery, but what was clear from Floyd (2010) is that straight phalangeal alignment was the goal of all farriery intervention and that deviation away from alignment exponentially increases linear forces on the DDFT, load/pressure on the navicular bone and caudal inflammation with resultant hard and soft tissue changes.
Fig.11 Craig (2020) also suggested that 50/50 dorso palmar balance around COR is not obtainable without compromising tissue and therefore shouldn’t be used as an ideal in practice. Dorso-palmar ratios will vary from 50/50 to 60/40 dorso/palmar from COR, in proportion to the genetic conformation of the slope of the pastern in the individual, but the bony alignment remains constant and does not break-back.
Fig.12 Using the top left image above, you can see that by aligning P3 with P1 and P2 would result in a 50/50 balance around COR while providing a very safe and optimal toe breakover bevelled towards the white line without compromising sole depth or sole cup. This highlights that balance around the COR on every axis is needed in order to achieve ideals, in contradiction to simplistic explication of Craig (2020) which discourages any elevation of the heels.
Other studies and articles that echo the principles of correct phalangeal alignment include Logie (2017) which stated “In the negative HPA more force is placed on the flexor tendons, which is transmitted as pressure into the navicular region; the horse may try to alleviate this pressure by changing its stance, so that its feet are in front of the perpendicular. The hoof capsule is overloaded in the posterior portion and may crush as a result, this and the change in stance can create a vicious pain cycle creating collapsed feet which are slow to recover, if at all.”
Turner (2020) “there is no reason not to shoe for a correct hoof axis and a broken hoof axis can
predispose to lameness problems and it has been associated with a greater risk of breakdown in
Witte (2014) “In order for the horse to perform optimally, it is important that the foot is in balance. A balanced foot requires medio-lateral and dorso-palmar balance, with a straight foot-pastern axis. A balanced foot allows for the correct distribution of force within the foot and limb and reduces the likelihood of injury.”
Brown (2020), “HPA determines the alignment of the bony column and therefore, the digit's ability to perform mechanically to its potential.”
Zani et al (2015), “A reduced palmar angle and increased angle between the middle and distal phalanx were observed in horses with alterations of collateral ligaments of the distal interphalangeal joint and navicular bone spongiosa, respectively.”
Logie (2017) raises an important point, the effect of this bony orientation/conformation does not end in the hoof, it affects the posture of the horse. This can be seen even more clearly in the hind end where the alleged “normal” broken back phalangeal conformation suggested by Craig (2020), has been linked to postural adaptation and pathologies all the way up the hind limb and anecdotally into the trunk of the horse; in the fore feet it is ubiquitously linked with navicular syndrome, ringbone, osteoarthritis, shoulder asymmetry and limb discomfort.
Fig.13 This figure shows all the anatomical points that have been pathologically linked to a broken phalangeal alignment in the hind digit.
Fig. 14 This shows the effect of correcting a misaligned HPA, by the application of elevation by the author, on the general posture of the animal. The goal is to create more ideal phalangeal alignment of the digit by encouraging positive morphology, not to remain in the elevation for ever. A misaligned HPA is not just a hoof issue.
These studies show us that the potential negative effects of a broken phalangeal alignment can affect the entire musculoskeletal system. Understanding that the horse is a tensegrity structure with a network of myofascial lines and kinetic chains, where the position and orientation of any anatomical point can affect every other, means that even deeper importance is given to the correct position and orientation of the HPA.
Fig.15 Schematic diagram of the hind end myofascial lines adapted from Elbrond and Shultz (2015) Many of the myofascial lines end or are connected directly to the hoof, all the anatomy along a myofascial line has a pull force on every other point, so when the phalangeal alignment is broken, the pull force of this misalignment is transferred throughout the entire myofascial system.
So in critical review, the Craig (2020) statistics support repeatable findings that Broken-back phalangeal alignment, aka Broken-back HPA, aka Low Palmar/plantar angles are endemic in our horse population. Keep in mind, the large majority of these hoof radiographs were selected from cases involved in veterinarian workup, so statistical results would be influenced by the large cross-section of horses being examined for investigative diagnostics.
Critical thinking begs to ask the question how correlation of such data can be used to determine an interpretive conclusion. Is a "common" finding of broken-back phalangeal alignment equivalent to what is "normal or healthy" dynamics in phalangeal orientation?
Common perhaps expresses a new normal but does not and should not imply acceptable; familiarity breeds complacency.
In conclusion, in the opinion of the author, backed by current and past peer-reviewed research, and in-the-field empirical evidence working under horses, the statistical interpretation of Craig (2020) study can be extremely useful to highlight the prevalence of a broken phalangeal alignment amongst the domestic horse population, something that should and must be addressed to reduce the genesis of the associated pathologies.
The reality is that the data pool is what it is, but the interpretation of those numbers is concerning. Concluding broken-back HPA is a normality in this uncontrolled study means all of the peer-reviewed studies linking that normality to pathology are obsolete.
Striving towards bony column alignment aims to keep joint spacing healthy, minimizes extensor and flexor moments acting on the distal limb, and positions the hoof under the limb to best distribute stress and strain within healthy functional ranges which reduces incidence of pathology and injury.
Accepting the conclusion of the Craig (2020) study could potentiate an increase in mediocre farriery practices that support poor address to biomechanical needs and lead to further escalation in poorly configured hoof care and its associated pathologies.
Further comprehensive controlled studies are needed to look at a broad range of variables that affect bony column alignment and what tolerances outside of perfect alignment are acceptable and healthy. IE. within what phalangeal range can the distal limb orientation compensate before pathology and long-term compensation sets in. In the meantime, bony alignment assessed during the static stance phase, when four limbs are standing equally loaded with cannon bones vertical to the ground-serves a useful guideline towards long-term health and soundness.
Some of the authors previous articles further discuss the subject on phalangeal alignment.
Craig, 2020, How Deep Learning is Helping to Measure the Hoof and Creating ‘Big Data’ for Us to Analyze, https://www.eponamind.com/club-nov19-event accessed 02/04/2020
HILARY M. CLAYTON, 1990, The effect of an acute angulation of the hind hooves on diagonal synchrony of trotting horses
HILARY M. CLAYTON , 1990, The effect of an acute hoof wall angulation on the stride kinematics of trotting horses
Turner, 2020, Developing Treatment Strategies for Palmar Foot Pain,
O’grady 2018, Proper Physiological Horseshoeing: it starts with the trim, http://www.queencommunicationsllc.com/SCAVCD/Digital%20Proceedings/O'Grady-2-Proper%20Phys%20Horseshoeing.pdf accessed 02/04/2020
Waguaspack and hanson, 2011, treating navicular syndrome in equine patients,
https://pdfs.semanticscholar.org/d1db/f7122c6cbca37544466b188e600c39ce4dc5.pdf, accessed 02/04/2020
Waguespack and hanson, 2014, navicular syndrome in equine patients: anatomy, causes and diagnosis, https://www.vetmed.auburn.edu/wp-content/uploads/2015/01/PV1110_waguespack_Surgical.pdf, accessed 02/04/2020
Waguespack and Hanson, 2010, Navicular Syndrome in Equine Patients: Anatomy, Causes and Diagnosis, Surgical Views, https://www.vetmed.auburn.edu/wp-content/uploads/2015/01/PV1110_waguespack_Surgical.pdf
Ruff. K.C, Osborn. M.L, Uhl. E.W, 2016, Analysis of Forces Acting on the Equine Navicular Bone in Normal and Dorsiflexed Positions
Uhl. E.W, Blas-Machado. U, Kirejczyk. S.G.M, Osborn. M.L, 2018, Correlating Increased Mechanical Forces with Tissue Lesions in Equine Navicular Disease
Moleman, M, et al (2006) ‘Hoof growth between two shoeing sessions leads to a substantial increase in the moment about the distal, but not the proximal, interphalangeal joint.’ Equine Veterinary journal, volume 38, No. 2, pp. 170-174
van HEEL, M, et al (2004) ‘Dynamic pressure measurements for the detailed study of hoof balance: the effect of trimming” Equine Veterinary Journal, volume 36, No.8, pp. 778-782
van HEEL, M, 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, volume 37, No.6, pp. 536-540
Redden, R.F. (2010) 'Evaluating the Navicular Bone' Indepth Equine Podiatry Symposium Notes
Elbrond. V, Shultz. R, 2015, MYOFASCIA – THE UNEXPLORED TISSUE: MYOFASCIAL KINETIC LINES IN HORSES, A MODEL FOR DESCRIBING LOCOMOTION USING COMPARATIVE DISSECTION STUDIES DERIVED FROM HUMAN LINES, Medical Research Archives, issue 3