The Truth about Hoof Pastern Axis
Updated: Feb 27
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. Mechanical theoretical studies, and experiential opinion state a forward movement of the COP, or point of force trajectory (Fig.8).
Fig.8 Showing the forward migration of the COP or Point of force application with poor phalangeal alignment. leading to an increase in extensor moment and a resultant increase in flexor strain.
Either theory clearly demonstrates increased load on the DDFT, distal phalanx and DIPJ structures. 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.