There is much debate about the ideal angle for the pedal bone to sit at in relation to the ground. Some sects suggest a palmar angle of 0 degrees as an ideal, while the widely accepted normal range lies between 2 – 7 degrees.
But what are the implications for palmar angles, what plays a role in creating them and what decides what they should be?
Fig.1 The palmar angle (PA) is shown in purple.
The PA has a fluid relationship with the other angles of the digit. Heel angle will directly affect PA and PA will directly affect phalangeal alignment and hoof angle. Understanding this and the influences on the factors directly affecting PA begin to tell us what the ideal for our horse should be.
Firstly lets establish, as the opening paragraph states, there is a widely accepted range of palmar angles (PA’S) that fall within the ideal, this is down to natural variation and biodiversity.
Fig.2 Hoof Angle is made up of PA and bone angle.
If we accept phalangeal alignment as ideal, which we will explore later, with the knowledge that horses will have different pedal bone angles (Fig.1.2) as a matter of natural biodiversity, then it would suggest that they would require different PA’s in order to achieve the same alignment. Fig.2 shows some palmar angles both within the normal accepted range and some outside of that range. While the majority of horses will fall within the range stated previously there will always be some outside of this range that require much higher palmar angles in order to still have a straight alignment, some of these will possibly be pathological some not.
Alignment becomes our first and perhaps most important factor in deciding on appropriate palmar angle. Studies stating a range of normal palmar angles are limited, Parkes (2003) stated a normal range anywhere between 2-10 degrees and Professor Weller expresses a palmar angle below 2 degrees as pathological. However studies suggesting alignment as an ideal are widespread. This has direct implications for what becomes acceptable as a range for PA because as PA becomes lower it becomes increasingly difficult and rare to have a straight bone column alignment as a result.
Dorner et al. (2017) looked at the relationship between PA and radiographic changes in the navicular, it found a negative correlation between the navicular score and the palmar angle and negative correlation between the navicular score and the hoof angle. Importantly what it showed was a lower palmar angle was more likely to show radiographic changes in the navicular bone. This study shows the same results as many studies before it.
Fig.3 Studies linking navicular disease with conformations exhibiting increased dorsiflexion. i.e. a broken phalangeal alignment from a low PA.
Studies linking poor bone alignment in the digit with navicular changes go way back, Fig.3 shows just a few. Waguespack et al. 2010 through 2014 explores the biomechanical considerations of navicular, Waguespack showed that the trabecular bone in the navicular bone suggested the primary force acting on it was from the deep digital flexor tendon (DDFT) and the amount of compressive force on the navicular bone was dependent on the conformation, so more misaligned, more compression. It also suggested 2 theories for the pathogenesis of navicular syndrome, vascular changes and repetitive biomechanical forces from increased strain from the tendon due to poor bone alignment. Thompson et al and Ruff et al support the findings of Waguespack. They again stated osseous changes to the navicular corresponded with greater compression from the DDFT created by poor bone alignment. Poor bone alignment doesn’t just affect the navicular bone itself, the increased strain on the DDFT showed to correlate with lesions of the DDFT, ligaments and navicular bone (Uhl et al. 2018). Although these studies do not suggest ideal palmar angles, they collectively point toward straight phalangeal alignment as an ideal and a broken alignment as predisposing to pathology. Going back to the suggestion of ground parallel pedal bones being ideal, the near impossibility of them having a straight bone alignment suggests each of these studies would disagree.
The plethora of studies linking a low palmar angle with navicular extends far beyond this list, Mieszkowska et al. stated that the alignment of the digit was important in hoof mechanics, blood flow and haemodynamics. Holroyd et al (2013) suggested a larger palmar angle was associated with a smaller probability in presenting with navicular bone or DDFT lesions. An earlier study from Holroyd et al (2012) cited and agreed with Eliashar (2004) stating that for every degree away from the ideal there was an increase in strain on the DDFT of 4%, considering the previously cited studies this would equate to an increased predisposition to pathology with every degree. Holroyd (2012) also supported the findings that there is a three way relationship between palmar angle, DDFT strain and navicular changes. This paper also discussed the physics of increased flexor strain.
Fig. 4 increase in extensor moment results in a direct increase in flexor strain to counteract the collapsing force.
This raises another consideration for palmar angle. By simple Pythagoras if you reduce the angle of the hypotenuse without reducing its length, the base length must increase. This means that for every degree of palmar angle reduction there will be a resultant increase in the distance from the centre of rotation to the point of force, a resultant increase in extensor moment and therefore a counteracting increase in flexor structure strain, which as we have outlined leads to a navicular predisposition in the front feet. Van Heel et al. (2004,2005) and Moleman et al. (2006) and an earlier study Clayton et al. (1990) all discussed this movement of the point of force or centre of pressure.
When we consider the studies of van Heel et al (2004,2005) and Moleman et al. (2006) showing the effects of hoof growth reducing the palmar angle and the subsequent strain on the DDFT, we also have to question whether the advocates of a ground parallel pedal bone are also suggesting negative palmar angles are also acceptable. These studies showed a reduction in palmar angle of around 3-4 degrees over a shoeing period which would result in a significant negative palmar angle starting at 0. Floyd (2010) discussed negative palmar angles again linking this poor bone alignment with navicular syndrome and expressing the importance of returning these feet to a positive palmar angle. In the hind feet negative plantar angles have been linked to pathology all the way up the hind limb (Mannsman et al. 2010, Pezzanite et al 2018, Clements et al. 2019). What Floyd also discussed was hoof morphology that leads to the creation of this inevitable end point of feet starting a shoeing cycle at 0 degrees. Weak and collapsed heels.
The health of the caudal hoof and its individual structures, the frog, the digital cushion and the bars, is directly responsible for the heel height and angle and as a result the palmar angle. This relationship is discussed by Dyson et al. (2011) which cites many other studies exploring the interconnected relationship of the internal hoof structures and the external hoof proportions. Dyson et al. (2011) highlights the link between poor hoof conformation and lameness, horses with increased heel to toe height ratios were more likely to be lame, heel to toe height ratios are directly responsible for palmar angle, so we can see that a low palmar angle, for an individual horse, predisposes that horse to lameness.
Fig. 5 Heel to toe height ratios by breed.
Dyson et al. (2011) prescribes a 3:1 heel to toe height ratio as a minimum for a healthy hoof, anything above this ratio suggested caudal hoof failure. Fig.5 shows some cases from my small pilot cohort study that showed the population of horses I studied had a range of heel to toe ratios with an average range between 3:1 and 2:1 to present with a straight hoof pastern axis (HPA). Although we do not have radiographs of these feet experience tells me that they would certainly have a positive palmar angle. Again it would be very rare to impossible, although evidence based research would have to be done, to have a PA of 0 with a heel to toe ratio above 3:1, and it certainly wouldn’t have a straight HPA.
So having said that it would suggest that for a hoof to have a PA of 0 degrees, either the heels have completely failed and collapsed or have been trimmed beyond an appropriate height and fore the most part would have to be lower then the frog height.
All of this brings us onto the haemodynamic system, which when understood also lends itself to the extrapolation of a positive palmar angle as an ideal. When the horse loads the hoof it descends under the load. The Digital cushion, in the heathy hoof is under the caudal aspect of the pedal bone and the navicular and middle phalanx to some extent.
Fig. 6 Sagittal Section of the digit.
If we look at the anatomy of the hoof and understand that the digital cushion is designed as part of a hydraulic dampening suspension system, to absorb concussion and protect the bones, its easy to understand how its position would create a positive angle to the pedal bone. We can see from Fig.6 how the pedal bone is sitting up on the digital cushion. In order to create a 0 degree palmar angle there would need to be significant changes in ordered anatomy. The real question here is when there is a 0 degree palmar angle where has the pedal bone got room to go under load!? Suddenly everything is automatically subject to strictly compressive forces and the displacement mechanisms of shock absorption are curbed.
In conclusion, if we look at the feet where a 0 degree palmar angle is present, for the most part we will be looking at feet that have collapsed heels, a poor heel to toe ratio and a long toe low heel presentation, all of which have been strongly linked to pathology and catastrophic injury. For these to become an ideal to aim toward, having inevitable implications on trimming protocol, is illogical in the light of the studies in this article, which are a small representation of studies that could be used to argue the case for phalangeal alignment and therefore a positive palmar angle.
This article briefly outlines the implications for and of palmar angles, many of my other writings support this article and are linked within this paper or can be found at the website. Videocasts available from my YouTube channel also discuss this subject in depth.
Parks A. 2003.The foot and shoeing. In: Diagnosis and Management of Lameness in the Horse, 2nd edn.,Eds: M. Ross and S. Dyson, W.B. Saunders, St Louis. pp 250-271.
Dorner, Cristobal & Fueyo, Pablo & Olave, Rodrigo. (2017). Relationship between the Distal Phalanx Angle and Radiographic Changes in the Navicular Bone of Horses: A Radiological Study. Global Journal of Medical Research. 17. 7-13. 10.17406/GJMRG.
Weaver M, Shaw D, Munaiwa G, Fitzpatrick D, Bellenger C. 2009.Pressure distribution between the deep digital flexor tendon and the navicular bone, and the effect of raising the heels in vitro.Vet. Comp. Orthop .Traumatol. 22, 278-282.
Waguespack and Hanson, 2010, Navicular Syndrome in Equine Patients: Anatomy, Causes and Diagnosis, https://www.vetmed.auburn.edu/wp-content/uploads/2015/01/PV1110_waguespack_Surgical.pdf
Waguespack and Hanson, 2011, Treating Navicular Syndrome in Equine Patients, http://vetfolio-vetstreet.s3.amazonaws.com/14/d3c47053df11e0a4050050568d17ce/file/PV0111_waguespack_Surgical.pdf
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
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
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