30/11/2023
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WHAT DO WE ACTUALLY KNOW?
I was planning to write about orthopedic horseshoes, but it is impossible to move on to this topic without presenting the substantive chaos and misinformation prevalent in the world of equine orthopedics. And while there is nothing wrong with the fact that we don't know so much yet, there is nothing good in the fact that we live and act as if everything was known.
According to my observations, the approach to the most common injuries affecting the equine distal limb in Poland and parts of Europe is based on two assumptions.
🔷 THE STEEPER THE ORIENTATION OF THE HOOF, THE MORE STRAIN ON THE SUSPENSORY LIGAMENT (SL) AND THE SUPERFICIAL DIGITAL FLEXOR TENDON (SDFT), WHILE THE LESS STEEP THE ORIENTATION OF THE HOOF, THE MORE STRAIN ON THE NAVICULAR REGION AND THE DEEP DIGITAL FLEXOR TENDON (DDFT).
🔷 RELIEVING STRAIN ON ONE STRUCTURE MEANS OVERLOADING ANOTHER, HENCE BY RELIEVING THE DDFT AND THE NAVICULAR AREA, WE OVERLOAD THE SDFT AND SL, AND VICE VERSA.
It's obvious, everyone knows it! After all, it has been investigated!
Obvious? Investigated?
Well, let's start from the beginning.
Over the last hundred years, dozens, if not hundreds, of studies have been conducted on the biomechanics of the horse's leg, including the distribution of loads within tendons and ligaments, the relationship between the load on individual structures and the geometry of the hoof, also using various modifications of horseshoes.
For science not to be faith, we should have at least a basic understanding of the research methodology and its limitations.
Because to believe it at its word, one would probably have to have a split or deranged personality. The divergence of conclusions often resulting from research on the same issue is difficult to comprehend. I have the impression that everyone can choose what they like best and draw the conclusions that suit them at the moment.
It is worth considering what this may be due to.
🔶Research methods
🔹According to my current knowledge, there are not yet fully reliable and non-invasive methods of examining the load on tendons and ligaments in vivo.
🔹 Most often, in the case of live horses, mathematical calculations are used, taking into account the distances between designated characteristic points on the skin surface and the angles between the sections they create, e.g. the change in the angle of hyperextension of the coffin or fetlock joint is taken into account, and on this basis the values for changes in the length of individual structures or the values of forces exerted on individual structures are calculated[1]. Unfortunately, at this moment I do not have sufficient knowledge to verify the reliability of the developed calculation methods, so I can only believe in them.
🔹 In short: it is often assumed that the greater the hyperextension in the coffin joint, the greater the load on the DDFT and the navicular apparatus (for the navicular, calculations are used taking into account the length of the lever arm and the value of the ground reaction force (GRF) at breakover, and the greater the hyperextension in the fetlock joint, the greater the load on the SDFT and SL.
🔹Ultrasonography is sometimes used to measure the cross-sectional area of individual structures, and the change in the value of this surface area is extrapolated to the change in the load value of a given structure: the smaller the cross-sectional area (thinner the tendon), the greater the stretch. Although everything here seems simple, it turns out that it is not entirely so. There are studies on the properties of tendons indicating their auxetic properties: this means that when stretched in one direction, they also increase in size in transverse directions.[2] However, it is not known whether these properties apply to all tendons and the only study I could find conducted on horse tendons (SDFT) did not show such a relationship. [3]
🔹 Many studies have been carried out on dead horse legs, which on one hand provides much greater opportunities to monitor the forces affecting individual structures, and on the other hand has obvious limitations, such as changes occurring in tissues after death [4] or obviously... lack of the horse attached to the subjected structures.
🔹 Several studies have constructed advanced 3D models of the distal limb, with the most detailed possible representation of all anatomical structures (e.g. CT scans). On this basis, computer simulations were performed and the values of forces exerted on individual structures were calculated. The conclusions drawn from such computer simulations, although very interesting and certainly bring a lot to the table, unfortunately (like studies on dead legs) do not take into account the impact of the horse itself on the way the hoof interacts with the ground (including the work of the flexor muscles) and individual conformation variables, such as the length of individual bones, the length of muscle-tendon units, the flexibility of tendons and ligaments, and the range of motion in individual joints.
🔹Lots of research ignores the proximal interphalangeal joint (PIPJ) altogether, assuming that the range of motion there is so small that it does not matter - however, there are studies that show that it is statistically significant. [5]
🔹 Invasive (and probably more reliable) methods used in in vivo studies of tendon and ligament loading include implanting strain gauges in the tendons/ligaments [4, 6, 7] or implanting markers in the tendons/ligaments/bones to avoid errors related to movement of the skin or when using fluoroscopy [8].
🔶Number of horses
In most of the most frequently cited studies, only a few or a dozen or so horses are used, often of the same breed (information about the type of horses is not always provided). There is usually no information about the conformation, posture and initial morphology of the hooves of the examined horses. Sometimes it is mentioned that the horses were "healthy" and their hooves were "balanced". The research carried out on the Holy Grails! 🫣
Extrapolating the results of research conducted on a few Thoroughbreds or a few ponies to the general horse population seems to me to be a strong abuse, just like extrapolating measurements made on the basis of a computer model of one specific limb - especially when faced with such great discrepancies in the results.
However, for some reason, the world (or at least much of Europe) seems not to notice these inconsistencies and operates in a reality described by the two equations mentioned at the beginning of this post.
🔶One of the most important problems I see in this regard is the inconsistency of these assumptions with anecdotal evidence and observations of a large group of experienced practitioners. In my practice, I also see correlations that contradict these assumptions.
Experienced farriers of high-performance horses report that in their observations, most damage to theSL in the hind limbs occurs in horses with NPLA (negative plantar angles), while according to the theory mentioned above, such a positioning of the phalanxes should relieve the SL as much as possible. In the front limbs, repeated or chronic damage to the SDFT and SL seems to occur frequently in horses with a strongly broken back hoof pastern axis and a low palmar angle, which also seems illogical in the light of these assumptions.
In such cases, the use of suspensorix type horseshoes, causing an even greater reduction in hoof angulation while moving on soft ground, seems at least controversial to me, as is the fear of restoring the correct hoof morphology to such horses (either through corrective trimming or shoeing).
Here as well, anecdotal evidence suggests that such actions (increasing the hoof angle towards the straight hoof pastern axis) do not hinder the regeneration of damaged structures or even support it - and it turns out that there are studies that confirm their validity, more on that later.
I have not yet managed to find a common denominator explaining the divergent results of similar studies, but I would look for it, among others: in differences regarding the conformation of the examined horses and the initial condition of the hoof in the examined limb. There is no consistent calibration of the results of individual studies: it is often not known what the initial hoof angulation was and what it looked like after applying the modifications described in the study. There is no information on the flexibility and length of the deep and superficial flexor muscle-tendon units, the length and angulation of the pastern and the hoof pastern axis.
🔹 Do we really know when the load distribution within the flexor tendons and SL is optimal?
🔹What orientation of phalanges is optimal for a given horse?
🔹 How significant is the individual conformation of a given horse, including the length of flexor muscle-tendon units and SL length for determining optimal phalangeal alignment?
🔹What if there is a (even slight) DDFT or SDFT contracture?
🔹What is conformation and what is posture caused by compensation (for example, a broken back hoof pastern axis despite the correct hoof angulation)?
🔹 Is preventive or long-term relief (underload) of any structure beneficial for it, or can it reduce its strength over time and predispose it to damage?
🔹Where is the line between underload and overload?
I often see that the ‘here and now’ is assumed as the default state for a given horse and any changes are seen as ‘risky interference’. What if this condition is caused by neglect or a pathology that not only cannot regenerate in this setting (vicious circle), but also leads to overload of other structures?
Recently, Wayne Turner (Progressive Equine Services & Hoof Care Centre) published an interesting post on this topic:
https://www.facebook.com/hoofscanandhoofcarecentre/posts/837759551478989
🔶I also see a few logical problems here.
🔹 In all this fear of changing the hoof angle, we forget that maintaining a constant hoof angle in most horses is practically impossible. Hooves very rarely grow and wear out evenly. It has been shown that in an 8-week cycle, hoof angulation decreases by an average of 3-4 degrees[9, 10]. (Of course, again - the study included a small group of horses and there are also some that will increase in the angulation.) If we do not correct the angulation of such a hoof by at least 3-4 degrees during trimming, its angulation will decrease with each cycle. We are afraid of changing the angulation of the hoof by 2 degrees when using a wedge pad, not caring at all that sometimes the angulation of the hooves can be changed just with trimming by several or over a dozen degrees!
🔹 Why do we assume that the sum of the loads on the flexor tendons and SL is always equal (‘less load on the DDFT = more load on the SDFT and SL’ and vice versa)? After all, the horse's weight is carried both by tendons and ligaments, as well as by the boney column, and how much of this weight is suspended on the tendons depends, among other things, on the length of the lever arm of the pastern, which depends among other factors, on the fetlock's position.
Professor Denoix, asked in February 2023 whether the sum of the loads on SDFT, DDFT and SL is always equal, replied: "I don't know."
🔹 Why do we extrapolate the proportions of DDFT, SDFT and SL load between different limbs and between different horses, when in all studies (that I know of), these proportions are relative and checked only within a given specific limb? (They do not concern changes in hoof angulation relative to one specific baseline value, only changes within a given limb at specific values of angulation change.)
For example, let's assume (because this is not the case) that for each limb tested, it has been shown that increasing the hoof angle reduces the load on the DDFT and increases the load on SDFT and SL. This does not mean that if we take two different legs, and one has a steeper hoof angle than the other, that in the steeper one the SDFT and SL tension will be higher and DDFT lower than in the less steep one.
Especially in the case of a club foot with DDFT contracture - recently I even came across a question in a knowledge test: which structure in such a hoof is most overloaded? The ‘correct’ answer was: SDFT and SL. And how do we know this? What is known for sure is that in the case of DDFT contracture, the too short deep flexor, its accessory ligament and the navicular area are overloaded, and the load on the SL and SDFT will depend on many other factors, including the length of these structures.
🔶Preparing this post, I have so far studied over 50 research papers on the biomechanics of the equine distal limb, initially hoping to summarize the conclusions drawn from them in an easy-to-read table. However, this is probably a very-long-term project🥲
I’ve collected several studies conclusions of which seemed most interesting to me. I encourage you to interpret them yourself.
🔷CREVIER-DENOIX, N., ROOSEN, C., DARDILLAT, C., POURCELOT, P., JERBI, H., SANAA, M., & DENOIX, J.-M. (2001). Effects of heel and toe elevation upon the digital joint angles in the standing horse. Equine Veterinary Journal, 33(S33), 74–78.
The study was conducted on 5 horses at rest. The left forelimb was placed on a platform allowing for the hoof angulation change by -15, -10, -5, 0, +5, +10, +15 degrees, respectively, while the opposite limb was held up. (The horses were sedated and, as I understand it, had no way to change their posture as the angle of the hoof changed.) There markers were attached to the skin. X-rays were taken and the angles between the segments were measured.
A linear relationship was observed between increasing hoof angle and greater hyperextension of the fetlock joint (in the range of 6.9 degrees +/- 2 degrees for fetlock angulation, 7.3 degrees +/- 1 degree for the pastern joint and 29.5 degrees +/- 1.8 degrees for the coffin joint).
It was hypothesized that this principle applies to the entire horse population.
🔷CHATEAU, H., DEGUEURCE, C., & DENOIX, J.-M. (2010). Three-dimensional kinematics of the distal forelimb in horses trotting on a treadmill and effects of elevation of heel and toe. Equine Veterinary Journal, 38(2), 164–169.
It was assumed that research methods based on markers attached to the skin surface may be unreliable, as well the ones where the changes occurring at the level of the pastern joint were neglected - in particular, discrepancies in the response of the fetlock joint (and therefore the SDFT and SL load) to changes in hoof angulation.
The study was conducted on 4 French trotters on a trotting treadmill. 6-degree wedges were used to raise the heels and the toe. To avoid the above-mentioned errors, kinematic markers were implanted in the metacarpal bones, long pastern bone, short pastern bone and the lateral quarter of the hoof of the left front limbs. Measurements of one horse were not collected due to technical errors.
It was observed that increasing the hoof angle by 6 degrees resulted in significant decrease
of extension in the coffin joint and an increase in flexion in the pastern joint (the opposite effects were observed in the case of a 6-degree decrease in the hoof angle). Neither increasing nor decreasing hoof angulation had an impact on the maximum extension of the fetlock joint (and therefore on the SL and SDFT load).
🔷HAGEN, J., KOJAH, K., GEIGER, M., & VOGEL, M. (2018). Immediate effects of an artificial change in hoof angulation on the dorsal metacarpophalangeal joint angle and cross-sectional areas of both flexor tendons. Veterinary Record, 182(24), 692–692.
Study conducted on 30 horses, front limbs. 5, 10 and 20 degree wedges were used to raise the heels and the toe, and the SDFT and DDFT cross-sectional areas were measured and calculated using ultrasonography.
When using a toe wedge of 5 degrees, in 33% of horses the fetlock angle decreased and its extension increased (greater SL load), and 35% of horses showed no changes in the position of the fetlock.
The position of the fetlock joint changed in most horses only when using a 10- and 20-degree heel wedge and when using a 20-degree toe wedge (22% of horses in the case of 10 degrees toe wedge and 10% of horses in the case of 20 degrees toe wedge responded by lowering the fetlock).
The cross-sectional areas of both SDFT and DDFT consistently increased as the hoof angulation increased and decreased as the hoof angulation decreased. It was concluded that a higher hoof angulation relieves both the DDFT and SDFT.
🔷PEHAM, C., GIRTLER, D., KICKER, C., & LICKA, T. (2006). Raising heels of hind hooves changes the equine coffin, fetlock and hock joint angle: a kinematic evaluation on the treadmill at walk and trot. Equine Veterinary Journal, 38(S36), 427–430.
A study conducted on 8 warmbloods at a walk and trot on a treadmill using markers attached to the hind limb. It showed that increasing the angulation of the hind hooves by 8 and 16 degrees resulted in a decrease in the maximum extension of the fetlock joint (which is interpreted as a decrease in the load on the SL and SDFT), a decrease in the extension in the coffin joint and an increase in the maximum flexion in the hock joint. Doubling the angulation of the wedge used resulted in doubling the observed results, both at walk and trot.
🔷SCHEFFER CJ, BACK W. Effects of 'navicular' shoeing on equine distal forelimb kinematics on different track surfaces. Vet Q 2001;23:191-195.
The study was conducted on 11 warmbloods (front limbs) using regular shoes, eggbar shoes and shoes with 5-degree wedge pad on 3 different surfaces (asphalt, Agterberg-type surface and soft sand). The lowest SL load (least fetlock extension) was observed when using a wedge pad of 5 degrees and on soft sand. (!!!)
🔷KEEGAN KG, BAKER GJ, BOERO MJ, et al: Measurement of suspensory ligament strain using a liquid mercury strain gauge: Evaluation of strain reduction by support bandaging and alteration of hoof wall angle. Proc Am Assoc Equine Pract 37:243-244, 1991
A study cited in Jean Marie Denoix's publication "FUNCTIONAL ANATOMY OF TENDONS AND LIGAMENTS IN THE DISTALS LIMBS (MANUS AND PES)" conducted using implanted strain gauges showed that changing the hoof angulation between 40 and 70 degrees resulted in a decrease in the load in the DDFT with increasing hoof angle, but had no effect on SDFT and SL loading.
🔷LOCHNER FK, MILNE OW, MILLS EJ, et al: In vivo and in vitro measurement of tendon strain in the horse. Am J Vet Res 41:1929-1937,1980
The same script cites a study, also conducted using implanted strain gauges, according to which a 10-degree increase in hoof angulation results in 0.6% more load on the SL. (That is, for 1 degree of hoof angle there is 0.06% more load on the SL - for comparison, another study (Eliashar et al. 2004) showed that for 1 degree of hoof angle there is 4% less load exerted by the DDFT on the navicular bone. )
🔷TURNER TA, POULOS PW, HARWELL NM: The effect of hoof angle on coffin, shepherd and fetlock joint angles. Proc Am Assoc Equine Pract 33:729-738, 1987
The same script quotes a study which shows that increasing the hoof angle by 10 degrees changes the angulation of the fetlock by only 1 degree.
🔷NILSSON G, FREDERICSON I, DREVEMO S. Some procedures and tools in the diagnostics of distal equine lameness. Acru ver. Scand. Suppl 1973;44:63.
The study showed that when the hoof angulation increases, the fetlock angulation also increases and, consequently, the load on the SDFT and SL decreases.
🔷RIEMERSMA, D. J., BOGERT, A. J., JANSEN, M. O., & SCHAMHARDT, H. C. (1996). Influence of shoeing on ground reaction forces and tendon strains in the forelimbs of ponies. Equine Veterinary Journal,
28(2), 126–132.
A study conducted on 5 ponies using implanted strain gauges showed a slight increase in the load on the SL (by 0.24%) with a simultaneous decrease in the load on the DDFT and ICL (by 0.19% and 0.4%, respectively) using a 7-degree wedge pad. For comparison, when using an eggbar shoe, the SL load was increased by 0.22% and the DDFT load was decreased by 0.13%.
Reverse wedge pad (reducing the hoof angle by 7 degrees) resulted in an increase in ICL load by 0.8%.
🔷NOBLE, P., LEJEUNE, J.-P., CAUDRON, I., LEJEUNE, P., COLLIN, B., DENOIX, J.-M., & SERTEYN, D. (2010). Heel effects on joint contact force components in the equine digit: a sensitivity analysis. Equine Veterinary Journal, 42, 475–481.
The study was conducted on 4 Thoroughbred horses using 6 and 12 degree wedge pads at the trot. When the hoof angle was increased by 12 degrees, the forces exerted on the coffin joint were significantly decreased, while the forces exerted on the fetlock joint were slightly increased.
🔷WILLEMEN, M. A., SAVELBERG, H. H. C. M., & BARNEVELD, A. (1999). The effect of orthopedic shoeing on the force exerted by the deep digital flexor tendon on the navicular bone in horses. Equine Veterinary Journal, 31(1), 25–30.
A study conducted on 12 Dutch Warmbloods trotting on a treadmill with flat open shoes, eggbar shoes and shoes with 6-degree wedge pads. Compared to regular shoes, when using wedge pads, there was a 24% reduction in the force exerted on the navicular by the DDFT (eggbar shoes had no effect on this parameter) and a simultaneous reduction in fetlock extension by 3.7%.
🔶The significant discrepancy in the results of studies on similar issues suggests that at the moment we do not have sufficient knowledge of the biomechanics of the equine distal limb to develop universal protocols for the treatment of specific injuries. Both this discrepancy in research results and numerous anecdotal evidence indicate that horses with similar injuries respond differently to similar applications, which means that each case should be considered individually.
Jenny Hagen describes that in order to check how a given horse will react to a change in the angulation of the hoof (e.g. whether it will raise or lower the fetlock), she uses, for example, taped wedge pads in various positions, special platforms that allow to change the angulation of the hoof, or e.g. devices such as " Digital Extension Device” constructed by Hans Castelijns.
In the case of a similar injury, depending on the horse's reaction, in one case the best solution may turn out to be, for example, suspensorix type shoes and lowering the hoof angle, and in another case, wedge pads and caudal extensions! [11, 12]
Environmental conditions should also be taken into account, as well as the amount of recommended exercise and the surface on which the horse is exercised (if the horse is recommended to move on hard surfaces, two-dimensionally modified horseshoes will have little - or no - effect!), or behavior and lifestyle of the horse. For example, if the horse lives outside 24/7 and is strongly bonded with its herd and shoeing its hind limbs will create a necessity of separating it from the herd, it is crucial to consider whether the benefits of the recommended solutions certainly outweigh the undesired consequences that may result from them.
I’m going to write more about individual distal limb problems and the shoeing solutions used for treating them in the following posts.
1. Bartel, D.L., Schryver, H.F., Lowe, J.E. and Parker, R.A. (1978) Locomotion in the horse: a procedure for computing the internal forces in the digit. Am. J.vet. Res. 39, 1721-1727.
2. Gatt R, Vella Wood M, Gatt A, Zarb F, Formosa C, Azzopardi KM, Casha A, Agius TP, Schembri-Wismayer P, Attard L, Chockalingam N, Grima JN (2015). "Negative Poisson's ratios in tendons: An unexpected mechanical response" (PDF). Acta Biomater. 24: 201–208. doi:10.1016/j.actbio.2015.06.018. PMID 26102335.
3. Vergari, C., Pourcelot, P., Holden, L., Ravary-Plumioën, B., Gerard, G., Laugier, P., … Crevier-Denoix, N. (2011). True stress and Poisson's ratio of tendons during loading. Journal of Biomechanics, 44(4), 719–724.
4. RIEMERSMA, D. J., BOGERT, A. J., JANSEN, M. O., & SCHAMHARDT, H. C. (1996). Tendon strain in the forelimbs as a function of gait and ground characteristics and in vitro limb loading in ponies. Equine Veterinary Journal, 28(2), 133–138.
5. Lawson, S. E. M., Chateau, H., Pourcelot, P., Denoix, J.-M., & Crevier-Denoix, N. (2007). Effect of toe and heel elevation on calculated tendon strains in the horse and the influence of the proximal interphalangeal joint. Journal of Anatomy, 210(5), 583–591.
6. Brown, T. D., Sigal, L., Njus, G. O., Njus, N. M., Singerman, R. J., & Brand, R. A. (1986). Dynamic performance characteristics of the liquid metal strain gage. Journal of Biomechanics, 19(2), 165–173.
7. Stephens, P.R. et al., 1989.
Application of Hall-effect transducer formasurement of tendon strains in horses. Am. J. Vet. Res. 50,1089–1095.
8. Wagner, F. C., Gerlach, K., Geiger, S. M., Gittel, C., Böttcher, P., & Mülling, C. K. W. (2021). Biplanar High-Speed Fluoroscopy of Pony Superficial Digital Flexor Tendon (SDFT)—An In Vivo Pilot Study. Veterinary Sciences, 8(6), 92.
9. Moleman, M., Heel, M. C. V., Weeren, P. R., & Back, W. (2010). Hoof increase between two shoeing sessions leads to a substantial increase of the moment about the distal, but not the proximal, interphalangeal joint. Equine Veterinary Journal, 38(2), 170–174.
10. HEEL, M. C. V., MOLEMAN, M., BARNEVELD, A., WEEREN, P. R., & BACK, W. (2010). Changes in location of center of pressure and hoof-unrollment pattern in relation to an 8-week shoeing interval in the horse. Equine Veterinary Journal, 37(6), 536–540.
11. https://www.americanfarriers.com/articles/12297-shoeing-to-support-fetlock-joints-affected-by-tendon-and-ligament-injuries
12. https://www.americanfarriers.com/articles/13123-toe-angulation-and-extensions-can-relieve-suspensory-and-flexor-tendon-injuries
Image source:
Denoix, J.-M. (1994). Functional Anatomy of Tendons and Ligaments in the Distal Limbs (Manus and Pes). Veterinary Clinics of North America: Equine Practice, 10(2), 273–322.
