Heel Wedges: Their Effect on Tendon and Ligament Strain by Kent M. Thompson PhD
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Heel Wedges: Their Effect on Tendon and Ligament Strains
Originally published in ANVIL Magazine February 1998 © Kent M. Thompson, PhD
http://www.anvilmag.com/farrier/heelwedg.htm
An introduction to basic mechanical properties which govern movement in the lower limb, primarily those which are exerted by the tendinous/ligamentous structures
To understand the mechanical functioning of the lower limb, we must first discuss the forces which initiate or prevent movement. Several structures are involved in the function of the lower limb including the deep digital flexor tendon, superficial digital flexor tendon, suspensory ligament, digital extensor tendon, extensor branch of the suspensory ligament, laminae and the ground reaction force. To model the lower limb, forces are generated by muscle contraction and relaxation which are transferred by the elastic tissue of tendon and ligament.Each tendon or ligament generates a turning force (moment) about one joint or several joints.
One example of this is the coffin joint which is acted upon by the ground reaction force, deep digital flexor tendon, extensor branches of the suspensory ligament and common extensor tendon. In the standing horse, the following mathematical relationship can serve to define the equilibrium which exists. However, during movement, the equation would change to represent different parts of the stride and it would not be in equilibrium.
To simplify the present mechanical definition of the lower limb, we will consider only the forces as they are in the standing horse. If at any point part of the equation reaches its threshold level, then damage to the associated structure will likely occur. For example, if the deep digital flexor tendon component increases by a change in the toe angle, then that structure or related structures may be prone to damage under normal loading conditions.An illustration is a case where the toe angle has changed to increase the force present in the deep digital flexor tendon.
In order to maintain equilibrium, the other tendinous/ligamentous components must change their relative distribution of forces.Once a mathematical relationship like this is formulated, empirical or derived data must be collected which will either support or disprove the hypothesis. To do this, it is now possible to measure the contribution of each tendon, or ligament structure in the lower limb with devices which measure strain. From these we can calculate the direction and magnitude of the forces which can be used to build a mathematical model of the lower limb.
One use of this model is to compare the effect that different farriery techniques have on the relative loads that associated structures must carry.Hoof balance is a term frequently used, yet the question still remains regarding how to precisely define a properly balanced foot. Balance in the sagittal plane has been associated with the shoulder and pastern angle, yet medio-lateral balance is somewhat more difficult to quantify. Alterations in hoof balance through the addition of wedge pads have been used for many years for therapeutic and performance reasons.
Hoof angle is frequently implicated as one cause of lameness in horses. A large toe angle (>55 degrees) is not generally associated with an increased incidence of musculo-skeletal damage. However, a low toe angle has been implicated as a factor in the onset of lower limb disorders including degenerative bone disease, navicular disease, bone chips, tendon tearing and ligament strains. Once the tendon is damaged, the treatment administered is the subject of debate.
Moment equations for the coffin and fetlock joints:
Coffin: DFb-F1-EBa=0
Fetlock: DFr+SF2r+SL.5r-Ff+EB.5r=0
However, one part of the treatment should include the removal of as much of the load borne by the damaged structures as possible. This can be done by a combination of splints, bandaging and changes in the toe angle. The study which I will discuss here was designed to determine the influence that changes in toe angle had on strain in the suspensory ligament, extensor branch of the suspensory ligament, deep digital flexor tendon, superficial digital flexor tendon and the surface of the hoof wall. This information will help us to determine normal mechanical function of the lower limb as well as to determine treatment of lameness disorders including laminitis and various tendon injuries.
For this study, each leg was trimmed to a constant angle (55 degrees). Heel wedge pads were then added to create the following toe angles: 58 degrees, 61 degrees, 64 degrees, 67 degrees, 70 degrees, and 78 degrees.Strain on the deep flexor tendon was reduced with increases in toe angle. The reduction in strain followed a linear pattern with the increasing toe angle at two locations on the deep flexor tendon.
Deep digital flexor tendon strains decreased from 2.49 for the 55 degree toe angle to 1.42% and 1.02% for the 70 degree and 78 degree toe angles, respectively. This represented a 59% decrease in tendon strain between the 55 degree and 78 degree treatments. Strain decreased in a similar fashion at the interphalangeal site, as there was a 64% decrease between the 55 degree and 78 degree angles. Strain measured in the superficial flexor tendon was not affected by an increase in toe angle. Within the range of toe angle used in this study, there was no change in strain on the superficial flexor tendon. Initial strain for the control treatment was 2.90% and varied very little with the increase in toe angle. This is similar to that observed by others. The suspensory ligament acted very similarly to the superficial flexor tendon; there was not any change in suspensory ligament strain with the addition of the toe wedges. Strain in the suspensory ligament had a small numerical increase in strain as the toe angle was increased, but the change was not significant.
However, a large increase was noted for the extensor branch of the suspensory ligament. There was very little strain present on the extensor branches with the 55 degree, 58 degree, 61 degree and 64 degree toe angles. However, when the toe angle was increased to 67 degrees, extensor branch strain increased dramatically. At the 55 degree toe angle, strain on the extensor branches was .02% and when toe angle was increased to 78 degrees, strain increased to 1.40%.
Strains on the surface of the hoof wall remained in compression during loading and the magnitude of compression increased on the medial and lateral walls with increases in toe angle, and decreased on the dorsal hoof wall. A similar but opposite trend was observed for the dorsal hoof wall as strain decreased 52.8% as toe angle increased. Strain on the medial hoof wall followed the same pattern as the lateral hoof wall. The trend in this case was for a higher strain on the hoof wall as toe angle increased. The difference was noted between the 78 degree toe angle and the 58 degree and 55 degree toe angles. Heel wedges had no effect on reducing strain in the superficial flexor tendon and suspensory ligament. However, heel wedges are recommended in cases where reduction of strain on the deep flexor tendon is needed, such as laminitis and tendon flexor tendon injury.
One other effect noted with the addition of a heel wedge is the large increase in strain of the extensor branch of the suspensory ligament.
This may be of particular interest in the treatment of laminitic horses, as reduction of deep flexor tendon strain and increased extensor branch strain both occur with heel wedges and will work concurrently to stabilize the coffin bone in normal and laminitic horses. Heel wedges also raised compressive strains on the surface of the hoof wall at both the medial and lateral quarters, which may affect the occurrence and treatment of quarter cracks.Thus, heel wedges have a great influence on redirecting the forces that are generated by the tendon and ligament structures in the lower limb. The use of heel wedges will be of great benefit in treating laminitic horses, due to their effect on reducing strain in the deep flexor tendon and increasing strain in the extensor branch of the suspensory ligament.
Christina,
The article heel wedges seems odd being here.
“Thus, heel wedges have a great influence on redirecting the forces that are generated by the tendon and ligament structures in the lower limb. The use of heel wedges will be of great benefit in treating laminitic horses, due to their effect on reducing strain in the deep flexor tendon and increasing strain in the extensor branch of the suspensory ligament.”
Isn’t this contradictory to what Strasser would find? I’m not sure how to understand it being on your blog.
Dan
Dan,
You’re absolutely correct, this does seem out of place in a site advocating for a low-heeled barefoot trim. I believe he draws conclusions that are not supported by his data. In particular, the statement below, he notes that heel wedges increase the strain in the extensor branch of the suspensory ligament. It is my contention that the great number of suspensory injuries is caused by high heels, and the increase in strain is the reason for it. Thsi is my primary reason for including this article. He does not do an adequate job of explaining how increased strain in the suspensory ligament would stabilize a coffin bone.
“One other effect noted with the addition of a heel wedge is the large increase in strain of the extensor branch of the suspensory ligament. This may be of particular interest in the treatment of laminitic horses, as reduction of deep flexor tendon strain and increased extensor branch strain both occur with heel wedges and will work concurrently to stabilize the coffin bone in normal and laminitic horses. ”
I also see no explanation for how increased strain in the hoof wall via raised heels can be a treatment for quarter cracks. Rather, it sounds like the cause for them.
Excellent observation. Thanks!
“Heel wedges also raised compressive strains on the surface of the hoof wall at both the medial and lateral quarters, which may affect the occurrence and treatment of quarter cracks.”
The statement you quote from the article is implying that wedges are, indeed, a possible cause of quarter cracks.
Please note the angles he mentions when strain is increased on the extensor branches of the suspensory ligament… 67 degrees and higher. How many 67 degree angles do you see out there? And I may be wrong here, but the vast majority of suspensory injuries occur in Thorougbred race horses, not particularly known for their high heel angles.
Hi Tom,
Thanks for your comment; you make some good points.
I have to admit my ‘field of vision’ for angles tends to stop at around 55 degrees so I’m not sure I’d recognize if a hoof is 67 degrees. But you’re right, that is quite steep.
I’m not too familiar with the occurrence of injuries in TB race horses, but IME sporthorses in regular work do encounter this problem with regularity. There always seems to be one in my direct personal experience that’s on layup from this injury and several more in the extended circle of competition horses I know.
You’re right that racers aren’t knows for their tall heel heights but a lot of them do have long toe/’low’ heel syndrome, and I think mechanically a ‘low’ heel (which is in reality quite long, just at a different angle) can have the same effect on the suspensory as well as the sesamoids, since the flight path, or breakover, is extended beyond an efficient motion.
The other thing I never understood about wedging the heels on a foundered horse is this: If the toe of the coffin bone has dropped, a heel wedge will increase the angle of rotation and intensify, even more, the weight of the bony column onto the tip of the coffin bone. This seems totally counter-indicated, to me.
I agree with you Jean, that raising the heels (and concurrently lowering the toes) is counter-intuitive, because it exacerbates the existing pathological condition. However, I can say that the reasoning behind it (flawed, in my opinion), is that the DDFT is ‘pulling’ on the back of the coffin bone, rotating it. Thus, shortening the tendon is supposed to take the pull out of the tendon and stop it from rotating the bone.
my friends horse was fitted with wedge shoes to help treat a suspensory ligament injury.The leg was mega hyper flexing and the farrier suggested to try wedges.He says she must stay in these forever.I notice that the mare now has other issues which seem to stem from the back but not being an expert cant say exactly where.I am wondering if the unnatural angle is causing problems elsewhere.
I cant help wondering if going barefoot would help this horse and wonder how the change to wedge shoes to no shoes could be made.Would it firstly be feasible to try this and if so would one lower the heel gradually by the use of lower and lower wedges as she seems unhappy putting a foot to the floor with no shoe on.Any comments will be appreciated.BTW the mare is a 16 hh. 15yo Anglo arab with a distant history of laminitis but none for eight years.Thanks
Hi Barbara,
There is a lot more information that would be needed to really understand the situation here and make good recommendations, so in the absence of that I will have to make some assumptions.
Presumably, the injury was on only one leg (a front one?), but the wedges were on the pair of feet? So what is the logic of having a wedge shoe on the uninjured leg, other than putting them both at the same angle? IOW why should an uninjured leg have the same treament?
Further, I don’t really understand how the wedge shoe, i.e. changing the angle, would alleviate this injury. Has a vet been involved?
I do think the horse would be better off barefoot, because I suspect the trim is not optimal, there may beheel pain due to overlong bars and the horse avoids stepping onto them. This transfers the weight into the pastern and overstresses the ligaments and tendons in the fetlock and eventually you get the situation you describe with an overflexing joint. Attaching a shoe does not address the problem. Raising the angle only puts more weight onto the toe, overstressing it and the laminar attachment which can lead to laminitis/founder if allowed to go on long enough.
The transition to barefoot is much less involved than what one might expect. The mare was most likely put into wedges of the final height all at once, not in stages. Returning to the natural angle is easier and usually the horse exhibits great relief immediately upon doing so. It is the correct trim that will allow her to weight her feet properly.
I recommend a second opinion from a trimmer that will explain how to better balance the foot as the imbalance is what causes injuries.
If you like you can send me pictures at the email address listed on the home page for a cosultation.
Incidentally the mare’s reluctance to weight her foot without a shoe is a VERY STRONG indicator of the above scenario, that there is a great deal of overgrown horn (sole and bar) impinging on the soft tissues and causing pain, and needs to be removed.
I hope this helps.
Christina
Hi Christina,sorry I did not make things clearer.The mare was ex-rayed on the hind leg on which she was lame.They didnt seem able to make a clear diagnosis but thought it most likely suspesory ligament damage.At this point there did not seem to be too much hyper flexion.At about the same time she went lame on the other foot.This turned out to be some old infection and the foot was poulticed on and off for a period approaching several weeks.During this time she started favouring the infected foot and the hyperflexion became much more marked on the original lame leg.The farrier suggested wedge heel shoes and thses were applied.She has been wearing these type of shoes around 18 months now.The owner could do with getting another opinion but we are in a part of the UK where there are no barefoot trimmers so apart from the vets the only advice we can get is from farriers.The owner knows there is a problem but dosnt know how to get anything other than conventional advice.I am the yard owner and have said I would try to help get some advice.
Hi Barbara,
There is not much more I can tell you without more information. I suggest you email me from the ‘About Us’ contact link on the home page. Include pictures and a timeline of her history. The pictures should include conformation shots as well as foot pictures, lateral views from ground level (like xrays) sole shots with the camera lens perpendicular to the sole, views of the heels looking down on the foot as you hold it at the pastern and let the hoof drop loosely, the hairline from the front at ground level to determine balance.
Also if you let me know where in the UK you are, I can ask some colleagues if they know anyone in your area.
Regards,
Christina
I have a retired race horse, he is now 22 raced until 11. Shortly after I had him he tore off a quarter of his foot including some of the frog, all the way up to & including the root [ed. note: there is no ‘root’ in a foot]. The Vet thought he needed [to be] put down at first, then he said he may need [to be] stalled the rest of his life, gues what it grew back all except the heal (front left inside heal). I had to put egg bar shoes on, than regular shoes, burned on, now it is a regular shoe. I am not pleased with the farrier (he is a new one & young. It seems that my horse is very flat footed. The back of the hoof gets bruised when I ride him for long periods on trails ( not real long maybe 2 hrs.) The farrier now says he has side bone? Can any one explain this to me.
He may have healed his ‘heels’ just as well if not better without shoes, but you’ll never know. It’s possible or likely though that the shoeing is contributing to both the flat feet and the sidebone, by increasing tension on the laminae pulling them away from the coffin bone, letting it ‘drop’ inside the foot and being closer to the sole making it look flat; and the sidebone from concussion from the shoes or the way he is trimmed or both. Sidebone is ossification of the lateral cartilages.
Hi Christina,
I’m a civil engineer in structural design and the fact that the “Strain on the deep flexor tendon was reduced with increases in toe angle” and that “The reduction in strain followed a linear pattern” doesn’t come as a surprise to me. No need to do all these tests – I could have told him so from the start. It’s basic mechanics.
It’s true that increasing the toe angle will reduce both the tensile force in the DFT and the tensile stress in the lamina. However, there is a big flaw with all of this. The author is a farrier or at least he thinks like a farrier. He assumes that the horse is wearing shoes and that therefore the horse is carrying the entire load by the hoof wall.
There is a much better way to deal with a laminitic horse. Take off the shoes and trim the hoof wall at the toe area so that the hoof wall doesn’t have any ground contact. You will still have the tensile force in the DFT but the tensile stress in the lamina will be gone. Problem solved!
Since the horse will have to carry the load by its sole and frog for a while until a new strong lamina has grown down it may be tender-footed (laminitis also affects the sole corium) boots and pads are recommended.
Lars
Hi Christina, I was actually looking for photos of front hooves to determine the prevalence of medial or lateral flaring when I found this….;-)
I think a lot of misunderstanding in this area comes from incorrect or incomplete understanding/description of the biomechanics of the musculos skeletal system 🙂
I often wonder, why is the extensor tendon in most articles about laminitis and founder for example completely ignored?? Yet the extensor and the DDFT keep each other more or less in balance, depending on corresponding muscle strength. Doris Halstead , the author of Release the Potential, even says that the EXTENSOR muscles are the weightbearing muscles when a horse stands, which means they have to work against gravity and tend to be stronger than the flexors. Looking at the bulk of the extensor muscles on the front legs compared to the flexor muscles certainly seems to support this 🙂
How can then the flexor muscle, via the DDFT, ever have enough strength to have a negative affect on the coffin bone??! One can’t simply take body parts out of [body] context and expect accurate results, let alone theories!
Muscle contraction is only transfered onto bone via tendons, not ligaments. The primary function of ligaments is to stabilize joints. They passively react to movement in order to protect the joint, but will not actively move bones/joints.
In my book there’s only one dorsal hoofwall angle any horse should have, and that is the one the horse WANTS to grow, which is usually indicated right below the coronet band for about 1/2 inch or so.
If the horse cannot maintain that angle, then the hoof is most likely affected by internal/metabolic issues (diet, gut health, disease. etc.)that need to be addressed. It is typically a sign, that the the connective tissue in the hoof is weaker than it should be and when the horse loads the heel, the laminae simply stretch and give way. How much they give, depends on how much they have been affected. It is just simple mechanical loading forces at work and how they affect compromised connective tissue.
Wedging a hoof potentially shortens the flexor muscle, not unlike calf muscles in humans who tend to wear high heels. Although one hears it a lot, tendons do NOT contract, only muscles do and if muscles are in a constant (more or less)state of contraction they become hypertonic (overdeveloped and short). But those muscles can also be released with body work and stretching as needed. Flexor muscles should always be addressed with bodywork while lowering heels on a horse with high heels.
So wedging actually potentially increases the possibility of muscle (and DDFT) strain, by artificially making the flexor muscle hypertonic (shorter). Instead of releasing the supposed pull on the coffin bone, it theorhetically actually increases it.
The suspensory developed from a muscle, so it is not really a true tendon per say, more like a modified muscle, somewhat in between a muscle and a tendon. It’s main function is to support the staying aperatus in the front and fetlocks. This makes it somewhat of a double edged sword – due to its higher number of muscle cells, it can still be more easily damaged than a tendon, yet that also increases its ability to heal 🙂
Generally speaking I think we tend to see more suspensory or other tendon strain in legs that have a hoof with low heel and long toe (delayed breakover). My gelding certainly fit that picture when he strained his right front suspensory while running on a sugar high in his pasture and sliding in the mud, hyperextending that RF suspensory tendon.
It was his more under-run, flatter right front hoof (his dominant hoof) that was affected. It would be interesting to see whether or not in most cases the dominant leg is affected whne the suspensories are strained. It can also happen in the hinds, which tend to have lower heels to begin with. In hinds I think the issue is often more repetitive strain that eventually causes the damage, as it seems to be often found in Dressage horses, compared to other disciplines.
I wish horse care professionals would be more observant and note what the hoof shape was of the affected (injured) leg, whether or not it was the horse’s dominant leg, other potential contributing factors, etc. This would show trends that would easily tell us what the most likely common root causes are, without doing any extensive research.
Keep up the good work – happy trails 🙂
Ute