Written and presented April 2012 by R.F. (Ric) Redden, DVM
To better understand the club foot syndrome, we must be familiar with the mechanical formula and how it greatly influences the various degrees of hoof capsule distortion and bone remodeling associated with this syndrome. There appears to be a direct relationship between the degree of tension increase or contributive force of the DDF muscle and these two very distinct alterations from the normal healthy foot. This paper will describe that relationship and the soft tissue and coffin bone alterations that are found in the four basic categories of club feet.1 These characteristics are unique for each grade, however several variables can influence the stereotype mechanical model.
The Healthy Foot
First let's briefly describe a healthy foot on a light boned breed such as a thoroughbred, quarter horse, Arabian and other similar breeds. A healthy foot will have a relatively constant growth pattern heel vs. toe, especially when left barefoot. This uniform toe to heel growth rate is clearly revealed by the relatively even spaces between the growth rings, which routinely occur approximately every 30 days. Shoeing styles, trim and reset timeframes can alter this natural pattern to some degree, but it remains well within a range that can quickly adjust back to its original pattern. The hoof wall has a relatively straight, linear appearance and the toe angle has a very large range depending on bone angle (BA) and palmar angle (PA), which can also vary considerably. The approximate angle found along the growth rings when the toe and heel grow at a different rate will closely mimic the PA on the foot that has not been trimmed for 30-45 days.
Figure 1A Left: This horse has very healthy, sound feet despite a grade 1 right front club. Figure 1B Right: The right front growth rings indicate a tendency for a negative PA in the right hind due to the club in front.
Figure 1C Left: Radiographs of same horse taken prior to 5 week reset. The left front has 25mm of sole, a 0 PA and a very healthy horn wall.
Figure 1D Right: The right front has approximately 22mm of sole, a positive PA and very healthy horn wall. Note difference in bone angles. Both feet are sound and healthy even though strikingly different.
Sole depth maintains around 15mm plus with a few millimeters of natural cup. This is the goal on trim day. Strong feet and healthy digital cushion go hand in hand - you won't find one without the other. Domestic horses with strong, intact heel tubules will have a positive PA that will fall into a range of 2 to 5°. Contrary to what we have thought in the past, hind feet of light breeds do not have a larger PA or hoof angle than front feet. Observing many foals as they mature, the large majority have a 0° PA behind, which may explain why their heels can quickly crush and a negative PA develops once put into training.
With this very basic description of a healthy foot we can start comparing one foot to another, from horse to horse as well as feet on the same horse. Observing the external characteristics and soft tissue parameters before and after trimming or shoeing helps us better understand the interconnectedness of each component as well as how we can enhance the natural healing mode with shoeing mechanics.
Club syndrome influence on the opposite foot
The foot opposite a club also appears to be greatly influenced by the club syndrome. The hoof capsule silhouette has several distinguishing characteristics that apparently occur due to a significant laxity of DDF muscle tension, which reduces suspension function. In these feet the pastern sits back well away from the face of the dorsal half of the wall but can remain parallel to the face of the body when digital alignment is present. Imagine pushing the pastern forward just above the heel on the club foot. The tension on the DDFT displaces the pastern on the club and the lack of tension on the opposite foot lets it sag, creating the distinct difference. The growth ring pattern on this low foot is often wider at the toe than the heel, indicating the toe is outgrowing the heel. This ratio steadily increases from one shoeing to the next, as blood flow is exceptionally good to the toe but impaired in the heel due to excessive heel load that results from lack of adequate suspension.
Figure 2A: Photo of a hind foot with a negative PA and crushed heel. Note the growth ring pattern, very low heel angle, bull nose and coronary band angle.
Figure 2B: Right of the same foot. Note the negative PA and digital alignment imbalance. Heel tubules on the low profile foot have a very low angle as compared to feet with even growth ring patterns. The ground surface contact point may be folded forward to the widest point of the foot. The bars are very thin and also folded inward with the heel tubules. The frog is very prominent and extends well past the ground surface of the heels and most often hangs out the bottom well below the load surface of the heels in shod feet. The digital cushion, which can be estimated by placing your forefinger on the frog and thumb in the cleft of the heel, is also compressed. Comparing its thickness to that of the club, it is very obvious that the cushion is all but maxed out.
As load is steadily passed to the growth centers the vascular supply is greatly compromised and heel growth shuts down. This can be demonstrated with comparative venograms. As a rule, the higher the scale the greater the imbalance of toe to heel growth on the low profile foot. This DDFT tension laxity closely resembles the relationship of the pastern to the hoof capsule that occurs following a DDFT tenotomy, a procedure that allows the pastern bones to sag, shifting load to the posterior aspect of the coffin joint/navicular area and associated components of the heel.
Figure 3: Left: Estimating depth of heel and digital cushion. Right: Frayed heel tubules are one of the first signs of heel crush.
How does this happen? The physics of the syndrome are very simple and quite clear if we focus on the function of the DDFT. The club foot results from increased DDFT tension and suspension function. The opposite foot apparently has the same degree of hypo-function; lack of suspension allows excessive internal heel loading that quickly surpasses the limitations of cushion load and recall. As the cushion fails the tubules fail, and the capsule quickly alters the natural growth patterns. One component does not fail alone but affects adjoining support structures.
The hind foot on the same side of the horse as the club foot also has distinct characteristics that clearly distinguish it from the opposite hind foot. Even the hind foot that follows a grade 1 club will have a lower profile hoof angle, lower heel and less than healthy digital cushion depth, much like that of the low heel in front. Why this occurs is unknown at this time, but it is the author's hypothesis that both are closely related and the result of the club syndrome.
A low beam, soft tissue lateral radiograph taken 4-6 weeks post trim or shoeing is very helpful to evaluate the severity of a club foot and determine optimum management options. Radiographs made shortly after trimming or shoeing often fail to clearly describe the significance of the syndrome as the PA, sole depth, digital breakover and HL zone are usually altered with the trimming process.
Figure 4: Standing the horse on two blocks with the head held straight and low beam penetration can help assure a more accurate radiographic image.
Figure 5: Lateral low beam soft tissue view vs. high beam. Note the lower beam projection reveals one branch of the shoe, which is vital for measuring sole depth and PA.
This protocol allows for accurate evaluation of sole depth and other valuable soft tissue parameters. It is also helpful to see the dermal/epidermal (DE) zone on all lateral soft tissue detail film. This zone describes the horn (H) and laminae (L) and as a rule evenly divides the HL zone. If the H zone appears quite narrow closer to the toe, this indicates that the toe was backed up (rasped off) by the farrier.
Figure 6: Measuring soft tissue parameters on a foot with a positive PA and negative PA.
Radiopaque paste along the face of the foot describes the wall proper, which is not visible on any radiographs without an opaque marker. Contrary to popular belief, digital images do not reveal the outermost layer of the horn wall as this small area is always overexposed. Radiopaque paste also describes growth rings that can provide valuable information. Using a nail, needle or wire only touches the high points and therefore does not reveal the growth rings or the true wall margin.
It is important to note that a common practice is to trim and shape a club foot to make it appear more normal. This practice can drastically alter several key soft tissue parameters, including the horn-lamellar (HL) zone, sole depth and PA, which can cause confusion when evaluating a foot and lead to misinterpretation. The best time to observe the foot's natural characteristics is 4-6 weeks post trim or shoeing.
When interpreting film made shortly after shoeing we must focus on the joint space (lateral view), which in most cases will be very tight along the dorsal aspect and wider along the distal articular zone. This is an indication of increased DDFT tension. Even though PA may be 2 to5° and appear quite healthy, an 8-10° PA will allow for a more even joint space. This is also an indication that increased tension is present due to lowering of the heel, which is a very common practice. The face of the bone of a club foot develops a distinct bulge. The bone shape can also allude to the club foot syndrome even when the foot is freshly shod. In young horses, the distal HL zone is narrower than the proximal measurement.
In newborn foals, the coffin bone is quite small and has a very distinct shape and the bone and foot grow rapidly in a relatively natural growth plane. Apparently the overall shape of the bone is greatly influenced by the forces exerted by the DDFT, laminae and ground load, all of which directly influence the nutrient supply to the bone and associated components.
Radiographs of foals only a few days old suggest that bone angles certainly do not always match, nor does the overall stereotype shape of the bones match. Feet with higher bone angles are prone to grade 2 or higher clubs, however those with relatively matching bone angles can progress to various grades of club that are noticeable within weeks or months of birth. This syndrome has been previously referred to as an acquired syndrome, however the author is very satisfied that there is enough clinical evidence to support the theory that the club syndrome is congenital with very strong genetic influence.
Figure 7: 3 week old foal with a 60° bone angle (BA).
Note bone angle shape and digital alignment. As the foal grows and attains body weight the forces at play are certainly magnified. When all forces remain within their natural range, a healthy, sound foot as described earlier evolves.
Foals with low grade clubs can escalate to a higher grade, whether due to genetic influence or to increased tension placed on the DDFT by excessive heel lowering. The genetic component seems to have potent influence on the mechanical model of the foot just as it does for other unique characteristics of the individual. When low grade clubs are trimmed or shod in a fashion that increases DDFT tension, they often escalate to a higher grade on the scale due to neuromuscular reflex that is apparently triggered by pain response in the toe area. Simply put, trying to solve the problem of excessive heel growth using the excessive forces that created the problem is often futile and precipitates the common ill effects of the club foot that have been recognized for years. Toe bruising, fragmented terminal laminae, thin soles, lack of toe horn growth and frequently subsequent toe abscesses commonly occur when treatment is designed to eliminate the unique capsular characteristics of the club. Soundness issues frequently plague athletic horses even with lower grade clubs.
Figure 8: Foal with a grade 4 club, fragmenting toe, thin sole and no toe growth. This frequently leads to abscesses and ongoing soundness issues.
Toe abscesses are very common in foals with grade 2 or higher club feet, but can be prevented by monitoring sole and wall junction integrity. Note a seemingly harmless abscess that often breaks at the coronary band can be a precursor to white line disease in the mature foot.
The Mechanics of the Club Foot
Removing the heel from a low grade club with a PA that is 5° greater than the opposite foot may appear to immediately correct the club syndrome. However, the heel grows back in 10-15 days. Why does it regenerate so quickly? This brings us to a very decisive point of understanding mechanics. If the heel were growing and pushing the horse upward, simply removing the heel would be a reliable solution. And if this were the case, the freshly trimmed heel would set firmly on the ground once the excess heel was removed. However, even with lower grade clubs this practice can create a remarkable air gap along the ground surface of the foot from heel to toe. Even when it looks to be loaded, one can often slide a business card under the heel almost through to the toe. This foot is now being loaded over a very small part of the toe as the heel is suspended and unable to share digital load.
Though an inferior check desmotomy is accepted worldwide as a recommended treatment for mid-grade club feet, the theory that the DDFT is the source of the forces that cause the syndrome seems to get lost. Literature continues to teach that regular, proper farrier care is needed to prevent the heel from growing. Unfortunately this concept violates the mechanical principles of the club foot and can set off a cascading series of events that have the potential to haunt the horse throughout its career.
The trigger mechanism of the club syndrome is not known, but it is obvious that whatever it is has a large scale of intensity. Clinical evidence from venograms support the theory that increased suspension forces of the DDFT directly contribute to increased tension and shear on the very elastic laminae and compressive forces of the palmar rim on the sole corium, an easily compressed source of nutrient supply. This domino effect remarkably reduces sole proliferation and sole and toe horn growth. Increased DDFT forces rotate the coffin bone around the articular surface of PII, increasing the PA and internal load on the apex and sole corium as well as additional stress on the distal lamellar attachment and the dorsal face of PIII. As force increases, load on the digital cushion, heel and frog (which are all located below the suspension structures) decreases, resulting in tight, narrow heels; small recessed frog; excessive heel height, etc. These are all typical distinguishing characteristics seen with various grade clubs.
Figure 9: Balance = harmony between suspension and support structures.
Growth rings become remarkably wider at the heel than the toe, clearly demonstrating the imbalance of forces between the DDFT and its opposing antagonist component, the laminae. The laminae directly oppose the force of the DDFT. The wall then receives load from the laminae, which is passed to the toe at ground contact. Therefore the length of digital breakover demands great respect when attempting to relieve DDFT tension. To better understand the potent effects of the DDFT tension, we need only to greatly reduce the forces at play and observe the medical benefits seen as accelerated sole growth, return of healthy papillae (as seen on the venogram), reduced heel growth and increased toe growth. This treatment concept is referred to as reversal therapy. However, positive results require a few weeks as the foot begins to overcome the ill effects of imbalanced DDFT tension. Unfortunately, much of the horse world continues to want immediate results and simply cannot visualize the DDFT as being the seat of hoof and bone distortion.
Grading Club Feet
Redden defined 4 basic categories of club feet based on increasing severity of hoof capsule distortion from grade 1 at the lowest to grade 4 at the highest.1 Radiographic evidence can also be correlated to the various categories. As a rule, the higher the grading scale, the greater the degree of capsule distortion and soft tissue parameter alterations.
Figure 10: Photo and x-ray of grade 1 club foot. Note the linear alignment between pastern and hoof. This is a very healthy, sound foot.
A grade 1 club is often considered to be a healthy, robust foot by most horsemen and professionals. However when we take a closer look at distinct internal as well as external characteristics there is often conclusive evidence that increased suspension force is directly responsible for the subtle but distinct alterations that clearly distinguish it from the opposite foot.
In the foal, the most defining characteristics of the grade 1 club with similar bone angle to the opposite foot are is a hoof angle 5°greater than the opposite foot and greater PA relative to the discrepancy in bone angle. The face of the pastern is pushed dorsally in a straight plane with the face of the hoof and the heel bulb will have a fuller appearance, though this is difficult to detect at a very young age as the foot is smaller than the pastern.
At 6-12 months of age the grade 1 club is quite easy to identify as the features explained above are simply magnified. The PA will be greater, the frog not as well formed as the opposite frog and the heel and foot width will be slightly narrow. Growth rings will be quite uniform or slightly wider at the heel, especially the lateral heel.
A foot that remains a grade 1 club throughout the life of the horse can be easily maintained as long as it is trimmed and/or shod in a fashion that does not drastically reduce PA and thereby increase tension on the DDFT, making it look like the lower profile opposite foot that has a 0° PA and less DDF suspension. The grade 1 club will have a 3 to 5° PA and can maintain 15-20mm of sole on shoeing day. Dropping the PA with every trim to match the opposite foot invariably increases tension on all structures and will slowly remodel the face of PIII, the L zone, sole depth and articular surface.
Figure 11: Photo and x-ray of grade 2 club foot. Note wider growth rings at the heel than the toe and the pastern/toe relationship, The 10° PA is evidence of excessive toe loading, In the radiograph, note debris that has invaded the toe.
A grade 2 club foot is the most common grade and most often missed, as it can appear to be a healthy foot compared to the opposite grade 2 low heel (slam dunk) foot. Grade 1 and grade 2 clubs have a very similar pastern and dorsal hoof capsule alignment. The pastern is displaced dorsally, creating a linear plane along the pastern and face of the hoof capsule. This characteristic can be detected when foals are quite young. The pastern of the opposite foot sits back away from the linear plane along the dorsal face, creating a slight dip just proximal to the coronary band. The heel shape on the club will have fullness between the bulbs with less of a pocket in the space between the heel bulbs. The lateral view profile of the club heel will be more vertical than the opposite heel.
Foals with grade 2 club feet may have a bone angle that is 5-10°greater than the opposite foot. This appears to be unique to the club syndrome as the larger than average bone angle (50 to 51°) has not been documented in the low foot. The apex is always convex in the newborn, but starts to remodel quickly if subjected to excessive internal bending forces. As a yearling the apex will have developed a distinct lip, referred to as a load induced lesion, caused by the unrelenting pressure of the DDF as it presses the apex into the sole corium. Sole depth will be diminished compared to an untrimmed foot that has natural growth. The cup of the foot may be absent radiographically unless the sole has been trimmed by the farrier to remove sole pressure.
The face of the coffin bone will begin to develop a slight bulge along the mid face. Distal to this point the bone will have a dish appearance that increases the distance between the opaque DE zone and the face of the bone. This resorption of bone can be confused with rotation of the coffin bone. Note the parallel arrangement of the bone and wall above the bulge. This differentiates the alterations from capsular rotation, which is caused by loss of horn integrity (white line disease, laminitis or both) and occurs along the entire dorsal face. The resorption or remodeling found in the club foot apparently falls under Wolff's law, which states that bone remodels along lines of stress. This lip of remodeling is also accompanied by a higher PA than we find in the healthy foot described above.
Figure 12: At first glance this foot might appear to have rotation, but note the parallel arrangement between horn wall and proximal face of coffin bone. Distal to the bulge bone remodeling gives the appearance of rotation.
As this foot matures and the forces at play are continually reinforced by traditional heel lowering, the bone continues to remodel along the lines of stress. The lip is resorbed and the palmer surface of the bone is no longer a straight plane. The anterior third of the bone is no longer in line with the wings. This can be considered a load induced lesion and is a typical characteristic of grade 2 club feet.
Before trimming, most all grade 2 club feet will have a 5 to 8° PA and can be as high as 10 to 12°. Even if the heel is removed and the PA is subsequently lowered, it will drift back up over 4-6 weeks. This tells us a lot about the foot. If a foot naturally has an 8 to 10° PA but is maintained at 2 to 3° we know without a doubt that the tension on the DDFT is greatly increased on a regular basis. Every time the heel is taken off load is transferred to the toe.
Sole depth revealed by radiographs may appear to be adequate, however if this foot was shod 6 weeks previously and only 10 to 12mm of sole is present, the foot is not happy. If the foot has 10 to 12mm of sole and had only ben shod a few days, interpretation would be totally different. When a cup is present, consider whether it is natural or manmade. If the foot was shod a few days prior to the x-ray, we can assume it is manmade and have no clue what it was before shoeing. Here lies the value of noting the last trim or shoe date.