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.