49th Annual Convention of the American Association of Equine Practitioners, 2003, New Orleans, Louisiana
THE EQUINE FOOT, IN-DEPTH
Clinical and Radiographic Examination of the Equine Foot (21-Nov-2003)
R. F. Redden
Versailles, KY, USA.
1. Introduction Lameness is one of the most frequently encountered problems in equine practice. The foot is involved, either directly or indirectly, in the large majority of lameness cases, as it is the first line of defense for the animal. The health of the foot plays a major role in the fight or flight response that has preserved this noble species for several thousand years. "No foot, no horse" is an adage that has been used across the world for centuries. This indisputable statement encapsulates the importance of a healthy foot; yet we know less about the foot than about almost any other part of the horse, and it is the one piece of anatomy that is dependent on a lay profession for the preservation of its health and function.
Worldwide, farriers bear much of the responsibility for maintaining or restoring the health of the horse's foot. For centuries their knowledge and skills have been self-taught, without the benefit of a formal educational program. Their basic job description is to keep the foot healthy by using effective but primitive methods to control the ill effects of horn growth and of wear and tear on the hoof capsule, with little or no information about the effects of these procedures on the sensitive soft tissues, vascular supply, or bone. Veterinarians, on the other hand, have been taught anatomy, physiology, and basic examination techniques; however, they often have limited working knowledge of the foot and little or no farriery skills.
Both professions play important and complementary roles. Veterinarians and farriers alike are often asked to examine the foot for a variety of reasons, including developmental problems, gait analysis, lameness exams, and prepurchase exams. In many cases, the opinions that result are as diverse as the backgrounds and areas of expertise of the respective professionals. Combining the knowledge and skills of a competent farrier with the medical and surgical training of the veterinarian greatly enhances the diagnostic and prognostic potential of both clinical and radiographic examinations. Working together also advances the professional standing of veterinarians and farriers.
Clinical and radiographic examinations of the foot are simply discovery exercises. Numerous authors have described their methods and techniques in detail. But despite the vast amount of written material on the subject, obtaining meaningful information about the foot remains a challenge for veterinarians and farriers. The key is to use a disciplined, methodical approach that is designed to disclose and define the various normal soft tissue parameters, normal bone anatomy, normal hoof capsule anatomy, and how each component is interrelated. The protocol should also reveal the response of these structures to the forces imposed by ground contact, supporting tissues, and the horse's body weight.
Seeking and defining specific pieces of information in a consistent, repeatable manner for each foot, in each horse, greatly enhances the practitioner's understanding and knowledge bank regarding the vast range of normal-which is the real information you want. Whether examining a foot or a radiograph, look for all the normal areas first; what's left over points to the problem you seek. This simple approach effectively helps avoid misinterpretation, a common result of forming an opinion without sufficient diagnostic information; for example, making presumptions concerning the clinical relevance of a radiographic lesion without consideration of the history or physical findings.
2. Clinical Examination Regardless of the purpose of the examination, the physical exam is the most important aspect of evaluating the equine foot. The extent and nature of the exam must be tailored to the situation, however, taking into account the demands of the client. Good horsemanship, a good working knowledge of the foot, and some basic farriery skills are other prerequisites for a proper and safe examination.
A complete history which clearly describes the complaint complements the physical exam and adds context to any clinical findings. Listen to the history as you examine the foot, but do not jump to conclusions nor be swayed by the opinions or conclusions of others. Visually inspect the foot before picking it up, and feel the hoof capsule with your hands, noting its many unique characteristics.
Although certain generalities can be made, there is a range of normal for hoof characteristics which is influenced by the horse's breed, age, environment, and use. Considering the variability imposed by these factors, the range of normal can be very broad. The importance of understanding the variability in structure of the healthy equine foot lies in identifying subtle deviations from normal which are of clinical significance. These early distortions are easily missed if the normal parameters for a horse of that breed, age, environment, and use are not appreciated.
The following example details the requirements for adequately defining normal for a particular horse. Let us consider the forefoot of a 3-yr-old Thoroughbred horse, bred for racing but used as a noncompetitive riding horse in central Kentucky. That foot would probably have the following characteristics: a hoof angle between 50 degrees and 58 degrees, and a heel angle perhaps 15-20 degrees less; a relatively straight wall (i.e. no flaring, dishing, or bulging); width approximately 5 in. (measured at the widest point); mass of digital cushion 2-3 in. (discussed later); hoof wall thickness of 3/8-1/2 in. at the toe and the bars;a hoof wall perhaps one-half as thick at the quarters; a sole with a moderate cup (3-5 mm in height); a frog in contact with the ground (although it would also be normal for this horse to have a relatively flat sole, i.e., little or no cup, and a large, flat frog); and a hoof wall with a solid appearance and a glossy surface.
We must leave behind the "ideal" of the normal equine foot depicted by artists in veterinary and farriery texts for the past century or more. Hoof angles and heel angles do not match on any normal foot. And the "ideal" toe angles of 45 degrees for front feet and 50 degrees for hind feet are far from normal as they do not match the pastern angles. One must become a connoisseur of horses' feet and begin to build a personal data bank of normal for particular breeds, age groups, environments, and uses.
The Seat of Pain When dealing with a lame horse, most authors consider the physical exam simply a means of reaching a diagnosis, i.e., of giving the problem a name. While this approach certainly satisfies one of the goals of the exam (to identify the problem), years of experience as an equine podiatrist have made me very aware that most owners want a fix and could care less about a diagnosis. As I'm going through a lameness work-up, I focus on identifying the area(s) of pain rather than specific pathology. Dividing the foot into two halves, front and back, then dividing further into quadrants (medial and lateral, front and back) offers a simple way of isolating the specific area of inflammation or seat of pain (Fig. 1, A and B).
Figure 1. (A) Imagine dividing the foot in half.
Figure 1. (B) Then in quarters.
Dividing the foot into four basic zones helps me determine whether the components in each zone fit within the range of normal for that particular animal. With my understanding of radiographic anatomy (again bearing in mind the range of normal), I visualize the bone and associated soft tissues superimposed over the hoof (Fig. 2). Any finding that falls outside the range of normal is considered relevant, as it contributes to the dysfunction of the foot as an integrated unit and thus probably plays a role in the current lameness problem.
Simply cleaning the ground surface of the hoof can reveal areas of possible concern. (Fig. 3). Each of these areas is a map of a potential problem: examine each thoroughly before moving on. Remember to look for all the normal areas first, and what is leftover often points to the problem that you are attempting to identify.
Figure 2. Visualize the bone and associated soft tissues superimposed over the hoof.
Figure 3. Simply cleaning the ground surface of the hoof can reveal areas of possible concern.
After a quick visual exam, I palpate, using thumb pressure to locate areas of increased sensitivity along the coronary band, the bulbs of the heel, and even over the sole on thin-soled feet. Hoof testers should be used with great care, because inappropriate use causes the horse to anticipate further pain and show an exaggerated response to even light pressure.
When applying hoof testers, use a very soft touch. All that is needed to identify areas of increased sensitivity is just enough pressure to cause slight movement of thin horn (e.g., the sole in a thin-soled horse). Also be aware of how you are holding the horse's leg. If, by positioning the limb between your knees so that you are comfortable, the horse is made uncomfortable, you may elicit a response that has nothing to do with the foot.
It is easy to abduct the limb too far when placing the horse's lower limb between your knees. To avoid this situation, note where the horse's body in relation to the foot when you first pick up the leg. Try to maintain that orientation when placing the limb between your knees-i.e., put yourself where the foot is or have someone hold the limb for you (Fig. 4 A-D).
Figure 4. (A) Note relaxed position of foot.
(B) Position yourself to horse’s relaxed position.
(C) Avoid abducting limb for your comfort.
(D) Proper stance when using hoof testers.
The Failing Structure Distinguishing the abnormal area(s) allows me to identify which part(s) of the system is failing and affecting the integrity of the whole. Simply recognizing the failing structure(s) as the primary problem-the underlying cause of any secondary bone and/or soft tissue disease-gives new meaning to the discovery exercise and places new emphasis on the findings. Following is an example of this concept. Race horses, or in fact any speed horse, with less than 10 mm of sole, zero or negative palmar angle (the angle of the palmar margin of PIII relative to the ground surface), loss of cushion mass (see below), obvious medial-lateral imbalance, and a history of foot pain are often diagnosed with navicular disease, pedal osteitis, or bruised feet. Any of these diagnoses may be correct and the associated pathology may be contributing to the present lameness. However, more important is the fact that the essential protective function of the hoof capsule and the shock-absorbing features of the cushion network are seriously compromised, and the cumulative effects of these failing systems are now of paramount importance.
The "diagnosis" in this case is thus, multifaceted. However, it can be simplified by describing the situation as one of mild, moderate, or excessive horn loss associated with mild, moderate, or excessive compromise of the soft tissues. Instead of being focused on a medical diagnosis (which may well be challenged by another veterinarian or farrier) and a quick fix to satisfy the immediate demands of the client, identifying the failing systems allows the focus to be placed on a solution, which in this case involves restoring the much-needed hoof mass.
Figure 5. Use thumb and finger to guesstimate depth of digital cushion.
The depth of the digital cushion can be estimated by placing your thumb in the shallow depression between the heel bulbs and placing the index finger of the same hand on the center of the frog (Fig. 5). In light breed horses with strong, healthy heels, the distance between thumb and fingertip is in the range of 3-3.5 inches.
Figure 6. (A) Typical Thoroughbred hind foot. Note coronary band relationship with the ground. (B) Front foot, American Saddlebred. Growth ring patterns, coronary
band conformation, heel tubule angles, toe angles, and horn quality offer insight to sole depth, palmar angle, and overall state of balance.
When this distance is well short of the normal range, one can expect to see evidence of soft tissue compromise radiographically. This simple observation, coupled with noting the slope of the coronary band relative to the ground, also allows an estimation of sole depth and palmar angle.
Figures 6 and 7 illustrate how these observations correlate with radiographic findings. Note the difference in slope of the coronary band, angle of the horn tubules at the heel, and depth of cushion between the two horses (Fig. 6A, 6B). Compare these photographs with lateral radiographs of the same feet (Fig. 7A, 7B).
Figure 7. Radiographs of feet shown in Figure 6. (A) Note negative 6 degree palmar angle. (B) Note H-L zone and positive 6 degree palmar angle.
Incidentally, in my experience hind feet with a zero or negative plantar angle (wings of PIII level with or lower than the apex) are often associated with pain in the lumbar area or croup. Back pain in these horses frequently diminishes once heel mass is improved and a normal plantar angle is restored.
Other Notes It is necessary to remove the horse's shoe in order to thoroughly examine the foot, especially when the primary problem cannot be identified with the shoe on. At the very least, the shoe prevents examination of the bearing surface of the wall, the terminal laminae, and the perimeter of the sole. However, care must be taken when pulling shoes. In feet with fragile walls, raised nail clinches, or a special shoeing package, the shoe is best removed by a competent farrier unless you have considerable farriery expertise.
A good sense of smell can be a valuable aid in examining the foot. We all know the smell of a foot with thrush. But your olfactory sense can also help you identify digital sepsis. With experience, it is even possible to distinguish soft tissue necrosis from septic processes involving bone. Again, attention to detail is the key to refining one's examination skills.
physical examination is the most important part of evaluating the foot
develop a methodical approach, and use it every time
look for normal first (bearing in mind the range of normal for that horse's breed, age, environment, and use); what's left over points to the problem you seek
localize the seat of pain to one or more quadrants
visualize the underlying bone and associated soft tissues when looking at the hoof
think in terms of identifying the failing structure(s)
3. Radiographic Examination Much has been written about specific views for imaging the equine foot. Almost without exception, the primary objective of these views is examination of bone (PIII, navicular bone, and/or coffin joint surfaces). Little or no attention is paid to the soft tissues within the hoof capsule. This approach seriously limits the scope and accuracy of the radiographic examination and thus its value in developing an action plan for managing lameness involving the foot.
The coffin bone is suspended within its protective shell by soft tissues whose health is crucial to the structural and functional integrity of the foot as a whole. Dysfunction is inevitable when any of the soft tissues are compromised or strained beyond their normal limits. Over the many years I have worked as an equine podiatrist, I've come to appreciate the fact that soft tissue pathology is present to some degree in every footsore horse. Thus, evaluation of the soft tissue zones within the hoof capsule is an extremely important part of radiographic examination of the foot.
Evaluating the Soft Tissues While it is true that radiography is relatively poor at imaging soft tissues, a lot of information about the soft tissues within the foot can be gleaned from good quality radiographs taken with soft tissue detail in mind (discussed in the next section). At the very least, the width of the corium and horn can be accurately measured for both hoof wall and sole, provided the outer surface of the dorsal hoof wall is delineated using radiopaque material and the ground surface is defined either by the shoe or by a radiopaque marker in the surface of the positioning block. I measure the following indices on all routine lateral films (Fig. 8).
Sole depth is defined as the vertical distance between the palmar/plantar margin of PIII and the outer surface of the sole. It is routinely measured at the distal tip, or apex, of PIII (Fig. 8). A normal, healthy foot has a sole depth of at least 15 mm. Based on venographic studies in a wide variety of horses, I consider a sole depth of less than 15 mm to be clinically significant. In a normal foot, the papillae of the solar corium appear to need a space of at least 10 mm between the palmar surface of PIII and the cornified layer of the sole for adequate vascular filling; and at least 5 mm of cornified sole is required to protect the solar corium. Venograms in horses with a sole depth <15 mm show solar papillae that are bent, compressed, or even absent. This distortion or compression surely inhibits sole growth, creating a vicious cycle of thin, tender soles.
Figure 8. Standard low beam, soft tissue view with opaque wall marker and ground surface marker offers a consistent means of accurately measuring soft tissue parameters. Progressive farriers often use this view as a blueprint for pathological shoeing.
Dorsal Horn-Lamellar Zone Width Dorsal horn-lamellar (H-L) zone width is defined as the distance between the dorsal surface of PIII and the outer surface of the dorsal hoof wall, measured with the ruler perpendicular to the dorsal surface of PIII (Fig. 8). Dorsal H-L zone width can be measured anywhere along the dorsal face of PIII, but I routinely measure it at two locations: just below the extensor process, and near the distal tip of PIII. I record the measurements as proximal/distal (e.g. 15/15, meaning that the dorsal H-L zone is 15 mm at both locations). In a normal adult foot, the measurements should be the same proximally as distally (i.e. both numbers are identical). In the immature foot, the proximal value may be greater than the distal value.
Normal dorsal H-L zone width in Quarter Horses, Thoroughbreds, and most other light horse breeds is 15-16 mm. In Standardbreds, the H-L zone normally is a little wider, averaging 20 mm. Normal H-L zone width for Warmbloods depends on the size of the foot; in many cases it is similar to that for light breeds. Once again, an appreciation of the range of normal for that type and size of horse is essential for accurately interpreting this area.
Figure 9. (A) White line disease. Note the lucent lesion starts at the ground surface of the wall, has a very irregular border, often is superimposed over the bone, and often contains dirt, stone, and other debris.
(B) Chronic laminitis. The lucent lesion is within the laminae and stops abruptly at the innersole margin even when penetration has occurred. The sides of the lesion are smooth and the proximal distal border of the lesion has a smooth radius. Capsular rotation is the only common finding. Significant information can be gained by using the soft tissue parameters as a measurable unit to describe displacement.
Dorsal H-L zone width is an important measurement, as this zone widens in conditions that affect the laminar corium, laminar attachments, and wall thickness. Laminitis and white line disease are two common and clinically important conditions in which the dorsal H-L zone widens. Widening as one moves down the hoof wall from proximal to distal (i.e. H-L zone wider distally than proximally) may also be seen with other conditions.
This assessment, when used with the palmar angle (Fig. 9B), provides a meaningful way to identify and describe displacement of PIII. The conventional method of identifying and quantitating PIII rotation is inaccurate and misleading. The fact that the hoof capsule can be substantially altered by the farrier reduces evidence of rotation. Drawing straight lines along the irregular hoof wall and irregular face of PIII is subjective at best and the wall is constantly being altered by growth and the disease process. Therefore the whole basis of this measurement (PIII-hoof wall angle) is seriously flawed.
Traditionally measuring capsule rotation as a means to diagnose laminitis has also created the misconception that simply rasping the horn wall back to a parallel relationship with the face of PIII is an effective means of treating the syndrome. Very serious life threatening lamellar swelling often occurs without even a subtle hint of rotation. Therefore the significance of rotation as it relates to pathology is questionable.
On a good soft-tissue-detail lateral film, one can readily identify the linear radiopaque zone that equally divides the H-L zone in most normal horses. For example, in a foot with a dorsal H-L zone width of 15 mm, each zone measures 7.5 mm. When widening of the dorsal H-L zone is found, evaluation of the width of each zone is important, as it can provide diagnostically and prognostically valuable information. For example, the lamellar zone widens in laminitis, (Fig. 9B) whereas it is the horn zone that widens in white line disease (Fig. 9A). (Note: The outer surface of the dorsal hoof wall must be accurately represented by radiopaque material in order for measurement of the horn zone to be accurate.)
Coronary-Extensor Process Distance Coronary-extensor process (C-E) distance is the vertical distance between the most proximal extent of the outer hoof wall and the top of the extensor process of PIII (Fig. 8). In most normal horses it is 0-15 mm. The C-E distance can be accurately measured only if the radiopaque marker on the dorsal hoof wall extends all the way to the proximal limit of the wall. This measurement can be important in confirming displacement of PIII, provided a baseline is established for that horse prior to, or at the onset of the disease process.
Palmar Angle Palmar angle refers to the angle of the palmar or plantar margin of PIII relative to the ground surface. It can be measured relative to (a) the ground surface of the hoof capsule, or (b) the ground itself. In the first instance, (a), the angle is largely unrelated to the mechanics of the shoe or other device that may be attached to the foot. It provides information about the structural integrity of the soft tissues in the heel area, especially the digital cushion. With the second method, (b) the palmar angle is also indicative of the mechanical effect of any shoe/device that is attached to the foot (Fig. 18A).
In most healthy feet with strong heels and a robust digital cushion, the palmar angle is positive, meaning that the wings of PIII are higher than the apex (Fig. 7B). As with most other indices, the range of normal for palmar angle is dependent, in part, on the horse's breed. Breeds that tend to have upright hooves typically have higher palmar angles than breeds with naturally lower hoof angles. The shoeing package can also affect the palmar angle, which must be borne in mind when measuring palmar angle relative to the ground.
A high palmar angle (relative to the range of normal for that breed) may be found in horses with club feet, laminitis, and certain other pathological conditions. A negative palmar angle (wings of PIII lower than the apex) indicates substantial loss of structural integrity in the heel area, a situation that can usually be predicted simply by looking at the foot and estimating the depth of the digital cushion.
Qualitative Assessment In addition to these measurements, a high-quality radiograph taken at a soft exposure (see below) can reveal variations in radiodensity within these soft tissue zones. For example, even in a normal foot there is a subtle yet distinct change in radiodensity between the laminar corium and the cornified inner layers of the dorsal hoof wall. Evaluating the soft tissue zones around PIII is particularly important in the diseased foot, as congestion, edema, or accumulations of inflammatory exudate or gas can alter the radiodensity of the tissue, in addition to altering its thickness.
Thus, a lot of useful information regarding the soft tissues of the hoof can be obtained, either directly or by inference, if one only looks for it. This approach is particularly useful in the lame, footsore horse that has no radiographic abnormalities on "standard" foot films (i.e. no obvious bone pathology). Careful evaluation of the soft tissue zones surrounding PIII often reveals interesting details to the trained eye. As with clinical examination, it is important to develop an eye for fine detail and an appreciation for the range of normal (relative to breed, age, environment, and use) in order to get the most out of a radiographic examination.
Exposure Settings The coffin bone differs greatly from other bones in the limb, in that it is surrounded by a dense, cornified shell whose thickness, density, and water content affect radiographic detail of the bones and soft tissues it encases, and even of the capsule itself. These characteristics of the hoof capsule must be factored in to the radiographic technique. Another unique feature of PIII that must be considered is its shape. The thickness and density of the bone differs markedly from proximal to distal, the bone being very thin and lacelike at its palmar/plantar margin. In certain pathological conditions, this area can become even less radiodense. The radiographic technique must factor in this normal variation in bone thickness and density. The coffin bone cannot be thoroughly evaluated from just a handful of films, no matter how good the films.
Adequately imaging the navicular bone and adjacent structures presents yet another challenge. Not only are the navicular bone and related structures encased within the hoof capsule, they are surrounded on three sides by PIII (and, on some views, overlaid by PII), so superimposition of bone also must be factored in to the radiographic technique. Thus, thoroughly examining all of the structures within the foot requires several views and different exposure settings, each one tailored to best image the structure of primary interest.
Figure 10. (A) This soft exposure has farrier interest; it clearly defines soft tissue parameters as well as soft tissue lesions. Note opaque lesions proximal to the navicular bone. Palmar angle of navicular bone can be measured from this view as well.
(B) This 65 degree DP was taken with a very soft exposure, clearly defining the thin margin and multiple cystic lesions.
Selecting Exposure Settings For any radiographic view, the ideal exposure setting will depend on the equipment used (x-ray machine, screens and film, processor, etc.), hoof mass, and the structure of primary interest. I use the terms soft, medium, and hard to describe the exposure settings I select for a particular view, depending on which tissue I am most interested in evaluating. (Note: Specific values for kVp and mAs will depend on the equipment used and the size of the foot being examined, so it is not possible to provide even general guidelines here.)
Soft exposures are for nonbony tissues, such as the horn and corium of the hoof wall and sole, and for the palmar margin of PIII (Fig. 10A). A good soft exposure can reveal differences in radiodensity within the hoof wall which allows differentiation between the laminar corium and the keratinized layers of the hoof wall. When looking for abnormalities at the palmar margin of PIII on the 65 degree dorsopalmar (DP) view, a very soft exposure is needed. If the perimeter of PIII cannot be seen without the use of a hot light, the view should be retaken at an even lower mAs (Fig. 10B).
Medium and hard exposures are used when the structure of interest is bone. Medium exposure is used for bony structures of moderate density or thickness, such as the body of PIII, and for articular surfaces. Hard exposure is used for denser bone or superimposed structures, in particular the navicular bone. I always use a 6:1 parallel grid when using a hard exposure setting.
As this approach illustrates, it is important to tailor the settings to the goal of the examination-to the structure you are most interested in evaluating. "Underexposed" is a relative term. What may seem grossly underexposed to others may be the perfect exposure to show soft tissue detail within the hoof wall or sole, or the palmar margin of PIII.
Unless taking radiographs simply to guide farriery decisions, I take at least two exposures for each view: one soft and one bone detail (medium or hard) exposure. Soft tissue detail is essential, as the nonbony structures surrounding PIII are an integral part of virtually every foot problem.
Factors Affecting Image Quality The diagnostic value of any radiographic examination is determined by the capability of two basic factors: the equipment and the examiner. The skills and knowledge of the examiner are as important as the choice and maintenance of the equipment (x-ray machine, cassettes, screens, film, developing and marking systems, positioning blocks). Not only does the examiner need a good working knowledge of clinical and radiographic anatomy of the foot (including an understanding of the range of normal), s/he needs good radiography skills, from a basic grasp of the geometry of radiation to experience with taking routine radiographic views. Good horsemanship skills are also important.
A disciplined, methodical protocol, designed to provide as much information as possible, is of primary importance if one is to get the most diagnostic value from radiographic examinations. Techniques for ensuring high quality radiographs of the equine foot are described in detail elsewhere1 and will only be summarized here.
Several different factors can affect image quality, and thus limit the amount of accurate information you can obtain from your films:
Preparation of the foot-thoroughly clean the foot of all debris, paying particular attention to the frog sulci.