WSVMA Annual Conference

The Equine Digit: Functional Anatomy and Clinical Implications

Spokane Convention CenterSpokane, Washington

October 1-3, 2010

Andrew Parks, VetMB, MS, DACVSUniversity of Georgia

College of Veterinary Medicine

Andrew Parks, Vet MB, MS, Diplomate ACVS University of Georgia

Athens, GA Biography: Dr. Parks received his Vet MB from the University of Cambridge in 1981. Dr. Parks is a Diplomate of the American College of Veterinary Surgeons and received his certificate in Veterinary Radiology from the Royal College of Veterinary Surgeons in 1982. He currently serves as Professor of large animal surgery at the University of Georgia. Dr. Parks has made over 160 professional presentations and has peer reviewed and authored articles and book chapters. His clinical interests are large animal surgery lameness and diseases of the foot.

Contact Email: parksa@uga.edu

Form and Function of the Equine Digit Andrew Parks, DVM

Dept. Large Animal Medicine, University of Georgia, Athens, GA 30602

The equine foot is not strictly defined by the NAV, but is generally taken to mean the hoof and all the structures contained within it. The digit refers to all structures distal to the metacarpophalangeal joint. For the purposes of this discussion, functional anatomy is used to both describe the anatomical structures as static entities and how these structures interact in motion. Functional anatomy of the foot is important to clinicians because it influences how surgical approaches to structures within go the foot are made, how the foot is stabilized after injury, and how lameness problems arising within the foot are treated. Additionally, increasingly sophisticated diagnostic techniques necessitate increasingly precise knowledge of the structures within the foot.

The anatomical structures within the foot/distal digit include the middle and distal phalanx, and distal sesamoid bone, ligaments, interphalangeal joints, common digital extensor and deep digital flexor tendons, the integument, and associated vessels and nerves. For the purposes of this discussion these structures will be discussed as part of two functional units, the distal interphalangeal joint, and the hoof.

Distal Interphalangeal Joint

The distal interphalangeal joint is complex. It has three articulations, between the middle phalanx and the distal phalanx, between the middle phalanx and the distal sesamoid, and between the distal phalanx and the distal sesamoid bone. The bones are maintained in apposition by the collateral ligaments, the collateral sesamoidean ligaments, and the distal sesamoidean impar ligament. The head of the middle phalanx, has two condyles separated by a shallow groove that conform to the opposing articular surfaces of the distal phalanx and distal sesamoid bone. There is minimal movement in the articulation between the distal phalanx and the distal sesamoid so that they essentially function as a singular articular surface. The shape of the articular surfaces and the constraints of the ligaments ensure that the primary plane of motion is flexion and extension, but the shallowness of the intercondylar grove of the middle phalanx and corresponding low saggital ridge of the distal phalanx permit significant rotation, sliding and collateromotion.

Hoof

The hoof itself is the integument of the foot. As such, it has three principle layers, the subcutaneous tissue, dermis, and epidermis. The epidermis is further subdivided into the stratum basale, stratum spinosum, and stratum coreum. The stratum basale and stratum spinosum are frequently referred to collectively as the stratum germinativum. The stratum corneum of the hoof forms the hoof capsule. The hoof is divided into 5 regions, limbic (perioplic), coronary, parietal (lamellar), solar, and cuneate. Each region shows specialization within each layer. For example, the subcutaneous tissue of the coronary integument forms the coronary cushion, the parietal subcutaneous tissue forms the periostium of the parietal surface of the distal phalanx and ungual cartilage, and the subcutaneous tissue of the cuneate integument forms the digital cushion. The specialized junction of the epidermis and dermis to form papillae and rete pegs in the limbic, coronary, solar and cuneate regions and intergitating lamellae in the parietal region is well documented. The three layers of the wall, stratum internum, stratum medium, and stratum

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externum, are formed by the stratum corneum of the limbic, coronary, and parietal integument respectively. The distal phalanx is considered to be suspended within the hoof capsule by the lamellae around approximately 80% of the circumference of the foot. Palmarly, the deep digital flexor tendon supports the distal phalanx and the distal sesamoid bone.

The hoof wall is not uniform through out its thickness. The density, size, and structure of the horn tubules vary substantially from the outer wall to the inner wall. But more importantly, the alignment of the keratin fibrils within the intertubular horn varies across the substance of the wall, and it is the intertubular horn that froms the basis for much of the mechanical properties of the hoof capsule. The properties of the wall are comparable around the circumference of the hoof, but because it is thickest dorsally and thinnest palmarly, it is more flexible palmarly than dorsally. The hoof wall is viscoleastic. The outer wall is stiffer than the inner wall. The hoof capsule is more fracture resistant than bone, and it is structurally designed to deflect all microfractures away from the underlying sensitive tissues regardless of which plane the fracture is made in. The stride and weight-bearing

The stride is divided into suspension and stance phases. The stance phase is further subdivided into initial contact, impact, support, and breakover phases. Initial contact usually occurs laterally at either the heel or quarter. Less frequently the foot lands flat, and toe or medial first landings are rare in healthy horses. As the body descends up to the midpoint of the weight-bearing phase of the stride, the metacarpophalangeal joint dorsiflexes and the distal interphalangeal joint flexes. Therefore, structures that extend across the flexor surface of the the MCPJ, but not the DIPJ, namely the superficial digital flexor tendon and the suspensory ligament/distal sesamoidean ligaments, come under tension, lengthen, and absorb energy. In contrast, a structure that extends across both joints lengthens minimally. During the second half of the weight-bearing phase of the stride, the distal interphalangeal joint extends, so that structures that extend across both joints, or just the distal interphalangeal joint come under greater tension, structures such as the deep digital flexor tendon and its associated accessory ligament, and the collateral sesamoidean ligaments of the distal sesamoid and the distal sesamoidean impar ligament.

Weight-bearing by the limb during the stride is discussed in the context of the ground reaction force. The ground reaction force is broken into 3 components: vertical, horizontal in the direction of travel, and horizontal perpendicular to the direction of travel. The vertical component can be considered to be weight-bearing, and the horizontal component in the direction of travel can be considered to be breaking and propulsion. The graph of the vertical component of the ground reaction force against time forms an inverted U in a trotting horse; i.e. the force is least at the beginning and end of the stride and greatest at mid-stance. Superimposed on this graph during the impact phase of the stride are high frequency vibrations, which are thought to be responsible for much of the injury associated with athletic performance. There are several mechanisms to dampen the forces of impact: hoof viscoelasticity, a hydrodynamic mechanism involving the vascular channels within the ungular cartilages, and the viscoelasticity of the articular cartilage of the interphalangeal joints. Although, this damping occurs at several levels, the majority of it occurs within the hoof, that is, in the hoof capsule and the underlying layers of the integument that attach the hoof capsule to the distal phalanx.

Examination of the Foot Andrew Parks, DVM

Dept. Large Animal Medicine, University of Georgia, Athens, GA 30602 DIAGNOSIS AND CLINICAL SIGNS

The presenting symptom is one manifestation of abnormal form or function, usually lameness or abnormal appearance. This initial symptom directs the course of clinical investigation for more subtle symptoms to better define the disease process to obtain the most accurate diagnosis possible. The presenting symptom may obviously identify the horse's foot as the site of disease, However, the source of the problem is not obvious in many horses with lameness originating from the foot. Therefore the breadth of the examination may be determined by the specificity or vagueness of the presenting symptom(s).

The basic principles or physical examination of the foot follow those for the rest of the musculoskeletal system whenever possible: visual observation, palpation for pain and heat, flexion and extension. Emphasis is given to identifying the cardinal signs of acute or chronic inflammation, the commonest causes of disease in the musculoskeletal system.

It is the structure of the integument of the foot that makes it different from the rest of the limb. The hoof offers clues that are not present elsewhere, yet the rigid hoof capsule inhibits basic palpation of the structures within the foot. Therefore it is important to develop an ability to "read" the hoof capsule (which seems to be a life long process). Visual examination of the hoof capsule in conjunction with the phalangeal axis from the lateral, medial, dorsal, palmar and solar aspects to compare the balance and conformation of the distal limb with an "ideal balance and conformation" is straightforward. Closer observation reveals more subtle signs. Examination of the coronary band should show any local areas of proximal or distal displacement. Examination of the growth rings below the coronary band should show local variations in the spacing of the growth rings. By sweeping ones hand on the outside of the hoof from the coronary band to the weightbearing surface, convexities or concavities in a proximal to distal direction can be identified. Similarly, by sweeping an outstretched hand around the hoof from the medial to lateral heel, deviations from the normal smooth curvature can be identified as increased or decreased convexity and even local concavity can be identified. Palpation of the coronary band in a similar manner may reveal lipping when the coronary band extends abaxial to the wall. Taking any one of these more subtle symptoms alone can be misleading, but taken together they can provide supporting evidence. For example, local coronary band displaced proximally at the junction of the toe and the quarter may be accompanied by growth rings that are closer together and an exaggerated convexity in the hoof capsule at that point in the horizontal plane. Taken together, these findings are very suggestive that there is excessive compressive vertical stresses within the hoof wall at that point. But if the coronary band is displaced without the difference in spacing of the growth rings or convexity, more caution is needed before this assertion can be made.

Having examined the outside of the foot, is important to maximize the information we can learn about the inside of the foot. Heat within the foot can often be readily appreciated. However, because pain within the hoof is rarely appreciable on digital palpation, compression of the hoof with hoof testers or percussion with a hammer is necessary to localize pain. Cleaning the weightbearing surface of the foot by lightly removing the surface layer of the sole, frog and

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distal wall indicates any potential entry sides for infection, usually seen as small dirt filled cavities. Also visible may be speckled red discoloration of the horn indicating hemorrhage into the hoof capsule from underlying trauma. Interestingly, hemorrhage within the hoof maintains its blood red color rather than turning black and blue as bruises do elsewhere. Hemorrhage within the wall represents a past event and not necessarily an ongoing process, though of course, it may still be.

Various ancillary diagnostic aids are used to gain further information about the disease process: exploration, regional and intra-articular anesthesia, radiography, ultrasound, nuclear scintigraphy and thermography. Removal of exfoliating hoof and careful exploration of tracts is indicated if infection is suspected.

In summary, examination of the equine foot offers many challenges. While there are a finite number of tissues within the foot and a limited number of pathological processes that can affect these tissues, this knowledge does us no good unless we can put the two together. With the advent of ultrasound and scintigraphy, we can now make diagnoses that were impossible 20 years ago; for example, insertional tenopathy of the deep digital flexor tendon. With the development and application of newer technology, the obvious examples being computed tomography and magnetic resonance imaging, we will be able to make diagnoses in the future that are currently impossible; for example, sprains of the many small ligaments within the foot.

The Horse Shoe: Design and Function Andrew Parks, DVM

Dept. Large Animal Medicine, University of Georgia, Athens, GA 30602

A. Form of a Horse Shoe 1. Materials

Steel Aluminum Synthetic polymer Composite

2. Dimensions and weight

Width of web Thickness of web Length of stock Density of material

3. Cross-sectional profile

Edges – rolling / beveling Rims / swedging Creases / fullering Seating out / slippering

4. Extensions

Primary function as a lever Static and dynamic effects Can be placed anywhere around the periphery of the shoe Medial and lateral function similarly Dorsal and palmar function differently

5. Bars

A piece of stock that extends from one branch of the shoe to the other Stability Increase ground surface area Protection Support Extension

6.i Pads: Form

Full Rim Wedge Custom

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6.ii Pads: Function Protection Shock absorption Change angle Change length Recruit weightbearing Add weight

7. Shoe Attachment

Nails Adhesives Clips Casting tape

B. Function of a Horse Shoe

1. Protection

Increase the width of the web Add a bar Pad, full or partial

2. Balance:

Trimming Branches of different thickness Wedge pads Shims Acrylic

3. i Traction: Determining factors

Materials of opposing surfaces Contour of opposing surfaces Surface area of contact

3.ii Traction: Methods of changing

Creases Rims Calks/grabs

4. Breakover

Length of toe Angle of distal phalanx Position of breakover

5. Animation

Weight of shoe Length of toe

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6. Support To maintain anatomical alignment To reduce stress

C. The Practice of Shoeing

1. Effect of shoes on foot function

Decrease hoof expansion Increase maximal stress of impact Change pattern of wear May influence foot growth in young horses? Change the shape of the distal phalanx? Change the quality of the hoof wall

2. Shoeing horses for performance

Maintain basic principles Discipline specific Compromises common Knowledge by experts in field

3. Corrective Shoeing

Symptomatic shoeing Disease specific shoeing

4. Lameness associated with shoeing

Poor trimming Poor shoe fit Poor shoe selection Poor nail placement

Balance, Support, and Foot Wounds Andrew Parks, DVM

Dept. Large Animal Medicine, University of Georgia, Athens, GA 30602 Balance

Balance is a controversial subject. Traditionally, it is primarily determined by a set of anatomic guidelines related to the symmetry of the foot and distal limb in the frontal plane and the angles of the pastern, dorsal hoof wall, and heels in the median plane. This is traditionally supplemented by symmetry of landing with the horse in motion. While these may still act as approximate guidelines, it is now known that absolute symmetry in the frontal plane and symmetry of landing are not the norm. Therefore, it is better to consider balance as a concept. Additionally, it is helpful to separate out conformation and balance, albeit somewhat arbitrarily. As such, conformation represents the shape and size of the musculoskeletal structures of the limb as seen at rest. Correspondingly, balance refers to the relationship between the hoof and the underlying structures and between the hoof and the ground. Therefore, balance is both a static and dynamic concept. This division between conformation and balance is advantageous because conformation in adult horses changes little if at all and then only slowly, and attempts to change it in adult horses usually have deleterious consequences. In contrast, balance can change quickly in adult horses, and modifying balance is commonly performed to reduce lameness and improve performance. Support

There are many injuries to the distal limb in which the stresses must be reduced in the injured tissues. The object of treatment is to support the tissue. However, the term support is widely misused. Additionally, with the best of intentions, support is misapplied. A shoeing practice may reduce the stress in one tissue but not another, yet the same practice is widely used for both. How to use shoeing to help an injured foot can usually be worked out from first principles by determining whether the structure is under stress from compression or tension. For example, it is common to use a mild abaxial extension to support one side of the foot. If the structure is stressed under compression, e.g. bone, the support should be on the contralateral side of the limb to shift weightbearing to the other side of the limb and reduce compression in the bone. In contrast, if the structure is stressed under tension, e.g. a collateral ligament, then the extension should be placed on the same side of the limb to prevent the two sides of the joint separating and the ligament being stretched. Wounds and Wound Healing

Foot wounds take many forms, from simple abscesses, to hoof wall avulsions, to punctures of structures deep within the foot. Despite the varied appearance and presentation of these wounds, there are certain common features in the way they heal and are treated that warrant consideration in general terms to avoid repetition.

Abrasions to the coronary band and iatrogenic hoof wall avulsions heal in a similar way to

partial thickness skin wounds. Remnants of germinal epithelium are distributed across the surface of the wound so healing is primarily by epithelialisation. Defects that involve the full thickness of the coronary band, wall, sole, or frog follow the classical 4 phases of wound healing.

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Following an injury, however, the hoof wall does not retract nor in the repair phase do the wound margins contract. The epithelium that covers the wound may have diverse characteristics depending upon whether it originated from the coronary band, wall, sole, frog, or pastern and excessive granulation tissue on the surface of the foot is uncommon.

Laminitis I & II: Acute and Chronic Laminitis – An Overview Andrew Parks, DVM

Dept. Large Animal Medicine, University of Georgia, Athens, GA 30602 Laminitis is usually classified by its duration and the morphological changes that have occurred.1 As such, developmental laminitis is the initial phase between the initial insult and the onset of clinical symptoms. Acute laminitis begins with the onset of clinical symptoms and is frequently cited as lasting until 72 hours after the onset of symptoms or the development of displacement of the distal phalanx, whichever is the sooner. If the distal phalanx does not displace after 72 hours, the disease enters the sub-acute phase. Chronic laminitis begins with the displacement of the distal phalanx regardless of whether the horse was in the acute or sub-acute phase of the disease. Chronic laminitis is further subdivided based on time and the amount of healing that has taken place. Early chronic laminitis is that phase immediately after the distal phalanx has displaced; the wall of the hoof capsule is grossly unchanged. This phase is succeeded by chronic active laminitis in which the distal phalanx remains unstable, but secondary changes in the hoof capsule are beginning to become evident. Chronic stable laminitis, as its name implies is that stage of the disease in which the deformed shape of the foot is accompanied by stability of the distal phalanx. The classification of laminitis into phases is a convenience to enhance comprehension and assist in the diagnosis, treatment and prognosis of the disease, but the disease is a continuum. Horses tend to sequentially pass from one stage to the next on an unpredictable basis, and the boundaries between the stages are blurred. However, the continuum varies greatly between horses as they may take different entry points into the disease and different paths once affected. Pathophysiology The pathogenesis has yet to be clearly defined. Several mechanisms have been implicated over the last 30-40 years including dysfunction of the digital vasculature, a primary inflammatory response, toxin induced activation of metalloproteinases, and coagulopathy and thrombus formation.2 It is quite likely that all of these mechanism may be involved in some manner. Recently, unequivocal evidence confirms an inflammatory response very early in the disease has before other changes are present, suggesting that the vascular changes, thrombi formation, and metalloproteinase degradation of the basement membrane are “downstream” events.2 As the strength of the lamellae becomes decreased during the disease process, the capacity of the lamellae to maintain the position of the distal phalanx suspended within the hoof capsule decreases, the distal phalanx becomes unstable, and the distal phalanx displaces within the hoof capsule. The mechanical collapse of the lamellae can occur at any point around the circumference of the foot leading to different patterns of collapse. When the dorsal lamellae collapse, (dorsal) rotation ensues, when all the lamellae appear to collapse simultaneously the distal phalanx displaces distally, and when the lamellae on one side of the foot collapse, usually medial, unilateral distal displacement occurs.

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Following displacement of the distal phalanx, the newly developed cavity between the hoof capsule and distal phalanx fills with blood and necrotic material. The lamellae in the damaged area become hyperkertotic and hyperplastic forming what is frequently referred to as a “lamellar wedge.” With healing the distal phalanx may become progressively more stable within the hoof capsule, but frequently the hoof capsule becomes distorted and an abnormal relationship between the distal phalanx and hoof capsule and phalangeal axis persists. The outcome is dependent on several factors including the alignment of the dermal papillae of the coronary integument, the character of the lamellar wedge, the growth patterns of the capsule, the tension in the deep digital flexor tendon, and the pattern of weight-bearing. Diagnosis and Assessment The diagnosis of laminitis is usually straightforward. In horses with acute disease the diagnosis is made based on the characteristic gait, palpation of heat within the foot, increased digital pulses, and localization of pain with hoof testers. In horses with chronic disease, visual inspection of changes in the hoof capsule, palpation of the coronary band and radiographs provide additional information that allow the disease to be further characterized, characterization which, in conjunction with the clinical symptoms, is important in forming a prognosis and strategy for treatment. In rare instances, acute or subacute laminitis may present with sufficiently subtle symptoms to present a difficulty diagnosis, though with time the symptoms are likely to become more classic. Routine radiographic examination fo the foot should include at least three views, a lateromedial, a dorsopalmar, and a dorsopalmar oblique. The lateromedial view is used to assess the position of the distal phalanx within the hoof capsule in the saggital plane, the presence of gas within the wall and sole, and secondary changes to the distal phalanx. Of particular importance is the thickness of the sole, the angle of the solar margin of the distal phalanx to the ground, the degree of capsular rotation, and the vertical distance from the firm proximal border of the wall to the extensor process.3.4 The dorsopalmar view is used to assess the position of the distal phalanx in the hoof capsule in the frontal plane. Normally, the width of the distal interphalangeal joint space should be symmetrical, the articular surface of the distal phalanx approximately parallel to the ground, and the thickness of the medial and lateral walls adjacent equal. Marked tilting of the distal phalanx to one side accompanied by an increase in hoof width on the same side and narrowing of the distal interphalangeal joint on the opposite side are strongly suggestive of asymmetrical distal displacement. Care should be taken to assess the positioning before determining this because tilting of the distal phalanx and asymmetry of the distal interphalangeal joint can be induced if the limb is not close to vertical. Venography has been used to assess perfusion of the digit. Decreased perfusion in the coronary, parietal, and subsolar vasculature have been associated with a decreased prognosis.5 Medical Therapy Medical therapy is aimed at preventing, limiting progression, and reversing the underlying pathophysiological processes and controlling pain by adminstration pharmacological agents.2,6 As such, the drugs used to affect the underlying pathophysiological processes are directed at the four mechanisms postulated to cause the disease. To counter vascular dysfuntion in the digit,

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various vasodilators and rheologic agents have been used, and of these the most commonly used and likely to be beneficial is acepromazine. The main pharmacologic agents used to treat the inflammatory response in early laminitis are the non-steriodal anti-inflammatory drugs and DMSO, and because of the overlap between endotoxemia and inflammation, these non-steriodal anti-inflammatory drugs are also beneficial in horses with endotoxemia. Antiendotoxin therapy is frequently used in horses that clinically appear to be endotoxic. Of these, endotoxin antiserum and polymixin B are most frequently used. Heparin and aspiring have both been used to treat coagulopathy and prevent thrombus formation n early laminitis. Unfortunately, no pharmacological agents are of proven benefit once the initiating events have occurred. Of all these agents, the most frequently used to treat uncomplicated laminitis are phenylbutazone, flunixin meglumine, DMSO, and acepromazine. However, many other drugs are used to treat diseases such as diarrhea, pleuropneumonia, and metritis that are commonly associated with the onset of laminitis, such as antibiotics, fluids, anti-endotoxin serum, and polymixin B. The pharmacologic control of pain is an area that until recently has not received sufficient attention other than the traditional mainstay of non-steroidal anti-inflammatory drugs. More recently, interest in using other agents such as lidocaine and ketamine given as an intravenous infusion, narcotics either as a continuous rate infusion or epidural, and gabapentin has developed.6 The use of these drugs to control pain must be paralleled by advances in assessing and monitoring pain.7 Concurrent pituitary pars intermedia dysfunction and equine metabolic syndrome must be managed appropriately. Supportive Care The goals of supportive therapy shift during the course of the disease, though the principles used to achieve the goals remain remarkably similar. In the acute and subacute stages of the disease, the goals are to prevent displacement of the distal phalanx by attempting to stabilize the distal phalanx within the hoof capsule and to control pain. Immediately following distal displacement the goals are to limit additional displacement and control pain. As the disease progresses further and the distal phalanx becomes stable within the hoof capsule, the goals become realignment of the distal phalanx with the other phalanges and the ground, and realignment of the hoof capsule with the distal phalanx. Additionally, other complications of chronic laminitis must be managed. The supportive care of horses with acute and chronic laminitis follows similar principles, however there are differences in application.8,9 The distal phalanx is stabilized by reducing stress on the most severely affected lamellae and by decreases the stresses within the lamellae associated with locomotion. This is achieved by moving the center of pressure, adjusting the distribution of pressure, and the rate at which the load is applied. The latter is accomplished by altering the contour of shoes or shoe like devices to both diminish and smooth out the moment about the distal interphalangeal joint. There are numerous ways to achieve these goals and many of them are interchangeable, and clinician preference is frequently based on past experience. In the early stages of the disease devices applied to the feet to accomplish these goals tend to be temporary in nature so that they

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are easy to apply, adjust, and remove as circumstances change. This also allows rapid assessment of the effect of the intervention. Therefore, the most frequently used types of support are customized Styrofoam board, moldable silicone putty, and commercial cuff and wedge combinations, all of which can be taped in place. There is a general assumption that the pain a horse is experiencing is related to both the stress in the lamellae, and the pressure on the sole distal to the distal phalanx. There may be other as yet unidentified sources of pain. Thus the effect of a treatment may be assessed by changes in comfort of the horse. By extension, it may be possible to glean information about the distribution of lamellar injury by observing a horse’s response to different treatments; this is particularly useful in horses with acute or subacute laminitis (in horses with chronic laminitis, the pattern of displacement provides you with similar information). The center of pressure can be shifted by the use of extensions or wedges; the center of pressure moves towards the side of the extension or elevated side of the wedge. The distribution of pressure can be spread out over a greater area by changing the width of the shoe or by filling all or part of the space between the branches of the shoe with a material contacts the sole and the ground; it can be done selectively to load one part or the sole and frog over other areas. Elastic/viscoelastic materials can extend the duration of loading to diminish the shock of contact with the ground. Easing the moment about the distal interphalangeal joint is most important in the saggital plane, the natural plane of motion of the distal interphalangeal joint, but to a lesser extent in the frontal and transverse planes in which collateromotion and rotation also occur within the joint. This is accomplished by rounding or beveling the outer rim of the shoe, or the addition of rails. When a device is applied that covers the entire ground surface of the foot such as a wooden shoe,10 then the entire ground surface can be modified to roll or bevel the margins on one or more sides, and can extend as far as needed towards the center of the device. Other aspects of supportive care include ice therapy,11 limb wraps to control edema, and care of pressure sores. Additionally, appropriate nutritional support must be provided. Surgery Deep digital flexor tenotomy is performed when the distal phalanx continues to rotate after all other measures to stablize it have failed, there is persistent pain in horses with phalangeal rotation that does not respond to other treatment, or in horses that have developed a secondary contracture. Drainage of purulent material is performed as necessary, and is best achieved through the distal wall to preserve the integrity of the sole. Debridement of the distal phalanx must be undertaken with great caution and only in circumstances in which probing the distal aspect of the limb demonstrates exposure of the bone. This is because radiographic evidence of distal phalangeal lysis is far from diagnostic for septic pedal osteitis. Hoof wall resections and resections are techniques that have been used to improve the alignment of the wall with the distal phalanx.

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Prognosis The prognosis for horses with acute laminitis varies greatly with the severity of the initial disease and the ensuing progression of lamellar injury, displacement of the distal phalanx, and distortion of the hoof capsule. It is prudent to give a guarded prognosis regardless of the symptoms because of the propensity of the disease to recrudesce, but in horses with severe disruption of the lamellae with correspondingly severe clinical signs, the prognosis for survival must be poor. For those horses that have successfully progressed to chronic stable laminitis, the prognosis for survival is better, and the severity of the secondary changes to the hoof capsule are likely to give an indication regarding the necessity for indefinite hoof care. References: 1. Parks AH, Mair TS. Laminitis: A call for unified terminology. Equine Vet Education, 2009;

21: 102. 2. Belknap JK, Parks AH. Laminitis associated with acute abdominal disease, In The Acute

Equine Abdomen, Eds. White, Moore and Mair 3. Cripps, P.J. and Eustace, R.A. Factors involved in the prognosis of equine laminitis in the

UK. Equine Vet. J. 1999; 31: 433 4. Redden, R.F. Clinical and Radiographic Examination of the Equine Foot. Proc. Am. Assoc.

Equine Pract.2003; 49: 174. 6. Peroni JF. Pharmacologic management of laminitis, In: Current Therapy in Equine Medicine,

6th edition, Eds Robinson and Sprayberry. 2009; 543. 5. Redden RF. A technique for performing digital venography in the standing horse. Equine Vet

Education, 2001; 13: 172. 7. Bussières G, Jacques C, Lainay O, et al. Development of a composite orthopaedic pain scale

in horses. Res Vet Sci 2008; 85 :294. 8. Parks AH. Treatment of acute laminitis. Equine Vet Education, 2003; 15: 273. 9. Parks AH. O’Grady SO. Chronic laminitis, In: Current Therapy in Equine Medicine, 6th

edition, Eds Robinson and Sprayberry. 2009; 550 10. O’Grady SE , Steward ML. The wooden shoe as an option for treating chronic laminitis.

Equine Vet Education, 2009; 21: 107 11. van Eps AW, Pollitt CC: Equine laminitis: cryotherapy reduces the severity of the acute

lesion. Equine Vet J 2004; 36: 255.

Equine Lameness Andrew Parks, DVM

Dept. Large Animal Medicine, University of Georgia, Athens, GA 30602 THE MECHANICS OF EQUINE LAMENESS We are all taught that when a horse that is lame in a forelimb trots, the head rises when the lame foot is on the ground and the head drops when the sound foot is on the ground. However, if you watch a sound horse jog, its head drops when each foot hits the ground. So the head movement appears monophasic when the horse is lame, but biphasic if the horse is sound. How can we explain this dichotomy? To best explain this it is simplest to look at what the head, trunk and limbs are doing during a sound stride and then compare that to a lame stride. During a normal trot, as the horse lands from the suspension phase of the stride, the weight of the horse causes the metacarpophalangeal joint to dorsiflex in proportion to the amount of weight on the limb. The dorsiflexion of the limb causes the vertical height of the limb to shorten. The limb is shortest approximately at the middle of the weight bearing phase of the stride. Therefore, the height of the trunk in relation to the ground is lower because the horse has descended from the flight phase of the stride and the forelimb has shortened. After the middle of the weight bearing phase of the stride, by a combination of elastic shortening of tendons and ligaments supporting the metacarpophalangeal joint and muscular contraction, the metacarpophalangeal joint extends (straightens) and the animal lifts of the ground. This happens twice each stride, once for each forelimb. At the trot in a sound horse, the head more or less moves in unison with the trunk. A lame horse at a trot is reluctant to load the lame limb as much as a sound horse. Therefore the trunk does not descend as much. Furthermore, there is a compensatory increase in the load of the lame limb in the stance phase. Therefore, the trunk of the horse does not descend as much when the horse is bearing weight on the lame limb as it is when it is on the sound limb. Therefore, the biphasic nature of the movement of the trunk remains, but it is no longer symmetrical, and it does not explain why the head appears to go up when the lame leg lands. The explanation for the latter is in large part explained by a change in the relationship between the movement of the head and the withers. In a lame horse, the movement of the head is in the same direction as the movement of the withers, but the magnitude of the movement is now greatly exaggerated. The end result is that the movement of the head is biphasic for a lame horse, but the drop of the head is much reduced (varying with the severity of lameness) so that to the clinician, the drop is not perceptible. Therefore, the horse’s head appears to stay high during the weight bearing phase on the lame limb, giving the impression that the movement of the head is monophasic, which explains the traditional mantra. There is another twist to the explanation. That is, it assumes the horse feels greatest pain during the midpoint of the weight bearing phase of the stride. If the pain is greatest at the beginning or end of the weight bearing phase of the stride or during protraction of the limb, then the movement of the head and trunk will be differ, though not necessarily enough for the clinician to appreciate. In addition to changes in the movement of the head and trunk, the stride length is reduced and the suspension phase of the stride following weight bearing by the lame limb may be a significantly shorter in proportion to the weight bearing phase.

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For detecting hind limb lameness we were all taught to look for a hip hike when the lame leg is on the ground, a hip drop on the lame side when the sound leg is on the ground or for the side with the greatest excursion of the tuber coxae during the course of the stride. But what is really going on. Again, it is best to examine the movement of the pelvis of a sound horse at a trot. The front end differs from the back in that the structure that the clinician focuses on, the head, is in the midline, whereas the structures the clinician focuses on in the hind limbs are the tuber coxae (hips), which are not on the midline. Consequently, the pelvis as a whole moves up and down twice a stride, once when each limb is weight bearing, but the pelvis rotates only once a stride, so the overall motion of the tuber coxae is the result of combining the two. It is simplest to start by examining the movement of the pelvis as a whole represented by the position of the tuber sacrale. In a sound horse the tuber sacrale show a biphasic rise and drop much like the whithers, and of similar magnitude. In a lame horse, as in the forelimb, the lame limb is loaded less during weight bearing than the sound limb, so that the whole pelvis does not drop as much. Therefore, there is a trend for the movement of the pelvis to more closely resemble the monophasic movement of the head. However, because there is no equivalent amplification that the head provides in the front, this is harder for the clinician to see. Therefore, clinicians watch the tuber coxae. The tuber coxae do provide some amplification of the movement of the movement of the tuber sacrale. In the sound horse the pelvis is moving up and down rhythmically in a biphasic fashion with each stride and the pelvis rotates through a cycle once a stride. However, because the whole pelvis is moving up and down twice a stride the tuber coxae moves up and down each stride, but how much the one side of the pelvis drops with each half of the stride depends on where in its rotation the pelvis is. The pelvis tilts down away from the weight bearing limb in the stance phase. Therefore, when the right leg is bearing weight the pelvis is tilted down to the left and the left tuber coxae is lower than the right, and when the left leg is bearing weight, the pelvis tilts down to the right so the right tuber coxae is lower than the left. Therefore the left tuber coxae has two low points, but the one during which the right limb is bearing weight is lower. When a horse is lame in a hind limb, the leg doesn’t shorten as much as the sound when it is on the ground, therefore the pelvis doesn’t drop as much as when the horse is bearing on the sound limb. Consequently, the tuber coxae of the lame limb shows the most pronounced dip when the sound limb is on the ground. This is in line with the observation that the lame side has the greatest hip dip and greatest range of excursion. Inter-relationship between hind and forelimbs in a lame horse. If a horse is lame in a forelimb, it may cause sufficient asymmetry in the movement of the pelvis to mimic a lameness in the contralateral hindlimb, but this is an inconsistent finding. If a horse is lame in a hind limb, it frequently changes the action of the head to suggest a concurrent lameness, though of lesser magnitude, in the ipsilateral forelimb; this is a much more consistent finding. Both these findings are likely to be more noticeable the more marked the original lameness. This information may be helpful to differentiate whether a horse has lameness originating from two limbs or not. If a horse has a primary (more noticeable) lameness in a hind limb and a milder lameness (meaning apparent lameness) in the contralateral forelimb, it is very

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likely that the horse has two problems. Likewise if a horse has a marked lameness in a forelimb and a milder lameness in the ipsilateral hindlimb, it is likely that the horse has two problems. However, if the horse has a marked lameness in a forelimb and a milder “lameness” in the contralateral hindlimb or a marked hindlimb lameness associated with a milder “lameness” in the ipsilateral forelimb, it is not possible to tell if there is one or two problems without blocking out the primary problem. Bilateral forelimb lameness When horses are jogged at a straight trot bilateral forelimb lameness may be difficult to detect. If the lameness in each forelimb is approximately equal in magnitude, the movement of the head may be symmetrical, though usually reduced in excursion, and accompanied by the appearance of stiffness. If one side’s lameness is more marked than the other at a straight trot, the marked lameness will mask the milder problem. Lunging in a circle and digital nerve blocks may be needed to determine the nature of the lameness. Lameness Scales

AAEP Scale 0. Lameness not perceptible under any circumstances I. Lameness is difficult to observe; not consistently apparent regardless of circumstances II. Lameness is difficult to observe at a walk or trotting in a straight line, but is consistently

apparent under certain circumstances III. Lameness is consistently observable at a trot under all circumstances IV. Lameness is obvious; marked nodding, hitching, and/or shortened stride V. Lameness is obvious; minimal weight-bearing in motion or at rest; inability to move Ross and Dyson Scale (based on observation of horse trotting in hand on firm/hard

surface) 0. Sound 1. Mildly/intermittently lame, subtle head/pelvic asymmetrical movement 2. Obvious lameness, head/pelvic movement consistent and excursion several cm 3. Pronounced head/pelvic asymmetry 4. Severely lame with extreme head nod/pelvic hike, but can still trot (noticeable at walk) 5. Does not bear weight on limb and if trotted, carries limb. I much prefer the latter and use it independent of circumstance. I.e. a horse can be a 2 at a straight trot and a 3 on the lunge. THE LAMENESS EXAM Signalment The signalment is useful as an indicator of risk factor, i.e. age, sex, or breed association with a particular disease. It is information to be noted and returned to as needed. History

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No two histories are ever the same because one question leads to another, and so each history takes a different path. However, there is a common core of questions frequently asked. The duration of ownership gives the clinician some idea of the owner’s awareness of this or other past problems. The speed of onset, severity at onset, and progression of lameness, effect of exercise, work and shoeing history, and prior treatments should all be determined. Physical examination The horse should be examined as a whole. Then each limb should be observed from a distance and close up for changes in shape/contour and discoloration. The limb is palpated for heat, swelling, pain, moisture on the surface, and texture of surface. The joints are flexed to determine passive range of motion and detect pain associated with it. Hoof testers are applied to detect foci of pain within the foot. Lunging Lunging changes the loading of the limbs, increasing the load on the inner limb and decreasing the load on the outer limb. Additionally, the outer limb has to travel further than the inner limb. Most forelimb lamenesses are increased when the lame limb is on the inside of the circle. Conventional wisdom has it that horses with upper limb lamenesses and lameness originating in the suspensory ligament are more lame when the injured limb is on the outside of the circle. Comparison should be made between the lameness evident on each rein and at the straight trot. Effect of surface It is frequently beneficial to trot a horse on both soft and hard surfaces. Horses with diseases that are likely to be more painful when the limb is subject to concussion such as pedal osteitis, navicular disease, or osteoarthritis are usually more lame when worked on a hard surface. To a lesser extent horses with injury to soft tissue structures that support the fetlock may be lamer on soft surfaces. Changing the surface is frequently used in conjunction with lunging. Flexion tests Flexion tests are a very useful adjunct to the lameness exam, but they can be problematic to interpret. The are several papers that have critically analyzed the use of flexion tests. Firstly, there is poor inter-clinician consistency, but individual clinician repeatability is good. The response obtained is related to the amount of force applied and how long the force is applied for. Older horses are more likely to flex positive than young horses. Repetition of a test during a single examination is likely to result in changing results, and there is inconsistency when the tests are repeated after long intervals. Lastly, many sound horses flex positively. Overall, some common sense needs to be used in interpretation. If a mildly lame horse flexes moderately to markedly sore, the test is probably quite useful in localizing a source of pain. However, if a sound horse exhibits a mild response, the long term significance of that test is in doubt. Diagnostic Analgesia Perineural and intrasynovial diagnostic analgesia have long been the mainstay of ancillary diagnostic techniques for the lameness exam. Both techniques have their merits. Perineural analgesia must be performed in a distal to proximal direction whereas intrasynovial analgesia can be used at any point in the limb at any time during the exam. Intrasynovial analgesia is usually considered more specific than perineural, though there are limitations to this specificity. The

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risks of either technique are very low when performed in an aseptic manner, but the consequences of infection in a synovial structure are greater when they occur. Perineural analgesia tends to be longer acting than intrasynovial analgesia. There is some variability in the onset and duration of analgesia of synovial cavities depending on the structure and volume used that is not well defined. Local analgesia is best employed in the diagnostic process in horses with mild to moderate lameness of 3- 4 weeks or greater duration, as mild injuries may spontaneously recover quickly. The horse must be lame enough to detect an improvement. Sometimes the abolition of a response such as hoof tester pain or a positive flexion test may be needed. Horses that are acutely severely lame should not be blocked diagnostically until the clinician can be sure that there are no occult fractures present, which may include performing a bone scan first or re-radiographing the horse after a period of rest. Diagnostic analgesia should be tested before the horse is re-evaluated. With perineural blocks it is usually assumed that skin desensitization indicates the block has worked. When a suspensory ligament is blocked, the absence of any response to firm pressure is usually an indication that the block has worked. Intra-synovial blocks cannot be checked in the same manner. They are best judged by a change in response, i.e. lameness changes or a flexion test changes. With intra-synovial blocks, it is the certainty that the local anesthetic was placed in the appropriate structure that is most important. Obviously, synovial fluid in the hub of the needle is proof that it is in a cavity, though not always the right cavity. If synovial fluid is not obtained, back pressure on the syringe plunger after injection of the anesthetic is a strong indicator that the needle was in a synovial cavity. Some structures can’t be blocked, and sometimes diagnostic analgesia doesn’t work. Nerves cannot be blocked for a diagnostic examination at any point proximal to their last muscular branch. Therefore, extra-synovial structures proximal to the last muscular branch of the nerve that innervates them must be blocked by direct infiltration. The most likely reason a nerve block hasn’t worked is that the analgesic has not been placed appropriately. After repeated blocks, it is the subjective impression of some clinicians that perineural analgesia doesn’t work as well; this is thought to be due to perineural fibrosis. At other times there is no clear cut explanation, for example, some horses with laminitis are difficult to block regardless of the accuracy and amount of local anesthetic used. Miscellaneous If you are unsure of the nature of a horse’s lameness, do not commit yourself until you have had an opportunity to get more information. If the circumstance under which a lame horse is observed changes, i.e. lunging, post flexion test, post block, determine which leg is lame before assigning a grade to the lameness; if you are tempted to assign a grade first, you may miss a switch in the lameness to another limb. All observations should be written down as soon as possible.