Separation of the lamellae is a consequence of the severity of the original pathologic processes in the face of the mechanical stresses superimposed on the lamellae. Displacement occurs when the load applied exceeds the strength of the lamellae supporting the distal phalanx within the hoof capsule. Three recognized variants of displacement of the distal phalanx have been described (Figure 201-1). The exact reasons for these different patterns are unknown, but it would appear reasonable to assume that the sites of displacement are subject to the greatest stress, the most lamellar injury, or a combination of the two. Clinical observation suggests that conformation is one factor to be considered; horses in which the foot is offset from the axis of the limb, usually laterally, should have greater stress on the side of the foot that is most in line with the metacarpus and displacement on that side of the foot is more likely. When lamellar breakdown occurs dorsally, the resultant rotation of the distal phalanx about the distal interphalangeal joint is such that the angle of the distal phalanx changes in relation to both the ground and the dorsal hoof capsule; this is routinely referred to as rotation, but in this text is referred to as dorsal rotation to differentiate it from other forms of displacement. Capsular rotation refers to deviation of the parietal surface of the distal phalanx with regard to the hoof capsule. Phalangeal rotation refers to rotation of the distal phalanx with respect to the axis of the proximal phalanges (i.e., distal interphalangeal joint flexion). In an acutely rotated horse, both these types of rotation occur simultaneously. When the structural breakdown occurs evenly around the hoof capsule, the distal phalanx displaces symmetrically within the hoof capsule, referred to here as symmetrical distal displacement, but often called “sinking.” When one quarter is affected more than the other, the distal phalanx displaces distally unilaterally, a less frequent scenario than dorsal capsular rotation; here it is referred to as asymmetrical distal displacement, but is sometimes referred to as “unilateral rotation” or “medial/lateral sinking.” In reality it is unlikely that lamellar injury is confined to any one area around the perimeter of the foot, so most horses display a combination effect.

Presentation, Diagnosis, and Evaluation

The diagnosis of chronic laminitis is seldom a challenge because the gait and appearance of the foot are characteristic of the disease, and radiographs usually remove all doubt. Occasionally, nerve blocks are necessary to localize the pain to the foot in mildly affected horses and should be interpreted in conjunction with the results of hoof tester application and radiographic findings.

Horses with chronic laminitis have several possible clinical manifestations: (1) as the continuation of acute laminitis, (2) as a recurrence of past chronic laminitis, or (3) with an unknown history because the acute phase was never observed or the horse was purchased without knowledge of the disease.

Full evaluation of the history, examination of the entire horse, and examination of the digits are all important in determining the prognosis and treatment. The severity of the initial acute phase is the best indicator of the extent of the original damage and constitutes a major factor in the prognosis. The duration of the disease since the onset of the acute phase gives some indication as to how much the lamellar repair process may have destabilized the distal phalanx within the hoof capsule. In horses with chronic laminitis, the degree of lameness (manifested by willingness of the horse to walk, how stilted it walks, how willing it is to lift a foot, and how much it lies down) does not always correlate with the prognosis as well as it does with the prognosis in acute laminitis, and does not necessarily correlate with the clinical appearance of the foot or the radiographic changes. How the animal preferentially places its foot may indicate the distribution of lamellar injury; for example, a horse that preferentially bears weight on its heels reduces the tension in the dorsal lamellae and the compression of the sole beneath the dorsal margin of the distal phalanx. Similarly, a horse that is landing on one side of the foot may be protecting the contralateral wall or sole, although this asymmetrical landing may also be caused by conformation. The digital pulse should always be evaluated for intensity. Evaluation of the coronet, hoof wall, sole, and white line, combined with the response to hoof tester examination and palpation of the coronary band, provides clinical indicators of the pathology that has occurred within the hoof. This pathology may include capsular distortion, displacement of the distal phalanx, infection, and secondary contraction of the hoof or flexor apparatus.

Radiology

The radiographic features of chronic laminitis have been well documented. High-quality lateral, dorsopalmar, and dorsopalmar oblique projections are necessary. The exposure must include good images of the hoof capsule. A radiopaque marker is invaluable in determining the position of the distal phalanx in relation to the dorsal surface of the hoof capsule on the lateral radiograph; it should be placed on the midsagittal dorsal hoof wall, starting at the coronary band and extending distally. Additionally, for the dorsopalmar radiograph, linear markers may be placed on the medial and lateral walls of the hoof capsule. Additionally, to ensure the maximal information is obtained from the radiographs, they should be obtained with the horse standing on a flat level surface and bearing weight evenly and with the metacarpal bone vertical to the ground. The horse should be standing on two wooden blocks of equal height that have embedded radiodense markers to indicate the plane of the ground surface.

The lateral radiographic view should be examined to determine the thickness of the dorsal hoof wall, the degree of capsular rotation, the angle between the solar margin of the distal phalanx and the ground, the distance between the coronet and the proximal margin of the extensor process, and the distance between the dorsal margin of the distal phalanx and the ground. The dorsopalmar view is most useful for determining whether there is asymmetric distal displacement of the distal phalanx within the hoof capsule; specifically, asymmetric displacement is associated with a decrease in the distance from the distal phalanx to the ground, an increase in the distal interphalangeal joint space, and increased wall thickness on the affected side. The location of gas pockets is determined by correlating the findings of the lateral and dorsopalmar views. The margin of the distal phalanx is evaluated for pedal osteitis, sequestra, and marginal fractures on the oblique dorsopalmar view.

Positive-contrast venography of the foot may indicate filling defects in the digital vasculature that signify a poor prognosis—namely, defects in filling of the lamellar vessels, the circumflex area, and the terminal arch. It is important that the venogram is performed with appropriate technique, including application of an effective tourniquet and infusion of the contrast medium while the foot is unloaded to prevent artifactual filling defects. The venogram, when used, should always be performed and interpreted by a clinician that has experience in use of this modality.

Outcome and Prognosis

The eventual outcome of treatment for horses with chronic laminitis can be divided into functional and morphologic outcomes. The functional outcome is most likely to dictate whether the horse can return to athletic activities, can regain only pasture soundness, or must be euthanized. The mor­phologic outcome is more likely to determine the degree to which continued and potentially lifelong therapeutic measures will be necessary. Although there is fair correlation between the functional and morphologic outcomes, some horses with seemingly modest morphologic changes are euthanized, whereas others are performing more strenuous athletic activity than the morphologic appearance would lead one to expect.

At the onset of chronic laminitis, the eventual outcome is hard to predict, but the most important indicator for survival remains the severity of the initial insult to the lamellae. The appearance of the initial radiographs does not necessarily correlate with either the functional or the morphologic outcome. However, the thickness of the sole and the angle between the solar surface of the distal phalanx and the ground are fair indicators of the difficulty of treatment of horses with dorsal phalangeal rotation, and both are more useful than the degree of capsular rotation in successfully predicting the rehabilitation of the horse. The thickness of the soft tissues dorsal to the parietal surface of the distal phalanx and the distance between the coronet and extensor process are suggestive of the prognosis in horses with distal displacement of the distal phalanx because these indicate the severity of the original injury. In horses with capsular rotation, the eventual morphologic outcome only becomes apparent as the new dorsal hoof grows down through the middle third of the wall.

Treatment

The desired outcomes of treatment of horses with chronic laminitis are freedom from pain and feet with normal appearance and function. Obviously there are major limitations that interfere with achieving these ideals. The treatment of chronic laminitis is multifaceted and includes providing supportive care of the feet, medical therapy, surgical intervention, and nutritional management. In contrast to treating horses with acute laminitis, in which medical therapy frequently assumes priority, supportive therapy or therapeutic farriery is the most important element for success in treating horses with chronic laminitis.