White line disease in a 19-year-old appendix mare

Abstract

A 19-year-old appendix mare was presented with severe, acute right forelimb lameness and a history of significant hoof wall defect. The defect began as progressive toe separation affecting the dorsal hoof wall, which was eventually resected by a farrier. Placement of bar shoes by a farrier to stabilize the hoof was ineffective. Radiographs showed hoof wall separation, palmar rotation, and displacement of the coffin bone, consistent with failure of the laminar structures. Treatment included phenylbutazone, radiographic-guided therapeutic farriery consisting of derotation with a wedge shoe, and restriction to a small, dry paddock.

A 19-year-old appendix mare was examined on farm due to a recent history of worsening lameness. The mare had a history of a significant hoof wall defect for several months, when a farrier diagnosed the mare with thrush and resected a large portion of the dorsal hoof wall, from the toe, on the right front hoof. During routine vaccine appointment 3 months before presentation, the affected hoof was assessed by the veterinarian who recommended placement of a bar shoe and repair of the hoof defect in order to stabilize the hoof wall and allow healthy hoof wall growth. White line disease was considered to be the likely etiology of the hoof wall disease rather than thrush. At the time of presentation, the mare was lame in the right front leg which had become markedly worse over the previous 2 d.

Case description

On physical examination, the mare was bright, alert, and responsive with a body condition score of 6/9. The lameness in the right front was apparent at the walk, scored on the AAEP scale as grade 4/5. All physical examination parameters were within normal limits and there was no heat, pain, swelling, or resentment on palpation of the right front leg. The mare had been shod 6 weeks earlier. The foot had a long toe, low heel conformation (Figure 1a). The left shoe was missing and the right set far forward and extremely loose. The hoof wall defect was still present and affected an area of the hoof wall 10 to 12 cm wide across the toe and extending 60% up the hoof wall (Figure 1b). The right front hoof was soaked and cleaned, allowing better examination of the defect. The resected hoof wall had exposed laminae which were hard and dry. The coronary band was soft and deformed, especially above the resected area. The medial hoof wall had an exaggerated dished appearance. A vertical crack had formed on the lateral aspect of the resected hoof wall extending proximally toward the coronary band, giving the hoof wall above the resected area an undermined appearance. Another vertical crack was beginning to form at the medial aspect of the resected hoof wall. To avoid damaging the hoof wall further, the shoe was left on and a duct-tape bandage was applied to keep it in place. Following the physical examination, the patient was administered phenylbutazone (Phenylbutazone 20%; Rafter 8 Products, Calgary, Alberta), 4.4 mg/kg body weight (BW), IV, for pain. Due to the extensive hoof wall damage and lameness, radiographs of the front feet were recommended to assess the internal structures of the hoof, particularly the hoof wall-coffin bone suspension system.

Figure 1.

Right front hoof viewed from lateral (a) and dorsal (b) aspects. The lateral view (a) shows the mare’s low heel-long toe conformation, with the shoe set very far forward on the hoof, extending well past the toe. The hoof wall defect (resected hoof wall) had exposed underlying laminae tissue, which appeared well-cornified. The coronary band was soft and deformed (b, black arrowheads). The medial aspect of the hoof wall (right side in photograph) has an exaggerated concave conformation. A vertical crack had begun to form just proximal to the dorso-medial aspect of the resected hoof wall (b, white arrowhead). A vertical crack had also formed at the lateral aspect of the defect (a, white arrowheads), extending proximally to the coronary band, giving the dorsal hoof wall an undermined appearance.

Orthogonal radiographic views (lateral and dorso-palmar) of the foot were taken with the feet on blocks. Radiopaque paste (barium) was applied along the midline of the dorsal hoof wall, identifying the level of the coronary band. The lateral and dorso-palmar views of the left front foot were within normal limits. The lateral and dorso-palmar views of the right hoof revealed abnormalities consistent with failure of the hoof wall-coffin bone suspension system. On the lateral view (Figure 2a), the dorsal hoof wall defect and undermining of the hoof wall were visible as a gas opacity extending from the toe proximally. The horn-lamellar zone (HLZ) of the hoof wall increased abnormally proximal to distal, measuring 33 mm and 44 mm, respectively (normal range ≤ 18 mm, remaining constant between proximal and distal measurements) (1,2). Palmar rotation of the coffin bone had caused its angle to change, resulting in an increased palmar angle (PA) of 10°. The PA of the coffin bone varies between normal horses, ranging from 5° to 10° (1). In this case the PA was considered increased as the left front (unaffected) hoof had a PA of 4°. The affected foot had a coronary to extensor process distance (CED) of 20 mm, which is at the high end of normal (< 18 to 20 mm) (2). This indicated distal displacement of the coffin bone (sinking) (2). Distal displacement and palmar rotation of the coffin bone had also decreased the sole depth under the tip of the coffin bone to 4 mm, compared to 10 mm of sole depth in the left hoof. The tendon surface angle (TSA) of the navicular bone was 45°. Remodeling of the tip of the coffin bone was also present, giving it a “ski-tip” appearance, indicating chronic solar pressure in that area (1). On the dorso-palmar view, the hoof wall defect and undermining of the hoof wall were visible as gas opacity at the lateral aspect of the hoof wall. The concave appearance of the medial hoof wall and the deformity of the coronary band were highlighted in this view.

Figure 2.

Lateral radiograph of the right front hoof prior to treatment (a), and after treatment (b). a — demonstrates several abnormalities consistent with failure of the hoof wall-coffin bone suspension apparatus. Rotation and sinking of the coffin bone have resulted in decreased sole depth below the palmar tip of the coffin bone. The rotation of the coffin bone has increased its palmar angle compared to the unaffected hoof. The horn lamellar zone measurements of dorsal hoof wall thickness had become increased overall and moving proximal to distal (these measurements should be similar). Sinking of the coffin bone has increased the distance from the dorsal aspect of the extensor process to the level of the coronary band. The undermining of the dorsal hoof wall proximally to the resected hoof wall is visible on radiograph as a gas opacity within the hoof wall extending proximally from the toe. b — the palmar angle was brought to 0° relative to the solar surface of the wedge shoe (Nanric Modified Ultimate, Nanric, Lawrenceburg, Kentucky, USA) using the 7/8″ toe wedge. The overall palmar angle of the coffin bone with the ground surface was brought to 23° by building the heel of the shoe with Super Fast glue (Vettec Hoofcare, Orange, California, USA).

Palmar rotation and displacement of the coffin bone can be associated with laminitis (1), where the laminae suspending the coffin bone within the hoof capsule fail in an inflammatory process (2). This mare also had a disturbance in her laminar tissue due to the progression of her white line disease and hoof wall defect, contributing to palmar rotation and displacement of the coffin bone in addition to inflammatory laminitis. Displacement of the coffin bone results in marked lameness in cases of laminitis (2), which this mare had during the farm visit. The blunted tip of the coffin bone reflected the chronicity of the laminar failure (rotation and displacement of the coffin bone) as the mare had been treated for several months with little success.

The goal of treatment was to stabilize and improve the mechanics of the right front hoof through therapeutic farriery and to provide analgesia for the lameness. Phenylbutazone (Phenylbutazone 20%; Rafter 8 Products), 2.2 mg/kg BW, PO, q24h, was administered while the mare was in the clinic. Radiography during shoeing allowed measurements to be taken as the shoes were modified to achieve ideal therapeutic angles. To reduce strain on the dorsal hoof wall laminae surrounding the defect, the mechanics of the hoof were improved by easing and shifting breakover caudally, elevating the heel, and shifting support to the soles (3). The Nanric Modified Ultimate (Nanric, Lawrenceburg, Kentucky, USA) is a 20° glue-on wedge shoe chosen for derotation of the coffin bone and support of the sole. To facilitate shoeing, xylazine (XYLAMAX 100 mg/mL; Bimeda, Cambridge, Ontario), 0.5 mg/kg BW, IV, was administered.

The defect and diseased hoof wall of the right front hoof were further debrided with a probe and burr. To achieve a 0° PA relative to the solar surface of the shoe, a 3/4-inch plastic wedge pad was placed under the toe and a radiograph was taken to check the angle. The resulting PA (to the shoe surface) was 3°, so a 7/8-inch toe pad was used to elevate the toe further. This brought the PA (to the shoe surface) to 0° and the overall PA (to the ground surface) to 14°. Before being glued to the shoe, the toe pad was cut into a crescent shape and notched under the midline of the toe to eliminate any pressure. Behind the toe pad, cushioning putty (Advanced Cushioning Support; Nanric) was packed under a felt pad to support the sole surface. Super Fast glue (Vettec Hoofcare, Orange, California, USA) was applied to the bottom of the shoe to further improve its mechanics and increase the overall PA to approximately 20°. The last radiograph was taken to confirm that the PA (22°) and TSA (61°) were satisfactory (Figure 2b). The hoof wall defect was dusted with copper sulphate powder (Allen’s Blue Powder; Allen Farrier Products, Vandergrift, Pennsylvania, USA) then packed with medicated hoof crack paste (Keramend; Kinetic Vet, Lexington, Kentucky, USA). The hoof was waxed and the shoe glued on using the cuff on the shoe. To further secure the shoe, it was casted onto the hoof (Equicast, Lawrence, Kansas, USA). After the cast cured, it was trimmed around the coronary band and Lightplast Pro (BSN Medical, Laval, Quebec) was applied to prevent debris from falling in the cast. The unaffected hoof was shod in an Ultimate 20° shoe (Nanric) and casted similarly.

After being shod, the mare walked comfortably on concrete, with a tendency to land heel first. The following day, the mare was quite comfortable bearing full weight and walking with full strides on concrete, and it was decided to reduce the phenylbutazone (Phenylbutazone 20%; Rafter 8 Products) to 2.2 mg/kg BW, PO, as needed for pain. The mare was discharged with instructions to monitor her for increasing lameness or shoe loss. To reduce the chance of the shoes becoming loose, it was advised to confine the mare to a smaller, dry pasture or a stall, if the pasture was wet. The shoes required resetting in 6 wk, again assisted by radiography. After the mare received the first set of glue-on shoes, the mare’s owners struggled with aftercare of the shoes and the mare soon needed one to be replaced. Six months after the beginning of treatment, sufficient derotation and support of the hoof capsule was achieved, and the mare was put into nail-on full rocker shoes, requiring less aftercare. Sole depth on the affected hoof had improved significantly from 4 mm to 19 mm at the time she was put into nail-on rocker shoes, and the mare has remained comfortable and sound at pasture.

Discussion

This mare’s severe hoof pathology likely began as minor hoof wall separation at the toe, known as white line disease, progressing to a large area of diseased hoof wall. This process was likely propagated by poor mechanics (long toe, low heel conformation) and instability of the hoof capsule after major hoof wall resection without placement of a therapeutic shoe (4). Microscopic epidermal laminae attach to the hoof capsule and interdigitate at the white line to form an attachment with the dermal laminae of the coffin bone (2–5). This suspends the distal phalanx within the hoof capsule, allowing the horse to bear weight using the hoof wall. When the white line becomes damaged by keratinolytic micro-organisms and/or excessive mechanical stress, the hoof wall can begin separating at the toe (2,4). The severity of white line disease is variable, ranging from mild toe separation not affecting soundness, to causing major hoof wall defect and separation as seen in this case (4). Large areas of damage to the hoof wall and laminar suspension system can result in the rotation and displacement of the coffin bone within the hoof capsule as it did in this mare (2,4). Pain, due to marked lameness in this patient, occurred secondary to stress on the laminae in areas with large separations and increased solar pressure under the displaced coffin bone, where the sole depth is reduced (4).

A radiographic podiatry study involves taking lateral and dorso-palmar views of the hooves while on blocks. These views help reveal the degree of hoof wall damage, separation and coffin bone rotation or displacement. Capturing an accurate lateral view is important when assessing coffin bone rotation, as palmar rotations > 10° may be underestimated if the beam is oblique (1). Useful radiographic measurements taken from the lateral view of the foot include the PA, measured as the angle of the solar surface of the coffin bone in relation to the ground, the sole depth beneath the tip of the coffin bone, and the HLZ, which measures the thickness of the dorsal hoof wall proximally and distally (1,2,6). The CED measures the distance from the base of the extensor process of the coffin bone to the coronary band and serves as a repeatable measurement reflecting distal displacement of the coffin bone (2). These parameters are affected by palmar rotation or distal displacement of the coffin bone, regardless of the etiology of laminar failure, such as laminitis or this mare’s severe white line disease. These measurements can be used to guide therapeutic farriery treatment and estimate prognosis (4).

The deep digital flexor tendon (DDFT) passes distally on the palmar/plantar aspect of the distal limb and inserts on the palmar/plantar aspect of the coffin bone, passing along the tendon flexor surface of the navicular bone. The tension of the DDFT opposes the coffin bone’s laminar attachment to the hoof capsule (7,8). Raising the heel (and increasing the PA) decreases DDFT tension (6,9), and since the tendon flexor surface of the navicular bone acts as a fulcrum, its position also affects DDFT tension (7,8). Increasing the tendon surface angle (TSA) by raising the heel reduces its fulcrum effect and decreases DDFT tension on the coffin bone and laminae of the dorsal hoof wall, allowing the coffin bone to remain de-rotated and reduce laminar pain.

The therapeutic approach to treating the displacement and rotation of the coffin bone is essentially the same as that for a laminitic horse. The goal is to stabilize and realign the coffin bone within the hoof capsule, improve the mechanics of the hoof by placing breakover closer to the point of rotation of the joint, and to transfer load from the diseased dorsal hoof wall to the heels, bars, and frog (2,3). Realignment of the coffin bone with the ground and reducing excess mechanical forces at the toe will allow new hoof wall to grow with normal attachment to the coffin bone. Supporting the frog and sole with a cushioned shoe relieves weight-bearing on the hoof wall and stress on the laminae (3,10). The beveled toe, placement, and the added rocker built onto the bottom of the shoe improve mechanics of the hoof by easing breakover and shifting it caudally. Increasing mechanical advantage of the hoof reduces stress within the laminae of the dorsal hoof wall (10). Ensuring a 0° PA with the shoe surface will result in even support of the sole and frog and raising the heel to an overall 20° PA with the ground relieves DDFT tension opposing the dorsal laminae (3,10). The hoof wall is slow-growing; therefore, this mare would require several treatments over a period of months before efficacy of treatment and prognosis could be evaluated externally and radiographically. Three months after the initiation of treatment the mare remained comfortable at the walk and had transitioned from the Modified Ultimate (Nanric) glue-on shoes to a nail-on steel rocker shoe with mechanics easing breakover.

Treatment of podiatry cases with laminar separation and displacement or rotation of the coffin bone relies on common principles, regardless of the cause of laminar dysfunction. The approach to treatment of each case will depend on the type and severity of changes to the coffin bone position and is reflected in radiographic measurements. Radiographic guidance during therapeutic farriery treatment is essential to make appropriate changes for each individual case and allows progression to be tracked.