History
A 10-year-old 454-kg (1,000-lb) Quarter Horse gelding was admitted to the hospital for evaluation of an intermittent lameness of the left forelimb of 1 year's duration. The horse had been rescued 15 months ago and the medical history prior to rescue was unknown. During the course of the past year, the horse had recurrent subsolar abscesses. The left forelimb lameness varied in severity from a grade 1/5 to a grade 4/5 to 5/5 on the American Association of Equine Practitioners lameness scale and often worsened when exercised.1
On physical evaluation, there was no palpable joint effusion or soft tissue swelling in any limb. The patient had a positive response to hoof testers on the left forelimb sole, most evident on the medial solar surface where a concave defect on the sole was evident. No gait abnormalities were observed at the walk. When the horse was trotted in a straight line on a hard surface, a grade 3/5 left forelimb lameness was evident and characterized by a mild head nod. When the horse was lunged toward the left at a trot on a hard surface, the head nod was exacerbated. A left lower forelimb flexion test yielded a mild positive response. Following a palmar digital nerve block of the left forelimb, the lameness resolved completely. Radiography of the left front foot was performed (Figure 1).
Lateromedial (A) and dorsopalmar (B) radiographic views of the left front foot of a 10-year-old Quarter Horse gelding evaluated for an intermittent lameness of the left forelimb of 1 year's duration.
Citation: Journal of the American Veterinary Medical Association 250, 8; 10.2460/javma.250.8.841
Determine whether additional imaging studies are required, or make your diagnosis from Figure 1—then turn the page →
Radiographic Findings and Interpretation
Lateromedial and dorsopalmar radiographic images of the left front foot were reviewed. On the dorsopalmar image, asymmetry of the medial solar margin is evident. An obvious lesion is not observed on the lateromedial image (Figure 2).
Same radiographic dorsopalmar image as in Figure 1. Notice the deformity of the distomedial aspect of the third phalanx (white arrow).
Citation: Journal of the American Veterinary Medical Association 250, 8; 10.2460/javma.250.8.841
For complete radiographic evaluation of the distal phalanx and navicular bone, the following additional radiographic images were obtained: dorsal 65° proximal-palmarodistal and dorsal 45° proximal-palmarodistal obliques, palmar 50° proximal-palmarodistal oblique, dorsal 65° proximal 45° medial-palmarodistolateral oblique, and dorsal 65° proximal 45° lateral-palmarodistomedial oblique (not shown). Comparative radiographic images of the right front foot were also obtained (Figure 3).
Additional radiographic images of the left (A, B, C) and right (D) front feet of the horse in Figure 1. The lesion is conspicuous on the dorsal 65° proximal-palmarodistal (A) and dorsal 65° proximal 45° medial-palmarodistolateral (B) oblique views of the left front foot and is seen as a solitary, well-defined, smoothly marginated, ovoid region of lysis surrounded by sclerosis along the medial palmar process of the distal phalanx (black arrowheads). The lesion distorts the medial contour of the third phalanx (black arrows). On the palmar 50° proximal-palmarodistal oblique view of the left foot (C), a large, irregularly ovoid gas opacity can be seen immediately adjacent to the region of lysis (white arrowhead) consistent with associated abscess formation and gas-producing bacteria and necrosis. Notice the dilation of the vascular solar canal and vascular channels (black arrowhead) of the left front foot (C), compared with that in the same radiographic view (ie, palmar 50° proximal-palmarodistal oblique) of the normal distal phalanx of the right front foot (D).
Citation: Journal of the American Veterinary Medical Association 250, 8; 10.2460/javma.250.8.841
The complete study of the left front foot revealed an approximately 3 × 2-cm, solitary, ovoid, well-defined and smoothly marginated region of lysis surrounded by sclerosis along the medial palmar process of the distal phalanx adjacent to the solar hoof irregularity. A large, 3-cm diameter, irregularly ovoid, gas opacity on the dorsal 65° proximal 45° medial-palmarodistolateral view was observed to be associated with the lucent irregular lesion of the distal medial aspect of the third phalanx.
Given the smooth nature of the region of lysis, history of recurrent subsolar abscesses, and the mild clinical lameness, the primary differential diagnosis for the large region of lysis within the medial palmar process of the left front foot was pressure resorption secondary to a keratoma or primary bone neoplasm. Lesser consideration for neoplastic differential diagnoses included fibroma, malignant melanoma, squamous cell carcinoma, mast cell tumor, neurofibroma, or hemangioma. Septic osteitis of the distal phalanx or a bone cyst are other differential diagnoses were considered less likely. Multiple gas opacities superimposed over the hoof wall including a large gas opacity adjacent to the lytic defect of the distal phalanx were consistent with gas-producing bacteria and associated necrosis, presumed to be secondary to the horse's history of subsolar abscesses.
Treatment and Outcome
Surgery was performed while the horse was standing and sedated. An abaxial sesamoidean nerve block was used for regional anesthesia. The epidermal sole was thinned with a hoof knife over and around the solar mass. An elliptical incision was made around the mass with a scalpel blade and was bluntly dissected from the surrounding dermis. A conical 3 × 2-cm, firm, keratinized, tan mass was removed. The mass was submitted for histologic evaluation, which revealed regularly arranged laminar proliferations of epithelial cells and keratin consistent with a keratoma. The horse was discharged from the hospital the following day with instructions for bandage changes every other day until healthy granulation tissue filled the defect. The horse was sound 3 days after surgery, and the defect in the sole had filled in with a healthy bed of granulation tissue by 10 days after surgery. At 13 months after surgery, the horse was still sound.
Comments
Keratomas are a benign mass formed from keratin-producing epidermal cells of the hoof that can result in lameness and hoof wall deformities.2 They are an uncommon but well-recognized condition affecting the equine hoof. Common clinical signs and physical examination findings include lameness (ranging from mild to severe), recurrent subsolar abscesses, and deformity of the hoof wall or white line. The etiology of keratoma formation is unknown, but is believed to be due to chronic inflammation or trauma that causes abnormal epidermal proliferation into the dermis. Keratomas most commonly occur within the lamina of the hoof situated between the external wall and distal phalanx. The case described in the present report was unusual because keratomas of the sole of the hoof are infrequently reported.3–5
Radiographic findings in the horse of the present report included a smoothly marginated lytic area in the distal phalanx, classic of a keratoma, resulting from pressure placed by the keratinized mass on the bone within a confined space.2 The subtlety of the lesion on the dorsopalmar and lateromedial radiographic images highlights the diagnostic importance of using multiple radiographic projections for complete evaluation of the foot. In hindsight, upon reevaluation of the lateromedial image, the solar canal was deemed mildly widened and more angular in shape than normal, a subtle finding.
It is important to note that, although rare, primary osseous neoplasms may have a similar lytic and smoothly marginated appearance, and histologic evaluation of affected tissue should be performed when possible to obtain a definitive diagnosis.6 Keratomas may also result in an irregularly marginated area of bone lysis that may be mistaken for septic or nonseptic pedal osteitis, or in many instances, there may be no abnormalities found on radiographic evaluation.2,7 When there is an absence of radiographic abnormalities and the suspicion of a keratoma exists, advanced imaging such as MRI and CT are extremely valuable for both diagnosis and surgical planning.7,8 Ultrasonography is useful for cases in which the keratoma extends proximal to the coronary band and thus was not performed in the patient of the present report. The recovery period following surgical removal of a solar keratoma is much shorter than the 6 to 12 months reported for hoof wall resections necessary to remove keratomas within the hoof wall.9 The surgical procedure to remove a solar keratoma is less aggressive than hoof wall resection and requires less postoperative bandaging and care. The recurrence rates of keratomas have been reported to range from 11% to 40%.7,9 Assuming complete removal of the keratoma, the prognosis for the resolution of lameness and return to function is good to excellent.2,9
References
1. Stashak TS. Examination for lameness. In: Adam's lameness in horses. Philadelphia: Lippincott Williams & Wilkins, 2002;122.
2. Honnas C. Keratomas of the equine digit. Equine Vet Educ 1997; 9: 203–207.
3. O'Grady S, Horne P. Lameness caused by a solar keratoma: a challenging differential diagnosis. Equine Vet Educ 2001; 13: 87–89.
4. Miller SM, Katzwinkel RH. Solar keratoma: an atypical case. J S Afr Vet Assoc 2015; 86: 1257.
5. Raghuvanshi P, Sharma Y. Surgical treatment of lameness caused by solar keratoma in a mare. Indian Vet J 2010; 87: 174–175.
6. Honnas CM, Liskey CC, Meagher DM, et al. Malignant melanoma in the foot of a horse. J Am Vet Med Assoc 1990; 197: 756–758.
7. Mair T, Linnenkohl W. Low-field magnetic resonance imaging of keratomas of the hoof wall. Equine Vet Educ 2012; 24: 459–468.
8. Getman LM, Davidson EJ, Ross MW, et al. Computed tomography or magnetic resonance imaging-assisted partial hoof wall resection for keratoma removal. Vet Surg 2011; 40: 708–714.
9. Boys Smith SJ, Clegg PD, Hughes I, et al. Complete and partial hoof wall resection for keratoma removal: postoperative complications and final outcome in 26 horses (1994–2004). Equine Vet J 2006;38: 127–133.
