History
A 28-year-old 537-kg American Quarter Horse mare was referred for evaluation of lameness in the left forelimb of 1 month’s duration. Recently, the horse had shoeing changes to manage suspected navicular pain. Neither the shoeing changes nor a course of phenylbutazone (3.7 mg/kg, PO, q 12 h for 3 days) improved the lameness. Two weeks prior to referral, swelling on the lateral aspect of the coronary band of the left forelimb was detected, and results of radiographic examination of the affected foot performed by the referring veterinarian were suggestive of a subsolar abscess (not shown). The affected foot was soaked in magnesium sulfate (Epsom salt) solutions once daily for 4 days, after which the coronary band swelling increased in size and drained mucopurulent fluid. A sample of this fluid was submitted for aerobic bacterial culture, which yielded growth of a Staphylococcus sp. Despite drainage and treatment with trimethoprim sulfadiazine powder (Uniprim; Niogen Vet; 75 mg/kg, PO, q 24 h for 3 days), the left forelimb lameness did not improve. A basisesamoid nerve blockade was performed by the referring veterinarian and confirmed that the lameness originated from the foot. The horse was referred for further diagnostic evaluation and treatment.
On referral examination, the horse had a grade 4/5 (on the American Association of Equine Practitioners lameness scale1) lameness in the left forelimb at the walk. A large, focal soft tissue swelling was centered over the lateral aspect of the left forelimb coronary band, and the horse retracted its foot during palpation of the area. At the affected site, the skin surface was macerated, and there was purulent exudate draining from the central portion of the swelling. The remaining findings on physical examination were unremarkable. The left forelimb shoe was removed, and radiography of the left forelimb foot was performed (Figure 1).
Lateromedial (A), horizontal dorsopalmar (B), flexed dorsoproximal-palmarodistal oblique (C), and dorsal 45° lateral-palmaromedial oblique (D) radiographic views of the left forelimb foot of a 28-year-old 537-kg American Quarter Horse mare evaluated because of left forelimb lameness of 1 month’s duration. A—The round marker indicates the level of the coronary band. B and C—Lateral is toward the left. D—Lateral is toward the right.
Citation: Journal of the American Veterinary Medical Association 259, 11; 10.2460/javma.20.03.0107
Lateromedial (A), horizontal dorsopalmar (B), flexed dorsoproximal-palmarodistal oblique (C), and dorsal 45° lateral-palmaromedial oblique (D) radiographic views of the left forelimb foot of a 28-year-old 537-kg American Quarter Horse mare evaluated because of left forelimb lameness of 1 month’s duration. A—The round marker indicates the level of the coronary band. B and C—Lateral is toward the left. D—Lateral is toward the right.
Citation: Journal of the American Veterinary Medical Association 259, 11; 10.2460/javma.20.03.0107
Lateromedial (A), horizontal dorsopalmar (B), flexed dorsoproximal-palmarodistal oblique (C), and dorsal 45° lateral-palmaromedial oblique (D) radiographic views of the left forelimb foot of a 28-year-old 537-kg American Quarter Horse mare evaluated because of left forelimb lameness of 1 month’s duration. A—The round marker indicates the level of the coronary band. B and C—Lateral is toward the left. D—Lateral is toward the right.
Citation: Journal of the American Veterinary Medical Association 259, 11; 10.2460/javma.20.03.0107
Formulate differential diagnoses, then continue reading.
Diagnostic Imaging Findings and Interpretation
Lateromedial and 0° (horizontal) dorsopalmar radiographic projections were obtained and reviewed first. A large, ovoid, smoothly marginated concave defect was superimposed with a radiolucent region in the lateral aspect of the digital cushion (lateral heel; Figure 2). There was a smooth area of osseous proliferation at the distolateral aspect of the proximal phalanx, blunting of the lateral palmar process of the distal phalanx, and moderately increased soft tissue opacity overlying the lateral coronary band, consistent with the site of drainage. Additional radiographic projections included dorsal 45° lateral-palmaromedial oblique (D45L-PaMO), dorsal 45° medial-palmarolateral oblique (D45M-PaLO), and a flexed dorsoproximal-palmarodistal oblique obtained with the use of a foot stand. The aforementioned radiolucency was ovoid with smooth margins at its junction with the lateral palmar process of the distal phalanx indicative of osteolysis. There was a thin area of sclerosis at the rim of the concave osseous defect. A thin rim of mineral opacity surrounded the ovoid radiolucency (palmarly and distally) with multifocal and ill-defined central mineral opacities. Additional radiographic findings included mild osteophytosis of the proximal and distal interphalangeal joints. Considering the smooth margins of the area of increased radiolucency and the mare’s history of chronic lameness, we suspected pressure resorption and osteolysis of the lateral palmar process as sequelae to a soft tissue mass confined to the hoof capsule. Differential diagnoses included atypical keratoma, granuloma, primary chronic abscess formation, epidermal inclusion cyst, and soft tissue neoplasia (eg, chondroma or chondrosarcoma). Because of the smooth margination of the abnormal radiolucency with a sclerotic rim, septic osteitis was considered unlikely. The enthesopathy of the distolateral aspect of the proximal phalanx was at the attachment of the middle scutum and likely caused by previous trauma. The following day, the horse underwent general anesthesia for CT of the left front foot examination to fully characterize the lesion and facilitate planning for surgical resection.
Same images as in Figure 1. There is a large, ovoid, smoothly marginated concave defect (arrows) that is confluent with a region of increased radiolucency at the lateral aspect of the digital cushion (lateral heel), with a sclerotic rim of the concave osseous defect. There is blunting of the lateral palmar process of the distal phalanx (dashed ovals), moderately increased soft tissue opacity (dashed arrows) overlying the lateral aspect of the coronary band (consistent with the site of drainage observed on clinical examination), and a smooth area of osseous proliferation at the distolateral aspect of the proximal phalanx (arrowheads).
Citation: Journal of the American Veterinary Medical Association 259, 11; 10.2460/javma.20.03.0107
Same images as in Figure 1. There is a large, ovoid, smoothly marginated concave defect (arrows) that is confluent with a region of increased radiolucency at the lateral aspect of the digital cushion (lateral heel), with a sclerotic rim of the concave osseous defect. There is blunting of the lateral palmar process of the distal phalanx (dashed ovals), moderately increased soft tissue opacity (dashed arrows) overlying the lateral aspect of the coronary band (consistent with the site of drainage observed on clinical examination), and a smooth area of osseous proliferation at the distolateral aspect of the proximal phalanx (arrowheads).
Citation: Journal of the American Veterinary Medical Association 259, 11; 10.2460/javma.20.03.0107
Same images as in Figure 1. There is a large, ovoid, smoothly marginated concave defect (arrows) that is confluent with a region of increased radiolucency at the lateral aspect of the digital cushion (lateral heel), with a sclerotic rim of the concave osseous defect. There is blunting of the lateral palmar process of the distal phalanx (dashed ovals), moderately increased soft tissue opacity (dashed arrows) overlying the lateral aspect of the coronary band (consistent with the site of drainage observed on clinical examination), and a smooth area of osseous proliferation at the distolateral aspect of the proximal phalanx (arrowheads).
Citation: Journal of the American Veterinary Medical Association 259, 11; 10.2460/javma.20.03.0107
On CT, a smoothly marginated, ovoid, mixed soft tissue and mineral-attenuating mass (approx 3.0 X 2.2 X 3.8 cm) was evident and presumed to have caused resorption of the lateral palmar process of the distal phalanx (Figure 3). The unossified lateral ungual cartilage was thickened and heterogeneously attenuating compared with the medial ungual cartilage. Normal laminar distinction with mild deformation and laminar thickening was present at the site of the mass. The findings on CT suggested that atypical keratoma, soft tissue neoplasia, and epidermal inclusion cyst were the most likely differential diagnoses and refuted primary chronic abscess formation and septic osteitis. Neither the mass nor the resorptive bone lesion contacted the navicular bone, navicular bursa, or distal interphalangeal joint. Radiopaque markers placed on the hoof wall confirmed the margins of the mass and guided the placement of surgical transection sites for partial hoof wall resection. Contrast-enhanced CT could have been used to further characterize the etiology of the soft tissue mass; however, this information would also have been obtained through histologic assessment.
Transverse (A and B) and dorsal (C and D) plane and dorsoproximolateral-palmarodistomedial 3-D reconstruction (E) CT images of the left forelimb foot of the horse described in Figure 1, with lateral toward the right in all images. A smoothly marginated, ovoid, mixed soft tissue and mineral-attenuating mass (approx 3.0 X 2.2 X 3.8 cm) has replaced much of the lateral palmar process of the distal phalanx (arrows). The junction of the mass with the bone (chevron) has a smooth sclerotic rim, most consistent with pressure resorption. A and C—Metal markers (dotted rectangle) have been placed on the hoof wall to help in planning the surgical approach. The images are displayed in a bone algorithm (window width, 1,340 HU; window level, 480 HU; A and C) or a soft tissue algorithm (window width, 400 HU; window level, 40 HU; B and D), with a 2-mm slice thickness. Lines across images represent the plane of the paired orthogonal image.
Citation: Journal of the American Veterinary Medical Association 259, 11; 10.2460/javma.20.03.0107
Transverse (A and B) and dorsal (C and D) plane and dorsoproximolateral-palmarodistomedial 3-D reconstruction (E) CT images of the left forelimb foot of the horse described in Figure 1, with lateral toward the right in all images. A smoothly marginated, ovoid, mixed soft tissue and mineral-attenuating mass (approx 3.0 X 2.2 X 3.8 cm) has replaced much of the lateral palmar process of the distal phalanx (arrows). The junction of the mass with the bone (chevron) has a smooth sclerotic rim, most consistent with pressure resorption. A and C—Metal markers (dotted rectangle) have been placed on the hoof wall to help in planning the surgical approach. The images are displayed in a bone algorithm (window width, 1,340 HU; window level, 480 HU; A and C) or a soft tissue algorithm (window width, 400 HU; window level, 40 HU; B and D), with a 2-mm slice thickness. Lines across images represent the plane of the paired orthogonal image.
Citation: Journal of the American Veterinary Medical Association 259, 11; 10.2460/javma.20.03.0107
Transverse (A and B) and dorsal (C and D) plane and dorsoproximolateral-palmarodistomedial 3-D reconstruction (E) CT images of the left forelimb foot of the horse described in Figure 1, with lateral toward the right in all images. A smoothly marginated, ovoid, mixed soft tissue and mineral-attenuating mass (approx 3.0 X 2.2 X 3.8 cm) has replaced much of the lateral palmar process of the distal phalanx (arrows). The junction of the mass with the bone (chevron) has a smooth sclerotic rim, most consistent with pressure resorption. A and C—Metal markers (dotted rectangle) have been placed on the hoof wall to help in planning the surgical approach. The images are displayed in a bone algorithm (window width, 1,340 HU; window level, 480 HU; A and C) or a soft tissue algorithm (window width, 400 HU; window level, 40 HU; B and D), with a 2-mm slice thickness. Lines across images represent the plane of the paired orthogonal image.
Citation: Journal of the American Veterinary Medical Association 259, 11; 10.2460/javma.20.03.0107
Treatment and Outcome
The horse underwent general anesthesia, abaxial sesamoid nerve blockade with liposomal bupivacaine suspension (Nocita; Aratana Therapeutics Inc), and regional limb perfusion with 2 g amikacin for partial resection of the lateral hoof wall. The surgery revealed a thick-capsuled mass filled with caseous exudate, which was removed and submitted for histologic examination. The defect was packed with sterile polyhexamethylene biguanide-impregnated gauze (Kerlix AMD; Cardinal Health) that had been soaked in sterile saline (0.9% NaCl) solution, and a foot bandage was placed. The horse recovered without complication. Phenylbutazone (1 g, PO, q 12 h for 5 days), trimethoprim-sulfamethoxazole (30 mg/kg, PO, q 12 h for 14 days), and omeprazole (1 mg/kg, PO, q 24 h) were administered, and subsequent bandage changes for the treated foot were performed with calcium alginate dressings.
Histologic results indicated that the mass lining was composed of redundant flakes of orthokeratotic hyperkeratosis with no identifiable histoanatomic structures and that the mass was composed of abundant collagen and degenerate bone admixed with minimal inflammation, consistent with a keratoma. By 4 days after discontinuation of all medications (18 days after surgery), the horse’s lameness was not detectable at a walk, granulation tissue had covered the exposed bone, and there was no purulent drainage; however, the horse developed acute colitis, characterized by profuse watery diarrhea and dehydration, and was euthanized.
Comments
Keratomas are uncommon lesions in horses that form from an overproduction of keratin by the stratum basale epidermal cells derived from the coronary band, the inner cornified hoof wall layer, and, rarely, the sole.1,2 Keratomas grow between the hoof wall and distal phalanx, causing little discomfort until they impinge on the weight-bearing surface of the foot or laminar dermis or are associated with local infection.2 Although the etiology of keratomas remains unknown, they have been associated with chronic irritation, trauma, or infection.3 Clinical signs include moderate to severe lameness and may be concurrent with hoof wall deformation4 or supracoronary swellings.5 The most common locations for keratomas are at the toe or dorsomedial and dorsolateral aspects of the foot.4 Keratomas at the lateral quarter may have a solar component.1,6 This keratoma in the horse of the present report was atypical because it did not involve the sole and completely obliterated the lateral palmar process of the distal phalanx.
Radiographic findings for the horse of the present report that were supportive of a keratoma were a soft tissue opacity similar to the keratinized hoof wall and a smoothly marginated osteolytic region in the distal phalanx.4 Multiple oblique radiographic projections were critical in this case to fully characterize the lesions and prioritize differential diagnoses. To formulate an effective treatment plan, the distinction between a soft tissue mass, primary chronic abscess formation, and septic osteitis of the distal phalanx was required. The proximal-to-distal oblique projections best outlined the smoothly marginated junction between the keratoma and the distal phalanx. Along with the identification of the sclerotic rim, these projections effectively ruled out primary chronic abscess formation and septic osteitis and thus confirmed that surgical resection was a necessary treatment to resolve the lameness and local infection. Computed tomography was advantageous to the diagnostic and treatment plan in this case by providing more sensitive diagnostic information, refining the differential diagnosis list, and aiding in surgical planning.7,8 Although ultrasonography can be used with keratomas that extend proximal to the coronary band,9 this diagnostic technique was not required. In addition, by accurately identifying the location of the keratoma, a smaller hoof wall defect was able to be created and reduced postoperative complications.7,10 Given the proximal location of this horse’s keratoma, as established by CT examination, removal of the sole was not required.
Although diagnostic imaging can commonly be used to establish a presumptive diagnosis of keratoma, definitive diagnosis requires histologic examination.2 Recurrence rates after surgical resection range from 12% (3/26) to 38% (6/16).10,11 The prognosis for return to function at the previous performance level after surgical removal of a keratoma is very good.10
References
- 1. ↑
Keegan KG, Dent EV, Wilson DA, et al. Repeatability of subjective evaluation of lameness in horses. Equine Vet J. 2010;42(2):92–97. doi: 10.2746/042516409X479568
- 2. ↑
O'Grady S, Horne P. Lameness caused by a solar keratoma: a challenging differential diagnosis. Equine Vet Educ. 2001;13(2):87–89. doi: 10.1111/j.2042–3292.2001.tb01894.x
- 4. ↑
Wagner P, Balch-Burnett O, Merrit F. Surgical management of keratomas in the foot of the horse. Equine Pract. 1986;8(5):12–14.
- 5. ↑
Reeves MJ, Yovich JV, Turner AS. Miscellaneous conditions of the equine foot. Vet Clin North Am Equine Pract. 1989;5(1):221–242. doi: 10.1016/s0749-0739(17)30612-0
- 6. ↑
Miller SM, Katzwinkel RH. Solar keratoma: an atypical case. J S Afr Vet Assoc. 2015;86(1):1257. doi: 10.4102/jsava.v86i1.1257
- 7. ↑
Getman LM, Davidson EJ, Ross MW, Leitch M, Richardson DW. Computed tomography or magnetic resonance imaging-assisted partial hoof wall resection for keratoma removal. Vet Surg. 2011;40(6):708–714. doi: 10.1111/j.1532-950X.2011.00864.x
- 8. ↑
Anderson JDC. Characterisation of keratomas of the equine digit using contrast enhanced computed tomography (CECT) to facilitate surgical removal and provide prognostic information regarding post operative morbidity. Equine Vet J. 2013;45(suppl 44):2–19.
- 9. ↑
Morris SL, Nabors BE, Gambino JM, Fontenot RL. What Is Your Diagnosis? J Am Vet Med Assoc. 2017;250(8):841–844. doi: 10.2460/javma.250.8.841
- 10. ↑
Boys Smith SJ, Clegg PD, Hughes I, Singer ER. Complete and partial hoof wall resection for keratoma removal: post operative complications and final outcome in 26 horses (1994–2004). Equine Vet J. 2006;38(2):127–133. doi: 10.2746/042516406776563288
- 11. ↑
Mair TS, Linnenkohl W. Low-field magnetic resonance imaging of keratomas of the hoof wall. Equine Vet Educ. 2012;24(9):459–468. doi: 10.1111/j.2042–3292.2011.00367.x
