A4-year-old Quarter Horse stallion was referred to the Veterinary Teaching Hospital at Washington State University for MR imaging of the forelimb feet. The horse was in training as a reining horse and had a 10-month history of left forelimb lameness. On previous occasions, the lameness had been localized to the left forelimb foot by use of a palmar digital nerve block. Multiple radiographic views of the proximal and middle phalanges (pastern region) and the foot had been obtained in which no abnormalities were detected. The horse had been previously treated with intra-articular injections of cortisone into the distal interphalangeal joint, which failed to improve the lameness. Ultrasonography of the soft tissues of the palmar aspect of the pastern region prior to admission revealed no abnormalities.
On initial examination at the Veterinary Teaching Hospital, the horse had moderate (grade 3/51) lameness of the left forelimb when trotted over a smooth, hard surface. The horse was not responsive to the application of hoof testers on either forelimb foot. No palpable abnormalities were detected in the horse's limbs. Administration of a palmar digital nerve block in the horse's left forelimb foot eliminated the lameness. Physical examination revealed no other abnormalities. Radiographs of the left forelimb foot were obtained, including lateromedial and dorsal 60-degree proximal-palmarodistal oblique views of the foot and the proximal and middle phalanges and a palmaroproximal-palmarodistal oblique (skyline) view of the distal sesamoid (navicular) bone; no pathologic changes were detected radiographically. To further investigate the cause of the lameness, MR imaging was performed on both forelimb feet.
The horse was sedated with xylazine and butorphanol (administered IV), and anesthesia was induced via IV administration of ketamine and guaifenesin. Anesthesia was maintained with 2% to 4% isoflurane in oxygen. By use of a 1.0-T imaging system,a MR evaluation of both forelimb feet was performed. Standard equine foot sequences that had been developed at Washington State University2 were used, including sagittal and transverse PD-weighted, T2-weighted, and short tau inversion recovery sequences, as well as a transverse gradient echo sequence focused over and oriented perpendicular to the long axis of the navicular bone.
Magnetic resonance images of the feet of the right and left forelimbs revealed a high number of synovial invaginations and mild edema of the navicular bone bilaterally. There was also a circumscribed soft tissue mass in the distal aspect of the digital flexor tendon sheath dorsal to the lateral aspect of the deep digital flexor tendon in the left forelimb foot (Figure 1). This cylindrical lesion measured 1.0 cm in diameter and extended 1.7 cm proximally from the level of the ligamenta sesamoidea collateralia (navicular suspensory ligament; Figure 2). Compared with tendons (which appear black in imaging sequences), the soft tissue mass had increased signal intensity on PD images. It had low signal intensity (gray-black appearance) on T2-weighted and short tau inversion recovery MR images, indicating no free water content. The increased signal intensity on PD-weighted MR images suggested that the mass contained a high level of available free protons and may have high protein content. On the basis of the MR images, a diagnosis of a space-occupying proteinaceous mass within the digital flexor tendon sheath of the left forelimb was made.
Transverse MR images of the distal portion of the left forelimb of a horse with left forelimb lameness of 10 months' duration. A—Proton density image of the middle phalanx (P2) just proximal to the ligamenta sesamoidea collateralia (navicular suspensory ligament) of the left forelimb foot. Notice the moderate signal intensity space-occupying circular mass (arrows) located dorsal and lateral to the lateral lobe of the deep digital flexor (DDF) tendon; a similar mass was not detected in the right forelimb foot. B—T2-weighted image of the left forelimb at the level of P2, just proximal to the navicular suspensory ligament. The mass (arrows) is located dorsal and lateral to the deep digital flexor tendon and has low signal intensity, indicating an absence of fluid. The mass is impinging on the lateral lobe of the deep digital flexor tendon.
Citation: Journal of the American Veterinary Medical Association 228, 12; 10.2460/javma.228.12.1918
Sagittal PD MR images of the left (A) and right (B) forelimbs of the horse in Figure 1. Within the foot of the left forelimb, the mass (arrows) can be seen just proximal to the distal sesamoid bone and distal to the level of the lateral heel bulb. The same image slice of the unaffected right forelimb foot is provided for comparison.
Citation: Journal of the American Veterinary Medical Association 228, 12; 10.2460/javma.228.12.1918
One day after the MR evaluation, the digital flexor tendon sheath of the left forelimb was injected with a local anesthetic agent.3 When the horse was evaluated 30 minutes after the injection, this diagnostic local anesthetic technique had eliminated 90% of the forelimb lameness. Following this intrasynovial injection, normal skin sensation in the distal portion of the left forelimb was apparent.
On the basis of the findings of MR imaging and the horse's response to diagnostic local anesthesia of the digital flexor tendon sheath, a tenoscopic evaluation of the distal aspect of the digital flexor tendon sheath of the left forelimb was performed. The horse was anesthetized routinely and positioned in right lateral recumbency. The distal portion of the left forelimb was aseptically prepared and draped for surgery. An 18-gauge, 4-cm needle was placed into the digital flexor tendon sheath (distal to the proximal annular ligament), and the sheath was distended with 50 mL of sterile saline (0.9% NaCl) solution. A No. 11 scalpel blade was used to make a 3-mm stab incision into the sheath lateral to the deep digital flexor tendon (2.5 cm distal to the base of the lateral sesamoid bone). The approach used was a modification of that previously described4 for tenoscopy of the digital flexor tendon sheath. An arthroscopic cannula and blunt trocar for a 30°-angled, 2.7-mm rigid endoscopeb were inserted in a proximodistal direction into the digital flexor tendon sheath. The blunt trocar was removed, and the endoscope was inserted. The distal extent of the digital flexor tendon sheath was examined, and a soft tissue mass adjacent to the lateral border of the deep digital flexor tendon was identified. This mass corresponded to the lesion observed on the MR images. An 18-gauge, 4-cm needle was placed into the digital flexor tendon sheath proximal to the mass, and a 1-cm instrument portal was made in the lateral side of the sheath with a No. 11 scalpel blade. Because the mass impinged on the deep digital flexor tendon and digital flexor tendon sheath, its removal was achieved with tenoscopic guidance. The soft tissue mass was removed by use of a 4 × 10-mm Ferris Smith rongeur.c An outer chitinous capsule with a central lumen was grasped and removed. Multiple fragments of the mass were removed from the sheath until the distal lateral portion appeared similar to the portion medial to the deep digital flexor tendon. After mass removal, the distal digital annular ligament was transected with a hook knifed just lateral to midline from within the digital flexor tendon sheath. This was done to relieve pressure over the deep digital flexor tendon in this area. The surgical incisions were closed with polypropylene suture in a simple interrupted pattern. The lower portion of the left forelimb was placed in a sterile compression bandage, and the horse was allowed to recover from anesthesia.
Fragments of the mass, including the chitinous shell, were submitted for histologic examination. Tissues were fixed in neutral-buffered 10% formalin, embedded in paraffin, sectioned at 6-mm intervals, and stained with H&E. Fragments of the cyst wall consisted of well-differentiated stratified squamous epithelium of variable thickness (5 to 20 cell layers); toward the cyst lumen, keratinization was evident. The entire cyst lumen was not visible, but fragments consisted of densely packed laminated keratin. Other fragments were comprised of dense, fibrous connective tissue that was partially lined by synoviocytes. Numerous clusters of small blood vessels (neovascularization) were detected throughout the collagen fragments.
The left forelimb of the horse was maintained in a compression bandage until suture removal 14 days after surgery. The horse was treated with phenylbutazone (4.4 mg/kg [2.0 mg/lb], PO, q 24 h) for 10 days to decrease pain and postoperative inflammation. The digital flexor tendon sheath was injected with hyaluronic acide (20 mg) and methylprednisolone acetatef (120 mg) at 7 days after the surgery to decrease inflammation and minimize the risk of the development of scar tissue and adhesions in the sheath.
The horse was discharged, and the owner was given instructions for a rehabilitation program. Stall confinement for 30 days from the date of surgery was recommended; hand walking was started beginning with a few minutes each day and slowly increasing the amount of time that the horse was walked to 20 or 30 min/d. After 60 days, the horse began jogging and was allowed free movement in a small paddock (9 × 9 m [30 × 30 feet]). The horse was kept in a small paddock for 120 days and started a walk-jog program under saddle at the end of this period. Six months after surgery, the horse was returned to full training.
Follow-up information was obtained from the referring veterinarian and trainer and via a follow-up lameness evaluation. Having returned to training, the horse subsequently competed as a reining horse 9 months after surgery. Except for a sole abscess in the left forelimb foot, forelimb lameness had not recurred, and the horse remained in competition without the need for medication for at least 2 years after the surgery.
Discussion
To the authors' knowledge, epidermoid cysts in the digital flexor tendon sheath of horses have not been reported previously. Epidermoid cysts (also known as epidermal inclusion cysts, follicular cysts, or infundibular cysts) originate from stratified squamous epithelium and are common skin masses in animals. In horses, cutaneous epidermal cysts have been located primarily on the head and distal portions of the limbs; with regard to limbs, such cysts have been identified on the dorsal aspect of the metatarsophalangeal region and the lateral aspect of the metatarsus.5
Epidermoid cysts have also been reported in noncutaneous tissues, most commonly in nervous, skeletal, and reproductive tissues. Osseous epidermoid cysts have been identified in the distal phalanx in dogs6,7 and the mandible of horses.8 Nervous system epidermoid cysts occur most commonly in the brain and have been reported in horses, dogs, and cats.9–12 There have been single reports of epidermoid cysts in large animals other than horses, including ovarian epidermoid cysts in 3 bovids.13 These previous reports indicate that epidermoid cysts can occur in locations other than skin, most likely as embryologic remnants.
Magnetic resonance imaging is a valuable diagnostic tool for use in humans and horses with musculoskeletal problems; in the horse of this report, it enabled distinct demarcation of the space-occupying cyst. Magnetic resonance evaluation should be considered in all lame horses in which a definitive diagnosis cannot be made via radiography, ultrasonography, or other imaging techniques, especially when the lameness has been localized to a specific anatomic region by use of diagnostic local anesthesia. The information gained through MR imaging of the foot changed the treatment of the horse of this report to include tenoscopic removal of the mass, an option that would not have otherwise been considered.
Tenoscopy and tenoscopic mass removal allowed the horse to successfully resume a performance career. The procedure of tenoscopy has been described, but the portals used in this horse were different because of the location of the lesion in the distal aspect of the sheath. Diagnostic local anesthesia in the digital flexor tendon sheath helped confirm the source of the horse's lameness to the sheath or surrounding structures that could be desensitized by local anesthetic absorbed from the sheath. Injection of the digital flexor tendon sheath with hyaluronic acid can decrease adhesion formation14; therefore, injection of hyaluronic acid into the sheath was performed following surgery in the horse of this report. Although the cyst in this horse was unusual, this case highlights the value of MR imaging in providing data from which an accurate diagnosis can be made, resulting in the selection of the optimal treatment.
ABBREVIATIONS
| MR | Magnetic resonance |
| PD | Proton density |
Philips Gyroscan NT, Philips Medical Systems, Best, The Netherlands.
Storz rigid endoscope, Karl Storz Vet, Goleta, Calif.
Ferris-Smith rongeurs, Karl Storz Vet, Goleta, Calif.
Storz hook knife, Karl Storz Vet, Goleta, Calif.
Hylartin, Pharmacia & Upjohn Co, Kalamazoo, Mich.
Depomedrol, Pharmacia & Upjohn Co, Kalamazoo, Mich.
References
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