What Is Your Diagnosis?

Steven S. Trostle Blue Ridge Equine Clinic, 4510 Mockernut Ln, Earlysville, VA 22936

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 DVM, MS, DACVS
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Lisa L. Casinella Blue Ridge Equine Clinic, 4510 Mockernut Ln, Earlysville, VA 22936

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 DVM
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Karen H. Sussman Blue Ridge Equine Clinic, 4510 Mockernut Ln, Earlysville, VA 22936

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 DVM
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Norm Rantanen PO Box 1351, Fallbrook, CA 92028.

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 DVM, MS, DACVR

History

A 2-year-old Thoroughbred filly was evaluated for lameness of the left hind limb of 4 months’ duration. An osteochondral fragment had been arthroscopically removed from the sagittal ridge of the left metatarsophalangeal joint when the horse was a yearling. The horse was initially referred for a nuclear medicine examination of the hind limbs and pelvis; no abnormalities were detected. The following day, a lameness examination was performed and the horse was lame (grade 3/4)1 when trotting. The lameness became more pronounced after the tibiotarsal and stifle joints were flexed. Intra-articular anesthesia of the metatarsophalangeal joint and perineural anesthesia of the peroneal and tibial nerves did not affect the lameness; however, the lameness improved by 80% after intra-articular anesthesia of the femoropatellar joint. Radiographs of the left stifle joint were obtained (Figure 1).

Figure 1—
Figure 1—

Craniocaudal (A) and lateral (B) radiographic views of the left stifle joint of a 2-year-old Thoroughbred filly evaluated for lameness of the left hind limb of 4 months’ duration.

Citation: Journal of the American Veterinary Medical Association 230, 5; 10.2460/javma.230.5.663

Determine whether additional imaging studies are required, or make your diagnosis from Figure 1—then turn the page

Diagnostic Imaging Findings and Interpretation

On the lateral radiographic view, a circular small focal area of radiolucency is evident in the patella (Figure 2).

Figure 2—
Figure 2—

Same radiographic view as in Figure 1B. Notice the small circular area of bony lysis (black arrow) and adjacent sclerosis of the patella.

Citation: Journal of the American Veterinary Medical Association 230, 5; 10.2460/javma.230.5.663

Comments

A cranioproximal-craniodistal (skyline) radiographic view of the patella and trochlear ridges was obtained (Figure 3). A circular radiolucent bony defect is evident on the medial articulating surface of the patella. The radiolucent defect is circumscribed by sclerosis. Differential diagnoses for this bony lesion included osteochondrosis dissecans, osteomyelitis, or an osseous cystlike lesion of the patella.

Figure 3—
Figure 3—

Cranioproximal-craniodistal radiographic view of the left patella and trochlear ridges of the horse in Figure 1. Notice a circular (approx 8-mm-diameter) radiolucent osseous defect (black arrow) in the medial articular aspect of the patella. In addition, marked sclerosis is evident adjacent to the radiolucent area.

Citation: Journal of the American Veterinary Medical Association 230, 5; 10.2460/javma.230.5.663

The cranioproximal-craniodistal radiographic view substantially aided in locating the lesion radiographically. This radiographic view is not typically considered as part of a standard series of radiographs for the stifle joint, but it is indicated for detection of bony lesions of the patella or femoral trochlear groove in horses.2 Cranioproximal-craniodistal patellar radiographic views can be obtained with the horse standing with the stifle joint flexed and the radiographic film cassette held horizontal to the ground surface.2 Alternatively, the horse can be positioned in dorsal recumbency with the stifle joint flexed and the radiographic film cassette held vertically.2

Osseous cystlike lesions are commonly recognized conditions in bones and joints of horses and other species.3 The pathophysiologic causes of osseous cystlike lesions are controversial and divided between 2 theories: developmental and traumatic.3 In the developmental theory, abnormal cartilage is believed to infold into the underlying bony spongiosa. The infolded cartilage becomes necrotic and matrix remains nonmineralized so that osteoclasts and blood vessels do not migrate into the defect to enable repair.4 In the traumatic theory, damage to the articular cartilage may be associated with focal subchondral ischemia or pressure necrosis of the subchondral bone.5 Others suggest that if there is damage to the articular cartilage and subchondral bone, synovial fluid is forced under pressure (caused by weight bearing and exercise) against the subchondral bone, resulting in resorption of the subchondral bone and subsequent formation of an osseous cyst.3

To the best of our knowledge, osseous cystlike lesions of the patella have not been reported in horses. In the horse reported here, no abnormalities were detected during radiography of the patella in the contralateral limb. It is difficult to determine whether the cystlike lesion was secondary to trauma or was developmental. External trauma resulting in fracture of the patella has been reported.6,7 Most patellar fractures have occurred secondary to direct trauma such as a kick or the impact of the stifle joint hitting a fence.6 If trauma occurs with the stifle joint in flexion, the patella can be fixed against the trochlear groove of the femur. Although the morphology of fractures may vary, sagittal fractures on the proximal medial aspect of the patella are most common.7 In the horse of this report, the osseous cystlike lesion was in the same area as many medial sagittal fractures of the patella. Therefore, cartilage or subchondral bone damage or both may have been caused by the same mechanism that would have caused a fracture.

Radiographically, the osseous cystlike lesion of the patella does have adjacent bony sclerosis. Sclerosis is the localized formation of new bone within bone.8 Sclerosis is the response to bone remodeling and can occur secondary to several stimuli.8 In the horse reported here, no abnormalities were detected in the patella during nuclear scintigraphy, suggesting that bony changes in the patella may have been caused by chronic stimuli, potentially supporting a developmental pathophysiologic process. The scintigraphic findings in horses with osseous cystlike lesions are inconsistent.9

  • 1

    American Association of Equine Practitioners. Guide to veterinary service and judging of equine events. 4th ed. Lexington, Ky: American Association of Equine Practitioners, 1991;19.

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  • 2

    Butler JA, Colles CM, Dyson SJ, et al. The stifle and patella. In: Clinical radiology of the horse. 2nd ed. Malden, Mass: Blackwell Scientific Publications, 2000;285326.

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  • 3

    Baxter GM. Subchondral cystic lesions in the horse. In: McIlwraith CW, Trotter GW, eds. Joint disease in the horse. Philadelphia: WB Saunders Co, 1996;384396.

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  • 4

    Jeffcott LB, Kold SE, Melsen F. Aspects of the pathology of stifle bone cysts in the horse. Equine Vet J 1983;15:304311.

  • 5

    Kold SE, Hickman J. An experimental study of the healing process of equine chondral and osteochondral defects. Equine Vet J 1986;18:1824.

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  • 6

    Marble GP, Sullins KE. Arthroscopic removal of patellar fracture fragments in horses: five cases (1989–1998). J Am Vet Med Assoc 2000;216:17991801.

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  • 7

    Dyson S, Wright I, Kold S, et al. Clinical and radiographic features, treatment and outcome in 15 horses with fracture of the medial aspect of the patella. Equine Vet J 1992;24:264268.

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  • 8

    Dyson SJ. Radiography and radiology. In: Ross MW, Dyson SJ, eds. Diagnosis and management of lameness. Philadelphia: WB Saunders Co, 2003;153165.

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  • 9

    Dyson SJ, Martinelli MJ, Pilsworth R, et al. Equine scintigraphy. Newmarket, UK: Equine Veterinary Journal Ltd, 2003;93.

  • Figure 1—

    Craniocaudal (A) and lateral (B) radiographic views of the left stifle joint of a 2-year-old Thoroughbred filly evaluated for lameness of the left hind limb of 4 months’ duration.

  • Figure 2—

    Same radiographic view as in Figure 1B. Notice the small circular area of bony lysis (black arrow) and adjacent sclerosis of the patella.

  • Figure 3—

    Cranioproximal-craniodistal radiographic view of the left patella and trochlear ridges of the horse in Figure 1. Notice a circular (approx 8-mm-diameter) radiolucent osseous defect (black arrow) in the medial articular aspect of the patella. In addition, marked sclerosis is evident adjacent to the radiolucent area.

  • 1

    American Association of Equine Practitioners. Guide to veterinary service and judging of equine events. 4th ed. Lexington, Ky: American Association of Equine Practitioners, 1991;19.

    • Search Google Scholar
    • Export Citation
  • 2

    Butler JA, Colles CM, Dyson SJ, et al. The stifle and patella. In: Clinical radiology of the horse. 2nd ed. Malden, Mass: Blackwell Scientific Publications, 2000;285326.

    • Search Google Scholar
    • Export Citation
  • 3

    Baxter GM. Subchondral cystic lesions in the horse. In: McIlwraith CW, Trotter GW, eds. Joint disease in the horse. Philadelphia: WB Saunders Co, 1996;384396.

    • Search Google Scholar
    • Export Citation
  • 4

    Jeffcott LB, Kold SE, Melsen F. Aspects of the pathology of stifle bone cysts in the horse. Equine Vet J 1983;15:304311.

  • 5

    Kold SE, Hickman J. An experimental study of the healing process of equine chondral and osteochondral defects. Equine Vet J 1986;18:1824.

    • Search Google Scholar
    • Export Citation
  • 6

    Marble GP, Sullins KE. Arthroscopic removal of patellar fracture fragments in horses: five cases (1989–1998). J Am Vet Med Assoc 2000;216:17991801.

    • Search Google Scholar
    • Export Citation
  • 7

    Dyson S, Wright I, Kold S, et al. Clinical and radiographic features, treatment and outcome in 15 horses with fracture of the medial aspect of the patella. Equine Vet J 1992;24:264268.

    • Search Google Scholar
    • Export Citation
  • 8

    Dyson SJ. Radiography and radiology. In: Ross MW, Dyson SJ, eds. Diagnosis and management of lameness. Philadelphia: WB Saunders Co, 2003;153165.

    • Search Google Scholar
    • Export Citation
  • 9

    Dyson SJ, Martinelli MJ, Pilsworth R, et al. Equine scintigraphy. Newmarket, UK: Equine Veterinary Journal Ltd, 2003;93.

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