JPC SYSTEMIC PATHOLOGY

Musculoskeletal System

March 2025

M-M01 (NP)

 

Signalment (JPC #2314368): 3-day-old foal 

 

History: This foal died of unknown causes. This section is from a 2 cm raised nodule on the shaft of the radius. Similar nodules were present on the ribs and other long bones.

 

HISTOPATHOLOGIC Description: Bone, radius (per contributor): Elevating the periosteum, extending from the periosteum into the underlying more normal cancellous bone, are multiple 1 to 5 mm diameter nodules composed of an outer rim of hyaline cartilage undergoing variably organized endochondral ossification with thin zones of chondrocyte proliferation, hypertrophy, and calcification with formation of primary and secondary spongiosa progressing into bony trabeculae that blend into the more normal underlying cancellous bone. The underlying cancellous bone trabeculae contain variably sized islands of cartilage (retained cartilage cores) and are moderately thickened. The marrow cavities contain sparse or absent hematopoietic cells. 

 

MORPHOLOGIC DIAGNOSIS: Bone, radius: Osteochondromas, multiple (cartilaginous exostosis), breed unspecified, equine.

 

Condition: Multiple cartilaginous exostoses (MCE) or osteochondromatosis

 

SYNONYMS: Single: Osteochondroma; Multiple: Hereditary multiple exostoses, osteocartilaginous exostoses; also known as diaphyseal aclasis, metaphyseal aclasis, dyschondroplasia, hereditary deforming chondrodysplasia

 

GENERAL DISCUSSION:

• Benign, cartilage-capped tumor-like exostosis arising from the metaphysis of any bone that undergoes endochondral ossification
• May be single (monostotic) or multifocal (polyostotic or multiple cartilaginous exostoses)

• In humans, dogs, and horses, the polyostotic form is inherited, occurs in young animals, and would be more accurately classified as a skeletal dysplasia rather than a neoplasm

 

PATHOGENESIS:

• Dog, horse:

• Genetic defect is unknown in animals
• In humans, EXT1 and 2 genes has been associated with multiple exostoses: Decreased heparan sulfate -> Increased responsiveness to bone morphogenic protein -> Excessive chondrogenesis at the perichondrial ring
• Tend to be bilaterally symmetrical, and grow corresponding to period active bone growth

• Growth of osteochondroma in an adult is suggestive of transformation
• Cat:
  • Cause undetermined, but feline leukemia virus and transmissible feline sarcoma implicated; associated with a frameshift mutation in the EXT1 gene; although spontaneous osteochondromatosis is reported (Gómez, J Comp Pathol. 2023)
  • Occurs primarily in skeletally mature animals within flat bones that develop from intramembranous ossification
  • Behaves more like an actual neoplasm and may undergo malignant transformation

 

TYPICAL CLINICAL FINDINGS:

• Dog, horse:
  • Young animals; lesions tend to be bilaterally symmetrical and increase in size with active bone growth

1. Often animals are asymptomatic or presented for cosmetic reasons
2. Clinical signs often follow impingement of the lesion on adjacent tissue (Impinge nerves, tendons, muscles, or blood vessels; vertebral exostoses may protrude into the spinal canal and compress the spinal cord, leading to neurologic signs)
3. Horse: Usually present at birth; caudal distal radius most common monostotic site in Thoroughbreds (deep digital flexor tendon impingement)
4. Radiographically, the majority of lesions appear as cancellous, bony nodules with radiolucent cartilaginous caps

 

TYPICAL GROSS FINDINGS:

• Dog, horse: Exophytic nodules that typically occur at metaphyses of long bones, the pelvis, ribs, scapula, and vertebrae; Affected bone may be bowed or attenuated

 

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Dog, horse:
  1. During expansion, consists of an outer cap of hyaline cartilage resembling a poorly organized growth plate which undergoes endochondral ossification from its deep surface
  2. Trabecular bone and bone marrow within the mass are continuous with the marrow cavity of the parent bone
  3. Once growth ceases, the cartilage cap is replaced by bone
  4. Trabecular bone is not remodeled, and mineralized cartilage cores persist
• Cat:
  1. Multifocal progressively enlarging areas of osteocartilaginous hyperplasia within the periosteum
  2. Not continuous with marrow cavity; the underlying cortex is often intact

 

DIFFERENTIAL DIAGNOSIS:

• Osteosarcoma or chondrosarcoma: Malignant tumors with anaplastic cells
• Ecchondroma with extensive endochondral ossification: Absence of cartilaginous cap and medullary communication with marrow cavity

 

COMPARATIVE PATHOLOGY:

• Dogs:
  1. Tracheal or laryngeal chondroma and osteochondroma are uncommonly reported in young dogs; similar to this condition, them may represent a dysplastic cartilaginous mass rather than a true neoplasm
• Pigs: Case report of osteochondromatosis
• Koalas: Craniofacial osteochondromas that often extend into the nasal and paranasal sinuses; they do not appear to be malignant but can be quite expansile 
• African hedgehogs: Case reports of retrovirus-induced osteochondromatosis
• White-tailed deer: Cranial osteochondromas reported 
• Avian: Rarely reported in pet birds, including cockatiels; case report of tracheal osteochondroma in a Fisher’s Lovebird and synovial osteochondromatosis in great horned owls and red-tailed hawk

 

REFERENCES:

1. Agnew D, Nofs S, Delaney MA, Rothenburger JL. Xenartha, Erinacoemorpha, Some Afrotheria, and Phloidota. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. London, UK: Academic Press; 2018:525. 
2. Caswell JL, Williams KJ. Respiratory System. In: Maxie MG, ed. Jubb, Kennedy & Palmer's Pathology of Domestic Animals. Vol 2. 6th ed. St. Louis, MO: Elsevier; 2016:483. 
3. Craig LE, Dittmer KE, Thompson KG. Bones and joints. In: Maxie MG, ed. Jubb, Kennedy and Palmer’s Pathology of Domestic Animals. Vol 1, 6th ed. St. Louis, MO: Elsevier; 2016:54, 116-117.
4. Gómez Á, Rodríguez-Largo A, Pérez E, Calvo-Sánchez N, Loomans S, Chiers K, Monteagudo L, Luján L, Pérez M. Feline osteochondromatosis in a 12-year-old feline leukaemia virus-negative cat. J Comp Pathol. 2023;205:24-26.
5.  Higgins D, Rose K, Spratt D. Monotremes and Marsupials. In: Terio KA, McAloose D, St. Leger JA, eds. Pathology of Wildlife and Zoo Animals. San Diego, CA: Academic Press. 2018: 463. 
6. Kim RK. Musculoskeletal system. In Schmidt R, Struthers JD, Phalen DN. Pathology of Pet and Aviary Birds. 3rd ed. Ames, IA: John Wiley & Sons, Inc.; 2024: 363-64.
7. Lopez A, Martinson SA. Respiratory System, Thoracic Cavities, Mediastinum, and Pleurae. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:633.
8. Olson EJ, Dykstra JA, Armstrong AR, Carlson CS. Bones, Joints, Tendons, and Ligaments. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:1074-1076-1094.
9. Wunschmann A, Armien AG, Hofle U, Kinne J, Lowenstine LL, Shivaprasad HL. Birds of Prey. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. London, UK: Academic Press; 2018:728. 

 

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