JPC SYSTEMIC PATHOLOGY

MUSCULOSKELETAL SYSTEM

March 2025

M-M05

 

Signalment: Stillborn piglet of unknown gender

 

HISTORY (JPC #1810706): Tissue from 1 of 3 stillborn piglets which had thickened forelegs.

 

HISTOPATHOLOGIC DESCRIPTION: Radius and ulna, cross section, with adjacent skeletal muscle and tendons: Radiating diffusely and circumferentially from the surface of the cortical bone and markedly elevating the overlying periosteum is a circumferential, up to 4mm thick, layer of thin bony trabeculae oriented perpendicular to the cortex composed of woven bone separated by loose, sparsely vascular, edematous, fibromyxomatous connective tissue lacking bone marrow elements. This connective tissue contains numerous loosely and haphazardly arranged spindle to stellate cells. Diffusely, the periosteum is markedly thickened up to 250 µm by fibrous connective tissue, and the inner periosteal margin is lined by up to 5 layers of osteoblasts separated by variable amounts of osteoid. Connective tissue from the thickened, reactive periosteum merges and fuses with adjacent tendons and separates and surrounds markedly atrophied skeletal muscle. The connective tissue surrounding the periosteum is thickened and edematous. The adjacent skeletal muscle myofibers are often shrunken (atrophy) and occasionally are multinucleated with increased sarcoplasmic basophilia and central nuclei (regeneration) and are surrounded by edema. There is multifocal mild scattered hemorrhage and perivascular edema.

 

MORPHOLOGIC DIAGNOSIS: Long bones, radius and ulna: Periosteal new bone formation (hyperostosis), diffuse, severe, with edema and marked muscle atrophy, breed unspecified, porcine.

 

CONDITION: Congenital hyperostosis

 

SYNONYMS: Congenital porcine cortical hyperostosis, diaphyseal dysplasia, congenital thick foreleg, “thick legs”

 

GENERAL DISCUSSION:

• Hyperostosis in general refers to excessive bone formation as nonspecific response to various forms of bone injury, i.e., trauma, infection, metabolic imbalance, etc.
• Congenital hyperostosis: 
  • Rare autosomal recessive congenital disease of newborn swine
  • Excessive periosteal bone formation (thickened limbs) within long bones
  • Axial skeleton not affected
  • Radius and ulna are most severely affected
  • Only cortical bone affected; growth plates and trabecular bone not involved

 

PATHOGENESIS:

• Unknown; presumed to be an autosomal recessive trait
• Proposed pathogenesis: Local circulatory disturbance due to positioning in the uterus > extensive periosteal reaction; arteriosclerosis of small arteries and arterioles has been observed in vessels supplying the radioulnar region in affected pigs > disorganization of the perichondrial ossification groove of Ranvier (the chondrogenic membrane surrounding the growth plate enabling expansion of the growth plate)

 

TYPICAL CLINICAL FINDINGS:

• Piglets are either stillborn or die within the first days of life. 
• One or both forelimbs are firm and enlarged; occasionally affects hindlimbs
• Overlying skin: Hyperemic, tense and fixed to underlying tissue

 

TYPICAL GROSS FINDINGS:

• Enlarged limbs up to twice normal diameter; thick layer of extracortical bone extends along the diaphysis of the radius, ulna and tibia
• Marked edema and swelling of surrounding soft tissues
• Normal joints and epiphyses 

 

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Radiating spicules of woven bone extend from the surface of preexisting cortical bone, under a thickened periosteum; no intervening marrow; no evidence of excessive bone resorption or remodeling
• Bone spicules are oriented perpendicular to the long axis of the cortex and arise from the cambium layer of the periosteum
• Overlying thickened periosteum is lined by multiple layers of active osteoblasts; periosteum merges with surrounding edematous, poorly vascular connective tissue; merged tissues fuse with adjacent muscle and the overlying dermis

 

DIFFERENTIAL DIAGNOSIS:

• Reactive periosteal hyperostosis secondary to infection, neoplasia, trauma, or metabolic disease
• Osteopetrosis: Thickening of spongiosa due to decreased osteoclast resorption
• Hypertrophic osteopathy (M-N05, formerly hypertrophic pulmonary osteopathy): does not affect neonatal animals; diffuse, periosteal new bone formation along the diaphysis and metaphysis of certain limb bones; associated with inflammatory and/or neoplastic conditions typically in the thoracic cavity and less often the abdominal cavity
  • New bone growth is usually confined to limbs
  • Characterized by hyperemia, edema, granulation of periosteum > osteoid deposition on existing cortical bone > trabecular bone

 

COMPARATIVE PATHOLOGY:

• Humans: Resembles Caffey's disease in children (diaphyseal dysplasia, infantile cortical hyperostosis); human disease is self-limiting and regresses
• Dog: Craniomandibular osteopathy (M-M18),“lion jaw”, most common in West Highland White terrier
  • Also seen in Scottish terrier, Labrador retrievers, great Danes, doberman pinschers, cairn terriers, German wirehaired pointers, and others
  • Proliferative disorder mostly confined to bones of skull, particularly mandible, and occipital and temporal bones 
  • Seen microscopically as endosteum, periosteum, and trabecular bones
• Dog: Calvarial hyperostosis of Bullmastiffs: asymmetrical hyperostosis of skull without involvement of mandible 
• Dog: Canine hepatozoonosis (H. americanum) may cause a similar lesion to hypertrophic osteopathy but generally occurs more proximal on the limb
• Avians: Polyostotic hyperostosis occurs in aged female psittacines (particularly budgerigars); pathophysiology is linked to estrogen; gross thickening and exophytic masses of bones that lead to deformation; in the orbit can lead to exophthalmos, and blindness (compression of optic nerve)

 

REFERENCES:

1. Fletcher OJ, Barnes HJ, Abdul-Aziz T. Chapter 3: Skeletal System. In: Abdul-Aziz T, Fletcher OJ, Barns HJ, eds. Avian Histopathology. 4th ed. Madison, WI: Omnipress; 2016: 74.
2. 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. Philadelphia, PA: Elsevier; 2016: 53, 91-94.
3. Decker S, Volk HA. Dorsal vertebral column abnormalities in dogs with disseminated idiopathic skeletal hyperostosis (DISH). Vet Rec. 2014;174(25):632.
4. Doize B, Martineau G. Congential hyperostosis in piglets: A consequence of a disorganization of the perichondrial ossification groove of Ranvier. Can J Comp Med. 1984;48:414-419.
5. 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.
6. Pritzker KPH, Kessler MJ. Arthritis, muscle, adipose tissue, and bone diseases of nonhuman primates. In: Abee CR, Mansfield K, Tardif S, Morris T, eds. Nonhuman Primates in Biomedical Research, Vol 2. San Diego, CA:Elsevier. 2012: 658.
7. Slovak JE, Gilmour LJ, Miles KG. What is your diagnosis? Idiopathic calvarial hyperkeratosis. J Am Vet Med Assoc. 2015;246(11):1187.

 

 

 

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