Signalment:  

6-month-old intact female Yorkshire sow (Sus scrofa domestics)Part of a traumatic bone injury study involving a comminuted fracture of the femoral shaft with stabilization via surgical pinning. This particular animal developed a suppurative infection along the pins, but was maintained until the end of the study.

Gross Description:  

The mid femur was markedly swollen with a dense fibrous capsule surrounding the fractured bone, sequestrate and woven bone. Multifocal abscesses were noted tracking along the surgical pins.

Histopathologic Description:

Bone, femur (per contributor): Focally extensive, pre-existing cortical bone is discontinuous (fractured) and thinned with scalloped edges, and replaced by irregular trabeculae of woven bone in varying degrees of maturation. Multifocally, woven bone is contiguous with the remodeled cortical bone and intertwined trabeculae are perpendicular to the cortical bone (reactive bone). Both cortical bone and woven bone are frequently surrounded and bounded by disorganized islands of cartilage undergoing endochondral ossification (callus) composed of tightly packed chondrocytes in a basophilic matrix as well as densely packed fibroblasts and collagen fibers which diffusely fill and obscure the marrow cavities (myelophthisis). Diffusely, bone trabeculae are lined by plump reactive osteoblasts which are frequently in close apposition to the bone or in regions which are scalloped and discontinuous, and frequent multinucleated osteoclasts rest in resorption bays (Howships lacunae). Multifocally deep to the reactive bone is a swath of loose fibrosis admixed with myriad neutrophils, fewer macrophages and lymphocytes, high numbers of plump reactive fibroblasts, eosinophilic cellular and karyorrhectic debris (necrosis), eosinophilic proteinaceous material (edema), fibrin, hemorrhage and moderate numbers of small caliber vessels with reactive endothelium (granulation tissue). Multifocally, within this area there are frequent multinucleated osteoclasts which are either free or surround fragments of osteolysis. These are also often centered on sequestrae characterized by sharp angulated fragments of necrotic bone with empty lacunae. The larger sequestrae are surrounded by the previously described granulation tissue (involucrum).

Morphologic Diagnosis:  

Bone, femur (per contributor): Osteomyelitis, necrotizing, neutrophilic, focally extensive, marked with cortical fracture, reactive bone, callus formation, myelophthisis and sequestrae, Yorkshire sow, Sus scrofa domesticus

Lab Results:  

Staphlococcus aureus was cultured from the open pin lesions, however, no bacteria was visualized in the submitted tissue samples.

Condition:  

Staphylococcal osteomyelitis

Contributor Comment:  

Cases of acute osteomyelitis are most frequently associated with aerobic bacteria such as Staphylococcus sp. and Streptococcus sp. resulting from hematogenous spread in younger animals in which the physis is still open, or resulting from sepsis secondary to omphalitis. The veterinary literature describes less pathogenic agents such as Fusobacterium necrophorum or coliforms which can be opportunist infections causing acute osteomyelitis.

In pigs, Actinomyces pyogenes is frequently isolated from osteomyelitic lesions as well as in cattle and sheep, while Salmonella sp. are commonly isolated from foals.

Additionally, anaerobic bacteria, such as Peptostreptococcus anaerobicus or Bacteroides asaccharolyticus, as well as mixed aerobic and anaerobic infections are also frequent causes of acute osteomyelitis.(1)

Bacteria such as Staphylococcus aureus or Pseudomonas aeruginosa contain fibronectin-binding surface proteins, which have the ability to interact with collagen, or the surface proteins contain binding sites that attach to sialoprotein, a noncollagenous bone matrix protein.(5)

Posttraumatic osteomyelitis usually represents a form of exogenous infection in which the bacteria are introduced through a traumatic wound as in this case, or through a surgical wound. Entrance of infection can occur following simple wounding in which the formation of hematomas under the skin may allow bacterial contamination and infection to develop. Such an infectious process may proceed without obviously apparent clinical signs. Posttraumatic osteomyelitis is frequently avascular as in this case, resulting from a nidus of necrotic bone and the probable inoculation of environmental or skin bacteria secondary to penetrating wounds, such as those caused by bullet, tooth or other foreign bodies.

As the fractured femur in this case was stabilized through open treatment of closed fractures, the posttraumatic or iatrogenic infection may have resulted from the placement of the pins or may have contaminated a sterile wound through that portal of entry. This pathogenesis represents the most common forms of osteomyelitis reported in small animals. When the skin is devitalized to the point that bacterial contamination is possible, infection of bone is an ever present danger.

Subsequent to inadequate treatment of acute osteomyelitis, the condition can obviously progress to become chronic and therefore represents an infection that is well established in bone and has been present for several weeks, months, or even years. Chronic infection can occur after bony union and structural stabilization has occurred or prior to union of the fracture. If the infection occurs before bone union, the treatment of the condition is made more difficult by the presence of the non-union or delayed union. An infected non-union is the worst possible treatment scenario in which the clinician is first required to allow for healing of the fracture and then to deal with the infection.(3)

JPC Diagnosis:  

Long bone: Osteomyelitis, suppurative, chronic, with medullary and periosteal new bone growth.

Conference Comment:  

There is marked variations in the slides submitted for this case. Lesions identified within the conference which were not present on all slides include callus formation, (either from the fracture repair or a pathologic fracture from the osteomyelitis), reactive periosteal changes without antecedent callus formation, extensive chondrous metaplasia, medullary osteosclerosis, and cortical osteopenia. Additionally, although the contributor did not find bacteria present histologically in the lesion, conference participants were able to find cocci bacteria within the area of suppuration.

It was difficult to determine the presence of a fracture from review of the submitted slides alone. Some slides had areas of very regular periosteal new bone growth arranged perpendicularly and radiating outwards from the periosteum with endosteal perpendicular reactive woven bone formation, with no callus formation. Other slides had very haphazard spicules of reactive woven bone and periosteum with extensive areas of disorganized cartilage, and this is likely the callus formed from the original fracture. Conference participants interpreted the islands of cartilage as chondrous metaplasia in reaction to decreased oxygen tension rather than endochondral ossification, which in the moderators experience is generally minimal in a callus and occurs late in the process.

Ideal fracture repair begins with the formation of a hematoma from locally disrupted periosteum, blood vessels, and tissue. Macrophages, platelets, and the dead bone release various growth factors, including bone morphogenic proteins (BMPs), transforming growth factor-beta, platelet derived growth factors, which stimulate the proliferation of woven bone and is the initial scaffolding laid down to stabilize the area. Proliferating undifferentiated mesenchymal cells invade the area, which eventually undergo osseous or chondrous metaplasia to form the haphazard reactive woven bone and hyaline cartilage, and is termed the primary callus. The primary callus is eventually replaced by lamellar bone and may be remodeled by osteoclasts over a long period of time. If the fracture is unstable and there is excessive movement, the area instead forms mature fibrous tissue, which does not serve as a substrate for bone formation and becomes a nonunion. With time, pseudoarthrosis can occur(2).

In this case, we prefer the use of the term medullary osteosclerosis (a condition in which increased fibrous connective tissue fills spaces between spicules of woven bone) rather than myelophthisis, (which is usually reserved to describe bone marrow suppression secondary to marrow infiltration by malignant cells or local production of myelosuppressive cytokines).

The contributor mentioned the presence of sequestrum formation within the lesion. Conference participants interpreted the small fragments of necrotic bone as the result of surgical trauma from the initial surgery, as an involucrum (a dense layer of fibrous connective tissue or reactive bone which walls off a focus of necrotic bone) was not observed in this case. An involucrum separates the sequestrum from its vascular supply and thus prohibits its resorption; however, in this case, the small fragments of dead bone appear to have access to a vascular supply and conference participants predicted that they should eventually be resorbed(2).

In regards to the observed osteopenia, cortical bone normally compacts from the endocortical surface and progresses outward, (a finding not observed in this case). Conference participants discussed the potential reasons for this increased cortical porosity, such as osteoclastic resorption due to the elucidation of cytokines such as tumor necrosis factor (TNF), interleukin 1 (IL-1), IL-16, prostaglandin E2 from the suppurative inflammation and triggering the expression of receptor activator for nuclear factor kappa B ligand (RANKL), also known as osteoclast differentiation factor (ODF), which stimulates osteoclasts upon binding their RANK receptors; or periosteal new bone growth in response to disuse osteopenia from the surgical implant, called stress shielding. In accordance with Wolffs Law, the reduction of stresses relative to normal would cause bone to adapt by reducing its mass, either by becoming more porous, called internal remodeling, or by getting thinner, called external remodeling(2,4).

Lesions of skeletal muscle which were not present in all slides include myofiber atrophy, edema, reactive fibrosis, or foci of coagulation necrosis and regeneration of skeletal muscle, and multifocal areas minimal lymphocytic inflammation.

References:

1. http://vetpath.wordpress.com/systemic-pathology/bone-pathology-steve-weisbrode-2/

2. Carlson CS, Wesibrode, SE. Bones, joints, tendons, and ligaments. In: McGavin MD, Zachary JF, eds. Pathologic Basis of Veterinary Disease. 5th ed. St. Louis, MO:Mosby; 2011:921-3, 932, 951-2, 961-2.

3. Newton C and Nunamaker D: Textbook of Small Animal Orthopaedics, Chapter 37, Osteomyelitis: David M Nunamaker. J.B. Lippincott Company. Online: http://cal.vet.upenn.edu/projects/saortho/chapter_37/37mast.htm

4. Thompson K. Bones and joints. In: Maxie MG, ed. Jubb, Kennedy and Palmers Pathology of Domestic Animals. 5th ed. Vol 1, New York, NY: Elsevier Saunders; 2007:92-8.

5. Woodard JC: Outline of Veterinary Skeletal Pathology, Online: http://www.cldavis.org/woodard_bone/contents.htm

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