Porth's Essentials of Pathophysiology, 4e

Disorders of the Skeletal System: Trauma, Infections, Neoplasms, and Childhood Disorders 1091

C h a p t e r 4 3

direct penetration or contamination of an open fracture or wound (exogenous origin), or extension from a contiguous site. The specific agents isolated in pyogenic bacterial osteomyelitis are often associated with the age of the per- son or the inciting condition (e.g., trauma or surgery). Staphylococcus aureus is the most common cause, but organisms such as Escherichia coli, Neisseria gonor- rhoeae, Haemophilus influenzae, and Salmonella species are also seen. 19,20 Staphylococcus aureus has two charac- teristics that favor its ability to produce osteomyelitis: (1) it has the ability to produce a collagen-binding adhesion molecule that allows it to adhere to the connective tis- sue elements of bone, and (2) it has the ability to evade host defenses, attack host cells, and colonize bone per- sistently. 31 Also, infection caused by methicillin-resistant S. aureus is becoming an increasingly common problem. 31 Hematogenous Osteomyelitis. Hematogenous osteo- myelitis originates with infectious organisms that reach the bone through the bloodstream. 8,19,20,30–32 Acute hematogenous osteomyelitis occurs predominantly in children. In adults, it is seen most commonly in debili- tated persons and in those with a history of chronic skin infections, chronic urinary tract infections, and intrave- nous drug use and in those who are immunologically suppressed. Intravenous drug users are especially at risk for infections with Streptococcus and Pseudomonas. The pathogenesis of hematogenous osteomyelitis dif- fers in children and adults. 30 In children, the infection usually affects the long bones of the appendicular skele- ton. It starts in the metaphyseal region close to the growth plate, where termination of nutrient blood vessels and sluggish blood flow favor the attachment of blood-borne bacteria (Fig. 43-8). With advancement of the infection, purulent exudate collects in the rigidly enclosed bony tis- sue. Because of the bone’s rigid structure, there is little room for swelling and the purulent exudate finds its way beneath the periosteum, shearing off the perforating arteries that supply the cortex with blood, thereby lead- ing to necrosis of cortical bone. The necrotic bone that is formed may separate from the viable surrounding bone

■■ The complications of fractures include a loss of skeletal continuity (malunion or nonunion), pressure from swelling and hemorrhage (fracture blisters and compartment syndrome), and fat embolism syndrome (FES). Compartment syndrome is a condition of increased pressure in a muscle compartment that compromises blood flow and potentially leads to death of nerve and muscle tissue. Fat embolism syndrome is a constellation of signs and symptoms including a petechial skin rash, respiratory failure, and cerebral dysfunction due to the presence of fat droplets in small blood vessels after a fracture.

Bone Infections and Osteonecrosis

Bone like other body tissues is susceptible to infection due to invasion by microorganisms. It also is susceptible to osteonecrosis (bone death) when it loses its blood supply. Infections—Osteomyelitis Osteomyelitis represents an acute or chronic infection of the bone and marrow. 18,19 Despite the common use of antibiotics, bone infections remain difficult to treat and eradicate. All types of organisms, including parasites, viruses, bacteria, and fungi, can cause osteomyelitis, but certain pyogenic bacteria and mycobacteria are the most common. Pyogenic Osteomyelitis Pyogenic osteomyelitis is almost always caused by bacteria. Organisms may reach the bone by seed- ing through the bloodstream (hematogenous spread),

Sinuses

Trapped necrotic bone (sequestrum) Periosteum Reactive bone (involucrum)

A C FIGURE 43-8. Hematogenous osteomyelitis. (A) Infectious organisms reach the metaphysis through a nutrient blood vessel. (B) Bacterial growth results in bone destruction and abscess formation. From the abscess cavity, the infection spreads between the trabeculae into the medullary cavity of compact bone and then through the cortex to the outside. (C) The purulent exudate finds its way beneath the periosteum, shearing off the perforating arteries that supply the cortex with blood, thereby leading to necrosis of cortical bone and formation of devascularized fragments, called sequestra. B