Pathogenesis of Musculoskeletal Infections

An understanding of the pathogenesis of musculoskeletal infections facilitates management.

Portals of Entry

Most infections are hematogenous, with the primary site of entry in the ear; oropharynx; or respiratory, gastrointestinal (GI), or genitourinary (GU) tracts. Skin infections such as occur after chicken pox, penetrating injuries such as nails in the sole of the foot, or infections from surgical procedures are less common. Extension of contiguous infections are least common, although adjacent joint infections are relatively common when associated with adjacent osteomyelitis.

Initial Deposition

Bacteremia is a common event but rarely results in tissue infections. When infections occur, bone and joints are vulnerable. The reason for this vulnerability is unknown.

Bone is usually infected in the metaphysis. Bacteria are deposited in capillary loops adjacent to the physeal plate. Nearly always, these bacteria are quickly destroyed by phagocytosis. Trauma is a factor that reduces resistance by causing the formation of a hematoma. Bacterial proliferation is enhanced by the elaboration of a biofilm, which enhances bacterial adhesion to bone and provides protection from phagocytosis or antibiotics.

Joints may be infected by hematogenous spread via the synovium, by a penetrating injury of a joint, by direct spread from a contiguous infection, or by bacterial transport by way of transphyseal vessels.

Natural History of Infection

It is probable that the vast majority of bacterial colonies are destroyed by systemic and local mechanisms. The likelihood of progression is based on the balance between organism virulence and host resistance.

Spontaneous resolution is common. Host resistance exceeds the virulence of the organism.

Subacute osteomyelitis is less common. Host resistance and virulence are about equal. A bone abscess forms reactive sclerotic bone walls off the abscess. No equivalent of this subacute form exists for septic arthritis.

Classic acute osteomyelitis, or septic arthritis, results from a virulent organism and a normal host. The patient becomes systemically ill, and if untreated may develop septicemia and die, or extensive local bone necrosis may occur, and chronic osteomyelitis follows.

Impaired host may allow development of a bone or joint infection from organisms of relatively low virulence, as seen in such conditions as sickle cell disease.

Organisms

The organisms that infect the musculoskeletal system are numerous, varied, continually changing, and have predilection for the site, tissue, and age of the host.

Staphylococci

The incidence is declining, however. Staphylococcus aureus includes a variety of pathogenic strains. New strains may be methicillin or vancomycin resistant. Staphylococcus epidermidis may cause infection in impaired hosts.

Streptococci

Streptococcal infections cause soft tissue and bony infections. Common pathogens are b-hemolytic streptococci. Streptococcus pneumonia may cause septic arthritis. Streptococcus agalactiae is a common cause of osteomyelitis. Streptococcus pyogenese is a less common pathogen.

Neisseria meningitidis

Meningococcal septicemia causes acute and chronic orthopedic problems. The disseminated intravascular coagulation acutely cause necrotizing fascitis and damage physeal circulation, causing physeal arrest and limb deformities.

Pseudomonas aeroginosa

This gram-negative rod is chondrophilic, and a cause of joint infections of the foot from penetrating injuries.

Salmonella

This gram-negative rod is most likely to be encountered in sickle cell osteomyelitis.

Mycobacteria tuberculosis

This acid-fast organism has a resurgence of worrisome drug resistant strains. It causes bone infections in children. Tuberculous spondylitis with kyphosis is a common and serious deformity.

Kingella kingae

This gram-negative coccobacillus is common in the respiratory system, slow growing, aerobic, and fastidious. It is difficult to culture. Only recently has it been found to cause musculoskeletal infections. It remains susceptible to most antibiotics.

Haemophilus influenzae

This was previously a cause of septic arthritis in infants. Now it is rare due to immunization programs.