Osteomyelitis is an inflammatory condition of bone secondary to infection; it may be acute or chronic. Symptoms of acute osteomyelitis include pain, fever, and edema of the affected site, and patients typically present without bone necrosis in days to weeks following initial infection. Chronic osteomyelitis develops after months to years of persistent infection and may be characterized by the presence of necrotic bone and fistulous tracts from skin to bone.^1,2 Osteomyelitis is further classified by mechanism of infection as hematogenous or nonhematogenous. With hematogenous osteomyelitis, bacteria are seeded into bone secondary to a bloodstream infection and the condition is most common in children, older adults, and immunocompromised populations.^1–3 Nonhematogenous osteomyelitis occurs from direct inoculation in the setting of surgery or trauma or with spread from contiguous soft tissue and joint infections.^1,2

Etiology

Methicillin-sensitive Staphylococcus aureus is the most frequently identified pathogen across all types of osteomyelitis, followed by Pseudomonas aeruginosa and methicillin-resistant S. aureus. Hematogenous osteomyelitis is often monomicrobial and can occur from aerobic gram-negative rods or from P. aeruginosa or Serratia marcescens in injection drug users.⁴ Vertebral osteomyelitis is the most common type of hematogenous osteomyelitis and is polymicrobial in 5% to 10% of cases.¹ Blood cultures may be negative if osteomyelitis develops following bacterial clearance from the bloodstream. Nonhematogenous osteomyelitis can be polymicrobial; S. aureus is the most common pathogen in addition to coagulase-negative staphylococci and gram-negative aerobes and anaerobes. Polymicrobial diabetic foot infections and decubitus ulcers may include Streptococcus species and Enterococcus species.¹ Less common pathogens can be associated with certain clinical conditions, including immunocompromise (Aspergillus species, Mycobacterium tuberculosis, Candida species), sickle cell disease (Salmonella species), HIV infection (Bartonella henselae), and tuberculosis (M. tuberculosis).^1,5,6

Clinical Features

The clinical presentation of nonhematogenous osteomyelitis varies and symptoms are often non-specific. Signs and symptoms common to all sub-types may include pain, edema, and erythema. Acute osteomyelitis may present with a more rapid onset of symptoms (development over days) and is more likely to be associated with fever. Systemic symptoms are not common in chronic osteomyelitis, and the presence of fistulous tracts from skin to bone is diagnostic. Long-standing, nonhealing ulcers and nonhealing fractures may also be associated with chronic osteomyelitis.

Patients with diabetic neuropathy are at higher risk of developing osteomyelitis secondary to local spread from diabetic foot infections and unrecognized wounds.² Smoking increases the risk of osteomyelitis from diabetic foot infections and healing fractures.⁷ Peripheral vascular disease and poorly healing wounds (e.g., decubitus ulcers) are more likely to lead to bone inflammation. Osteomyelitis secondary to diabetic foot ulcers can be difficult to diagnose given chronic changes from vascular insufficiency and ischemia.⁸

Hematogenous osteomyelitis often presents similarly to nonhematogenous disease. The most common form of hematogenous osteomyelitis is vertebral; patients often have back or neck pain and muscle tenderness, sometimes followed by fever. Hematogenous osteomyelitis may also occur in the sternoclavicular, pelvic, and long bones.⁹ When hematogenous osteomyelitis affects prepubertal children, it typically occurs in the metaphysis of long bones adjacent to growth plates, with a predilection for the tibia and femur.¹

Diagnosis

A diagnosis of osteomyelitis should be considered in any patient with acute onset or progressive worsening of musculoskeletal pain accompanied by constitutional symptoms such as fever, malaise, lethargy, and irritability. Constitutional symptoms do not always occur in adults, especially in the setting of immunocompromise. The index of suspicion for osteomyelitis should be higher in patients with underlying conditions, including poorly controlled diabetes mellitus, neuropathy, peripheral vascular disease, chronic or ulcerated wounds, history of recent trauma, sickle cell disease, history of implanted orthopedic hardware, or a history or suspicion of intravenous drug use. A dedicated physical examination can increase the likelihood of diagnosing osteomyelitis if findings include erythema, soft tissue infection, bony tenderness, joint effusion, decreased range of motion, or exposed bone. The probe-to-bone test may be useful to rule out diabetic foot osteomyelitis in low-risk patients.^10,11