Although bone is part of the vascular system (and therefore fractures can cause hemorrhage and metastatic cells to lodge in the skeleton), why might antibiotics have trouble reaching areas of infected bone in osteomyelitis?
Osteomyelitis is an infection of bone that is caused by either hematogenous spread, contiguous spread from adjacent soft tissues or joints, or direct inoculation (i.e. during trauma or surgery).
The majority of cases of osteomyelitis are caused by Staph aureus bacteria. Other implicated pathogens include Staph epidermidis, gram negative rods, and salmonella species (in patients with sickle cell disease), although these are much less common.
In addition to bacteria, the region of bone with osteomyelitis contains dead bone, local inflammation, and granulation tissue.
In an attempt to “wall off” the infection, bone grows over the focus of osteomyelitis. This bony growth forms a thick sheath known as the involucrum. The area surrounded by the involucrum, the center of infection, is called the sequestrum (Figure 1).
This process of sequestration is similar to that of an abscess formation in soft tissue. Sequestering the site of infection from the circulation has evolved to protect the body from further spread of bacteria. Unfortunately, the process by which the bacteria are segregated also prevents blood flow to the focus of osteomyelitis. Thus, antibiotics intended to treat the infection have difficulty reaching their target.
Figure 1: Osteomyelitis of the tibia. The image on the right is modified to show the focus of infection with the sequestrum highlighted in red and the involucrum highlighted in blue. (Image modified from Radiopedia.org, rID: 29506)
Surgical debridement is a major component of treatment in osteomyelitis. Surgery not only physically removes infected tissue, but it also facilitates the delivery of antibiotics.
Surgeons will debride the infected bone and soft tissues. Additionally, during surgical debridement, antibiotic beads can be placed at the site of infection to directly introduce antibiotics to the tissues.
RECAP: when bone is infected, the area is sequestered from the surrounding bone by the growth of new bone. This sequestration might prevent further spread of the infection but also prevents antibiotics from penetrating the infection. Debridement might therefore be needed.
Additional Points to Consider
Although antibiotics may have trouble reaching the infected area and thus are limited in their ability to eradicate the infection, oral antibiotics may be used to suppress chronic osteomyelitis, to prevent it from worsening.