Infections are nothing out of the ordinary. Nearly everyone has experienced some time of infection at one time or another, and as podiatrists, we seem them in our office on a daily basis. Most bacterial infections are treated with an antibiotic and the patient ends up no worse for wear.
What causes osteomyelitis?
The Waldvogel Classification System of Osteomyelitis is based on the ways a bone may become infected. The first mechanism of infection is when osteomyelitis develops from a direct implantation of bacteria into the bone from outside the body, (a process known as 'direct extension'). A good example of this would be a patient stepping on a nail, with the nail transmitting an infection directly into the bone.
Another common way of developing osteomyelitis is from spread from elsewhere in the body. In this case the infection is seeded into the bone from the blood supply. This is often categorized as 'Acute Hematogenous Osteomyelitis', with 'hematogenous' a reference to its having been spread via the blood.
Osteomyelitis as a result of peripheral vascular disease (poor circulation) is most commonly seen in diabetics.
The most common way of developing osteomyelitis in the feet is from contiguous spread. This is where an existing soft-tissue infection eventually spreads into the bone from those infected surrounding tissues.
How can osteomyelitis be diagnosed?
Osteomyelitis may sometimes be diagnosed by clinical appearance (though this is often difficult), but imaging techniques make the job much easier.
A simple X-ray is usually enough to make a tentative diagnosis of osteomyelitis in most cases. It is cheap and fast, but because osteomyelitis doesn't show up on X-ray until somewhere between 30-50% of the bone has been destroyed, X-rays are not the most sensitive technique for early diagnosis of osteomyelitis. When one suspects osteomyelitis based on an X-ray, it must be differentiated from conditions such as trauma, avascular necrosis (where the bone dies from lack of circulation), arthritis, tumours and other pathologies.
When early detection is desired, bone scans are probably the most sensitive imaging technique that can be done. They can detect very subtle and very early changes. The difficulty with standard bone scans, though, is that they are expensive, invasive (requiring the patient be injected with a special dye), and time consuming for the patient, (requiring multiple visits to the hospital over as much as a 24-hour period.) Bone scans are also limited by the fact that while they can detect changes in bone activity, but can't differentiate between possible causes for that change in activity. For example, trauma, surgery, tumours and other conditions also show up as a positive bone scan.
To get around this somewhat, a new type of bone scan known as a seratec scan has been developed. Not available everywhere at this time, seratec scans use labeled white blood cells (the cells that fight infection), and so are more sensitive to discerning infections.
MRI and CT are probably the most useful imaging techniques we have to fully analyze the extent the osteomyelitis is involved in a given bone. CT is particularly useful in examining the outer, cortical layer; MRI is more useful in examining the inside portions of bones.
After all the imaging is done and the working diagnosis is made, the definitive diagnosis is made by bone culture (taking a portion of the bone and sending it to a lab for testing). This is the single most accurate way to know for certain whether a bone is infected.
How is osteomyelitis treated?
Intravenous antibiotics are usually preferred to oral medications. But often antibiotics are simply not enough by themselves, and surgical intervention may be required.
Surgery usually involves removing the infected bone, followed by a thorough cleaning of the site. The wound is then often packed with antibiotic-impregnated PMMA beads or materials like Bioset. to provide antibiotic coverage directly to the infected area.
After the infection has been addressed, further surgery such as osteotomies (bone cuts to move bone from one spot to another) bone grafts, bone transport or other means may be necessary to resolve any resulting defect in the bone.