osteomyelitis

This can occur in three different ways and historically osteomyelitis has been classified based on how it occurs. Hematogenous osteomyelitis is generally a disease of the very young and very old. It generally occurs from a soft tissue infection elsewhere in the body, that then gets into the bloodstream and travels to a bone somewhere else in the body and invades that bone. In the young it has a greater tendency to invade “long bones” and lodge itself in the most vascular part of the bone known as the metatphyseal region.

In the elderly the most common site for hematogenous osteomyelitis is the vertebrae. This type of osteomyelitis is generally not caused by staph aureus but rather by other organisims.

For the sake of completeness one other type of hematogenous osteomyelitis is that caused by IV drug users who basically end up injecting the bacteria into their circulation.

The next type of classification is contiguous osteomyelitis. In this instance the bone becomes infected from an external contaminated source such as penetrating trauma, open fracture, bone surgery or joint replacement. This type can occur at any age and in any bone. This is one type of osteomyelitis that is very common in the foot.

The last classification is that of vascular insufficiency, or poor circulation. This is quite frequently seen in diabetics with diabetic neuropathy. (See my discussion on neuropathy and diabetic foot). Foot ulcers serve as an entrance for infection and bacteria to gain access to the bone by contiguous spread. The general rule of thumb is that if an ulcer fails to heal in six weeks, and assuming has been treated with meticulous wound care, then one should suspect osteomyelitis of the underlying bone.

Note the x-ray below of a right foot. The blue arrow points to the area of bone destruction on the great toe. Notice the mottled appearance of the bone and the irregularities at the edges of the bone. Compare this to the normal bone of the first metatarsal which is noted by the red arrow. For those of you perceptive enough, you will notice the second toe has been amputated.

As previously stated staph aureus is the most common bacteria associated with osteomyelitis, but is not the only one. Pseudomonas aeruginosa is also a very common bacteria and this is typically seen as a result of puncture wounds. Diabetics and others with compromised circulation and immune systems may present with osteomyelitis by multiple strains of bacteria.

Early detection of osteomyelitis is very important as to limit the destruction of bone and to stand a better chance of resolving the infection with antibiotics. This is where the problem arises for the doctor. This is a disease that can be difficult to detect and there are other conditions which may give a false positive to the tests for osteomyelitis.

Since this is a bone infection one would think that an x-ray would show osteomyelitis. The problem is that many times they are initially inconclusive. Bone has to lose upwards of fifty percent of its density before changes will be seen on x-ray. By the time it takes the bone to lose this much density (2 to 6 weeks) and show up on x-ray the bacteria has fairly well infiltrated the bone.

Many doctors will order a bone scan to make the diagnosis. There have mixed results with this method. Bone scans may be “hot” or positive for conditions other than osteomyelitis such as trauma, (surgical trauma included), as well as diabetic osteolysis which is bone destruction due to the ravages of diabetes.

An MRI is the most sensitive and specific imaging study for defining bone infection but they are not universally available and they are expensive.

Absolute definitive diagnosis is made based on bone culture and biopsy. Using various techniques depending on location, a piece of the diseased bone is actually removed and cultured for bacteria. It is important that the surgeon probe down to bone to get the culture as opposed to taking soft tissue samples as there can be a difference in organisms found at each site. A patient should also be off all antibiotics for at least 48 hours to allow for the most accurate results.

In most cases osteomyelitis is a surgical disease with the exception of hematogenous osteomyelitis which many times can be treated successfully with antibiotics for upwards of six weeks. This is usually a combination of parenteral antibiotics (intravenous or injectable) followed by oral antibiotics. In some cases antibiotic beads have been implanted in the infected area is an effort to disperse a continual saturation of antibiotic to the infected bone. However, this modality is still considered controversial. A full course of antibiotics must be taken in order to destroy the bacteria otherwise you run the risk of a chronic infection.

Most other cases of osteomyelitis will require surgical debridement (removal) of all the diseased bone particularly because some organisms form a gel layer over and in the bone protecting them from the effects of antibiotics. The chief consideration is what will be the resulting function at level of surgery. Having the end of a toe amputated generally will not hinder a patient in their ability to walk, but more extensive surgery in other parts of the foot may make walking afterward very difficult, so all this has to be planned for prior to surgery. It should also be noted that in cases of surgical debridement, antibiotic therapy will also be necessary.

Acute osteomyelitis that is either not treated or improperly treated will go on to becoming a chronic osteomyelitis. Another scenario is one where the circulation to the extremity is so bad to begin with that surgical intervention for the osteomyelitis would almost certainly lead to a non-healing surgical wound with the potential for loss of limb or even life. In fact may infectious disease experts never consider osteomyelitis as cured but rather as “arrested”. A person can live with a chronic osteomyelitis assuming it does not adversely affect the body part in question. However, chronic disease does have its pitfalls. The infected bone as well as sinus tracts may undergo malignant changes, there may be constant pain in the site and the everyday care of the infected site may cause more problems than it is worth.