Osteomyelitis:
Diagnostic Clues from a Clinician
I've just attended one of the most important meetings of the year, the 72nd annual Scientific Conference of the American Diabetes Association in Philadelphia. Several thousand physicians from all over the world were present and several hundred were attendees of our Foot Care/ Lower Extremities section sessions. One of the major topics of our sessions was diabetic foot infection as well as a specific session dealing with diabetic foot osteomyelitis (DFO). Perhaps no topic in our discipline is more controversial – and at the same time it seems to be such a conundrum for the practitioner caring for patients with this common complication. So I thought it would be apropos to discuss DFO at this juncture in our series. I will approach this primarily from a clinician's standpoint, based on my experience and understanding of this entity in the hopes that I might clarify some of the mystique about DFO.
Of course, osteomyelitis refers to infection of bone, specifically the medullary cavity of bone, and is diagnosed by positive bone cultures and histopathology indicating inflammation with some degree of osteonecrosis. While this seems rather self-explanatory, most studies of osteomyelitis are not specific in this regard. Some take just bone cultures, some take cultures of the tissue immediately surrounding the bone, and some take just histopathology from bone biopsy to substantiate the diagnosis. Hence, when we read studies on the treatment of DFO, we can often not be sure that osteomyelitis was indeed present. Therefore, when "cure" is reported we really cannot be sure if soft tissue infection was cured or if bone infection was indeed cured. This is the problem with most of the literature dealing with the treatment of DFO. We also do not yet have randomized trials on DFO because of the difficulties with randomizing patients to surgery versus medical treatments alone. Rather than taking on this subject in its entirety, let's focus on diagnosis of osteomyelitis at this time and leave management of DFO to a future issue of this eZine.
Studies have, however, given us good insight into the predictors of osteomyelitis; especially those associated with diabetic foot ulcers and diabetic foot infections. One fairly recent study published in 2009 indicated that the exposure of bone and a history of prior ulcer or recurrent ulcers were significant predictors of osteomyelitis in patients with diabetic foot infections. The same group reported that ulcers with a duration of >30 days had a significant risk for infection compared to those with a shorter duration. Experienced clinicians will recognize that chronic, deeper ulcers are those most likely to be associated with DFO – especially when they have not responded to customary diabetic foot ulcer (DFU) treatments (debridement, offloading, etc.). Perhaps the best clinical diagnostic clue to detect DFO is one that I have used for many years and is, in effect, supported by most studies and reviews on this topic. The probe to bone (PTB) test can easily be done by any clinician without the need for specialized equipment. PTB or as I call it, the "5 cent bone scan" is highly predictive for underlying osteomyelitis, especially in the presence of an infected foot ulcer. Although Grayson (1995) first reported on this technique using a sterile metal probe, for years prior and even subsequent to his report, many clinicians (including myself) have simply used the back of a wooden applicator stick or other blunt instrument. The PTB test is performed by gently inserting the probe into the base of a wound to determine the depth of penetration (Figure 1) . Specifically, the probe test seeks to determine if bone can be palpated at the base of the wound (or at any other deep location to the periphery of the wound). When the hard or gritty surface of the bone is directly touched by the probe the PTB test is positive (This is best used on wounds where bone can be directly visualized; try it for yourself). This simple test is very easy to master and can be done within seconds of your examination of any DFU.
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Figure 1
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There is literally not a wound that my residents or I do not probe to determine the depth and penetration to bone. Since patients with foot infections have a higher probability for having osteomyelitis, this test is actually more accurate in patients with infected DFUs. While false positive results have been reported to reduce the positive predictive value of the PTB test in non-infected wounds, I still recommend that all wounds be gently probed for bone involvement. Remember that osteomyelitis is usually a very indolent, occult, and chronic complication that does not always exhibit acute signs of inflammation. Therefore, even a DFU that has not healed in months that has no signs of infection or ischemia might have underlying osteomyelitis as the reason for its recalcitrance. These are the very wounds that need to be probed and if you can directly probe to bone in the recesses of the wound, it is very likely that osteomyelitis is present. At this point, with your suspicions aroused, further diagnostic imaging will be indicated to assess for its presence – usually to confirm your clinical impression. If clinical signs of infection are present, a tissue culture (or better yet, bone culture) will assist in directing your therapy for the infection. To keep things simple, and clinically relevant, we will limit our present discussion to the PTB test. Learn this simple technique and you will be surprised by how quickly you will be able to make the diagnosis of DFO earlier than you might have otherwise done. Remember that plain X-rays are very insensitive to early osteomyelitis, as changes do not become evident until a good deal of bone infection has occurred. We will speak more about imaging in our next issue since it is a subject deserving its own eZine. Nonetheless, I have provided you with several pertinent references below that will support much of what has been discussed thus far. They will also serve as a great resource to help further your understanding of this common complication of diabetic foot ulcers.
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References are provided below that can expand upon many of the points made above. We welcome your opinions, concerns, and suggestions. If you have an interesting case or a troubling circumstance that you would like to share with fellow PRESENT Diabetes members, please feel free to comment on eTalk.
Best regards,
Robert Frykberg, DPM, MPH
PRESENT Editor,
Diabetic Limb Salvage
REFERENCES
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