Bone biopsy has long been considered the gold standard in determining the diagnosis of osteomyelitis. However, bone biopsy is a procedure and not an exam. Clinicians send the collected specimen to pathology or microbiology. Unfortunately, there can be discrepancies between pathology and microbiology findings on these specimens.1 Accordingly, the question arises of whether one should send a bone biopsy specimen to pathology or microbiology.
Microbiology has the advantage of not only providing a positive or negative result, but also identifies the organism or organisms present and antibiotic susceptibility. Researchers have shown that identification of the causative bacteria improves successful antibiotic treatment of osteomyelitis.2 However, obtaining an uncontaminated specimen through the ulceration site is difficult, if not impossible. In many cases, the bone is exposed and is likely at least colonized with bacteria. If not, simple contamination by adjacent tissue is still probable.
Percutaneous biopsies may aid in the reduction of this false positive result but then one risks missing the infected bone. This leads to a decrease in confidence in the accuracy of a negative result. Furthermore, most patients have already been on an antibiotic prior to the biopsy, which may also lead to a false negative. Given the duration of treatment required to eradicate osteomyelitis, it is unlikely the antibiotic would eliminate all bacteria. However, the collected bone specimen may also contain antibiotics and thus possibly interfere with the growth of the culture on a new medium.
Lesens and colleagues retrospectively reviewed 80 patients with osteomyelitis and found 96 percent had a positive bone culture.3 About half of the patients received an antibiotic within two weeks of the bone specimen collection. This suggests antibiotic cessation may not be as critical as once thought to get a positive result. The 2012 Infectious Diseases Society of America (IDSA) guidelines recommend a two week absence of antibiotics in appropriate cases in order to obtain ideal culture results but this is usually not practical.4
Finally, bone culture may be in some situations a self-fulfilling prophesy as the medium used to grow the pathogen has been predetermined to select known pathogenic bacteria. In at least one study, using ribonucleic acid (RNA) testing has identified significantly more bacteria than traditional techniques.5 The importance of these additional bacteria is still uncertain and requires additional research, but certainly highlights the shortcomings of traditional culture techniques.
A Closer Look At Pathology Examination For Osteomyelitis
Microscopic evaluation of the bone specimen by a pathologist bypasses the concerns of antibiotics and cross-contamination, but is not without its own limitations. A landmark study by Meyr and colleagues pointed out that concordance between pathologists is low.6 Thus, complete reliance on blinded histopathology results is also questionable.
Some of the most frequently used histological criteria were extrapolated from studies on prosthetic joint infections. The Musculoskeletal Infection Society uses five polymorphonuclear neutrophils (PMNs) per high-power field (HPF) as the cutoff point for diagnosing prosthetic joint infections. This is because it is the most frequently-used microscopic diagnostic marker worldwide and because several studies have shown that there is no significant difference between using five or 10 PMNs as the threshold.7,8,9
However, this definition appears to be oversimplified. In recent years, studies suggest that this cutoff point is too high for the diagnosis of many periprosthetic joint injections.10,11,12 Certain microorganisms, particularly coagulase-negative staphylococci and P. acnes, can cause infection with a PMN infiltration rate below five.13-17 Some disorders can histologically mimic infection, resulting in a false positive. One may see this with periprosthetic fractures, diabetes exhibiting perivascular fibrosis and lymphoplasmacytic infiltrations, and rheumatoid arthritis, which commonly reveals more than five PMNs per high-power field.18-20 Often, the pathologist uses available clinical data to help make the diagnosis on the slide so the histologic diagnosis is not completely independent. Likewise, the foot and ankle surgeon should be compiling all the data from a case to best determine the diagnosis.
Combining The Best Of Multiple Techniques For Optimal Results
Generally, a combination of both pathology and microbiology findings is feasible to obtain. Strategic use and collection can improve your treatment by increasing the likelihood of obtaining the data you actually desire. Many scenarios may arise but often the primary goal of the specimen is to identify the pathogen. In the case of obvious osteomyelitis, false positives by possible cross-contamination are not of concern. However, one must still be cautious to obtain a true bone specimen and not deep soft tissue as concordance rates are low between soft tissue and bone in diabetic foot osteomyelitis.21,22
Consider using a sterile rongeur to collect the area of bone most likely infected and send the specimen to microbiology. Alternately, one might consider multiple cultures from slightly different locations to reduce the chance of missing the offending pathogen. One may then send the remainder of the specimen to pathology.
In cases in which the anatomic location prohibits complete resection or would result in major amputation, pathogen identification is likely key as long-term antibiotics are often required. Typically, the risk of failure is too high in such cases if osteomyelitis is not treated. However, in cases of unlikely osteomyelitis, pathology may be preferred in order to more reliably exclude the diagnosis.
Finally, depending on the situation, obtaining evidence of a clearance margin may be possible. Multiple authors have demonstrated that obtaining a clean clearance margin improves outcomes and changes treatment.4,23,24 One should strive for clean margins whenever possible. Some have suggested a one cm clearance margin from suspected MRI involvement for metatarsal osteomyelitis.25
One should send the clearance margin to pathology as it may be contaminated from extraction through an ulceration or infected wound.26 Virtually everything in the sterile field may now present itself as a potential fomite. In fact, one study found that when the sagittal saw blade was cultured, it was positive in 15 of 16 cases after initial resection of osteomyelitis.27 Thus, prior to establishing a clearance margin, the surgeon should attempt to re-establish a sterile field with clean tools and blades. Once resection and irrigation are complete, consider primary or delayed primary closure whenever feasible. Distally, this may involve amputation of a toe or possible plastic techniques such as a rotational skin plasty in more proximal locations (see second photo above).
Some Words Of Caution About Extended IV Antibiotics
As we noted above, clinicians may see discrepancies between pathology and microbiology biopsy results. Although one would ideally use a conglomerate of all observations to come to the most likely conclusion, typically the fear of missing osteomyelitis and the resultant catastrophic outcome leads to treatment in the majority of cases unless all biopsy results are negative.
Although extended-duration intravenous antibiotics are often perceived as a safer route in questionable cases, this is not true for all individuals. Certainly, many foot and ankle surgeons have witnessed patients develop kidney concerns resulting in dialysis.28-34 Acute renal failure has been reported in about one quarter of patients with diabetes and osteomyelitis treated with vancomycin plus piperacillin-tazobactam or vancomycin plus cefepime.30 However, the decisions surrounding antibiotic treatment are often not exclusively made by the surgeon, and frequently involve infectious disease specialists or a consensus of all team members.
Diagnosis and treatment of osteomyelitis continue to be highly debated and ever-evolving. Likely, we will be collecting biopsies in a slightly different fashion in the future, whether by RNA or another technology that is yet to be discovered. However, understanding recent research on bone biopsy in the diabetic foot may improve outcomes now through purposeful collection of the appropriate specimens. Surgeons are urged to consider the limitation of each type of collection, obtain bone culture from areas most likely to grow the specimen and send an appropriate clear margin to pathology whenever feasible. n
Dr. Thorud is a Diplomate of the American Board of Podiatric Medicine, and the American Board of Foot and Ankle Surgery. He currently practices in McHenry, Ill., and is associated with Mercy Health System.
Dr. Seidel is a first-year resident at Cambridge Health Alliance in Cambridge, Mass.
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