Diagnosis is key, but not always easy

Maureen Leahy

Diagnosis of orthopaedic infections is the first step to effective treatment. However, some infections are easier than others to diagnose. During the recent AAOS/Orthopaedic Research Society research symposium, Musculoskeletal Infection: Where are we in 2014? experts discussed diagnostic challenges associated with orthopaedic infections, including those in the spine, shoulder, and foot and ankle.

As with any orthopaedic infection, an essential component to managing postoperative spinal infections is timely diagnosis, according to Jung U. Yoo, MD, chair and professor in the department of orthopaedics and rehabilitation and codirector of the Spine Center at Oregon Health & Science University.

“Nearly all postoperative spinal infections are acute and occur within 6 weeks of surgery,” said Dr. Yoo. “Because the wound often looks pristine, however, the average time to diagnosis of a deep wound infection is 30 days. Only two-thirds of patients have any wound drainage, and very few have constitutional symptoms.”

Surgeons should suspect infection if the patient experiences an increase in pain, changing neurologic symptoms, or has an elevated C-reactive protein (CRP) level, according to Dr. Yoo. “CRP should return to normal 2 to 3 weeks after surgery. An elevated CRP after the third week, therefore, is a strong indicator of infection,” he explained.

The gold standard for identifying the infecting organism is formal surgical débridement; swabbing or draining the wound will likely be useless, Dr. Yoo pointed out. He added that computed tomography–guided biopsy may also be helpful. “However, the culture sensitivity of the biopsy drops significantly if the patient is already on antibiotics,” he said.

Dr. Yoo also noted that multiple tissue sampling and prolonged culture can help identify whether a low-virulent organism, such as Propionibacterium or Staphylococcus epidermidis, is the cause of infection.

PSI and P acnes
“Periprosthetic shoulder infections (PSIs) are a growing problem; they can also be difficult to diagnose due to the unique microflora of the shoulder,” said Grant E. Garrigues, MD, a shoulder and elbow surgeon at Duke University Medical Center.

For example, a large number of infected shoulder arthroplasties are caused by the P acnes organism, according to Dr. Garrigues. He explained that P acnes is an aerotolerant anaerobe, a commensal microbe with a predilection for the pilosebaceous glands of the head and neck region and around the axilla. Testosterone dependent, there is a higher bacterial burden in the skin of males, and not surprisingly, male gender is a risk factor for P acnes infection.

“P acnes is very difficult to diagnose because it generally does not evoke a suppurative inflammatory response. Clinical symptoms such as erythema, purulence, and drainage are rarely observed; pain is the only consistent, though entirely nonspecific, presenting symptom,” Dr. Garrigues said. “In addition, no commercially available, pre- or intraoperative test can reliably predict culture growth for P acnes.

“P acnes is a very slow-growing organism that takes, on average, 7 to 13 days to culture, and in some cases up to 3 weeks. However, the longer a culture is held, the higher the risk of growing contaminants, which can increase the false-positive rate,” Dr. Garrigues continued. “Additionally, without a confirmatory test, the clinical implications of positive cultures are unclear—especially if only a small percentage of the cultures are positive.”

Although it is the most common cause of PSI, P acnes is not the only cause. When diagnosing PSI, therefore, Dr. Garrigues recommends that all the standard diagnostic tests for infection be performed. “Orthopaedic surgeons should always have a high index of suspicion for P acnes, even if all of the standard tests for periprosthetic joint infection are normal. However, an elevated erythrocyte sedimentation rate or CRP level, for example, could indicate the presence of an S aureus infection,” he said.

Foot and ankle infections
Diagnosis of foot and ankle infections after elective surgery is fairly straightforward, said David N. Garras, MD, of Midwest Orthopaedics at Rush University Medical Center, Chicago. “Foot and ankle infections show up very well—the surgical sites are not covered by a lot of soft tissue. However, infection rates are higher than in other parts of the body, mainly due to the unique environment of the foot and resident organisms,” he said.

With superficial infections, the skin is red, warm, and tender, but joint motion is painless, according to Dr. Garras. Some streaking, lymphadenopathy, leukocytosis, and fever may also be present. The most common organisms for cellulitis or other superficial infections are S aureus and Group A Streptococcus; treatment typically consists of 5 to 7 days of oral antibiotics. “In some cases these infections require surgical drainage, soaks, and culture-specific antibiotics,” he said.

With deep foot and ankle infections, the skin is also warm, tender, and swollen, but deep infections can also be fluctuant and may or may not involve leukocytosis or fever, Dr. Garras noted. Diagnostic tools include radiographs, magnetic resonance imaging, and, in some cases, needle aspiration. “I aspirate anything that feels a little big to try and get a sample to send to the lab,” he admitted.

Deep infections are limb-threatening and require aggressive treatment, Dr. Garras stressed. “Early surgical débridement, broad-spectrum antibiotics, and later, targeted culture-based antibiotics, are warranted. Most of the time, deep infections will require some sort of soft-tissue reconstruction,” he said.

Disclosure information: Dr. Garrigues—Synthes, Tornier, Arthrex Inc, Zimmer, DJ Orthopaedics; Dr. Yoo—Osiris Therapeutics; Dr. Garras—no conflicts.

Maureen Leahy is assistant managing editor of AAOS Now. She can be reached at leahy@aaos.org

Bottom Line

• Timely diagnosis is an essential component of managing postoperative orthopaedic infections.
• The gold standard for diagnosing postoperative spinal infections is formal surgical débridement; however, long-acting cultures may help identify whether low-virulent organisms are the cause.
• A large number of infected shoulder arthroplasties are caused by the P acnes organism, which can be difficult to diagnose.
• Although diagnosis of foot and ankle infections after elective surgery is fairly straightforward, deep infections can be limb threatening.