Sunday, July 24, 2011

Revision surgery for failed total shoulder replacement arthroplasty for infection - our approach, Part 6

We will now proceed with a discussion of some of the specific causes of failed shoulder arrthroplasty.  Surely one of the most insidious is infection. Surgical cultures obtained from shoulders requiring revision of a prior arthroplasty because of pain, stiffness, or loosening are frequently positive despite the absence of preoperative evidence of joint sepsis. Often these cultures are positive for organisms, such as Propionibacterium acnes and Coagulase-negative Staphylococci, that are less commonly recovered from revision surgery in other sites of joint replacement such as the hip or knee. These organisms are distinguished by (1) their presence on normal skin, (2) their failure to engender systemic manifestations of infection, such as elevated C reactive protein, sedimentation rate, and white blood cell count, (3) their failure to produce local clinical evidence of infection, such as redness, swelling and tenderness, (4) the low yield of cultures of joint aspiration, and (5) the difficulty in eradicating infections with these organisms. We recently completed a study regarding cultures obtained at the time of revision arthroplasty. The abstract from this as yet unpublished study is reproduced below.

Background: P. acnes is a gram-positive anaerobic bacterium not infrequently cultured at the time of revision shoulder arthroplasty, even when common clinical and laboratory signs of sepsis are absent. Because several weeks of culture incubation can be required to recover this organism, clinical decisions regarding the type of revision surgery and the post-operative antibiotic treatment program must be made before the culture results are finalized. Our goal is to seek prognostic information that would help guide necessary decision-making at the time of surgery.
Methods: The culture results from 193 consecutive shoulder arthroplasty revisions for pain or stiffness, but without clinical evidence of infection, were correlated with a host of preoperative and intraoperative observations. Univariate and multivariate logistic regression models were used to identify factors predictive of a positive culture for P. acnes or other organisms. Results: One hundred and eight of the 193 surgeries had positive cultures; 70% of the positive cultures grew P. acnes. While 97% of the P. acnes cultures were positive by 3 weeks after surgery, only 45% were positive after one week. The prognosis for a positive P. acnes culture was statistically significantly increased for male patients, shoulders with humeral loosening and osteolysis on preoperative x-ray, surgical findings of glenoid wear, osteolysis, membrane formation, and cloudy fluid
as well as cases in which there was a surgical suspicion of infection. We developed a multivariate model to predict P. acnes with 92% sensitivity and 47% specificity.
Conclusions: Preoperative and intraoperative factors can be used to prognosticate the risk of a positive culture for P. acnes. This evidence may be useful in decision making at the time of revision shoulder arthroplasty before the definitive culture results become available.

Thus the diagnosis of infection is usually made from culture at the time of revision surgery, where multiple samples are taken along with frozen sections.  This is why it is so important to avoid administering antibiotics until multiple cultures are obtained to minimize the risk of sampling errors. For this reason we do not start antibiotics after an aspiration in the office or emergency room because of the possibility that this culture may not yield a representative result (i.e., it may be falsely negative or contaminated by skin flora).  If the infection is acute, the organism sensitive to antibiotics, and the patient healthy, we may elect a vigorous debridement of soft tissue inflammation, a surgical scrub of the joint surfaces, and irrigation with copious volumes of antibiotic saline solution.  If the infection is established, we will usually remove all components and cement and then replace only an uncemented humeral component, smoothing the residual glenoid surface if needed. We no longer use antibiotic impregnated spacers, because (1) they seem no more effective in resolving infection than a primary exchange and (2) they obligate the patient to a revision procedure which is usually not necessary in a primary exchange. Culture-specific intravenous antibiotics are used for a minimum of six weeks.

Updated information on the prevalence and diagnosis of Propionibacterium can be found here.


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