Saturday, January 21, 2017

Positive cultures at revision arthroplasty, expecting the unexpected

Future surgery after revision shoulder arthroplasty: the impact of unexpected positive cultures

These authors studied 117 patients having revision shoulder arthroplasty without obvious evidence of infection. The average time from the initial surgery to revision was 4.3 years. 28 of 117 (23.9%) had what they referred to as 'unexpected positive cultures ( UPCs).

The diagnoses at time of revision surgery were rotator cuff dysfunction (32/117; 27.3%), glenoid wear after shoulder hemiarthroplasty/painful shoulder hemiarthroplasty (23/117; 19.7%), glenoid loosening (17/117; 14.5%), dislocation (11/117; 9.4%), malunion/nonunion (11/117; 9.4%), instabil- ity (10/117; 8.6%), arthrofibrosis (7/117; 6.0%), and humeral loosening (6/117; 5.1%). It is not clear from the manuscript what type of revision surgeries were performed for these shoulders, specifically how many had complete single stage exchange of the prosthesis. 

Interestingly, these authors appear to routinely administer at least 2 weeks of empirical oral antibiotics after each revision surgery, so each patient received some antibiotic treatment for a possibly positive culture. The antibiotics used are not described. 18 of 28 (64.3%) patients received antibiotics for 6 weeks postoperatively without complications compared with 10 of 28 (35.7%) who received the routine 2-week empirical antibiotic regimen. The decision to continue antibiotics beyond the 2-week threshold was based on culture results, clinical presentation, and intraoperative findings. Patients with positive cultures for Propionibacterium were variably treated with no antibiotics in 8 cases, with 6 weeks of IV Vancomycin in 5 cases, and with IV Penicillin in two cases.

28 of 117 (23.9%) had what they referred to as 'unexpected positive cultures ( UPCs). 15 (57.1%) of these cultures grew Propionibacterium acnes. However, review of the data (Table II) indicate that three cases had only one specimen submitted for culture and 10 had only two cultures submitted. Over half of the cases had four or fewer cultures sent. It is recognized that the presence of Propionibacterium may be overlooked if a small number of specimens are submitted. The number of specimens submitted for culture in the 89 cases without positive cultures is not presented.

2 of 28 (7.1%) patients with UPCs required future surgery, and only 1 (3.6%) had a recurrent infection. This reinfection was in a patient who grew 1 of 6 cultures positive for P. acnes, and this reinfection was 2.2 years after the index revision. This patient did not receive a course of postoperative antibiotics (outside of the routine 2-week empirical antibiotic regimen). In addition, there were 3 patients without UPCs (3.4%; P = .959) who presented with reinfection. These 3 patients did not receive a course of postoperative antibiotics outside of the routine 2 weeks of empirical postoperative antibiotics.
18 of 89 (20.2%) patients without UPCs required 25 additional surgeries. The difference in reoperation rate between those shoulders with and without UPCs was not statistically significant.

Comment: It is critical to recognize that the need for re-revision after a shoulder arthroplasty revision may be prompted by a wide variety of mechanical factors, so that the overall re-revision rate is not a useful indicator of the importance of culture results. It is also import to recognize that the 28 cases with positive cultures presented years after the index procedure without obvious evidence of infection. This observation makes it impossible to know when a revision surgery with positive cultures has successfully eliminated bacteria from the shoulders.

In cases where these authors performed a single stage revision followed by two weeks of antibiotics, they may have adequately treated the presence of Propionibacterium as pointed out in this article,  Failed shoulder joint replacement: single stage revision when cultures are positive for Propionibacterium. The key in managing Propionibacterium appears to be the removal the implant carrying the infected biofilm.

Shoulder arthroplasty failure may or may not be associated with positive cultures. The role of bacteria in the failure remains unclear, as emphasized in this article, Glenoid loosening - is it predictive of positive cultures? and in this article,  How do revised shoulders that are culture positive for Propionibacterium differ from those that are not? In the latter article the authors reviewed records of 132 shoulders that underwent surgical revision of a shoulder arthroplasty, 66 of which became culture positive for Propionibacterium and 66 did not. The authors found that Propionibacterium-positive and Propionibacterium-negative shoulders were similar with respect to many characteristics; however, Propionibacterium-negative shoulders were revised sooner after the index procedure and were significantly more likely to be female, to have sustained a fall, to have instability, and to have rotator cuff deficiency. Intraoperatively, Propionibacterium-positive shoulders demonstrated more glenoid erosions, glenoid osteolysis, glenoid loosening, and a higher incidence of a soft tissue mem- brane between the humeral component and humeral endosteum. Shoulders culture positive for Propionibacterium were more likely to be culture positive for another bacteria. 
These authors concluded that although Propionibacterium-positive and Propionibacterium-negative shoulders have many similarities, factors such as male gender, delayed presentation, glenoid osteolysis and loosening, humeral membrane, and the absence of instability or cuff failure should arouse suspicion of Propionibacterium and suggest the need for deep cultures and consideration of aggressive surgical and medical treatment.

We conclude that this article demonstrates the need for a standardized approach to culturing and reporting quantitative culture results in cases of revision arthroplasty as emphasized in this article,  Considering the Load of Propionibacterium in Revision Shoulder Arthroplasty. Correlating the standardized quantitative culture results obtained with the management strategy and clinical outcome will inform our future understanding and management of failed shoulder arthroplasty.


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