The non-infection group included all fifteen patients who underwent revision shoulder arthroplasty from September 2011 to September 2012 who met the criteria for the category with no evidence of infection (Table II). All patients during this period of time had intraoperative samples submitted for frozen section and culture, no matter the preoperative indications. No patients were excluded. This allowed an accurate assessment of frozen section specificity by eliminating possible selection bias.
The P. acnes infection group consisted of all eighteen patients from 2005 to 2012 who fit the criteria for definite infection (twelve patients) or probable infection (six patients) (Table II), and for whom the only organism that grew on tissue or aspirate culture was P. acnes. Two patients were excluded because ESR, CRP, and frozen section were not obtained.
The other infection group consisted of all twelve patients from 2005 to 2012 who fit the criteria for definite infection (eight patients) or probable infection (four patients) (Table II), and for whom any organism besides P. acnes grew on culture. Patients with multiple-organism infections were placed in this category, even if one of the organisms was P. acnes. Two patients were excluded because ESR, CRP, and frozen section were not obtained.
Forty-five patients were included in this study across the three groups. The electronic medical records of all forty-five patients were reviewed for preoperative and intraoperative cultures and preoperative ESR and CRP. Because of the slow-growing nature of P. acnes, anaerobic tissue and fluid cultures for the shoulder are incubated for fourteen days at our institution. Information was also collected on patient demographics and operative procedure.
The intraoperative frozen section histology specimens for the forty-five patients were collected for reanalysis. All tissue samples that were evaluated were deep periprosthetic specimens, obtained from along the interfaces of the glenoid or humeral components or adjacent to the implants. These samples included intramedullary tissue if a humeral stem was removed, tissue off the glenoid surface and glenoid holes if a glenoid component was removed, and deep capsular tissue or deep bursal or pseudocapsular tissue in a cuff-deficient revision. None of the analyzed samples were from a superficial location. One senior anatomic pathologist (T.W.B.) analyzed all specimens and used four different thresholds to diagnose infection or non-infection (Table III)8,19. The total number of polymorphonuclear leukocytes in the five high-power fields (400×) with the highest concentration of polymorphonuclear leukocytes was determined. The anatomic pathologist was blinded to infection category during analysis of each specimen.
We compared the sensitivity of frozen section histology for the P. acnes infection group and the other infection group for each of the four diagnostic thresholds (Table III). In addition, a receiver operating characteristics (ROC) curve based on the total number of polymorphonuclear leukocytes in the five high-power fields with the highest concentration of polymorphonuclear leukocytes was utilized to determine a potential new optimal threshold. We also compared the means and sensitivities of ESR and CRP between these two groups. Institutional cutoff values were used to determine a positive ESR (>10 mm/h) and CRP (>1 mg/dL).
Chi-square analysis was used to compare sensitivities of the various tests. Student t tests were used to compare means. A p value of <0.05 was considered to be significant.
Source of Funding
There was no external funding source for this study.
The mean ESR and CRP for the non-infection, P. acnes infection, and other infection groups are reported in Table IV. For the eighteen patients in the P. acnes infection group, the sensitivity was 61% (eleven patients) for ESR and 33% (six patients) for CRP. For the twelve patients in the other infection group, the sensitivity was 83% (ten patients) for ESR and 58% (seven patients) for CRP. For the thirteen patients in both groups, the specificity was 85% (eleven patients) for both ESR and CRP.
Frozen Section Analysis
Differences in the sensitivity of frozen section histology between the P. acnes infection group and other infection group are reported in Table V. On the basis of our current institutional guidelines, for the eighteen patients in the P. acnes infection group, the sensitivity was 50% (nine patients), and for the twelve patients in the other infection group, the sensitivity was 67% (eight patients) (p = 0.15). Specificity was 100% (0 false positives).
There was minimal variation in the frozen section sensitivity for the P. acnes infection group between the previously established guidelines (Table V). An absolute polymorphonuclear leukocyte concentration of twenty-three or more in ten high-power fields19 was the most sensitive guideline for P. acnes (56%), and the American Academy of Orthopaedic Surgeons (AAOS) Guideline of ten polymorphonuclear leukocytes per high-power field for five or more fields was the least sensitive guideline (39%). There were no false positive interpretations using any of the thresholds, yielding a specificity of 100% using all four guidelines (Table V).
The total number of polymorphonuclear leukocytes in five high-power fields was used to create an ROC curve (Fig. 1). The majority of specimens (60% [nine patients]) in the fifteen patients in the non-infection group had zero polymorphonuclear leukocytes in five high-power fields, and the highest number of polymorphonuclear leukocytes was six. The ROC indicates that any cutoff from seven to ten polymorphonuclear leukocytes will improve sensitivity of frozen section histology for P. acnes infections (72%), while maintaining 100% specificity (Fig. 1).
Identifying infection in revision total shoulder arthroplasty is critical for determining appropriate medical and surgical management. The slow-growing P. acnes is a unique, but common, shoulder pathogen. Evidence suggests that the indolent nature of this organism inhibits the ability of common diagnostic tests to identify infection. Intraoperative frozen section histology is particularly important because of the potential immediate impact on medical and surgical management. The objectives of this study were to calculate the sensitivity and specificity of frozen section histology for patients with a P. acnes infection based on histologic guidelines that have been advocated for diagnosing periprosthetic infections of the hip and knee, and then to calculate a new threshold that might improve the sensitivity and specificity of frozen section diagnosis of periprosthetic shoulder infections. Our results showed that the guidelines recommended by the AAOS Practice Guidelines Committee for Diagnosis of Periprosthetic Joint Infections of the Hip and Knee8 had high specificity (no false positives) but relatively low sensitivity when applied to revision shoulder arthroplasty. Our ROC curve analysis suggests that a new threshold of a total of ten or more polymorphonuclear leukocytes in five high-power fields may help increase the sensitivity of frozen sections to diagnose the commonly indolent periprosthetic infections of the shoulder with minimal impact on specificity.
In revision shoulder arthroplasty, results from frozen section histology can change medical and surgical management. Although the performance of frozen section in the lower-extremity joints is well studied11,12, few studies have evaluated the sensitivity in the shoulder, and no studies have examined the performance of frozen section in P. acnes infections. Our data support a low sensitivity of frozen section in P. acnes infections (50%) using the diagnostic thresholds currently recommended for revision hip and knee arthroplasty, and highlight the need for improved tests for identifying infection with this pathogen. On the basis of our data as well as previous studies, ESR, CRP, and preoperative aspirate cultures also have lower sensitivity for diagnosing periprosthetic shoulder infections than when those tests are applied to periprosthetic knee or hip infections.
Poor frozen section performance can impact surgical and postoperative medical management. For example, a patient with negative preoperative signs of infection (normal ESR, CRP, negative aspirate) will be assumed to have aseptic failure and, without evidence of acute inflammation on frozen section, will undergo a one-stage revision. If positive tissue culture occurs late, as is common for P. acnes, antibiotic therapy may be considerably delayed, usually after a patient’s discharge. At our institution, treatment for later positive culture results includes six weeks of intravenous antibiotics, followed by potential extended oral suppressive antibiotic therapy. This situation is not uncommon. Indeed, for our eighteen patients with P. acnes infection, 39% (seven patients) underwent a one-stage procedure followed by delayed medical management (Table II). The length of delay before initiation of intravenous antibiotic therapy was based on P. acnes culture times and ranged from four to eight days (mean, 5.2 days), but can be even longer for this slow-growing organism5. For one patient, culture time took twenty-one days, but intravenous antibiotics were initiated earlier because of gross intraoperative signs of infection. The long-term impact of this potential delay in treatment is not known, nor is the potential difference in long-term outcome between this approach and a two-stage reimplantation that would be performed if the diagnosis of infection was confirmed at the time of surgery.
Our new proposed threshold of ten polymorphonuclear leukocytes per five high-power fields calculated from the ROC curve resulted in the frozen section for a P. acnes infection having an increased sensitivity (73%) compared with its previous sensitivity (50%). We believe that use of this new criterion has utility for intraoperative decision-making that ideally will lead to improved outcomes. The presence of a positive frozen section, depending on the other associated clinical data, may lead the treating surgeon to maintain antibiotic therapy postoperatively until culture results are final to avoid a delay in treatment, or it may be the final piece of evidence to convince a surgeon to perform a two-stage exchange. In addition, by maintaining a high specificity (100%) with this new threshold, the risk of these potential additional treatments (extended antibiotic therapy and/or two-stage exchange) being unnecessary remains low. Finally, it should be noted that the predictive value of any laboratory test, including frozen section interpretation, reflects in part the prevalence of a condition. For example, although our specificity of 100% reflects no false-positive results, that result might be different if the population undergoing frozen sections was different.
We chose to create categories of infection based on likelihood because of the lack of consensus in defining infection in shoulder arthroplasty and the controversy that exists in labeling P. acnes positive cultures as genuine or a contaminant. For this study, we chose to only include those patients with a high probability of having true infections to reduce the probability that the lower sensitivity of frozen section in the P. acnes infection group was a result of increased false-positive infections. Therefore, we included patients who were classified as definite infection and probable infection. As there is no gold-standard criterion for infection, it was reasonable and most consistent with the shoulder literature for us to use these categories6,16-18. We further analyzed the performance of frozen section in patients with an extremely high likelihood of infection (definite infection only). We again found an increased sensitivity in the P. acnes infection group between our current institutional criteria (66%) and the new threshold criteria (83%) (data not shown).
Because this was a retrospective study, we recommend caution in using this new threshold to decide one-stage or two-stage revision with no other supporting criteria. A prospective study with a larger number of both infected and non-infected cases is needed. The risks and benefits of a two-stage revision also need to be balanced with a better understanding of the natural history of P. acnes in patients with positive revision shoulder arthroplasty. For example, in a previous study with a two-year follow-up, we found a low recurrence rate of infection for patients with at least one positive intraoperative P. acnes culture and no other signs of infection who did not undergo a two-stage revision13. However, the patient population in that study would fit mostly into the possible infection category (Table I) and, therefore, could not be appropriately matched with the P. acnes infection group used in this study. This study may help to further define this category of patients. The presence of acute inflammation in periprosthetic tissue can support the diagnosis of infection in cases with one unexpected positive culture. If the frozen section (and permanent histology) does not show acute inflammation, then it further supports that a single unexpected positive culture after revision may not require further treatment.
Periprosthetic infection with P. acnes following shoulder replacement surgery can be a devastating complication3,6,7. Early identification of a joint infection is essential in guiding surgical and medical management. Frozen section histology can be a helpful tool for identifying infection intraoperatively. We found a high specificity of frozen section, but a low sensitivity for those patients with P. acnes infection when using currently recommended diagnostic thresholds. We improved sensitivity by utilizing a new histologic threshold value of ten polymorphonuclear leukocytes per five high-power fields, while maintaining 100% specificity.
Investigation performed at the Cleveland Clinic, Cleveland, Ohio
A commentary by Pierre Mansat, MD, PhD, is linked to the online version of this article at jbjs.org.
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Disclosure: None of the authors received payments or services, either directly or indirectly (i.e., via his or her institution), from a third party in support of any aspect of this work. One or more of the authors, or his or her institution, has had a financial relationship, in the thirty-six months prior to submission of this work, with an entity in the biomedical arena that could be perceived to influence or have the potential to influence what is written in this work. No author has had any other relationships, or has engaged in any other activities, that could be perceived to influence or have the potential to influence what is written in this work. The complete Disclosures of Potential Conflicts of Interest submitted by authors are always provided with the online version of the article.Copyright 2014 by The Journal of Bone and Joint Surgery, Incorporated