The term "unexpected positive cultures" may not be the best term: if cultures were not expected to be positive, why were they taken in the first place?
These authors note that many studies have found a high rate of positive cultures after revisions of failed shoulder arthroplasties that did not show the classic signs of infection (fever, chills, and elevated inflammatory markers in blood and joint fluid)
The purpose of this study of arthroplasty revisions performed on shoulders without obvious preoperative evidence of infection was to compare the patient-reported outcomes for those patients with and without substantially positive intra-operative cultures.
Preoperative antibiotics were withheld until biopsy specimens were obtained. Thereafter, an intravenous (IV) dose of either 1.5 g cefuroxim or 1 g dicloxacillin was administered. At surgery an "infection-type" revision was performed with debridement of scar and cement, antibiotic irrigation, and single stage exchange of implants. After surgery an "infection-type" antibiotic regimen was implemented: postoperative IV antibiotics for the first 24 hours followed by either oral or IV antibiotic treatment covering at least C acnes until the final culture result was available. If cultures exhibited growth at readings up to the fourth postoperative day, then 2 weeks of IV treatment were initiated, followed by 4 weeks of oral treatment. If cultures were positive at the reading on the 14th postoperative day, then an additional 4 weeks of oral treatment were recommended.
Five biopsies were obtained from 124 revisions. 27 shoulders (22%) had 3 or more positive cultures for the same organism. Cutibacterium acnes accounted for 67% of these (18/27).
All patients were assessed using the Oxford Shoulder Score (OSS) and range of motion preoperatively and after at least a 2-year follow-up (median follow-up of 29 months. At baseline, the OSS was 22 in both the culture-negative and the culture positive groups. At follow-up, the OSS was similar for the two groups: 37 in the culture-negative group and 35 in the culture positive group. One might conclude from this result that the presence of positive cultures was unimportant. A more attractive conclusion is that the shoulders with three or more out of five positive cultures were adequately managed by the surgical removal of potentially biofilm-coated implants, thorough debridement and a vigorous postoperative antibiotic protocol.
The results of this study are remarkably similar to those of another recent publication: Single-Stage Revision Is Effective for Failed Shoulder Arthroplasty with Positive Cultures for Propionibacterium
In that study, 55 shoulders without obvious clinical evidence of infection had a single-stage revision arthroplasty. The patient self-assessed functional outcomes for shoulders with ≥2 positive cultures for Propionibacterium (the culture positive group) were compared with shoulders with no positive cultures or only 1 positive culture (the control group).
The surgery consisted of complete prosthesis removal, debridement, antibiotic irrigation and reimplantation of a new prosthesis After all culture specimens were obtained, 15 mg/kg of vancomycin and 2 g of ceftriaxone were administered intravenously. In cases in which there was a high index of suspicion, the patient was maintained on intravenous ceftriaxone through a peripherally inserted central catheter (PICC) (the Red Protocol). If the cultures were negative at 3 weeks, all antibiotics were discontinued. In cases in which there was a low index of suspicion for infection, patients were placed on oral amoxicillin and clavulanate (the Yellow Protocol) either until the cultures were negative at 21 days or until 2 cultures with the same bacterial species became positive.
The culture-positive group had 89% male patients, with a mean age (and standard deviation) of 63.5 ± 7.2 years. The mean Simple Shoulder Test (SST) scores for the 27 culture-positive shoulders improved from 3.2 ± 2.8 points before the surgical procedure to 7.8 ± 3.3 points at a mean follow-up of 45.8 ± 11.7 months after the surgical procedure (p < 0.001), a mean improvement of 49% of the maximum possible improvement.
The control group had 39% male patients, with a mean age of 67.1 ± 8.1 years. The mean SST scores for the 28 control shoulders improved from 2.6 ± 1.9 points preoperatively to 6.1 ± 3.4 points postoperatively at a mean follow-up of 49.6 ± 11.8 months (p < 0.001), a mean improvement of 37% of the maximum possible improvement.
Subsequent procedures for persistent pain or stiffness were required in 3 patients (11%) in the culture positive group and in 3 patients (11%) in the control group; none of the revisions were culture-positive.
The authors concluded that the clinical outcomes after single-stage revision for Propionibacterium culture-positive shoulders were at least as good as the outcomes in revision procedures for control shoulders and that two-stage revision procedures may not be necessary in the management of these cases.
In sum, both groups of authors treated all of these revisions with a type of surgery and postoperative antibiotic regimen anticipating that a substantial percentage of the intraoperative cultures would be positive - even though there was not compelling preoperative evidence of infection. This is important because it is recognized that many, if not most, shoulder periprosthetic infections have a "stealth" rather than an "obvious" presentation.