A Study of 137 Culture-Positive Cases
There has been a tendency in the recent literature to report cultures obtained at revision shoulder arthroplasty as being 'positive' or 'negative' or to assign revised shoulders to arbitrarily defined categories, in some cases distinguished by a single culture result (see below).
These authors took a different approach, one that considered the semi-quantitative results of all the cultures obtained from a revised shoulder - the 'load' of bacteria recovered from the shoulder using a defined culturing protocol.
They studied 137 revision shoulder arthroplasties from which a minimum of 4 specimens had been submitted for culture and that had at least 1 was positive for Propionibacterium. Standard microbiology procedures were used to assign a semiquantitative value (0.1, 1, 2, 3, or 4), called the Specimen Propi Value, to the amount of growth in each specimen. The sum of the Specimen Propi Values for each shoulder was defined as the Shoulder Propi Score, which was then divided by the total number of specimens to calculate the Average Shoulder Propi Score.
They found that the number and percentage of positive specimen-specific cultures (of material obtained from the stem explant, head explant, glenoid explant, humeral membrane, collar membrane, other soft tissue, fluid, or other) per shoulder ranged from 1 to 6 and 14% to 100%. A high percentage of specimens (mean, 43%; median, 50%) from the culture-positive shoulders showed no growth. These observations indicate that Propionibacterium are not evenly distributed through the tissues and implants of a failed shoulder arthroplasty so that more than a few samples are necessary to detect the presence of bacteria.
Another interesting finding was that the type of specimen submitted for culture affected the likelihood of culture positivity. Only 32.6% of the fluid cultures were positive in comparison with 66.5% of the soft-tissue cultures and 55.6% of the cultures of explant specimens. The average Specimen Propi Value (and standard deviation) for fluid specimens (0.35 ± 0.89) was significantly lower than those for the soft-tissue (0.92 ± 1.50) and explant (0.66 ± 0.90) specimens (p < 0.001). This finding provides a possible explanation for the limited utility of a culture-negative joint fluid aspiration in ruling out an infection.
A third intesting finding was the inter-sex difference in Propionibacterium load recovered from revised shoulders. The Shoulder Propi Score was significantly higher in men (3.56 ± 3.74) than in women (1.22 ± 3.11) (p < 0.001). Similarly, men had a significantly higher Average Shoulder Propi Score (0.53 ± 0.51) than women (0.19 ± 0.43) (p < 0.001).
Finally. the authors found that the percent of cultures positive for Propionibacterium varied widely among the cases with no apparent 'threshold' that could be used to distinguish 'definite infection' from 'probable infection' or 'probable contaminant' (see below).
Comment: This study provides an approach for standardizing:
(1) the harvesting of specimens = five samples of explants or tissue
(2) the culturing of specimens = aerobic and anaerobic media
(3) the period of observation = at least 17 days
(4) the reporting of culture results = Specimen Propi Value, Shoulder Propi Score and Average Shoulder Propi Score.
Such standardization and objective presentation of the results may facilitate comparison among investigators with respect to the characteristics of revised shoulder arthroplasties and the effectiveness of different surgical and medical approaches to their management.
An informed commentary on this article can be found at "The Emperor May Truly Have New Clothes" J Bone Joint Surg Am, 2017 Jan 18; 99 (2): e7 . https://doi.org/10.2106/JBJS.16.01148
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