Plasma Cell Infiltration on Histopathological Samples of Chronic Bone and Joint Infections due to Cutibacterium acnes: A series of 21 Cases
Histopathological definition of bone and joint infection (BJI) is based on Mirra’s
criterion (≥ 5 polymorphonuclears (PMNs) per field in 5 high power fields (HPFs)). However, this
definition does not seem appropriate for chronic BJIs caused by slow-growing germs such as
Cutibacterium acnes (C. acnes). The aim of this study was to confirm that Mirra’s criterion is not
adequate for diagnosis of BJIs due to C. acnes. The second objective was to determine if plasma cell
infiltration could be useful for the diagnosis of chronic BJIs due to C. acnes.
We retrospectively selected 25 consecutive patients from 2009 to 2013 with chronic
BJIs due to C. acnes. Histological analysis was performed on the 21 cases with at least two C. acnes
positive cultures. In addition of Mirra’s criterion, the number of plasma cells (≥5 plasma
cells/5 HPFs, defined as “CRIOAc Lyon’s criterion”) was implemented in the histopathological
analysis. Patients were defined as infected, if at least one of the two criteria were present.
Results: According to Mirra’s and CRIOAc Lyon’s criteria, positive histopathology was observed in
12 (57.1%) and 15 (71.4%) cases respectively. Considering the 9 cases with negative Mirra’s
criterion, high plasma cell infiltration (≥5 plasma cells per field/5 HPFs) was observed in 5 cases
(55.6%), and low plasma cells infiltration (2-5 plasma cells per field/5 HPFs) was observed in 4 other
Adding CRIOAc Lyon’s criterion to Mirra’s criterion might restore some
histopathological diagnosis of chronic BJIs due to C. acnes when a chronic BJI is clinically suspected.