Inflammatory and healing environment in synovial fluid after anterior cruciate ligament reconstruction: Granulocytes and endogenous opioids as new targets of postoperative pain

Abstract

Background: Biological processes after anterior cruciate ligament reconstruction (ACLR) is crucial for recovery. However, alterations in the of synovial fluid cell population during the acute phase following ACLR and the relationship between these cells and postoperative pain is unclear. The goal of this study was to reveal alterations in synovial fluid cell population during the acute phase following ACLR and relationship between postoperative pain and proportion of synovial fluid cells.

Methods: Synovial fluids were obtained from all patients (n = 50) before surgery and from patients who showed hydrarthrosis at days 4 (n = 25), and 21 (n = 42) post-surgery. The cell population was analyzed by flow cytometry. IL1β, IL8, and met-enkephalin in synovial fluid were quantitated by enzyme-linked immunosorbent assay. Patients answered numerical rating scale (NRS) questionnaire at 4 days and approximately 4 weeks postoperatively.

Results: The granulocyte population was significantly higher at 4 days after surgery than at any other time points. The population of macrophages was 3.2 times and 7.7 times as high as at surgery on days 4 and 21, respectively. T cell population was significantly higher 21 days after surgery compared to 4 days after surgery. All NRS 4 weeks after surgery showed a significant negative correlation with the granulocyte population in synovial fluid 4 days after surgery. Granulocyte population in synovial fluid significantly correlated with the levels of IL1β and IL8. Postoperative pain at rest tended to decrease with an increase in met-enkephalin concentration 4 days after ACLR.

Conclusions: Synovial fluid after ACLR had an inflammatory environment at early time points and a healing environment in the subsequent phase about concerning to the cellular composition. A proportion of synovial fluid cells and endogenous opioids affected postoperative pain.

Keywords: Anterior cruciate ligament reconstruction; Granulocyte; Inflammatory mediators; Knee arthroscopic operation; Synovial fluid cells.

Fig. 1

The time course of this experiment. A: Synovial fluids were obtained just before surgery and 4, and 21 days after surgery. Patients answered numerical rating scale (NRS) questionnaire at 4 days and approximately 4 weeks after surgery. B: List of CD markers for fluorescence activated cell sorting (FACS).

Fig. 2

Cellular context of synovial fluid: Representative results of cellular context of synovial fluid. Forward-scattered light (FSC): proportional to cell-surface area or size. Side-scattered light (SSC): proportional to cell granularity or internal complexity. Large and high-complexity, small and high-complexity, and small and low-complexity cell populations are presented within blue, green, and red ovals, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 3

Populations of synovial fluid cells. A: Granulocyte population in synovial fluid. B: Monocyte/macrophage population in synovial fluid. Population of CD11c-positive monocytes/macrophages and population of CD206-positive monocytes/macrophages were shown separately. C: The total population of T cells, helper T cells, and cytotoxic T cells in synovial fluid.

Fig. 4

Relationship between granulocyte population and pain intensity. A: Correlation of granulocyte population 4 days after surgery and numerical rating scale (NRS) for pain during movement (Q1), at rest (Q2), at first movement in the morning (Q3), and during climbing up and down stairs (Q4) at approximately 4 weeks after surgery. (N = 13–14) B: Correlation of granulocyte population and concentration of IL1β and IL8 4 days after surgery (N = 24).

Fig. 5

Relationship between met-enkephalin and pain intensity. A: Correlation of met-enkephalin concentration and numerical rating scale (NRS) for pain during movement (Q1), at rest (Q2), at first movement in the morning (Q3), and during climbing up and down stairs (Q4) 4 days after surgery. (N = 6–10) B: Correlation of granulocyte population and concentration of met-enkephalin 4 days after surgery (N = 10).