Acute Pain After Total Knee Arthroplasty: 2-Arachidonoylglycerol Tone and Endocannabinoid/Eicosanoid Crosstalk

Abstract

Background: Total knee arthroplasty (TKA) is a surgical procedure that induces intense acute postoperative pain, but the mechanisms that amplify post-TKA pain remain incompletely understood. Endocannabinoids, such as 2-arachidonoylglycerol (2-AG), are endogenous lipids that can produce antinociceptive effects. However, hydrolysis of 2-AG by monoacylglycerol lipase (MAGL) generates arachidonic acid, the precursor to a host of eicosanoids that enhance pain. The presence of this metabolic pathway suggests that individuals with elevated 2-AG levels may be primed to develop greater postoperative pain.

Methods: The primary goal of this prospective study was to determine if intraoperative 2-AG levels in the synovial fluid (SF) and cerebrospinal fluid (CSF) of TKA patients are associated with the magnitude of acute postoperative pain at rest and with ambulation. The secondary goal was to determine whether 2-AG metabolism contributes to prostaglandin E2 (PGE2) biosynthesis in synovial tissue ex vivo.

Results: Ninety subjects were enrolled in the study. SF 2-AG was positively correlated with pain at rest (r = 0.2644; P = .0157) and with ambulation (r = 0.3856; P = .0005) while CSF 2-AG was associated with pain at rest (r = 0.3312; P = .0017) but not with ambulation (r = 0.1454; P =.1871). Stratification of the results by sex revealed positive correlations between 2-AG and pain in females, which were markedly weaker or not observed in males. Ex vivo analysis demonstrated coexpression of MAGL and cyclooxygenase-2 in synovial membranes, with MAGL inhibition by MJN110 elevating 2-AG levels (median, vehicle: 0.165 nmol/g vs MJN110: 0.325 nmol/g, P = .0269) and concomitantly reducing PGE2 (median, vehicle: 5.645 nmol/g vs MJN110: 3.440 nmol/g, P = .0425).

Conclusions: Our findings demonstrate that patients presenting with an elevated 2-AG tone develop greater postoperative pain and position MAGL as an enzymatic node linking 2-AG metabolism with eicosanoid biosynthesis in perioperative human tissue.

Figure 1.

Flow diagram of subjects included in the analysis. 2-AG indicates 2-arachidonoylglycerol; CSF, cerebrospinal fluid; SF, synovial fluid.

Figure 2.

Associations between SF 2-AG and acute postoperative pain. Correlation between SF 2-AG and (A) postoperative pain at rest, (B) postoperative pain with ambulation, (C) preoperative NRS pain, (D) preoperative KOOS JR interval, and (E) postoperative opioid MME. 2-AG indicates 2-arachidonoylglycerol; KOOS, Knee Injury and Osteoarthritis Outcome Score; NRS, numerical rating scale; SF, synovial fluid.

Figure 3.

Associations between CSF 2-AG and acute postoperative pain. Correlation between CSF 2-AG and (A) postoperative pain at rest, (B) postoperative pain with ambulation, (C) preoperative NRS pain, (D) preoperative KOOS JR interval, and (E) postoperative opioid MME.

Figure 4.

Expression of MAGL and COX-2 in human synovium. A, Western blots depicting MAGL and COX-2 expression in the synovium of male and female subjects (n = 6, 3 men and 3 women). B, Immunolocalization of MAGL and COX-2 in the synovium of male and female subjects (n = 4, 2 men and 2 women, scale bar =100 μm).

Figure 5.

Effect of MAGL inhibition on 2-AG, AA, and PGE₂ levels in human synovium. Synovial explants from male (circles) or female (triangles) subjects were incubated with vehicle (0.1% DMSO) or 10 μM MJN110 for 6 hours and tissue levels of (A) 2-AG, (B) AA, and (C) PGE₂ were quantified (n = 12, 6 men and 6 women). Note that the metabolites are color-coded such that 2-AG, AA, and PGE₂ levels for each subject are depicted with the same symbol and color in each panel.