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 = 0.0157) and with ambulation (r = 0.3856; p = 0.0005) while CSF 2-AG was associated with pain at rest (r = 0.3312; p = 0.0017) but not with ambulation (r = 0.1454; p 0.1871). Stratification of the results by gender revealed positive correlations between 2-AG and pain in females, which were markedly weaker or not observed in males. Ex vivo analysis demonstrated co-expression 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 = 0.0269) and concomitantly reducing PGE₂ (median, vehicle: 5.645 nmol/g vs MJN110: 3.440 nmol/g, p = 0.0425).

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.