Background: Despite acute knee injury being a major risk factor for osteoarthritis (OA), the factors that initiate and maintain this risk for longer-term knee symptoms are poorly understood. Bioactive lipids derived from omega-3 and -6 polyunsaturated fatty acids have key roles in the regulation of the inflammatory response, and have been linked to joint damage and osteoarthritis pain in translational models.

Hypothesis/Purpose: We hypothesised that there would be associations between systemic levels of bioactive lipids and knee symptoms longitudinally following acute knee injury and related knee surgery.

Study Design: This study analysed a subset of serum samples collected at three timepoints after injury from a prospective cohort study of young, active, healthy adults who had sustained acute knee injury (recruited via a surgical care pathway), and in age- and sex-matched controls. Surgery, where it occurred, was performed after the baseline sample was taken and prior to 3 month and 2 year visits.

Methods: An LC-MS/MS assay of 41 bioactive lipids was carried out in sera of i) 47 individuals (median age 28; sampled at baseline (median 24 days post-injury), at 3 months and 2 years) with paired Knee Injury and Osteoarthritis Outcome Scores (KOOS)-4, and ii)
age- and sex-matched controls.

Results: Levels of omega-3 polyunsaturated fatty acids (PUFAs) eicosapentaenoic acid (EPA; p=<0.0001) and docosahexaenoic acid (DHA; p=<0.0001) and the pro-resolution intermediaries 17-, and 14-hydroxydocosahexaenoic acid (HDHA) were all significantly greater at baseline compared with later times, and were also higher than levels in healthy
controls (p=0.0019, & p=<0.0001, respectively). Pro-inflammatory prostaglandins (E2 and D2), leukotriene B4, and thromboxane B2 were significantly lower at the time of injury compared to later times. Higher levels of 8,9-, 11,12-, and 14,15-dihydroxyeicosatrienoic acid (DHET) were cross-sectionally associated with more severe knee pain/symptoms (KOOS-4) at 2 years in this sub-cohort (p=0.0004, R2=0.251; p=0.0002, R2=0.278; p=0.0012,
R2=0.214).

Conclusion: The profile of pro-resolution versus pro-inflammatory lipids at baseline suggests an initial activation of pro-resolution pathways, followed by a later activation of pro-inflammatory pathways. The association of DHETs with knee symptoms at 2 years provides
rationale for further mechanistic investigation into the role of this pathway in chronic pain in this population.

Clinical Significance: In this largely surgically managed cohort, the association of soluble epoxide hydrolase metabolites, the DHETs, with more severe knee symptoms at 2 years provides a rationale for further investigation into the role of this pathway in persisting knee symptoms in this population including potential therapeutic strategies.

What is known about the subject: There is an initial inflammatory response following acute knee injury. However, how these pathways change over time and whether they contribute to development of chronic knee symptoms is not yet understood.

What this study adds to existing knowledge: This study adds insight into the systemic inflammatory response over time following acute knee injury and surgery, and suggests new pathways to investigate ways to treat chronic knee symptoms.