Modulation of inflammatory pathways by omega-3 fatty acids in knee joint health for the management of post-traumatic osteoarthritis: a review

Abstract

The knee joint facilitates movement essential for daily activities. Its intricate anatomy makes it particularly prone to injury, specifically to the anterior cruciate ligament (ACL). ACL injury is a leading cause of post-traumatic osteoarthritis (PTOA), a degenerative joint disease that affects millions and results in pain, decreased mobility, and reduced quality of life. The cascade of initial inflammatory activity triggered by ACL injury plays a critical role in the pathogenesis of PTOA, as elevated levels of pro-inflammatory cytokines and catabolic enzymes lead to ongoing joint damage and dysfunction.

Omega-3 fatty acids, an essential component of our diet, have been demonstrated to exert protective effects on joint tissues by modulating inflammatory pathways and promoting tissue repair. Specifically, these fatty acids diminish the production of inflammatory cytokines and enhance the resolution of inflammation, thereby potentially reducing the progression of PTOA. Given the lack of effective preventative therapies for PTOA, there is a pressing need for strategies that target early inflammatory processes to slow disease progression. This review provides a comprehensive analysis of the mechanistic and biochemical pathways through which these fatty acids influence knee joint health, with a focus on their impact in PTOA. By investigating the roles of omega-3 fatty acids and their metabolites, eicosanoids and specialized pro-resolving mediators, we highlight the potential for a nutrition based therapeutic application in managing PTOA.

Background

Knee post-traumatic osteoarthritis (PTOA), primarily characterized by cartilage degeneration, subchondral bone changes, and altered joint biomechanics, is a form of osteoarthritis (OA) that arises following joint injury. PTOA occurs commonly after anterior cruciate ligament (ACL) injury, with ACL injury occurring in 68.6 per 100,000 people each year [1]. This accounts for up to 90% of individuals that develop knee PTOA [2]. The comprehensive cascade of events that contribute to PTOA development post ACL injury is relatively unknown. It is postulated that the initial ACL injury triggers an acute inflammatory response involving synovial cellular infiltration, elevated inflammatory cytokines, and activation of catabolic enzymes that degrade the extracellular matrix (ECM) [2]. If left unresolved, these inflammatory processes lead to undesired sustained inflammation that has been identified as a key factor in the progression of PTOA [3]. Preventative strategies that target and resolve early, acute inflammation post-ACL injury could mitigate chronic inflammation and secondary PTOA development. Despite the burden of PTOA, there are currently no therapies to prevent or slow progression [4], which affects 6 million people in the United States and imposes a 7-billion-dollar economic burden [5].

Fatty acids are long-chain hydrocarbons that play critical roles in various physiological processes and in many diseases such as cardiovascular disease [6] and chronic inflammation [7]. Fatty acids are categorized into saturated, monounsaturated, and polyunsaturated fatty acids (PUFAs). PUFAs are vital for the synthesis and maintenance of cell membranes, energy production, and modulation of inflammatory responses [8]. These modulatory functions can be broadly grouped into biophysical effects, receptor-mediated mechanisms, intracellular signaling mechanisms, and cellular responses, as discussed in detail below. Among PUFAs, omega-3 fatty acids are of particular interest due to their potent anti-inflammatory and pro-resolving properties [7]. These fatty acids act as ligands for nuclear and G-protein coupled receptors, modulate intracellular signaling pathways, and influence immune cell behavior, including macrophage polarization, while also serving as substrates for the production of specialized pro-resolving mediators (SPMs) that actively resolve inflammation.

This review provides a comprehensive analysis of the mechanistic and biochemical pathways through which fatty acids may influence joint health, with a focus on their impact in knee PTOA development following ACL injury. Many of these mechanisms have also been implicated in other OA subtypes, such as primary, age-related OA and PTOA induced through other injuries, however knee PTOA following ACL injury is the focus in this review. By elucidating the roles of key inflammatory and resolution factors, we highlight the potential therapeutic applications of dietary modulation of fatty acids in managing knee PTOA.

This narrative review was informed by a literature search conducted using PubMed/MEDLINE and Web of Science. Searches were performed using combinations or keywords related to post-traumatic osteoarthritis, knee joint injury, ACL, polyunsaturated fatty acids, specialized pro-resolving mediators, and inflammatory and resolution pathways. Key terms included and combinations of “post-traumatic osteoarthritis,” “ACL injury,” “knee osteoarthritis,” “omega-3 fatty acids,” “polyunsaturated fatty acids,” “inflammation,” “NF-κB,” “metabolism”, “macrophage polarization,” “G-protein coupled receptors,” “PPAR,” “eicosanoids,” and “specialized pro-resolving mediators.” This approach yielded approximately 1,200 hits, of which 180 articles were included based on relevance to this review.

Knee joint structure and nutrient supply

The knee joint is the largest and one of the most complex joints in the human body [9]. Bony structures of the knee joint—including the femur, tibia, and patella—provide structural support, while the articular cartilage that lines these opposing bony surface functions to reduce friction and absorb shock. Articular cartilage consists of a dense extracellular matrix (ECM) and highly specialized cells, chondrocytes, that are responsible for the production and maintenance of the ECM. The ACL plays a pivotal role in joint function [10], resisting anterior and rotational displacement of the tibia relative to the femur providing 85% of total knee stabilization [11]. Therefore, injury to the ACL is detrimental to joint integrity, leading to instability and increasing the risk of further damage to other joint structures. Without proper intervention, ACL injuries can severely impair joint function, leading to long-term consequences including chronic mechanical instability and loss of function, cartilage degeneration, and an increased likelihood of PTOA.

The synovium, or synovial membrane, lines the inner surface of the joint capsule and maintains homeostasis by producing synovial fluid, which lubricates and nourishes the intra-articular joint structures while reducing cartilage friction [12]. Articular cartilage relies heavily on the synovium and other surrounding structures, including the infrapatellar fat pad and underlying subchondral bone, for nourishment via diffusion [13, 14]. This reliance on passive nutrient diffusion makes cartilage susceptible to poor healing after injury; therefore, any disruption to nutrient delivery by mechanical damage and/or inflammation can exacerbate tissue degeneration. The synovium also serves as an important site for immune cell recruitment during inflammation, highlighting its role in immune regulation following joint injury [15].

Pathophysiology of knee post-traumatic osteoarthritis (PTOA) and current treatments

PTOA is a degenerative whole joint disease characterized by structural deterioration, pain, swelling, and reduced mobility. Knee joint trauma, such as ACL injury, initiates an inflammatory response with release of pro-inflammatory cytokines and matrix metalloproteinases (MMPs), leading to ECM degradation [4, 16], cartilage thinning, cartilage fibrillation and fissures [17, 18]. Concurrently, subchondral bone undergoes remodeling, resulting in sclerosis (hardening of bone) and osteophytes formation (bone spurs) [4, 19], contributing to joint stiffness and pain. The synovium becomes inflamed and thickened, synovitis, [19, 20] which exacerbates cartilage breakdown and further drives degeneration. Figure 1 summarizes the end stage structural pathologies of PTOA which culminate in joint space narrowing, altered mechanics, and functional decline [21, 22]. The specific mechanisms, involvement of omega-3 fatty acids, and effects on specific knee joint structures are explored further in subsequent sections.

Fig. 1.

Schematic comparison of a healthy joint (left) and a PTOA joint (right) illustrating injury-induced structural changes in joint tissues. The healthy joint exhibits intact articular cartilage, a normal synovial membrane, and organized subchondral bone that support joint homeostasis. In contrast, the PTOA joint shows cartilage degradation, synovitis, and osteophyte formation, and subchondral bone remodeling

During knee PTOA, the acute inflammatory response begins within minutes to hours post-ACL injury and is marked by vascular changes such as vasodilation and increased permeability, facilitating immune cell infiltration. Neutrophils are the first responders, peaking in abundance at one day post-injury [23]. They release proteases (e.g., MMPs) and reactive oxygen species (ROS), degrading the ECM and work to clear cellular debris to enable tissue repair [24, 25]. While essential for initial clean up, excessive neutrophil activity exacerbates cartilage and ECM damage. Macrophages follow, phagocytosing debris and orchestrating the transition to the proliferative phase of healing.

Post-injury, synovial fluid exhibits elevated levels of pro-inflammatory cytokines including Tumor Necrosis Factor Alpha (TNF-α) interleukin(IL)−1β(IL-1β), IL-6, and IL-8 within 48 h [3, 26, 27]. After ACL injury specifically, IL-6, inducible nitric oxide synthase (iNOS), A Disintegrin and Metalloproteinase (ADAM) ADAMTS-4, ADAMTS-5, MMP-3, MMP-13, Transforming growth factor beta (TGF-β), TNF-α, and IL-1α are upregulated within 24 h [28–30]. IL-6, IL-8, and IL-1β remain significantly elevated at 48 hours [27], and soluble Cd11b, IL-6, IL-1β, and TNF-α persist over 1–3 days, indicating sustained inflammation [28–30]. These cytokines originate from synoviocytes, chondrocytes, and infiltrating immune cells, including macrophages and neutrophils [31]. The nuclear factor-kappa B (NF-κB) signaling pathway is activated in chondrocytes and synoviocytes, driving inflammation and cartilage catabolism [32, 33], while oxidative stress induces ROS, further damaging joint tissues [34]. Given these multi-tissue interactions, therapeutic strategies should consider whole-joint involvement and systemic approaches to PTOA treatment.

The progression from the acute to chronic phase of inflammation is critically influenced by the balance between pro-inflammatory and anti-inflammatory signals. If the acute inflammatory response, which is initially protective for tissue homeostatic maintenance, is not properly resolved, persistent inflammation, fibrosis, and subsequent joint degeneration will follow. This chronic inflammatory state is a key contributor to the pathogenesis of PTOA, highlighting the need for therapies that can effectively resolve the acute inflammatory phase post-injury and prevent the progression to chronic joint degeneration.

Current treatments for PTOA mainly involve anti-inflammatory medications and corticosteroids, which have been associated with adverse effects such as a reduction in bone mineral density [35, 36], increased risk of osteoporosis and fracture [37], local bone calcification and synovitis [38], and severe complications such as osteonecrosis [39–41]. Non-steroidal anti-inflammatory drugs (NSAIDS) are also accompanied with significant risks such gastrointestinal complications [42] and renal failure [43], and even accelerate PTOA progression [44] by increasing chondrotoxicity [45], synovial hyperplasia [46], and fibrosis [46]. These side effects underscore the need for safer, more effective treatment options for managing PTOA.

Polyunsaturated fatty acids: an overview

Fatty acids are organic molecules consisting of long hydrocarbon chains with a carboxyl group at one end, are classified based on the presence or absence of double bonds in their hydrocarbon chain into saturated (no double bonds) and unsaturated (one or more double bonds) fatty acids. Saturated fatty acids are typically found in animal fats and plant oils and are a significant source of energy [47], but have different metabolic and physiological effects compared to polyunsaturated fatty acids (PUFAs) and thus will not be discussed within this review. Unsaturated fatty acids, found in fish, nuts, and vegetable oils [48], can be further divided into monounsaturated and polyunsaturated fatty acids based on the number of double bonds. PUFAs, particularly omega-3 and omega-6, are essential components of the human diet as they cannot be endogenously synthesized and must be consumed through the diet. PUFAs have attracted attention for their potential impact on modulating inflammatory processes across the body and within many diseases and pathologies. While PUFAs are traditionally recognized for their roles in cellular membrane structure maintenance [8], evidence suggests they participate in influencing inflammatory responses in the joint and are a potential therapeutic for mitigating PTOA development and progression.

Omega-6 PUFAs are commonly consumed via vegetable oils, seeds, nuts and animal products [48–50]. Omega-3 PUFAs are found in leafy vegetables, walnut, soybeans, flaxseed, fish, and krill oils [48, 50]. Most adults are deficient in omega-3 PUFAs [51] which has led to an encouragement of increased dietary intake and supplementation of omega-3 PUFAs, which are widely popular and available in many different formulations, including but not limited to fish oil, krill oil, and algal oil. Omega-3 PUFA supplementation in humans has been shown to reduce inflammation [7] anxiety and depression [52], risk of cardiovascular disease [6], insulin resistance [53], and OA related joint pain [54–58].

Metabolism

Omega-3 and omega-6 PUFA metabolism and conversion into bioactive lipid mediators are essential in modulating inflammatory responses that contribute to joint health. Through a series of conversions omega-3 PUFAs, most common being alpha-linoleic acid (ALA) form EPA/DHA and omega-6 PUFAs, most common being linoleic acid (LA) form arachidonic acid (AA) which are then further metabolized via desaturation and elongation reactions into bioactive signaling molecules, called eicosanoids and docosanoids [59, 60] (Fig. 2).

Fig. 2.

Schematic overview of omega-6 and omega-3 polyunsaturated fatty acid metabolic pathways. Linoleic acid (LA) and α-linolenic acid (ALA) are released from membranes via cytosolic phospholipase A2 (cPLA2), producing arachidonic acid (AA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), which are subsequently metabolized through cyclooxygenase (COX), lipoxygenase (LOX), and cytochrome P450 (CYP450) pathways. These shared enzymatic pathways generate eicosanoids and docosanoids, specialized pro-resolving mediators (SPMs). Abbreviations: 14-hydroxydocosahexaenoic acid (14-HDHA); 17-hydroxy docosahexaenoic acid (17-HDHA); 18-hydroxyeicosapentaenoic acid (18-HEPE)

Eicosanoids and docosanoids have unique roles in immune responses, inflammatory modulation, vascular, renal, gastrointestinal, and reproduction regulation [61]. These bioactive lipid mediators are derived by oxidation from AA, and EPA/DHA by cyclooxygenase (COX), lipoxygenase (LOX) and cytochrome P450 (CYP) pathways. When converted, AA gives rise to eicosanoids with general pro-inflammatory properties such as prostaglandins and thromboxanes via the COX pathway and leukotrienes via the LOX pathway, as well as several pro-resolution lipid metabolites including lipoxins [62, 63]. In contrast, EPA/DHA leads to production of lipid mediators with many pro-reparative/resolution and anti-inflammatory properties such as resolvins, protectins and maresins [64] (Fig. 2).

Fatty acid delivery and uptake in the joint

Two primary pathways for fatty acid incorporation into the joint are generally accepted: subchondral bone exchange, demonstrated in horses [65], mice [65, 66] and non-human primates [67], and synovial fluid diffusion. Although, synovial fluid diffusion is widely recognized as the dominant source of nutrient supply for the articular cartilage [13, 14, 68]. In the knee joint, PUFAs are delivered through fatty acid transport mechanisms, specifically fatty acid translocase Cluster of differentiation 36 (CD36), which specifically facilitates the uptake of free fatty acids into chondrocytes [69] (Fig. 3b). Chondrocytes express other fatty acid receptors, including free fatty acid receptor (FFAR) 1 and 4 [69, 70], which have been shown to be colocalized with markers of matrix catabolism [71], suggesting their potential involvement in disease progression.

Fig. 3.

Overview of the suspected role of omega-3 PUFAs in immunomodulation with green indicating positive interventions of omega-3 PUFAs, and green triangles representing EPA/DHA. A Omega-3 PUFAs maintain cell membrane permeability and structure via lipid rafts and influence macrophage polarization toward an anti-inflammatory (M2) phenotype. B Overview of omega-3 PUFA metabolites, EPA/DHA, inhibiting NF-κB and therefore decreasing the transcription of pro-inflammatory molecules through binding to FFAR4 and blocking the binding of DAMPs to TLR’s and activating PPARα. These pathways have been associated with a reduction in extracellular matrix catabolism and local inflammation, and a subsequent reduction in PTOA. Abbreviations: Cluster of differentiation 36 (CD36); Damage associated molecular patterns (DAMPs); Docosahexaenoic acid (DHA); Eicosapentaenoic acid (EPA); Free Fatty Acid Receptor 4 (FFAR4); G protein coupled receptors (GPCR); Interleukin-1 beta (IL-1β); Interleukin-6 (IL-6); Interleukin-8 (IL-8); Matrix Metalloproteinases (MMPs); Nuclear Factor kappa-light-chain-enhancer of activated B cells (NF-kB); Peroxisome proliferator-activated receptor alpha (PPARα); Polyunsaturated fatty acids (PUFAs); Post-traumatic osteoarthritis (PTOA); Prostaglandin E2 (PGE2); Toll-like receptor (TLR); Tumor Necrosis Factor-alpha (TNF-α)

Dietary PUFA consumption has a notable impact on joint health, specifically the balance of PUFA intake affects systemic lipid levels and alters the fatty acid composition within cartilage. Interestingly, dietary PUFA intake can modify the fatty acid profile of cartilage [72–74], demonstrating a direct link between diet and joint composition.

Mechanisms of immunomodulation

Omega-3 PUFAs play a crucial role in restoring homeostasis across various disease states, including wound healing [75], cardiovascular disease [6], neurodegenerative disease [76], musculoskeletal disease [77], and inflammation [78], and are well recognized for their immunomodulation properties through multiple interconnected mechanisms. These include alterations in plasma membrane composition and biophysical properties, activation of free fatty acid receptors (FFARs) and peroxisome proliferator-activated receptors (PPARs), modulation of toll-like receptor (TLR) receptor pathways, suppression of nuclear factor kappa B (NF-κB) mediated transcription, regulation of macrophage polarization, and serving as substrates for the biosynthesis of specialized pro-resolving mediators (SPMs). Together, these pathways represent key mechanisms through which omega-3 PUFAs influence inflammatory processes relevant to PTOA.

Membrane effects

Fatty acids incorporated into cell membranes impact fluidity and the functionality of membrane-bound proteins, influencing nutrient uptake, signal transduction, and inflammatory responses – key processes for cartilage and joint health. PUFAs, particularly EPA/DHA, enhance fluidity [79–82] and permeability [83] by disrupting the tight lipid packing [84]. In chondrocytes this supports ECM-related protein function (e.g., integrins, MMPs) [85], and mechanotransduction, aiding in cartilage maintenance [86].

PUFAs also regulate lipid raft organization, essential for maintaining cellular membrane integrity and function. [87, 88] (Fig. 3a). These lipid raft microdomains facilitate the assembly of various signaling molecules, including G-protein coupled receptors (GPCRs), such as FFAR1 and FFAR4 which are crucial for mediating responses to free fatty acids [89, 90]. Several GPCRs have been identified as key regulators of cartilage homeostasis and degeneration [91, 92], subchondral bone remodeling, synovitis and inflammation [92, 93], and modulating chronic pain [94]. By maintaining membrane fluidity and optimizing GPCR function, fatty acids may help preserve the balance between anabolic and catabolic processes within the joint observed with PTOA.

Receptor mediated mechanisms

Activation of FFAR and PPAR signaling pathways

FFARs, specifically FFAR1 and FFAR4, are activated by long chain fatty acids, omega-3 PUFAs being the most potent [95, 96], and initiate intracellular signaling cascades that lead to anti-inflammatory effects. Through activation by EPA/DHA, FFAR4 inhibits the NF-κB signaling pathway, reducing the production of pro-inflammatory cytokines, and promoting the expression of anti-inflammatory genes [97–99] (Fig. 2b). Recently, activation of FFAR4 by omega-3 PUFAs in chondrocytes and synovial cells led to the suppression of inflammatory mediators and the promotion of cartilage homeostasis [92, 100]. Indicating FFAR4 plays a crucial role in maintaining joint health by modulating inflammatory responses in joint microenvironment.

Peroxisome Proliferator-activated Receptors (PPARα, β/δ,γ), fatty-acid activated transcription factors of nuclear receptor hormone superfamily, function as transcription factors by binding to DNA and regulating gene expression, and act as intracellular receptors by binding lipid molecules [101]. PPARα(liver, heart, and skeletal muscle), activated by EPA/DHA, illicit an anti-inflammatory response by inactivation of NF-κB signaling [102, 103] (Fig. 3b) and regulates lipid homeostasis, specifically inducing the expression of genes involved in fatty acid catabolism [104]. PPAR β/δ regulates lipid metabolism and inflammatory reactions [101]. PPARγ, activated by DHA, reduces pro-inflammatory cytokines and may be protective against OA in mice [105–107].

Modulation of toll-like receptor (TLR) signaling

Toll-like receptors (TLRs), particularly TLR2 and TLR4, detect DAMPs (damage-associated molecular patterns) produced during joint injury by stressed or injured cells [108–110] and lead to the activation of the NF-κB pathway and subsequent production of pro-inflammatory cytokines (TNF-α, IL-1β and IL-6) [108]. TLR signaling in synovial fibroblasts [111], synovial macrophages [112], and chondrocytes [113], have been implicated in the upregulation of MMPs and cytokines, exacerbating cartilage degradation [114–116]. Inhibition of TLR4 has demonstrated chondroprotective effects in mouse OA [117], suggesting a potential therapeutic strategy.

Omega-3 PUFAs can inhibit TLR activation, suppressing NF-κB signaling, leading to a reduction in the production of pro-inflammatory cytokines and chemokines (Fig. 3b). Conversely, omega-6 PUFAs are known to promote inflammation by enhancing TLR signaling and NF-κB activation [118], contributing to the amplification of inflammatory responses and exacerbation of joint damage. By influencing TLR-mediated pathways, fatty acids can play a significant role in the delayed progression and management of PTOA.

Intracellular signaling mechanisms

Regulation of NF-κB signaling

The transcription factor nuclear factor κB (NF-κB) is a central regulator of inflammatory responses, controlling the expression of key pro-inflammatory cytokines, including TNF-α, IL-1β, IL-6, and IL-8, amongst others [119]. This process drives the inflammatory response and if not properly regulated or resolved, can lead to chronic inflammation [120].

EPA/DHA mitigate inflammation by modulating NF-κB activity and thereby decreasing the expression of pro-inflammatory genes [121] (Fig. 3b). Specifically, in-vitro studies have demonstrated that EPA/DHA can attenuate NF-κB activation in macrophages [122, 123] and human endothelial cells [124]. This suppression of NF-κB activity has also been observed in-vivo through dietary supplementation of EPA/DHA in rats [102]. The ratio of omega-6:omega-3 PUFAs appear to be important in NF-κB activity, as a 20:1 ratio significantly activated NF-κB activity [125]. Related to PTOA, omega-3 PUFA derivatives were found to suppress activation of NF-κB and subsequent factors associated with PTOA pathogenesis, in-vitro [126] and in a rat treadmill model [127].

Enzymatic competition with eicosanoid pathways

Omega-3 PUFAs modulate inflammation by competing with arachidonic acid (AA) for cyclo‐oxygenase (COX) and lipoxygenase (LOX) enzymes, thereby reducing the synthesis of AA-derived eicosanoids, including prostaglandins, thromboxanes, and leukotrienes [128] (Fig. 2). Upon stimulation, cytosolic phospholipase A₂ (cPLA₂), liberates membrane-bound PUFAs, including AA, EPA, and DHA, providing substrate for downstream COX and LOX pathways [129, 130] (Fig. 2). When EPA/DHA are present, they compete for these enzymes, acting as competitive inhibitors to AA for the COX and LOX enzymes [131, 132]. Increased dietary intake of EPA/DHA leads to a shift in lipid mediator production, decreasing pro-inflammatory leukotriene B₄ (LTB₄) [133], thromboxane B2 (TXB2), and prostaglandin E2 (PGE2) [134–137]. Conversely, AA supplementation enhances production of AA-derived metabolites, particularly PGE2 and LTB4, exacerbating inflammation [136, 138].

Therefore, increase of consumption and thereby percent membrane EPA/DHA composition reduces metabolism of AA, and decreases the production of the AA-derived proinflammatory eicosanoids [124, 132, 139, 140]. This competition between AA and EPA/DHA for enzymatic activity determines the balance of pro-inflammatory and anti-inflammatory bioactive lipid mediators synthesized.

Cellular responses

Regulation of macrophage polarization

Macrophages shift from pro-inflammatory M1 to anti-inflammatory M2 phenotype to resolve inflammation and promote tissue repair across the body [141]. In M1 phenotype, cytokines such as IL-6, IL-1β and TNF-α [142] are activated to initiate the inflammatory response and clear debris from damaged tissue. Over time, as the inflammatory phase resolves, macrophages are then polarized to the M2 phenotype, anti-inflammatory response [143] by secreting anti-inflammatory cytokines such as IL-10 and TGF-β [144], promoting ECM remodeling [145] and angiogenesis through factors like VEGF and fibronectin [146], all together aiding in tissue repair.

This transition from M1 to M2 is essential for resolving inflammation and initiating the processes of tissue regeneration and healing and has been identified as a potential strategy for treating PTOA [147]. In synovium, macrophages have been characterized to be a major regulator of the production of MMPs and proinflammatory cytokines [148]. Omega-3 PUFAs influence the M1/M2 macrophage phenotype balance, by promoting the polarization of macrophages to the M2, anti-inflammatory phase, and encourage tissue repair and reduce chronic inflammation (Fig. 3a). Further, a combination EPA/DHA supplementation in human THP-1 macrophages in-vitro inhibited polarization towards M1 and subsequent pro-inflammatory cytokines (IL-6, TNF-α, and IL-1β), compared to just EPA or DHA alone [123]. Additionally, DHA supplementation enhanced polarization towards M2 with increased production of anti-inflammatory cytokines such as TGF-β and IL-10 [149], also observed in a high omega-3 PUFA mouse model [150]. Moreover, specialized pro-resolving mediators (SPMs) have been shown to promote M2 polarization, including MaR1, RvD2 and RvD1 [151]. Therefore, therapeutic strategies like dietary modulation with fatty acids that enhance M2 macrophage function and promote their polarization could improve healing outcomes and mitigate the progression of PTOA.

Promotion of pro-resolution mediators

The resolution phase of inflammation is a highly orchestrated and active series of events which are mediated in part by specialized bioactive lipid metabolites known as specialized pro-resolving mediators (SPMs) [64, 152, 153].

EPA/DHA serve as precursors for SPMs biosynthesis, creating four major classes of SPMs: lipoxins, resolvins, protectins, and maresins (Fig. 2), each playing distinct roles in the resolution of inflammation. Resolvins, specifically RvD1(derived from DHA), and RvE1 (derived for EPA), resolve inflammation by reducing neutrophil infiltration and decreasing the production of pro-inflammatory cytokines [126, 154–158]. Maresins, such as Maresin 1, primarily modulate macrophage activity, promoting the clearance of apoptotic cells and debris, essential for the resolution of inflammation and tissue repair [153, 159]. Protectins, derived from DHA, have specific actions on T-cells and neural tissues [152, 160, 161]. Lipoxins, derived from AA, are particularly effective in inhibiting neutrophil recruitment to inflamed tissues, thereby limiting the initial inflammatory response. [162–164]. SPMs are primarily mediated through GPCRs, which are critical for transducing external signals into cellular responses. Each class of SPMs interacts with specific GPCRs, activating anti-inflammatory and pro-resolution pathways. Resolvin D1 (RvD1) binds FPR2 and GPR32 [165], triggering pathways that inhibit neutrophil migration and promote the clearance of apoptotic cells. Similarly, resolvin E1 (RvE1) acts through ChemR23 to reduce pro-inflammatory cytokine production and enhance the resolution of inflammation at the injury site [166]. Maresins, particularly Maresin 1 (MaR1), binds LGR6, which is involved in tissue repair and regeneration [167]. In addition to its GPCR-mediated actions, MaR1 has been identified as a ligand for the nuclear receptor RORα, suggesting a broader regulatory role in inflammation [168]. The interaction of SPMs with these GPCRs is highly specific and essential for their pro-resolving functions. This receptor mediated mechanism not only promotes the resolution of inflammation but also ensures the restoration of tissue homeostasis and prevention of excessive tissue damage.

Integrated effects on joint structure and function

Omega-3 PUFAs and their metabolites, EPA and DHA, have been shown to exert significant protective effects on various joint structures and tissues, including bone, synovium, cartilage, and collagen, as detailed in Table 1.

Table 1.

Effects of PUFAs on joint structures

Structure	PUFA	Effect	Species	Ref
Clinical, Mechanical, and Functional	Omega-3	Green lipid muscle extract (omega-3) supplementation resulted in ↓joint stiffness and ↓ NSAID consumption compared to placebo	Human	[176]
		Green lipid muscle extract (omega-3) supplementation reported ↓ pain and greater improvement of patient assessment of arthritis	Human	[56]
	Omega-3/Omega-6	Phytalgic ® (Omega-3 and Omega-6) users: ↓ NSAIDs use, ↓ pain, ↓ joint stiffness, better WOMAC function scores (compared to placebo)	Human	[55]
	EPA/DHA	↓ pain, joint stiffness, ↑ function	Human	[57]
	PUFAs	Higher consumption associated with ↓ joint space loss	Human	[177]
	EPA/DHA	↓ Pain and better WOMAC function scores	Human	[58]
	RvD1	↓ Arthritis severity	Mouse, K/BxN serum-transfer arthritis (STA) model, n-3 diet, IA SPM injection (RvD1)	[154]
Bone	n-3 rich diet	Ca:P ratio increase (Calcium to phosphorus ratio, indicative of bone health)	Guinea Pig	[169]
		↓ Incidence of heterotopic ossification	Mouse DMM	[170]
		↓ Osteophyte severity	Mouse DMM	[170]
	DHA	↓ Bone mass loss compared to vehicle	Rat ACLT	[171]
Synovium	n-3 rich diet	↓ Joint synovitis compared to n-6 rich diet fed mice	Mouse DMM	[170]
	RvD1	↓ Joint synovitis	Mouse, K/BxN serum-transfer arthritis (STA) model, n-3 diet, IA SPM injection (RvD1)	[154]
		↓ Joint Synovitis	Mouse DMM, IA SPM injection	[172]
Cartilage	EPA/DHA	↓ IL-1β mediated GAG loss	In vitro bovine cartilage explants	[173]
	EPA	↓ fibroblastic-like cells, cell debris, and cell death indicating a protective effect	In vitro chondrocytes	[174]
	Total n-3 Levels	Associated with less cartilage loss	Human (retrospective study looking at plasma concentrations)	[175]
	Low n-6/n-3 ratio diet	↓ Articular cartilage damage	Mouse	[125]
		Significantly inhibited the synthesis of cartilage matrix catabolism enzymes and promoted cartilage anabolism	Mouse	[125]
	DHA	↓ Cartilage degeneration	Rat ACLT	[171]
	RvD1	↑ Expression of cartilage matrix synthesis genes. Protects from cartilage degradation	Mouse, K/BxN serum-transfer arthritis (STA) model, n-3 diet, IA SPM injection (RvD1)	[154]
		↓ Cartilage degradation	Mouse DMM, IA SPM injection	[172]
Collagen	EPA	↑ Percentage of collagen synthesized (compared to AA enriched cells)	In vitro bovine cartilage explants	[173]
		↑ Collagen as a percentage of total cell protein (compared to AA)	In vitro	[178]
		↑ Expression of NF-κB collagen formation-associated genes (IL-6, iNOS, and MCP-1)	In vitro	[178], [179]
	Maresin 1	↑ Type II collagen in cartilage	Rat treadmill, monosodium iodoacetate (MIA) OA induction	[127]
Cellular	RvD1	↓ Synovial pro-inflammatory markers	Mouse DMM, IA SPM injection	[172]
		Inhibition of NF-κB ↓ MMP-13, COX-2, iNOS, PGE2 expression (catabolic mediators in OA) Prevents apoptosis	In vitro human chondrocytes	[126]
		↓ Neutrophil recruitment	Mouse, K/BxN serum-transfer arthritis (STA) model, n-3 diet, IA SPM injection (RvD1)	[154]
	LXA4	↓ MMP-3 and −13 expression ↓ NF-κB activity	Rat treadmill, monosodium iodoacetate (MIA) OA induction	[180]
	Maresin 1	↓ MMP13 Activation ↓ NF-κB activity	Rat treadmill, monosodium iodoacetate (MIA) OA induction	[127]

ACL Transection (ACLT), Arachidonic Acid (AA), Cyclooxygenase-2 (COX-2), Destabilization of medial meniscus (DMM), Docosahexaenoic Acid (DHA), Eicosapentaenoic Acid (EPA), Glycosaminoglycans (GAGs), Inducible Nitric Oxide Synthase (iNOS), Interleukin-1 beta) IL-1B, Interleukin-6 (IL-6), Intraarticular (IA), K/BxN serum-transfer arthritis (STA), Lipoxin A4 (LXA4), Matrix Metalloproteinase-13 (MMP-13), Monocyte Chemotactic Protein-1 (MCP-1), Monoiodoacetate (MIA), Non-Steroidal Anti-Inflammatory Drugs (NSAIDs), Nuclear Factor-Kappa B (NF-κB), Omega-3 (n-3), Osteoarthritis (OA), Polyunsaturated Fatty Acids (PUFAs), Prostaglandin E2 (PGE2), Resolvin D1 (RvD1), Specialized pro-resolving mediators (SPMs), Western Ontario and McMaster Universities Arthritis Index (WOMAC)

Bone, particularly the subchondral bone beneath the cartilage, provides structural support to the joint and assists in load bearing and shock absorption. In the context of PTOA, the subchondral bone often experiences pathological changes such as increased sclerosis, development of osteophytes, and overall bone mass loss [4, 19] which can exacerbate cartilage damage and joint dysfunction. Omega-3 PUFAs have been shown to aid in maintenance of the subchondral bone [169–171]. In an ACL transection model of rats, DHA reduced bone loss by altering expression of genes related to bone activity indicating that DHA may slow the progression of subchondral bone remodeling [171], additionally a decrease in osteophyte severity was observed with omega-3 PUFA dietary intervention in mice [170].

The synovium is responsible for joint lubrication and nutrient transport. In PTOA, the synovium often becomes inflamed (i.e. synovitis), leading to excessive production of pro-inflammatory cytokines, which accelerate cartilage degradation and worsen joint damage. Omega-3 PUFAs can mitigate joint synovitis [154, 172], ultimately preserving an environment to promote inflammatory resolution. Specifically, omega-3 PUFA fed mice who underwent destabilization of medial meniscus to induce PTOA demonstrated significantly lower synovial inflammation than omega-6 PUFA fed mice who showed thickened synovium and high density of infiltrated cells [170].

Cartilage, primarily composed of chondrocytes embedded in the dense ECM of collagen and proteoglycans. The ECM provides the structural framework of cartilage, and the preservation of this matrix is crucial for maintaining joint integrity. Omega-3 PUFAs have been shown to support chondrocyte viability and protect the matrix from degradation, which is essential in preventing full cartilage breakdown [125, 154, 171–175].

Overall, the beneficial impact of omega-3 PUFAs on cartilage, bone, and synovium underscores the importance of dietary PUFAs in modulating inflammatory processes and promoting joint health. Omega-3 PUFAs offer significant benefits for overall joint function by reducing inflammation, alleviating pain, improving mobility, decreased joint stiffness and improved physical function [55–58, 154, 176, 177]. This evidence provides a unique opportunity to target these structures with dietary PUFAs to ultimately encourage inflammatory resolution and limit progression of PTOA.

Conclusions and future directions

The role of dietary fatty acids in modulating PTOA progression post-ACL injury remains largely unexplored. ACL injury initiates an acute inflammatory response, setting the stage for chronic joint degeneration. Omega-3 PUFAs may influence this trajectory by attenuating early inflammation, reducing the risk of chronic inflammation and cartilage damage. Additionally, omega-3 PUFAs help preserve joint structures including bone, synovium, and cartilage by mitigating inflammation and supporting tissue integrity, highlighting their potential in promoting inflammatory resolution and slowing PTOA progression.

In conclusion, elucidating the biochemical and mechanistic roles of omega-3 PUFAs in knee PTOA provides a foundation for developing dietary strategies to manage and mitigate PTOA progression. Existing studies vary widely in experimental design, including differences in injury models, dosing regimens, timing of supplements, and outcomes measured, limiting direct comparison across studies. Future research should aim to optimize omega-3 fatty acid supplementation protocols, considering timing relative to joint injury, dose optimization, and omega-3 to omega-6 fatty acid ratio. Route of delivery should also be considered, specifically systemic versus local delivery, as local delivery may provide a more targeted effect within the joint microenvironment. Investigating the impact of these protocols on key molecular pathways, such as, macrophage polarization, SPM activity, and receptor-mediated signaling via FFARs, PPARs and TLRs, may provide further insight into their immunomodulatory effects. Closing this knowledge gap could lead to innovative, non-invasive treatments that enhance the quality of life for individuals with knee PTOA and may be relevant for other OA pathologies.

Abbreviations

ADAM

A Disintegrin and Metalloproteinase

AA

Arachidonic Acid

ACL

Anterior Cruciate Ligament

COX

Cyclooxygenase

CYP

Cytochrome P450

DHA

Docosahexaenoic Acid

EPA

Eicosapentaenoic Acid

ECM

Extracellular Matrix

CD36

Fatty Acid Translocase

FFAR1

Free Fatty Acid Receptor 1

FFAR4

Free Fatty Acid Receptor 4

GPCRs

G-Protein Coupled Receptors

IL

Interleukin

LTB4

Leukotriene B4

LOX

Lipoxygenase

MaR1

Maresin 1

MMPs

Matrix Metalloproteinases

NSAIDs

Non-Steroidal Anti-Inflammatory Drugs

NF-κB

Nuclear Factor-Kappa B

OA

Osteoarthritis

PPARα, β/δ, γ

Peroxisome Proliferator-Activated Receptors

PUFAs

Polyunsaturated Fatty Acids

PTOA

Post-Traumatic Osteoarthritis

PGE2

Prostaglandin E2

ROS

Reactive Oxygen Species

RvD1

Resolvin D1

RvD2

Resolvin D2

TXB2

Thromboxane B2

TKA

Total Knee Arthroplasty

TGF-β

Transforming Growth Factor Beta

TNF-α

Tumor Necrosis Factor Alpha

Authors’ contributions

AMP: Conceptualization, literature search and original draft and preparation LHB: Review and editing KSS: Review and editing KJS: Review and editing TMN: Review and editing.

Funding

R01HL158926.

Data availability

No datasets were generated or analysed during the current study.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.