After anterior cruciate ligament (ACL) reconstruction, patient-reported outcomes are improved 10 years post-surgery. Cytokine concentrations, however, remain elevated years after surgery; over 80% of patients with combined ACL and meniscus injuries have post-traumatic osteoarthritis (PTOA) within 10-15 years after injury. Since pain nociceptors are not located in the articular cartilage, patient-reported outcomes improve despite progressive, irreversible cartilage loss, thus making PTOA a "silent killer." Because early cartilage loss progresses without pain and dysfunction, the prevalence of PTOA continues to increase. PTOA now represents the most common cause of military disability.

Our recent results illustrate the downstream cytokine and degradative enzyme activity following ACL reconstruction. ACL and meniscus injury initiate a biochemical cascade resulting in cartilage degradation and this process involves an up-regulated pro-inflammatory response with a dysregulated anti-inflammatory response. Single-dose intra-articular anti-inflammatory treatment appears to reduce hyaline cartilage degradation shortly after the time of injury based on synovial fluid measures of type II collagen degradation. The intra-articular inflammatory milieu at the time of surgery appears to predict the patient symptom state two years later; however, the effectiveness of preoperative anti-inflammatory treatments in impacting patient symptoms or slowing long-term PTOA progression is as yet unclear. A lack of efficacy in preoperative interventions may be attributed to a profound inflammatory stimulus from surgical reconstruction of the ACL. The postoperative inflammatory cascade results in articular cartilage and meniscus degradation due to matrix degrading enzymes, especially those which breakdown type II collagen.

PTOA affects the whole joint organ including the cartilage, synovium, and bone. PTOA progression is multifaceted and includes activation of the pro-inflammatory Nuclear Factor Kappa-B (NFkB) pathway, an increase in pro-inflammatory M1 macrophages, cell senescence, and bone remodeling. Limiting the biochemical cascade through an innovative disease modifying treatment to target upstream activity will potentially treat all components of the knee, thereby lessening the inflammatory response, reducing cartilage catabolism, and potentially improving pathologic bony remodeling observed after ACL reconstruction. The early proteomic PTOA response is more similar to inflammatory rheumatoid arthritis than idiopathic OA; thus, long-acting agents which better regulate pro-inflammatory cytokine activity may more successfully limit tissue destruction. By re-purposing approved therapeutics with proven immune efficacy, a readily-available and cost-effective strategy for disease modification may be possible.

Montelukast was first approved for clinical use in 1998 for prophylaxis and chronic treatment of asthma. The drug selectively inhibits the cysteinyl leukotriene receptor 1 (CysLT1). Montelukast blocks the actions of cysteinyl leukotriene D4 (LTD4), which is produced through the arachidonic acid pathway. This pro-inflammatory signal is released from several cells including the inflammatory mast cells and eosinophils. Montelukast also appears to address multiple PTOA mechanisms by inhibiting cysteinyl leukotrienes. Cysteinyl leukotriene inhibition in animal and laboratory models of PTOA resulted in the elimination of senescent cells, reduced NFkB activation, decreased concentrations of pro-inflammatory and catabolic factors and reactive oxygen species (ROS) while increasing expression of anti-inflammatory factors (inducing anti-inflammatory M2 macrophage infiltration), inhibiting osteoclastogenesis and improving bone quality. The novel use of oral montelukast offers the potential of a disease modifying treatment to prevent irreversible cartilage loss after ACL injury.