The anterior cruciate ligament (ACL) is a critical structure in the knee joint, providing stability and preventing excessive forward movement of the tibia relative to the femur1. ACL injuries are among the most common knee injuries, particularly in sports that involve sudden stops, jumps, or changes in direction2. These injuries can range from partial tears to complete ruptures and often necessitate surgical intervention for optimal recovery3. However, even after surgical treatment, ACL injuries often lead to long-term joint changes, including chondral wear, osteophyte formation, and structural alterations of the subchondral bone. These changes collectively contribute to joint degeneration, commonly referred to as posttraumatic osteoarthritis (PTOA)4,5.
While the progression from initial ACL injury to symptomatic PTOA can span anywhere from as soon as 2 years to as long as 20 years following the injury, early joint changes can be detected within just a few weeks6,7,8,9. One key feature of these early changes is the elevated presence of inflammatory cytokines (e.g., interleukin (IL)-1, IL-6, tumor necrosis factor (TNF)) and markers of extracellular matrix breakdown (e.g., metalloproteinases (MMPs) and CTX-II). Synovial fluid assessments of these markers of inflammation and degradation provide crucial insights into the acute and chronic phases of joint pathology. For instance, IL-1 and IL-6 levels peak in the synovial fluid of the injured joint within the first month post-injury. Although these levels decline over time, they rarely return to the normal, baseline level, even years after treatment10,11,12. This initial pro-inflammatory response triggers a cascade of joint damage, including synovial inflammation, cartilage degradation, and biomechanical alterations, perpetuating tissue breakdown and accelerating joint degeneration13. Additionally, acute inflammation following ACL injury often results in joint effusion and knee edema, exemplified by the swelling and fluid accumulation within the joint14.
To replicate ACL injury and investigate the progression of PTOA, the surgically induced ACL transection (ACLT) model in mice is widely used15,16. This model involves cutting the ACL to simulate injury and assessing the resulting biological and mechanical consequences. The ACLT model is a valuable tool for studying the inflammatory response, cartilage degradation, and biomechanical alterations post-injury, allowing insights into the molecular and cellular mechanisms driving PTOA. Furthermore, it enables the evaluation of therapeutic strategies aimed at mitigating joint degeneration and preserving function. In humans, synovial fluid analysis is widely used to monitor inflammation and cartilage degradation over time, as obtaining synovial fluid is relatively straightforward17,18. However, in mice, collecting synovial fluid after injury models is challenging. In this manuscript, we describe our surgical technique for ACLT and a knee lavage method for synovial fluid collection to assess inflammatory and degradative markers. We also present a method for measuring knee size to quantify edema following joint injury. Additionally, we describe our microCT assessment method to grade the severity of PTOA post-ACLT. These approaches provide complementary tools for evaluating the acute and chronic effects of ACL injury in a preclinical setting.