Ex vivo robotic measurement of the acute biomechanical impact of anterior cruciate ligament rupture in rats.

Orthopaedic injuries, such as anterior cruciate ligament (ACL) rupture, cause worsened biomechanical function and sometimes debilitating pain, ultimately reducing patient quality of life. Preclinical animal models are often used to understand the effect of orthopaedic injuries on musculoskeletal health. A variety of injury models, including ACL rupture, have been established in rodent knees to understand risk factors, injury mechanisms, and healing responses in vivo. Previous research in small animal models has heavily focused on multi-omics, in vivo imaging, and histology, with less emphasis on biomechanical testing methods which are critical to understand changes in joint function after injury. To address this gap, we have developed characterization methods with a miniature multiaxial robotic testing system to study in situ rat knee biomechanics, and demonstrated the ability of this approach to measure the effects of ACL injury on translational and rotational joint stability in cadaveric rat knees. Following ACL transection there was a 2.5-fold increase in anterior tibial translation (p < 0.001) with an accompanying 2.5-fold decrease in anterior drawer joint stiffness (p < 0.001). Additionally, ACL transection resulted in 1.7-fold increases in varus rotation (p = 0.016), and even greater 2.5-fold increases in valgus rotation (p = 0.012). This multiaxial approach captured changes in knee joint stability in off-axis planes of motion, expanding on prior assessments of uniaxial joint testing approaches. The implications of this robotic testing approach are widespread, with benefits to better understanding both normative biomechanical function and injury responses in rodent musculoskeletal research.