Modeling physiological tasks and their impact on cartilage deformation and recovery: An intact porcine knee joint model

Articular cartilage supports joint function through interstitial fluid pressure which effectively stiffens the cartilage and reduces interfacial friction and compressive deformation. Prior work has shown that interstitial fluid pressure changes over time, and it remains unclear whether and by what mechanisms cartilage can restore baseline levels of interstitial pressure, fluid content, frictional behavior, and deformation after they are altered during normal physiological activities. In this work we develop a controllable cadaveric whole-joint porcine knee model to simulate daily activities and quantify axial cartilage deformation. We hypothesized that simulated standing would lead to compressive deformation of the cartilage while simulated walking and lying supine would lead to deformation recovery. Seven adolescent porcine stifle joints were used in this study. Each joint was mounted to a six-degree-of-freedom robotic test frame. Three unique tasks were simulated in a fixed order at near-physiological kinematics and kinetics: standing for 30 min (Standing 1), walking for 10 min (Walking), standing for 30 min (Standing 2), and lying supine for 60 min (Supine). A fixed measurement location was used to track displacements along the superior-inferior axis which was associated with cartilage deformation and recovery. Standing 1 led to compressive deformation, Walking produced partial recovery, Standing 2 re-established the compressive deformation, and Supine produced partial recovery. The mean deformation for each task relative to baseline was -0.78 mm, -0.46 mm, -0.96 mm, and -0.57 mm respectively. Three technical replicates showed no systematic differences following an additional freeze-thaw cycle, demonstrating a repeatable response and negligible freeze-thaw effects. Following resection of the menisci and ligaments we observed similar trends; however, the deformation magnitude during Standing 1&2 was greater, as was the recovery during Walking. The recovery magnitude during the Supine task remained similar to the intact state. Taken together, these ex vivo results provide strong evidence that cartilage will experience time and task dependent deformation and recovery in vivo.