Rafael Claramunt

Estimating joint pressure distribution is crucial for understanding how pathologies disrupt natural biomechanics and for optimizing rehabilitation outcomes. While numerous computational, cadaveric, and “in vivo” studies have investigated joint mechanics, most have focused on the hip and knee, leaving the ankle underrepresented. Flexible matrix sensors offer a compelling solution for intra-articular pressure mapping due to their thin profile and sensing networks. This study presents a single experimental synthetic model of the human ankle joint—characterized by high-curvature joint surfaces—incorporating ligamentous constraints, a custom Velostat™ piezoresistive 6 × 4 sensor array, and low-cost microcontroller electronics. Joint kinematics was evaluated in the sagittal plane across the range of motion from dorsiflexion to plantarflexion in five angular positions under four clinical conditions: intact joint, unreduced posterior malleolar fracture, and fracture fixation using two distinct syndesmotic stabilization techniques. Motion was passively induced without external axial loading to isolate the mechanical contribution of the ligaments. Qualitative intra-articular contact maps revealed distinct differences among surgical approaches. These preliminary results demonstrate the viability of flexible, low-cost piezoresistive sensing systems as a proof of concept for intra-articular contact monitoring in complex anatomical joints.

Background/Objectives: Screw loosening and vertebral fractures remain common after vertebral body tethering (VBT). Because tightening torque sets screw preload, its biomechanical effect warrants explicit modeling. In this paper, a Finite Element (FE) model, supported by ex-vivo porcine vertebrae tests, was developed and validated that incorporates torque-induced pre-tension to quantify vertebral stress, aiming toward customizable VBT planning. Methods: An FE model with pre-tension and axial extraction failure was parameterized using ex vivo tests on five porcine vertebrae. A laterally inserted surgical screw in each specimen was tightened to 5.9±0.80 [N·m]. Axial extraction produced failure loads of 2.1±0.31 [kN]. This is also considered in the FE model to validate the failure scenario. Results: Torque alone generated peak von Mises stresses of 16.1 [MPa] (cortical) and 2.1 [MPa] (trabecular), lower than prior reports. With added axial load, peaks rose to 141.1 [MPa] and 19.7 [MPa], exceeding typical ranges. However, predicted failure agreed with experiments, showing 0.58 [mm] displacement and a conical displacement distribution around the washer. Conclusions: Modeling torque-induced pre-tension is essential to reproduce realistic stress states and anchor failure in VBT. The framework enables patient-specific assessment (bone geometry/density) to recommend safe tightening torques, potentially reducing screw loosening and early fractures.