Joint-specific distance thresholds for patient-specific approximations of articular cartilage modeling in the first ray of the foot

The addition of cartilage elements in a finite element model prevents bone-on-bone articulation during simulation, thus providing more accurate information about joint kinematics. We present a semi-automated method for identifying joint articulation surfaces and creating volumetric articular cartilage elements based on patient-specific bone information. The approach identifies contact surfaces based on a joint-specific, user-specified distance threshold criterion applied to a polygonized set of bones. Volumetric cartilage elements are generated using half of the minimum inter-joint distance. We present the method and then apply it to the first ray of the human foot, which includes the medial cuneiform, first metatarsal, and first proximal and distal phalanges. Distance thresholds for the first ray ranged from 3.0 to 4.25 mm depending on the joint and the desired contact surface coverage. Inter-joint distances were found and applied to the contact surfaces to generate uniformly thick cartilage models. Average inter-joint distances of 0.46, 0.72 and 0.51 mm were found for the first interphalangeal, metatarsophalangeal, and cuneometatarsal joints, respectively. The values cited are one half of the minimum inter-joint difference, as identified by the proximity algorithm. This is taken to represent the (uniform) cartilage thickness at each joint.