Forward

This is part 2 of a multi-part series which goes through the custom joint replacement to a finger due to rheumatoid arthritis.

Introduction

In Giving Arthritis the Finger: Part 1, Synopsys’ Simpleware™ ScanIP software (Synopsys Inc., Mountain View, CA) was used to process and convert the CT images, segment the anatomy and exported/imported into the 3DExpereince as point clouds with CATIA. A patient specific joint replacement and tissue removal was designed.

Finite Element Model, FEA (Abaqus)

The CATIA CAD geometry was imported into Abaqus/CAE by way of a STEP file.

Figure 1: Imported CAD model

The cortical bone material model was a simple linear elastic model with a Young’s modulus of 14,800MPa and an assumed Poisson’s ratio of 0.3. The silicone implant was modeled with an Ogden material model, as defined in table and plot below.

Table 1: Silicone Hyperelastic Ogden Materialmodel for Silcone-A

Figure 2: Stress strain curve for Silicone-A

Due to the complex geometry of the bone tissue a hybrid second order tetrahedron mesh (C3D10H) was chosen. A 2mm mesh seed was chosen with a mesh refinement around the stems of 1mm. As the optimization progresses this mesh seed will be reevaluated.

Figure 3: Hybrid second order tetrahedron mesh

Surface to surface contact was defined between the bone and implant for both sides with a slave adjustment to remove overclosure. Surface to surface contact was also defined for self contact of the implant. The distal end of the stem has a kinematic coupling with RP-1 and is rotated in coordinate system where the x-direction is the natural joint bending direction. The proximal end of the stem is fixed.

Figure 4: Proximal end fixed, distal end constrained to rotate in defined coordinate system’s x-direction

The distal end is rotated to extend the finger by 20° then flexed for 70° and then returned to neutral. Every 10 degrees are associated with 0.1s in this quasi-static analysis. This will consist of 1 fatigue cycle.