Absorbable (Poly-L-lactic acid) bone screws are used commonly in anterior cruciate ligament (ACL) reconstruction, where polymer screws attach the tendon graft to the tibia, absorbing back into the body as the tendon naturally attaches to the bone.
Proper bone screw design includes analysis of the holding force of the screw and the stresses within the screw. Nonlinear deformation and relaxation within the polymer, however, makes prediction of these forces and stresses over time very difficult, complicating their design.
Veryst used its unique ability to test a PLLA material, match the test data to nonlinear material models, and simulate the installation of a PLLA bone screw. Veryst measured the nonlinear properties of the PLLA polymer including the effect of large deformations and relaxation. We then fit the data to an advanced proprietary constitutive model from the PolyUMod® material software library (developed by Veryst and now available through PolymerFEM.com) and implemented that model within a commercial nonlinear finite element code.
We used the material model within a finite element model of a tendon held within a tibia using the absorbable polymer screw. The figure shows the bone screw as installed within the tibia. The model not only predicted important short term pull-out forces but also the evolution of stresses over time.