RSA-measured inducible micromotion and interface modeling with finite element methods

Area of Science:

• Orthopedic surgery
• Biomaterials science
• Biomechanics

Background:

• Osteolysis is a primary cause of aseptic loosening and hip implant failure.
• The precise mechanisms driving osteolysis, particularly the role of prosthesis-cement interface micromotion, remain incompletely understood.

Purpose of the Study:

• To investigate the occurrence of dynamically inducible micromotion at the prosthesis-cement interface in vivo.
• To develop and validate a finite element model representing this interface using clinical data.

Main Methods:

• Radiostereometric analysis (RSA) was employed to measure micromotion in 21 hips with Exeter stems at 3 and 12 months post-surgery.
• Loading conditions were altered from double-leg stance to single-leg stance to induce micromotion.
• A Coulomb friction nonbonded model was used to represent the stem-cement interface and fit the RSA data.

Main Results:

• Dynamically inducible micromotion was present at both 3 and 12 months postoperatively, with no significant difference between time points.
• At 3 months, stem head displacement occurred posteriorly (0.10 +/- 0.16 mm) and inferiorly (0.08 +/- 0.12 mm) upon loading.
• The developed finite element model successfully predicted gap formation and closure at the implant-cement interface.

Conclusions:

• Dynamically inducible micromotion occurs in vivo at the prosthesis-cement interface, persisting up to 12 months post-implantation.
• This micromotion may generate pressure and distribute wear debris, potentially contributing to osteolysis and aseptic loosening.
• The validated finite element model provides a tool for further investigation into implant-cement interface mechanics and failure mechanisms.