A fatigue assessment technique for modular and pre-stressed orthopaedic implants.

Area of Science:

• Orthopaedic biomechanics
• Materials science
• Implant design

Background:

• Orthopaedic implants face significant cyclic loads, necessitating robust pre-clinical fatigue analysis.
• Acetabular cup designs increasingly focus on bone conservation through reduced wall thickness, inducing high pre-stress in modular implants.

Purpose of the Study:

• To propose and justify a novel technique for preclinical fatigue strength assessment of modular orthopaedic implants.
• To incorporate mean stress, stress concentrations, and material processing into fatigue analysis.
• To evaluate the impact of these factors on implant longevity and bone conservation.

Main Methods:

• Computational prediction of stress distributions in a modular acetabular cup under assembly and in vivo loads.
• Calculation of cyclic residual stress and stress amplitude.
• Verification against low-cycle-fatigue (LCF) and high-cycle-fatigue (HCF) using yield strength and the Soderberg criterion, respectively.

Main Results:

• Mean stress significantly influences fatigue life predictions; neglecting it lowers the reserve factor.
• A low cyclic load ratio (Rl=0.1) resulted in a high cyclic stress ratio (Rs=0.80).
• The minimum predicted reserve factors were 1.96 for HCF and 2.08 for LCF, indicating adequate safety margins when mean stress is considered.

Conclusions:

• The proposed fatigue analysis methodology accurately assesses modular implant strength by accounting for mean stress.
• Accurate fatigue assessment is vital for optimizing implant design, maximizing modularity, and conserving bone.
• This method is applicable to various pre-stressed prosthesis concepts in early-stage development.