Viscoelastic behaviour of acrylic bone cements

Area of Science:

• Biomaterials Science
• Orthopedic Surgery
• Mechanical Engineering

Background:

• Component loosening at the bone-cement interface is linked to local contact stresses.
• Reduced-modulus bone cement may alleviate these stresses.
• The viscoelasticity of reduced-modulus cement raises concerns about long-term implant subsidence.

Purpose of the Study:

• To compare the viscoelastic properties of a reduced-modulus bone cement (PBMMA) with standard polymethylmethacrylate (PMMA).
• To evaluate stress relaxation behavior under unconstrained conditions.
• To assess potential long-term subsidence implications for total hip arthroplasty.

Main Methods:

• Unconstrained stress relaxation tests were conducted on PBMMA and PMMA specimens at 37°C in an aqueous environment.
• Applied strains were 1%, 2.5%, and 5%, with load monitored for 100 hours.
• Analysis focused on initial (first hour) and steady-state (15-100 hours) stress relaxation, alongside length change measurements.

Main Results:

• PBMMA exhibited faster initial stress diminution, suggesting quicker stress transfer and distribution.
• Significant differences in steady-state stress relaxation were observed for 2.5% and 5% strains (P < 0.05) between PBMMA and PMMA.
• PBMMA showed minimal strain recovery, indicating higher potential for subsidence compared to PMMA's substantial recovery.

Conclusions:

• Reduced-modulus PBMMA offers potential benefits in stress transfer at the bone-cement interface.
• Unconstrained tests suggest PBMMA may lead to greater subsidence than PMMA.
• Constrained creep tests are essential for accurately predicting in vivo subsidence in total hip arthroplasty.