Fatigue crack propagation under variable amplitude loading in PMMA and bone cement.

Area of Science:

• Biomaterials Science
• Orthopedic Engineering
• Mechanical Engineering

Background:

• Fatigue failure of polymethyl methacrylate (PMMA) bone cement is a primary cause of cemented joint replacement failure.
• Existing research on bone cement fatigue primarily uses constant amplitude loading, neglecting the variable amplitude loading (VAL) experienced in vivo.

Purpose of the Study:

• To investigate the effects of variable amplitude loading (VAL) on fatigue crack propagation in PMMA and Palacos R bone cements.
• To compare the fatigue behavior of bone cements under constant amplitude loading (CAL) versus VAL, including overloads and simulated daily activity spectra.

Main Methods:

• Fatigue crack propagation tests were conducted on compact tension (CT) specimens of pure PMMA and Palacos R bone cement.
• Tests included CAL at various R-ratios (0.1-0.9) and VAL with single overloads and simplified load spectra simulating daily activities.
• Crack growth rates (da/dN) were measured and analyzed under different loading conditions.

Main Results:

• In PMMA, R-ratio significantly affected crack propagation, consistent with slow crack growth under constant load.
• Single overloads caused notable crack retardation in PMMA, particularly at low da/dN.
• In Palacos R, overloads had minimal effect, while low da/dN overloads accelerated crack growth; spectrum loading retarded crack growth compared to Paris Law predictions.

Conclusions:

• Variable amplitude loading (VAL) significantly influences bone cement fatigue crack growth, demonstrating a more complex behavior than observed under constant amplitude loading (CAL).
• The findings highlight the inadequacy of CAL for accurately predicting bone cement fatigue life in joint replacements and emphasize the need to incorporate VAL in testing.
• These results have critical implications for the design, testing, and longevity assessment of cemented joint replacements.