The friction of explanted hip prostheses

Area of Science:

• Biomaterials Science
• Orthopedic Surgery
• Tribology

Background:

• Total hip replacement (THR) longevity is influenced by friction at the articulating surfaces.
• Charnley prostheses are widely used, and understanding wear and friction is crucial for improving implant design and patient outcomes.
• The tribological behavior of retrieved implants can provide insights into in vivo performance and failure mechanisms.

Purpose of the Study:

• To evaluate the frictional resistance of retrieved Charnley prostheses.
• To compare friction in retrieved implants with new prostheses under simulated physiological conditions.
• To investigate factors influencing friction, such as cement ingress and socket fixation.

Main Methods:

• Frictional resistance was measured on explanted Charnley prostheses using the Durham hip function simulator.
• Tests were conducted under both dry and lubricated (0.01 Pa s) conditions.
• Friction factor (f) was calculated and compared between new and retrieved implants, and between different fixation states.

Main Results:

• Retrieved prostheses exhibited higher median friction factors (0.13 dry, 0.06 lubricated) compared to new prostheses (0.11 dry, 0.04 lubricated).
• A statistically significant increase in friction was observed for retrieved implants in the lubricated regime.
• Cement ingress into the worn cup significantly increased friction under dry conditions; socket looseness did not significantly increase friction.

Conclusions:

• Friction increases in Charnley prostheses after in vivo use, particularly under dry conditions, and is exacerbated by cement ingress.
• While friction may decrease over time, suggesting a fatigue process, it is unlikely to be a primary driver of long-term socket loosening.
• Understanding tribological changes in retrieved hip implants is vital for optimizing future THR designs and surgical techniques.