Enhanced lubricant film formation through micro-dimpled hard-on-hard artificial hip joint: An in-situ observation of dimple shape effects.

This study evaluates the impact of dimple shapes on lubricant film formation in artificial hip joints. Micro-dimples with 20-50 µm lateral size and 1 ± 0.2 µm depths were fabricated on CrCoMo hip joint femoral heads using a picosecond laser. Tribological studies were performed using a pendulum hip joint simulator to apply continuous swing flexion-extension motions. The results revealed a significantly enhanced lubricant film thickness (≥ 500 nm) with micro-dimpled prosthesis heads at equilibrium position after the lubricant film has fully developed. The average lubricant film thickness of dimpled prostheses with square- and triangular-shaped dimple arrays over time is about 3.5 that of the non-dimpled prosthesis (204 nm). Remarkably, the prosthesis with square-shaped dimple arrays showed a very fast lubricant film formation reaching their peak values within 0.5 s of pendulum movement, followed by prosthesis with triangular-shaped dimple arrays with a transition period of 42.4 s. The fully developed lubricant film thicknesses (≥ 700 nm) are significantly higher than the surface roughness (≈ 25 nm) demonstrating a hydrodynamic lubrication. Hardly any scratches appeared on the post-experimental prosthesis with square-shaped dimple array and only a few scratches were found on the post-experimental prosthesis with triangular-shaped dimple arrays. Thus, prostheses with square-shaped dimple arrays could be a potential solution for durable artificial hip joints.