Reduced wear of cross-linked UHMWPE using magnesia-stabilized zirconia femoral heads in a hip simulator

Abstract

Background: To reduce wear, the ideal bearing surface in joint arthroplasty should be smooth and hydrophilic. Ceramics generally offer better wettability than metals and can be polished to a smoother finish. However, clinical studies have found no reduction in liner wear when using yttria-stabilized zirconia (Y-TZP) instead of cobalt chromium alloy (CoCr) femoral heads.

Question/purposes: We (1) determined whether a hard, diamond-like carbon (DLC) coating would enhance the wettability of CoCr and magnesia-stabilized zirconia (Mg-PSZ) femoral heads without increasing roughness, and (2) compared their wear performance.

Methods: In an observational study limited to CoCr and Mg-PSZ heads, we measured roughness and contact angle on as-received and DLC-coated heads. Eight heads then were subjected to 11 million cycles of wear in a hip simulator against cross-linked ultrahigh molecular weight polyethylene (XLPE) liners.

Results: Mg-PSZ femoral heads were smoother and more hydrophilic than CoCr heads. Although DLC coatings did not reduce roughness, they reduced the contact angle of CoCr and Mg-PSZ substrates, which may provide enhanced lubrication in vivo. In hip simulator tests, liners bearing against CoCr heads wore at a greater rate compared with Mg-PSZ heads. The DLC coating on Mg-PSZ heads did not reduce wear further.

Conclusions: The wear rate of XLPE versus Mg-PSZ was seven times less than CoCr heads, probably owing to lower roughness and greater wettability of Mg-PSZ heads.

Clinical relevance: The use of Mg-PSZ femoral heads should lead to reduced wear in vivo compared with CoCr heads, but the clinical benefit of DLC-coated Mg-PSZ is unclear.

Fig. 1A–D

Representative surface plots of femoral heads from each group examined in this study are shown: (A) CoCr, (B) DLC-CoCr, (C) Mg-PSZ, and (D) DLC-Mg-PSZ. DLC coatings appeared to fill in small imperfections, especially for CoCr substrates, but did not change the overall topography. For all images, the spherical form has been removed. Lateral axes are 633 μm × 475 μm; the vertical axes of all images are fixed at 680 nm for comparison purposes (Magnification, ×10).

Fig. 2A–D

Representative droplets and contact angle (θ_c, average ± S.D.) for each specimen type are shown: (A) CoCr, θ_c = 93.0º ± 1.0º, (B) DLC-CoCr, θ_c = 72.3º ± 2.1º, (C) Mg-PSZ, θ_c = 78.6º ± 3.2º, and (D) DLC-Mg-PSZ, θ_c = 70.5º ± 1.4º. DLC coatings reduced contact angle relative to uncoated CoCr and Mg-PSZ substrates (p < 0.0001 and p = 0.004, respectively).

Fig. 3

The plot shows the average gravimetric XLPE liner bearing wear against CoCr, Mg-PSZ, and DLC-Mg-PSZ femoral heads. All liners initially gained weight (“negative” wear) compared with the soak controls. Liners bearing against CoCr heads exhibited an overall regression curve (dotted line) best represented as a second-order polynomial. Liners bearing against Mg-PSZ and DLC-Mg-PSZ heads exhibited linear wear rates and lower steady-state wear rates compared with CoCr.

Fig. 4A–D

These photographs of typical XLPE liner surfaces show the articular surface of a (A) soak control with machining marks, and test liners after 11 million cycles bearing against (B) CoCr alloy, (C) Mg-PSZ, and (D) DLC-coated Mg-PSZ femoral heads (Magnification: ×300).

Fig. 5A–C

Typical surface topography is shown before wear testing (top row) and after 4 million cycles (bottom row) for (A) CoCr, (B) Mg-PSZ, and (C) DLC-Mg-PSZ femoral heads (Magnification, ×10). Lateral axes are 554 μm × 418 μm.