Quantitative Surface Properties for Retrieved Metal-on-UHMWPE and Ceramic-on-Ceramic THR Femoral Heads

Introduction: Femoral head surface roughness has been recognized as an important determinant of linear and volumetric polyethylene (UHMWPE) wear in total hip replacement (THR), particularly for metal heads. Fisher et al 1 found that a 2- μm scratch with a 1- μm buildup of metal debris produced a 70-fold increase in the wear rate. Ceramic materials and hard-on-hard bearing couples have been introduced to provide more scratch resistance. However, THR bearing surfaces of all materials can become damaged during in vivo function, potentially impacting wettability. The purpose of this study is to quantify surface roughness as related to distinct damage types on retrieved femoral heads and to assess wettability of common bearing materials. Materials and Methods: An IRB-approved archive of retrieved THR bearing components was queried, identifying 29 metal (CoCr) and ceramic (alumina, zirconia) femoral heads that had articulated with UHMWPE and ceramic acetabular liners, respectively. Mean in vivo functional duration was 5.5 ± 4.5 years (range, 1 month to 27 yrs), and patient age and weight averaged 68 ± 12 years (range, 36–81) and 72 ± 21 kilograms (range, 52–123), respectively. Retrieval reasons were dislocation (18), loosening (4), polyethylene wear (2), infection (1), squeaking (1) and unknown (3). Damage appearance and surface roughness were evaluated in a grid pattern of 49 zones distributed across the bearing surface of each head using optical microscopy to identify Mode-1 or Mode-2 wear and a non-contact interferometer (NT2000 & NPFLEX, Bruker Corp.) Wettability of smooth and rough CoCr and alumina materials was assessed using contact angle measurements (DSA30, Kruss USA). Results: Among the metal heads, scratching was the most common damage (Mode-1) and was visualized on over 90% of all heads. Metal transfer and discoloration (Mode-2) damage, confirmed as titanium deposits, was visualized on 79% of dislocated heads. Zones with metal transfer had significantly higher surface roughness compared to zones without this damage (Table 1, p 2 and metal transfer (Mode-2) was visualized as dark smears similar to previous studies. 3 Areas of roughening (Mode-1 & Mode-2) exhibited significantly higher surface roughness than areas without such damage (Table 2). When comparing wettability, the contact angle for alumina (32.8°) was lower than CoCr (54.5°) and both materials had significantly higher contact angles with increased surface roughness. Conclusions: Damage mechanisms leading to Mode-1 and Mode-2 wear were visualized on both metal and ceramic heads, leading to significant increases in surface roughness. However, the surface morphology of each damage mode differed between material types (Figure 3). These results, along with the empirical evidence relating surface roughness and wettability, have implications for wear and the ability to maintain suitable fluid films between bearing surfaces. Continued investigation into clinical phenomena possibly related to these parameters, such as squeaking in ceramic-on-ceramic bearings 4 or accelerated wear in metal-on-metal bearings, 5 is warranted.