Micro-grooved Surface Topography Does Not Influence Fretting Corrosion of Tapers in THA
Authors:   , Arnholt Christina Marie , Kurtz Steven M. 
Published: July 2021

Surface topography of the femoral stem trunnion has been suggested as a factor in fretting corrosion of the taper interface in total hip arthroplasty (THA). The purpose of this study was to identify if femoral stem taper morphology was correlated with taper fretting and corrosion. This was accomplished by first developing a method to characterize a broad range of machined smooth stems and intentionally micro-grooved stems. Through multivariate analysis of covariance, different factors were tested for correlation with fretting corrosion. Finally a matched cohort was designed controlling all related factors to deduce weather surface topography was related with fretting corrosion. From a multi-institutional retrieval collection of over 3,000 THAs, 398 stems paired with CoCr (ASTM-F75) alloy heads were collected as part of a multi-center, IRB-approved retrieval program. Stems were fabricated from CoCr (ASTM-F75) or Titanium (Ti6V4Al) alloys and were used in metal on polyethylene (M-PE) bearing total hip devices. Other stem, femoral head alloys, and different bearing combinations were removed from this study. The devices had a single location of modularity at the head neck junction. In order to classify the surface topography into two groups visual inspection, microscopy, white light interferometer (Zygo New View 5000) and a stylus profilometer (Talyrond 585, Taylor Hobson, UK) were used. The final method employed a stylus profilometer with which linear profiles were measured using a diamond tip stylus capturing a 10 mm line trace. Commercial software (Ultra, Taylor Hobson, UK) was used to analyze a 1 mm representative as-manufactured region. Three parameters were calculated from the profiles: average surface roughness, amplitude and wavelength of micro-grooves (if any). Surface observations led to a classification system in which a surface had to contain a periodic pattern, a wavelength > 140 [mu]m and an amplitude of > 5 [mu]m to be considered micro-grooved. Fifty percent (200/398) of the femoral stem taper surfaces were classified as smooth tapered stems. The remaining 50% (198/398) femoral stem taper surfaces were classified as micro-grooved tapered stems. A semi-quantitative scoring method was used to classify the stem-head pairs into four groups based on quantity of damage on the female and male taper surfaces [1, 2]. For this study the femoral head taper was characterized. With this scoring system, a score of 1 is assigned when the damage is considered minimal and corresponds to fretting damage occurring on less than 10% of the surface with no pronounced evidence of corrosion. A score of a 2 indicates mild damage where more than 10% of the surface has fretting damage or there is corrosion attack confined to small areas. A score of a 3 reflects moderate damage where more than 30% of the surface has fretting damage or localized corrosion attack. A score of 4 corresponds to severe damage over the majority of the taper (>50%) with abundant corrosion debris. The femoral head taper was independently evaluated by three trained investigators (C. A., G. B. H, and D. W. M.). Any differences between investigators damage scores were resolved in a conference, resulting in a final damage score for both the femoral head and stem. Using multivariate analysis of covariance we found implantation time (p<0.0001), apparent engagement length (p<0.0001), flexural rigidity (p=0.008) and head size (p=0.01) were significant factors in fretting corrosion head damage scores. Surface topography (i.e. smooth or micro-grooved, p=0.78), surface wavelength (p=0.60), and surface amplitude (p=0.23) were not associated with femoral head fretting corrosion damage score. Overall, the results of this study do not support trunnion surface morphology as a contributing factor to fretting and corrosion damage at the modular head-neck interface. This was deduced through an initial classification system, followed by statistical analysis, which showed correlated factors and finally a controlled matched cohort that showed no correlation between surface topography and fretting corrosion. This study was limited by the use of a semi quantitative scoring method. Future work would include the use of quantified volume of metal released from each cohort.