Abstract
Introduction:
Degradation of modular head-neck tapers was raised as a concern in the 1990s (Gilbert 1993). The incidence of fretting and corrosion among modern, metal-on-polyethylene and ceramic-on-polyethylene THA systems with 36+ mm femoral heads remains poorly understood. Additionally, it is unknown whether metal debris from modular tapers could increase wear rates of highly crosslinked PE (HXLPE) liners.
The purpose of this study was to characterize the severity of fretting and corrosion at head-neck modular interfaces in retrieved conventional and HXLPE THA systems and its effect on penetration rates.
Patients & Methods:
386 CoCr alloy heads from 5 manufacturers were analyzed along with 166 stems (38 with ceramic femoral heads). Metal and ceramic components were cleaned and examined at the head taper and stem taper by two investigators. Scores ranging from 1 (mild) to 4 (severe) were assigned in accordance with the semi-quantitative method adapted from a previously published technique. Linear penetration of liners was measured using a calibrated digital micrometer (accuracy: 0.001 mm). Devices implanted less than 1 year were excluded from this analysis because in the short-term, creep dominates penetration of the head into the liner.
Results:
The majority of the components were revised for instability, infection, and loosening. Mild to severe taper damage (score ≥2) was found in 77% of head tapers and 52% of stem tapers. The extent of damage was correlated to implantation time at the head taper (p = 0.0004) and at the stem taper (p = 0.0004). Damage scores were statistically elevated on CoCr heads than the matched stems (mean score difference = 0.5; p < 0.0001; Figure 2) and the two metrics were positively correlated with each other (ρ = 0.41). No difference was observed between stem taper damage and head material (CoCr, ceramic) (p = 0.56), nor was a correlation found between taper damage and head size (p = 0.85; Figure 3). The penetration rate across different formulations of HXLPE was not found to be significantly different (p = 0.07), and therefore grouped together for further analysis. Within this cohort, penetration rate was not found to be associated with head size (p = 0.08) though a negative correlation with implantation time was noted (ρ = −0.35). When analyzed with taper damage scores, a correlation was not observed between head damage scores and HXLPE penetration rates (p = 0.51).
Discussion:
The results of this study do not support the hypothesis that 36+ mm ceramic or CoCr femoral heads articulating on HXLPE liners are associated with increased risk of corrosion among HXLPE liners when compared with smaller diameter heads. A limitation of this study is the semi-quantitative scoring technique, heterogeneity of the retrieval collection and short implantation time of the larger diameter heads. Because corrosion may increase over time in vivo, longer-term follow-up, coupled with quantitative taper wear measurement, will better assess the natural progression of taper degradation in modern THA bearings.
