Summary Statement. Fretting and corrosion has been identified as a clinical problem in modular metal-on-metal THA, but remains poorly understood in modern THA devices with polyethylene bearings. This study investigates taper damage and if this damage is associated with polyethylene wear. 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 characterise 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) 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). 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 along with taper damage scores, a correlation was not observed between head taper 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

Carbonate-substituted hydroxyapatite (CHA) is more osteoconductive and more resorbable than hydroxyapatite (HA), but the underlying mode of its action is unclear. We hypothesised that increased resorption of the ceramic by osteoclasts might subsequently upregulate osteoblasts by a coupling mechanism, and sought to test this in a large animal model.

Defects were created in both the lateral femoral condyles of 12 adult sheep. Six were implanted with CHA granules bilaterally, and six with HA. Six of the animals in each group received the bisphosphonate zoledronate (0.05 mg/kg), which inhibits the function of osteoclasts, intra-operatively.

After six weeks bony ingrowth was greater in the CHA implants than in HA, but not in the animals given zoledronate. Functional osteoclasts are necessary for the enhanced osteoconduction seen in CHA compared with HA.