Abstract
Controversy exists over the role of fretting-corrosion in modular junctions of large-diameter metal-on-metal (MOM) heads given the many design plus alloy mix-and-match variations. Overall data was also scant regarding a) fitting stem trunnions to head tapers, b) role of taper angles, c) role of smooth vs threaded trunnion junctions, d) role of head neck-lengths and e) role of head diameters. While the “12: 14” taper has been used with small CoCr heads for 40 years, we could not find retrieval analyses on this European ‘gold-standard’. We therefore selected 10 femoral stems with 28 mm modular heads for analysis (3–8 years follow-up). Unique to this study were the threaded taper profiles on both stems and heads (Fig. 1).
Six stems were cemented Ti6Al4V (Alize, FH-Orthopedics, France) with 12/14 taper angle defined as 5° 42′. These represented Ti64: CoCr combinations from 2 vendors. The other four were CoCr stems including the CoCrMo (Protasul-2) and CoNiCrMo (Protasul-10) alloys (cemented and HA-coated; Sulzer, Switzerland). These CoCr: CoCr combinations from one vendor had “12/14” stem-taper defined as 5° 38′. Anatomical positioning of Metasul heads (Sulzer, Switzerland) was identified by main-wear zone maps. Femoral heads were then bi-valved in horizontal plane for direct imaging by interferometry (WLI) and SEM. Visual corrosion mapping (3) was recorded digitally in 4 anatomical views. Quantitative analysis used 1 to 5 taper zones with 6-replicate measurements per zone (Fig. 1).
The WLI and SEM studies showed that non-contacting taper zones inside CoCr heads (Fig. 2) were threaded with pitch of 70 μm (PV: peak-valley depth = 5–7 μm). The non-contact zones on Sulzer stems had 130 μm pitch (PV = 4–8 μm) whereas Alize stems had 210 μm pitch (PV = 10–12 μm). Threads on both stem types were much coarser than CoCr heads; Ti64 stem threads were much coarser than CoCr stems. In contact zones, the Metasul threads had flattened (avg. roughness = 0.45 μm Ra). With CoCr stems there was little difference. Difference in pitch of stem-threads vs head-threads indicated there was no imprinting onto head tapers. Nor were there statistically significant differences evident in the contact zones along CoCr or Ti64 tapers. Small damaged areas (Fig. 3: arrows) may have been due to alternatively; initial machining, surgical impaction, in-vivo cold-welds, fretting, corrosion, or from surgical-removal. The as labeled “corrosion damage” was well within the “mild” grade for all implants.(3) Thus even with this considerable variety of design and material parameters, we were satisfied that these gold-standard taper junctions with threaded interfaces had performed very well with 28 mm MOM at 3–8 years follow-up.
