Modular necks allow intraoperative adjustment of neck length, offset, and version, enabling the surgeon to better match leg length and accommodate anatomical differences. However, there have been recent reports of early fatigue failures of the neck initiating from the neck and stem taper, and some retrieved components exhibit severe fretting corrosion. Fatigue testing in the 10°/9° orientation according to ISO 7206-6 has been shown to replicate the clinical fatigue failures. The location of fractures in the testing was consistent with the highest stress location observed in the finite element analyses. Thus, although the testing reproduced clinical fracture mode, it resulted in relatively minor fretting and corrosion. The purpose of this pilot study was to evaluate techniques for accelerating fretting corrosion with the goal of replicating the most severely corroded clinical retrieval cases.
Constructs tested in this study consisted of a single stem and neck design (PROFEMUR® modular, MicroPort Orthopedics, formerly Wright Medical Technology). The worst-case long varus neck design was evaluated in two materials: Ti-6Al-4V and wrought chromium molybdenum (CoCrMo). In vitro fatigue testing in the 10°/9° configuration was conducted at frequency of 10 Hz in unbuffered, aerated saline. Fretting mass loss, distraction force, and assessment of taper corrosion via scanning electron microscopy and energy-dispersive X-ray spectroscopy were measured.
All of the constructs impacted per surgical technique survived 5 Mc and were similar to constructs tested under standard conditions. In contrast, some hand-assembled Ti neck constructs failed in fatigue after a surprisingly small number of cycles. The hand-assembled constructs that survived 5 Mc of loading exhibited lower distraction forces, higher than normal fretting mass loss, and moderate to severe corrosion of the taper. The qualitative appearance of corrosion was similar to corroded retrievals. The CoCrMo necks that were hand assembled survived 10 Mc, but one of the necks fractured during distraction, and they exhibited higher fretting and corrosion at 5 Mc. The effect of adding acidified saline, bone, or fat to the stem pocket was unclear, but elevating the temperature and adding a rest period resulted in a substantial increase in fretting and corrosion (p = 0.048; one-way analysis of variance [ANOVA]). The constructs that were assembled by gentle impaction (1500 N) exhibited moderate fretting. The qualitative appearance of corrosion at 5 Mc for both Ti and CoCrMo constructs was worse than that of the well-impacted devices. The CoCrMo necks showed a trend toward better performance (in cyclic fatigue) than Ti-6Al-4V necks after hand-assembly although Pearson contingency analysis showed no significant difference (p = 0.35; χ2 = 0.89).
This study has shown conclusively that failure to impact modular neck connections can have a substantial effect on taper fretting (p = 0.018, one-way ANOVA) and corrosion, leading to early fatigue failure. Applicability to other designs requires further study.