Abstract
Introduction:
Modular necks allow intra-operative 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/stem taper, and some retrieved components exhibit severe fretting corrosion.1 Fatigue testing according to ISO 7206-6 (10/9 orientation) has been shown to replicate the clinical fatigue failures, but results 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.
Methods:
Constructs tested in this study consisted of a single stem and neck design (PROFEMUR modular, Wright Medical Technology). The worst case long varus neck design was evaluated in two materials: Ti6Al4V and wrought CoCr. In vitro fatigue testing in the 10/9 configuration was conducted at 10 Hz in unbuffered, aerated saline. Fretting mass loss, distraction force, and assessment of taper corrosion via SEM/EDS were measured. Methods used to exacerbate fretting corrosion are shown in Figure 2.
Results:
Test results & conditions are shown in Figure-2. All of the constructs impacted per surgical technique survived 5 Mc and were similar to constructs tested under standard conditions (Figure-1a). In contrast, Ti neck constructs that were hand assembled 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. In particular, constructs that were hand-assembled and tested at elevated temperature with a rest period had much higher fretting mass loss and a level of corrosion that is qualitatively similar to corroded retrievals (Figures-1b, d). The CoCr necks that were hand assembled survived 10 Mc, but one of them fractured during distraction, and they exhibited higher fretting and corrosion at 5 Mc (Figure-1c). The effect of adding acidified saline to the stem pocket was unclear, but elevating the temperature and adding a rest period resulted in a significant increase in fretting and corrosion.
Discussion:
Pallini et al2 recommended light tapping based on tests that showed distraction forces were equal between hammer impacted necks and hand assembly followed by simulated normal walking. The simulated gait loads in Pallini's study were applied in line with the neck taper's axis, whereas in the present study, they were applied in the 10/9 orientation, which is considerably off-axis. It is also notable that while the CoCr necks performed significantly better than Ti necks after hand-assembly, their performance was still markedly reduced compared to well-impacted constructs.
Conclusion:
This study has shown conclusively that failure to impact modular neck connections can have a devastating effect on taper fretting and corrosion, leading to early fatigue failure. Applicability to other designs and to lightly impacted necks requires further study.
