Abstract
Introduction
Tribo-chemical damage of modular taper junctions is often observed at revision THR and may be a contributing factor to chronic inflammation of peri-prosthetic tissues through generation of chromium rich corrosion products. At the time of revision, surgeons may elect to leave the primary femoral stem in situ and replace the original femoral head with a new component. This decision is based on the assumption that the interface formed between the original trunnion and the new bore is capable of withstanding the loads and torques applied during use, without failure of the new interface. This study was performed to determine the extent to which the mechanical properties of the taper interface are degraded with varying degrees of tribo-chemical damage secondary to prior implantation.
Materials and Methods
Fifteen CoCr femoral heads (DePuy: 6, Smith & Nephew: 5; Zimmer: 4) were retrieved at revision THR and were examined with stereomicroscopy. The surface of each bore was scored for the presence of fretting and corrosion using the grading system of Goldberg et al. Nine additional heads in original (unimplanted) condition (3 per manufacturer) were also selected to act as controls. Each head was manually assembled on a matching unimplanted TiAlV trunnion in a mechanical testing machine (MTS Bionix) and loaded at 500N/sec to a maximum assembly load of 4000N. The head/trunnion specimen was then mounted in a torsional loading fixture and immersed in bovine serum. A cyclic torque was applied to the head with an initial maximum value of 2 Nm. The specimen was unloaded and held for a 30 sec wait period and the torsional loading was repeated to a peak value of 4 Nm. With each torsional cycle the peak torque was increased by 2 Nm until the taper junction underwent rotational failure. During testing, relative motion between the femoral head and the trunnion was measured with a displacement transducer (DVRT-3, MicroStrain, accuracy = ± 0.1%, resolution = 1.5 μm, hysteresis & repeatability = ± 1 μm). A separate disassembly test was performed by first assembling each specimen with 4000N and then applying a distraction force at 0.008 mm/sec until separation.
Results
The maximum torque applied at 3 μm permanent displacement of the taper junction averaged 13.2 ± 0.7 Nm. This value characterizes the resistance of each taper junction to slippage under repetitive loading. The relationship between the severity of taper corrosion and resistance to torsional displacement was not statistically significant (p = 0.1343). The ultimate torque capacity of the taper junctions averaged 17.1 ± 0.7 Nm and did not vary significantly with corrosion score (p = 0.2317). There was no difference between manufacturers in terms of resistance to torsional displacement, ultimate torque capacity, and disassembly force (1352 ± 114N).
Conclusions
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1.
Slippage of 3 μm at the CoCr-TiAlV taper junction during cyclic loading requires peak torques of only 12–14 Nm.
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2.
The torsional properties of taper junctions are not significantly degraded in the presence of mild or moderate grades of corrosion damage on the Goldberg scale.
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3.
This supports the practice of retaining a TiAlV stem and replacing a CoCr head during a revision procedure to avoid disrupting the femoral component.
