AN ASSESSMENT OF SEVEN DESIGN, MATERIAL AND SURGICAL FACTORS IN THE FRETTING CORROSION OF HEAD-NECK MODULAR TAPER JUNCTIONS: A MULTIFACTORIAL DESIGN OF EXPERIMENT STUDY

Introduction

Mechanically assisted crevice corrosion (MACC) of head-neck modular taper junctions is prevalent in virtually all head neck tapers in use today. To date, no clear in vitro tests of design, material or surgical elements of the modular taper system have been reported that show which factors principally affect MACC in these tapers. Possible elements include seating load, head-neck offset, surface roughness, taper engagement length, material combination, angular mismatch, and taper diameter. The goals of this study were to use an incremental fretting corrosion test method¹ to assess the above 7 elements using a design of experiments approach. The hypothesis is that only one or two principal factors affect fretting corrosion.

Methods

A 2^7-2 design of experiment test (7 factors, ¼ factorial, n=32 total runs, 16 samples per condition per factor) was conducted. Factors included: Assembly Force (100, 4000N), Head Offset (1.5, 12 mm), Taper Locking Position (Mouth, Throat), Stem Taper Length (0.44, 0.54 in), Stem Taper Roughness (Ground, Ridged), Taper Diameter (9/10, 12/14), and Stem Material (CoCrMo, Ti-6Al-4V). The heads were CoCrMo coupled with taper coupons (DePuy Synthes, Warsaw, IN). Test components were assembled wet and seated axially with 100 or 4000N assembly force. The assemblies were immersed in PBS and potentiostatically held at −50mV vs. Ag/AgCl. Incremental cyclic loads were applied vertically to the head at 3Hz until a 4000N maximum load was reached (See Fig. 1). Fretting currents at 4000 N cyclic load were used for comparisons while other parameters, including onset load, subsidence, micromotion and pull off load were also captured. Statistical analysis was performed using Pareto charts and Student's T-tests for single factor comparisons (P < 0.05 was statistically significant).

Results

Average fretting corrosion currents at 4000 N cyclic load ranged from 0 to 23 µA for all test specimens. The primary factors that statistically affected fretting corrosion currents were head-neck offset (P<0.05) and assembly load (P<0.05). Test factors with the most significance are shown in the Pareto chart of effects (Fig 2). Assembly force, head offset, and the interaction between these two factors were the most significant effects (see Fig 3). All other factors had diminishing effects on fretting current. Note that there is a correlation between fretting currents and pull off load (Fig. 3c). A number of interactive effects were seen between factors on various output parameters (e.g., subsidence, pull off load, onset load) as well.

Discussion

This work demonstrates that the principal factors affecting fretting corrosion are seating load and head-neck offset. Material combination, taper diameter, engagement length, roughness and angular mismatch were less significant effectors of fretting corrosion. This test assesses early fretting corrosion response but does not necessarily predict long-term performance where crevices and solution changes may be important.

Significance

This work shows a relative comparison of the effects of multiple design, material and surgical elements on the early fretting corrosion behavior of modular tapers in vitro. Head offset and seating loads represent the most significant factors amongst those studied.

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