The vast majority of total hip replacements (THR) implanted today enable modularity by means of a tapered junction; based on the Morse taper design introduced for cutting tools in the 19^th Century ¹. Morse-type tapers at the head-stem junction provide many benefits, key for a successful surgical outcome such as wider component selection and restoration of better biomechanics ². However, moving from mono-block to modular designs has not been without its issues. Fluid ingress and motion at the interface has led to a complex multifactorial degradation mechanism better known as fretting-corrosion ³. Fretting-corrosion products created at the junction are commonly associated with adverse local tissue reactions ⁴.

There is a wide variation in the taper junction of THR differing quite significantly from Morse's original design. Performance of the taper junction has been found to vary with different designs ^5,6. However, there is still a lack of common understanding of what design inputs makes a ‘good’ modular taper interface. The aim of this study was to better understand the links between implant design and fretting-corrosion initially focussing on the role of angular mismatch between male and female taper. A combination of experimental approaches with the aid of computational models to assist understanding has been adopted. A more descriptive understanding between taper design, engagement, motion and fretting-corrosion will be developed.

Three different sample designs were created to represent the maximum range of possible angular mismatches seen in clinically available THR modular tapers (Matched: 0.020 ±0.002 °, Proximal: 0.127 ±0.016 °, Distal: −0.090 ±0.002 °). Head-stem components were assembled at 2 kN. Motion and fretting-corrosion at the interface was simulated under incremental uniaxial sinusoidal loading between 0.5–4 kN at 8 intervals of 600 cycles. The different types of motions at the interface was measured using a developed inductance circuit composed of four sensing coils, digital inductance converter chip (LDC1614, Texas Instruments, US) and microcontroller (myRIO, National Instruments, US). Fretting-corrosion was measured using potentiostatic electrochemical techniques with an over potential of +100 mV vs OCP (Ivium, NL). Complimentary finite element (FE) models were created in Ansys (Ansys 19.2, US).

Under uniaxial loading, the ‘matched’ modular taper assemblies corroded most and allowed the greatest pistoning motion due to a seating action. ‘Distal’ and ‘proximal’ engaged modular tapers showed reduced corrosion and seating when compare to the ‘matched’ components. However the kinetics of corrosion and motion were interface dependent. It is hypothesized, and complimented by FEA analysis, that lower initial contact stress in the ‘matched’ modular tapers allows for greater subsidence and depassivation of the oxide layer and higher corrosion. ‘Matched’ modular tapers allowed less rotational and toggling motions compared to mismatched tapers, suggesting a reduced mismatch might perform better once the heads have seated over time. Future work involves tests conducted under a surgically relevant impaction force and physiological loading kinematics to develop this descriptive link between taper design, engagement and performance.

Introduction

Titanium and its alloys are attractive biomaterials attributable to their desirable corrosion, mechanical, biocompatibility and osseointegration properties. In particular, β – titanium alloys like the TMZF possess other advantages such as its lower modulus compared to Ti6Al4V alloy. This reduces stress shielding effect in Total Hip Arthroplasty (THA) and the replacement of V in the Ti6Al4V alloy, eliminates in-vivo V-induced toxicity. Unfortunately, implants made of TMZF were later recalled by the FDA due to higher than acceptable revision rates. The purpose of this study was to compare the fretting corrosion characteristics of Ti6Al4V and TMZF titanium alloys. It is hoped the findings will inform better design of β – titanium alloys for future applications in THA.

Introduction

Titanium and its alloys are attractive biomaterials attributable to their desirable corrosion, mechanical, biocompatibility and osseointegration properties. Ti6Al4V alloy in particular remains a prominent biomaterial used in Total Hip Arthroplasty (THA) today. This is partly due to biocompatibility and stress shielding issues with CoCrMo alloys, resulting in its increasing side-lining from the THA construct. For several decades now, research efforts have been dedicated to understanding wear, corrosion and surface degradation processes in implant materials. Only recently have researchers shown interest in understanding the subsurface implications of fretting and the role it plays on implant fracture. The purpose of this study was to utilise advanced microscopy and spectroscopy techniques to characterise fretting-induced subsurface transformations in Ti6Al4V. This makes mapping specific regions that are most prone to wear and fatigue failures at the modular taper interface of THA probable. Thus, informing a proactive approach to component design and material selection.

Introduction

Fretting corrosion at the Head-Neck taper interface of Large Metal on Metal (MoM), Metal on Polymer (MoP) and Ceramic on Ceramic (CoC) total hip arthroplasty (THA) remains a clinical concern. Ceramic femoral heads have gained a lot of attention more recently as a possible way to mitigate/reduce the dissolution of Cobalt Chromium ions. The objective of this study is to assess the fretting corrosion currents emanating from four material combinations for which Ti6Al4V and Co28Cr6Mo are the neck components of Co28Cr6Mo and BIOLOX®delta femoral heads at three different cyclic loads.

Surgical interventions for the treatment of chronic neck pain, which affects 330 million people globally, include fusion and cervical total disc replacement (CTDR). Most of the currently clinically available CTDRs designs include a metal-on-polymer (MoP) bearing. Numerous studies suggest that MoP CTDRs are associated with issues similar to those affecting other MoP joint replacement devices, including excessive wear and wear particle-related inflammation and osteolysis. A standard ISO testing protocol was employed to investigate a device with a metal-on-metal (MoM) bearing. Moreover, with findings in the literature suggesting that the testing protocol specified by ISO-18192-1 may result in overestimated wear rates, additional tests with reduced kinematics were conducted.

Six MoM CTDRs made from high carbon cobalt-chromium (CoCr) were tested in a six-axis spine simulator, under the ISO-18192-1 protocol for a duration of 4 million cycles (MC), followed by 2MC of modified testing conditions, which applied the same axial force as specified in ISO-18192-1 (50-150N), but reduced ranges of motion (ROM) i.e. ±3° flexion/extension (reduced from ±7.5°) and ±2° lateral bending (reduced from ±6°) and axial rotation (reduced from ±4°). Foetal bovine serum (25% v/v), used as a lubricant, was changed every 3.3×10⁵ cycles and stored at −20°C for particle analysis. Components were measured after each 1×10⁶ cycles; surface roughness, damage modes and gravimetric wear were assessed. The wear and roughness data was presented as mean ±95% confidence interval and was analysed by one-way analysis of variance (ANOVA) (p=0.05).

The mean wear rate of the MoM CTDRs tested under the ISO protocol was 0.246 ± 0.054mm³/MC, with the total volume of wear of 0.977 ± 0.102mm³ lost over the test duration (Fig. 1). The modified testing protocol resulted in a significantly lower mean volumetric wear rate of 0.039 ± 0.015mm³/MC (p=0.002), with a total wear volume of 0.078 ± 0.036mm³lost over the 2MC test duration. Under both test conditions, the volumetric wear was linear; with no significant bedding-in period observed (Fig. 1). The mean pre-test surface roughness decreased from 0.019 ± 0.03µm to 0.012 ± 0.002µm (p=0.001) after 4MC of testing, however surface roughness increased to 0.015 ± 0.002µm (p=0.009) after the additional 2MC of modified test conditions. Following 4MC of testing, polishing marks, observed prior to testing, had been removed. Consistently across all components, surface discolouration and multidirectional, criss-crossing, curvilinear and circular wear tracks, caused by abrasive wear, were observed. Reduced ROMs testing caused similar types of damage, however the circular wear tracks were smaller in size, compared to those produced during testing under the ISO protocol.

The wear rates exhibited by MoM CTDRs tested under ISO-18192-1 testing protocol (0.246mm³/MC) were lower, when compared to CTDR designs incorporating MoP bearings, as well as MoM lumbar CTDRs. Wear rates generated under a modified ISO testing protocol were reduced tenfold, similarly to findings that have previously been reported in the literature, and support the hypothesis that the testing protocol specified by ISO-18192-1 may overestimate wear rates. Characterisation of particles generated by MoM CTDRs and biological consequences of those remain to be determined.

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Introduction

Cobalt-Chromium-Molybdenum (CoCr) and Titanium-Aluminium-Vanadium (Ti) alloys are the most commonly used alloys used for Total Hip Replacement due to their excellent biocompatibility and mechanical properties. However, both are susceptible to fretting corrosion In-vivo. The objective of this study was to understand the damage mechanism of both combinations through a sub-surface damage assessment of the alloys at various fretting amplitudes using the Transmission Electron Microscopy (TEM – CM200 FEGTEM). The TEM was used to attain a cross sectional view of the alloys in orderto see the effect of high shear stress on the grain structure.