In vitro corrosion testing of modular hip tapers

Influence of Taper surface topographies on contact deformation and stresses

The role of bore and trunnion surface topography on the failure rate of total hip joint replacements due to trunnionosis is not clear despite significant variations in the design of taper components between manufacturers. Taper surface topography, along with other taper design parameters such as clearance, diameter, and assembly force, determine the initial interlock of the contacting surfaces after assembly; this has been related to relative motions that can cause fretting and corrosion at the taper interface. However, in most in-silico parametrical taper studies associated with taper micromotions, the bore and trunnion surfaces have been simplified using a flat surface and/or sinusoidal functions to mimic the surface roughness. The current study tests the hypothesis that the use of simple geometrical functions for the taper surface topography can predict the surface mechanics developed in assembled tapers. Measured and simulated surfaces of bores and trunnions were characterised using common roughness parameters and spectral density estimations. Using the same characterised surface profiles, 2D Finite Element (FE) models of CoCr alloy femoral heads and Ti alloy trunnions were developed. Models simulated assembly conditions at different resultant forces ranging from 0.5 to 4.0 kN, contact conditions were determined and associated with their topographical characteristics. Measured surfaces of bore and trunnion components comprise up to seven dominant spatial frequencies. Flattening of the trunnion microgrooved peaks was observed during the assembly of the taper. When the femoral head bore and trunnion topography were both considered a reduced number of microgrooved peaks were in contact, from 51 in an idealised taper surfaces to 35 in measured surfaces using an assembly reaction force of 4 kN. The contact points in the models developed high plastic strains, which were greater than that associated with failure of the material. Results showed that line and sine wave functions over estimate contact points at the taper interface compared to those surfaces that consider roughness and peak variation. These findings highlight the important role of modelling the full surface topography on the taper contact mechanics, as surface variations in the roughness and waviness change the performance of tapers.

Metallosis in Total Hip Arthroplasty

» Metallosis is a rare but significant complication that can occur after total hip arthroplasty (THA) for a variety of reasons but most commonly in patients with metal-on-metal implants.» It is characterized by the visible staining, necrosis, and fibrosis of the periprosthetic soft tissues, along with the variable presence of aseptic cysts and solid soft tissue masses called pseudotumors secondary to the corrosion and deposition of metal debris.» Metallosis can present with a spectrum of complications ranging from pain and inflammation to more severe symptoms such as osteolysis, soft tissue damage, and pseudotumor formation.» Workup of metallosis includes a clinical evaluation of the patient's symptoms, imaging studies, serum metal-ion levels, and intraoperative visualization of the staining of tissues. Inflammatory markers such as erythrocyte sedimentation rate and C-reactive protein along with intraoperative frozen slice analysis may be useful in certain cases to rule out concurrent periprosthetic joint infection.» Management depends on the severity and extent of the condition; however, revision THA is often required to prevent rapid progression of bone loss and tissue necrosis.

Has Wrought Cobalt-Chromium-Molybdenum Alloy Changed for the Worse Over Time?

BACKGROUND

Total hip arthroplasty (THA) failure due to tribocorrosion of modular junctions and resulting adverse local tissue reactions to corrosion debris have seemingly increased over the past few decades. Recent studies have found that chemically-induced column damage seen on the inner head taper is enabled by banding in the alloy microstructure of wrought cobalt-chromium-molybdenum alloy femoral heads, and is associated with more material loss than other tribocorrosion processes. It is unclear if alloy banding represents a recent phenomenon. The purpose of this study was to examine THAs implanted in the 1990s, 2000s, and 2010s to determine if alloy microstructure and implant susceptibility to severe damage has increased over time.

METHODS

Five hundred and forty-five modular heads were assessed for damage severity and grouped based on decade of implantation to serve as a proxy measure for manufacturing date. A subset of heads (n = 120) was then processed for metallographic analysis to visualize alloy banding.

RESULTS

We found that damage score distribution was consistent over the time periods, but the incidence of column damage significantly increased between the 1990s and 2000s. Banding also increased from the 1990s to 2000s, but both column damage and banding levels appear to recover slightly in the 2010s.

CONCLUSION

Banding, which provides preferential corrosion sites enabling column damage, has increased over the last 3 decades. No difference between manufacturers was seen, which may be explained by shared suppliers of bar stock material. These findings are important as banding can be avoidable, reducing the risk of severe column damage to THA modular junctions and failure due to adverse local tissue reactions.

Metal-on-metal total hip arthroplasty: does increasing modularity affect clinical outcome?

BACKGROUND

Modularity of metal-on-metal (MoM) implants has come under scrutiny due to concerns regarding additional sources of metal debris. This study is a retrieval analysis of implants from the same manufacturer with the same MoM bearing surface. The difference between the implants was presence or absence of modular junctions.

METHODS

This is a retrospective study of 31 retrieved implants from 31 patients who received a Conserve Wright Medical MoM hip prosthesis. The 31 implants consisted of 16 resurfacings and 15 implants with modular junctions; 4 conventional THAs and 11 modular-neck THAs.

RESULTS

43% of pre-revision MRI scans performed on resurfacing implants and 91% performed on the modular implants illustrated evidence of an adverse local tissue reaction. There was no difference in pre-revision blood metal ion levels or bearing surface wear between the resurfacings and modular implants. The neck-head tapers of the modular group showed low levels of material loss. However, the neck-stem tapers showed increased severity of corrosion and material loss.

CONCLUSIONS

The modular implants had an increased incidence of adverse local tissue reaction. This could be related to the presence of modular junctions, particular the neck-stem junction which showed increased susceptibly to corrosion.

Pre-clinical evaluation of fretting-corrosion at stem-head and stem-cement interfaces of hip implants using in vitro and in silico models

Prior to clinical use, the corrosion resistance of new prosthesis system must be verified. The fretting-corrosion mechanisms of total hip arthroplasty (THA) implants generate metal debris and ions that can increase the incidence of adverse tissue reactions. For cemented stems, there are at least two interfaces that can be damaged by fretting-corrosion: stem-head and stem-cement. This investigation aimed to evaluate, through in vitro and in silico analyses, fretting-corrosion at the stem-head and stem-cement interfaces, to determine which surface is most affected in pre-clinical testing and identify the causes associated with the observed behavior. Unimodular stems and femoral heads of three different groups were evaluated, defined according to the head/stem material as group I (SS/SS), group II (CoCr/SS), and group III (CoCr/CoCr). Seven pairs of stems and heads per group were tested: three pairs were subjected to material characterization, three pairs to in vitro fretting-corrosion testing, and one pair to geometric modeling in the in silico analysis. The absolute area of the stem body degraded was more than three times higher compared with the trunnion, for all groups. These results were corroborated by the in silico analysis results, which revealed that the average micromotion at the stem-cement interface (9.65-15.66 μm) was higher than that at the stem-head interface (0.55-1.08 μm). In conclusion, the degradation of the stem-cement interface is predominant in the pre-clinical set, indicating the need to consider the fretting-corrosion at the stem-cement interface during pre-clinical implant evaluations.

In vitro testing for hip head-neck taper tribocorrosion: A review of experimental methods

In vitro test methods are challenged by the multi-factorial nature of head-neck taper connection tribocorrosion due to the consequences of simplification. Incorrect study design and misinterpretation of results has led to contradictory findings regarding important factors affecting head-neck taper tribocorrosion. This review seeks to highlight important considerations when developing in vitro test methods, to help researchers strengthen their study design and analyze the implications of others' design decisions. The advantages, disadvantages, limitations and procedural considerations for finite element analyses, electrochemical studies and in vitro simulations related to head-neck taper connection tribocorrosion are discussed. Finite element analysis offers an efficient method for studying large ranges of mechanical parameters. However, they are limited by neglecting electrochemical, biological and fluid flow factors. Electrochemical studies may be preferred if these factors are considered important. Care must be taken in interpreting data from electrochemical studies, particularly when different materials are compared. Differences in material valence and toxicity affect clinical translation of electrochemical studies' results. At their most complex, electrochemical studies attempt to simulate all aspects of headneck taper connection tribocorrosion in a bench top study. Effective execution requires in-depth knowledge of the tribocorrosion phenomenon, the involved mechanisms, and their measures such that each study design decision is fully informed.

Predictors of Adverse Local Tissue Reaction in a High-Risk Population

Background

Adverse local tissue reaction (ALTR) is a recognized complication of total hip arthroplasty (THA) with metal-on-polyethylene (MoP) bearing surface implants. Specific models of THA implants have been identified as having a higher incidence of ALTR. The purpose of this study is to determine if serum metal levels, patient symptoms, implant factors, and imaging findings can be predictive of ALTR within this high-risk population.

Methods

We retrospectively reviewed an observational cohort of 474 patients who underwent MoP THA and were at increased risk of having ALTR. Patients were stratified based on the presence or absence of ALTR. Patient symptoms, serum metal ions, implant head offset, and imaging findings were compared.

Results

Patients with ALTR were more likely to be symptomatic (52.9% vs 9.9%, P < .0001). The presence of ALTR was associated with significantly higher serum cobalt and chromium levels (6.2 ppb vs 3.6 ppb, P < .0001; 2.3 ppb vs 1.2 ppb, P < .0001). Head offsets greater than 4 mm were associated with a higher prevalence of ALTR (53% vs 38%, P = .05). On metal artifact reduction sequence magnetic resonance imaging, patients with ALTR had larger effusions (4.7 cm vs 2.1 cm, P < .001) and a higher incidence of trochanteric bursitis (47% vs 16%, P < .001).

Conclusions

In high-risk MoP implants, serum cobalt and chromium levels are elevated, even in patients without ALTR. A larger femoral head offset is a risk factor for the development of ALTR. Our study suggests that patients presenting with painful THA and elevated metal ions require risk stratification based on patient symptoms, metal artifact reduction sequence magnetic resonance imaging findings, and implant factors.

The role of fretting-frequency on the damage modes of THR modular junction: In-vitro study

According to the National Center for Health Statistics, currently, more than 250,000 total hip replacements annually in the US alone, with an estimated increase to 500,000 by the year 2030. The usage of tapered junctions between the femoral neck and head gives the surgeon flexibility in implant assembly. However, these modular junctions are subjected to micro-motion that may cause chemical and fretting-corrosion at the modular junction. Therefore, it is imperative to study these forces to mitigate their effects. The current study aims to understand the effects of fretting-corrosion as a function of fretting frequencies caused by common physical activities in an in-vitro model of hip modular junctions. The fretting system has a tribological contact condition of flat-on-flat, mounted to a load frame. CoCrMo pins were polished and immersed in a synovial fluid-like electrolyte solution (Bovine calf serum 30 g/l). Electrochemical measurements were made using a potentiostat. Samples then undergo 3600 cycles at 50 μm (to simulate gross slips), with a horizontal load at 200 N, and a frequency of 0.5 Hz, 0.7 Hz, 1 Hz, and 1.5 Hz to simulate Sit Down-Stand Up, Stair Climb, Walking, and Jogging, respectively. Worn surfaces were then examined under optical and scanning electron microscopy. The evolution of free potential as a function of time for tested frequencies shows the initial potential drop and stabilized trend in the potential evolution. The sample group at a higher frequency displays a higher tendency of corrosion than a lower frequency; however, the dissipation energy decreases as a function of fretting frequency. Both electrochemical and mechanical responses correlate to the variation in the fretting frequencies. Organometallic complexes were found on the surfaces of the samples that were subjected to a slower frequency of fretting, whereas mechanical grooving was noticed on samples with a faster frequency. Hence, these preliminary studies suggest that implant failure rates may be altered based on fretting-frequencies induced by physical activity. Further studies will be required to verify the findings and explore the potential role of fretting frequency in the damage modes of the modular junction.

Self-reinforced poly(ether ether ketone) and polyethylene composite gaskets for prevention of mechanically-assisted corrosion in modular taper junctions: Seating, micromotion and short-term fretting corrosion

Mechanically-assisted crevice corrosion (MACC) is a phenomenon known to cause complications in modular orthopedic implants, particularly at metal-metal taper junctions. Previous studies of the properties and corrosion performance of an interfacial polymeric self-reinforced composite (SRC) gaskets have shown its capability as a high-strength, insulating barrier against oxide abrasion and metal degradation of metal-metal (or metal-hard) contacts in MACC conditions. This study characterizes the short-term tribocorrosion performance of poly (ether ether ketone) SRCs (SRC-PEEK) and polyethylene SRC (SRC-PE) films under in vitro test conditions for head-neck modular junction designs in hip replacement devices. SRC films composed of SRC-PEEK and SRC-PE were seated between 9/10 femoral head bores and stem tapers as thin interfacial gaskets and tested against metal-metal controls under short-term cyclic loading conditions in a custom in vitro test setup. Head-neck seating mechanics were measured, followed by incremental cyclic fretting corrosion testing with monitoring of fretting current, force, and relative micromotion between head and neck components during cyclic loading. SRC-PEEK tapers had a seating subsidence that was approximately three times that of the SRC-PE tapers and nine times that of controls. SRC-PE tapers, likely due to low friction, partially failed to lock during seating resulting in a pushing up of the head on the taper. Average fretting currents were significantly lower for both SRC groups (less than 0.3 μA at 4000 N) compared to control tapers experiencing fretting corrosion currents between 1.7 μA and 32 μA, (p < 0.05). SRC-PEEK gaskets exhibited similar subsidence and micromotion performance as controls while SRC-PE tapers experienced over 240 μm of subsidence during seating and loading conditions. The SRC-PE low-friction properties likely caused insufficient taper locking, which may increase the risk of improper head seating or head disassociation. These results show that SRC-PEEK gaskets, unlike SRC-PE gaskets, can maintain adequate frictional locking at the taper junction and prevent the onset of MACC. SRC-PEEK gaskets improve the performance of modular taper junctions and could be considered as a potential solution to mitigate fretting corrosion.

"Off-label" Usage of an Oxidized Zirconium Femoral Head in Revision of a Total Hip Arthroplasty with Mechanically Assisted Crevice Corrosion and a Legacy Taper

We present a case of a 72-year-old male with a history of a late 1980s metal-on-polyethylene total hip arthroplasty who presented with unilateral leg vascular compromise, joint pain, and stiffness and subsequently underwent revision for adverse local tissue reaction secondary to mechanically assisted crevice corrosion. His stable and extensively porous coated femoral implant had a legacy taper with no currently manufactured option for a non–Co-alloy femoral head. After shared decision-making with the patient, we opted to use an oxidized zirconium femoral head from another manufacturer with a similar taper during his revision surgery and documented that his vascular compromise resolved and his serum Co was undetectable 3 years after the revision.

Incidence of Pseudotumors in a Dual Modular Stem Construct With and Without Metal-on-Metal Bearing Surface

BACKGROUND

The purpose of this study was to compare the incidence of pseudotumors in metal-on-metal (MoM) titanium modular neck hip arthroplasties to non-MoM modular neck hip arthroplasties. A secondary goal was to determine whether a correlation exists between elevated metal concentrations and pseudotumor incidence.

METHODS

The data were collected and evaluated from 49 MoM joints and 26 non-MoM joints between the years 2012 and 2017. Hip ultrasonography was done after a minimum of 5 years postimplantation. Whole serum metal (titanium, cobalt, and chromium) concentrations were measured at the hip ultrasonography study.

RESULTS

The average time elapsed between surgery and ultrasonography visit was 7.6 years. In the 49 patients with MoM joints, 22.4% (n = 11) had a pseudotumor. In the 26 patients with non-MoM joints, 9.1% of metal-on-polyethylene hips (n = 1) and 0% of ceramic-on-ceramic/polyethylene hips developed a pseudotumor. No significant statistical correlation was observed between serum metal concentrations and pseudotumor formation (P > 0.05). A significant correlation was observed of serum titanium concentration to pseudotumor size (P = 0.024).

CONCLUSION

The incidence of pseudotumor formation in MoM total hip arthroplasties was more than five times the incidence associated with non-MoM bearings. The correlation between serum titanium concentration and pseudotumor size suggests that titanium levels may be a useful indicator for pseudotumor formation in patients with this particular titanium modular neck femoral implant. No notable correlation was observed between serum cobalt and chromium concentration and pseudotumor formation or size.

A comparative study on the physicochemical characteristics of nanoparticles released in vivo from CoCrMo tapers and cement-stem interfaces of total hip replacements

The good biocompatibility and corrosion resistance of the bulk CoCrMo alloy has resulted in it being used in the manufacture of implants and load bearing medical devices. These devices, however, can release wear and corrosion products which differ from the composition of the bulk CoCrMo alloy. The physicochemical characteristics of the particles and the associated in vivo reactivity are dictated by the wear mechanisms and electrochemical conditions at the sites of material loss. Debris released from CoCrMo hip bearings, taper junctions, or cement-stem interfaces can, therefore, have different chemical and morphological characteristics, which provide them with different in vivo toxicities. Here, we propose to assess and compare the characteristics of the particles released in vivo from CoCrMo tapers and cement-stem interfaces which have received less attention compared to debris originating from the hip bearings. The study uses state-of-art characterization techniques to provide a detailed understanding of the size, morphology, composition, and chemistry of the particles liberated from the wear and corrosion flakes from revised hip replacements, with an enzymatic treatment. The phase analyses identified Cr₂ O₃ nanoparticles released from tapers and cement-stem interfaces, whose composition did not vary with origin or particle morphology. The size distributions showed significantly smaller particles were released from the stems, compared to the particles originating from the corresponding tapers. The investigation demonstrates that the tribocorrosive processes occurring at the taper and stem interfaces both result in Cr₂ O₃ nanoparticle formation.

Fretting corrosion of Si3 N4 vs CoCrMo femoral heads on Ti-6Al-V trunnions

Fretting corrosion at the head-neck taper junction was compared between silicon nitride (Si₃ N₄ ) and commercially available cobalt chrome (CoCrMo) femoral heads on titanium (Ti-6Al-4V) trunnions. An electrochemical setup was used to capture the fretting currents (characterized by oxide abrasion and repassivation) during cyclic loading. Onset load, pull-off force (disassembly load), short term and long term (1 million cycles) fretting currents were used to compare the fretting corrosion performance between the test group (Si₃ N₄ /Ti-6Al-4V) and the control group (CoCrMo/Ti-6Al-4V). Incremental cyclic fretting corrosion tests showed that the Si₃ N₄ /Ti-6Al-4V combination had statistically lower (P < .05) average fretting current of 0.189 µA (SD = 0.114 µA) compared to 0.685 µA (SD = 0.630 µA) for CoCrMo/Ti-6Al-4V for cyclic load of 3200 N. Similarly, for the one million cycle fretting corrosion tests, the Si₃ N₄ /Ti-6Al-4V couples had statistically lower (P < .05) average current (0.048 µA, SD = 0.025 µA) vs CoCrMo/Ti-6Al-4V couples (0.366 µA, SD = 0.143 µA). The Si₃ N₄ heads also had higher onset loads (P < .05) for fretting (vs CoCrMo, 2200 N vs 1740 N) indicating a difference in surface contact mechanics between the two groups. Scanning electron microscopy with energy dispersive spectroscopy confirmed material transfer from the trunnions to the heads for both groups tested, and from head to trunnion for the CoCrMo heads. Minimal Si₃ N₄ transfer was noted. The electrochemical, mechanical, and microscopic inspection data supported the hypothesis that Si₃ N₄ /Ti-6Al-4Vcombination had better fretting corrosion performance compared to CoCrMo/Ti-6Al-4V.

Standardizing terms for tribocorrosion-associated adverse local tissue reaction in total hip arthroplasty

Recognizing and adopting standardized terms for adverse local tissue reaction associated with tribocorrosion in total hip arthroplasty are essential for clear scientific discourse and clinical communication. Our goal was to develop terms that can be broadly applied to characterize the local tissue response to tribocorrosion debris, based on current evidence regarding the etiology of this failure mode and its consequences. The proposed standardized terms will improve the understanding and interpretation of analytical tests, advance diagnostic and treatment algorithms, and reduce confusion in research by maintaining consistent nomenclature.

Influence of Different Damage Patterns of the Stem Taper on Fixation and Fracture Strength of Ceramic Ball Heads for Total Hip Replacement

Background

Modularity finds frequent application in total hip replacement, allowing a preferable individual configuration and a simplified revision by retaining the femoral stem and replacing the prosthetic head. However, micromotions within the interface between the head and the stem taper can arise, resulting in the release of wear debris and corrosion products. The aim of our experimental study was to evaluate the influence of different taper damages on the fixation and fracture stability of ceramic femoral heads, after static and dynamic implant loading.

Methods

Ceramic ball heads (36 mm diameter) and 12/14 stem tapers made of titanium with various mild damage patterns (intact, scratched, and truncated) were tested. The heads were assembled on the taper with a quasistatic load of 2 kN and separated into a static and a dynamic group afterwards. The dynamic group (n = 18) was loaded over 1.5 million gait cycles in a hip wear simulator (ISO 14242-1). In contrast, the static group (n = 18) was not mechanically loaded after assembly. To determine the taper stability, all heads of the dynamic and static groups were either pulled off (ASTM 2009) or turned off (ISO 7206-16). A head fracture test (ISO 7206-10) was also performed. Subsequent to the fixation stability tests, the taper surface was visually evaluated in terms of any signs of wear or corrosion after the dynamic loading.

Results

In 10 of the 18 cases, discoloration of the taper was determined after the dynamic loading and subsequent cleaning, indicating the first signs of corrosion. Pull-off forces as well as turn-off moments were increased between 23% and 54% after the dynamic loading compared to the unloaded tapers. No significant influence of taper damage was determined in terms of taper fixation strength. However, the taper damage led to a decrease in fracture strength by approximately 20% (scratched) and 40% (truncated), respectively.

Conclusion

The results suggest that careful handling and accurate manufacturing of the stem taper are crucial for the ceramic head fracture strength, even though a mild damage showed no significant influence on taper stability. Moreover, our data indicate that a further seating of the prosthetic head may occur during daily activities, when the resulting hip force increases the assembly load.

Chronic Systemic Metal Ion Toxicity from Wear on a Revised Cobalt-chromium Trunnion

Introduction:

Metal toxicity secondary to corrosion and wear has been reviewed in the arthroplasty literature with evidence supporting dermatologic, neurologic, and cardiac involvement. This is the first report of a novel case with the occurrence of systemic pathology with only mildly elevated serum cobalt (Co) and chromium (Cr) levels due to trunnion wear in a metal-on-polyethylene articulation.

Case Report:

A 45-year-old female who had undergone a revision total hip arthroplasty developed concerning dermatologic, neurologic, and cardiac symptoms. Symptoms were initially attributed to an unknown autoimmune etiology. She had mildly elevated cobalt and chromium levels and subsequently underwent a second revision, this time with complete exchange of the cobalt/chromium components. By her 2-month follow-up, she reported partial resolution of symptoms. She continues to improve more than a year postoperatively.

Conclusion:

Consideration should be given to metal ion toxicity due to trunnion wear in patients with cobalt/chromium femoral components, even with relatively low ion levels.

The effect of manufacturing tolerances on the mechanical environment of taper junctions in modular TKR

Taper design is known to influence corrosive behavior in taper junctions used in modular orthopaedic devices. Manufacturing tolerance of bore-cone tapers is a critical design parameter due to the effect on taper fit, but the effect of variations in manufacturing tolerance on the mechanics of taper junctions has not been well characterized, particularly in modular total knee replacement (TKR). The purpose of this study was to investigate the effect of manufacturing tolerance on stress and micromotion of modular TKR taper junctions. A 3D finite element (FE) model of a modular TKR taper junction was developed and assigned elastoplastic material properties. Model taper geometry was varied by perturbing the angle mismatch by 0.05° between ±0.25° and represented expected variation in manufacturing tolerance. Stress and micromotion were calculated during dynamic FE simulations for each taper junction geometry under varying activity loads and material combinations. Although an increase in angle mismatch generally resulted in higher stress and micromotion, plastic material behavior disrupted this trend for larger angle mismatches. Model predictions corresponded with corrosion behavior evident in vitro. If the FE results obtained here apply in vivo, the absence of elastoplastic material properties in a taper model may grossly overestimate the micromotion and underestimate corrosion behavior and ion release. It is recommended that manufacturing tolerances of bore-cone tapers in modular TKR designs should produce angle mismatches within 0.1° at the taper junction.

Design, Material, and Seating Load Effects on In Vitro Fretting Corrosion Performance of Modular Head-Neck Tapers

BACKGROUND

The short-term corrosion and micromechanical behavior of 32 unique head-neck taper design/material/assembly conditions was tested using an incremental cyclic fretting corrosion (ICFC) test method previously developed.

METHODS

Seven materials, design, and simulated surgical parameters were evaluated, each being assigned 2 conditions for testing, using a 2^7-2 (7 factor, quarter factorial) design of experiments test matrix. The factors explored were (1) seating load, (2) head-neck offset, (3) material combination, (4) taper diameter, (5) taper roughness, (6) angular mismatch/engagement, and (7) taper length. Each sample underwent assembly, ICFC testing, pull off.

RESULTS

Low seating load and high head offset correlated with increased fretting corrosion (P < .05). High head offset also contributed to a lower onset load for fretting current and higher micromotion (P < .05). Head subsidence measured over the ICFC test for samples seated at 100 N was significantly higher than samples seated at 4000 N. Micromotion for 12-mm head offsets was statistically higher than samples with a 1.5-mm head offset. A number of interactive effects were observed. For example, samples seated at 4000 N were less sensitive to head offset than samples seated at 100 N in terms of the resulting fretting current.

CONCLUSION

Taper locking position, material combination, taper engagement length, taper roughness, and taper dimensions all had weak or no correlation with fretting current and taper micromotion. This test method and experimental design is a versatile means of assessing potential new taper designs in the future.

A case example and literature review of catastrophic wear before catastrophic failure: identification of trunnionosis and metallosis in metal-on-polyethylene hip arthroplasty prior to frank failure or fracture

Though rare, there are documented failures of femoral prosthesis due to corrosion of the head-neck interface in total hip arthroplasty (THA), a phenomenon known as trunnionosis. This wear can result in metallosis, whereby metal debris scatters the surrounding soft tissues. We present on a 58-year-old female who presented with increase in hip and back pain 10 years following right THA using a metal-on-polyethylene construct with a large femoral head (44 mm). Aspiration withdrew metallic fluid, and intraoperative findings showed corrosion of the head-neck taper with surrounding metallosis and pseudocapsule formation. Despite advances in THA design, corrosion and wear between components still exists and may be cause for failure. We present on both the subtle clinical findings and the recommended workup when suspicion is high for trunnionosis, metallosis, or wear, ideally with identification prior to catastrophic failure such as component dislocation or fracture as previously reported.

Material loss at the femoral head taper: a comparison study of the Exeter metal-on-polyethylene and contemporary metal-on-metal total hip arthroplasty

AIMS

There are limited published data detailing the volumetric material loss from tapers of conventional metal-on-polyethylene (MoP) total hip arthroplasties (THAs). Our aim was to address this by comparing the taper wear rates measured in an explanted cohort of the widely used Exeter THA with those measured in a group of metal-on-metal (MoM) THAs.

PATIENTS AND METHODS

We examined an existing retrieval database to identify all Exeter V40 and Universal MoP THAs. Volumetric wear analysis of the taper surfaces was conducted using previously validated methodology. These values were compared with those obtained from a series of MoM THAs using non-parametric statistical methodology. A number of patient and device variables were accounted for using multiple regression modelling.

RESULTS

A total of 95 Exeter MoP and 249 MoM THAs were examined. The median volumetric loss from the MoM cohort was over four times larger than that from the MoP cohort (1.01 mm³ vs 0.23 mm³, p < 0.001), despite a significantly shorter median period in vivo for the MoM group (48 months vs 90 months, p < 0.001). Multiple regression modelling indicated that the dominant variables leading to greater female taper material loss were bearing diameter (p < 0.001), larger female taper angles (p < 0.001), and male titanium stem tapers (p < 0.001).

CONCLUSION

Consistent with the long-term clinical success of the device, the volumetric material loss from Exeter femoral head tapers was, in general, small compared with that from larger-diameter MoM head tapers. Cite this article: Bone Joint J 2018;100-B:1310-9.

Dual-taper modular hip implant: Investigation of 3-dimensional surface scans for component contact, shape, and fit

Background

The etiology of wear particle generation and subsequent corrosion in modular total hip arthroplasty implants likely begins with mechanical fretting. The purpose of this study was to determine geometric features of the male and female taper surfaces that drive stability within the neck-stem junction.

Methods

Eighteen modular hip components received 3-dimensional surface scans to examine the neck-stem taper junction using an optical scanner. The normal distance between the surfaces of the neck taper as seated in the stem slot was measured and produced a color map of the contact proximity. Contour plots identified surface shape variation and contact. Angle measurements and neck seated depth were analyzed by regression.

Results

The typical features observed were (1) a vertical line of contact at one end of the transition from the flat surface to the radius surface; (2) a vertical line of contact in the radius surface just past the centerline; (3) a concavity along the flat surface between the neck and stem components; and (4) one of the neck flat surfaces was closer to its mating surface on the stem. The seated depth of the neck was dependent on the taper angles in the flat section of the neck (R² = 0.5000, P = .0332).

Conclusions

The shape of the neck and stem tapers deviate from ideal design dimensions, contributing to relative motions between the neck and stem. While these processes are not proven to directly cause implant failure, they may place the implants at higher risk for failure.

Effects of Seating Load Magnitude on Incremental Cyclic Fretting Corrosion in 5°40' Mixed Alloy Modular Taper Junctions

BACKGROUND

Mechanically assisted crevice corrosion of modular tapers continues to be a concern in total joint arthroplasties. A surgical factor that may affect taper fretting corrosion during cyclic loading is seating load magnitude. In this study, modular head-neck taper junctions were seated, capturing load-displacement, over a range of axially oriented loads, and electrochemical and micromotion data were captured during short-term incremental cyclic fretting corrosion (ICFC) tests. The hypothesis is low seating loads result in greater motion and fretting corrosion in ICFC tests. The effect of assembly load on pull-off force post-ICFC testing was also evaluated.

METHODS

The study employed custom-built test fixtures which measured head-neck micromotion and an electrochemical chamber to monitor electrochemical reactions. Head-neck motion measurements were captured using 2 noncontact differential variable reluctance transducers mounted to the head. Seating experiments ranged from 1000 to 8000 N.

RESULTS

Significant differences due to seating loads were reported in seating displacement, ICFC subsidence, and fretting current at 4000 N cyclic load. Seating load decreased but did not eliminate currents. Fretting onset load remained fixed (approximately 1200 N) for tapers seated above 2000 N. Fretting subsidence was negligible for seating loads of 4000 N or higher, and increased subsidence was observed below 4000 N.

CONCLUSION

This short-term test method evaluated the acute performance of modular implants which were assembled under various loads and demonstrated the link between seating loads, fretting motions, and electrochemical reactions. While increased seating loads reduced fretting corrosion and taper subsidence, it did not prevent fretting corrosion even at 8 kN seating.

The onset of fretting at the head-stem connection in hip arthroplasty is affected by head material and trunnion design under simulated corrosion conditions

Mechanically assisted crevice corrosion (MACC) is a mechanism for trunnion damage in total hip arthroplasties (THAs). Retrieval studies have shown reduced MACC-related damage for ceramic heads compared with cobalt-chromium (CoCr) heads. We propose that ceramic heads demonstrate fretting at higher cyclic compressive loads than CoCr heads on titanium alloy trunnions in a simulated corrosion model. A closed electrochemical chamber was used to measure fretting current onset loads for two modern titanium alloy trunnions (Zimmer 12/14 and Stryker V-40) in which trunnion failure has been reported. Ceramic and CoCr alloy 36 + 0 mm heads were impacted on each trunnion and cyclically loaded at 3 Hz with increasing magnitude from 100 to 3,400 N for 540 cycles. Onset load was the cyclic compressive load at which the slope of the average fretting current increased significantly. A CoCr head with V40 trunnion demonstrated the lowest onset load (1,400 N), while the V40 trunnion with a ceramic head showed the highest onset load (2,200 N). Significant differences occurred in average fretting current between head materials for V40 trunnions (p < 0.001) at loads over 2,000 N. CoCr-12/14 and ceramic-12/14 couples demonstrated similar onset loads (2,000 N). All head-trunnion combinations showed cyclical fretting response to loading at 100 N. Head material composition was observed to increase fretting at the taper junction but the effect was taper geometry dependent. Using ceramic heads may reduce the phenomena of trunnion fretting and corrosion but the effect of both trunnion geometry and metallurgy warrants further investigation. Statement of clinical significance: Trunnion corrosion may occur with titanium alloy stems regardless of the head material used. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1630-1636, 2018.

Investigation of nano-sized debris released from CoCrMo secondary interfaces in total hip replacements: Digestion of the flakes

The in vivo release of wear debris and corrosion products from the metallic interfaces of total hip replacements is associated with a wide spectrum of adverse body reactions and systemic manifestations. The origin of debris and the electrochemical conditions at the sites of material loss both play a role in determining the physicochemical characteristics of the particles, and thus influence their in vivo reactivity. Debris retrieved from revised CoCrMo tapers and cement-stem interfaces consists of heterogeneous flakes that comprise mechanically mixed metal particles, corrosion products and organic material. Detailed investigation of the size and composition of the metal debris contained within these composites requires the digestion of the flakes to release the small metal particles. Here, we compare alkaline and enzymatic digestion methods that both aim to fragment the flakes and reveal their smallest building blocks. The characterization of debris cleaned with both methods revealed crystalline Cr oxide nanoparticles and clusters. Comparison between the treatments showed that the alkaline method is more efficient in fragmenting the flakes and provided cleaner and generally smaller nanoparticles than exhibited in debris released with the enzymatic treatment. The provision of cleaner nanoparticles from the alkaline method also allows the physicochemical properties of the particles to be more clearly identified. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 107B: 424-434, 2019.

Maximizing the fixation strength of modular components by impaction without tissue damage

Objectives

Taper junctions between modular hip arthroplasty femoral heads and stems fail by wear or corrosion which can be caused by relative motion at their interface. Increasing the assembly force can reduce relative motion and corrosion but may also damage surrounding tissues. The purpose of this study was to determine the effects of increasing the impaction energy and the stiffness of the impactor tool on the stability of the taper junction and on the forces transmitted through the patient’s surrounding tissues.

Methods

A commercially available impaction tool was modified to assemble components in the laboratory using impactor tips with varying stiffness at different applied energy levels. Springs were mounted below the modular components to represent the patient. The pull-off force of the head from the stem was measured to assess stability, and the displacement of the springs was measured to assess the force transmitted to the patient’s tissues.

Results

The pull-off force of the head increased as the stiffness of the impactor tip increased but without increasing the force transmitted through the springs (patient). Increasing the impaction energy increased the pull-off force but also increased the force transmitted through the springs.

Conclusions

To limit wear and corrosion, manufacturers should maximize the stiffness of the impactor tool but without damaging the surface of the head. This strategy will maximize the stability of the head on the stem for a given applied energy, without influencing the force transmitted through the patient’s tissues. Current impactor designs already appear to approach this limit. Increasing the applied energy (which is dependent on the mass of the hammer and square of the contact speed) increases the stability of the modular connection but proportionally increases the force transmitted through the patient’s tissues, as well as to the surface of the head, and should be restricted to safe levels.

Cite this article: A. Krull, M. M. Morlock, N. E. Bishop. Maximizing the fixation strength of modular components by impaction without tissue damage. Bone Joint Res 2018;7:196–204. DOI: 10.1302/2046-3758.72.BJR-2017-0078.R2.

The seating mechanics of head-neck modular tapers in vitro: Load-displacement measurements, moisture, and rate effects

The mechanically assisted crevice corrosion performance of head-neck modular tapers is a significant concern in orthopedic biomaterials. Fretting crevice corrosion processes in modular tapers are thought to be influenced by a wide array of factors including seating mechanics of the junction, hence there is a need for in vitro test methods that can assess their performance. This study presented a test method to directly measure the load-displacement seating mechanics of modular tapers and used this method to compare the seating mechanics for different tapers, moisture, seating loads and seating rates. Seating mechanics were explored whereby the instantaneous load-displacement behavior of the head seating onto the neck is captured and used to define the mechanics of seating. Two distinct taper design/material combinations were assembled wet or dry using axially applied loads (500, 1,000, 2,000, and 4,000 N) at two loading rates of 100 and 10⁴  N/s (n = 5 for each condition) using a servohydraulic test frame. The results showed that pull-off strength scaled with seating load and ranged between 43% and 68% of seating load depending on sample and wetness. Tapers seated wet had higher pull-off strengths (2,200 ± 300 N) than those seated dry (1,800 ± 200 N, p < 0.05). Seating mechanics (load-displacement plots) varied due to sample type and due to wetness with differences in seating energy, seating stiffness, and seating displacement. These results show the detailed mechanics of seating during assembly and provide significant insight into the complex interplay of factors associated with even "ideal" seating (axial, quasistatic) loading. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1164-1172, 2018.

Fretting-corrosion at the modular tapers interface: Inspection of standard ASTM F1875-98

Interest in the degradation mechanisms at the modular tapers interfaces has been renewed due to increased reported cases of adverse reactions to metal debris and the appearance of wear and corrosion at the modular tapers interfaces at revision. Over the past two decades, a lot of research has been expended to understand the degradation mechanisms, with two primary implant loading procedures and orientations used consistently across the literature. ASTM F1875-98 is often used as a guide to understand and benchmark the tribocorrosion processes occurring within the modular tapers interface. This article presents a comparison of the two methods outlined in ASTM F1875-98 as well as a critique of the standard considering the current paradigm in pre-clinical assessment of modular tapers.

Bead debonding from a modern cementless total hip femoral component with concomitant taper corrosion

We report a case of bead shedding from a cylindrical extensively porous-coated cementless femoral component with concomitant taper corrosion at the modular head-neck junction of a metal-on-polyethylene total hip prosthesis. The patient presented with chronic thigh pain 4 years after primary total hip arthroplasty, and radiographs revealed significant osteolysis and metallic debris around the femoral stem. Intraoperatively, the patient had a grossly loose femoral component with debonding of sintered beads from the femoral stem, as well as evidence of taper corrosion. We identify a failure of a modern beaded femoral component in conjunction with taper corrosion.

Evidence based recommendations for reducing head-neck taper connection fretting corrosion in hip replacement prostheses

INTRODUCTION

This systematic review seeks to summarise the published studies investigating prosthetic design, manufacture and surgical technique's effect on fretting corrosion at the head-neck taper connection, and provide clinical recommendations to reduce its occurrence.

METHODS

PubMed, MEDLINE and EMBASE electronic databases were searched using the terms taper, trunnion, cone and head-neck junction. Articles investigating prosthetic design, manufacture and surgical technique's effect on fretting corrosion were retrieved, reviewed and graded according to OCEBM levels of evidence and grades of recommendation.

RESULTS

The initial search yielded 1,224 unique articles, and 91 were included in the analysis.

CONCLUSIONS

There is fair evidence to recommend against the use of high offset femoral heads, larger diameter femoral heads, and to pay particular consideration to fretting corrosion's progression with time and risk with heavier or more active patients. Particular to metal-on-metal hip prostheses, there is fair evidence to recommend positioning the acetabular component to minimise edge loading. Particular to metal-on-polyethylene hip prostheses, there is fair evidence to recommend the use of ceramic femoral heads, against use of cast cobalt alloy femoral heads, and against use of low flexural rigidity femoral stems. Evidence related to taper connection design is largely conflicting or inconclusive. Head-neck taper connection fretting corrosion is a multifactorial problem. Strict adherence to the guidelines presented herein does not eliminate the risk. Prosthesis selection is critical, and well-controlled studies to identify each design parameter's relative contribution to head-neck taper connection fretting corrosion are required.

Modularity in Total Hip Arthroplasty: Benefits, Risks, Mechanisms, Diagnosis, and Management

Modular implants are currently widely used in total hip arthroplasty because they give surgeons versatility during the operation, allow for easier revision surgery, and can be adjusted to better fit the anatomy of the specific patient. However, modular implants, specifically those that have metal-on-metal junctions, are susceptible to crevice and fretting corrosion. This can ultimately cause implant failure, inflammation, and adverse local tissue reaction, among other possible side effects. Surgeons should be aware of the possibility of implant corrosion and should follow a set of recommended guidelines to systematically diagnose and treat patients with corroded implants. Ultimately, surgeons will continue to use modular implants because of their widespread benefits. However, more research is needed to determine how to minimize corrosion and the negative side effects that have been associated with modular junctions in total hip arthroplasty. [Orthopedics. 2017; 40(6):355-366.].

Alarmingly High Rate of Implant Fractures in One Modular Femoral Stem Design: A Comparison of Two Implants

BACKGROUND

Reports of implant fracture at the modular junction have been seen in modular neck designs, stem-sleeve modular femoral stems, and diaphyseal engaging bi-body modular stems. To date, however, there has never been a direct comparison between 2 different implant designs from the same modular family. The purpose of this study is to compare the rate of implant failure of 2 such stem-sleeve modular femoral stem designs, the S-ROM and Emperion, to further identify factors which increase the risk of this mode of failure.

METHODS

A retrospective, single surgeon, review of our institutional database was performed to compare the 2 groups of patients.

RESULTS

A total of 1168 total hip arthroplasty procedures were included in our analysis, 547 (47%) with Emperion and 621 (53%) with S-ROM. Eight (1.5%) fractures in 7 patients occurred in the Emperion group compared to 1 (0.2%) fracture in the S-ROM group (P = .015).

CONCLUSION

The precise cause of the stem fractures in our study remains unknown and is likely multifactorial. Given the unexpectedly high rate of catastrophic implant failures in the form of stem fracture at the stem-sleeve junction, we recommend more judicious use of modularity in primary total hip arthroplasty.

The Influence of Contamination and Cleaning on the Strength of Modular Head Taper Fixation in Total Hip Arthroplasty

BACKGROUND

Intraoperative interface contamination of modular head-stem taper junctions of hip implants can lead to poor fixation strength, causing fretting and crevice corrosion or even stem taper fracture. Careful cleaning before assembly should help to reduce these problems. The purpose of this study was to determine the effect of cleaning (with and without drying) contaminated taper interfaces on the taper fixation strength.

METHODS

Metal or ceramic heads were impacted onto titanium alloy stem tapers with cleaned or contaminated (fat or saline solution) interfaces. The same procedure was performed after cleaning and drying the contaminated interfaces. Pull-off force was used to determine the influence of contamination and cleaning on the taper strength.

RESULTS

Pull-off forces after contamination with fat were significantly lower than those for uncontaminated interfaces for both head materials. Pull-off forces after application of saline solution were not significantly different from those for uncontaminated tapers. However, a large variation in taper strength was observed, pull-off forces for cleaned and dried tapers were similar to those for uncontaminated tapers for both head materials.

CONCLUSION

Intraoperative contamination of taper interfaces may be difficult to detect but has a major influence on taper fixation strength. Cleaning of the stem taper with saline solution and drying with gauze directly before assembly allows the taper strength of the pristine components to be achieved. Not drying the taper results in a large variation in pull-off forces, emphasizing that drying is essential for sufficient and reproducible fixation strength.

Excellent mid-term results of a new polished tapered modular cemented stem: a study of 113 hip replacements with minimum 5-year follow-up

BACKGROUND

This prospective cohort study reports the medium-term clinical and radiographic results of 113 hips treated with a hybrid total hip replacement (THR) with a new cemented tapered cobalt-chrome (Co-Cr) stem with a titanium (Ti) modular neck (ProfemurXm®).

METHOD

Between October 2008 and December 2010 we performed 115 consecutive hybrid THR with the ProfemurXm® in 105 patients.

RESULTS

Survivorship of the implant (stem and modular neck) at a mean of 6.5 years (min 5-max 8) was 100% with the endpoint revision for any reason. No implant was at risk for revision or showed signs of loosening. The mean Harris Hip Score was 89/100, mean Oxford Hip Score was 43/48, mean WOMAC was 91/100. No patient had thigh pain, no patient reported squeaking. There were no dislocations in this cohort. No implant showed development of radiolucent lines (RLL), either at the stem-cement or cement-bone interface. No hip showed osteolysis or calcar resorption. The mean femoral subsidence of the stem within the cement mantel was 0.31 mm (range 0-0.6 mm) after 6.5 years. With the use of this modular stem, 93% of hips showed no measurable leg length difference after THR, and leg length could be restored within a 5-mm limit in 99% of hips.

CONCLUSIONS

The mid-term results of this new polished stem were excellent, without adverse effects from the use of modularity.

Effects of hip implant modular neck material and assembly method on fatigue life and distraction force

Hip implant neck fractures and adverse tissue reactions associated with fretting-corrosion damage at modular interfaces are a major source of concern. Therefore, there is an urgent clinical need to develop accurate in vitro test procedures to better understand, predict and prevent in vivo implant failures. This study aimed to simulate in vivo fatigue fracture and distraction of modular necks in an in vitro setting, and to assess the effects of neck material (Ti6Al4V vs. CoCrMo) and assembly method (hand vs. impact) on the fatigue life and distraction of the necks. Fatigue tests were performed on the cementless PROFEMUR® Total Hip Modular Neck System under two different loads and number of cycles: 2.3 kN for 5 million cycles, and 7.0 kN for 1.3 million cycles. The developed in vitro simulation setup successfully reproduced in vivo modular neck fracture mode and location. Neck failure occurred at the neck-stem taper and the fracture ran from the distal lateral neck surface to the proximal medial entry point of the neck into the stem. None of the necks failed under the 2.3 kN load. However, all hand-assembled Ti6Al4V necks failed under the 7.0 kN load. In contrast, none of the hand-assembled CoCrMo necks and impact-assembled necks (Ti6Al4V or CoCrMo) failed under this higher load. In conclusion, Ti6Al4V necks were more susceptible to fatigue failure than CoCrMo necks. In addition, impact assembly substantially improved the fatigue life of Ti6Al4V necks and also led to overall higher distraction forces for both neck materials. Overall, this study shows that the material and assembly method can affect the fatigue strength of modular necks. Finally, improper implant assembly during surgery may result in diminished modular neck survivability and increased failure rates. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:2023-2030, 2017.

[Endoprostheses in the elderly : Biomaterials, implant selection and fixation technique]

The replacement of hip and knee joints is one of the greatest success stories in orthopedics. Due to continuous improvement of biomaterials and implant design, patient-associated problems are now mostly multifactorial and only rarely caused by the implant. Abrasion was significantly reduced by the introduction of highly cross-linked polyethylene (PE), antioxidant stabilized PE, new ceramics and the development of ceramic and protective surfaces. It is assumed that further reduction of frictional resistance will not lead to a significantly better clinical result: however, the problem of periprosthetic infections and implant-related incompatibility is still unsolved and remains challenging for biomaterial research. For the knee joint PE will be irreplaceable for joint articulation even in the future due to the contact situation. Mobile bearings and fixed bearings are two established successful philosophies, which have shown comparably good clinical results. For the hip joint, it is forecasted that ceramic-on-ceramic will be the system of the future if the correct positioning and mounting of the components can be solved so that the problems, such as development of noise and breakage can be reduced to a minimum. An in-depth understanding and detailed knowledge of the biomaterials by the surgeon can prevent implant-related problems. For elderly patients it is assumed that the economic burden on the public healthcare system will have the strongest impact on implant selection.

Assessment of Corrosion, Fretting, and Material Loss of Retrieved Modular Total Knee Arthroplasties

BACKGROUND

Modular junctions in total hip arthroplasties have been associated with fretting, corrosion, and debris release. The purpose of this study is to analyze damage severity in total knee arthroplasties of a single design by qualitative visual assessment and quantitative material loss measurements to evaluate implant performance and patient impact via material loss.

METHODS

Twenty-two modular knee retrievals of the same manufacturer were identified from an institutional review board-approved database. Junction designs included tapers with an axial screw and tapers with a radial screw. Constructs consisted of 2 metal alloys: CoCr and Ti6Al4V. Components were qualitatively scored and quantitatively measured for corrosion and fretting. Negative values represent adhered material. Statistical differences were analyzed using sign tests. Correlations were tested with a Spearman rank order test (P < .05).

RESULTS

The median volumetric material loss and the maximum linear depth for the total population were -0.23 mm³ and 5.84 μm, respectively. CoCr components in mixed metal junctions had higher maximum linear depth (P = .007) than corresponding Ti components. Fretting scores of Ti6Al4V alloy components in mixed metal junctions were statistically higher than the remaining groups. Taper angle did not correlate with material loss.

CONCLUSION

Results suggest that CoCr components in mixed metal junctions are more vulnerable to corrosion than other components, suggesting preferential corrosion when interfacing with Ti6Al4V. Overall, although corrosion was noted in this series, material loss was low, and none were revised for clinical metal-related reaction. This suggests the clinical impact from corrosion in total knee arthroplasty is low.

In Vivo Damage of the Head-Neck Junction in Hard-on-Hard Total Hip Replacements: Effect of Femoral Head Size, Metal Combination, and 12/14 Taper Design

Recently, concerns have been raised about the potential effect of head-neck junction damage products at the local and systemic levels. Factors that may affect this damage process have not been fully established yet. This study investigated the possible correlations among head-neck junction damage level, implant design, material combination, and patient characteristics. Head-neck junctions of 148 retrieved implants were analysed, including both ceramic-on-ceramic (N = 61) and metal-on-metal (N = 87) bearings. In all cases, the male taper was made of titanium alloy. Damage was evaluated using a four-point scoring system based on damage morphology and extension. Patient age at implantation, implantation time, damage risk factor, and serum ion concentration were considered as independent potential predicting variables. The damage risk factor summarises head-neck design characteristics and junction loading condition. Junction damage correlated with both implantation time and damage factor risk when the head was made of ceramic. A poor correlation was found when the head was made of cobalt alloy. The fretting-corrosion phenomenon seemed mainly mechanically regulated, at least when cobalt alloy components were not involved. When a component was made of cobalt alloy, the role of chemical phenomena increased, likely becoming, over implantation time, the damage driving phenomena of highly stressed junctions.

Influence of the compliance of a patient's body on the head taper fixation strength of modular hip implants

BACKGROUND

The strength of the modular fixation between head and stem taper of total hip replacement implants should be sufficient to minimise relative motion and prevent corrosion at the interface. Intraoperatively the components are assembled by impaction with a hammer. It is unclear whether the effective compliance of the patient's body modifies the strength of the taper interface under impaction assembly. The purpose of this study was to assess the influence of the compliance of the patient's body on the taper fixation strength.

METHODS

Cobalt-chrome and ceramic femoral heads were assembled with titanium alloy stem tapers in the laboratory under impaction. Impaction forces were applied with a constant energy, defined by the drop height of the impactor, according to standard experimental procedure. The compliance of the patient was simulated in the laboratory by varying the stiffness of springs mounted below the stem taper. Pull-off forces between head and neck were measured to determine fixation strength.

FINDINGS

Decreasing spring stiffness had no effect on the applied peak impaction forces during assembly or on the pull-off forces. Pull-off forces showed no difference between metal and ceramic head materials.

INTERPRETATION

Pull-off forces and impaction forces were independent of the spring stiffness below the stem taper, indicating that the compliance of the patient has no effect on the taper fixation strength. Impaction testing in the laboratory can therefore be performed under rigid fixation, without accounting for the compliance of the patient.

The taper corrosion pattern observed for one bi-modular stem design is related to geometry-determined taper mechanics

Bi-modular primary hip stems exhibit high revision rates owing to corrosion at the stem-neck taper, and are associated with local adverse tissue reactions. The aim of this study was to relate the wear patterns observed for one bi-modular design to its design-specific stem-neck taper geometry. Wear patterns and initial geometry of the taper junctions were determined for 27 retrieved bi-modular primary hip arthroplasty stems (Rejuvenate, Stryker Orthopaedics) using a tactile coordinate-measuring device. Regions of high-gradient wear patterns were additionally analyzed via optical and electron microscopy. The determined geometry of the taper junction revealed design-related engagement at its opening (angle mismatch), concentrated at the medial and lateral apexes (axes mismatch). A patch of retained topography on the proximal medial neck-piece taper apex was observed, surrounded by regions of high wear. On the patch, a deposit from the opposing female stem taper-containing Ti, Mo, Zr, and O-was observed. High stress concentrations were focused at the taper apexes owing to the specific geometry. A medial canting of the components may have augmented the inhomogeneous stress distributions in vivo. In the regions with high normal loads interfacial slip and consequently fretting was inhibited, which explains the observed pattern of wear.

A combined experimental and finite element approach to analyse the fretting mechanism of the head-stem taper junction in total hip replacement

Fretting corrosion at the taper interface of modular hip implants has been implicated as a possible cause of implant failure. This study was set up to gain more insight in the taper mechanics that lead to fretting corrosion. The objectives of this study therefore were (1) to select experimental loading conditions to reproduce clinically relevant fretting corrosion features observed in retrieved components, (2) to develop a finite element model consistent with the fretting experiments and (3) to apply more complicated loading conditions of activities of daily living to the finite element model to study the taper mechanics. The experiments showed similar wear patterns on the taper surface as observed in retrievals. The finite element wear score based on Archard’s law did not correlate well with the amount of material loss measured in the experiments. However, similar patterns were observed between the simulated micromotions and the experimental wear measurements. Although the finite element model could not be validated, the loading conditions based on activities of daily living demonstrate the importance of assembly load on the wear potential. These findings suggest that finite element models that do not incorporate geometry updates to account for wear loss may not be appropriate to predict wear volumes of taper connections.

The unstable total hip arthroplasty

One of the most common causes for revision surgery following total hip arthroplasty (THA) is dislocation.

Dislocation is associated with a considerable amount of suffering and risks for the patient, and extra costs for the health care system.

Compared with degenerative arthritis, the dislocation rate is doubled for avascular necrosis and multiplied by three times for congenital dislocation, four for fracture, five for nonunion, malunion or a failed hip arthroplasty, and eleven times after surgery for prosthetic instability.

In analysing instability the cause may be assessed as 1) locally caused within the hip with explanatory radiographic findings, 2) locally caused without explanatory radiographic findings or 3) non-locally caused, i.e. non-compliant patient, neuromuscular or cognitive disorders.

Revision strategies for instability are typically directed to correct the underlying aetiology, but also to strive for an upsizing of the head and liner.

Cite this article: Ullmark G. The unstable total hip arthroplasty. EFORT Open Rev 2016;1:83-88. DOI: 10.1302/2058-5241.1.000022.

Fretting corrosion of CoCr alloy: Effect of load and displacement on the degradation mechanisms

Fretting corrosion of medical devices is of growing concern, yet, the interactions between tribological and electrochemical parameters are not fully understood. Fretting corrosion of CoCr alloy was simulated, and the components of damage were monitored as a function of displacement and contact pressure. Free corrosion potential (E_corr), intermittent linear polarisation resistance and cathodic potentiostatic methods were used to characterise the system. Interferometry was used to estimate material loss post rubbing. The fretting regime influenced the total material lost and the dominant degradation mechanism. At high contact pressures and low displacements, pure corrosion was dominant with wear and its synergies becoming more important as the contact pressure and displacement decreased and increased, respectively. In some cases, an antagonistic effect from the corrosion-enhanced wear contributor was observed suggesting that film formation and removal may be present. The relationship between slip mechanism and the contributors to tribocorrosion degradation is presented.