Enhanced wear and corrosion in modular tapers in total hip replacement is associated with the contact area and surface topography

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.

What Patient and Implant Factors Affect Trunnionosis Severity? An Implant Retrieval Analysis of 664 Femoral Stems

BACKGROUND

Corrosion at the modular head-neck taper interface of total and hemiarthroplasty hip implants (trunnionosis) is a cause of implant failure and thus a clinical concern. Patient and device factors contributing to the occurrence of trunnionosis have been investigated in prior implant retrieval studies but generally with limited sample sizes and a narrow range of models. The purpose of the present investigation was to determine which patient and device factors were associated with corrosion damage on the femoral stem taper across a large collection of different implant models retrieved following revision hip arthroplasty.

METHODS

A retrieval study of 664 hip arthroplasty modular stem components was performed. Patient and device information was collected. Trunnions were imaged under digital microscopy and scored for corrosion damage using a scaling system. Damage was related to patient and device factors using regression analyses.

RESULTS

Greater duration of implantation (P = .005) and larger head size (P < .001) were associated with an elevated corrosion class. Older age at index surgery (P = .035), stainless steel stem material (P = .022), indication for revision as bone or periprosthetic fracture (P = .017), and infection (P = .018) and certain larger taper geometries were associated with a decreased corrosion class.

CONCLUSION

Factors identified as contributing to a higher or lower risk of more severe corrosion are consistent with most prior smaller retrieval studies. Surgeons should be aware of these risk factors when selecting implants for their patients and when diagnosing trunnionosis in symptomatic hip arthroplasty patients.

Metal-on-Metal Hips: Ten-Year Clinical and Radiographic Outcomes of the ADEPT Metal-on-Metal Hip Resurfacing and Modular Total Hip Arthroplasty

BACKGROUND

The aim of this study is to update the 10-year follow-up survivorship and metal ions levels of a cohort of metal-on-metal (MoM) hip resurfacing (HR) and large-diameter-head (LDH) total hip arthroplasty (THA).

METHODS

The study is a retrospective analysis of prospectively collected data that compared the outcomes of 24 MoM HR (21 patients) and 15 (11 patients) modular LHD MoM THA at >10 years follow-up. Baseline characteristics as well as intraoperative and postoperative information were collected, including complications, revisions, clinical and radiographic outcomes, and serum metal ions level (Cobalt, Chromium). Metal ion levels were compared using a two-tailed unpaired t-test and Wilcoxon signed-rank test (jamovi v2.3.3.0, Sydney, NSW, AU).

RESULTS

No significant differences were detected in gender, BMI, and ASA score between the two groups. Patients in the modular THA group were significantly older (57 years vs. 46 years; p < 0.05). The HR overall survivorship was 91.7% (22 of 24 hips) with survivorship from implant failure and/or aseptic loosening and/or metal debris related 100% of problems. The modular THA overall survivorship was 86.7% (13 of 15 hips) with survivorship from implant aseptic loosening and metal ions complications of 93.4% (14 of 15 hips). No significant difference was noted when comparing clinical outcomes. Metal ions were significantly lower in the HR group (Co 25.8 nmol/L vs. 89 nmol/L; p < 0.001-Cr 33.5 nmol/L vs. 55.2 nmol/L; p = 0.026).

CONCLUSION

Both implants reported excellent and comparable clinical outcomes at >10 years follow-up. The Adept HR reported remarkable survivorship, in line with the registry data, proving once again its reliability in young active males. The modular LDH THA, despite being discontinued, presented higher reliability and a lower failure rate when compared with similar withdrawn MoM implants. Trunnionosis did not appear to be a significant problem in this particular modular design.

An Overview of the Stability and Fretting Corrosion of Microgrooved Necks in the Taper Junction of Hip Implants

Fretting corrosion at the head-neck interface of modular hip implants, scientifically termed trunnionosis/taperosis, may cause regional inflammation, metallosis, and adverse local tissue reactions. The severity of such a deleterious process depends on various design parameters. In this review, the influence of surface topography (in some cases, called microgrooves/ridges) on the overall performance of the microgrooved head-neck junctions is investigated. The methodologies together with the assumptions and simplifications, as well as the findings from both the experimental observations (retrieval and in vitro) and the numerical approaches used in previous studies, are presented and discussed. The performance of the microgrooved junctions is compared to those with a smooth surface finish in two main categories: stability and integrity; wear, corrosion, and material loss. Existing contradictions and disagreements among the reported results are reported and discussed in order to present a comprehensive picture of the microgrooved junctions. The current research needs and possible future research directions on the microgrooved junctions are also identified and presented.

Model validation for estimating taper microgroove deformation during total hip arthroplasty head-neck assembly

Total hip arthroplasty (THA) failure and the need for revision surgery can result from fretting-corrosion damage of the head-neck modular taper junctions. Prior work has shown that implant geometry, such as microgrooves, influences damage on retrieved implants. Microgroove deformation within the modular taper junction occurs when the female head taper meets the male stem taper during THA surgical procedure. The objective of this work was to validate microgroove deformation after head-neck THA assembly as calculated by finite element analysis (FEA). Four 28 mm CoCrMo head tapers and four Ti6Al4V stem tapers were scanned via white light interferometry. Heads were assembled onto stem tapers until 6kN reaction force was achieved, followed by head removal using a cut-off machine. The stem tapers were then rescanned and analyzed. Simultaneously, a 2D axisymmetric FEA model was developed and assembled per implant geometries and experimental data. For experiments and FEA, the mean change in microgroove height was 1.23 µm and 1.40 µm, respectively. The largest microgroove height change occurred on the proximal stem taper due to the conical angles of the head and stem tapers. FEA showed that the head-stem assembly induced high stresses and microgroove peaks flattening. 76-89% and 91-100% of the microgrooves in the experiments and FEA, respectively, showed height changes along the contact length of the stem taper. A validated FEA model of THA head-neck modular junction contact mechanics is essential to identifying implant geometries and surface topographies that can potentially minimize the risk of fretting and fretting-corrosion at modular junctions.

Stand up! Are normal weight-bearing forces sufficient for a 12/14 Morse taper locking in total hip arthroplasty?

BACKGROUND

The aim of this study was to investigate total hip arthroplasty (THA) Morse taper pull-off strengths after impaction prior to cyclical loading compared to cyclical loading alone. The practical relevance of the experiment is to provide a perspective on what may be clinically satisfactory taper assembly given the spectrum of head tapping patterns used by surgeons, as well as compare traditional impaction performed in standard THA with alternate methods of taper engagement such as 'in situ assembly' used in micro-invasive techniques.

METHODS

36 taper constructs utilising a combination of cobalt-chrome alloy and ceramic-titanium alloy junctions were investigated in vitro in wet and dry conditions with cyclical loading of the constructs. Taper disengagement strengths with and without impaction were compared. Secondary investigation of the surface roughness of the heads and tapers was also assessed.

RESULTS

An impaction to a wet taper resulted in a greater pull off force compared to a dry taper with a CoCr head and taper combination. Impacting the head and dryness of the taper did not affect pull off forces of a ceramic femoral head on titanium taper. Pulling a head off a taper significantly alters the head surface roughness.

CONCLUSION

Impaction of a taper does not provide any benefit over cyclical loading of a taper assembly alone for pull-off strength.

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.

Intraoperative assembly of anatomical shoulder prosthesis frequently results in malalignment of the modular taper junction

Anatomical shoulder arthroplasties (ASA) may fail because of micromotion at the modular taper junction causing wear due to fretting. Sufficient taper strength can reduce micromotion and potential reasons for failure. However, there are no normative standards for a safe assembly process performed intraoperatively by the surgeon. The purpose of this study is to determine the effect of common intraoperative assembly strategies and to identify critical influencing factors on taper stability. ASA with standard and stemless humeral component in combination with concentric Al₂ O₃ heads and eccentric CoCr28Mo6 alloyed humeral heads were tested. Taper angles and surface roughness were determined. Force magnitudes and impact directions were recorded using a sensorized head impactor and a three-dimensional force measuring platform. Subsequently, the axial pull-off forces were measured and taper engagement areas were macroscopically evaluated. In comparison to standard stem tapers that were impacted with an assembly device, stemless tapers were impacted into the artificial bone with significantly lower forces. Taper strength correlates to maximum impact force and was higher for CoCr28Mo6 heads with a mean pull-off ratio of 0.56 than for Al₂ O₃ heads with 0.37. Interestingly, all tapers showed an asymmetric clamping behavior, due to tilting during impaction. This is caused by the variation of the resulting force vector and further promoted by humeral head eccentricity. Assembly technique markedly influences the force magnitude, impact direction, impulse, and consequently taper strength. The resulting force vector and head eccentricity were identified as potential risk factors for taper malalignment.

An integrated benefit-risk assessment of cobalt-containing alloys used in medical devices: Implications for regulatory requirements in the European Union

In 2017, the European Union (EU) Committee for Risk Assessment (RAC) recommended the classification of metallic cobalt (Co) as Category 1B with respect to its carcinogenic and reproductive hazard potential and Category 2 for mutagenicity but did not evaluate the relevance of these classifications for patients exposed to Co-containing alloys (CoCA) used in medical devices. CoCA are inherently different materials from Co metal from a toxicological perspective and thus require a separate assessment. CoCA are biocompatible materials with a unique combination of properties including strength, durability, and a long history of safe use that make them uniquely suited for use in a wide-range of medical devices. Assessments were performed on relevant preclinical and clinical carcinogenicity and reproductive toxicity data for Co and CoCA to meet the requirements under the EU Medical Device Regulation triggered by the ECHA re-classification (adopted in October 2019 under the 14th Adaptation to Technical Progress to CLP) and to address their relevance to patient safety. The objective of this review is to present an integrated overview of these assessments, a benefit-risk assessment and an examination of potential alternative materials. The data support the conclusion that the exposure to CoCA in medical devices via clinically relevant routes does not represent a hazard for carcinogenicity or reproductive toxicity. Additionally, the risk for the adverse effects that are known to occur with elevated Co concentrations (e.g., cardiomyopathy) are very low for CoCA implant devices (infrequent reports often reflecting a unique catastrophic failure event out of millions of patients) and negligible for CoCA non-implant devices (not measurable/no case reports). In conclusion, the favorable benefit-risk profile also in relation to possible alternatives presented herein strongly support continued use of CoCA in medical devices.

Innovative Coatings of Metallic Alloys Used as Bioactive Surfaces in Implantology: A Review

Metallic implants are widely used in the field of implantology, but there are still problems leading to implant failures due to weak osseointegration, low mechanical strength for the implant, inadequate antibacterial properties, and low patient satisfaction. Implant failure can be caused by bacterial infections and poor osteointegration. To improve the implant functionalization, many researchers focus on surface modifications to prepare the proper physical and chemical conditions able to increase biocompatibility and osteointegration between implant and bone. Improving the antibacterial performance is also a key factor to avoid the inflammation in the human body. This paper is a brief review for the types of coatings used to increase osseointegration and biocompatibility for the successful use of metal alloys in the field of implantology.

Variability in stem taper surface topography affects the degree of corrosion and fretting in total hip arthroplasty

Degradation at the modular head-neck interface in total hip arthroplasty (THA) is predominately expressed in the form of corrosion and fretting, potentially causing peri-prosthetic failure by adverse reactions to metal debris. This retrieval study aimed to quantify variations in stem taper surface topographies and to assess the influence on the formation of corrosion and/or fretting in titanium alloy stem tapers combined with metal and ceramic heads. Four hip stem designs (Alloclassic, CLS, Bicontact and SL-Plus) were characterized using high-resolution 3D microscopy, and corrosion and fretting were rated using the Goldberg scoring scheme. Quantification of the taper surface topographies revealed a high variability in surface characteristics between threaded stem tapers: Alloclassic and CLS tapers feature deeply threaded trapezoid-shaped profiles with thread heights over 65 µm. The sawtooth-shaped Bicontact and triangular SL-Plus taper are characterized by low thread heights below 14 µm. Significantly lower corrosion and fretting scores were observed in lightly threaded compared to deeply threaded tapers in ceramic head combinations. No significant differences in corrosion or fretting scores with thread height were found in pairings with metal heads. Understanding the relationship between stem taper surface topography and the formation of corrosion and fretting could help to improve the performance of modern THAs and lead to longer-lasting clinical results.

The mechanics of head-neck taper junctions: What do we know from finite element analysis?

Modular hip implants are widely used in hip arthroplasty because of the advantages they can offer such as flexibility in material combinations and geometrical adjustments. The mechanical environment of the modular junction in the body is quite challenging due to the complex and varying off-axial mechanical loads of physical activities applied to a tapered interface of two contacting materials (head and neck) assembled by an impact force intraoperatively. Experimental analogies to the in-vivo condition of the taper junction are complex, expensive and time-consuming to implement; hence, computational simulations have been a preferred approach taken by researchers for studying the mechanics of these modular junctions that can help us understand their failure mechanisms and improve their design and longevity after implantation. This paper provides a clearer insight into the mechanics of the head-neck taper junction through a careful review on the finite element studies of the junction and their findings. The effects of various factors on the mechanical outputs namely: stresses, micromotions, and contact situations are reviewed and discussed. Also, the simulation methodology of the studies in the literature is compared. Research opportunities for future are scrutinised through tabulating data and information that have been carefully retrieved form the reported findings.

Taper corrosion: a complication of total hip arthroplasty

The focus on taper corrosion in modular hip arthroplasty increased around 2007 as a result of clinical problems with large-head metal-on-metal (MoM) bearings on standard stems. Corrosion problems with bi-modular primary hip stems focused attention on this issue even more.

Factors increasing the risk of taper corrosion were identified in laboratory and retrieval studies: stiffness of the stem neck, taper diameter and design, head diameter, offset, assembly force, head and stem material and loading.

The high variability of the occurrence of corrosion in the clinical application highlights its multi-factorial nature, identifying the implantation procedure and patient-related factors as important additional factors for taper corrosion.

Discontinuing the use of MoM has reduced the revisions due to metal-related pathologies dramatically from 49.7% (MoM > 32 mm), over 9.2% (MoM ⩽ 32 mm) to 0.8% (excluding all MoM).

Further reduction can be achieved by omitting less stiff Ti-alloys and large metal heads (36 mm and above) against polyethylene (PE).

Standardized taper assembly of smaller and ceramic heads will reduce the clinical occurrence of taper corrosion even further. If 36 mm heads are clinically indicated, only ceramic heads should be used.

Taper-related problems will not comprise a major clinical problem anymore if the mentioned factors are respected.

Cite this article: EFORT Open Rev 2020;5:776-784. DOI: 10.1302/2058-5241.5.200013

Geometric Variations of Modular Head-Stem Taper Junctions of Total Hip Replacements

Taper degradation in Total Hip Replacements (THR) has been identified as a clinical concern, and the degradation occurring at these interfaces has received increased interest in recent years. Wear and corrosion products produced at the taper junction are associated with adverse local tissue responses, leading to early failure and revision surgery. Retrieval and in-vitro studies have found that variations in taper design affect degradation. However, there is a lack of consistent understanding within the literature of what makes a good taper interface. Previous studies assessed different design variations using their global parameters assuming a perfect cone such as: taper length, cone angle and diameters. This study assessed geometrical variations of as-manufactured head and stem tapers and any local deviations from their geometry. The purpose of this study was to provide a greater insight into possible engagement, a key performance influencing parameter predicted by Morse taper connection theory. This was achieved by taking measurements of twelve different commercially available male tapers and six female tapers using a coordinate measurement machine (CMM). The results suggested that engagement is specific to a particular head-stem couple. This is subject to both their micro-scale deviations, superimposed on their macro-scale differences. Differences in cone angles between female and male tapers from the same manufacturer was found to create a predominately proximal contact. However, distally mismatched couples are present in some metal-on-metal head-stem couples. On a local scale, different deviation patterns were observed from the geometry which appeared to be linked to the manufacturing process. Future work will look at using this measurement methodology to fully characterise an optimal modular taper junction for a THR prosthesis.

Specimen-Specific Finite Element Models for Predicting Fretting Wear in Total Hip Arthroplasty Tapers

Products from fretting wear and corrosion in the taper junction of total hip arthroplasty (THA) devices can lead to adverse local tissue reactions. Predicting damage as a function of design parameters would aid in the development of more robust devices. The objectives of this study were to develop an automated method for identifying areas of fretting wear on THA taper junctions, and to assess the predictive ability of a finite element model to simulate fretting wear in THA taper junctions. THA constructs were fatigue loaded, thus inducing damage on the stem taper. An automated imaging and analysis algorithm quantified fretting wear on the taper surfaces. Specimen-specific finite element models were used to calculate fretting work done (FWD) at the taper junction. Simulated FWD was correlated to imaged fretting wear. Results showed that the automated imaging approach identified fretting wear on the taper surface. Additionally, finite element models showed the greatest predictive ability for tapers exhibiting distal contact. Finite element models predicted an average of 30.3% of imaged fretting wear. With additional validation, the imaging and finite element techniques may be useful to manufacturers and regulators in the development and review of new THA devices.

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.

What the Surgeon Can Do to Reduce the Risk of Trunnionosis in Hip Arthroplasty: Recommendations from the Literature

Trunnionosis, defined as wear and corrosion at the head–neck taper connection, is a cause of failure in hip arthroplasty. Trunnionosis is linked to a synergistic combination of factors related to the prosthesis, the patient, and the surgeon. This review presents analytical models that allow for the quantification of the impact of these factors, with the aim of providing practical recommendations to help surgeons minimize the occurrence of this failure mode. A tighter fit reduces micromotion and, consequently, fretting of the taper connection. The paramount parameters controlling the fixation force are the coefficient of friction and the impaction force. The influence of the head diameter, as well as of the diameter and angle of the taper, is comparatively small, but varus alignment of the taper and heads with longer necks are unfavourable under physiologic loads. The trunnion should be rinsed, cleaned, and dried carefully, while avoiding any contamination of the bore—the female counterpart within the head—prior to assembly. Biological debris, and even residual water, might critically reduce the fixation of the taper connection between the head and the neck. The impaction force applied to the components should correspond to at least two strong blows with a 500 g hammer, striking the head with an ad hoc impactor aligned with the axis of the taper. These strong blows should correspond to a minimum impaction force of 4000 N.

Taper Design, Head Material, and Manufacturer Affect the Onset of Fretting Under Simulated Corrosion Conditions

BACKGROUND

We investigated the effect of taper design, head material, and manufacturer on simulated mechanically assisted crevice corrosion (MACC).

METHODS

Six pristine C-taper stems coupled with alumina-zirconia or cobalt-chromium (CoCr) heads were tested in a mechanical/electrochemical setup to measure average fretting currents and fretting current onset loads. Outcomes were compared with previous data from V40 tapers from the same manufacturer and 12/14 tapers from another manufacturer.

RESULTS

Within a single manufacturer, differences in average fretting current between V40 and C-taper designs were dependent on head material. Only with V40 tapers did CoCr heads show higher average fretting currents than ceramic heads. Between manufacturers, differences were found between similar taper designs, as 12/14 taper couples showed higher average fretting currents than C-taper couples, regardless of head material.

CONCLUSION

Taper design, head material, and factors inherent to different manufacturers influence fretting current in simulated MACC. Unlike clinical and retrieval studies, this experimental design allows for investigations of factors affecting MACC in a controlled environment. Taper design, independent of manufacturer, contributes to the observed differences in average fretting current between head materials. In some taper designs, head composition, specifically ceramic, should not be considered alone to reduce risk of corrosion.

Model-Based Roentgen Stereophotogrammetric Analysis to Monitor the Head-Taper Junction in Total Hip Arthroplasty in Vivo-And They Do Move

Model-based Roentgen stereophotogrammetric analysis (RSA) using elementary geometrical shape (EGS) models allows migration measurement of implants without the necessity of additional attached implant markers. The aims of this study were: (i) to assess the possibility of measuring potential head–taper movement in THA in vivo using model-based RSA and (ii) to prove the validity of measured head–taper migration data in vitro and in vivo. From a previous RSA study with a 10 years follow-up, retrospectively for n = 45 patients head–taper migration was calculated as the relative migration between femoral ball head and taper of the femoral stem using model-based RSA. A head–taper migration of 0.026 mm/year can be detected with available RSA technology. In vitro validation showed a total migration of 268 ± 11 µm along the taper axis in a similar range to what has been reported using the RSA method. In vivo, a proof for interchangeable applicability of model-based RSA (EGS) and standard marker-based RSA methods was indicated by a significant deviation within the migration result after 12-month follow-up for all translation measurements, which was significantly correlated to the measured head–taper migration (r from 0.40 to 0.67; p < 0.05). The results identified that model-based RSA (EGS) could be used to detect head–taper migration in vivo and the measured movement could be validated in vitro and in vivo as well. Those findings supported the possibility of applying RSA for helping evaluate the head–taper corrosion related failure (trunnionosis).

How to Select 2D and 3D Roughness Parameters at Their Relevant Scales by the Analysis of Covariance

In this paper, a multi-scale methodology is proposed to model and characterize the effect of two lubricants on changes in surface morphology during a running-in test. The test concerns two steels samples, mounted on a twin-disc tribometer to test each of lubricants A and B for a period of 42 h. The changes are characterized by the standardized roughness parameters given in ISO 25178. A technique involving replication is used to monitor wear during the test. Using all these replication measurements, a multi-scale methodology is applied. These selected models highlighted the relevant parameters for quantifying wear during lifespan, and also showed that lubricant A was better able to preserve surface integrity during wear than lubricant B.

Burst Strength of BIOLOX®delta Femoral Heads and Its Dependence on Low-Temperature Environmental Degradation

Zirconia-toughened alumina (ZTA) currently represents the bioceramic gold standard for load-bearing components in artificial hip joints. ZTA is long known for its high flexural strength and fracture toughness, both properties arising from a microscopic crack-tip shielding mechanism due to the stress-induced tetragonal-to-monoclinic (t→m) polymorphic transformation of zirconia. However, there have been concerns over the years regarding the long-term structural performance of ZTA since the t→m transformation also spontaneously occurs at the material’s surface under low-temperature environmental conditions with a concomitant degradation of mechanical properties. Spontaneous surface degradation has been extensively studied in vitro, but predictive algorithms have underestimated the extent of in vivo degradation observed in retrievals. The present research focused on burst-strength assessments of Ø28 mm ZTA femoral before and after long-term in vitro hydrothermal ageing according to ISO 7206-10. An average burst strength of 52 kN was measured for pristine femoral heads. This value was ~36% lower than results obtained under the same standard conditions by other authors. A further loss of burst strength (~13% in ultimate load) was observed after hydrothermal ageing, with increased surface monoclinic content ranging from ~6% to >50%. Nevertheless, the repetitively stressed and hydrothermally treated ZTA heads exceeded the minimum burst strength stipulated by the US Food and Drug Administration (FDA) despite severe test conditions. Lastly, Raman spectroscopic assessments of phase transformation and residual stresses on the fracture surface of the femoral heads were used to clarify burst-strength fluctuations and the effect of hydrothermal ageing on the material’s overall strength degradation.

Effects of corrosion and cleaning method on taper dimensions: an in vitro investigation

PURPOSE

Taper corrosion related revisions have recently been reported in the orthopaedic literature. Cleaning procedure of the trunnions during hip revision is not standardised. The purpose of this bench top investigation was to understand the alterations in the trunnion dimensions and surface roughness characteristics as a result of corrosion product build-up.

METHODS

8 titanium alloy trunnions and CoCr femoral heads assemblies were cyclically tested in a mechanical simulator. Following disassembly of the tested constructs, the trunnions were cleaned using 2 methods. The trunnion dimensions were measured using coordinate measuring machine, and surface roughness was measured using white light interferometry. The trunnions were reassembled with ceramic femoral heads and titanium sleeves following cleaning. Head/sleeve pull-off testing was conducted to understand the effects of cleaning methods on the pull-off strength.

RESULTS

Grade 4 corrosion was observed on all trunnions after mechanical testing. The aggressive cleaning methods had a larger impact on the surface roughness when compared to the light cleaning method. The aggressive cleaning method also decreased the taper cone angle. The pull-off strength was not affected by the cleaning method and the pull-off values were approximately 50% of the assembly loads.

CONCLUSIONS

The study suggests that trunnion cleaning method may alter the surface roughness and taper cone angle of the existing trunnion. However, the effects of these changes on the pull-off strength did not reach statistical significance. Complex corrosion testing under cyclic loading conditions are warranted to understand the long-term effects of these changes.

Metal Ion Release after Hip and Knee Arthroplasty - Causes, Biological Effects and Diagnostics

All metal implants in human bodies corrode which results in metal ions release. This is not necessarily a problem and represents for most patients no hazard. However, if a critical metal ion concentration is exceeded, local or rarely systemic problems can occur. This article summarizes the mechanisms of metal ion release and its clinical consequences. Several situations can result in increased metal ion release: metal-on-metal hip arthroplasties with increased wear, increased micromotion at taper interfaces, direct metal-metal contact (polyethylene wear, impingement), erroneously used metal heads after ceramic head fracture. Possible problems are in most cases located close to the concerned joint. Furthermore, there are reports about toxic damage to several organs. Most of these reports refer to erroneously used metal heads in revisions after a broken ceramic head. There is currently no evidence of carcinogenic or teratogenic effects of implants but data is not sufficient to exclude possible effects. Cobalt and chromium blood levels (favorably in whole blood) should be measured in patients with suspected elevated metal ions. According to current knowledge levels below 2 µg/l seem to be uncritical, levels between 2 and 7 µg/l are considered borderline with unknown biological consequences and levels above 7 µg/l indicate a local problem which should be further diagnosed. Metal ion levels always need to be interpreted together with clinical symptoms and imaging results.

In vivo corrosion and damages in modular shoulder prostheses

Wear and corrosion at taper junctions of orthopaedic endoprostheses remain of great concern and are associated with adverse clinical reactions. Whereas tribocorrosion of hip tapers was extensively investigated, there is only little knowledge regarding the clinical performance of modular total shoulder prostheses. This retrieval study evaluated 35 modular taper junctions of anatomical shoulder explants using stereomicroscopy, confocal microscopy, as well as optical and scanning electron microscopy to determine the damage modes as well as the effects of taper topography and alloy microstructure. Among all humeral head tapers, 89% exhibited material degradation. Different overlapping wear mechanisms were identified such as plastic deformation, adhesive material transfer, microploughing, and fretting damage. Only CoCrMo cast alloy heads showed a susceptibility to electrochemically dominated fretting in comparison to CoCrMo wrought alloy. Moreover, corundum blasted stem tapers show a significantly increased incidence rate for microploughing. To date, this is the most comprehensive study on the damage types of modular taper junctions of anatomical shoulder arthroplasty proving the existence of fretting even on less weight-bearing implants. This study revealed critical fretting factors, such as the surface finish and the alloy type that are essential for the development of countermeasures that avoid any taper corrosion.

Curative effect and prognosis of 3D printing titanium alloy trabecular cup and pad in revision of acetabular defect of hip joint

Curative effect and prognosis of 3D printing titanium alloy trabecular cup and pad in revision of acetabular defect of hip joint were investigated. Forty-two patients who underwent acetabular revision in the Second Affiliated Hospital of Luohe Medical College were divided into observation and control groups according to different methods of acetabular revision and revision materials. 3D printed titanium alloy trabecular cups and pads were used in the observation group, and non-3D printed titanium trabecular cups and pads were used in the control group. Preoperative and postoperative pain visual analog scale (VAS score), hip Harris scores and quality of life Health Survey Scale (SF-36) scores were compared between the groups. At 3, 6 and 12 months after operation, Harris score and SF-36 score of the observation group were significantly higher than those of the control group, and VAS score was significantly lower than that of the control group (P<0.05). Stability and bone ingrowth of prosthesis in the observation group were better than those in the control group. Revision of the hip prosthesis with 3D printed titanium trabecular metal cups and pads resulted in satisfactory outcomes. Short-term prognosis is satisfactory but the long-term prognosis remains to be further investigated.

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.

Influence of flexural rigidity on micromotion at the head-stem taper interface of modular hip prostheses

Fretting corrosion as one reason for failure of modular hip prostheses has been associated with micromotion at the head taper junction. Historically the taper diameter was reduced to improve the range of motion of the hip joint. In combination with other developments, this was accompanied by increased observations of taper fretting, possibly due to the reduced flexural rigidity of smaller tapers. The aim of the study was to investigate how the flexural rigidity of tapers influences the amount of micromotion at the head taper junction. Three different stem and two different taper designs were manufactured. Experimental testing was performed using three different activity levels with peak loads representing walking, stair climbing and stumbling. The relative motion at the head-stem taper was measured in six degrees of freedom. Micromotion was obtained by subtraction of the elastic deformation derived from monoblock and finite element analysis. Less rigid tapers lead to increased micromotion between the head and stem, enlarging the risk of fretting corrosion. The influence of the stem design on micromotion is secondary to taper design. Manufacturers should consider stiffer taper designs to reduce micromotion within the head taper junction.

Surface integrity of polyethylene liners following trunnionosis of a dual modular neck total hip implant

BACKGROUND

There has been a trend in the evolution of total hip arthroplasty towards increased modularity; with this increase in modularity come some potentially harmful consequences. Modularity at the neck shaft junction has been linked to corrosion, adverse reaction to metal debris and pseudotumour formation.

AIM

The aim of this retrieval study is to assess whether the surface integrity of the polyethylene (PE) liner is affected by metal wear debris in a single implant design series of THA revised for trunnionosis.

METHODS

A retrieval analysis of thirty dual-taper modular neck hip prostheses was performed; the mean time from implantation to revision was 2.7 years (1.02-6.2). PE liners were analysed using a scanning electron microscope with an energy dispersive spectrometer to assess for metal particles embedded on the liner surface. Serum metal ion levels and inflammatory markers were also analysed.

RESULTS

There were small numbers of metal particles present on the PE liners. The mean number of metal particles per liner was four and the particles varied in size from 0.5 µm to 122 µm mean 16 µm. All patients had elevated metal ion levels: cobalt 6.02 µg/l, chromium 1.22 µg/l, titanium 3.11 µg/l. The cobalt:chromium ratio was 7.55:1. Inflammatory markers were also marginally raised (ESR 17; CRP 10).

CONCLUSION

These results suggest that retention of PE liners may be reasonable when performing isolated revision of the femoral component in cases of failure at the modular neck stem junction, especially when the inner diameter of the liner is already optimised for head size and stability.

A Survey of the Prevalence of and Techniques to Prevent Trunnionosis

Trunnionosis of total hip arthroplasty (THA) components has been an increasingly reported complication. Consensus is lacking regarding preventive practices and the overall incidence of trunnionosis. In this study, fellowship-trained adult reconstruction orthopedic surgeons were surveyed to identify expert opinions. A 25-question, web-based survey regarding trunnionosis incidence, prevention, and biomaterials was sent to 345 fellowship-trained adult reconstruction orthopedic surgeons in North America. The survey yielded 151 (43.8%) responses from surgeons with a mean of 11.97±9.49 years of experience. These surgeons believe that the material composite of the head-neck junction is the most important contributor to trunnionosis. They often choose a ceramic head with a metal alloy stem to reduce trunnionosis. They more commonly impact the femoral head 3 times than once. Fifty-one percent believe that trunnionosis leads to THA failure for between 0% and 2% of all THA revisions, whereas 48.3% believe that the failure rate is greater than 2%. More than half (53.6%) of these surgeons recommend a revision THA if a patient's serum cobalt level is greater than 10 µg/L, regardless of symptom presence. The incidence of trunnionosis appears to be increasing due to changes in implants and/or an increased awareness of the problem, with 48.3% of these surgeons believing that trunnionosis is the primary cause of THA failure for more than 1 in 50 patients. Some suggested preventive measures include cleaning and drying the trunnion, using ceramic femoral heads, matching THA components, and adding titanium sleeves on well-fixed stems that are retained during revision surgery. [Orthopedics. 2018; 41(4):e557-e562.].

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.

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.

Fretting and Corrosion Damage in Retrieved Metal-on-Polyethylene Modular Total Hip Arthroplasty Systems: What Is the Importance of Femoral Head Size?

BACKGROUND

Fretting and corrosion at the modular femoral head-femoral neck (taper) interface have been reported in retrieved total hip arthroplasty (THA) prostheses. This study investigated associations among implant design, radiographic factors, and patient factors with corrosion and fretting at the taper interface in retrieved metal-on-polyethylene modular THA prostheses.

METHODS

Ninety-two retrieved primary metal-on-polyethylene THA implants were evaluated and graded for fretting, corrosion, and damage at the taper interface, including the femoral stem trunnion and femoral head. Preoperative radiographs were assessed for osteolysis and femoral stem alignment; and medical records were reviewed for demographic data.

RESULTS

Male patients had greater head corrosion (P = .037), patient age at revision had a weak, negative correlation with trunnion corrosion (ρ = -0.20, P = .04), and both body mass index and duration of implantation had weak, positive correlations with head fretting (ρ = 0.26, P = .01 and ρ = 0.33, P = .001, respectively). A weak, negative correlation was found between femoral head size and both head fretting and head corrosion (ρ = -0.26, P = .007 and ρ = -0.21, P = .028, respectively), and a weak, positive correlation was found between head offset and trunnion fretting (ρ = 0.23, P = .030). Varus femoral stem alignment was associated with greater head fretting (P = .038).

CONCLUSION

Larger femoral head sizes were correlated with less severe head corrosion and head fretting, with 28-mm heads exhibiting more moderate-to-severe damage. Other factors, such as head-taper engagement and geometry, rather than head size, may affect rates of corrosion and fretting damage at the taper interface.

Mixing of Head-Stem Components in Total Hip Arthroplasty

BACKGROUND

Implant manufacturers proclaim that the tapers of modular total hip arthroplasty are not standardized and can vary from manufacturer to manufacturer. That is why the combination of various components from different manufacturers ("Mix and Match") is not permitted. In this study, different taper combinations were investigated experimentally to assess the effect of "Mix and Match" regarding the taper connection strength.

METHODS

Torque-off tests using hip stems and metal femoral heads from 6 different implant manufacturers were performed. First the components were tested as intended and afterwards the stems were combined with metal heads from other manufacturers.

RESULTS

There was no significant difference in taper connection strength when stems from the manufacturers Link, Smith & Nephew, and Zimmer were combined with heads from other manufacturers. The Biomet stems showed a significantly reduced taper connection strength if femoral heads of Aesculap, DePuy, or Smith & Nephew were used. On the contrary, the DePuy stems in combination with the originally intended femoral heads showed a significantly lower taper connection strength compared to the use of heads from Link, Biomet, and Zimmer. The same was observed for the Aesculap stems in combination with Zimmer heads.

CONCLUSION

The results of this study suggest that mixing components from different manufacturers may affect the taper connection strength and could reduce the stability. As safety should be a high priority in patient treatment, any potential risks should be avoided. Therefore, mixing and matching of heads and femoral stems from different manufacturers cannot be recommended.

Effect of impact assembly on the interface deformation and fretting corrosion of modular hip tapers: An in vitro study

Wear and corrosion at the modular head-neck junction has been recognised to be a potential clinical concern, with multiple reports on adverse local tissue reactions and subsequent early failure of metal-on-metal hip replacements. Furthermore, reports on head-neck taper corrosion are also being described with conventional metal-on-polyethylene bearings. Manufacturing tolerances, surgical technique, non-axial alignment, material combination, high frictional torque and high bending moment have all been implicated in the failure process. There is limited guidance on the force of impaction with which surgeons should assemble modular hip prostheses. This study aims to investigate the effect of impaction force on the deformation and corrosion of modular tapers. Short neck tapers with high surface roughness (average R_z  = 16.58 μm, R_a  = 4.14μm) and long neck tapers with low surface roughness (average R_z  = 3.82 μm, R_a  = 0.81μm), were assembled with CoCrMo alloy heads (smooth finish) under controlled conditions with 2, 4 or 8 kN of impaction force. Material combinations tested included CoCrMo-head/CoCrMo-neck and CoCrMo-head/Ti-6Al-4V-neck. Assessment of surface deformation before and after impaction was made using surface profilometry. Measurement of fretting current during sinusoidal cyclic loading evaluated mechanically assisted corrosion for each assembly load during short-term cyclic loading (1000-cycles) and long-term cyclic loading (5 million-cycles). Deformation on head and neck tapers increased with assembly load. Fretting currents during short term simulation testing showed significantly lower currents (p < 0.05), in 8 kN assemblies when compared to 2 and 4 kN, especially for the short-rough tapers. Long-term simulator testing demonstrated a progressive reduction in fretting corrosion for samples impacted with 4 and 8 kN; however, this reduction was greater for samples impacted at 8 kN even at the start of testing. Based on our results, surgeons could minimise mechanically assisted crevice corrosion by using higher impact loads when assembling the head to the stem in total hip arthroplasty. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:405-416, 2018.

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.

Same Same but Different? 12/14 Stem and Head Tapers in Total Hip Arthroplasty

BACKGROUND

Taper corrosion has been identified to be a major concern in total hip arthroplasty during the past years. So far, the mechanisms that lead to taper corrosion in modular taper junctions are not fully understood. However, it has been shown that corrosion is also influenced by the geometry and topography of the taper, and these parameters vary among the implant manufacturers. The purpose of this study was to investigate the variations of common stem and head tapers regarding design and surface characteristics.

METHODS

An analysis of selected commercially available 12/14 stem and head tapers was performed. As geometric parameters, the taper angle, the opening taper diameter, and the taper length were measured using a coordinate measuring machine. Several topographic parameters were determined using a tactile roughness measurement instrument.

RESULTS

Although all investigated tapers are so-called 12/14 tapers, this study showed that the stem and head tapers differ among the manufacturers. The stem tapers were clearly different in both geometry and topography, and the range in variation of the topographic parameter was greater than it was for the geometric parameter. In contrast, the head tapers were different in their geometry, although not in topography.

CONCLUSION

Ultimately, this study provides an overview on the characteristics and variations of modular hip taper connections, and in addition, a new classification system regarding the surface finish is presented. These findings could be further considered in experimental corrosion or retrieval studies.

Variation in taper surface roughness for a single design effects the wear rate in total hip arthroplasty

Material loss from the head-stem taper junction of total hip arthroplasty (THA) is implicated in adverse reactions to metal debris (ARMD); the mechanisms for this are multi-factorial. We investigated the relationship between the roughness of the "as manufactured" taper surface and the wear rate from this junction. Fifty retrieved Pinnacle metal-on-metal (MOM) bearings paired with a Corail stem were included in the study. Multivariable statistical analysis was performed to determine the influence of taper roughness on material loss rate after controlling for other confounding surgical, implant, and patient factors. The surface roughness of the "as manufactured" head taper surface was associated with the rate of material loss from this surface. Four of eighteen roughness variables taken from ISO 4,287 and ISO 13,565-2 were significant: The Reduced Peak Height (Rpk, the protruding peaks above the core) (p = 0.004), Material Ratio 1 (Mr1, the ratio of the protruding peaks above the core) (p = 0.002), Area of the Peak Region (A1, the area of the Abbott-Curve that contains the peaks from the profile) (p = 0.003) and the Skewness (Rsk, the asymmetry of the height distribution corresponding to the height or depth of surface features) (p = 0.03). We found a large variability in the measured values with a median (range) of 0.50 (0.05-2.98), 11.98 (0.46-39.98), 30.89 (0.15-581.00), and 0.04 (-0.73-0.84), respectively. A 1-unit increase in Rpk was associated with a 73% increase in the taper wear rate. The variability of "as manufactured" surface roughness has a significant effect on taper material loss. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1784-1792, 2017.

Assessment of the equivalence of a generic to a branded femoral stem

Aims

The aim of this study was to compare the design of the generic OptiStem XTR femoral stem with the established Exeter femoral stem.

Materials and Methods

We obtained five boxed, as manufactured, implants of both designs at random (ten in total). Two examiners were blinded to the implant design and independently measured the mass, volume, trunnion surface topography, trunnion roughness, trunnion cone angle, Caput-Collum-Diaphyseal (CCD) angle, femoral offset, stem length, neck length, and the width and roughness of the polished stem shaft using peer-reviewed methods. We then compared the stems using these parameters.

Results

We found that the OptiStems were lighter (p < 0.001), had a rougher trunnion surface (p < 0.001) with a greater spacing and depth of the machined threads (p < 0.001), had greater trunnion cone angles (p = 0.007), and a smaller radius at the top of the trunnion (p = 0.007). There was no difference in stem volume (p = 0.643), CCD angle (p = 0.788), offset (p = 0.993), neck length (p = 0.344), stem length (p = 0.808), shaft width (p = 0.058 to 0.720) or roughness of the polished surface (p = 0.536).

Conclusion

This preliminary investigation found that whilst there were similarities between the two designs, the generic OptiStem is different to the branded Exeter design.

Cite this article: Bone Joint J 2017;99-B:310–16.