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Chapter

A suggestive qualitative review of the tribological implications of total hip prostheses modularization

Lucian Capitanu1, Liliana-Laura Badita2*, Constantin Tiganesteanu1

1[anonimizat], Bucharest, Romania

2[anonimizat]-[anonimizat], Bucharest, [anonimizat], [anonimizat] a closed loop biotribosystem. The input factors of the biotribosystem are represented by the modular elements of the hip prosthesis. Modular components offer a variety of intraoperative options in primary arthroplasty and total hip revision. Modularity is a [anonimizat]. The use of prostheses with modular neck is an effective alternative in the treatment of developing hip dysplasia. The use of modular metal joints presents a [anonimizat] (THA). [anonimizat]. A major disadvantage for all modular orthopedic devices is that each modular interface of the components becomes a [anonimizat], fretting and fatigue. [anonimizat], are thought to cause tissue reactions that lead to weakening of the implant and subsequent failure of arthroplasty. Hip prostheses with modular neck have an additional interface that presents a risk of fretting and corrosion compared to monobloc prostheses. [anonimizat] / CoCr, CoCr / Ti, Ti / Ti and SS / CoCr couples. [anonimizat], [anonimizat]. [anonimizat], [anonimizat]. [anonimizat], [anonimizat], and the clinical factors. Even in the case of a [anonimizat], [anonimizat]. [anonimizat], to the release of debris that can cause local negative tissue reactions in the human body. Therefore, it must be borne in mind that each new modular element introduced for the purpose of a [anonimizat]. [anonimizat].

Keywords: THP, modularization, fretting, corrosion, fretting cracking, breaking

Introduction

Modularization of total hip prosthesis (THP) had been developeing from the need to reduce the inventory of instruments for implants and to simplify their subsequent revision by offering the option to retaining the stem and performing just the femoral head change. The head modularity allows the adjustment of the legs’ length and also allows the use of other materials (like ceramics) as an option of bearing [1].

This may be beneficial for subsequent bearing wear. Despite its potential benefits, an increase in the use of modular interfaces can lead to increased fretting corrosion and corrosive cracking at the taper junction [2].

Confirmation of corrosion at the femoral head – neck junction is performed by revision surgery. Head-neck junction refers to the taper between the femoral head hole and the femoral stem trunnion. Taper corrosion products can contribute to joint wear with the third body. Although taper corrosion is relatively rare in hips with metal-on-polyethylene (MoP) joints, corrosion products can lead to adverse local tissue reactions (ALTR) of the same type as those occurring in large diameter metal-to-metal bearings. [1].

H. Krishnan et al. [3] published a review that “aims to provide surgeons with an up-to-date summary of clinically relevant issues”. In this paper the authors described and analysed a series of important characteristcs of femoral modularity, like: development of femoral modularity, theoretical rationality for modularity and clinically relevant issues reported after using modular neck femoral stems. The authors present results of some international studies regarding failure rate of modular devices. All these data are important in modular prosthesis domain, but they cannot identify the failure mechanism. Future researches are needed because this information is necessary to determine if the modular neck femoral stems will be used in the future and how patients who have already implanted them should be monitored.

S. Hussenbocus et al., [4] have also shown that the modularity of the head – neck junction of the femoral component of THA has become popular as a design feature. It allows the adjustment of the legs’ length, the compensation and the balancing of the soft tissues through different head options. But a great disadvantage appears when a new modular interface is introduced to the femoral stems, which were previously monobloc or non-modular. It has potential for corrosion at the taper junction by mechanically assisted corrosive cracking. The authors of this study showed that "corrosion reduction appears to be related to its geometric parameters, material combinations and femoral head size. This review article describes the pathogenesis, risk factors, clinical assessment and management of taper corrosion at the head-neck junction".

The fretting and corrosion risks that appear when an additional interface is introduced in the case of modular neck hip prostheses, compared to monobloc prostheses, have been demonstrated by other researchers. Jauch S.Y., et al [5] observed the failures at the neck adapter of the modular prostheses for a number of different designs. It has been speculated and demonstrated that excessive micro-movements at the stem-neck interface were responsible for the implant failure process. They also realized experimental studies in order to investigate the influence of materials combinations and assembly conditions on the size of micro-movements at the stem-neck interface during cyclic loading. The largest micro-movements were observed at the lateral edge of the stem-neck taper connection, what is consistent with the crack location of clinically failed prosthesis. The titanium neck adapters showed significantly higher micro-movements than the cobalt-chromium neck adapters. Taking into account that contaminated interfaces also showed significantly higher micro-movements, an important conclusion was that special attention must be taken to clean the interface before assembling and in general, titanium neck adapters with titanium stem should be used with caution.

In another paper, S.Y. Jauch et al., [6] demonstrated that prostheses with Ti neck adapter caused significantly higher interface micromotions than those with CoCr adapter (5.1 ± 2.1 μm vs. 0.8 ± 1.6 μm), independent of the prosthesis design. This was realized by mechanically testing two different prosthesis designs: Metha prosthesis design (Aesculap AG) and H-Max M prosthesis design (Limacorporate). In the case of (Metha, Aesculap AG) prosthesis design a substantial number of in vivo fractures for Ti-Ti couplings, but there are no documented fractures for Ti-CoCr couplings. In contrast, for (H-Max M, Limacorporate) design with a Ti-Ti coupling, only clinical failure was reported. Finally, the authors concluded that “for the Ti-Ti couplings, the Metha prosthesis showed a trend towards larger micro-movements compared to the H-Max M (6.5 ± 1.6 μm versus 3.6 ± 1.5 μm). No differences in micro-movements between the Metha prosthesis with CoCr neck and H-Max M with Ti neck were observed (2.6 ± 2.0 μm)".

Thomas M. Grupp et al., [7] stated that "modular neck adapters for hip arthroplasty stems allow the surgeon to modify the collum-caput diaphysis (CCD) angle, offset and femoral anteversion intraoperatively. Fretting or corrosive cracking can lead to the failure of such a modular device due to high loads or surface contamination inside the modular coupling". In this study, a series of adapter failures of titanium alloy modular neck adapters in combination with a titanium alloy modular short hip stem was investigated. The failed neck adapters were implanted between August 2004 and November 2006 in 5000 devices. After this period, the titanium neck adapters were replaced with cobalt-chromium adapters. After biomechanical simulations and analysis, it has been shown that the primary micro-movements initiated the fretting inside the modular taper neck connection. “There was a continuous process of abrasion and repassivation, with a subsequent cold welding at the titanium alloy modular interface. Titanium oxide layers of 10-30 μm were observed on the surface. The surface crackings caused by fretting or fretting corrosion finally lead to fatigue fracture of the titanium alloy modular neck adapters. Cobalt-chromium alloy neck adapters present significantly reduced micro-movements, especially in the case of contaminated conical connection. With a cobalt-chromium neck, micro-movements can be reduced 3 times compared to the titanium neck. The incidence of fretting corrosion was also substantially lower with the configuration of the cobalt-chromium neck.”

A. Hernandez et al., [8] stated that there are several potential advantages when using modularity in total hip arthroplasty but there have also been complications that need to be addressed. “Various circumstances, such as it would be the large femoral head with a long varus neck, corrosion, and the patient's activity level, can participate in creating the environments that favor the fatigue failure of the implant".

Jacob M. Elkins et al., [9] analyzed the stability and wear potential of trunnions in large diameter metal-oo-metal (MoM) total hips, showing that "these have the theoretical advantages of joint stability and low bearing surface wear. However, the recent reports have indicated an unacceptably high rate of wear-associated failures in large diameter bearings, possibly due, in part, to increased wear at the trunnion interface. Thus, the deleterious consequences of using large heads may outweigh their theoretical advantages." Increased trunnion wear for larger diameter heads could plausibly occur due to the micro-movement that accompanies the lever arm increased between the head center and the pressure center of the spindle interface.

Georgios K. Triantafyllopoulos et al., [10] have shown that fretting and corrosion at the head-neck joints of total hip arthroplasties (THA) have been associated with adverse local tissues reactions in patients with metal on polyethylene (MoP) and metal on metal (MoM) prostheses. Fretting/ corrosion severity on the taper surfaces of the head and trunnion stem was examined at 154 MoP THAs during 3282 revision surgeries. The fretting and corrosion damage were subjectively classified by two independent observers on a scale of 1 to 4 and their relationships with the head size, alloy combinations, taper/ trunnion design, length of implant (LOI) and location were investigated. The main results showed that "fretting and corrosion are common in THA with MoP joints, but no damage was related to the size of the femoral head. On the contrary, the combination of taper design, LOI and alloy affected the severity of fretting and corrosion”.

JR Goldberg et al., [11] conducted an in-depth analysis of 231 modular hip implant recoveries to investigate the effects of materials combination, metallurgical state, bending rigidity, head and neck moment arm, neck length and implantation time, on corrosion and fretting of modular taper surfaces. Scores for corrosion and fretting were assigned to the medial, lateral, anterior and posterior quadrants of the necks and proximal and distal regions of the heads. Neck and head corrosion and fretting scores have been found to be significantly higher for couplings of mixed alloys versus similar alloys. Moderate to severe corrosion was observed in 28% of the heads of similar alloys couplings and 42% of the heads of mixed alloys couplings. Differences in corrosion scores between components made of the same base alloy, but with different metallurgical states, were observed. Corrosion and fretting scores tended to be higher for the heads than necks. Implantation time and the bending rigidity of the neck were predictors of head and neck corrosion and head fretting. The results of this study suggest that in vivo corrosion of the modular taper interfaces of the hip is attributed to a mechanically assisted corrosion process. Larger diameter necks will increase their rigidity and can reduce fretting and subsequent corrosion of the taper interface, regardless of the alloy used. However, the increase in the neck diameter must be balanced, with the decrease of the motion range and of stability resulting from the joint.

Three femoral failures occurring at the head-neck junction after a THA was presented in a study realized by Jonathon Spanyer and his team [12]. After experimental analysis it has been showed that catastrophic failures of femoral stems at the head-neck junction are a rare cause for revision after THA. Material and components design, surgical technique and patient factors may contribute to these.

J. Geringer and D.D. Macdonald [13] experimentally investigated the fretting corrosion at the AISI 316L SS coupling against poly(methylmethacrylate), under small displacements. The wear observed on the stainless steel "was modeled using the Point Defect Model (PDM). The originality of this approach in applying PDM to fretting corrosion consists of using a modified rate of the barrier layer dissolution, in the case of cyclic wear, so that the destruction rate of the barrier layer from the barrier of layer/ solution interface exceeds the growth rate of the barrier layer at the metal/ barrier layer interface at zero barrier layer thickness, this being the condition specified by PDM for depassivation".

L. Salles et al., [14] proposed a numerical treatment of the fretting wear under vibratory loading. "The method is based on Dynamic Lagrangian Frequency Time. It models unilateral contact by using Coulomb's friction law. The basic idea is to separate time into two scales, a slow scale for tribological phenomena and a fast scale for dynamics. A steady state is assumed for a certain number of vibration periods, and the variables are decomposed into Fourier series. An Alternating Frequency Time procedure is performed to calculate the nonlinear forces. Then, a hybrid Powell solver is used. Numerical investigations on a beam with friction contact interfaces illustrate the performances of this method and show the coupling between dynamic and tribological phenomena".

Francesco Traina et al., [15] showed that restoring the rotation centre of the hip in an anatomical position is considered to be relevant for the total hip prosthesis survival. "When the cup is implanted with a high rotation centre, the lever arm of the abductor muscles is decreased, causing higher joint-reaction forces. Modular stems with variable lengths and geometries can be used to balance soft tissues, and ceramic bearings surfaces to reduce the wear rate. 44 hip replacements performed with a high hip rotation centre have been matched with 44 replacements performed with an anatomical rotation centre. In all cases, the preoperative diagnosis was dysplasia of the hip (DDH) and cementless modular neck prostheses with ceramic bearing surfaces were used. At 9 years follow-up, the mean Harris score and WOMAC scores for the hip were not statistically different. All stems and cups were stable.”

Jeremy L. Gilbert et al., [16] tested stainless steel (ASTM F-1568) femoral hip stems combined with Co-Cr-Mo alloy heads (SS/CoCr) in an in vitro corrosion test to evaluate the tendency for mechanically assisted corrosion. In this study, three different aspects of modular design were evaluated: (1) comparison of CoCr/CoCr materials combinations, (2) wet versus dry assembly for SS/CoCr couplings and (3) 0- and 6-mm head offset for SS/CoCr couplings. Fretting corrosion tests were carried out on a range of cyclic loads up to 3300 N and continuous cyclic loading at 3300 N for 1 M cycles. The results showed that "SS / CoCr couplings were more susceptible to fretting corrosion than CoCr/CoCr couplings, and the dry assembly does not prevent fretting corrosion from occurring, but increases onset loading and that 6 mm offset heads had higher visual evidences of fretting damage". Also, "micro-movements measurements indicated fretting movements in the range of 10-25 μm, where 0 mm offset heads tended to piston on the stem trunnion, while 6 mm offset heads tended to break."

Marco Viceconti et al., [17] performed in vitro cyclic load fretting tests on a prototype of an uncemented hip prosthesis, with modular neck. The study had three major objectives: "to determine the amount of fretted material in the tapered-neck joint under various load cycle amplitudes, to determine the fretting deterioration evolution and to determine the effect of the stems bodies with different sizes, on the production of debris. All the tests produced some fretting micro-deteriorations on the tapered surface, although their degree was quite different among the testing groups. The amount of abraded material increased almost linearly with the applied load magnitude, but not with the number of loading cycles. The amount of weight loss was higher in the large stem bodies than in the small ones. Weight loss ranged from 0.28 ± 0.10 mg for small stem bodies loaded 5.5 million times up to 2300 N, to 2.54 ± 0.53 mg for large stem bodies, loaded 20 million times up to 3300 N."

M. Baxmann et al., [18] have shown that the modularity of the components of the femoral stems and neck has become a more frequently used tool for an optimized restoration of the hip joint center and improvement of the patient's biomechanics. But the additional taper interface increases the risk of mechanical failure due to fretting and corrosion cracking, being documented in case reports, more failures of the titanium alloy neck adapters. In this study, the authors developed an experimental fretting device for the systematic investigation of the micro-movement effect and contact pressure on the fretting damage in contact situations similar to the taper interfaces of the modular hip prostheses under cyclic loading, representative for in vivo loading conditions.

M. Ollivier et al., [19] realized a complete study in order to evaluate the revision-free survival of modular titanium neck components after a minimum follow-up period of 5 years and of the possible complications associated with the use of these components. The main result was that "the use of titanium cemented implants with a modular titanium stem is safe for a 5 years follow-up. The modular design does not prevent discrepancies in the limbs length, but restores the femoral offset".

P. Wodecki et al., [20] showed that total hip replacement (THR) with modular femoral components (stem-neck interface) makes it possible to adapt to extramedullary femoral parameters (anteversion, offset and length) theoretically improving muscle function, and stability. However, "adding a new interface has its disadvantages: reduced mechanical strength, fretting corrosion and material fatigue fracture". The authors report the case of a femoral stem fracture of the female part of the component, where the modular morse taper of the neck is inserted. An extended trohanteric osteotomy was necessary during revision surgery because the femoral stump could not be grasped for extraction, so that a long stem had to be used. This report shows that "the female part of the stem of a small femoral component may also be at increased failure risk and should be added to the list of risk factors."

Russell English et al., [21] published a computational approach for fretting wear prediction in total hip replacements. Both axisymmetric and 3D designs of a total commercial hip prosthesis are used to demonstrate the method and to highlight the key features of a proven wear algorithm. The designs together with the wear algorithm can be used effectively to study certain aspects of the design of the taper junction. The presented method can be used subsequently to identify the key factors leading to the release of the residues at the taper junction, so that an appropriate design of the prostheses and surgical procedural modifications can be made to alleviate this deterioration problem. The proposed method is also independent of design geometry and can be used for any finite element models (not just prosthetic devices) to predict the fretting wear.

Brent A. Lanting et al., [22] have shown that “recent literature indicates the potential for elevated revision rates of modular neck systems and the potential for local pseudotumors and metallosis formation at the modular neck / stem site. The authors analyzed the recovery of one modular neck implant design, including Scanning Electron Microscopy (SEM) evaluation and correlated with finite element analysis (FEA), as well as clinical characteristics of patient demographics, implant, and laboratory analysis. Correlation of the consistent corrosion locations to FEA indicates that the materials and design characteristics of this system can lead to a biomechanical reason for failure. The stem aspect of the modular neck / stem junction may present a particular risk.

Sherwin L. Su et al., [23] compared the fretting and corrosion behavior of a group of modular neck designs with that of a design that was recalled for the risks associated with fretting and corrosion at the modular neck junction. The authors first analyzed fretting and corrosion of more than 60 Rejuvenate modular implants recovered. They compared the results with the results obtained from 26 implants taken from other 7 modular neck designs. Multivariate analyzes were performed to evaluate the differences in fretting and corrosion, adjusting for confounding factors (e.g. implant length). Fretting and corrosion occurred on all modular neck-stem recoveries, regardless of design. However, mixed metal couplings suffered more corrosion than homogeneous couplings. This may be due to the lower modulus of the titanium alloy used for the stem, allowing for increased metal transfer and surface damage when loaded on a modular cobalt alloy modular neck, which, in turn, could be the higher scores of the side effects of tissue and corrosion. Due to the increased corrosion risk with mixed metals and increased neck fracture risk, with unmixed metallic stems and necks, the authors suggested that doctors avoid implanting modular neck-stem systems.

Rasmus T. Mikkelsen et al., [24] performed an analysis of the modular neck femoral stems (MNFS) against non-modular femoral stems (NFS) in total hip arthroplasty (THA), regarding clinical outcome, and metal ions levels. They showed that introduction of the modular neck femur stem (MNFS) raised concerns about pain, high levels of metal ions in the blood and adverse reactions to metal debris, such as pseudotumors (PTs), related to the corrosion between the femoral neck and the stem. Levels of pain, serum cobalt, and serum chromium were determined after comparative study between 33 patients with unilateral MNFS THA versus 30 patients with unilateral NFS THA. The main results showed that more patients in the NFS group had pain. Serum chromium and serum cobalt levels were higher in the MNFS group. Prevalence of PTs was twice higher in the MNFS group, but the difference was insignificant.

Clinical advantages of modular neck, respectively intraoperative adjustment of legs length and femoral anteversion via the neck-stem taper, have been also shown in the paper of Alan M. Kop and Eric Swarts, [25]. Beside this, they analysed 16 cases of double-tapered cone of the recovered Margron hip prostheses, in order to elucidate the failure mechanisms. Necks of these prostheses showed significant fretting and corrosive cracking of the neck-stem taper with an average time of 39 months after implantation. They were compared with the remaining recoveries, which showed no corrosion after an average time in situ of 2.7 months. The main conclusion of this study was that, even in the case of a modern taper design and corrosion resistant materials, increased modularity can lead to fretting and corrosive cracking, metal ions generation and particle debris that can contribute to periprosthetic osteolysis and loss of fixation.

Another complication of increasing modularity, respectively components fracturing, was analysed and presented by Michael B. Ellman et al., [26]. This has been done on a case of modular femoral neck fracture, which required a revision surgery to treat this complication. Careful preoperative planning during revision of these failures is essential to avoid morbidity and unnecessary subsequent revision surgeries, as demonstrated in this case. "The combined effects of fretting cracking and fretting corrosion of the large diameter femoral head, long modular neck of the metal-on-metal joint, patient size, and activity level, may have all played integral roles in creating an environment susceptible to this classic model of fatigue fracture".

I. P. S. Gill et al., [27] presented the results of the analysis of a series of 35 patients who underwent total hip replacement using the ESKA dual-modular short stem, with metal-on-polyethylene bearing surfaces. This implant has a modular neck section, in addition to the modular head. Of these patients, three had increased postoperative pain due to pseudotumor formation that resulted from corrosion at the modular neck-stem junction. These patients underwent a further surgery and aseptic lymphocyte vasculitis associated lesions were demonstrated by histological analysis. Recovery analysis of two modular necks showed corrosion at the neck-stem level. Cobalt and chromium levels in blood were measured at an average of nine months after surgery. These were compared with the levels of seven control patients with a mean age of 53.4 years, who had an identical joint prosthesis, but with a prosthesis that did not have modularity at the stem-neck junction. The mean blood levels of cobalt in the study group increased to 50.75 nmol/l compared with 5.6 nmol/l in control patients’ group. Corrosion at neck-stem tapers has been identified as an important source of metal ions release and pseudotumors formation that requires revision surgeries. Finite element modeling of the dual modular stem has shown high efforts at the modular stem-neck junction. Dual modular cobalt-chromium hip prostheses should be used with caution due to these concerns.

H.H. Ding et al., [28] investigated the influences of diamond like carbon (DLC) coatings and roughness on fretting behaviors of Ti-6Al-4V, using a fretting wear testing device with a cylindrical-on-flat surface contact. The results showed that "without the DLC coating, the friction coefficient was high, and under high displacement conditions, the wear volume was high. Smoother surfaces extended the gross sliding regime to the smaller displacement and higher-normal-force conditions. For tests on DLC coatings, the wear maps of the coating response can be divided into three areas: the coating working area (small displacement and low normal force conditions), the coating failure area (large displacement and high normal force conditions) and the transition area. In the coating working area, DLC coatings could protect the substrate with reduced friction, low wear volume and mild damage in the coating; the operating state occurred under the gross sliding regime". Increased normal force and displacement accelerated the coating failure process.

S. Fouvry et al., [29] quantified the wear rates, applying the classical Archard approach. It reports the wear volume to the product between the sliding distance and the normal load. Thereafter, a wear coefficient is extrapolated, assuming that it determines the wear resistance of the material studied. "This paper shows that this approach does not work when the friction coefficient is not constant. It seems that it is more relevant to consider the mechanical work of interfacial shearing as a significant wear parameter. This approach is applied to study the wear response of the different steels, and then extended to different types of TiN, TiC hard coatings, under reciprocal sliding conditions. By identifying wear energy coefficients, the wear quantification can be rationalized, and the wear resistance of the studied tribosystems can be classified. This also seems to be a convenient approach to interpret the different wear mechanisms. Metallic materials that involve plastic strain are analyzed by FEM computations. The energy balance confirms that a minor part of the dissipated energy is consumed by plasticity, whereas the major part participates in the heat and debris flow through the interface".

Katsufumi Uchiyama et al., [30] reported the case of a 46-year-old woman who underwent a revision surgery approximately 4 years after total hip arthroplasty, because of a fracture of the modular neck of a MODULUS femoral stem. The fractured surfaces of the recovered implant were inspected using optical microscopy and scanning electron microscopy. A three-dimensional finite element analysis was also performed to identify the stresses that might have caused the failure. The authors concluded that "active, obese patients who are implanted with a small modular component, could experience stress-induced fractures of the modular neck, with adequate fixation and osteointegration of the distal stem, especially if residual bone or tissue is present on the inner surface of the neck, that could contribute to micro-movement and reduced proximal fixation."

Starting from the idea that micro-movements associated with the modular components can lead to fretting corrosion and consequently, to release of debris which can cause adverse local tissue reactions in the human body, Claudio T. dos Santos et al., [31] studied two designs of modular hip prosthesis. Uncemented SS/Ti design, whose stem was made of ASTM F136 Ti-6Al-4V alloy, and whose metallic head was made of ASTMF138 austenitic stainless steel was compared to the cemented SS/SS design, with both components made of ASTMF138 stainless steel. It has been demonstrated that the SS/Ti design was more resistant to fretting corrosion than the SS/SS design after 10 million experimental testing cycles.

Fretting corrosion differences of modular metal-metal and ceramic-metal joints of total hip replacements have been studied by Nadim James Hallab et al., [32]. They assumed that modular ceramic – metal connections from total hip arthroplasty release more metal through fretting corrosion than traditional modular metal connections. This was investigated using an in vitro comparison of the ceramic (zirconia, ZrO2) and metal (Co alloy) femoral head fretting upon Co alloy stem components. In vitro fretting corrosion testing consisted in potentiodynamic monitoring and analysis of metal release from zirconia and Co alloy of 28 mm femoral heads with similar surface roughnesses (Ra = 0.46 μm) on identical Co alloy stems at 2.2 kN for 1 x 106 cycles, at 2 Hz. The experimental results showed a higher metal release (about 11 times higher in Co and 3 times increase in Cr) and the potentiodynamic fretting of metal-metal modular junctions, compared to ceramic-metal. Finally, it was concluded that zirconia heads coupled with Co alloy stems produced less fretting than Co alloy heads coupled with Co alloy stems.

McTighe T., et al., [33] published a complete review of the risk factors and benefits of modular junctions in total hip arthroplasty as well as some basic engineering principles that can reduce the risk factors and can improve the functionality of modular junctions. They showed that two products (Rejuvanate ™ and ABGII ™) were withdrawn by Stryker Orthopedics, Mahwah, NJ due to fretting corrosion that produces the decline in the clinical acceptance of modular hip implants. A main mechanism behind fretting corrosion is the stress, whose increase, in the case of modular junction, will proportionally increase the friction corrosion. The products recalled from Stryker had reduced taper support (13 mm versus 15, and 17 mm), with increased bending and torsion moments. This produce much higher stresses at the modular junction and potentially lead to a faster fretting corrosion rate, compared to the style of the stems that keep the neck. As taper lengths and taper ratios have changed over the years, standardizing on a 12/14 Euro Ceramtec “off-the-shelf” style taper allows for more standard revision options as compared to using a taper neck sleeve adapter. Neck taper adapters may have design limitations due to the fact that they have skirts that may interfere with range of motion or can cause pushing, generating particles debris and/ or dislocations.

Timothy McTighe et al. [34] published a complete review about past, present, and potential future developments of short femoral stems and reported their main advantages. Taking into account that with some short stem designs, the majority of the femoral neck is preserved, the main advantage is that this facilitates a minimally invasive surgical approach and attenuates the soft and bone tissues damages. Thus, preservation of the femoral neck, which offers a more natural barrier to the migration of particles debris, is associated with less blood losses and less time and energy to rehabilitate the hip, reducing the stress shielding of the proximal femur (redistribution of load and subsequent loss of proximal femoral bone mass) and reducing end-of-stem thigh pain. In these conditions, the use of a short stem can make patient rehabilitation faster and less painful. Theoretically, revision surgery becomes easier if or when it is necessary due to preservation of proximal native bone and tissue – the new design feature inherent in short stem implants. For these reasons, short stems procedures also have wider indications compared to the restoration of the hip surface.

The importance of modularity in total hip arthroplasty (THA) has also been shown by Ronny Grunert et al., [35]. This is because modularity allows orthopedic surgeons to accurately reconstruct the biomechanical parameters of the hip, especially in revision and tumor arthroplasty. Starting from the idea that a new modular neck-stem interface may result in a smaller implant breakage compared to conventional femoral stems, the authors of this paper designed a new modular stem for THA. As a result, three different variants of interface mechanisms have been developed, which ensures a simple connection between the stem and the modular neck and allows intra-operatively adjustment. After dynamic fatigue testing (ISO 7206-6) was shown that realized modular implants prototypes have to be used with caution due to the high risk of breakage. Beside breakage, the taper fretting, corrosion and disconnection are the other risks that can appear in this context. With the new design, it should be possible to detach the stem and neck intra-operatively module to adapt them to the anatomical situation.

Qualitative illustration of the findings of the studies presented in first section

We present below some edifying images of the modular junctions’ failures presented in the works of these authors. In an attempt to shorten the length of this work, we will not always present the images of the damages after each mentioned work, but we will group them, mentioning the bibliographic indication.

Improvements in design and biomaterial characteristics, with the aim of reproducing the natural biomechanics of the hip to maximize functioning and longevity in the case of total hip replacement (THR) include modular neck systems. [3]

These allow a wide range of adjustments to the rotation centre of the femoral head. However, there were problems, and in July 2012, ABG2 and Rejuvenate modular neck stems (Stryker, Kalamazoo, Michigan) were withdrawn due to high overhaul rates as a result of metal debris at the modular junction. Over 30000 THR with these stems have been implanted all over the world, and in the United Kingdom more than 6000 modular neck stems have been registered at the National Joints Registry.

There are three broad types of modularity: proximal, mid-stem and distal [3]. Proximal modularity includes head-neck junctions, neck-stem junctions, anterior/posterior pads, modular collars, proximal shoulders, and stem sleeves (Figure 1).

In the 1990s a modular femoral component with a variety of neck-stem combinations was introduced, and in the last three decades different types of modular prostheses with specific design features have been developed (Figure 2) [3].

Higher heights and those that go out below the distal part of the femoral head lead to risk and movement limitation. Different stems use designs based on Feldmuhle specifications, ranging from 9/10 to 14/16 tapers. For example, a 9/10 stem has a conical side wall with a maximum diameter of 10 mm, which inclines to a circular end wall of 9 mm. It has been found that a high taper caused a larger impingement, which led to the development of smaller tapers with larger heads, but with the risk of fatigue fractures and fretting corrosion [3].

S. Hussenbocus et al., [4] stated that the Morse taper concept is that of a cone within the joint – trunnion (male portion) and the bore (female portion) – and the stresses created by compression of the bore wall by the trunnion causes interference matching (Figure 3) and cold welding between the two components, which increases during the physiological loading. The modular neck adapters were used with three different CCD angles (130°, 135° and 140°) in combination with neutral, anteversed and reversed versions (±75°) for intraoperative adjustment of the CCD angle – Figure 3.

The radial compression stress that causes fretting between the conical surfaces also ensures resistance to the separation of the head – neck junction. The original angle of the Morse taper cone angle is 2050', but in Orthopedics, the term "Morse taper" is freely used to encompass all the angles that lead to cold welding of one element to another. There are cones of various sizes and angles. Studies have shown that larger diameter and longer heights of circular necks result in reduced range of motion compared to a smaller diameter and a shorter cone with trapezoidal neck [4]. With the first, there is a reduced arc of movement before the onset of impingement and therefore an increased risk of dislocation (Figure 4). Now the trunnions are shorter produced, to reduce the risk of dislocation over time, 14 mm/16 mm trunnions being replaced by trunnions narrower than 12 mm/14 mm. An important concept to understand is that not all "12/14" tapers have the same cone angle, and that the angle is dependent on the trunnion length (Figure 5).

The micro-movement at the taper was independent of the angular mismatches up to 0.075°, but an increase of the micro-movement above this threshold was observed. The tolerance accepted by the industry is 0.0167°, which is lower than the threshold in the cited study.

Studying how fretting corrosion is affected by different materials couplings combinations (Ti-6Al-4V / Ti-6Al-4V, CoCr / Ti-6Al-4V and CoCr / CoCr), Ti-6Al-4V / Ti-6Al-4V coupling showed the most significant deterioration of the surface under axial loading (Figure 6).

Jauch S.Y., et al., [5] analyzed 24 modular hip prostheses (Metha, size 4, Aesculap AG, Tuttlingen, Germany) and 3 monoblock prostheses for mechanical testing. All the stems were made of Ti-6Al-4V alloy, the neck adapters were made of either Ti-6Al-4V alloy or Co-Cr29-Mo alloy with the usual collum-caput diaphysis (CCD) angle of 130° and a neutral version. The distal part of the prosthesis was incorporated into methyl methacrylate (Technovit 4004, Heraeus Kulzer GmbH, Wehrheim Ts., Germany) according to ISO 7206-4 (10° adduction and 9° flexion) for mechanical testing. The neck adapter head cone (Figure 7) was manually mounted with the cobalt-chrome head sphere (Co-Cr29-Mo) (diameter 32 mm and size L). The modular neck adapters together with the head sphere were assembled with the stems, using a servo hydraulic testing machine under force control (500 N/s) at a maximum load of 2000 N (MTS Mini BionixII, MTS, Eden Prairie, MN, USA). Twelve stems with Co-Cr29-Mo neck adapters and 12 with titanium neck adapters were combined. Half of the neck adapters were assembled with a clean interface (n = 6 + 6), the others being contaminated with pigs’ bone chips, before being inserted into the stem cone (n = 6 + 6) – Figure 7. The micro-movements at the stem-neck interface were determined based on the linear displacements measured by three turbulent current sensors with a measuring range of 500 μm and a resolution of 500 μm – U05 (78), Micro-Epsilon, Ortenburg, Germany). Turbulent current sensors were preferred over LVDTs (linear differential transformers) due to their size and weight and the principle of non-contact measurement. Marker-based 3D systems (e.g, markers system of optical reflector or ultrasound system) could not be used due to the precision and resolution required for measurement. The theoretical maximum measurement error of turbulent current sensors, taking into account linearity, resolution, temperature stability, long-term stability and a statistical error, was determined by the manufacturer to be less than 2 μm for a 100 μm movement.

S.I. Jauch et al., [6] reported that the design parameters and the coupled materials are decisive for the size of the micro-movement at the stem-neck interface of the bi-modular hip implants. They mechanically tested 6 bi-modular hip prostheses from two different designs – Metha (Aesculap AG, Tuttlingen, Germany) and H-Max M (Limacorporate, Villanova di San Daniele – Italy). Half of the Ti4Al4V stems were assembled with Ti neck adapters, and the other half with CoCr29Mo necks. After disassembly of the taper connection, the Ti neck adapters of the H-Max M prosthesis showed a slight deterioration of the taper’s laterodistal surface, while the CoCr tapers showed no evidence of surface deterioration such as discoloration and stress traces (Figure 8). None of the Metha neck adapters showed any obvious signs. The Metha prosthesis design with Ti-Ti material coupling has a higher reported neck fracture rate, showing a strong tendency towards larger micro-movements at the taper interface, compared to the H-Max M design, with a very low reported fracture rate and similar coupling of Ti-Ti material from this study. This confirms that not the coupling of the material by itself, but in its combination with the design, is related to the probability of fretting that induces fractures. The change made for the Metha design, of the Ti neck that was withdrawn from the market, with a CoCr adapter, led to a reduction of the micro-movement size in the H-Max M with Ti adapter. The micro-movements cannot be completely eliminated, but the results of this study can be seen as an indication that the micro-movements from the conical interfaces must be kept below 5 μm for being used in clinical applications.

Thomas M. Grupp et al., [7] analyzed the failures of the titanium alloy neck adapter in the hip replacement and analyzed the failure mode and the influence of the implant material. The failed neck adapters were implanted between August 2004 and November 2006. After the third incident, the titanium neck adapters were replaced with cobalt-chromium adapters. The failure of the neck did not cause further damages to the acetabulum. Subsequently, in all these cases, it was not necessary to review the cups. In some cases, the inserts of the cups have been replaced by modules, as a result of visible signs of deterioration caused by the failure of the neck or for precautionary reasons. About 5000 hip prostheses were implanted with a combination of material – titanium stem and titanium adapter. The adapters and modular stems used in this study were, in all cases, Metha Short Hip Stem Prosthesis (Aesculap AG, Tuttlingen, Germany). The adapters and stems were made of titanium alloys. The stem is designed for uncemented anchoring, with pure titanium porous coating and additional thin layer of dehydrated dicalcium phosphate (Plasmapore® μCaP, Aesculap, Tuttlingen, Germany), Figure 9. In order to quantify the relative movement of the modular hip stem interface made of titanium alloy, a total of 8 samples with titanium alloy and cobalt-chromium alloy neck adapters were examined. Because it appears that intraoperative contamination of the cone connection with bone particles has a considerable impact on the size of the fretting interface due to the increased micro-movements, a specific test was developed, and each device was tested with a joint area, clean and contaminated with particles (small bone grafts of pigs about 1 mm in diameter). To determine the relative movement between the neck adapter and the stem, a non-contact measuring system (Micro-Epsilon Type U05 (78) Messtechnik Ortenburg, Germany) was used. Two sensors with a radius of 500 μm and a sensitivity of 0.025 μm were fixed on an aluminum plate and mounted on the neck adapter. Two steel plates fixed on the stem resection plane served as sensor targets (Figure 10).

The stem was combined with 130° CCD angle neck adapters, embedded in bone cement (Palacos R, Heraeus Medical GmbH Wehrheim, Germany) and tested on a servo-hydraulic testing machine (MTS 850.2, MTS Systems Corporation Eden Prairie MN, USA). A sinusoidal axial force between 50 and 2500 N was applied through a ceramic head with neck length L, at a frequency of 1 Hz, for 2000 cycles, to measure the relative displacement between the neck adapter and the stem, as regards irreversible deposition and micro-movements.

A. Hernandez et al., [8] studied the modular neck fracture in total left hip arthroplasty performed in 2009 using a Profemur Modular stem and a Conserve cup (Wright Medical Technology, Inc., 5677 Airline Road, Arlington, TN 38002) with a long modular titanium neck with 8 degrees varus, the surface of the metal-to-metal bearing and a 50 mm femoral head (Figure 11).

Jacob M. Elkins, et al. [9], shows that several years ago, the focus was on increasing the failure rates associated with the large diameter femoral heads used in the MoA THA. There is growing evidence that the tissue wear reaction and corrosion at the head – neck interface (trunnionosis) – Figure 12(a), unlike the reaction to wear on the bearing surface, can play a significant role in the failure of large diameter THA – MoM. In addition, tissue reactions consistent with metallosis for THA metal-on-polyethylene have been reported, suggesting that trunnionose-type wear is not unique to MoM bearing couplings. Increasing of trunnion wear for larger diameter heads could appear plausibly due to the micro-movement that accompanies the increased lever arm between the head center and the pressure center of the trunnion interface – Figure 12(b).

Georgios K. Triantafyllopoulos et al., [10] examined the surface of the head and the head trunnion surface using an optical stereomicroscope (Wild Type 376788, Heerbruug, Switzerland) at magnifications from x6 to x12. The term "trunnion" was used for reference to the male component on the stem and "taper" to describe the female component on the femoral head. The head taper was divided into the proximal and distal regions. The stem trunnion was divided into eight quadrants with the proximal and distal regions subdivided into the posterior, anterior, superior and inferior regions (Figure 13).

The fretting deterioration is as important as the corrosion, because the fretting leads to the continuous repositioning of the oxide layer with the final consumption of oxygen available in the crack, the pH change and the subsequent initiation of the corrosion process. The discoloration, pits, scars and black debris were attributed to corrosion. Each region was classified for fretting and corrosion (Figure 14) using the scoring system developed by Goldberg et al. [11] with a minimum score of 1 and a maximum score of 4.

Jonathon Spanyer et al. analyzed the catastrophic failure at three patients with femoral neck after THA with Accolade 1® stem [12]. They analyzed the explant of a failed femoral stem following a femoral neck fracture (Figure 15).

Francesco Traina et al., [15] analyzed the influence of the rotation center on the survival of the implant using a modular stem hip prosthesis. In all cases, they used an uncemented titanium anatomical stem with modular neck (Anca Fit, Wright Medical Technology, Arlington, Tennessee, USA), a 28 mm ceramic head with a ceramic bushing (Biolox Forte, Ceramtec, Stuttgart, Germany) and an uncemented porous titanium cup (Anca Fit, Wright Medical Technology, Arlington, Tennessee). Anca Fit stem was an anatomical stem made of titanium alloy (Ti-6A-14V), coated with hydroxyapatite (HA) sprayed with 80 μm high crystalline plasma in the proximal third. At the proximal end, a double tapered frame has been provided for modular necks. The modular necks were made of titanium (Ti-6A-14V) and have an elongated section and a taper design. The neck was fixed to the stem taper by an elongated conical profile, but the neck was connected to the head by a standard Morse taper of 12/14. The necks were available in two different lengths: short (28 mm) and long (38.5 mm). For both lengths there were six different designs: straight, varus-valgus 8°, anteverted – retroverted at 8° and 15°, the combination of 6° varus and 4.5° retroversion for left and right side and lateralized – medialized. This wide range of different geometries, provided with the two neck lengths, offers a range of possible compensations of 13.5 mm (Figure 16).

The cup orientation (CO) was evaluated on an anteroposterior radiographic view of the hip. It was considered a cup in a neutral position when the angle between the intertearthropic line and the edge of the cup was between 40° and 50°. The differences between the two groups were also evaluated in terms of femoral fixation, leg length and lever arm of the abductor muscle (Figure 16(b)).

Jeremy L. Gilbert et al., [16] analyzed fretting cracking corrosion of stainless steel (SS) stem – CoCr femoral head connections, materials comparisons, initial humidity and compensatory length. Examples of the conicities appearance after testing can be seen in Figure 17(a-c). These are optical microphotographs after testing the representative samples from each of the four groups tested. It can be seen that each of the stainless-steel tapers shows some signs of fretting corrosion attack, while the CoCr taper presents little visual evidences of the corrosion attack.

Marco Viceconti et al., [17] performed in vitro fretting tests on the cyclic load, on a prototype of an uncemented hip prosthesis, with modular neck (An.C.A.Fit, Cremascoli, Milano, Italy) – Figure 18.

This new design is an evolution of the GSP prosthesis tested in a preliminary study. No significant design changes were introduced in the neck-body taper joint or in the proximal part of the stem. Thus, for the purpose of the present study, the two designs can be considered equivalent. As expected, the amount of material lost and the size of the damaged area increased with the applied load (see Figure 19). The highest loading level (4000N) also caused greater damages on the antero-posterior faces.

M. Baxmann et al., [18] observed that fretting fatigue appeared with reduced movement or increased contact pressure. At low micro-movements or high normal pressures, a low fretting deterioration was observed. They found different modes of deterioration depending on the normal load and the applied micro-movements (Figure 20).

All the surface degradation classifications described in the literature were observed for the different testing conditions. Fretting wear has been found to be the predominant mode of fretting damage due to large micro-movements in combination with low normal loads (x = 25 μm, FN = 25 N and x = 50 μm, FN = 25-50 N; Figure 20). The fretting area showed clear indications of wear by detaching the particles in the fretting direction. At reduced displacement (x = 25 μm, FN = 50 N) or increased normal load (FN = 100 N, x = 50 μm), fretting fatigue appeared. The wear morphology indicated a central gluing and a circular sliding field. At a relatively small movement (x = 10 μm) or large normal load (FN = 100 N, x = 25 μm and FN = 250 N, x = 50 μm), the contact surface was characterized by asperities articulated by adhesion and undamaged areas between local contact points.

M. Ollivier et al., [19] performed the same in all patients, using an Watson-Jones anterolateral approach, in patients in a supportive position under general anesthesia, in 119 cases (71%) or with blockage of the spine in 51 cases (29%). Preoperative 2D planning of THA was performed using TraumacadTM (Voyant Health, Petach-Tikva, Israel). All patients received an uncemented cup (Trilogy TMTTM [Zimmer, Warsaw, IN, USA]). The average diameter of the cup was 52 mm (48-60). The primary stability of the cup was obtained by screw fixing in 158 cases (93%). On the femoral side, a cemented anatomical stem made of Ti6A14V (cemented AptaTM, Adler-Ortho, Milan, Italy) was used in all cases. It was inserted by pressing into the femoral stem, a modular component of the neck, regulated on the three planes (Adler-Ortho, System ModulaTM, made of titanium Ti6A14V; Figure 21) and a 28 mm ceramic head was used.

P. Wodecki et al., [20] presented a new type of hip arthroplasty failure related to the modular femoral components: fracture of the neck – stem junction (Figure 22). The explants were subjected to a metallurgical evaluation by microscopic and optical analysis. The expert's report concluded that the materials used were in accordance with ISO standards for titanium. Microscopic analysis found that the implant failed due to the surface deterioration by fretting corrosion. The experts confirmed on the basis of the Swedish Prosthesis Registry that the fracture rate of the implant was stable since 1998, at 1.7%, and the fracture in this case corresponded to one of them. The report excluded the industrial responsibility.

Brent A. Lanting et al., [22] analyzed 6 consecutively recovered implants, along with two non-implanted reference samples, by scanning electron microscopy (LEO 440 SEM, Carl Zeiss SMT Inc., Peabody, Massachusetts) and energy dispersion X-ray analysis (SEM / EDX) (Quartz Xone EDX system, Quartz Imaging Corporation, Vancouver, British Columbia). All 6 modular necks were examined using SEM / EDX to highlight corrosion, fretting and material transfer – Figure 23. In addition, a stem was sectioned into two planes at the neck-stem interface, and a SEM / EDX analysis was also performed on these sections.

Sherwin L. Su, et al., [23] analyzed 26 modular neck implants, of which 7 SMF, 6 REDAPT, 5 Kinectiv, 4 ABGII, 2 PROFEMUR, 1 ARC and 1 ALFA II. Half (13 of 26) of the recoveries included both the femoral stem and the modular neck. The remaining cases were isolated changes of the modular neck in the absence of a adverse local tissue reactions (ALTR) diagnosis. Modular necks were laser scanned using a non-contact three-dimensional digitizer (Range 7, Konica Minolta, Ramsey, NJ) and dimensional using Geomagic Qualify 12 (version 12; Morrisville, NC) for calculating the bending stiffness. The dimensions were measured from the two-dimensional elliptical or circular sections (depending on the design) of the modular necks at the coupling diameter (Figure 24). The modular necks associated with different metal alloy stems (A) showed higher fretting and corrosion scores than the necks that were coupled with stems of the same metal alloy (B), Figure 25.

Alan M. Kop and Eric Swarts [25] analyzed 16 cases of Margron hip prostheses (Portland Orthopedics Pty Ltd, Matraville, NSW, Australia) with double-taper conical (DTC) – Figure 26. Macro-observations of the 16 recovered components highlighted several key features, including the 6 neck components, with significant fretting and corrosion cracking of the neck-stem taper. In contrast, only 3 neck components showed corrosion signs at the level of the neck-stem taper and had associated corrosion at the level of the neck-head taper. In this sense, the average in situ time of the 6 tapers indicating fretting corrosion was 39 months, compared with 2.7 months for the rest, which did not indicate corrosion. Seventeen months was the shortest in vivo time, before the perceptible corrosion. The corrosion, seen with a 30× stereomicroscope, showed significant areas of surface irregularities with associated black residues, holes and engraving marks, probably due to the cracks’ corrosion and of the fretting scars that indicate movement at the taper junction. The nature of the corrosion stripe, as shown in Figure 27A and Figure 27B, is a consequence of the taper geometry of the neck in the stem, being a relief step in the stem between approximately 3 to 7 mm below the proximal contact point of the taper.

I.P.S. Gill et al., [27] studied a total of 35 recoveries from patients undergoing THR using the femoral component Adaptor GHE/s Short Modular Stem (Eska Implants AG, Lubeck, Germany), which is dual modular, and the Bionik acetabular cup with polyethylene bush (Eska Implants AG). All femoral components were made of cobalt-chromium uncemented, with modular stem-neck junctions (Figure 28). They are coated with a three-dimensional surface coating (Spongiosa Metal II, Eska Implants AG) finished with TiNb alloy to allow inward growth of the bone. The stem has a cobalt-chromium trunnion for articulation with the modular neck (cobalt-chromium in our series) with a tapered Morse connection. The modular femoral head was also made of cobalt-chromium.

The authors describe the case of a healthy 67-year-old woman with osteoarthritis, who underwent a THR in the left hip without complications, using the modular stem with short dual stem, with a 28 mm modular cobalt-chromium head articulated with a bush from polyethylene. At 12 months postoperatively, she developed pain in the inguinal area and diffuse swelling over the anterior aspect of the hip. The symptoms became more severe and a CT scan showed that the femoral component was securely attached, but the acetabular component was not marginally fixed, with visible bubbles at the edge of the metal cup when it was tensioned. The modular head appeared macroscopically normal. There was a yellow discoloration of the polyethylene bush and some black deposits on the cone of the stem neck (Figure 29).

The neck end appeared normal. The acetabular component was removed, as well as the modular neck and head. The abnormal tissue was completely removed. The modular stem was left in situ. A new acetabular cup with a ceramic bush was implanted and a new modular titanium femoral neck was attached to the stem to which a ceramic head was added. The recovered prosthesis was sent for the wear analysis. The trunnion and taper of the head were in relatively clean condition, with a minor deformation of the machined surface. In contrast to this, the neck trunnion that entered in the recovered stem showed significant corrosion and fretting. It is interesting that the corrosion occurred at the neck – stem taper level, but not at the head – neck taper level. Modularity at the head – neck junction is common for hip prostheses, but much less for the neck – stem junction. It is possible that the increase of the micro-movement at the neck – stem junction to occur due to the effect of the lever arm. Moreover, the loading conditions of the cone for the neck – stem taper differ from that of the head-neck in that the forces are transmitted centrally through the head, leading to small stresses. In contrast, the neck – stem connection is eccentrically charged, leading to greater stresses.

Katsufumi Uchiyama et al., [30] studied the early fracture of the modular neck of the MODULUS femoral stem (Lima Corporate, Villanova di San Daniele del Friuli, Italy) made of titanium Ti6Al4V alloy – Figure 30(a), a small modular neck and a 32 mm Biolox strong ceramic femoral head (Lima Corporate, Villanova di San Daniele del Friuli, Italy) and a radial acetabular cup component for a 50 mm Mallory head (BIOMET, Warsaw, IN).

Because the surface at the taper junction of the stem seemed to be scratched – Figure 30(b), the surgeon tried to replace the distal stem, but it was not possible without risking the extensive bone damage. Leaving the distal stem in situ, the modular neck was replaced with a new standard modular neck, with 135° offset and a Delta ceramic head (32 mm). The large trohanter was then reassembled using a dowel sleeve system (AI-physician, Tokyo, Japan) and a very high molecular weight polyethylene fiber cable (although the fiber cable is not visible on X-ray images; NESPLON Cable System, Alfresa Pharma Co., Osaka, Japan). The properly attached acetabular cup was kept together with the highly cross-linked polyethylene bush – Figure 30(b). One year after the surgery, the patient's pain was resolved and he was able to return to work without using a walking aid.

In Figure 31 are presented the two types of new prostheses analyzed by Claudio T. dos Santos et al [31], with stainless steel head and Ti-6Al-4V alloy stem (top) and with stainless steel head and stem (bottom).

Corrosion tests on modular hip implants were performed in vitro in accordance with method I of standard ASTM F1875-98. The modular head was assembled dry in stem, by applying a load of 2 kN, using a hydraulic press with analog loading register. Deterioration produced in the interface of the stem trunnion with the head taper was evaluated by visual analysis and measurement of the released particles residues. Figure 32 presents the stereoscopic images of the taper surface of the SS / SS and SS / Ti designs, before and after testing. A significant corrosive attack was observed on the taper of the SS / SS design after 10 million cycles.

The surface of the neck showed a reddish corrosion products line (Figure 32(b)), located where the head-neck opening (the coupling edge or the distal region) was connected and whose interfaces suffered a relative movement. The lines of the corrosion products were observed in the SS / Ti design. These lines formed around the neck, where the edge of the hole was connected (Figure 32(d)). In this case, the damage due to fretting corrosion was smaller than that on the taper of the SS / SS design, indicating a lower degree of corrosion.

Figure 33 shows a significant corrosive attack in the head of the SS / SS design, compared to the SS / Ti design (Figure 34). This head showed a high degree of corrosion at the points along the hole surface and also at the edge of the hole (Figure 34). On the other hand, the head of the SS / Ti design showed a non-significant fretting corrosion attack (Figure 34). In conclusion, a significant degree of corrosion was observed located on the inner surface of the head and in the head – taper interface in the SS / SS prostheses, resulting from fretting corrosion after 10 million cycles, which also led to particles debris and corrosion products. There were no cracks or failures in the stem body or head – taper connection during the fretting corrosion test.

Nadim James Hallab et al, [32] analyzed the THR stems made of Co alloy (ASTM F-75) which were tested by fretting corrosion, with ceramic femoral heads and Co alloy (ASTM F-75) (6 versions of cemented stems, size 14, taper 12/14: 3 heads of 28 mm made of zirconia stabilized with yttria and 3 heads of 28 mm made of Co alloy, +0 mm neck from Zimmer Inc., Warsaw IN and Saint-Gobain Ceramiques Advances Desmarquest, France). The length of the interface suitable for all heads was about 10.5 mm, as verified by the staining of the counterfeit with comassie blue. Interfacial damage was assessed after corrosion testing using a stereo zoom microscope (Bausch & Lomb Stereozoom 7 Compound Dissection Microscope, Rochester, NY) to monitor surface damage characteristics.

After fretting corrosion testing, the deterioration of the contact surfaces was observed. After removal of the Co alloy heads, small macroscopic or microscopic damages were observed on the machining grooves, which represents the central interlocking mechanism of the taper shape, although some evidences of abrasion appeared (Figure 35). No corrosion residues have been associated with this wear of the grooves.

The heads made of ZrO2 did not show a phenomenon similar to the ridge wear (Figure 36), but the samples were harder to visualize. Visual inspection of the ZrO2 taper showed dark metallic debris embedded between the taper notches "probably from the adhesive – abrasive wear of the metal stem" (Figure 36). Because these debris may have been abraded from the stem during testing or possibly during post-testing removal of the head from the stem, they cannot be considered as released during fretting corrosion.

The tensile strength of the head-neck coupling of the ZrO2 alloy / Co alloy was significantly higher than that of the head-neck coupling of the Co / Co alloy (1018 ± 95 N and 895 ± 115 N, respectively).

Conclusions

The experimental studies and analysis presented in this work allow to draw clear conclusions on the way in which the unfavorable tribological implications of THP modularization are manifesting.

First of all, the possibility (quite rare) of fracturing the neck of the femoral stem as a result of its improper sizing was highlighted. Secondly, the evidence of fretting phenomena that occurred at the tapers' junctions (femoral head – femural stem trunnion; femoral stem – femur stem neck, femoral stem neck – femoral head taper) were mention. Thirdly, the appearance of fretting corrosion at the taper junctions, with negative consequences regarding the reaction of surrounding tissues to debris and foreign particles, should also be highlighted.

Thus, the main conclusion is that the causes of the fretting occurrence are the presence of micro-movements and the mechanical stress at these junctions.

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