Uncemented metal-on-polyethylene total hip arthroplasties (THAs) have had a modular cobalt-chrome alloy head since their introduction in the early 1980's. Retrieval analysis studies and case reports in the early 1990's first reported corrosion between the femoral stem trunnion (usually titanium alloy) and cobalt-chrome alloy femoral head. However, then this condition seemed to disappear for about two decades? There are now numerous recent case series of this problem after metal-on-polyethylene THA, with a single taper or dual taper modular femoral component. Metal ion elevation, corrosion debris, and effusion are caused by mechanically assisted crevice corrosion (MACC). These patients present with diffuse hip pain, simulating trochanteric bursitis, iliopsoas tendinitis, or even deep infection. Trunnion corrosion, with adverse local tissue reaction, is a diagnosis of exclusion, after infection, loosening, or fracture. The initial lab tests recommended are: ESR, CRP, and serum cobalt and chromium ions. With a metal-on-polyethylene THA, a cobalt level > 1ppb is abnormal. Plain radiographs are usually negative, but may show calcar osteolysis or acetabular erosion or cyst. MARS MRI may be the best imaging study to confirm the diagnosis. Hip aspiration for culture and cell-count may be necessary. The operative treatment is empiric, with debridement, and head exchange with a ceramic head-titanium sleeve (or oxidised zirconium head) placed on the cleaned trunnion. The femoral component may have to be removed if there is “whole trunnion failure”. This usually relieves the symptoms, but the complication rate of this procedure may be high

Introduction. There have been increased concerns with trunnion fretting and corrosion and adverse local tissue reactions (ALTR) in total hip arthroplasty. We report on 11 catastrophic trunnion failures associated with severe ALTR requiring urgent revision arthroplasty. Methods. We retrospectively reviewed 10 patients with gross trunnion failure (n=11) and an additional 3 patients with impending trunnion failure. Results. All patients presented to the emergency department with severe pain, an inability to bear weight, and dramatic radiographs demonstrating implant failure. Patients were an average of 7.8 years from the initial index procedure. Implants were a cementless component with metal on polyethylene bearing from a single manufacturer with a 36mm femoral head size and a range of extended offset of 2.5 to 5.5 and neck length of +0 (n=1), +5 (n=5), and +10 (n=5). The implant was used during a limited time (2.5 years) by single surgeon in our practice using a posterior approach, with the last implant placed 7 years ago. Prior to revision, serum cobalt levels were elevated, typically more than chromium levels. Radiographs demonstrated failure of the implant with a dissociation of the head from the taper and large radiolucent fluid collections from the metal debris (Fig 1). Intraoperative findings and magnetic resonance imaging confirmed a diagnosis of ALTR with loss of abductors, and severe material loss from the taper (Fig 2). We estimate a conservative incidence of catastrophic trunnion failure in our series to be 2.2% (n = 636 total implanted). A femoral revision with a modular Wagner stem was performed, and when necessary the acetabulum was revised secondary to destruction of the locking mechanism from mechanical wear. Discussion and Conclusion. ALTR can occur in patients with catastrophic failure of their trunnion. Radiographs are dramatic. Serum metal ion levels and magnetic resonance imaging is comparable to adverse local tissue reaction in metal on metal bearing surfaces, corrosion at the head neck taper, and corrosion of dual modular taper stems. The initiating events leading to this mechanism of trunnion failure is unknown. We observed trunnion failure at large head sizes and at increased femoral head offsets. Other groups have reported an association of trunnion failure at increased neck length and head size with a variety of manufacturers. Based on our experience, we have been able to identify impending failure based on subtle radiographic criteria and elevated metal ion levels. A failure rate of 2% at 7.8 years is non-trivial. Impending trunnion failure should be considered as a possible diagnosis in a painful total hip arthroplasty with unknown etiology. Serum metal ion levels and magnetic resonance imaging should be obtained as part of the initial evaluation

Introduction. The release of metallic debris can promote many adverse tissue reactions, as metallosis, necrosis, pseudotumors and osteolysis . 1–3. This debris is mainly generated by the fretting-corrosion mechanism due to the geometric difference in the head-stem interface . 4. Retrieval and in silico analysis showed the roughness of the stem-head interface appears to play an important role in the volume of material lost and THA failure . 5–7. The technical standard ISO 7206-2 recommends the measurement of average roughness (Ra) and max height of the profile (Rz) to control the quality of the surface finish of articulating surfaces on THA implants. However, despite the importance of the trunnion roughness, there is no specific requirement for this variable on the referred technical standard. The present study carried out a surface finish analysis of the trunnion of hip stems from five distinct manufacturers. Methods. Four stems (n = 4) from five (5) distinct manufacturers (A, B, C, D, and E) were used to evaluate the roughness of the trunnion. All the stems are similar to the classical Exeter stem design, with a 12/14 taper and a polished body surface. The roughness of trunnions was evaluated according to ISO 4287 and ISO 13565-2. The total assessment length was 4.8 mm with 0.8 mm cut-off. The first and last 8.33% of assessment length were not considered. The measurements of all samples were made in a rugosimeter with 2 µm feeler ITP (Völklingen, Germany), the velocity of 0.5 mm.s. -1. , and a force of 1.5 mN. The calibration was made at 20 ºC and relative humidity at 50%. The Kruskal Wallis with post hoc Nemenyi test was used to evaluate the difference of Ra among the manufacturers. The confidence level was set at 5%. Results and Discussion. The analysis of surface finish revealed different roughness among the manufactures (p < 0.005), with Ra between 0.061 µm to 3.184 µm and Rz varying of 0.41 µm to 12.69 µm. The manufacturers A and E had a Ra (2.587±0.050 µm and 3.146±0.031µm) of the trunnion similar to founded by Panagiotidou et al (2013). Within such range, the trunnion has shown a high presence of pit . 8. The manufacturer C, on the other hand, had the best surface finish of the trunnion (Ra = 0.069±0.010 µm and Rz = 0.505 ± 0.076 µm). This more smooth surface might increase the taper strength, reduce the shear stress and the susceptibility to the fretting-corrosion damage . 4,8. . Conclusion. The results were worrying because there is great variability of roughness among the manufacturers with the occurrence of trunnions with roughness too high. Nevertheless, the ISO technical standard does not recommend any procedure or minimum parameters acceptable for the surface finish of the trunnion. The revision of ISO 7206-2 would guarantee better control of trunnion roughness to reduce the amount of metallic debris and increase the safety of THA implants. Additional research is needed to determine a target value for this variable. For any figures or tables, please contact authors directly

Introduction. The failure rate of Total Hip Replacement (THR) has been shown to be strongly influenced by the nature of the articulating interfaces, with Metal-on-Metal (MoM) articulations having three times the failure rate of Metal-on-Polyethylene (MoP) components. It has been postulated that this observation is related to edge wear and increased bearing torque of large MoM heads, which would lead to increased loading and wear at the head taper junction and, subsequently, to the release of metal ions and corrosion products. This suggests that taper wear and corrosion should not be as prevalent in large head MoP implants as in large head MoM implants. This study was undertaken to test the hypotheses that: (i) MoM implants exhibit higher rates of corrosion and fretting at the head taper junction than MoP implants, and that (ii) the severity of corrosion and fretting is greater in components of larger head diameter. Materials and Methods. Our study included 90 modular implants (41 MoM; 49 MoP) retrieved during revision hip arthroplasties performed between 1992 and 2012. Only retrievals with head diameters greater than 32 mm were included, and trunnion sizes ranged from 10/12 mm to 14/16 mm with 12/14 mm being the most common size. The stem trunnion and head taper surfaces were examined under stereomicroscope by a single observer. Each surface was scored for both corrosion (using a modified Goldberg scoring system) and fretting (using the standard Goldberg scoring system). For both the trunnion and head tapers, the student's t-test was used to determine if differences exist in the severity of corrosion or fretting between the MoM and MoP groups and between different head sizes of the same articulation type. Results. Overall, there was no significant difference in the severity of corrosion or fretting damage of femoral head taper surfaces or in the fretting of stem trunions between articulation types (p values: 0.245 to 0.733) or head sizes (p values: 0.333 to 0.680). However, corrosion damage of the trunions did vary with the type of articulation (p = 0.0069) and with head size (p = 0.0145). MoP trunnions were found to have significantly more corrosion damage than MoM trunnions at head diameters greater than 40 mm (p = 0.005). Discussion. The surprising conclusion of this study is that the severity of trunnion corrosion in MOP articulations, which surpassed the tribo-corrosion of MOM joints, especially when the prosthetic head size exceeded 40 mm. This conclusion is consistent with the presence of moderate to severe third-body damage in many large diameter polyethylene liners which would lead to a large increase in the frictional torques generated during hip motion. In addition, only part of the loading of the trunnion arises from increased frictional torque. The increase in head size, especially in designs with an offset head center, will lead to increased toggling, and accelerated wear and corrosion of the taper junction, independent of the bearing surface material

Introduction. The Morse taper was adapted into orthopaedics as a connecting junction in total hip replacements. The benefit of modular systems includes the adjustment of leg length, offset and the ability to remove the head for acetabular exposure during revision surgery. The design of the Morse taper facilitates the intimate contact of the conical trunnion of the femoral stem (male component), with the conical bore of the femoral head (female component). Design consideration for tapers involve physical parameters (angle, length, diameter and contact points), and manufacturing finish (surface characteristics). Orthopaedic trunnion tapers are not standardized and vary in length, taper angle and base dimension. Variations in the design and surface characteristics of the trunnion, which directly reflects on the interface at the taper junction can influence the likelihood of subsequent wear, corrosion and ultimately effect longevity of the implant. Methods and Materials. We studied 11 clean un-used commercial stems from five different companies (DePuy, Stryker, Biomet, Wright Medical, Smith and Nephew). Trunnion surface was scanned with an optical profilometer (Bruker ContourGT-I 3D Optical Microscope (Karlsruhe, Germany), an interference microscopes with the capability to analyse 3 dimensional topographical features of materials. Three measurements of each trunnion were taken on the anterior surface at the proximal, mid-point, and distal segments. Each region scan was conducted under a 20x objective with the scan length of 1000 um and a back scan of 500 um. Results. Four of the eleven hip stems demonstrated a smooth surface finish and the remaining seven had a distinct threaded surface with the Corail and the SROM surface topography shown in figure 1. All the analysed trunnion surfaces had a Sa value than Sz value (p < 0.05), indicating each trunnion surface consists of repetitive peak heights and valley depth across the scanned surface area shown in figure two. The higher Sa parameter for the threaded trunnions indicates a rougher overall surface due to a recurring threads pattern. The mean dominant thread heights of the trunnions varied from 12.91 μm on the Profemur to 3.24 μm on the SecurFit Max. The Silent, Summit and the Tri-Lock stems demonstrated a variation in thread heights on their respective surfaces. The Summit stem also demonstrated variability in the pitch of its threads which was not seen on the other threaded stems. The Silent had the widest pitch at 0.304 μm whilst the Profemur had the narrowest pitch at 0.13 μm. Conclusion. Threaded trunnion surfaces for different stems are not uniformly patterned and demonstrate variability in thread height and pitch. These differences could affect micro motion at the taper junction, influencing corrosion and fretting. The study is limited in its capacity as only one stem of each type was analysed, therefore any variation between same stem designs cannot be commented on

There is renewed concern surrounding the potential for corrosion at the modular head-neck junction to cause early failure in modern hip implants. Although taper corrosion involves a complex interplay of many factors, previous studies have correlated decreasing flexural rigidity of the femoral trunnion with an increased likelihood of corrosion at retrieval. A multicenter retrieval analysis of 85 modular femoral stems was performed to calculate the flexural rigidity of various femoral trunnions. Stems were implanted between 1991–2012 and retrieved between 2004–2012. There were 10 different taper designs from 16 manufacturers. Digital calipers were used to measure taper geometries by two independent observers. Mean flexural rigidity was 262 Nm. 2. , however there was a wide range of values among the various stems spanning nearly an order of magnitude between the most flexible (80 Nm. 2. ) and most rigid (623 Nm. 2. ) trunnions, which was due in part to the taper geometry and in part to the material properties of the base alloy. There was a modest but significant negative correlation between flexural rigidity of the trunnion and release date of the stem. This wide variability in flexural rigidity may predispose particular stem designs to an increased risk of corrosion at the modular head-neck taper, and may in part explain why taper corrosion is being seen with increasing frequency in modern hip arthroplasty

Introduction:. Angular mismatch of the modular junction between the head and the trunion has been recognized as a contributing factor to fretting and corrosion of hip prostheses. Excessive angular-mismatch can lead to relative motion at the taper interface, and tribo-corrosion of the head-neck junction secondary to disruption of the passive oxide layer. Although manufacturing standards have been adopted to define acceptable tolerances for taper angles of mating components, recent investigations of failed components have suggested that stricter tolerances or changes in taper design may be necessary to avoid clinical failures secondary to excessive taper wear and corrosion. In this study we examine the effect of angular-mismatch on relative motion between the taper and bore subjected to normal gait load using finite element methods. Methods:. Computer simulations were executed using a verified finite element model (FEM), the results from which were determined to be consistent with literature. A stable, converging hexahedral mesh was defined for the trunnion (33648 elements) and a tetrahedral mesh for the femoral head (51182 elements). A friction-based sliding contact was defined at the taper-bore interface. A gait load of 1638N (2.34 × BW, BW = 700N) was applied at an angle of 30° to the trunnion axis (Figure 1) on an assembled FEM. A linear static solution was set up using Siemens NX-Nastran solver. Angular-mismatch was simulated by incrementing the conical half-angle of the bore to examine these cases: 0°, 0.005°, 0.010°, 0.015°, 0.030°, 0.050°, 0.075°, 0.100°, 0.200°and 0.300°. Results:. Relative interface micro-motion at the proximal-medial point of the taper demonstrated a lack of dependence upon angular-mismatch for tolerances up to 0.075° and a monotonic increase in micro-motion for higher tolerances (0.075–0.3 °; Figure 2). A similar trend was observed with respect to the average values of contact pressure, max von Mises stress and shear stress acting at the proximal-medial aspect of the taper (Figure 3). Non-linear correlation tests indicate a significant correlation (p < 0.0001) of mismatch angle with peak von Mises stress (r = 0.965) and relative micro-motion (r = 0.964). Discussion:. The FEA results corroborate the notion that high angular-mismatch tolerances have a deleterious effect of fretting at the trunnion-head interface. Although, stability of the implant did not appear to be compromised at relatively lower tolerances, the propensity for it is higher at higher mismatches. The simulation was, however, executed as a single-step static analysis ignoring the effect of cyclical loading often observed during gait. This abstract serves as a proof of concept to justify the further development of this FEA to study the effect of angular-mismatch tolerances on micro-motion at the trunnion-head interface. However, current results strongly indicate that tolerance for angular-mismatch can be more liberal without increasing the micro-motion and stresses at the trunnion-head interface. Significance: The effect of angular-mismatch suggests a threshold tolerance different from the industry accepted tolerance of 0.0167°. Mismatches smaller than 0.075° demonstrated only modest variation in the interface micro-motion. Additionally, the results corroborate recent clinical evidence that even with perfectly fit implants, the potential for interface micro-motion can lead to fretting-induced corrosion

Introduction. Corrosion of the femoral head-trunnion junction in modular hip components has become a concern as the corrosion products may lead to adverse local tissue reactions. A simple way to avoid trunnion corrosion is to manufacture the femoral head with a non-metallic material, such as ceramics that are widely. An alternative solution may lie in advanced polymers like polyaryletherketones (PAEKs). These thermoplastics have high mechanical strength necessary for use as femoral heads in hip arthroplasty, but they must be tested to ensure that they do not adversely affect the wear of the ultrahigh molecular weight polyethylene (UHMWPE) liner counterface. Pin-on-disc (POD) wear testing has been extensively used to evaluate the wear properties of UHMWPE prior to more extensive and costly analysis with joint simulators. We hypothesized that the wear of crosslinked UHMWPE would not be adversely affected in POD tests when articulated against an advanced thermoplastic counterface. Methods. 0.1 wt.% VitE blended UHMWPE stock was e-beam irradiated to 100, 125, 140, 160, and 175 kGy and machined into cylindrical pins for testing. An additional group of 100 kGy e-beam irradiated and melted UHMWPE (with no vitamin E) was also machined and tested. Three different counterface materials were tested: (1) Cobalt-chrome (CoCr) with a surface roughness (R. a. ) of <0.5 μm, (2) Biolox™ ceramic (CeramTec), and (3) Polyetheretherketone (PEEK), a member of the PAEK family (Fig 1). A bidirectional POD wear tester [1] was used to measure the wear rate of UHMWPE specimens, where the specimens moved in a 10 mm × 5 mm rectangular pattern under a Paul-type load curve [2] synchronized with the motion. The peak load of the loading curve corresponded to a peak contact pressure of 5.1 MPa between each UHMWPE pin specimen and the counterface disc. Each test was conducted at 2 Hz in undiluted bovine serum stabilized with ethylenediamine tetraacetate (EDTA) and penicillin. The pins were cleaned and weighed daily, and a wear rate was calculated at the end of each test by linear regression. Results. As expected, higher radiation doses led to lower wear rates against all counterface materials (Fig 2). The PEEK discs produced the lowest UHMWPE wear in each group and the CoCr discs produced the highest UHMWPE wear; however, the two UHMWPE groups with the lowest wear rates showed no difference between the three counterface materials. Conclusions. Even though the PEEK discs had visible machining marks – that is they were not polished to an implant surface finish – they still yielded the lowest wear rates for UHMWPE articulating against them when compared to the highly polished and smooth CoCr and ceramic materials. Implementing further steps to better the surface roughness of the PEEK counterface may yield even better wear rates. Using PEEK in femoral heads may alleviate issues with trunnion corrosion without increasing the incidence of osteolysis or other wear related issues. For any figures or tables, please contact authors directly (see Info & Metrics tab above).

It is now well recognised that adverse local tissue reaction (so called pseudotumor or ALTR) may follow the use of metal-on-metal arthroplasty, the source of the metal being the primary bearing surface (the articulation itself) or a modular junction within the construct, such as the trunnion. This is more likely seen if a large femoral head has been used. We are aware of this risk and have implemented surveillance systems worldwide to facilitate its early diagnosis and prompt management. Less well known, and silent in its initial development, is adverse local reaction following metal-on-polyethylene hip replacement. Initially the problem is unknown to the patient and the surgeon. Typically there follows unexplained pain (due to synovitis), loss of motion, sometimes the development of a mass, followed by increasing dysfunction as the inflammatory, destructive process invades the surrounding soft tissues. Radiographic features are late to follow (focal osteolysis), unless cross-sectional imaging is considered (ultrasound and/or metal suppressed MRI) early in the process. Serum metal ions are an important adjunct in its diagnosis, and it is now acknowledged that the previously used level of 7 ppb is much higher than that required to recognise the entity. Typically there is a reverse of the usual CrCo ratio, and a recent study has suggested that a raised serum Co alone is adequate to assist in the diagnosis. Four important lessons have been learned at our center in recent years and will be demonstrated in this case-based presentation designed for surgeons in practice. It is hoped that the cases and supportive literature will prove useful in advancing the early diagnosis of this troublesome and often silent condition

Introduction. Corrosion products from modular taper junctions are a potent source of adverse tissue reactions after THR. In an attempt to increase the area of contact and resistance to interface motion in the face of taper mismatches, neck trunnions are often fabricated with threaded surfaces designed to deform upon assembly. However, this may lead to incomplete contact and misalignment of the head on the trunnion, depending upon the geometry and composition of the mating components. In this study we characterized the effect of different femoral head materials on the strength and area of contact of modular taper constructs formed with TiAlV trunnions. Materials and Methods. Three groups of 36mm femoral heads (CoCr, Biolox ceramic; Oxinium) and matching Ti-6Al-4V rods with 12/14 trunnions were selected for use in this study. The surface of each trunnion was coated with a 20nm layer of gold applied by sputter-coating in vacuo. Each head/trunnion pair was placed in an alignment jig and assembled with a peak axial impaction force of 2000N using a drop tower apparatus. After assembly, each taper was disassembled in a custom apparatus mounted in a mechanical testing machine (Bionix. MTS. After separation of the components, the surface of each trunnion was examined with backscattered electron microscopy to reveal the area of disruption of the original gold-coated surface. Images encompassing the entire surface of the trunnion were collected and quantified by image processing. Results. The force required to disassemble the Oxinium and Biolox heads from their mating tapers were 2153±104N and 2200±145N, respectively (p-=0.5359). In contrast, the average disassembly force of the CoCr-TiAlV couples was 47% less (1165±156N, p<0.0001). Direct contact between the trunnion and the femoral head was only present over 3.7±0.3% of the nominal surface area of the modular junctions and was limited to the crests of the threads. Contact area did not vary as a function of head composition (p>0.4). However, there were noticeable differences in terms of the distribution of contact between the head and the trunnion. CoCr heads typically had large spans of noncontact immediately below the apex of the taper and opposite each other at the trunnion base. Biolox heads tended to have complete contact at the apex but only extended down 30% of the taper and intermittently at the base. Oxinium heads had comparable complete contact areas to Biolox at the apex but unlike Biolox and CoCr, a uniform band of contact existed at the base. Conclusions. CoCr heads provided only half the resistance to disassembly of Biolox and Oxinium heads. The total area of direct head-trunnion contact is minimal and is not affected by head composition. The heads studied had characteristic patterns of interface contact. This may be due to variations in the geometry of the bores within each head combined with cocking of the femoral head during seating as the thread peaks are being deformed. For any figures or tables, please contact authors directly

Taper corrosion and fretting have been associated with oxide layer abrasion and fluid ingress that contributes to adverse local tissue reactions with potential failure of the hip joint replacement. [1,2]. Both mechanisms are considered to be affected by the precise nature of the taper design. [3]. Indeed relative motion at the taper interface that causes fretting damage and wear effects, such as pistoning and rocking, have been described following analysis of implants at retrieval. [4,5]. However, there is much less reported about the mechanisms that allow the fluid ingress/egress at the taper interface which would drive corrosion. Thus the aim of the present study was to investigate the effect of trunnion design on the gap opening and taper relative motions under different load scenarios and taper designs. A 3-D finite element model of a 40mm CoCr modular femoral head and a Ti6Al4V trunnion was established in Abaqus CAE/2018. Femoral head and trunnion geometries were meshed with an element (C3D8) size of 0.17mm. Tapers were assembled by simulating a range of impact forces (AF); taper interface behaviour was evaluated under physiological forces and frictional moments simulated during walking activity. [6]. , assuming different coefficients of friction (CF), Figure 1. The output involved the total and normal relative motion of the surfaces at the taper interface. The model predicted for a taper mismatch of 0.36° which, when combined with an assembly force of 2kN, generated the largest taper gap opening (59.2mm) during walking, Figure 2. In all trunnion designs the largest normal relative motion coincided with heel strike in the gait cycle (0–5%). The taper gap and normal relative motions were related to the initial taper lock area. Furthermore, the direction of the total motion was different in all three taper mismatches, with a shift in the direction towards the normal of the surface as the taper mismatch increased, Figure 3. By contrast, the direction of the normal relative motions did not change with different trunnion designs. Contact patterns were asymmetrical and contact areas varied throughout the walking activity; contact pressure and the largest taper gap were located on the same side of the taper, suggesting toggling of the trunnion. The relationship between taper gap opening and initial taper lock contact area suggests that the taper contact area functions as a fulcrum in a lever mechanism. Large taper mismatches create larger relative motions that will not only create more wear and fretting damage but also larger normal relative motions. This may allow fluid ingress into the taper interface and/or the egress of fluid along with any metal wear particles into the body. This increased understanding of the taper motion will result in improved designs and ultimately taper performance. For any figures or tables, please contact the authors directly

Introduction. The bearing surfaces of ceramic-on-ceramic (CoC) total hip replacements (THR) show a substantially lower wear rate than metal-on-polyethylene (MoP) THR in-vitro. However, revision rates for CoC THR are comparable with MoP. Our hypothesis that an explanation could be adverse reaction to metal debris (ARMD) from the trunnion led us to investigate the wear at both the bearing surfaces and the taper-trunnion interface of a contemporary CoC THR in an in-vitro study. Methods. Three 36mm CoC hips were tested in a hip simulator for 5 million cycles (Mc). BIOLOX. ®. delta ceramic femoral heads were mounted on 12/14 titanium (Ti6Al4V) trunnions. Wear of femoral heads, acetabular liners and trunnions was determined gravimetrically using the analytical balance. Roughness measurements (Sa) were taken on the articulating surfaces (pre and post-test) and on the trunnion surfaces (worn and unworn). Furthermore, Energy Dispersive X-ray Spectroscopy (EDX) was used to identify and quantify the wear debris present in the lubricant using scanning electron microscope (SEM). Results and Discussion. The total volumetric wear was 0.25 mm. 3. for CoC joints and 0.29 mm. 3. for titanium trunnions. The total wear volume of the titanium trunnions was in agreement with an explant study (Kocagoz et al, 2016, CORR) which quantified the volumetric material loss from retrieved trunnions with the total wear ranging from 0.0–0.74 mm. 3. The Sa values, pre-and post-test, for heads were 0.003 ± 0.002 and 0.004 ± 0.001 µm and for liners were 0.005 ± 0.001 and 0.005 ± 0.001 µm. Pre-and post-test measurements for Sa of heads (p = 0.184) and liners (p = 0.184) did not show a statistically significant change. The Sa of the trunnions on the unworn and worn areas showed a statistically significant decrease from 0.558 ± 0.060 to 0.312 ± 0.028 µm respectively (p < 0.001). Analysis of wear debris within the lubricant confirmed the presence of titanium. A recent clinical study (Matharu et al, 2016, BMC Musc Dis) found more ARMD in CoC hips than MoP hips. This is despite there being fewer metallic components in a CoC hip than a MoP hip. This in vitro study has shown that one source of metal debris in a CoC hip is the taper-trunnion junction. Conclusion. An explanation for wear related failures in ceramic-on-ceramic hip arthroplasty, despite the low wear arising at the articulating surfaces, may now exist; namely that titanium wear particles are generated from the trunnion. No other long-term hip simulator studies have measured wear at the taper-trunnion junction

INTRODUCTION. In theory, Finite Element Analysis (FEA) is an attractive method for elucidating the mechanics of modular implant junctions, including variations in materials, designs, and modes of loading. However, the credence of any computational model can only be established through validation using experimental data. In this study we examine the validity of such a simulation validated by comparing values of interface motion predicted using FEA with values measured during experimental simulation of stair-climbing. MATERIALS and METHODS. Two finite element models (FEM) of a modular implant assembly were created for use in this study, consisting of a 36mm CoCr femoral head attached to a TiAlV rod with a 14/12 trunnion. Two head materials were modelled: CoCr alloy (118,706 10-noded tetrahedral elements), and alumina ceramic (124,710 10-noded tetrahedral elements). The quasi-static coefficients of friction (µ. s. ) of the CoCr-TiAlV and Ceramic-TiAlV interfaces were calculated from uniaxial assembly (2000N) and dis-assembly experiments performed in a mechanical testing machine (Bionix, MTS). Interface displacements during taper assembly and disassembly were measured using digital image correlation (DIC; Dantec Dynamics). The assembly process was also simulated using the computational model with the friction coefficient set to µ. s. and solved using the Siemens Nastran NX 11.0 Solver. The frictional conditions were then varied iteratively to find the value of µ providing the closest estimate to the experimental value of head displacement during assembly. To validate the FEA model, the relative motion between the head and the trunnion was measured during dynamic loading simulating stair-climbing. Each modular junction was assembled in a drop tower apparatus and then cyclically loaded from 230–4300N at 1 Hz for a total of 2,000 cycles. The applied load was oriented at 25° to the trunnion axis in the frontal plane and 10° in the sagittal plane. The displacement of the head relative to the trunnion during cyclic loading was measured by a three-camera digital image correlation (DIC) system. The same loading conditions were simulated using the FEA model using the optimal value of µ derived from the initial head assembly trials. RESULTS. For both head materials, the predicted values of axial displacement of the head on the trunnion closely approximated the measured values derived from DIC measurements, with differences of −0.17% to +6.5%, respectively. Larger differences were calculated for individual components of motion for the stair climbing activity. However, the predicted magnitude of interface motion was still within 10% of the observed values, ranging from −7% to −5%. CONCLUSIONS. Our simulations closely approximated physical testing using complex loading, coming within 7% of the target values. By generating a validated computational model of a modular junctions with varying head materials, we will be able to simulate additional activities of daily living to determine micromotion and areas of peak pressure and contact stresses generated. For any figures or tables, please contact authors directly

Introduction. Modern hip replacements all have encapsulated the design concept of proximal modularity. The factors contributing to the increased wear and corrosion at the taper junction are trunnion geometry, surface characteristics, head size, impaction forces, and material coupling. This study maps the inferior and superior region of the trunnion and bore to provide a visual identification of the corrosion severity. The corrosion/wear generated inferiorly and superiorly at the bore and trunnion will be quantified to understand how corrosion is affected by mechanical stresses in relation to anatomical orientation. Methodology. Three neck tapers generated from bar stock containing a threaded trunnion Ti-6Al-4V and 3× 32mm femoral heads (Co-Cr-Mo) with a +4 offset manufactured by Signature Orthopaedics were used within this study. Rectangular Rozzette strain gauges (Tokyo Sokki Kenkyujo Co., Ltd.) were adhered onto the inferior and superior sections of the neck section. The tapers were fatigued in accordance to ISO 7206 at 5Hz for 5 million cycles at 37 degrees Celsius in phosphate buffered saline. The tapers were sectioned from the center of the femoral head to split both trunnion and bore into superior and inferior components. SEM imaging of all surface areas for each component, per taper (4) was done under ×100 magnification. The images were used to quantify the corrosion present across the surface area using a MATLAB based program called Histomorph. To obtain a visual observation of the variation of corrosion across the bore and trunnion the proximal, medial, and distal regions were mapped together for both the superior and inferior sections. Results. The superior region of the trunnion had a dominant tensile strain in comparison to the inferior region, which had a dominant compressive strain. Corrosion/wear of the inferior section of the trunnion was significantly higher (p<0.05) in comparison to the superior section (Figure 1). The bore had more corrosion/wear on the superior side in comparison to the inferior side however the difference was not significant. The mapping of the trunnion shows corrosion/wear along the whole length of the inferior side and dominantly at the distal region for the superior side (Figure 2 & 3). The superior section of the trunnion had higher corrosion/wear damage across the center and distal regions of the trunnion. The subdivision of the superior section reveals that the majority of the distal section contains higher wear/corrosion damage. However the central region also has sufficient corrosion/wear extending across the width of the bore. Conclusion. The corroded regions have shown that the type of stress present on the regions of the taper junction determines the severity of corrosion. The inferior section of the trunnion under compressive stress has significantly (p<0.05) higher corrosion/wear in comparison to the superior section dominated by tensile stress

Trunnionosis in modular hip arthroplasty has recently been recognised to be clinically important. Gaining an understanding of how the material interface at the head-trunnion affects the tribology at the modular junctions has current clinical implications as well as an implication on future implant selection and material choice. This matched-cohort study aims to compare tribocorrosion between ceramic and cobalt-chromium trunnions and to investigate other factors that contribute to the difference in tribocorrosion if present. All hip prostheses retrieved between 1999 and 2015 at one centre were reviewed. Fifty two ceramic heads were retrieved, and these were matched to a cobalt-chromium cohort according to taper design, head size, neck length and implantation time in that order. The two cohorts were similar in male:female ratio (p=0.32) and body mass index (p=0.15) though the ceramic group was younger than the cobalt-chromium group (56.6 (+/−)13.5 years for ceramic group vs 66.3 (+/−14.4) years for cobalt-chromium group; p=0.001). There was no significant difference in the reasons for revision between the two groups (p=0.42). The femoral head trunnions were examined by two independent observers using a previously published 4-point scoring technique. The trunnions were divided into three zones: apex, middle and base. The observers were blinded to clinical and manufacturing data where possible. Ceramic head trunnions demonstrated a lower median fretting and corrosion score at the base zone (p<0.001), middle zone (p<0.001) and in the combined score (p<0.001). In a subgroup analysis by head size, ceramic heads had a lower fretting and corrosion score at 28mm head diameter (p<0.001). Within the ceramic group, taper design had a significant effect on fretting and corrosion in the apex zone (p=0.04). Taper design also had a similar effect in the cobalt-chromium group in the apex zone (p=0.03). For the ceramic trunnions, the largest effect was contributed by the difference between the 11/13 taper and the 12/14 taper. For the cobalt-chromium trunnions, the largest effect was contributed by the difference between the 5 degree 38′ 37″ taper and type 1 taper. Ceramic head trunnions showed a significantly lower fretting and corrosion score as compared to cobalt-chromium trunnions. Ceramic heads had a lower score than cobalt-chromium heads at 28mm head diameter. Taper design had an effect on fretting and corrosion within each cohort

Introduction. Manifestation of high interface stresses coupled with micromotion at the interface can render the taper lock joint in a modular hip replacement prosthesis at risk for failure. Bending can lead to crevice formation between the trunnion and the head and can potentially expose the interface to the biological fluids, generating interface corrosion. Additionally, development of high stresses can cause the material to yield, ultimately leading to irreversible damage to the implant. The objective of this study is to elucidate the mechanical response of taper junction in different material combination assemblies, under the maximum loads applied during everyday activities. Methods. Computer simulations were executed using a verified FE model. A stable hexahedral mesh (33648 elements) was generated for the trunnion (taper size: 12/14mm) and a tetrahedral mesh (51182 elements) for the head (CoCr, size: 32mm). An assembly load of 4000N was applied along the trunnion axis followed by the application of a load of 230–4300N at 25° and 10° angle to the trunnion axis in the frontal and sagittal planes. A linear static solution was set up using Siemens NX Nastran. Two material combinations were tested - cobalt-chrome head with a titanium alloy trunnion and cobalt chrome head with a cobalt-chrome trunnion. Results. Table1 compares the results obtained from the simulation to those observed in experimental simulations performed under similar loading conditions in our lab. Larger vertical interface displacement was observed in the CoCr-CoCr assembly during toggle-inducing loads. The trunnion bending inside the femoral head was higher in the Ti-CoCr assembly (0.056) compared to the CoCr-CoCr assembly (0.027) with the overall bending of the Ti-CoCr assembly also observed to be much higher (Fig.1). Negligible difference between the stress measured in the femoral head and taper was observed (Fig.2). Discussion. Bending could potentially lead to the development of higher stresses especially under multiple cycles of loading. Fatigue and plastic deformation could result in irreparable damage to the interface leading to implant failure. Additionally, bending causes a separation of the interfaces at the trunnion-head junction, leading to crevice formation, triggering corrosion by exposure to the surrounding physiological environment. Thus, it is crucial that we understand the mechanics of the trunnion-head junction especially under conditions of functional loading

Introduction. Metal-on-polyethylene (MoP) is the most commonly used bearing couple in total hip replacements (THRs). Retrieval studies (Cooper et al, 2012, JBJS, Lindgren et al, 2011, JBJS) report adverse reactions to metal debris (ARMD) due to debris produced from the taper-trunnion junction of the modular MoP THRs. A recent retrospective observational study (Matharu et al, 2016, BMC Musc Dis) showed that the risk of ARMD revision surgery is increasing in MoP THRs. To the authors' best knowledge, no hip simulator tests have investigated material loss from the taper-trunnion junction of contemporary MoP THRs. Methods. A 6-station anatomical hip joint simulator was used to investigate material loss at the articulating and taper-trunnion surfaces of 32mm diameter metal-on-cross-linked polyethylene (MoXLPE) joints for 5 million cycles (Mc) with a sixth joint serving as a dynamically loaded soak control. Commercially available cobalt-chromium-molybdenum (CoCrMo) femoral heads articulating against XLPE acetabular liners (7.5Mrad) were used with a diluted new-born-calf-serum lubricant. Each CoCrMo femoral head was mounted on a 12/14 titanium alloy trunnion. The test was stopped every 0.5Mc, components were cleaned and gravimetric measurements performed following ISO 14242-2 and the lubricant was changed. Weight loss (mg) obtained from gravimetric measurements was converted into volume loss (mm. 3. ) and wear rates were calculated from the slopes of the linear regression lines in the volumetric loss versus number of cycles plot for heads, liners and trunnions. Additionally, volumetric measurements of the head tapers were obtained using a coordinate measuring machine (CMM) post-test. The surface roughness (Sa) of all heads and liners was measured pre and post-test. At the end of the test, the femoral heads were cut and the roughness of the worn and unworn area was measured. Statistical analysis was performed using a paired-t-test (for roughness measurements) and an independent sample t-test (for wear rates). Results and Discussion. The mean volumetric wear rates for CoCrMo heads, XLPE liners and titanium trunnions were 0.019, 2.74 and 0.013 mm. 3. /Mc respectively. There was a statistically significant decrease (p<0.001) in the Sa of the liners post-test. This is in contrast to the femoral heads roughness in which no change was observed (p = 0.338). This head roughness result matches with a previous MoP in vitro test (Saikko, 2005, IMechE-H). The Sa of the head tapers on the worn area showed a statistically significant increase (p<0.001) compared with unworn, with an associated removal of the original machining marks. The mean volumetric wear rate of the head tapers obtained using the CMM (0.028 ± 0.016 mm. 3. /Mc) was not statistically different (p=0.435) to the mean volumetric wear rate obtained gravimetrically (0.019 ± 0.020 mm. 3. /Mc) for the femoral heads. Therefore, wear of the heads arose mainly from the internal taper. The mean wear rates of the CoCrMo taper and titanium trunnion are in agreement with a MoP explant study (Kocagoz et al, 2016, CORR). Conclusion. This is the first long-term hip simulator study to report wear generated from the taper-trunnion junction of MoP hips

Introduction. Little is known about the relationship between head-neck corrosion and its effect on periprosthetic tissues and distant organs in the majority of patients hosting apparently well-functioning devices. We studied the degree and type of taper damage and the histopathologic response in periprosthetic tissue and distant organs. Methods. A total of 50 contemporary THRs (34 primary, 16 revision) retrieved postmortem from 40 patients after 0.4–26 years were studied. Forty-three femoral stems were CoCrMo and 7 were Ti6Al4V. In every case, a CoCrMo-alloy head articulated against a cementless polyethylene cup (19 XLPE and 31 UHMWPE). H&E and IHC sections of the joint pseudocapsules and liver were graded 1–4 for the intensity of various inflammatory cell infiltrates and tissue necrosis. The nature of the tissue response in the joint capsule, liver, spleen, kidneys and lymph nodes was assessed. Wear and corrosion products in the tissues were identified using SEM and EDS. Taper surfaces were graded for corrosion damage using modified Goldberg scoring and examined by SEM to determine the acting corrosion mode. Correlations between damage scores and the histologic variables were generated using the Spearman test. Results. No correlation was seen between taper damage scores and the macrophage response in the joint pseudocapsule. The distribution of corrosion scores for heads and femoral trunnions is shown in Figure 1. Moderate or severe corrosion of the head and/or trunnion was present in 9 hips (8 CoCr/CoCr and 1 CoCr/TiAlV). One patient with bilateral hips had local ALVAL-like lymphocyte-dominated tissue reactions (Figure 2) and mild focal lymphocytic infiltrates in the liver and kidneys (Figure 3). This was associated with severe intergranular corrosion of the CoCrMo trunnion and column damage on the head taper. Particle-laden macrophages in pseudocapsules were significantly correlated with liver macrophages (r=.382, p=0.012) and liver lymphocytes (r=.367, p=0.013). Pseudocapsule macrophage responses to metallic and/or polyethylene wear particles ranged widely from minimal to marked. Focal tissue necrosis was related to high concentrations of particulate wear debris. A minimal number of metallic particle-laden macrophages were also detected in the liver and spleen; and macrophage granulomas were present in para-aortic lymph nodes, especially in revision cases. DISCUSSION. The generation of metal ions and particulates at corroded CoCrMo heads and CoCrMo or Ti6Al4V trunnions was a significant contributor to the presence of perivascular lymphocytes within the joint pseudocapsule, with 1 patient showing a histologic pattern consistent with ALVAL. Patient factors and the rate of corrosion are among variables influencing whether an ALVAL-type reaction will develop and whether or not it will become symptomatic. Macrophages in the joint pseudocapsules were positively correlated with inflammatory cells in the liver. In this study, the intensity of inflammatory infiltrates in distant organs was mild. However, several cases of organ dysfunction have been reported in association with catastrophic wear of CoCrMo components. It continues to be essential to minimize the generation of metal ions and particulates and to improve strategies for identifying and managing patients exposed to high levels of degradation products. For any figures or tables, please contact the authors directly

Introduction. Angular mismatch between the head and trunnion is recognized as a contributing factor to mechanically-assisted corrosion of modular hip prostheses. Although manufacturing standards have been adopted to define acceptable tolerances for taper angles of mating components, the relationship between the head and trunnion taper angles (positive or negative) differs between manufacturers. In this study, we investigated the effect of positive and negative angular mismatch on the interface mechanics of a standard design of taper junction using finite element analysis (FEA). Methods. Computer simulations were executed using an FE model which had been previously verified through direct comparison with experimental studies. The neck and trunnion of a Ti6Al4V femoral component (taper size: 12/14mm) were modelled using a stable hexahedral mesh (33,648 elements), while the femoral head (CoCrMo, size: 32mm) was modelled using a tetrahedral mesh (51,182 elements). Assembly of the head on the trunnion was simulated through the application of a load of 4000N along the trunnion axis. This was followed by the application of a gait load of 1638N (2.34×700N BW) at an angle of 30o to the trunnion axis. A friction-based sliding interface (mu=0.12) was simulated at the trunnion-head junction. A linear static solution was set up using Siemens NX Nastran. In addition to a perfect match, 7 positive and negative mismatch angles were simulated ranging from −0.100 to 0.100 degrees. Head taper interface motion, contact pressure and internal stresses (von Mises) were calculated for each mating condition. Results. During gait loading, the maximum tangential displacement at the head-trunnion interface was 34µm, virtually identical for positive and negative mismatches in taper angle. There was minimal change in interface motion (5 µm) with variations in taper mismatch. The maximum contact pressure between the trunnion and the head increased dramatically with the magnitude of the mismatch from 144Mpa for a per4fect fit to 924MPa and 555MPa for taper clearances of 0.1 degrees for −0.1degrees, respectively. Smaller changes were seen in the maximum internal stresses developed within the trunion, which rose by a factor of approximately X2.5 over the same range of taper mismatch (perfect fit: 85MPa vs 220Mpa for 0.1deg. and 228MPa for −0.1 deg.). One difference between cases examined was the magnitude of the dilation of the bore within the femoral head during gait loading which increased from 2.19µm to 4.23µm with a change in mismatch from +0.015o to −0.015o. Discussion. The results support the conclusion that, provided that manufacturing tolerances are maintained to within ±0.015o, both a positive and negative angular mismatch results in similar levels of contact stress and interface micromotion in modular junctions. As expected, peak stresses were located at the proximal edge of the contact zone with a negative taper mismatch and at the distal edge with a positive mismatch. This may have important implications in vivo, due to differences in the chemical environment (pH and oxygen tension) at each of these locations

Introduction. Relative motion at the modular head-neck junction of hip prostheses can lead to severe surface damage through mechanically-assisted corrosion. One factor affecting the mechanical performance of modular junctions is the frictional resistance of the mating surfaces to relative motion. Low friction increasing forces normal to the head-neck interface, leading to a lower threshold for slipping during weight-bearing. Conversely, a high friction coefficient is expected to limit interface stresses but may also allow uncoupling of the interface in service. This study was performed to examine this trade-off using finite element models of the modular head-neck junction. Methods. A finite element model (FEM) of the trunnion/ head assembly of a total hip prosthesis was initially created and experimentally validated. CAD models of a stem trunnion (taper size: 12/14mm) and a prosthetic femoral head (diameter: 28mm) were discretized into elements for finite element analysis (FEA). The trunnion (Ti6Al4V) was modelled with a hexahedral mesh (33,648 elements) and the femoral head (CoCrMo) with a tetrahedral mesh (51,182 elements). A friction-based sliding contact interface was defined between the mating surfaces. The model was loaded in 2 stages: (i) an assembly load of 4000N applied along the trunnion axis, and (ii) 500N applied along the trunnion axis in combination with a torque of 10Nm. A linear static solution was set up using Siemens NX-Nastran solver. Multiple simulations were executed by modulating the frictional coefficient at the taper-bore interface from 0.05 to 0.15 in increments of 0.01, the coefficient of 0.1 serving as the control case (Swaminathan and Gilbert, 2012). Results. The vertical and tangential displacements of the nodes on the taper of the trunnion relative to the femoral head demonstrated a strong inverse dependence upon the coefficient of friction at the interface (Fig. 1). A similar trend was observed with respect to the peak interface pressure (Fig. 2). The peak von Mises stress, however, increases with increasing coefficient of friction (Fig. 2). A Fisher's R to Z correlation test was performed on each output variable to determine its correlation with coefficient of friction. The coefficient of friction correlated significantly (p<0.0001) with both tangential displacement (r = −0.990) and vertical displacement (r = −0.974). Peak von Mises stress (r = 0.995) and peak contact pressure (r = −0.984) were also found to be significantly (p<0.0001) correlated to the coefficient of friction. Discussion. A higher coefficient of friction at the taper-bore interface led to lower contact pressure and sliding at the modular junction. However, higher coefficients of friction also led to increased von Mises stresses within the bore and the trunnion increasing the risk of yielding and fatigue failure. The current results strongly indicate that factors affecting the frictional coefficient at the interface likely influence the occurrence of and severity of mechanically-assisted corrosion in THA. Significance. The results from this study will help us set tolerances for the interlocking mechanism, identifying the minimum frictional coefficient required to obtain stable implant mechanics