Objectives. The use of ceramic femoral heads in total hip arthroplasty (THA) has increased due to their proven low bearing wear characteristics. Ceramic femoral heads are also thought to reduce wear and corrosion at the head-stem junction with titanium (Ti) stems when compared with metal heads. We sought to evaluate taper damage of ceramic compared with metal heads when paired with cobalt chromium (CoCr) alloy stems in a single stem design. Methods. This retrieval study involved 48 total hip arthroplasties (THAs) with CoCr V40 trunnions paired with either CoCr (n = 21) or ceramic (n = 27) heads. The taper junction of all hips was evaluated for fretting/corrosion damage and volumetric material loss using a roundness-measuring machine. We used linear regression analysis to investigate taper damage differences after adjusting for potential confounding variables. Results. We measured median taper material loss rates of 0.210 mm. 3. /year (0.030 to 0.448) for the metal head group and 0.084 mm. 3. /year (0.059 to 0.108) for the ceramic group. The difference was not significant (p = 0.58). Moreover, no significant correlation between material loss and implant or patient factors (p > 0.05) was found. Conclusions. Metal heads did not increase taper damage on CoCr trunnions compared with ceramic heads from the same hip design. The amount of material released at the taper junctions was very low when compared with available data regarding CoCr/Ti coupling in metal-on-metal bearings. Cite this article: A. Di Laura, H. Hothi, J. Henckel, I. Swiatkowska, M. H. L. Liow, Y-M. Kwon, J. A. Skinner, A. J. Hart. Retrieval analysis of metal and ceramic femoral heads on a single CoCr stem design. Bone Joint Res 2017;6:–350. DOI: 10.1302/2046-3758.65.BJR-2016-0325.R1

Objectives . We aimed to determine the effect of surgical approach on the histology of the femoral head following resurfacing of the hip. Methods. We performed a histological assessment of the bone under the femoral component taken from retrieval specimens of patients having revision surgery following resurfacing of the hip. We compared the number of empty lacunae in specimens from patients who had originally had a posterior surgical approach with the number in patients having alternative surgical approaches. Results. We found a statistically significant increase in the percentage of empty lacunae in retrieval specimens from patients who had the posterior approach compared with other surgical approaches (p < 0.001). . Conclusions. This indicates that the vascular compromise that occurs during the posterior surgical approach does have long-term effects on the bone of the femoral head, even if it does not cause overt avascular necrosis. Cite this article: Bone Joint Res 2013;2:200–5

Background. One of the potential complications of polyethylene liner (PL) is its dissociation from the metal shell. This is a rare but catastrophic complication of total hip replacement (THR). Objective. was to analyze the retrieved dissociated components (PL and shell) (Depuy Pinnacle, Warsaw, IN, USA) to evaluate the mechanism of failure. All these components were dissociated within four years of implantation. Methods. Components were retrieved from three different centers in Canada over the period from January 2011 to October 2016. The analysis was done at the Orthopaedic Innovation Centre (OIC) in Winnipeg Canada. Nine PLs were retrieved at the time of revision THR. Assessment using optical and scanning electron microscopies at magnification between 25× and 150× was performed. The following questions were asked: 1) were the liners correctly seated at the primary surgery? 2) Are there signs of impingement present which could have caused the liner to become dissociated? 3) Does the wear pattern indicate that the liner was failing prior to dissociation?. Results. All PLs dissociated in the inferior direction. Five PL were believed to have been seated properly at the time of indexed surgery. All PL displayed signs of post dissociation impingement. Only 1 PL had fractured resulting in failure prior to dissociation. Other PL showed signs of wear, however none of them reached thinness that would be a cause for concern. Eight PLs demonstrated shearing of the anti rotation tabs. Assessment of the anti rotation tabs revealed that a couple had sheared off suddenly while remaining anti rotation tabs sheared off in progressive fatigue resulting in the failure of the locking mechanism. Conclusions. Retrieval analysis was useful in identifying common patterns of failure such as anti-rotation tab damage. This was suggestive that the locking mechanism of the acetabular components has probably failed in 8 out of 9 of the retrieved liners

Arthroplasty registries are important for the surveillance of joint replacements and the evaluation of outcome. Independent validation of registry data ensures high quality. The ability for orthopaedic implant retrieval centres to validate registry data is not known. We analysed data from the National Joint Registry for England, Wales and Northern Ireland (NJR) for primary metal-on-metal hip arthroplasties performed between 2003 and 2013. Records were linked to the London Implant Retrieval Centre (RC) for validation. A total of 67 045 procedures on the NJR and 782 revised pairs of components from the RC were included. We were able to link 476 procedures (60.9%) recorded with the RC to the NJR successfully. However, 306 procedures (39.1%) could not be linked. The outcome recorded by the NJR (as either revised, unrevised or death) for a primary procedure was incorrect in 79 linked cases (16.6%). The rate of registry-retrieval linkage and correct assignment of outcome code improved over time. The rates of error for component reference numbers on the NJR were as follows: femoral head category number 14/229 (5.0%); femoral head batch number 13/232 (5.3%); acetabular component category number 2/293 (0.7%) and acetabular component batch number 24/347 (6.5%). . Registry-retrieval linkage provided a novel means for the validation of data, particularly for component fields. This study suggests that NJR reports may underestimate rates of revision for many types of metal-on-metal hip replacement. This is topical given the increasing scope for NJR data. We recommend a system for continuous independent evaluation of the quality and validity of NJR data. Cite this article: Bone Joint J 2015;97-B:10–18

Introduction. Three-dimensional (3D) printing of porous titanium implants marks a revolution in orthopaedics, promising enhanced bony fixation whilst maintaining design equivalence with conventionally manufactured components. No retrieval study has investigated differences between implants manufactured using these two methods. Our study was the first to compare these two groups using novel non-destructive methods. Materials and methods. We investigated 16 retrieved acetabular cups divided into ‘3D printed’ (n = 6; Delta TT) and ‘conventional’ (n = 10; Pinnacle Porocoat). The groups were matched for age, time to revision, size and gender (Table 1). Reasons for revision included unexplained pain, aseptic loosening, infection and ARMD. Visual inspection was performed to evaluate tissue attachment. Micro-CT was used to assess clinically relevant morphometric features of the porous structure, such as porosity, depth of the porous layer, pore size and strut thickness. Scanning electron microscopy (SEM) was applied to evaluate the surface morphology. Results. Significant differences (p = 0.0002) were found for all morphometric parameters (Table 2). Microscopic analysis revealed uniform beads over the backside of conventional implants, due to the manufacturing technique (Figure 1a). Conversely, beads of random size were found on 3D printed implants, representing a by-product of the manufacturing process, where some starting powder particles are not completely fused together (Figure 1b). The two groups showed similar tissue attachment (3D printed 76.9 ± 27.1%; conventional 73.8 ± 12.2%; p = 0.2635). Conclusion. This was the first study to analyse retrieved 3D printed orthopaedic implants. Differences were found between these and conventional implants, but both literature and registry data do not suggest a short-mid-term clinical issue with 3D printed components. Similar tissue on growth suggested a comparable behaviour with bone in situ. The key difference is the presence of the particles on 3D printed implants, whose clinical significance needs to be investigated. For any figures or tables, please contact the authors directly

Dual Mobility (DM) Total Hip Replacements (THRs) were introduced to reduce dislocation risk, which is the most common cause of early revision. The in-vivo mechanics of these implants is not well understood, despite their increased use in both elective and trauma settings. Therefore, the aim of this study was to comprehensively assess retrieved DM polyethylene liners for signs of damage using visual inspection and semi-quantitative geometric assessment techniques.

Retrieved DM liners (n=20) were visually inspected for the presence of seven established modes of polyethylene damage. If embedded debris was identified on the external surface, its material composition was characterised using energy-dispersive x-ray analysis (EDX). Additionally, each liner was geometrically assessed for signs of wear/deformation using a validated methodology.

Visual inspection of the liners revealed that scratching and pitting were the most common damage modes on either surface. Burnishing was observed on 50% and 15% of the internal and external surfaces, respectively. In addition, embedded debris was identified on 25% of the internal and 65% of the external surfaces. EDX analysis of the debris identified several materials including iron, titanium, cobalt-chrome, and tantalum. Geometric analysis demonstrated highly variable damage patterns across the liners.

The results of this study provide insight into the in-vivo mechanics of DM bearings. For example, the results suggest that the internal bearing (i.e., between the head and liner) acts as the primary articulation site for DM-THRs as evidenced by a higher incidence of burnishing and larger, more concentrated regions of penetration across the liners’ internal surfaces. Furthermore, circumferential, and crescent-shaped damage patterns were identified on the articulating surfaces of the liners thus providing evidence that these components can rotate within the acetabular shell with varying degrees of mobility. The mechanics of DM bearings are complex and may be influenced by several factors (e.g., soft tissue fibrosis, patient activities) and thus further investigation is warranted.

Finally, the results of this study suggest that DM liners may be susceptible to ex-vivo surface damage and thus caution is advised when handling and/or assessing these types of components.

3D printing acetabular cups offers the theoretical advantage of enhanced bony fixation due to greater design control of the porous implant surfaces. Analysing retrieved 3D printed implants can help determine whether this design intent has been achieved.

We sectioned 14 off-the-shelf retrieved acetabular cups for histological analysis; 7 cups had been 3D printed and 7 had been conventionally manufactured. Some of the most commonly used contemporary designs were represented in both groups, which were removed due to either aseptic loosening, unexplained pain, infection or dislocation. Clinical data was collected for all implants, including their age, gender, and time to revision.

Bone ingrowth was evaluated using microscopic assessment and two primary outcome measures: 1) bone area fraction and 2) extent of bone ingrowth.

The additively manufactured cups were revised after a median (IQR) time of 24.9 months (20.5 to 45.6) from patients with a median (IQR) age of 61.1 years (48.4 to 71.9), while the conventional cups had a median (IQR) time to revision of 46.3 months (34.7 to 49.1, p = 0.366) and had been retrieved from patients with a median age of 66.0 years (56.9 to 68.9, p = 0.999).

The additively and conventionally manufactured implants had a median (IQR) bone area fraction of 65.7% (36.4 to 90.6) and 33.9% (21.9 to 50.0), respectively (p < 0.001).

A significantly greater amount of bone ingrowth was measured into the backside of the additively manufactured acetabular cups, compared to their conventional counterparts (p < 0.001). Bone occupied a median of 60.0% and 5.7% of the porous depth in the additively manufactured and conventional cups, respectively.

3D printed components were found to achieve a greater amount of bone ingrowth than their conventionally manufactured counterparts, suggesting that the complex porous structures generated through this manufacturing technique may encourage greater osteointegration.

Trunnionosis is an important failure mechanism of total hip arthroplasties as has recently been reported by the England and Wales national joint registry. Adverse local tissue reaction has also recently been associated with total hip arthroplasty (THA) with metal on polyethylene and ceramic on polyethylene articulations. The contributing factors in the mechanism of this failure pattern have not been elucidated, however they are likely multifactorial to include corrosion, fretting, taper design, implantation time, metal particulate debris, and wear at the metal on metal interface. Furthermore, dissimilar metallic combinations have been shown to exacerbate tribocorrosion. Authors have also reported on the use of ceramic heads to reduce trunniononis, however, tribocorrosion is still present. The majority of the literature regarding modular head neck taper fretting and corrosion involves cobalt chrome (CoCr) alloy. Little is known about head neck fretting corrosion with Oxinium femoral heads. To measure fretting, corrosion, and wear on the female tapers of retrieved Oxinium femoral heads and to determine how demographic and device factors affect these measurements. Ninety-two (92) retrieved 12/14 Oxinium heads were graded using the modified Goldberg score for subjectively grading corrosion and fretting on the taper surface. A novel silicone molding technique was validated, then applied to the female tapers of the retrievals and of two pristine Oxinium femoral heads, sizes 32+0 and 32+4. The molds were scanned using a Konica Minolta 3D laser scanner for reconstruction of the topography, dimensions, and surface features of the tapers. Geomagic software was used to align the retrieved to the pristine 3D models, allowing measurement of surface deviations (from wear) that had occurred while the heads were implanted. Patient demographic and implant data were correlated with Goldberg scores and wear deviations. The mean Goldberg score was 1.6. Goldberg scores of 1 (minimal), 2 (mild), and 3 (moderate) were present in 41 of the 92 heads (45%), 43 heads (47%), and 8 heads (8%) respectively. No implants received a score of 4 (severe). A positive significant correlation was found between length of implantation and increased female taper fretting (R = 0.436, p < 0.01). Wear deviations were significantly greater with 36mm heads compared to 32mm heads (p < 0.01) and with +4 offsets compared to 0 offsets (p = 0.013). Similar to previous work analyzing ceramic heads, Oxinium heads demonstrated predominately mild tribocorrosion grades, however do not eliminate tribocorrosion. Tribocorrosion was increased with large heads and increased offsets. This finding is consistent perhaps with greater mechanical burden that larger implants with increased offsets experience. Further investigation is needed to elucidate if Oxinium femoral heads reduce fretting and corrosion when compared to CoCr femoral heads

Introduction. The National Joint Registry (NJR) for England, Wales and Northern Ireland contributes important information on the performance of implants and surgeons. However, the quality of this data is not known. This study aimed to perform an independent validation of primary metal-on-metal hip procedures recorded on the NJR through linkage to the London Implant Retrieval Centre (LIRC). Patients/Materials & Methods. Primary, metal-on-metal hip arthroplasties performed between 1st April 2003 and 5th November 2013 were recruited from the NJR (n=67045). Retrieved, metal-on-metal components were recruited from the LIRC (n=782). Data linkage and validation checks were performed. Results. 476 procedures (60.9%) on the LIRC were successfully linked to the NJR. However, 306 procedures (39.1%) could not be linked. The outcome recorded by the NJR (as either revised, unrevised or death) for a primary procedure was incorrect in 79 linked cases (16.6%). The rate of registry-retrieval linkage and correct assignment of outcome code improved over time. The rates of error for component reference numbers on the NJR were: femoral head category number 14/229 (5.0%); femoral head batch number 13/232 (5.3%); acetabular component category number 2/293 (0.7%) and acetabular component batch number 24/347 (6.5%). Discussion. Registry-Retrieval linkage provided a novel means for data validation, particularly for component fields. This study suggests that NJR reports may underestimate revision rates for many types of metal-on-metal hip. This is topical given the increasing scope for NJR data. We recommend a system for continuous, independent evaluation of NJR data quality and validity

The early failure and revision of bimodular primary total hip arthroplasty prostheses requires the identification of the risk factors for material loss and wear at the taper junctions through taper wear analysis. Deviations in taper geometries between revised and pristine modular neck tapers were determined using high resolution tactile measurements. A new algorithm was developed and validated to allow the quantitative analysis of material loss, complementing the standard visual inspection currently used. The algorithm was applied to a sample of 27 retrievals (in situ from 2.9 to 38.1 months) of the withdrawn Rejuvenate modular prosthesis. The mean wear volumes on the flat distal neck piece taper was 3.35 mm. 3. (0.55 to 7.57), mainly occurring in a characteristic pattern in areas with high mechanical loading. Wear volume tended to increase with time to revision (r² = 0.423, p = 0.001). Implant and patient specific data (offset, stem size, patient’s mass, age and body mass index) did not correlate with the amount of material loss observed (p >  0.078). Bilaterally revised implants showed higher amounts of combined total material loss and similar wear patterns on both sides. The consistent wear pattern found in this study has not been reported previously, suggesting that the device design and materials are associated with the failure of this prosthesis. Cite this article: Bone Joint J 2015;97-B:1350–7

Objectives

Previous studies have suggested that metal-on-metal (MoM) Pinnacle (DePuy Synthes, Warsaw, Indiana) hip arthroplasties implanted after 2006 exhibit higher failure rates. This was attributed to the production of implants with reduced diametrical clearances between their bearing surfaces, which, it was speculated, were outside manufacturing tolerances. This study aimed to better understand the performance of Pinnacle Systems manufactured before and after this event.

Aims

Surgeons have commonly used modular femoral heads and stems from different manufacturers, although this is not recommended by orthopaedic companies due to the different manufacturing processes.

We compared the rate of corrosion and rate of wear at the trunnion/head taper junction in two groups of retrieved hips; those with mixed manufacturers (MM) and those from the same manufacturer (SM).

This study compared component wear rates and pre-revision blood metal ions levels in two groups of failed metal-on-metal hip arthroplasties: hip resurfacing and modular total hip replacement (THR).

There was no significant difference in the median rate of linear wear between the groups for both acetabular (p = 0.4633) and femoral (p = 0.0872) components. There was also no significant difference in the median linear wear rates when failed hip resurfacing and modular THR hips of the same type (ASR and Birmingham hip resurfacing (BHR)) were compared.

Unlike other studies of well-functioning hips, there was no significant difference in pre-revision blood metal ion levels between hip resurfacing and modular THR.

Edge loading was common in both groups, but more common in the resurfacing group (67%) than in the modular group (57%). However, this was not significant (p = 0.3479). We attribute this difference to retention of the neck in resurfacing of the hip, leading to impingement-type edge loading. This was supported by visual evidence of impingement on the femur.

These findings show that failed metal-on-metal hip resurfacing and modular THRs have similar component wear rates and are both associated with raised pre-revision blood levels of metal ions.

Two Durasul highly crosslinked polyethylene liners were exchanged during revision surgery four and five years after implantation, respectively. The retrieved liners were evaluated macroscopically and surface analysis was performed using optical and electron microscopy. A sample of each liner was used to determine the oxidation of the material by Fourier transform infrared spectroscopy. Samples of the capsule were examined histologically.

The annual wear rate was found to be 0.010 and 0.015 mm/year, respectively. Surface analysis showed very little loss of material caused by wear. Histological evaluation revealed a continuous neosynovial lining with single multinucleated foreign-body giant cells. Our findings showed no unexpected patterns of wear on the articulating surfaces up to five years after implantation and no obvious failure of material.

Aims. Acetabular edge-loading was a cause of increased wear rates in metal-on-metal hip arthroplasties, ultimately contributing to their failure. Although such wear patterns have been regularly reported in retrieval analyses, this study aimed to determine their in vivo location and investigate their relationship with acetabular component positioning. Methods. 3D CT imaging was combined with a recently validated method of mapping bearing surface wear in retrieved hip implants. The asymmetrical stabilizing fins of Birmingham hip replacements (BHRs) allowed the co-registration of their acetabular wear maps and their computational models, segmented from CT scans. The in vivo location of edge-wear was measured within a standardized coordinate system, defined using the anterior pelvic plane. Results. Edge-wear was found predominantly along the superior acetabular edge in all cases, while its median location was 8° (interquartile range (IQR) -59° to 25°) within the anterosuperior quadrant. The deepest point of these scars had a median location of 16° (IQR -58° to 26°), which was statistically comparable to their centres (p = 0.496). Edge-wear was in closer proximity to the superior apex of the cups with greater angles of acetabular inclination, while a greater degree of anteversion influenced a more anteriorly centred scar. Conclusion. The anterosuperior location of edge-wear was comparable to the degradation patterns observed in acetabular cartilage, supporting previous findings that hip joint forces are directed anteriorly during a greater portion of walking gait. The further application of this novel method could improve the current definition of optimal and safe acetabular component positioning. Cite this article: Bone Joint Res 2021;10(10):639–649

Aims. The aim of this study was to develop a novel computational model for estimating head/stem taper mechanics during different simulated assembly conditions. Methods. Finite element models of generic cobalt-chromium (CoCr) heads on a titanium stem taper were developed and driven using dynamic assembly loads collected from clinicians. To verify contact mechanics at the taper interface, comparisons of deformed microgroove characteristics (height and width of microgrooves) were made between model estimates with those measured from five retrieved implants. Additionally, these models were used to assess the role of assembly technique—one-hit versus three-hits—on the taper interlock mechanical behaviour. Results. The model compared well to deformed microgrooves from the retrieved implants, predicting changes in microgroove height (mean 1.1 μm (0.2 to 1.3)) and width (mean 7.5 μm (1.0 to 18.5)) within the range of measured changes in height (mean 1.4 μm (0.4 to 2.3); p = 0.109) and width (mean 12.0 μm (1.5 to 25.4); p = 0.470). Consistent with benchtop studies, our model found that increasing assembly load magnitude led to increased taper engagement, contact pressure, and permanent deformation of the stem taper microgrooves. Interestingly, our model found assemblies using three hits at low loads (4 kN) led to decreased taper engagement, contact pressures and microgroove deformations throughout the stem taper compared with tapers assembled with one hit at the same magnitude. Conclusion. These findings suggest additional assembly hits at low loads lead to inferior taper interlock strength compared with one firm hit, which may be influenced by loading rate or material strain hardening. These unique models can estimate microgroove deformations representative of real contact mechanics seen on retrievals, which will enable us to better understand how both surgeon assembly techniques and implant design affect taper interlock strength. Cite this article: Bone Joint J 2020;102-B(7 Supple B):33–40

Introduction. In 2010, a widely used metal-on-metal hip implant was voluntarily recalled from the market by the manufacturer. Our lab performed detailed retrieval analysis of 548 of the explanted devices and 165 periprosthetic tissue specimens. We reported a high degree of variability in the multiple measurements that we performed on the components, including volumetric wear. Other studies using semi-quantitative tissue grading methods to describe the histology of the periprosthetic tissues from metal-on-metal hips have looked for relationships between component wear and histological features. Grammatopoulos et al found higher ALVAL scores in 45 hip resurfacing patients with pseudotumors compared to 11 without pseudotumors and a moderately positive correlation between wear and the histological rankings for tissue necrosis and lymphocytic response. We examined correlations between the component wear measurements from retrieval analysis and the tissue features in this large group of one metal-on-metal hip design. Materials and Methods. Tissues were processed routinely into paraffin sections, stained with H&E and characterized with light microscopic semi-quantitative scores for ALVAL features and for the number of macrophages and lymphocytes, extent of metal debris and the estimated percentage of necrosis per slide. Spearman correlation analysis was conducted to evaluate any correlations between retrieval wear measurements and histological findings. Results. The median combined cup and ball wear volume was 17.5mm3 in these 165 cases. The median tissue ALVAL score was 5.5 out of 10. Component total wear volume was not predictive of the ALVAL score (P = 0.09, R = −0.007) or the number of lymphocytes (P= 0.170, R= −0.077) but there was a positive correlation between wear volume and the ranking for macrophages in the tissues (P< 0.001, R=0.215). No other correlations were found for component wear. The median time to revision was 4.84 years. There was a negative correlation between tissue inflammation ranking (P=0.002, R=−0.183) and for lymphocyte ranking and time to revision (P=0.003, R=−0.173). There was a positive correlation between time to revision and the macrophage ranking (P=0.005, R=0.166). Conclusion. From this large-scale retrieval analysis of one design of metal-on-metal total hip, a high degree of variability was observed over the multiple analyses performed, including volumetric wear and tissue features. This likely reflects our previous observation that no single particular failure mechanism was predominant in this large cohort. Funded by DePuy Synthes Companies of Johnson & Johnson

Aims. We evaluated a large database with mechanical failure of a single uncemented modular femoral component, used in revision hip arthroplasty, as the end point and compared them to a control group treated with the same implant. Patient- and implant-specific risk factors for implant failure were analyzed. . Methods. All cases of a fractured uncemented modular revision femoral component from one manufacturer until April 2017 were identified and the total number of implants sold until April 2017 was used to calculate the fracture rate. The manufacturer provided data on patient demographics, time to failure, and implant details for all notified fractured devices. Patient- and implant-specific risk factors were evaluated using a logistic regression model with multiple imputations and compared to data from a previously published reference group, where no fractures had been observed. The results of a retrieval analysis of the fractured implants, performed by the manufacturer, were available for evaluation. Results. There were 113 recorded cases with fracture at the modular junction, resulting in a calculated fracture rate of 0.30% (113/37,600). The fracture rate of the implant without signs of improper use was 0.11% (41/37,600). In 79% (89/113) of cases with a failed implant, either a lateralized (high offset) neck segment, an extralong head, or the combination of both were used. Logistic regression analysis revealed male sex, high body mass index (BMI), straight component design, and small neck segments were significant risk factors for failure. Investigation of the implants (76/113) showed at least one sign of improper use in 72 cases. Conclusion. Implant failure at the modular junction is associated with patient- and implant-specific risk factors as well as technical errors during implantation. Whenever possible, the use of short and lateralized neck segments should be avoided with this revision system. Implantation instructions and contraindications need to be adhered to and respected. Cite this article: Bone Joint J 2020;102-B(5):573–579

Aims. The aim of this study was to evaluate fretting and corrosion in retrieved oxidized zirconium (OxZr; OXINIUM, Smith & Nephew, Memphis, Tennessee) femoral heads and compare the results with those from a matched cohort of cobalt-chromium (CoCr) femoral heads. Patients and Methods. A total of 28 OxZr femoral heads were retrieved during revision total hip arthroplasty (THA) and matched to 28 retrieved CoCr heads according to patient demographics. The mean age at index was 56 years (46 to 83) in the OxZr group and 70 years (46 to 92) in the CoCr group. Fretting and corrosion scores of the female taper of the heads were measured according to the modified Goldberg scoring method. Results. The OxZr-retrieved femoral heads showed significantly lower mean corrosion scores than the CoCr heads (1.3 (1 to 2.75) vs 2.1 (1 to 4); p < 0.01). Mean fretting scores were also significantly lower in the OxZr cohort when compared with the CoCr cohort (1.3 (1 to 2) vs 1.5 (1 to 2.25); p = 0.02). OxZr heads had more damage in the proximal region compared with the distal region of the head. Location had no impact on damage of CoCr heads. A trend towards increased corrosion in large heads was seen only in the CoCr heads, although this was not statistically significant. Conclusion. Retrieval analysis of OxZr femoral heads showed a decreased amount of fretting and corrosion compared with CoCr femoral heads. OxZr seems to be effective at reducing taper damage. Cite this article: Bone Joint J 2019;101-B:386–389

Introduction. Improper seating during head/stem assembly can lead to unintended micromotion between the femoral head and stem taper—resulting in fretting corrosion and implant failure. 1. There is no consensus—either by manufacturers or by the surgical community—on what head/stem taper assembly method maximizes modular junction stability in total hip arthroplasty (THA). A 2018 clinical survey. 2. found that orthopedic surgeons prefer applying one strike or three, subsequent strikes when assembling head/stem taper. However, it has been suggested that additional strikes may lead to decreased interference strength. Additionally, the taper surface finish—micro-grooves—has been shown to affect taper interference strength and may be influenced by assembly method. The objective of this study was to employ a novel, micro-grooved finite element (FEA) model of the hip taper interface and assess the role of head/stem assembly method—one vs three strikes—on modular taper junction stability. Methods. A two-dimensional, axisymmetric FEA model representative of a CoCrMo femoral head taper and Ti6Al4V stem taper was created using median geometrical measurements taken from over 100 retrieved implants. 3. Surface finish—micro-grooves—of the head/stem taper were modeled using a sinusoidal function with amplitude and period corresponding to retrieval measurements of micro-groove height and spacing, respectively. Two stem taper micro-groove geometries— “rough” and “smooth”—were modeled corresponding to the median and 5. th. percentile height and spacing measurements from retrievals. All models had a 3' (0.05°), proximal-locked angular mismatch between the tapers. To simulate implant assembly during surgery, multiple dynamic loads (4kN, 8kN, and 12kN) were applied to the femoral head taper in a sequence of one or three strikes. The input load profile (Figure 1) used for both cases was collected from surgeons assembling an experimental setup with a three-dimensional load sensor. Models were assembled and meshed in ABAQUS Standard (v 6.17) using four-node linear hexahedral, reduced integration elements. Friction was modeled between the stem and head taper using surface-to-surface formulation with penalty contact (µ=0.2). A total of 12 implicit, dynamic simulations (3 loads × 2 assembly sequences × 2 stem taper surface finishes) were run, with 2 static simulations at 4kN for evaluating inertial effects. Outcome variables included contact area, contact pressure, equivalent plastic strain, and pull-off force. Results. As expected, increasing assembly load led to increased contact area, pressures, and plasticity for both taper finishes. Rough tapers exhibited less total contact area at each loading level as compared to the smooth taper. Contact pressures were relatively similar across the stem taper finishes, except the 3-strike smooth taper, which exhibited the lowest contact pressures (Figure 2) and pull-off forces. The models assembled with one strike exhibited the greatest contact pressures, pull-off forces, and micro-groove plastic deformation (Figure 3). Conclusion. Employing 1-strike loads led to greater contact areas, pressures, pull-off forces, and plastic deformation of the stem taper micro-grooves as compared to tapers assembled with three strikes. Residual energy may be lost with subsequent assembly strikes, suggesting that one, firm strike maximizes taper assembly mechanics. These models will be used to identify the optimal design factors and impaction method to maximize stability of modular taper junctions. For any tables or figures, please contact the authors directly