Background. Fretting at modular junctions is thought to be a ‘mechanically assisted’ corrosion phenomenon, initiated by mechanical factors that lead to increased contact stresses and micromotions at the taper interface. We adopted a finite element approach to model the head-taper junction, to analyse the contact mechanics at the taper interface. We investigated the effect of assembly force and angle on contact pressures and micromotions, during loads commonly used to test hip implants, to demonstrate the importance of a good assembly during surgery. Methods. Models of the Bimetric taper and adaptor were created, with elastic-plastic material properties based on material tests with the actual implant alloy. FE contact conditions were validated against push-on and pull-off experiments. The models were loaded according to ISO 7206-4 and −6, after being assembled at 2-4-15kN, both axially and at a 30° angle. Average micromotions and contact pressures were analysed, and a wear score was calculated based on the contact pressures and micromotions. Results. The average contact pressure decreased when a higher assembly force was used, with loads being distributed over a larger contact area, but increased when tested at a 30° angle. Average micromotions reduced with a higher assembly load, except when assembled at a 30° angle. The wear score decreased with increasing assembly force, when applied perpendicularly, while when assembled at a 30° angle, the wear score did not reduce with assembly force. Conclusions. The location and patterns of micromotions were consistent with retrieved tapers and those generated in in-vitro test models. Increased impaction loads reduced the average amount of micromotion and fretting. We intend to apply more complex loading regimes in future analyses, enabling to study phenomena such as edge loading and frictional torque. Level of evidence. IIb - Experimental study. Disclosure. This study was financially supported by Biomet UK Healthcare Ltd

Objectives. Mechanical wear and corrosion at the head-stem junction of total hip arthroplasties (THAs) (trunnionosis) have been implicated in their early revision, most commonly in metal-on-metal (MOM) hips. We can isolate the role of the head-stem junction as the predominant source of metal release by investigating non-MOM hips; this can help to identify clinically significant volumes of material loss and corrosion from these surfaces. Methods. In this study we examined a series of 94 retrieved metal-on-polyethylene (MOP) hips for evidence of corrosion and material loss at the taper junction using a well published visual grading method and an established roundness-measuring machine protocol. Hips were retrieved from 74 male and 20 female patients with a median age of 57 years (30 to 76) and a median time to revision of 215 months (2 to 324). The reasons for revision were loosening of both the acetabular component and the stem (n = 29), loosening of the acetabular component (n = 58) and infection (n = 7). No adverse tissue reactions were reported by the revision surgeons. Results. Evidence of corrosion was observed in 55% of hips. The median Goldberg taper corrosion score was 2 (1 to 4) and the annual rate of material loss at the taper was 0.084 mm. 3. /year (0 to 0.239). The median trunnion corrosion score was 1 (1 to 3). Conclusions. We have reported a level of trunnionosis for MOP hips with large-diameter heads that were revised for reasons other than trunnionosis, and therefore may be clinically insignificant. Cite this article: H. S. Hothi, D. Kendoff, C. Lausmann, J. Henckel, T. Gehrke, J. Skinner, A. Hart. Clinically insignificant trunnionosis in large-diameter metal-on-polyethylene total hip arthroplasty. Bone Joint Res 2017;6:52–56. DOI: 10.1302/2046-3758.61.BJR-2016-0150.R2

Summary. Corrosion and fretting damage at the head-neck interface of artificial hip joints is more severe with larger head sizes. This is a concern, as the release of metal particles and ions can cause adverse tissue reactions, similar to those observed high wear metal-on-metal articulations. Introduction. In the last few years corrosion was increasingly observed at head-neck interfaces of artificial hip joints, especially in joints with larger heads. There has always been evidence of some corrosion at modular junctions of artificial joints, but except for few designs, it was not seen as a real problem. It is important to better understand the factors contributing to corrosion at modular interfaces, so that necessary improvements can be made to minimise or completely avoid corrosion, in order to avoid possible adverse tissue reactions. Methods. Over 100 retrieved stems and heads of 28, 32, 36, 40 and larger heads with metal-on-polyethylene (MoP) and metal-on-metal (MoM) articulations were scored for corrosion and fretting damage, in order to get a better picture of the magnitude of the problem. For some of the head sizes it was possible to assess the fretting and corrosion damage separately from implants from two different manufacturers. The tapers of the stem and head were subdivided into eight regions each, and scored for the severity of fretting and corrosion damage, as well as of the affected area within each sub-section. The scoring was undertaken by three persons with a fair intraclass correlation. The fretting and corrosion scores were also assessed based on the location of the center of the head with respect to the center of the taper. The distance between these two centers influences the toggling motion between the head and neck, as the main load is about 30 degrees out of axis during walking and other activities of daily living. Results. It was found that head-neck interfaces of two manufacturers of 36mm heads had significantly more corrosion than 28mm heads. There is a significant relationship between head and neck fretting damage, and between corrosion and fretting damage. There is also more corrosion damage in 32, 40 and larger heads, but these groups were from different manufacturers, so that it was not possible to perform statistical tests. More corrosion was observed when the centre of the head was at a larger distance from the centre of the head, leading to an increased toggling moment due to the out-of-axis loading. Discussion. It is of some concern that more corrosion is being observed with larger heads. Corrosion generally gets worse over time, which could negatively impact on the long-term behavior of these hip joints. Furthermore, it is possible that the metal particle and ion release due to corrosion and fretting could have adverse soft tissue reactions, similar to those observed at some MoM articulations. The fact that there are significant differences in the observed corrosion and fretting damage between the head-neck interfaces of two companies, indicates that even subtle changes in the geometry and the machined taper surface are important. A better understanding of these factors is required to make sure that the corrosion and fretting damage is minimised, or even better eliminated for all heads of artificial joints

Material loss at the head-stem taper junction may contribute to the high early failure rates of stemmed large head metal-on-metal (LH-MOM) hip replacements. We sought to quantify both wear and corrosion and by doing so determine the main mechanism of material loss at the taper. This was a retrospective study of 78 patients having undergone revision of a LH-MOM hip replacement. All relevant clinical data was recorded. Corrosion was assessed using light microscopy and scanning electron microscopy, and graded according to a well-published classification system. We then measured the volumetric wear of the bearing and taper surfaces. Evidence of at least mild taper corrosion was seen in 90% cases, with 46% severely corroded. SEM confirmed the presence of corrosion debris, pits and fretting damage. However, volumetric wear of the taper surfaces was significantly lower than that of the bearing surfaces (p = 0.015). Our study supports corrosion as the predominant mechanism of material loss at the taper junction of LH-MOM hip replacements. Although the volume of material loss is low, the ionic products may be more biologically active compared to the particulate debris arising from the bearing surfaces

Summary Statement. Fretting and corrosion has been identified as a clinical problem in modular metal-on-metal THA, but remains poorly understood in modern THA devices with polyethylene bearings. This study investigates taper damage and if this damage is associated with polyethylene wear. Introduction. Degradation of modular head-neck tapers was raised as a concern in the 1990s (Gilbert 1993). The incidence of fretting and corrosion among modern, metal-on-polyethylene and ceramic-on-polyethylene THA systems with 36+ mm femoral heads remains poorly understood. Additionally, it is unknown whether metal debris from modular tapers could increase wear rates of highly crosslinked PE (HXLPE) liners. The purpose of this study was to characterise the severity of fretting and corrosion at head-neck modular interfaces in retrieved conventional and HXLPE THA systems and its effect on penetration rates. Patients & Methods. 386 CoCr alloy heads from 5 manufacturers were analyzed along with 166 stems (38 with ceramic femoral heads). Metal and ceramic components were cleaned and examined at the head taper and stem taper by two investigators. Scores ranging from 1 (mild) to 4 (severe) were assigned in accordance with the semi-quantitative method adapted from a previously published technique. Linear penetration of liners was measured using a calibrated digital micrometer (accuracy: 0.001 mm). Devices implanted less than 1 year were excluded from this analysis because in the short-term, creep dominates penetration of the head into the liner. Results. The majority of the components were revised for instability, infection, and loosening. Mild to severe taper damage (score ≥2) was found in 77% of head tapers and 52% of stem tapers. The extent of damage was correlated to implantation time at the head taper (p=0.0004) and at the stem taper (p=0.0004). Damage scores were statistically elevated on CoCr heads than the matched stems (mean score difference=0.5; p<0.0001) and the two metrics were positively correlated with each other (ρ=0.41). No difference was observed between stem taper damage and head material (CoCr, ceramic) (p=0.56), nor was a correlation found between taper damage and head size (p=0.85). The penetration rate across different formulations of HXLPE was not found to be significantly different (p=0.07), and therefore grouped together for further analysis. Within this cohort, penetration rate was not found to be associated with head size (p=0.08) though a negative correlation with implantation time was noted (ρ=−0.35). When analyzed along with taper damage scores, a correlation was not observed between head taper damage scores and HXLPE penetration rates (p=0.51). Discussion. The results of this study do not support the hypothesis that 36+ mm ceramic or CoCr femoral heads articulating on HXLPE liners are associated with increased risk of corrosion among HXLPE liners when compared with smaller diameter heads. A limitation of this study is the semi-quantitative scoring technique, heterogeneity of the retrieval collection and short implantation time of the larger diameter heads. Because corrosion may increase over time in vivo, longer-term follow-up, coupled with quantitative taper wear measurement, will better assess the natural progression of taper degradation in modern THA bearings

It has been speculated that high wear at the head-stem taper may contribute to the high failure rates reported for stemmed large head metal-on-metal (LH-MOM) hips. In this study of 53 retrieved LH-MOM hip replacements, we sought to determine the relative contributions of the bearing and taper surfaces to the total wear volume. Prior to revision, we recorded the relevant clinical variables, including whole blood cobalt and chromium levels. Volumetric wear of the bearing surfaces was measured using a coordinate measuring machine and of the taper surfaces using a roundness measuring machine. The mean taper wear volume was lower than the combined bearing surface wear volume (p = 0.015). On average the taper contributed 32.9% of the total wear volume, and in only 28% cases was the taper wear volume greater than the bearing surface wear volume. Despite contributing less to the total material loss than the bearing surfaces, the head-stem taper junction remains an important source of implant-derived wear debris. Furthermore, material loss at the taper is likely to involve corrosion and it is possible that the material released may be more biologically active than that from the bearing surface

Summary. The required torque leading to an abrasion of the passive layer in the stem-head interface positively correlates to the assembly force. In order to limit the risk of fretting and corrosion a strong hammer blow seems to be necessary. Introduction. Modular hip prostheses are commonly used in orthopaedic surgery and offer a taper connection between stem and ball head. Taper connections are exposed to high bending loads and bear the risk of fretting and corrosion, as observed in clinical applications. This is particularly a problem for large diameter metal bearings as the negative effects may be enhanced due to the higher moments within the taper connection. Currently, it is not known how much torque is required to initiate a removal of the passive layer, which might lead to corrosion over a longer period and limits the lifetime of prostheses. Therefore, the purpose of this study was to identify the amount of torque required to start an abrasion of the passive layer within the interface dependent on the assembly force and the axial load. Materials and Methods. Titanium hip stems (Furlong H-AC, JRI, UK) and cobalt-chromium heads (⊘ 28mm, size L, JRI, UK) were assembled using a drop rig with peak forces of 4.5 kN (F. P,1. , n = 4) or 6.0 kN (F. P,2. , n = 4). The prostheses were inverted and then mounted with the head rigidly fixed to the base of a materials testing machine using a non-conducting (nylon) jig while submerged in Ringer's solution. The stems were attached to the machine actuator via non-conductive plates. An axial load (F. A,1. = 1 kN, F. A,2. = 3 kN, n = 4 each) was applied to the stems along the taper axis. After a period of equilibration a torque, increasing from 0 up to 15Nm, was manually applied. The galvanic potential at the taper interface was continuously recorded using a titanium electrode. The torque required to cause a drop in the potential of 5% was identified. For statistical analyses non-parametric tests were performed (α = 0.05). Results. Four different phases of the potential could be clearly differentiated during testing: equilibrium, removal of the passive layer leading to a drop of the potential, repassivation and then a second equilibrium. Prostheses assembled with a force of 6 kN required a significantly higher torque to start a removal of the passive layer compared to those with 4.5 kN (7.2 ± 0.5 Nm vs. 3.9 ± 1.0 Nm for F. A,1. , p = 0.029). In contrast, no influence of the axial load on the fretting behaviour of the prostheses could be found (8.0 ± 1.6 Nm for F. P,2. , p = 0.486). Discussion. Changes in the galvanic potential were observed at low torque levels for a small head diameter. With increasing head diameter the tangential force leading to a removal of the passive layer in the stem-head interface decrease resulting in a higher risk for corrosion. Component assembly with a high force reduces the risk of fretting and corrosion in the taper interface; however, it is feasible that the determined torque levels can still be reached, particularly in situations of large weight and high activity of the patient or malpositioning of the prosthesis in the body

In 2020 almost 90% of femoral heads for total hip implants in Germany were made of ceramic. Nevertheless, the cellular interactions and abrasion mechanisms in vivo have not been fully understood until now. Metal transfer from the head-neck taper connection, occurring as smear or large-area deposit, negatively influences the surface quality of the articulating bearing. In order to prevent metal transfer, damage patterns of 40 Biolox delta ceramic retrievals with CoC and CoPE bearings were analysed. A classification of damage type and severity for each component (n=40) was done according to an established scoring system. To investigate the physical properties, the surface quality was measured using confocal microscopy, quantitative analysis of phase composition were performed by Raman spectroscopy and qualitative analysis of metal traces was done by scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX). The periprosthetic tissue was analysed for abrasion particles with SEM and EDX. Both bearing types show different damage patterns. Dotted/ drizzled metal smears were identified in 82 % of CoC (n=16) and 96 % of CoPE (n=24) bearings. Most traces on the ceramic heads were identified in the proximal area while they were observed predominantly in the distal area for the ceramic inlays. The identified marks are similar to those of metallic bearings. Metallic smears lead to an increase of up to 30 % in the monoclinic crystalline phase of the ceramic. The roughness increases by up to six times to Ra=48 nm. Ceramic and metallic wear particles from the articulating surfaces or head neck taper junctions were found in the periprosthetic tissue. Damage patterns on CoC hip implants seem to be similar to those of metallic implants. More detailed analysis of CoC implants are needed to understand the described damage patterns and provide advice for prevention

Abstract. Objectives. Total hip replacement (THR) is one of the most successful and cost-effective interventions in orthopaedic surgery. Dislocation is a debilitating complication of THR and managing an unstable THR constitutes a significant clinical challenge. Stability in THR is multifactorial and is influenced by surgical, patient and implant related factors. It is established that larger diameter femoral heads have a wider impingement-free range of movement and an increase in jump distance, both of which are relevant in reducing the risk of dislocation. However, they can generate higher frictional torque which has led to concerns related to increased wear and loosening. Furthermore, the potential for taper corrosion or trunnionosis is also a potential concern with larger femoral heads, particularly those made from cobalt-chrome. These concerns have meant there is hesitancy among surgeons to use larger sized heads. This study presents the comparison of clinical outcomes for different head sizes (28mm, 32mm and 36mm) in primary THR for 10,104 hips in a single centre. Methods. A retrospective study of all consecutive patients who underwent primary THR at our institution between 1st April 2003 and 31st Dec 2019 was undertaken. Institutional approval for this study was obtained. Demographic and surgical data were collected. The primary outcome measures were all-cause revision, revision for dislocation, and all-cause revision excluding dislocation. Continuous descriptive statistics used means, median values, ranges, and 95% confidence intervals where appropriate. Kaplan-Meier survival curves were used to estimate time to revision. Cox proportional hazard regression analysis was used to compare revision rates between the femoral head size groups. Adjustments were made for age at surgery, gender, primary diagnosis, ASA score, articulation type, and fixation method. Results. 10,104 primary THRs were included; median age 68.6 years with 61.5% females. A posterior approach was performed in 71.6%. There were 3,295 hips with 28 mm heads (32.6%), 4,858 (48.1%) with 32 mm heads and 1,951 (19.3%) with 36 mm heads. Overall rate of revision was 1.7% with the lowest rate recorded for the 36mm group (2.7% vs. 1.3% vs. 1.1%). Cox regression analysis showed a decreased risk of all-cause revision for 32mm & 36mm head sizes as compared to 28mm; this was statistically significant for the 32mm group (p = 0.01). Risk of revision for dislocation was significantly reduced in both 32mm (p = 0.03) and 36mm (p = 0.03) head sizes. Analysis of all cause revision excluding dislocation showed no significant differences between head sizes. Conclusion. There was a significantly reduced risk of revision for all causes, but particularly revision for dislocation with larger head sizes (36mm & 32mm vs. 28mm). Concerns regarding increased risk of early revision for aseptic loosening, polyethylene wear or taper corrosion with larger heads appear to be unfounded in this cohort of 10,104 patients with a mean of 6.0-year follow-up. Declaration of Interest. (b) declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported:I declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project

This study compared the pullout forces of the initial implantation and the “cement-in-cement” revision technique for short and standard-length (125 mm vs. 150 mm) Exeter. ®. V40 femoral stems used in total hip arthroplasty (THA). The idea that the pullout force for a double taper slip stem is relative to the force applied to the femur and that “cement-in-cement” revision provides the same reproduction of force. A total sample size of 15 femoral stems were tested (Short, n = 6 and Standard, n = 9). 3D printed fixtures for repeatable sample preparation were used to minimize variance during testing. To promote stem subsidence and to simulate an in vivo environment, the samples were placed in an incubator at 37°C at 100% humidity and experienced a constant compressive loading of 1335 N for 14 days. The samples underwent a displacement-controlled pullout test. After the initial pullout test, “cement-in-cement” revision will be performed and tested similar to the initial implantation to observe the efficacy of the revision technique. To compare the pullout forces between the two groups, a Kruskal-Wallis test using a significance level of 0.05 was conducted. The mean maximum pullout force for the short and standard-length femoral stems were 3939 ± 1178 N and 5078 ± 1168 N, respectively. The Kruskal-Wallis test determined no statistically significant difference between the two groups for the initial implantation (p = 0.13). The “cement-in-cement” revision pullout force will be conducted in future testing. This study demonstrated the potential use of short stem designs for THA as it provides similar levels of fixation as the standard-length femoral stem. The potential benefits for using a short stem design would be providing similar load transfer to the proximal femur, preserving proximal metaphyseal femoral bone in primary replacement, and reducing the invasiveness during revision

Objectives. Taper junctions between modular hip arthroplasty femoral heads and stems fail by wear or corrosion which can be caused by relative motion at their interface. Increasing the assembly force can reduce relative motion and corrosion but may also damage surrounding tissues. The purpose of this study was to determine the effects of increasing the impaction energy and the stiffness of the impactor tool on the stability of the taper junction and on the forces transmitted through the patient’s surrounding tissues. Methods. A commercially available impaction tool was modified to assemble components in the laboratory using impactor tips with varying stiffness at different applied energy levels. Springs were mounted below the modular components to represent the patient. The pull-off force of the head from the stem was measured to assess stability, and the displacement of the springs was measured to assess the force transmitted to the patient’s tissues. Results. The pull-off force of the head increased as the stiffness of the impactor tip increased but without increasing the force transmitted through the springs (patient). Increasing the impaction energy increased the pull-off force but also increased the force transmitted through the springs. Conclusions. To limit wear and corrosion, manufacturers should maximize the stiffness of the impactor tool but without damaging the surface of the head. This strategy will maximize the stability of the head on the stem for a given applied energy, without influencing the force transmitted through the patient’s tissues. Current impactor designs already appear to approach this limit. Increasing the applied energy (which is dependent on the mass of the hammer and square of the contact speed) increases the stability of the modular connection but proportionally increases the force transmitted through the patient’s tissues, as well as to the surface of the head, and should be restricted to safe levels. Cite this article: A. Krull, M. M. Morlock, N. E. Bishop. Maximizing the fixation strength of modular components by impaction without tissue damage. Bone Joint Res 2018;7:196–204. DOI: 10.1302/2046-3758.72.BJR-2017-0078.R2

The poor outcome of large head metal on metal total hip replacements (LHMOMTHR) in the absence of abnormal articulating surface wear has focussed attention on the trunnion / taper interface. The RedLux ultra-precision 3D form profiler provides a novel indirect optical method to detect small changes in form and surface finish of the head taper as well as quantitative assessment of wear volume. This study aimed to assess and compare qualitatively tapers from small and large diameter MOMTHR's. Tapers from 3 retrieval groups were analysed. Group 1: 28mm CoCr heads from MOMTHRs (n=5); Group 2: Large diameter CoCr heads from LHMOMTHRs (n=5); Gp 3 (control): 28mm heads from metal on polyethylene (MOP) THRs; n=3). Clinical data on the retrievals was collated. RedLux profiling of tapers produced a taper angle and 3D surface maps. The taper angles were compared to those obtained using CMM measurements. There was no difference between groups in mean 12/14 taper angles or bearing surface volumetric and linear wear. Only LHMOMs showed transfer of pattern from stem trunnion to head taper, with clear demarcation of contact and damaged areas.3D surface mapping demonstrated wear patterns compatible with motion or deformations between taper and trunnion in the LHMOM group. These appearances were not seen in tapers from small diameter MOM and MOP THRs. Differences in appearance of the taper surface between poorly functioning LHMOMTHRs and well functioning MOP or MOM small diameter devices highlight an area of concern and potential contributor to the mode of early failure

Introduction. There is much current debate concerning wear and corrosion at the taper junctions of large head total hip replacements, particularly metal-on-metal hips. Is such damage a modern concern or has it always occurred in total hip replacement but not previously noted. To investigate this five explanted V40 Exeter femoral stems (Stryker Howmedica) were obtained following revision surgery at a single centre. In all cases, the 24–26 mm femoral heads were still attached. Hypothesis. In conventional ‘small head’ modular hip prostheses such as the Exeter, negligible wear and corrosion is seen at the taper junction of explanted devices. Methods. The articulating surface and the taper junction of each femoral heads was measured using a Mitutoyo LEGEX322 co-ordinate measuring machine (CMM) In each case a wear map was generated and the wear volume from the contact surface was calculated using a bespoke MATLAB program. The accuracy of such measurements has been previously shown to be within 0.5mm3. Results. Wear analysis of the articulating surfaces found unworn surfaces with each femoral head having less than 1 mm. 3. volumetric wear. All tapers had minimal wear with volumetric wear of less than 0.5 mm. 3. . Discussion and Conclusion. The wear volumes measured fall within the CMM measuring accuracy of 0.5mm. 3. Furthermore all tapers showed low wear volumes indicating that the taper junction had functioned correctly and distributed the load across the length of the truneon. Significance. This work adds to the growing evidence that taper problems in modern hip prostheses are associated with increasing head diameter and smaller taper dimensions

Introduction. The National Joint Registry has recently identified failure of large head metal on metal hip replacements. This failure is associated with the high torque at the interface of standard modular taper junction leading to fretting and corrosion. A number of manufacturers produce mini spigots, which in theory, provide a greater range of motion as the neck head junction is reduced. However, the relative torque to interface ratio at this junction is also increased. In this study we investigated hypothesis that the use of small spigots (minispigots) will increase wear and corrosion on modular tapers. Methods. Wear and corrosion of spigots were compared in-vitro when loaded with a force representative of the resultant force passing through the hip. The heads (female tapers) were made of cobalt-chrome-molybdenum (CoCrMo) and the stems (male tapers) of titanium alloy (Ti). Commercially available tapers and heads were used. The surface parameters & profiles were measured before & after testing. Electrochemical static and dynamic corrosion (pitting) tests were performed on minispigots under loaded and non-loaded conditions. Results. Post-testing the surface parameters Ra, Ry & Rz on the head taper associated with the minispigots had become greater compared with standard spigots. In all instances the profile of the titanium male tapers was unchanged. SEM showed the corroded region of the head was similar to the profile on the Ti male taper, with evidence of pitting in the cobalt chrome. In the CoCrMo/ Ti combinations, wear and corrosion were increased in minispigots compared with standard spigots. On minispigots the rough surface finishes were affected more severely than those with a smoother surface. Static corrosion tests showed evidence of fretting in the rough but not the smooth minispigots. Pitting scans showed a greater hysteresis with the rough surface finishes on the minispogot indicating potentially greater corrosion in the former. Conclusion. The relative size of the taper in comparison to the head combined with the surface finish was crucial. As the relative torque to interface ratio at this junction increased corrosion of the cobalt chrome head increases and is further enhanced if the surface finish on the tapers is rough

Summary Statement. In young, active patients cementless THR demonstrates excellent prosthetic stability by RSA and outstanding clinical outcomes at 5 years using a tapered titanium femoral stem, crosslinked polyethylene liners and either titanium or tantalum shells. Introduction. Early femoral implant stability is essential to long-term success in total hip replacement. Radiostereometric analysis (RSA) provides precise measurements of micromotion of the stem relative to the femur that are otherwise not detectable by routine radiographs. This study characterised micromotion of a tapered, cementless femoral stem and tantalum porous-coated vs. titanium acetabular shells in combination with highly cross-linked UHMWPE or conventional polyethylene liners using radiostereometric analysis (RSA) for 5 years following THR. Patients and Methods. This IRB-approved, prospective, double randomised, blinded study, involved 46 patients receiving a primary THR by a single surgeon. Each patient was randomised to receive a titanium (23) (Trilogy, Zimmer) or tantalum (23) (Modular Tantalum shell, Zimmer) uncemented hemispheric shell and either a highly-crosslinked or conventional polyethylene liner. Tantalum RSA markers were implanted in each patient. All patients had a Dorr A or B femoral canal and received a cementless, porous-coated titanium tapered stem (M/L Taper, Zimmer). All final femoral broaches were stable to rotational and longitudinal stress. RSA examinations, Harris Hip, UCLA, WOMAC, SF-12 scores were obtained at 10 days, 6 months, and annually through 5 years. Results. All patients demonstrated statistically significant improvement in Harris Hip, WOMAC, and SF-12 PCS scores post-operatively. Evaluation of polyethylene wear demonstrated that median penetration measurements were significantly greater in the conventional compared to the HXPLE liner cohorts at 1 year through 5 years follow-up (p<0.003). At 5 years, conventional liners showed 0.38 ± 0.05mm vertical wear whereas HXLPE liners showed 0.08 ± 0.02mm (p<0.003). Evaluation of the femoral stems demonstrated that the rate of subsidence was highest in the first 6 months (0.09mm/yr), with no other detectable motion through 5 years. Two outlying patients had significantly higher stem subsidence values at 6 months (0.7 mm and 1.0mm). One stem stabilised without further subsidence after 6 months (0.7mm), and the other stem stabilised at 1 year (1.5mm). Neither patient has clinical evidence of loosening. Evaluation of acetabular shells demonstrated less median vertical translation in tantalum than titanium shells at each time-point except at 3-years follow-up, however due to large standard errors, there was no significant difference between the two designs (p>0.05). These large standard errors were predominantly caused by two outliers, neither of which had clinical evidence of loosening. Discussion/Conclusion. In this RSA study of young THR patients, cementless tapered femoral stems, highly crosslinked polyethylene liners, and tantalum or titanium acetabular shells all demonstrated excellent performance through 5 years follow-up. Highly crosslinked polyethylene liners demonstrated significantly less wear than conventional liners. The femoral stem showed excellent stability through 5 years, with no clinical or radiologic episodes of failure. The small amount of micromotion seen is less than that previously reported for similar tapered, cementless stems and approaches the accuracy of RSA (0.05mm). Both acetabular shells demonstrated excellent stability with minimal micromotion at 5 years without significant differences in migration. All patients demonstrated significant clinical improvement in pain and function and additional RSA evaluation of these patients is planned

Summary. This work uses a mathematical method to correlate the forces calculated to push-on and pull off a femoral head from a stem and correlate the results of in vitro testing. Introduction. This work aimed to mathematically model the force needed to disassemble the THR unit for a given assembly load. This work then compared these results with the results of an in vitro experiment. The research presented aimed to determine the assembly forces necessary to prevent movement of the head on the stem through friction. By assessing the forces necessary to push the head onto the stem securely enough to prevent any movement of the head through friction, it is likely that the fretting and corrosion of the head taper interface will be reduced. Methods. Mathematical equations were used to define the relationship between the push-on force and the taper specification in terms of friction, contact area and taper angle. Similar relationships were determined for the pull-off force and torque-off force. Push-on loads of 1–4 kN were used to calculate the normal force and then the pull-off force and torque-off force for the combinations. Stems were chosen to represent the trunnion interface available at Corin Ltd. The stems used had a 12/14 taper. Stems were paired with a size 32 mm diameter metal modular head. For this analysis it was assumed that µ. 1. was equal to µ. 2. on the basis of no change in material or surface finish. In vitro testing was conducted according to ISO7206-10, with variable assembly loads. The stems were held inverted vertically above the head. Each stem was pre-assembled to 1, 2, 3and 4 kN and the pull-off force was measured at each load (n=3). The roughness of the male and female trunnions was measured before and after testing. The results determined mathematically were compared with those found experimentally. Results. Mathematical analysis showed that for an increasing push-on force the pull-off force also increases. Similarly, the same trend was seen for the torque-off force. Linear regression analysis provided a relationship between the push-on force and the pull-off force, pull off force equates to 0.508 multiplied by push-on force, the R. 2. value was calculated as 0.986. The roughness of the trunnion and female taper were not significantly different before and after experimentation. The coefficient of frictional between the two surfaces, calculated based on the experimental pull-off forces, varied from 0.2 to 0.35. Discussion. Correlation has been shown between the results generated mathematically and those generated experimentally; pull-off force increases linearly with increasing push-on force. Further work is required to correlate the torque-off force determined experimentally with that calculated mathematically. ISO testing uses a push-on force of 2 kN to assemble heads onto a stems, this allows comparison between stems, however, does not correlate with the clinical scenario. The optimum force of assembly is not known and there is no correlation between the assembly load used during in vitro testing and the impact load applied during operation. The force with which a THR is assembled is related to the possibility of fretting and corrosion which may occur over the life of the joint. Further work is required to ensure optimum fit between modular heads and stems

Although cemented fixation provides excellent results in primary total hip replacement (THR), particularly in patients older than 75 years, uncemented implants are most commonly used nowadays. We compare the rate of complications, clinical and radiological results of three different designs over 75-years-old patients. 433 hips implanted in patients over 75 years old were identified from our Local Joint Registry. Group A consisted of 139 tapered cemented hips, group B of 140 tapered grit-blasted uncemented hips and group C of 154 tapered porous-coated uncemented hips. A 28 mm femoral head size on polyethylene was used in all cases. The mean age was greater in group A and the physical activity level according to Devane was lower in this group (p<0.001 for both variables). Primary osteoarthritis was the most frequent diagnoses in all groups. The radiological acetabular shape was similar according to Dorr, however, an osteopenic-cylindrical femur was most frequently observed in group A (p<0.001). The pre- and post-operative clinical results were evaluated according to the Merle-D'Aubigne and Postel scale. Radiological cup position was assessed, including hip rotation centre distance according to Ranawat and cup anteversion according to Widmer. We also evaluated the lever arm and height of the greater trochanter distances and the stem position. Kaplan-Meier analysis was done for revision for any cause and loosening. The hip rotation centre distance was greater and the height of the greater trochanter was lower in group B (p=0.003, p<0.001, respectively). The lever arm distance was lower in group C (p<0.001). A varus stem position was more frequently observed in group B (p<0.001). There were no intra- or post-operative fractures in group A, although there were five intra-operative fractures in the other groups plus two post-operative fractures in group B and four in group C. The rate of dislocation was similar among groups and was the most frequent cause for revision surgery (8 hips for the whole series). The mean post-operative clinical score improved in all groups. The overall survival rate for revision for any cause at 120 months was 88.4% (95% CI 78.8–98), being 97.8% (95% CI 95.2–100) for group A, 81.8% (95% CI 64.8–98.8) for group B and 95.3% (95% CI 91.1–99.6) for group C (log Rank: 0.416). Five hips were revised for loosening. The overall survival rate for loosening at 120 months was 91.9% (95% CI 81.7–100), being 99.2%(95% CI 97.6–100) for group A, 85.5 (95% CI 69.9 −100) for group B and 100% for group C (Log Rank 0.093). Despite a more osteopenic bone in the cemented group, the rate of peri-prosthetic fractures was higher after uncemented THR in patients older than 75 years. Although the overall outcome is good with both types of fixation, the post-operative reconstruction of the hip, which might be more reliable after cemented fixation, may affect the rate of complications in this population

Summary Statement. Proximal femoral bony deficits present a surgical and biomechanical challenge to implant longevity in revision hip arthroplasty. This work finds comparable primary stability when a distally fixing tapered fluted stem was compared with a conical design in cadaveric tests. Introduction. Proximal bony deficits complicate revision hip surgery and compromise implant survival. Longer distally fixing stems which bypass such defects are therefore required to achieve stability compatible with bony ingrowth and implant longevity. Aims. It is hypothesised that a tapered stem will provide superior rotational stability to a conical design. This work therefore aims to compare the primary stability and biomechanical properties of a new design of tapered fluted modular femoral stem (Redapt®, Smith & Nephew) with that of a conical fluted stem (Restoration®, Stryker). Materials & Methods. 7 Pairs of cadaveric femora were obtained according to strict inclusion/exclusion criteria. Each underwent dual energy x-ray absorptiometry and calibration plain-film radiographs were taken. Digital templating was performed using TraumaCad (Voyant Health, Brainlab) to determine implant sizing. Both stems are fluted, modular and manufactured from titanium. The control stem (Restoration) featured a straight conical design and the investigation stem (Redapt) a straight tapered design. Implantation was performed by a revision arthroplasty surgeon familiar with both systems. Proximal bone deficiency was reproduced using an extended trochanteric osteotomy with removal of metaphyseal bone before reattaching the osteotomy. Primary stability in the axial, sagittal and coronal planes was assessed using micromotion transducers (HBM, Darmstadt, Germany) and also by Radiostereometric Analysis (RSA). RSA employs simultaneous biplanar radiographs to measure relative movement. Two 1mm tantalum beads were mounted on the prosthesis with the centre of the femoral head taken as the third reference point. Beads were placed proximally in the surrounding bone as rigid body markers. Each bone was potted according to the ISO standard for fatigue testing and cyclically loaded at 1Hz for at least 3 increments (750–350N, 1000–350N, 1500–350N) for 1000 cycles. RSA radiographs were taken at baseline and on completion of each cycle. A strain analysis was concurrently performed using a PhotoStress® (Vishay Precision Group, Raleigh, USA) photoelastic coating on the medial femoral cortex. Each bone was loaded intact and then with the prosthesis in-situ at 500N increments until strain fringes were identified. Once testing was completed, the stems were sectioned at the femoral isthmus and data is presented on the cross-sectional fit and fill observed. Results. Both stem designs showed comparable primary stability with all stems achieving clinically acceptable micromotion (<150 μm) when loaded at body weight. A larger proportion of the control stems remained stable as loading increased to x2-3 body weight. Transducer-recorded migration appeared greatest in the axial plane (y axis) with negligible distal movement in the coronal or sagittal planes. Point motion analysis (RSA) indicated most movement to be in the coronal plane (x-axis) whereas segment motion analysis showed rotation about the long axis of the prosthesis to be largest. Photoelastic strain patterns were transferred more distally in both designs, however substantial stress shielding was also observed. Discussion/Conclusion. Both designs achieved adequate distal fixation and primary stability under representative clinical loading conditions. This work supports the continued use of this novel stem design for revision surgery in the presence of extensive proximal bone loss

Background and aim. Recent proposals have been introduced to modify stem design and/or femoral fixation in total hip replacement (THR). New designs need to consider previous design features and their results. The aim of this study has been to evaluate the clinical and radiological results of six different designs of tapered uncemented stems implanted in our Institution. Methods. 1918 uncemented hips were prospectively assessed from 1999 to 2011 (minimum follow-up of five years for the unrevised hips). All hips had a 28 or 32 mm femoral head and metal-on-polyethylene or alumina-on-alumina bearing surface. Six uncemented femoral designs that shared a femoral tapered stem incorporating a coating surface were included in the study. The different design features included the type of coating, metaphyseal filling, and sectional shape. Results. Intra-operative proximal femoral crack was 6.7% in one of the designs (p=0.01), univariate analysis showing a lower risk of crack in the other designs. The position of the stem was neutral in 80% of the cases for all designs. Femoral canal filing was related to the stem design (p<0.001 at the three levels) and to the femoral level assessed (subset alpha=0.005). Twelve stems were revised for aseptic loosening (6 from two different designs). The survival rate for femoral aseptic loosening at 15 years was 96.6% (95% CI 93.8 to 99.4) for one of these two designs ad 97.4% (95% CI95.5 to 99.6) for the other. Regression analysis showed that stem design was the only factor related to aseptic loosening when adjusted for femoral canal filling (at the three levels) stem position (neutral or not) and femoral type (cylindrical or not). Conclusion. Tapered uncemented stems consistently provide excellent bone fixation. New designs need to avoid changing successful features and concentrate on the less successful aspects

Bone surface strains were measured in cadaver femora during loading prior to and after resurfacing of the hip and total hip replacement using an uncemented, tapered femoral component. In vitro loading simulated the single-leg stance phase during walking. Strains were measured on the medial and the lateral sides of the proximal aspect and the mid-diaphysis of the femur. Bone surface strains following femoral resurfacing were similar to those in the native femur, except for proximal shear strains, which were significantly less than those in the native femur. Proximomedial strains following total hip replacement were significantly less than those in the native and the resurfaced femur. These results are consistent with previous clinical evidence of bone loss after total hip replacement, and provide support for claims of bone preservation after resurfacing arthroplasty of the hip