Stability and trunnion wear potential in large-diameter metal-on-metal total hips: a finite element analysis

Is it feasible to develop a supervised learning algorithm incorporating spinopelvic mobility to predict impingement in patients undergoing total hip arthroplasty?

Aims

Precise implant positioning, tailored to individual spinopelvic biomechanics and phenotype, is paramount for stability in total hip arthroplasty (THA). Despite a few studies on instability prediction, there is a notable gap in research utilizing artificial intelligence (AI). The objective of our pilot study was to evaluate the feasibility of developing an AI algorithm tailored to individual spinopelvic mechanics and patient phenotype for predicting impingement.

Methods

This international, multicentre prospective cohort study across two centres encompassed 157 adults undergoing primary robotic arm-assisted THA. Impingement during specific flexion and extension stances was identified using the virtual range of motion (ROM) tool of the robotic software. The primary AI model, the Light Gradient-Boosting Machine (LGBM), used tabular data to predict impingement presence, direction (flexion or extension), and type. A secondary model integrating tabular data with plain anteroposterior pelvis radiographs was evaluated to assess for any potential enhancement in prediction accuracy.

Results

We identified nine predictors from an analysis of baseline spinopelvic characteristics and surgical planning parameters. Using fivefold cross-validation, the LGBM achieved 70.2% impingement prediction accuracy. With impingement data, the LGBM estimated direction with 85% accuracy, while the support vector machine (SVM) determined impingement type with 72.9% accuracy. After integrating imaging data with a multilayer perceptron (tabular) and a convolutional neural network (radiograph), the LGBM's prediction was 68.1%. Both combined and LGBM-only had similar impingement direction prediction rates (around 84.5%).

Conclusion

This study is a pioneering effort in leveraging AI for impingement prediction in THA, utilizing a comprehensive, real-world clinical dataset. Our machine-learning algorithm demonstrated promising accuracy in predicting impingement, its type, and direction. While the addition of imaging data to our deep-learning algorithm did not boost accuracy, the potential for refined annotations, such as landmark markings, offers avenues for future enhancement. Prior to clinical integration, external validation and larger-scale testing of this algorithm are essential.

Understanding the role of head size and neck length in micromotion generation at the taper junction in total hip arthroplasty

Modular hip implants allow intra-operative adjustments for patient-specific customization and targeted replacement of damaged elements without full implant extraction. However, challenges arise from relative micromotions between components, potentially leading to implant failure due to cytotoxic metal debris. In this study magnitude and directions of micromotions at the taper junction were estimated, aiming to understand the effect of variations in head size and neck length. Starting from a reference configuration adhering to the 12/14 taper standard, six additional implant configurations were generated by varying the head size and/or neck length. A musculoskeletal multibody model of a prothesized lower limb was developed to estimate hip contact force and location during a normal walking task. Following the implant assembly, the multibody-derived loads were imposed as boundary conditions in a finite element analysis to compute the taper junction micromotions as the relative slip between the contacting surfaces. Results highlighted the L-size head as the most critical configuration, indicating a 2.81 μm relative slip at the mid-stance phase. The proposed approach enables the investigation of geometric variations in implants under accurate load conditions, providing valuable insights for designing less risky prostheses and informing clinical decision-making processes.

Metal ion release after hip resurfacing arthroplasty and knee arthroplasty: a retrospective study of one hundred ninety-five cases

PURPOSE

Hip arthroplasty with metal-on-metal bearings like hip resurfacing results in the release of metallic ions. In parallel, like every metallic implant, knee arthroplasty implants undergo passive corrosion. We analyzed blood levels of cobalt and chromium ions in patients who have a hip resurfacing arthroplasty and compared them to patients who have undergone knee arthroplasty at a minimum follow-up of one year. The hypothesis was that there is no difference in the ion release between hip resurfacing and knee arthroplasty.

METHODS

Sixty-three patients who underwent knee arthroplasty were compared to a cohort of 132 patients who underwent hip resurfacing. The blood levels of cobalt and chromium ions were determined preoperatively and at six and 12 months postoperatively and then compared between groups. We analyzed the relationship between ion release and the change in clinical outcome scores (Harris Hip score, Oxford Hip score, Merle D'Aubigné Postel score, Oxford Knee score, International Knee Society score), the BMI, sex, physical activity, implant size and inclination of the acetabular implant (hip resurfacing patients only). Mixed linear models were used to assess the changes in ion blood levels over time.

RESULTS

The cobalt blood levels were higher in the first 6 months in the resurfacing group (0.87 ug/L vs 0.67 ug/L; p = 0.011), while it was higher in the knee arthroplasty group at 12 months (1.20 ug/L vs 1.41 ug/L; p = 0.0008). There were no significant differences in chromium levels during the follow-up period.

CONCLUSION

The increase in metal ion release after knee arthroplasty is as high as after hip resurfacing at the one year follow-up. The monitoring of this parameter probably should not be recommended in case of good clinicals outcomes.

Advancement in total hip implant: a comprehensive review of mechanics and performance parameters across diverse novelties

The UK's National Joint Registry (NJR) and the American Joint Replacement Registry (AJRR) of 2022 revealed that total hip replacement (THR) is the most common orthopaedic joint procedure. The NJR also noted that 10-20% of hip implants require revision within 1 to 10 years. Most of these revisions are a result of aseptic loosening, dislocation, implant wear, implant fracture, and joint incompatibility, which are all caused by implant geometry disparity. The primary purpose of this review article is to analyze and evaluate the mechanics and performance factors of advancement in hip implants with novel geometries. The existing hip implants can be categorized based on two parts: the hip stem and the joint of the implant. Insufficient stress distribution from implants to the femur can cause stress shielding, bone loss, excessive micromotion, and ultimately, implant aseptic loosening due to inflammation. Researchers are designing hip implants with a porous lattice and functionally graded material (FGM) stems, femur resurfacing, short-stem, and collared stems, all aimed at achieving uniform stress distribution and promoting adequate bone remodeling. Designing hip implants with a porous lattice FGM structure requires maintaining stiffness, strength, isotropy, and bone development potential. Mechanical stability is still an issue with hip implants, femur resurfacing, collared stems, and short stems. Hip implants are being developed with a variety of joint geometries to decrease wear, improve an angular range of motion, and strengthen mechanical stability at the joint interface. Dual mobility and reverse femoral head-liner hip implants reduce the hip joint's dislocation limits. In addition, researchers reveal that femoral headliner joints with unidirectional motion have a lower wear rate than traditional ball-and-socket joints. Based on research findings and gaps, a hypothesis is formulated by the authors proposing a hip implant with a collared stem and porous lattice FGM structure to address stress shielding and micromotion issues. A hypothesis is also formulated by the authors suggesting that the utilization of a spiral or gear-shaped thread with a matched contact point at the tapered joint of a hip implant could be a viable option for reducing wear and enhancing stability. The literature analysis underscores substantial research opportunities in developing a hip implant joint that addresses both dislocation and increased wear rates. Finally, this review explores potential solutions to existing obstacles in developing a better hip implant system.

Design and Performance Evaluation of a Novel Spiral Head-Stem Trunnion for Hip Implants Using Finite Element Analysis

With an expectation of an increased number of revision surgeries and patients receiving orthopedic implants in the coming years, the focus of joint replacement research needs to be on improving the mechanical properties of implants. Head-stem trunnion fixation provides superior load support and implant stability. Fretting wear is formed at the trunnion because of the dynamic load activities of patients, and this eventually causes the total hip implant system to fail. To optimize the design, multiple experiments with various trunnion geometries have been performed by researchers to examine the wear rate and associated mechanical performance characteristics of the existing head-stem trunnion. The objective of this work is to quantify and evaluate the performance parameters of smooth and novel spiral head-stem trunnion types under dynamic loading situations. This study proposes a finite element method for estimating head-stem trunnion performance characteristics, namely contact pressure and sliding distance, for both trunnion types under walking and jogging dynamic loading conditions. The wear rate for both trunnion types was computed using the Archard wear model for a standard number of gait cycles. The experimental results indicated that the spiral trunnion with a uniform contact pressure distribution achieved more fixation than the smooth trunnion. However, the average contact pressure distribution was nearly the same for both trunnion types. The maximum and average sliding distances were both shorter for the spiral trunnion; hence, the summed sliding distance was approximately 10% shorter for spiral trunnions than that of the smooth trunnion over a complete gait cycle. Owing to a lower sliding ability, hip implants with spiral trunnions achieved more stability than those with smooth trunnions. The anticipated wear rate for spiral trunnions was 0.039 mm³, which was approximately 10% lower than the smooth trunnion wear rate of 0.048 mm³ per million loading cycles. The spiral trunnion achieved superior fixation stability with a shorter sliding distance and a lower wear rate than the smooth trunnion; therefore, the spiral trunnion can be recommended for future hip implant systems.

Survival rates of different bearing surfaces with the same model of stem in total hip arthroplasty: predictive factors for revision surgery

INTRODUCTION

Few studies have examined the survival rates of total hip arthroplasty (THA) with the same femoral stem, and the predictive factors leading to the revision of stemmed metal-on-metal (MoM) THA remain unknown. We determined the long-term survival rate of stemmed MoM THA compared with that of metal-on-polyethylene (MoP) bearing THA, the effect of head size and cup placement angle on revision rate, and predictors of revision.

MATERIALS AND METHODS

A total of 130 hips in 110 patients who underwent primary THA by the same surgeon were included. M2a-RingLoc with a metal-on-polyethylene bearing (group P, 53 hips), M2a-Taper with MoM bearing (group T, 44 hips), and M2a-Magnum with MoM bearing (group M, 33 hips) were used. The mean age at surgery was 63.1 ± 9.5 years, and the mean postoperative follow-up duration was 133.7 ± 39.1 months. Whole blood metal ion concentrations were measured preoperatively and postoperatively, and magnetic resonance imaging was performed to identify aseptic local tissue reactions (ALTRs). Kaplan-Meier survivorship analysis and multiple logistic regression analysis were performed.

RESULTS

The THA survival rate up to the maximum postoperative follow-up period was 96.2% at 173 months, 46.6% at 179 months, and 47.8% at 145 months in groups P, T, and M, respectively, with revision as the endpoint. The stemmed MoM THA recorded a very low survival rate (p < 0.001). The ALTR rates were 70.5% and 63.6% in groups T and M, respectively. The risk factor for revision was the use of MoM bearing, and there was no difference in the results based on the head size in group M. Cobalt levels continued to increase postoperatively, although they were not accurate predictors of revision.

CONCLUSIONS

Stemmed MoM THA has a very low survival rate and should be used with caution. It is important to monitor the patient's symptoms and perform appropriate imaging to ensure timely revision.

In vitro testing for hip head-neck taper tribocorrosion: A review of experimental methods

In vitro test methods are challenged by the multi-factorial nature of head-neck taper connection tribocorrosion due to the consequences of simplification. Incorrect study design and misinterpretation of results has led to contradictory findings regarding important factors affecting head-neck taper tribocorrosion. This review seeks to highlight important considerations when developing in vitro test methods, to help researchers strengthen their study design and analyze the implications of others' design decisions. The advantages, disadvantages, limitations and procedural considerations for finite element analyses, electrochemical studies and in vitro simulations related to head-neck taper connection tribocorrosion are discussed. Finite element analysis offers an efficient method for studying large ranges of mechanical parameters. However, they are limited by neglecting electrochemical, biological and fluid flow factors. Electrochemical studies may be preferred if these factors are considered important. Care must be taken in interpreting data from electrochemical studies, particularly when different materials are compared. Differences in material valence and toxicity affect clinical translation of electrochemical studies' results. At their most complex, electrochemical studies attempt to simulate all aspects of headneck taper connection tribocorrosion in a bench top study. Effective execution requires in-depth knowledge of the tribocorrosion phenomenon, the involved mechanisms, and their measures such that each study design decision is fully informed.

Interfacial compliance, energy dissipation, frequency effects, and long-term fretting corrosion performance of Ti-6Al-4V/CoCrMo interfaces

Fretting corrosion in modular orthopedic implants is a well-documented process that may be associated with adverse local tissue reactions, pain, and revisions. Engineering modular junction interfaces to withstand applied fretting motion without surface abrasion could prevent implant degradation and surface damage. Previous work on geometrically modified Ti-6Al-4V/CoCrMo interfaces with increased compliance showed reduced fretting currents and surface damage during short term, variable-load in vitro testing. This study assesses the same interfaces under long-term conditions using an in vitro pin-on-disk fretting corrosion test apparatus. Preliminary variable-load frequency testing of typical control pin geometries showed a frequency-dependent current response, with underlying contact conditions of metal-metal interfaces that remained unchanged. One-million-cycle testing showed diminished fretting currents in all groups by 5 × 10⁵  cycles, but consistently lower currents in the high-compliance group. Corresponding fretting currents and work of fretting measurements of high-compliance pins confirmed that minimal fretting was experienced at the interface, with elastic bending of the pin accounting for almost all applied displacement. Debris generated during testing were composed of titanium and chromium oxides, small amounts of cobalt and molybdenum oxides, and sodium and phosphate originating from the surrounding test solution. Post-test analyses of sample surfaces revealed substantially more surface damage on CoCrMo disks than Ti-6Al-4V pins, thought to be a result of adhesive wear of mixed oxide debris on the pin and abrasion of the disk by the oxide debris layer. Surface damage to high-compliance pins suggests some abrasion is unavoidable with geometric modifications.

Delamination-fretting wear failure evaluation at HAp-Ti-6Al-4V interface of uncemented artificial hip implant

Present research aims to develop a finite element computational model to examine delamination-fretting wear behaviour that can suitably mimic actual loading conditions at HAp-Ti-6Al-4V interface of uncemented hip implant femoral stem component. A simple finite element contact configuration model based on fretting fatigue experimental arrangement subjected to different mechanical and tribological properties consist of contact pad (bone), HAp coating and Ti-6Al-4V substrate are developed using adaptive wear modelling approach adopting modified Archard wear equation to be examined under static simulation. The developed finite element model is validated and verified with reported literatures. The findings revealed that significant delamination-fretting wear is recorded at contact edge (leading edge) as a result of substantial contact pressure and contact slip driven by stress singularity effect. The delamination-fretting wear behaviour is promoted under higher delamination length, lower normal loading with higher fatigue loading, increased porous (cancellous) and cortical bone elastic modulus with higher cycle number due to significant relative slip amplitude as the result of reduced interface rigidity. Tensile-compressive condition (R=-1) experiences most significant delamination-fretting wear behaviour (8 times higher) compared to stress ratio R=0.1 and R=10.

Bilateral Mechanically-Assisted Crevice Corrosion Resulting in Femoral Stem-Head Dissociation in Metal-on-Polyethylene Total Hip Arthroplasty

There are emerging reports of complications related to corrosion in modular femoral components. We report a unique case describing an 83-year-old man with bilateral mechanically-assisted crevice corrosion in hip replacements performed 10 years previously, by the same surgeon using the same size 3 Accolade TMZF stem and same 44-mm CoCr LFIT V40 head (Stryker Orthopedics, Mahwah NJ). Our patient presented with complete femoral stem-head complex dissociation of his right hip and elevated serum cobalt and chromium levels. He subsequently underwent right hip revision arthroplasty while his contralateral hip is monitored closely as an outpatient. This case helps to define the poorly understood mechanisms and component design factors implicated in this emerging issue. We also hope to provoke discussions about guidelines for monitoring and revising failing metal-on-polyethylene arthroplasty systems.

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

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

A review on the finite element simulation of fretting wear and corrosion in the taper junction of hip replacement implants

Taperosis/trunnionosis is a scientific term for describing tribocorrosion (fretting corrosion) at the head-neck taper junction of hip implants where two contacting surfaces are undergone oscillatory micromotions while being exposed to the body fluid. Detached ions and emitted debris, as a result of taperosis, migrate to the surrounding tissues and can cause inflammation, infection, and aseptic loosening with an ultimate possibility of implant failure. Improving the tribocorrosion performance of the head-neck junction in the light of minimising the surface damage and debris requires a better understanding of taperosis. Given its complexity associated with both the mechanical and electrochemical aspects, computational methods such as the finite element method have been recently employed for analysing fretting wear and corrosion in the taper junction. To date, there have been more efforts on the fretting wear simulation when compared with corrosion. This is because of the mechanical nature of fretting wear which is probably more straightforward for modelling. However, as a recent research advancement, corrosion has been a focus to be implemented in the finite element modelling of taper junctions. This paper aims to review finite element studies related to taperosis in the head-neck junction to provide a detailed understanding of the design parameters and their role in this failure mechanism. It also reviews and discusses the methodologies developed for simulating this complex process in the taper junction along with the simplifications, assumptions and findings reported in these studies. The current needs and future research opportunities and directions in this field are then identified and presented.

Modelling changes in modular taper micromechanics due to surgeon assembly technique in total hip arthroplasty

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.

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

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

Influence of Different Damage Patterns of the Stem Taper on Fixation and Fracture Strength of Ceramic Ball Heads for Total Hip Replacement

Background

Modularity finds frequent application in total hip replacement, allowing a preferable individual configuration and a simplified revision by retaining the femoral stem and replacing the prosthetic head. However, micromotions within the interface between the head and the stem taper can arise, resulting in the release of wear debris and corrosion products. The aim of our experimental study was to evaluate the influence of different taper damages on the fixation and fracture stability of ceramic femoral heads, after static and dynamic implant loading.

Methods

Ceramic ball heads (36 mm diameter) and 12/14 stem tapers made of titanium with various mild damage patterns (intact, scratched, and truncated) were tested. The heads were assembled on the taper with a quasistatic load of 2 kN and separated into a static and a dynamic group afterwards. The dynamic group (n = 18) was loaded over 1.5 million gait cycles in a hip wear simulator (ISO 14242-1). In contrast, the static group (n = 18) was not mechanically loaded after assembly. To determine the taper stability, all heads of the dynamic and static groups were either pulled off (ASTM 2009) or turned off (ISO 7206-16). A head fracture test (ISO 7206-10) was also performed. Subsequent to the fixation stability tests, the taper surface was visually evaluated in terms of any signs of wear or corrosion after the dynamic loading.

Results

In 10 of the 18 cases, discoloration of the taper was determined after the dynamic loading and subsequent cleaning, indicating the first signs of corrosion. Pull-off forces as well as turn-off moments were increased between 23% and 54% after the dynamic loading compared to the unloaded tapers. No significant influence of taper damage was determined in terms of taper fixation strength. However, the taper damage led to a decrease in fracture strength by approximately 20% (scratched) and 40% (truncated), respectively.

Conclusion

The results suggest that careful handling and accurate manufacturing of the stem taper are crucial for the ceramic head fracture strength, even though a mild damage showed no significant influence on taper stability. Moreover, our data indicate that a further seating of the prosthetic head may occur during daily activities, when the resulting hip force increases the assembly load.

Analytical and computational sliding wear prediction in a novel knee implant: a case study

Osteoarthritis (OA) is a commonly occurring cartilage degenerative disease. The end stage treatment is Total Knee Arthroplasty (TKA), which can be costly in terms of initial surgery, but also in terms of revision knee arthroplasty, which is quite often required. A novel conceptual knee implant has been proposed to function as a reducer of stress across the joint surface, to extend the period of time before TKA becomes necessary. The objective of this paper is to develop a computational model which can be used to assess the wear arising at the implant articulating surfaces. Experimental wear coefficients were determined from physical testing, the results of which were verified using a semi-analytical model. Experimental results were incorporated into an anatomically correct computational model of the knee and implant. The wear-rate predicted for the implant was 27.74 mm³ per million cycles (MC) and the wear depth predicted was 1.085 mm/MC. Whereas the wear-rate is comparable to that seen in conventional knee implants, the wear depth is significantly higher than for conventional knee prostheses, and indicates that, in order to be viable, wear-rates should be reduced in some way, perhaps by using low-wear polymers.

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

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

Low failure rate at short term for 40 mm heads and second generation triple annealed HCLPE liners in hybrid hip replacements

INTRODUCTION

40 mm large diameter heads offer the advantages of lesser dislocation rates and better stability while highly cross linked polyethylene have lower wear rates than ultra high molecular weight polyethylene. Studies of the survivorship of 40 mm heads in hybrid hip replacements with Exeter stem and second generation highly cross linked polyethylene are limited. The purpose of the study is to report the short term of survivorship of the large diameter heads (40 mm) with Exeter stem with the secondary aim being the survival analysis of the thinnest second generation highly cross linked polyethylene.

METHODS

Retrospective case series of survivorship of patients with hybrid hip replacements of Exeter stems with 40 mm heads articulating with second generation triple annealed highly cross linked polyethylene liner on a uncemented acetabular shell was performed. As a subset, survival of thinnest second generation highly cross linked polyethylene survival (3.8 mm) at short term was assessed. Survival of the implants was confirmed from the hospital records and National joint registry as of 2015. Revision for any cause was taken as end point.

RESULTS

324 hybrid hip replacements with 40 mm heads had been performed for primary hip osteoarthritis. Of the 324 hip replacements, 154 hip replacements had thinnest second generation highly cross linked polyethylene (3.8 mm). Two patients had revision of components, one for periprosthetic fracture and one for deep infection. Mean age of the patients was 70.5 years (range 42-88 years, median 71, SD 8.3 years). None of the patients had revision due to trunion wear or loosening of components. The overall 5-year implant survival probability of hips with 40 mm heads was 99.4% (95% CI 98 to 100%) while the subset group of hip replacements with thinnest second generation highly cross linked polyethylene (3.8 mm) had 5-year implant survival probability of 99.3% (95% CI 97.1 to 100%).

CONCLUSION

Short term survivorship does not show significant evidence of early failure or higher rate of revision in our series of hybrid hip replacements with large diameter heads and second generation triple annealed highly cross linked polyethylene. Dislocation rate at the short term is none. Results from this series have to be carefully interpreted due to the relatively short follow up but so far results are encouraging. Long term follow up is required to conclude whether there is early or higher rate of failure. It is our intention to follow up this cohort and further publish our results at longer term.

Clinical Impact of Trunnion Wear After Total Hip Arthroplasty

Trunnionosis, characterized by corrosion and fretting of the taper, is a well-known entity commonly demonstrated in retrieval specimens. While there have been a number of recent reports regarding the potential for adverse local tissue reactions related to trunnionosis, it remains a relatively infrequent cause for failure of total hip replacement implants. A number of factors, including both biomechanical and bioelectrochemical factors, have a known impact on the development and severity of trunnionosis. Furthermore, specific implant design and material-related factors have been shown to influence the risk of trunnionosis leading to adverse local tissue reactions. Retention of a well-fixed femoral stem, in spite of corrosion of the male taper junction, is acceptable in the majority of cases. A ceramic head, often in combination with a titanium adaptor sleeve, is the most common replacement reported in the current literature to treat trunnionosis. In patients with modular-neck total hip replacements, revision of the femoral stem is likely required if corrosion at the modular neck-stem junction is encountered.

Impaction Force Influences Taper-Trunnion Stability in Total Hip Arthroplasty

BACKGROUND

This study investigated the influence of femoral head impaction force, number of head strikes, the energy sequence of head strikes, and head offset on the strength of the taper-trunnion junction.

METHODS

Thirty titanium-alloy trunnions were mated with 36-mm zero-offset cobalt-chromium femoral heads of corresponding taper angle. A drop tower impacted the head with 2.5J or 8.25J, resulting in 6 kN or 14 kN impaction force, respectively, in a single strike or combinations of 6 kN + 14 kN or 14 kN + 14 kN. In addition, ten 36-mm heads with -5 and +5 offset were impacted with sequential 14 kN + 14 kN strikes. Heads were subsequently disassembled using a screw-driven mechanical testing frame, and peak distraction force was recorded.

RESULTS

Femoral head pull-off force was 45% the strike force, and heads struck with a single 14 kN impact showed a pull-off force twice that of the 6 kN group. Two head strikes with the same force did not improve pull-off force for either 6 kN (P = .90) or 14 kN (P = .90). If the forces of the 2 impactions varied, but either impact measured 14 kN, a 51% higher pull-off force was found compared to impactions of either 6 kN or 6 kN + 6 kN. Femoral head offset did not significantly change the pull-off force among -5, 0, and +5 heads (P = .37).

CONCLUSION

Femoral head impaction force influenced femoral head trunnion-taper stability, whereas offset did not affect pull-off force. Multiple head strikes did not add additional stability, as long as a single strike achieved 14 kN force at the mallet-head impactor interface. Insufficient impaction force may lead to inadequate engagement of the trunnion-taper junction.

Short-term Outcomes of Ceramic Coated Metal-on-Metal Large Head in Total Hip Replacement Arthroplasty

Purpose

This study characterizes the short-term outcomes of ceramic coated metal-on-metal (MoM) large head total hip arthroplasty (THA) in prospectively selected patients aged 70 to 75 years.

Materials and Methods

Eighteen patients (18 hips) between the ages of 70 and 75-years old with THA using ceramic-coated MoM large heads between June 2014 and December 2014 were evaluated. We prospectively selected patients younger than 70 years for bipolar hemiarthroplasty and older than 75 years for conventional THA. There were one case of osteoarthritis, 8 cases of femur neck fracture, and 9 cases of intertrochanteric fracture. All patients underwent clinical and radiological follow-up at 6 weeks, 6 and 12 months, and every year postoperatively. The mean duration of follow-up was 24.2 months (range, 18–34 months).

Results

The average Harris hip score at the final follow-up was 81.0, except one case which was ultimately converted to conventional THA due to acetabular cup loosening. Radiographically, mean acetabular cup inclination was 45.8°(range, 38–56°) and anteversion was 20.1° (range, 11–25°). The average femoral head size was 48.7 mm. All stems were neutral-positioned except 1 varus-positioned stem. There was 1 case of a soft tissue infection, 3 patients complained of persistent groin pain, and no dislocations occurred.

Conclusion

Ceramic coated large MOM articulation (ACCIS) have many complications: cup loosening, groin pain, which can lead to fatal outcomes in the elderly patients. Especially in patients with communicated intertrochanter fracture (AO 31-A22, 23), careful attention should be paid to the choice of surgical option.

Evidence based recommendations for reducing head-neck taper connection fretting corrosion in hip replacement prostheses

INTRODUCTION

This systematic review seeks to summarise the published studies investigating prosthetic design, manufacture and surgical technique's effect on fretting corrosion at the head-neck taper connection, and provide clinical recommendations to reduce its occurrence.

METHODS

PubMed, MEDLINE and EMBASE electronic databases were searched using the terms taper, trunnion, cone and head-neck junction. Articles investigating prosthetic design, manufacture and surgical technique's effect on fretting corrosion were retrieved, reviewed and graded according to OCEBM levels of evidence and grades of recommendation.

RESULTS

The initial search yielded 1,224 unique articles, and 91 were included in the analysis.

CONCLUSIONS

There is fair evidence to recommend against the use of high offset femoral heads, larger diameter femoral heads, and to pay particular consideration to fretting corrosion's progression with time and risk with heavier or more active patients. Particular to metal-on-metal hip prostheses, there is fair evidence to recommend positioning the acetabular component to minimise edge loading. Particular to metal-on-polyethylene hip prostheses, there is fair evidence to recommend the use of ceramic femoral heads, against use of cast cobalt alloy femoral heads, and against use of low flexural rigidity femoral stems. Evidence related to taper connection design is largely conflicting or inconclusive. Head-neck taper connection fretting corrosion is a multifactorial problem. Strict adherence to the guidelines presented herein does not eliminate the risk. Prosthesis selection is critical, and well-controlled studies to identify each design parameter's relative contribution to head-neck taper connection fretting corrosion are required.

Mechanical Stability of the Taper Connection of Large Metal Femoral Heads With Adapter Sleeves in Total Hip Arthroplasty Analyzed Using Explicit Finite Element Simulations

BACKGROUND

Large diameter heads (LDHs) of metal-on-metal bearings in total hip arthroplasty provide increased range of motion and reduced dislocation rates. However, major concerns grew over high wear rates from the modular connection between femoral stem and head, especially in combination with adapter sleeves.

METHODS

A computational study on the taper connection stability of LDH (50 mm) with adapter sleeves of different lengths (S, M, L, and XL) compared with a standard femoral head (32 mm) without adapter sleeves was conducted using explicit finite element analyses. Four different impact configurations were considered resulting from varied mallet mass (0.5 vs 1.0 kg) and velocity (1.0 vs 2.0 m/s). The taper stability was evaluated by determination of the pull-off forces and micromotions due to simulated joint loads during walking (2 kN and 7.9 Nm, respectively). Moreover, the deformations of the adapter sleeves and the contact area in the taper connections were evaluated.

RESULTS

Although the pull-off forces of the LDH with different-sized adapter sleeves were comparable, contact area decreased and adapter sleeve deformations increased (up to 283%) with an increasing adapter sleeve length. Moreover, the micromotions of LDH with adapter sleeves were up to 7-times higher, as compared with the standard femoral head without an adapter sleeve.

CONCLUSION

The present numerical study confirms that the assembly technique of LDH with adapter sleeves reveals increased micromotions compared with standard femoral head sizes. We could demonstrate that deviations of the stem trunnion geometry and improper surgical instructions led to worse mechanical stability of the taper connection.

Prediction of Polyethylene Wear Rates from Gait Biomechanics and Implant Positioning in Total Hip Replacement

BACKGROUND

Patient-specific gait and surgical variables are known to play an important role in wear of total hip replacements (THR). However a rigorous model, capable of predicting wear rate based on a comprehensive set of subject-specific gait and component-positioning variables, has to our knowledge, not been reported.

QUESTIONS/PURPOSE

(1) Are there any differences between patients with high, moderate, and low wear rate in terms of gait and/or positioning variables? (2) Can we design a model to predict the wear rate based on gait and positioning variables? (3) Which group of wear factors (gait or positioning) contributes more to the wear rate?

PATIENTS AND METHODS

Data on patients undergoing primary unilateral THR who performed a postoperative gait test were screened for inclusion. We included patients with a 28-mm metal head and a hip cup made of noncrosslinked polyethylene (GUR 415 and 1050) from a single manufacturer (Zimmer, Inc). To calculate wear rates from radiographs, inclusion called for patients with a series of standing radiographs taken more than 1 year after surgery. Further, exclusion criteria were established to obtain reasonably reliable and homogeneous wear readings. Seventy-three (83% of included) patients met all criteria, and the final dataset consisted of 43 males and 30 females, 69 ± 10 years old, with a BMI of 27.3 ± 4.7 kg/m². Wear rates of these patients were determined based on the relative displacement of the femoral head with regard to the cup using a validated computer-assisted X-ray wear-analysis suite. Three groups with low (< 0.1 mm/year), moderate (0.1 to 0.2 mm/year), and high (> 0.2 mm/year) wear were established. Wear prediction followed a two-step process: (1) linear discriminant analysis to estimate the level of wear (low, moderate, or high), and (2) multiple linear and nonlinear regression modeling to predict the exact wear rate from gait and implant-positioning variables for each level of wear.

RESULTS

There were no group differences for positioning and gait suggesting that wear differences are caused by a combination of wear factors rather than single variables. The linear discriminant analysis model correctly predicted the level of wear in 80% of patients with low wear, 87% of subjects with moderate wear, and 73% of subjects with high wear based on a combination of gait and positioning variables. For every wear level, multiple linear and nonlinear regression showed strong associations between gait biomechanics, implant positioning, and wear rate, with the nonlinear model having a higher prediction accuracy. Flexion-extension ROM and hip moments in the sagittal and transverse planes explained 42% to 60% of wear rate while positioning factors, (such as cup medialization and cup inclination angle) explained only 10% to 33%.

CONCLUSION

Patient-specific wear rates are associated with patients' gait patterns. Gait pattern has a greater influence on wear than component positioning for traditional metal-on-polyethylene bearings.

CLINICAL RELEVANCE

The consideration of individual gait bears potential to further reduce implant wear in THR. In the future, a predictive wear model may identify individual, modifiable wear factors for modern materials.

Metal-on-Metal Compared With Metal-on-Polyethylene: The Effect on Trunnion Corrosion in Total Hip Arthroplasty

BACKGROUND

Trunnion tribocorrosion in total hip arthroplasties is concerning, but retrieval studies often are subjective or lack comparison groups. Quantitative comparisons of clinically relevant implants are required. The purpose of this investigation was to evaluate material loss in metal-on-metal (MoM) and metal-on-polyethylene (MoP) total hip articulations while controlling for trunnion design and head size.

METHODS

The 166 retrieved femoral heads from 2 manufacturers were analyzed. Four cohorts based on head size, trunnion design, manufacturer, and articulation type (MoM vs MoP) were created. Corrosion was measured by a coordinate measurement machine, and material loss was assessed (MATLAB).

RESULTS

Retrieved femoral heads from MoP articulations had 5 times less trunnion material loss than MoM articulations, on average, for both manufacturers. There was no difference in material loss between large modular head (>40 mm) and 36-mm MoM hip trunnion. Implants with a material loss above the detectable limit demonstrated a correlation with time in vivo only in MoP articulations.

CONCLUSION

Retrieved femoral heads from MoP bearing couples had a lower magnitude of material loss than MoM couples, independent of head diameter. A time in vivo effect was only seen in MoP bearings.

A combined experimental and finite element approach to analyse the fretting mechanism of the head-stem taper junction in total hip replacement

Fretting corrosion at the taper interface of modular hip implants has been implicated as a possible cause of implant failure. This study was set up to gain more insight in the taper mechanics that lead to fretting corrosion. The objectives of this study therefore were (1) to select experimental loading conditions to reproduce clinically relevant fretting corrosion features observed in retrieved components, (2) to develop a finite element model consistent with the fretting experiments and (3) to apply more complicated loading conditions of activities of daily living to the finite element model to study the taper mechanics. The experiments showed similar wear patterns on the taper surface as observed in retrievals. The finite element wear score based on Archard’s law did not correlate well with the amount of material loss measured in the experiments. However, similar patterns were observed between the simulated micromotions and the experimental wear measurements. Although the finite element model could not be validated, the loading conditions based on activities of daily living demonstrate the importance of assembly load on the wear potential. These findings suggest that finite element models that do not incorporate geometry updates to account for wear loss may not be appropriate to predict wear volumes of taper connections.

Hip joint geometry effects on cartilage contact stresses during a gait cycle

The cartilage surface geometry of natural human hip joint is commonly regarded as sphere. It has been widely applied in computational simulation and hip joint prosthesis design. Some new geometry models have been developed and the sphere assumption has been questioned recently. The objective of this study was to analyze joint geometry effects on cartilage contact stress distribution and investigate contact patterns during a whole gait cycle. Hip surface was reconstructed from CT data of a healthy volunteer. Three finite element (FE) models of hip joint were developed from different cartilage geometries: natural geometry, sphere and rotational ellipsoid. Loads at ten instants of gait cycle were applied to these models based on published in-vivo data. FE predictions of peak contact pressure during gait of natural hip were compared with sphere and rotational ellipsoid replaced hip joint. Contact occurs mainly in upper anterior region of both acetabulum and femur distributing along sagittal plane of human body. It moves towards inferolateral aspect as the resultant joint reaction force changes during walking for natural hip. Peak pressures at the instant with maximum contact force were 7.48 MPa, 14.97 MPa and 13.12 MPa for models with natural hip surface, sphere replaced and rotational ellipsoid replaced surface respectively. During the whole gait cycle, contact pressure of natural hip ranked lowest in most of the instants, followed by rotational ellipsoid replaced and sphere replaced hip. The results indicate that rotational ellipsoid is more consistent with natural hip cartilage geometry than sphere during normal walking. This means rotational ellipsoid prosthesis could give a better description of physiological structure compared with standard sphere prosthesis. Therefore, rotational ellipsoid would be a better choice for prosthesis design.

Experimental Measurement of the Static Coefficient of Friction at the Ti-Ti Taper Connection in Total Hip Arthroplasty

The modular taper junction in total hip replacements has been implicated as a possible source of wear. The finite-element (FE) method can be used to study the wear potential at the taper junction. For such simulations it is important to implement representative contact parameters, in order to achieve accurate results. One of the main parameters in FE simulations is the coefficient of friction. However, in current literature, there is quite a wide spread in coefficient of friction values (0.15 - 0.8), which has a significant effect on the outcome of the FE simulations. Therefore, to obtain more accurate results, one should use a coefficient of friction that is determined for the specific material couple being analyzed. In this study, the static coefficient of friction was determined for two types of titanium-on-titanium stem-adaptor couples, using actual cut-outs of the final implants, to ensure that the coefficient of friction was determined consistently for the actual implant material and surface finish characteristics. Two types of tapers were examined, Biomet type-1 and 12/14, where type-1 has a polished surface finish and the 12/14 is a microgrooved system. We found static coefficients of friction of 0.19 and 0.29 for the 12/14 and type-1 stem-adaptor couples, respectively.

Clinical and Wear Analyses of 9 Large Metal-on-Metal Total Hip Prostheses

Background

Metal-on-Metal (MoM) total hip arthroplasties (THA) are associated with pseudotumor formation and high revision rates. This prospective study analysed the clinical and wear analyses of 9 large Metal-on-Metal (MoM) total hip arthroplasties (THA) to understand the underlying mechanisms of failure. The MoM bearings were revised for multiple reasons; the main reason was pseudotumor formation.

Materials and Methods

From 2006 till 2010 the Reinier de Graaf Hospital implanted 160 large head M2a-Magnum™ (Biomet Inc. Warsaw, Indiana, USA) THAs in 150 patients. The first year, 9 bearings were revised and analysed at the Biomechanics Section, Hamburg University of Technology, Germany. We performed clinical (Harris Hip Score, radiographic analysis, blood cobalt and chromium) and wear analysis (implant, tissue and fluid) of the 9 bearings. Since this study did not fall under the scope of the Medical Research Involving Human Subjects Act in The Netherlands, no ethical approval was necessary. In this prospective study all patient details were anonymized by the corresponding author, all other authors were blinded during the research and wear analyses. Patients with bilateral MoM implants were excluded.

Results

The 9 bearings had a median (IQR) survival of 41.0 (25) months in situ. From these bearings, three showed no noticeable wear. The median (IQR) head wear volume was 3.2 (3.6) mm³ and maximum wear depth 0.02 (0.02) mm. For the cup the median (IQR) wear volume was 0.23 (0.3) mm³ with a maximum wear depth of 0.03 (0.05) mm.

Conclusion

An early identification of parameters related to failure of the MoM THA, such as pain, decreased range of motion, radiographic changes and high levels of blood cobalt and chromium is of great importance for patient’s quality of life. Especially now patients and surgeons face the long term effects of all these bearings still in situ. This study reports the clinical and wear analyses of 9 MoM THA. In the majority of this group the reason for revision was pseudotumor formation. Most bearings showed signs of wear, however with a great diversity in clinical analysis, in inclination angle, serum cobalt and chromium levels as well as wear analysis. For a better understanding of the underlying mechanisms related with failure, more wear analyses of revised MoM bearings are necessary as well as a frequent follow-up of the patients with a MoM bearing.

Current Concepts of Using Large Femoral Heads in Total Hip Arthroplasty

Instability and dislocation after total hip arthroplasty are the most common causes of revisions and major complications for failure of inserted prostheses, leading to a reduction in quality of life. Because the use of artificial femoral head sizes smaller than patient's own size is the important cause for dislocation, the use of large femoral head have increased. Femoral head sizes greater than 32 mm offer multiple advantages in physical function and activity levels of patients by improving hip stability, decreasing dislocation rate and increasing range of motion. However, various concerns are encountered including wear debris generation at the trunnion-bore interface and increases in frictional torque and stress over the component-bone interface when using larger head sizes. So, the use of femoral head sizes less than 40 mm is recommended.

Catastrophic Femoral Head-Stem Trunnion Dissociation Secondary to Corrosion

BACKGROUND

Modular femoral heads provide procedural enhancement by allowing accurate restoration of hip offset and limb-length equalization. However, corrosion may lead to adverse local tissue reactions. Severe trunnion corrosion can also lead to femoral head dissociation and catastrophic implant failure following primary total hip arthroplasty.

METHODS

We describe 5 cases, from our institution, in which the femoral head became dissociated from the femoral stem trunnion secondary to severe corrosion. Possible causes are evaluated.

RESULTS

Demographic commonalities among the 5 patients included a body mass index (BMI) of ≥30 kg/m(2) and male sex. All femoral heads were made of cobalt-chromium alloy and were larger-diameter implants (≥36 mm). Four of the 5 patients had a femoral head that increased the neck length above the default on a so-called standard head and 3 of the 5 had a stem with a 127° neck-shaft angle.

CONCLUSIONS

Although dissociation of the femoral head from the femoral trunnion following total hip arthroplasty is exceedingly rare, the prevalence may increase with longer follow-up. The dissociation is likely related to multiple factors, including a BMI of ≥30 kg/m(2), male sex, and corrosion resulting from the use of a larger metal head with a neck length of greater than the default and a stem with high offset. It is critical that surgeons be able to recognize this mode of implant failure and appropriately prepare to remove the femoral component during revision surgery.

LEVEL OF EVIDENCE

Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.

Large-diameter total hip arthroplasty modular heads require greater assembly forces for initial stability

Objectives

Modular junctions are ubiquitous in contemporary hip arthroplasty. The head-trunnion junction is implicated in the failure of large diameter metal-on-metal (MoM) hips which are the currently the topic of one the largest legal actions in the history of orthopaedics (estimated costs are stated to exceed $4 billion). Several factors are known to influence the strength of these press-fit modular connections. However, the influence of different head sizes has not previously been investigated. The aim of the study was to establish whether the choice of head size influences the initial strength of the trunnion-head connection.

Materials and Methods

Ti-6Al-4V trunnions (n = 60) and two different sizes of cobalt-chromium (Co-Cr) heads (28 mm and 36 mm; 30 of each size) were used in the study. Three different levels of assembly force were considered: 4 kN; 5 kN; and 6 kN (n = 10 each). The strength of the press-fit connection was subsequently evaluated by measuring the pull-off force required to break the connection. The statistical differences in pull-off force were examined using a Kruskal–Wallis test and two-sample Mann–Whitney U test. Finite element and analytical models were developed to understand the reasons for the experimentally observed differences.

Results

36 mm diameter heads had significantly lower pull-off forces than 28 mm heads when impacted at 4 kN and 5 kN (p < 0.001; p < 0.001), but not at 6 kN (p = 0.21). Mean pull-off forces at 4 kN and 5 kN impaction forces were approximately 20% larger for 28 mm heads compared with 36 mm heads. Finite element and analytical models demonstrate that the differences in pull-off strength can be explained by differences in structural rigidity and the resulting interface pressures.

Conclusion

This is the first study to show that 36 mm Co-Cr heads have up to 20% lower pull-off connection strength compared with 28 mm heads for equivalent assembly forces. This effect is likely to play a role in the high failure rates of large diameter MoM hips.

Cite this article: A. R. MacLeod, N. P. T. Sullivan, M. R. Whitehouse, H. S. Gill. Large-diameter total hip arthroplasty modular heads require greater assembly forces for initial stability. Bone Joint Res 2016;5:338–346. DOI: 10.1302/2046-3758.58.BJR-2016-0044.R1.

Corrosion of the Head-neck Junction After Total Hip Arthroplasty

Corrosion of the head-neck junction of implants used in total hip arthroplasty is a complex problem. Clinical severity appears to be multifactorial, and the predictive variables have yet to be consistently identified in the literature. Corrosion should be considered in the differential diagnosis of hip pain following total hip arthroplasty regardless of the type of bearing surface used. The most common presentation, pain followed by instability, is similar to complications associated with metal-on-metal articulations. The diagnosis of implant corrosion of the head-neck junction can be challenging; an infection workup should be performed along with analysis of serum metal ion levels and cross-sectional imaging. In the short term, a well-fixed stem may be retained, and the exchange of an isolated head with a ceramic femoral head seems to be a promising option for certain implants. Further research with longer follow-up is warranted, and high levels of evidence are needed to determine whether this approach is generalizable.

Higher Blood Cobalt and Chromium Levels in Patients With Unilateral Metal-on-Metal Total Hip Arthroplasties Compared to Hip Resurfacings

BACKGROUND

Adverse soft tissue reactions in metal-on-metal (MoM) hip replacements are associated with cobalt (Co) and chromium (Cr) ions in blood. We report the prevalence and risk factors for elevated blood Co and Cr levels in patients with a unilateral MoM hip.

METHODS

From a single institution, blood Co and Cr levels were analyzed in 1748 patients (692 hip resurfacings and 1056 total hip arthroplasties [THAs]). Concentrations exceeding 7 ppb were considered elevated, and the risk factors for elevated levels were calculated with binary logistic regression.

RESULTS

Elevated blood metal ion levels were more common in MoM THA than in resurfacing patients (17.4% vs 5.9%, P < .001), and in 5 of the 7 THA brands, more than 20% of patients had elevated metal ion concentrations, whereas the proportion was less than 10% in all hip resurfacings. In resurfacings, small femoral head (odds ratio [OR] 1.30 per millimeter decrease [CI, 1.12-1.49]), high acetabular inclination (OR 1.15 per degree increase [CI 1.09-1.22]), and young age (OR 1.05 per year decrease [1.02-1.10]) were independent risk factors for elevated ions. In the THA group, female gender (OR 2.04 [CI 1.35-3.06]), longer time between surgery and ion measurement (OR 1.19 per year increase [CI 1.05-1.34]), and large headsize (OR 1.07 per millimeter increase [CI 1.01-1.13]) were risk factors for elevated ions.

CONCLUSION

Given the high percentage of elevated levels, the systematic surveillance of especially large diameter MoM THAs seems justified.

Corrosion at the head-neck interface of current designs of modular femoral components

There is increasing global awareness of adverse reactions to metal debris and elevated serum metal ion concentrations following the use of second generation metal-on-metal total hip arthroplasties. The high incidence of these complications can be largely attributed to corrosion at the head-neck interface. Severe corrosion of the taper is identified most commonly in association with larger diameter femoral heads. However, there is emerging evidence of varying levels of corrosion observed in retrieved components with smaller diameter femoral heads. This same mechanism of galvanic and mechanically-assisted crevice corrosion has been observed in metal-on-polyethylene and ceramic components, suggesting an inherent biomechanical problem with current designs of the head-neck interface.

We provide a review of the fundamental questions and answers clinicians and researchers must understand regarding corrosion of the taper, and its relevance to current orthopaedic practice.

Cite this article: Bone Joint J 2016;98-B:579–84.

Large Metal Heads and Vitamin E Polyethylene Increase Frictional Torque in Total Hip Arthroplasty

PURPOSE

Trunnionosis has reemerged in modern total hip arthroplasty for reasons that remain unclear. Bearing frictional torque transmits forces to the modular head-neck interface, which may contribute to taper corrosion. The purpose of this study is to compare frictional torque of modern bearing couples in total hip arthroplasty.

METHODS

Mechanical testing based on in vivo loading conditions was used to measure frictional torque. All bearing couples were lubricated and tested at 1 Hz for more than 2000 cycles. The bearing couples tested included conventional, highly crosslinked (XLPE) and vitamin E polyethylene, CoCr, and ceramic femoral heads and dual-mobility bearings. Statistical analysis was performed using Student t test for single-variable and analysis of variance for multivariant analysis. P ≤ .05 was considered statistically significant.

RESULTS

Large CoCr metal heads (≥36 mm) substantially increased frictional torque against XLPE liners (P = .01), a finding not observed in ceramic heads. Vitamin E polyethylene substantially increased frictional torque compared with XLPE in CoCr and ceramic heads (P = .001), whereas a difference between conventional and XLPE was not observed (P = .69) with the numbers available. Dual-mobility bearing with ceramic inner head demonstrated the lowest mean frictional torque of all bearing couples.

CONCLUSION

In this simulated in vivo model, large-diameter CoCr femoral heads and vitamin E polyethylene liners are associated with increased frictional torque compared with smaller metal heads and XLPE, respectively. The increased frictional torque of vitamin E polyethylene and larger-diameter femoral heads should be considered and further studied, along with reported benefits of these modern bearing couples.

Comparison of Whole-Blood Metal Ion Levels Among Four Types of Large-Head, Metal-on-Metal Total Hip Arthroplasty Implants: A Concise Follow-up, at Five Years, of a Previous Report

Few studies of total hip arthroplasty (THA) implants with a large-diameter femoral head and metal-on-metal design have directly compared the progression of metal ion levels over time and the relationship to complications. As we previously reported, 144 patients received one of four types of large-diameter-head, metal-on-metal THA designs (Durom, Birmingham, ASR XL, or Magnum implants). Cobalt, chromium, and titanium ion levels were measured over five years. We compared ion levels and clinical results over time. The Durom group showed the highest levels of cobalt (p ≤ 0.002) and titanium ions (p ≤ 0.03). Both the Durom and Birmingham groups demonstrated significant ongoing cobalt increases up to five years. Eight patients (seven with a Durom implant and one with a Birmingham implant) developed adverse local tissue reaction. Six Durom implants and one Birmingham implant required revision, with one pseudotumor under surveillance at the time of the most recent follow-up. We found that ion generation and related complications varied among designs. More concerning was that, for some designs, ion levels continued to increase. Coupling a cobalt-chromium adapter sleeve to an unmodified titanium femoral trunnion along with a large metal-on-metal bearing may explain the poor performances of two of the designs in the current study.

Basic principles and uniform terminology for the head-neck junction in hip replacement

Recent problems with large head metal on metal hip replacements have spiked renewed interest in the head-neck junction. A thorough knowledge of the principles of the locking mechanism, the assembly technique and affecting factors on the strength of this junction is needed. Currently a confusing variability in terms is used to describe this junction. This overcomplicates an already complex issue. The purpose of this literature review is to collect and list the different terms used and to propose a uniform terminology. Two authors independently searched the electronic databases of PubMed, CINAHL and MEDLINE with specific key words and combinations according to the PRISMA guidelines. The initial search yielded a total of 518 articles with ultimately 53 articles included in the present analysis. No consensus for a uniform term for the 2 sides of the head-stem junction was found. Since there is already pronounced variability in taper designs between different manufacturers (even so similarly named, e.g. "12/14"), a uniform terminology could be the first step to simplify the situation. "Male" and "female taper" is proposed as the appropriate terminology for the stem and head junction in hip replacement, respectively. The importance of the assembly technique understanding the principles of the locking mechanism is emphasised.

Is There a Benefit to Head Size Greater Than 36 mm in Total Hip Arthroplasty?

This study compares the rate of dislocation and revision for instability between 36-mm and anatomic femoral heads (large diameter metal-on-metal THA, dual-mobility bearings, and hip resurfacing arthroplasty) in patients at high risk for dislocation. A total of 501 high-risk patients, over a 10-year period, were identified (282 36-mm THA, 24 dual-mobility bearings, 83 metal-on-metal arthroplasty, and 112 hip resurfacing arthroplasty). There were 13 dislocations in the 36-mm group compared to 1 in the anatomic group (4.6% vs 0.5%; P = .005). Four patients dislocated more than once in the 36-mm group (1.4% vs 0%; P = .04), and 2 patients in the 36-mm group required a revision for instability (0.7% vs 0%; P = .11). These results suggest that anatomic head sizes significantly lower the risk of dislocation in high-risk patients.

A Novel Approach for Dynamic Testing of Total Hip Dislocation under Physiological Conditions

Constant high rates of dislocation-related complications of total hip replacements (THRs) show that contributing factors like implant position and design, soft tissue condition and dynamics of physiological motions have not yet been fully understood. As in vivo measurements of excessive motions are not possible due to ethical objections, a comprehensive approach is proposed which is capable of testing THR stability under dynamic, reproducible and physiological conditions. The approach is based on a hardware-in-the-loop (HiL) simulation where a robotic physical setup interacts with a computational musculoskeletal model based on inverse dynamics. A major objective of this work was the validation of the HiL test system against in vivo data derived from patients with instrumented THRs. Moreover, the impact of certain test conditions, such as joint lubrication, implant position, load level in terms of body mass and removal of muscle structures, was evaluated within several HiL simulations. The outcomes for a normal sitting down and standing up maneuver revealed good agreement in trend and magnitude compared with in vivo measured hip joint forces. For a deep maneuver with femoral adduction, lubrication was shown to cause less friction torques than under dry conditions. Similarly, it could be demonstrated that less cup anteversion and inclination lead to earlier impingement in flexion motion including pelvic tilt for selected combinations of cup and stem positions. Reducing body mass did not influence impingement-free range of motion and dislocation behavior; however, higher resisting torques were observed under higher loads. Muscle removal emulating a posterior surgical approach indicated alterations in THR loading and the instability process in contrast to a reference case with intact musculature. Based on the presented data, it can be concluded that the HiL test system is able to reproduce comparable joint dynamics as present in THR patients.

The evolution of the trunnion

Implant modularity has recently come under increasing scrutiny due to concerns regarding wear, corrosion and potential adverse reactions to metal debris. This review outlines the evolution and development of the femoral stem trunnion and relates this to contemporary issues now encountered.Despite different manufacturers producing what appear to be similar trunnion designs, there is still a lack of standardisation, with small but significant design variations. Wear and corrosion is certainly not a new phenomenon, but recent changes in design and the use of larger metal head sizes has potentially made the problem more prevalent. These issues along with steps to avoid these problems are discussed.

Serum Metal Ions with a Titanium Modular Neck Total Hip Replacement System

The goal of this study is to evaluate serum levels of chromium (Cr), cobalt (Co), and titanium (Ti) within the first two years following total hip arthroplasty using a Ti modular neck system. Twenty-five patients were randomized to a metal-on-metal (MoM) bearing with an all CoCr shell, and the remaining 25 received a metal-on-polyethylene (MoP) with a Ti shell. Serum levels demonstrated increases for Cr, Co, and Ti at 1 year (P < .001). MoM had similar Ti levels to MoP hips at 1 year (P=0.11) but lower at 2 years (P=0.03). Results suggest that the passive corrosion (i.e., chemical, pitting, and crevice corrosion) of exposed non-articular metal surfaces may be a greater source of ions than the neck-stem or head-neck interfaces.

Midterm Results of Delta Ceramic-on-Ceramic Total Hip Arthroplasty

This study reports mid-term results of Delta ceramic on ceramic (COC) in total hip arthroplasty (THA). Subjects received Delta COC THA in a prospective multi-center study with either 28 mm (n=177) or 36 mm (n=168) articulations. Annual clinical and radiographic evaluations were performed, and patients were asked about hip noises. At mean 5.3-year follow-up there were 3 (0.9%) post-operative liner fractures. Nine revisions were performed (2 liner fracture, 4 stem loosening, 3 deep infection). Kaplan-Meier survivorship at 6 years was 96.9% (94.0-98.4). Twenty-six (7.5%) subjects reported squeaking, of whom none were revised. One (0.3%) subject could reproduce a sound in clinic. More patients reported squeaking with a 36 mm bearing (28 mm: 7/177, 36 mm: 19/168, P=0.013).

Modular tapered implants for severe femoral bone loss in THA: reliable osseointegration but frequent complications

BACKGROUND

Modular tapered stems have been suggested as the optimal implants for patients with severe femoral bone loss (Paprosky Type IIIB and IV) undergoing revision total hip arthroplasty (THA); however, there are few data describing survivorship and hip scores associated with this treatment.

QUESTIONS/PURPOSES

At minimum 2 years, we sought to determine the (1) survivorship of these implants; (2) radiographic evidence of osseointegration; and (3) the Harris hip scores of patients revised with a modular tapered stem for severe femoral bone loss including the complications associated with their use.

METHODS

Between 1999 and 2010, we performed 1124 femoral revisions; of those, 135 (12%) were performed using a modular tapered stem. General indications for use of this device included bone loss, poor quality bone, leg length discrepancy, difficulty adjusting version, proximal femoral remodeling, and ectatic medullary canals in the setting of revision THA. This report reviewed 70 of those that were performed in Type IIIB and IV femurs. Of those 70 patients, 11 had died and one was lost before 2-year followup; the remaining 58 (83%, 33 with a Type IIIB femur and 25 with a Type IV femur) were followed for a minimum of 24 months (average, 67 months; range, 24-151 months). Survivorship free from revision, radiographic analysis for signs of subsidence or stem loosening, and Harris hip scores were tallied.

RESULTS

One patient underwent stem revision for early subsidence followed by failure of ingrowth for a survivorship free from repeat revision of 98% (57 of 58 patients). Five other stems (for a total of six [10%]) subsided early but nonetheless achieved radiographic signs of ingrowth and were not revised again. All other stems demonstrated osseointegration without subsidence and remained radiographically well fixed. In patients with surviving implants, the mean Harris hip score improved from 34 (range, 11-49) preoperatively to 74 (range, 14-100; p<0.001) postoperatively. Overall, 15 of 58 patients (26%) experienced a complication, of whom 10 (17%) underwent reoperation.

CONCLUSIONS

We found that modular tapered stems resulted in 98% survivorship at a mean of 67 months in patients with Type IIIB and IV femurs undergoing femoral revision, although the overall complication rate in this complex subset of patients is high.

LEVEL OF EVIDENCE

Level IV, therapeutic study. See Guidelines for Authors for a complete description of levels of evidence.

The 2014 Frank Stinchfield Award: The 'landing zone' for wear and stability in total hip arthroplasty is smaller than we thought: a computational analysis

BACKGROUND

Positioning of total hip bearings involves tradeoffs, because cup orientations most favorable in terms of stability are not necessarily ideal in terms of reduction of contact stress and wear potential. Previous studies and models have not addressed these potentially competing considerations for optimal total hip arthroplasty (THA) function.

QUESTIONS/PURPOSES

We therefore asked if component positioning in total hips could be addressed in terms of balancing bearing surface wear and stability. Specifically, we sought to identify acetabular component inclination and anteversion orientation, which simultaneously resulted in minimal wear while maximizing construct stability, for several permutations of femoral head diameter and femoral stem anteversion.

METHODS

A validated metal-on-metal THA finite element (FE) model was used in this investigation. Five dislocation-prone motions as well as gait were considered as were permutations of femoral anteversion (0°-30°), femoral head diameter (32-48 mm), cup inclination (25°-75°), and cup anteversion (0°-50°), resulting in 4320 distinct FE simulations. A novel metric was developed to identify a range of favorable cup orientations (so-called "landing zone") by considering both surface wear and component stability.

RESULTS

When considering both wear and stability with equal weight, ideal cup position was more restrictive than the historically defined safe zone and was substantially more sensitive to cup anteversion than to inclination. Ideal acetabular positioning varied with both femoral head diameter and femoral version. In general, ideal cup inclination decreased with increased head diameter (approximately 0.5° per millimeter increase in head diameter). Additionally, ideal inclination increased with increased values of femoral anteversion (approximately 0.3° per degree increase in stem anteversion). Conversely, ideal cup anteversion increased with increased femoral head diameter (0.3° per millimeter increase) and decreased with increased femoral stem anteversion (approximately 0.3° per degree increase). Regressions demonstrated strong correlations between optimal cup inclination versus head diameter (Pearson's r=-0.88), between optimal cup inclination versus femoral anteversion (r=0.96), between optimal cup anteversion versus head diameter (r=0.99), and between optimal cup anteversion and femoral anteversion (r=-0.98). For a 36-mm cup with a 20° anteverted stem, the ideal cup orientation was 46°±12° inclination and 15°±4° anteversion.

CONCLUSIONS

The range of cup orientations that maximized stability and minimized wear (so-called "landing zone") was substantially smaller than historical guidelines and specifically did not increase with increased head size, challenging the presumption that larger heads are more forgiving. In particular, when the cup is oriented to improve not only stability, but also wear in the model, there was little or no added stability achieved by the use of larger femoral heads. Additionally, ideal cup positioning was more sensitive to cup anteversion than to inclination.

CLINICAL RELEVANCE

Positioning THA bearings involves tradeoffs regarding stability and long-term bearing wear. Cup positions most favorable to minimization of wear such as low inclination and elevated anteversion were detrimental in terms of construct stability. Orientations were identified that best balanced the competing considerations of wear and stability.

What is the trouble with trunnions?

BACKGROUND

Recent studies have attributed adverse local tissue reactions (ALTRs) in patients with total hip arthroplasties (THAs) to tribocorrosion debris generated by modular femoral stems. The presentations of ALTR are diverse, as are the causes of it, and the biological responses can be important reasons for failure after THA.

QUESTION/PURPOSES

(1) What clinical problems have been reported in patients with modular stems since 1988? (2) What THA design features are associated with tribocorrosion in taper junctions? (3) What are the microscopic and tribological characteristics of the debris produced at the taper junctions? (4) What are the cellular and immunological traits of the biological response to taper tribocorrosion debris?

METHODS

We conducted a systematic review using MEDLINE and EMBASE-cited articles to summarize failure modes associated with modular femoral stems. One hundred sixty-two of 1043 articles reported on the clinical performances or failure modes attributed to modular femoral stems. There were 10 laboratory studies, 26 case reports, 13 Level IV, 94 Level III, 18 Level II, and one Level I of Evidence papers. To address the remaining questions, we did a second review of 524 articles. One hundred twenty-seven articles met the eligibility criteria, including 81 articles on design features related to tribocorrosion, 15 articles on corrosion debris characteristics, and 31 articles on the biological response to tribocorrosion debris.

RESULTS

Sixty-eight of 162 studies reported failure attributed to modular femoral stems for one of these four modularity-related failure modes: tribocorrosion-associated ALTR, dissociation of a taper junction, stem fracture, and mismatch of a femoral head taper attached to a stem with a different trunnion size. The remaining 94 studies found no clinical consequences related to the presence of a taper junction. THA component features associated with tribocorrosion included trunnion geometry and large-diameter femoral heads. Solid tribocorrosion debris is primarily chromium-orthophosphate material of variable size and may be more biologically reactive than wear debris.

CONCLUSIONS

There has been an increase in publications describing ALTR around modular hip prostheses in the last 3 years. Implant design changes, including larger femoral heads and smaller trunnions, have been implicated, but there may also be more recognition of the problem by the orthopaedic community. Analyzing retrieved implants to understand the history of taper-related problems, designing clinically relevant in vitro corrosion tests to test modular junctions, and identifying biomarkers to recognize patients at risk of ALTR should be the focus of ongoing research to help surgeons avoid and detect tribocorrosion-related problems in joint replacements.