Does surface roughness influence the primary stability of acetabular cups? A numerical and experimental biomechanical evaluation

Finite element analysis of primary stability in cementless tibial components with varying interference fits

BACKGROUND

Cementless knee implants achieve initial fixation through an interference fit, where the tibial implant is press-fitted into an undersized bone cavity. The dimensions between the implant and bone cuts must be carefully balanced to achieve an optimal interference fit, ensuring good primary stability, which is crucial for long-term fixation and successful osseointegration. However, the ideal interference fit remains uncertain. Excessive interference fit may lead to bone plastic deformation, while insufficient fit can result in large micromotions, small movements at the bone-implant interface, that compromise stability. This study evaluates how interference fit affects bone plasticity and micromotions, and how different loading conditions influence primary stability using finite element analysis.

METHODS

Finite element models, based on experimentally implanted components, simulated interference fits of 350 μm and 700 μm. Micromotions, gap dynamics, and bone deformation were assessed during gait and squat activities under both simplified and complex loading conditions.

FINDINGS

Higher interference fits increased bone plastic deformation, limiting elastic energy accumulation, whereas lower interference fits exhibited a reduced effect. Micromotions and gaps were consistently larger in lower interference fit implants. Furthermore, simplified loading underestimated micromotions and gaps compared to the complex loading.

INTERPRETATION

These findings help explain why higher interference fits provided limited improvements in primary stability during experimental tests, despite differing predictions from simulations. This study enhances our understanding of bone-implant interactions and suggests that increasing interference fit does not necessarily improve implant stability. It also highlights the importance of incorporating complex loading conditions for more accurate primary stability assessment.

The research status and future direction of polyetheretherketone in dental implant -A comprehensive review

Currently, dental implants primarily rely on the use of titanium and titanium alloys. However, the extensive utilization of these materials in clinical practice has unveiled various problems including stress shielding, corrosion, allergic reactions, cytotoxicity, and image artifacts. As a result, polyetheretherketone (PEEK) has emerged as a notable alternative due to its favorable mechanical properties, corrosion resistance, wear resistance, biocompatibility, radiation penetrability and MRI compatibility. Meanwhile, the advancement and extensive application of 3D printing technology has expanded the range of medical applications for PEEK, including artificial spines, skulls, ribs, shinbones, hip joints, and temporomandibular joints. In this review, we aim to assess the advantages and disadvantages of PEEK as a dental implant material, summarize the measures taken to address its shortcomings and their effects, and provide insight into the future potential of PEEK in dental implant applications, with the goal of offering guidance and reference for future research endeavors.

Modeling the debonding process of osseointegrated implants due to coupled adhesion and friction

Cementless implants have become widely used for total hip replacement surgery. The long-term stability of these implants is achieved by bone growing around and into the rough surface of the implant, a process called osseointegration. However, debonding of the bone-implant interface can still occur due to aseptic implant loosening and insufficient osseointegration, which may have dramatic consequences. The aim of this work is to describe a new 3D finite element frictional contact formulation for the debonding of partially osseointegrated implants. The contact model is based on a modified Coulomb friction law by Immel et al. (2020), that takes into account the tangential debonding of the bone-implant interface. This model is extended in the direction normal to the bone-implant interface by considering a cohesive zone model, to account for adhesion phenomena in the normal direction and for adhesive friction of partially bonded interfaces. The model is applied to simulate the debonding of an acetabular cup implant. The influence of partial osseointegration and adhesive effects on the long-term stability of the implant is assessed. The influence of different patient- and implant-specific parameters such as the friction coefficient [Formula: see text], the trabecular Young's modulus [Formula: see text], and the interference fit [Formula: see text] is also analyzed, in order to determine the optimal stability for different configurations. Furthermore, this work provides guidelines for future experimental and computational studies that are necessary for further parameter calibration.

Determinants of the primary stability of cementless acetabular cup implants: A 3D finite element study

Primary stability of cementless implants is crucial for the surgical success and long-term stability. However, primary stability is difficult to quantify in vivo and the biomechanical phenomena occurring during the press-fit insertion of an acetabular cup (AC) implant are still poorly understood. The aim of this study is to investigate the influence of the cortical and trabecular bone Young's moduli E_c and E_t, the interference fit IF and the sliding friction coefficient of the bone-implant interface μ on the primary stability of an AC implant. For each parameter combination, the insertion of the AC implant into the hip cavity and consequent pull-out are simulated with a 3D finite element model of a human hemi-pelvis. The primary stability is assessed by determining the polar gap and the maximum pull-out force. The polar gap increases along with all considered parameters. The pull-out force shows a continuous increase with E_c and E_t and a non-linear variation as a function of μ and IF is obtained. For μ > 0.6 and IF > 1.4 mm the primary stability decreases, and a combination of smaller μ and IF lead to a better fixation. Based on the patient's bone stiffness, optimal combinations of μ and IF can be identified. The results are in good qualitative agreement with previous studies and provide a better understanding of the determinants of the AC implant primary stability. They suggest a guideline for the optimal choice of implant surface roughness and IF based on the patient's bone quality.

Off-the-shelf 3D printed titanium cups in primary total hip arthroplasty

Three-dimensional (3D)-printed titanium cups used in primary total hip arthroplasty (THA) were developed to combine the benefits of a low elastic modulus with a highly porous surface. The aim was to improve local vascularization and bony ingrowth, and at the same time to reduce periprosthetic stress shielding. Additive manufacturing, starting with a titanium alloy powder, allows serial production of devices with large interconnected pores (trabecular titanium), overcoming the drawbacks of tantalum and conventional manufacturing techniques. To date, 3D-printed cups have achieved dependable clinical and radiological outcomes with results not inferior to conventional sockets and with good rates of osseointegration. No mechanical failures and no abnormal ion release and biocompatibility warnings have been reported. In this review, we focused on the manufacturing technique, cup features, clinical outcomes, open questions and future developments of off-the-shelf 3D-printed titanium shells in THA.

Ceramic-on-ceramic articulation in press-fit total hip arthroplasty as a potential reason for early failure, what about the survivors: a ten year follow-up

PURPOSE

In press-fit total hip arthroplasty (THA), primary stability is needed to avoid micromotion and hereby aseptic loosening, the main reason for early revision. High aseptic loosening revision rates of the seleXys TH+ cup (Mathys Medical) with Ceramys ceramic-on-ceramic (CoC) bearing are seen in literature. Since CoC is presumed to overcome long-term wear-related revisions, the reason for early failure of this cup is important to clarify. The aim is to investigate its ten year outcomes and differentiate between potential causes and identify risk factors for aseptic loosening.

METHODS

Retrospective screening of a prospectively documented series of 315 THAs was performed. Primary outcome was cumulative incidence of cup revision due to aseptic loosening. Secondary outcomes were component revision and reoperation. Additionally, potential predictive factors for aseptic loosening were evaluated.

RESULTS

At the median follow-up of 9.7 years [IQR 4.4; 10.3], 48 TH+ (15.2%) were revised due to aseptic loosening. Competing risk analysis showed a ten year cumulative incidence of cup revision due to aseptic loosening of 15.6% (95% CI 12.0-20.2). Stabilization of early revision rates was observed, following a high rate of respectively 81.3% (n = 39) and 95.8% (n = 46) within the first two and three years. No significant predictive factors for aseptic loosening were found.

CONCLUSION

The ten year results of seleXys TH+ cup with Ceramys CoC bearing showed an unacceptable high aseptic loosening rate, which stabilized over time after a high early failure incidence. This could be attributed to a problem with osseointegration during the transition of primary to definitive stability.

Comparison of Test Setups for the Experimental Evaluation of the Primary Fixation Stability of Acetabular Cups

Sufficient primary fixation stability is the basis for the osseointegration of cementless acetabular cups. Several test methods have been established for determining the tilting moment of acetabular press-fit cups, which is a measure for their primary fixation stability. The central aim of this experimental study was to show the differences between the commonly used lever-out test method (Method 1) and the edge-load test method (Method 2) in which the cup insert is axially loaded (1 kN) during the tilting process with respect to the parameters, tilting moment, and interface stiffness. Therefore, using a biomechanical cup block model, a press-fit cup design with a macro-structured surface was pushed into three cavity types (intact, moderate superior defect, and two-point-pinching cavity) made of 15 pcf and 30 pcf polyurethane foam blocks (n = 3 per cavity and foam density combination), respectively. Subsequently, the acetabular cup was disassembled from the three artificial bone cavities using the lever-out and the edge-load test method. Tilting moments determined with Method 1 ranged from 2.72 ± 0.29 Nm to 49.08 ± 1.50 Nm, and with Method 2, they ranged from 41.40 ± 1.05 Nm to 112.86 ± 5.29 Nm. In Method 2, larger areas of abrasion were observed in the artificial bone cavity compared to Method 1. This indicates increased shear forces at the implant–bone interface in the former method. In conclusion, Method 1 simulates the technique used by orthopedic surgeons to assess the correct fit of the trial cup, while Method 2 simulates the tilting of the cup in the acetabular bone cavity under in situ loading with the hip resultant force.

A method to assess primary stability of acetabular components in association with bone defects

The objectives of this study were to develop a simplified acetabular bone defect model based on a representative clinical case, derive four bone defect increments from the simplified defect to establish a step-wise testing procedure, and analyze the impact of bone defect and bone defect filling on primary stability of a press-fit cup in the smallest defined bone defect increment. The original bone defect was approximated with nine reaming procedures and by exclusion of specific procedures, four defect increments were derived. The smallest increment was used in an artificial acetabular test model to test primary stability of a press-fit cup in combination with bone graft substitute (BGS). A primary acetabular test model and a defect model without filling were used as reference. Load was applied in direction of level walking in sinusoidal waveform with an incrementally increasing maximum load (300 N/1000 cycles from 600 to 3000 N). Relative motions (inducible displacement, migration, and total motion) between cup and test model were assessed with an optical measurement system. Original and simplified bone defect volume showed a conformity of 99%. Maximum total motion in the primary setup at 600 N (45.7 ± 5.6 µm) was in a range comparable to tests in human donor specimens (36.0 ± 16.8 µm). Primary stability was reduced by the bone defect, but could mostly be reestablished by BGS-filling. The presented method could be used as platform to test and compare different treatment strategies for increasing bone defect severity in a standardized way.

Clinical evaluation of a novel press-fit acetabular cup using "Ein-Bild-Roentgen-Analysis" (EBRA): A positive short-term prognosis

Introduction

Demographic change and demand for high quality of life lead to increasing implantation numbers. Aim of this study was to compare the Plasmafit® cup to Allofit® and Plasmacup®.

Methods

The study included 174 patients who had received 33 Plasmacup®, 68 Allofit® and 73 Plasmafit® cup implants. These were reviewed postoperatively, after 6 months control and after 12 months.

Results

No significant progressive migration could be discovered in any of the cup systems. At each follow-up the cups showed nearly constant values.

Conclusions

All examined acetabular cups showed excellent migration behavior within the first 12 postoperative months.

Effect of different motor tasks on hip cup primary stability and on the strains in the periacetabular bone: An in vitro study

BACKGROUND

Excessive prosthesis/bone motions and the bone strains around the acetabulum may prevent osteointegration and lead to cup loosening. These two factors depend on post-operative joint loading. We investigated how Walking (which is often simulated) and Standing-Up from seated (possibly more critical) influence the cup primary stability and periacetabular strains.

METHODS

Twelve composite hemipelvises were used in two test campaigns. Simplified loading conditions were adopted to simulate Walking and Standing-Up. For each motor task, a single-direction force was applied in load packages of increasing amplitude. Stable and unstable uncemented cups were implanted. Digital image correlation was used to measure implant/bone motions (three-dimensional translations and rotations, both permanent and inducible), and the strain distribution around the acetabulum.

FINDINGS

When stable implants were tested, higher permanent cranial translations were found during Walking (however the resultant migrations were comparable with Standing-Up); higher rotations were found for Standing-Up. When unstable implants were tested, motions were 1-2 order of magnitude higher. Strains increased significantly from stable to unstable implants. The peak strains were in the superior aspect of the acetabulum during Walking and in the superior-posterior aspect of the acetabulum and at the bottom of the posterior column during Standing-Up.

INTERPRETATION

Different cup migration trends were caused by simulated Walking and Standing-Up, both similar to those observed clinically. The cup mobilization pattern depended on the different simulated motor tasks. Pre-clinical testing of new uncemented cups could include simulation of both motor tasks. Our study could also translate to indication of what tasks should be avoided.

Dependence of the primary stability of cementless acetabular cup implants on the biomechanical environment

Biomechanical phenomena occurring at the bone–implant interface during the press-fit insertion of acetabular cup implants are still poorly understood. This article presents a nonlinear geometrical two-dimensional axisymmetric finite element model aiming at describing the biomechanical behavior of the acetabular cup implant as a function of the bone Young’s modulus Eb, the diametric interference fit ( IF), and the friction coefficient µ. The numerical model was compared with experimental results obtained from an in vitro test, which allows to determine a reference configuration with the parameter set: μ*  = 0.3, [Formula: see text], and IF*  = 1 mm for which the maximal contact pressure tN = 10.7 MPa was found to be localized at the peri-equatorial rim of the acetabular cavity. Parametric studies were carried out, showing that an optimal value of the pull-out force can be defined as a function of μ, Eb, and IF. For the reference configuration, the optimal pull-out force is obtained for μ  = 0.6 (respectively, Eb = 0.35 GPa and IF = 1.4 mm). For relatively low value of µ ( µ < 0.2), the optimal value of IF linearly increases as a function of µ independently of Eb, while for µ > 0.2, the optimal value of IF has a nonlinear dependence on µ and decreases as a function of Eb. The results can be used to help surgeons determine the optimal value of IF in a patient specific manner.

3D Printed Acetabular Cups for Total Hip Arthroplasty: A Review Article

Three-dimensional (3D) printed titanium orthopaedic implants have recently revolutionized the treatment of massive bone defects in the pelvis, and we are on the verge of a change from conventional to 3D printed manufacture for the mass production of millions of off-the-shelf (non-personalized) implants. The process of 3D printing has many adjustable variables, which taken together with the possible variation in designs that can be printed, has created even more possible variables in the final product that must be understood if we are to predict the performance and safety of 3D printed implants. We critically reviewed the clinical use of 3D printing in orthopaedics, focusing on cementless acetabular components used in total hip arthroplasty. We defined the clinical and engineering rationale of 3D printed acetabular cups, summarized the key variables involved in the manufacturing process that influence the properties of the final parts, together with the main limitations of this technology, and created a classification according to end-use application to help explain the controversial and topical issues. Whilst early clinical outcomes related to 3D printed cups have been promising, in-depth robust investigations are needed, partly because regulatory approval systems have not fully adapted to the change in technology. Analysis of both pristine and retrieved cups, together with long-term clinical outcomes, will help the transition to 3D printing to be managed safely.

[Clinical application of three-dimensional printed metal prosthesis in joint surgery]

Objective

To summarize the application progress of three-dimensional (3D) printed metal prosthesis in joint surgery.

Methods

The related literature was extensively reviewed. The effectiveness of 3D printed metal prosthesis in treatment of joint surgery diseases were discussed and summarized, including the all key issues in prosthesis transplantation such as prosthesis stability, postoperative complications, bone ingrowth, etc.

Results

3D printed metal prosthesis has good matching degree, can accurately reconstruct and restore joint function, reduce operation time, and achieve high patient satisfaction in short- and medium-term follow-up. Its application in joint surgery has made good progress.

Conclusion

The personalized microporous structure prostheses of different shapes produced by 3D printing can solve the problem of poor personalized matching of joints for special patients existing in traditional prostheses. Therefore, 3D printing technology is full of hope and will bring great potential to the reform of orthopedic practice in the future.

Influence of Synthetic Bone Substitutes on the Anchorage Behavior of Open-Porous Acetabular Cup

BACKGROUND

The development in implants such as acetabular cups using additive manufacturing techniques is playing an increasingly important role in the healthcare industry.

METHOD

This study compared the primary stability of four selectively laser-melted press-fit cups (Ti6Al4V) with open-porous, load-bearing structural elements on the surface. The aim was to assess whether the material of the artificial bone stock affects the primary stability of the acetabular cup. The surface structures consist of repeated open-porous, load-bearing elements orthogonal to the acetabular surface. Experimental pull-out and lever-out tests were performed on exact-fit and press-fit cups to evaluate the primary stability of the cups in different synthetic bone substitutes. The acetabular components were placed in three different commercially available synthetic materials (ROHACELL-IGF 110, SikaBlock M330, Sawbones Solid Rigid). Results & conclusions: Within the scope of the study, it was possible to show the differences in fixation strength between the tested acetabular cups depending on their design, the structural elements used, and the different bone substitute material. In addition, functional correlations could be found which provide a qualitative reference to the material density of the bone stock and the press-fit volume of the acetabular cups.

Calibration of crushable foam plasticity models for synthetic bone material for use in finite element analysis of acetabular cup deformation and primary stability

Polyurethane (PU) foam is a material often used in biomechanical experiments and demands for the definition of crushable foam plasticity (CFP) in numerical simulations of the primary stability and deformation of implants, to describe the crushing behaviour appropriately. Material data of PU foams with five different densities (10-40 pounds per cubic foot were ascertained experimentally in uniaxial compression test and used to calibrate CFP models for finite element modelling. Additionally, experimental and numerical deformation, push-out and lever-out tests of press-fit acetabular cups were carried out to assess the influence of the chosen material definition (linear elastic and CFP) on the numerical results. Comparison of the experimentally and numerically determined force-displacement curves of the uniaxial compression test showed a mean deviation of less than 3%. In primary stability testing, the deviation between the experimental and numerical results was in a range of 0%-27% for CFP modelling and 64%-341% for the linear elastic model. The material definition selected, highly influenced the numerical results in the current study. The use of a CFP model is recommended for further numerical simulations, when a deformation of the foam beyond the yield strength is likely to occur.

What we have achieved in the design of 3D printed metal implants for application in orthopedics? Personal experience and review

Purpose

This paper aims to review the latest applications in terms of three-dimensional printed (3DP) metal implants in orthopedics, and, importantly, the design of 3DP metal implants through a series of cases operated at The Second Hospital of Jilin University were presented.

Design/methodology/approach

This paper is available to practitioners who are use 3DP implants in orthopedics. This review began with the deficiency of traditional prostheses and basic concepts of 3DP implants. Then, representative 3DP clinical cases were summarized and compared, and the experiences using customized prostheses and directions for future potential development are also shown.

Findings

The results obtained from the follow-up of clinical applications of 3DP implants show that the 3D designed and printed metal implants could exhibit good bone defect matching, quick and safe joint functional rehabilitation as well as saving time in surgery, which achieved high patient satisfaction collectively.

Originality/value

Single center experiences of 3DP metal implants design were shared and the detailed technical points between various regions were compared and analyzed. In conclusion, the 3DP technology is infusive and will present huge potential to reform future orthopedic practice.

Experimental Characterization of the Primary Stability of Acetabular Press-Fit Cups with Open-Porous Load-Bearing Structures on the Surface Layer

Background: Nowadays, hip cups are being used in a wide range of design versions and in an increasing number of units. Their development is progressing steadily. In contrast to conventional methods of manufacturing acetabular cups, additive methods play an increasingly central role in the development progress. Method: A series of eight modified cups were developed on the basis of a standard press-fit cup with a pole flattening and in a reduced version. The surface structures consist of repetitive open-pore load-bearing textural elements aligned right-angled to the cup surface. We used three different types of unit cells (twisted, combined and combined open structures) for constructing of the surface structure. All cups were manufactured using selective laser melting (SLM) of titanium powder (Ti6Al4V). To evaluate the primary stability of the press fit cups in the artificial bone cavity, pull-out and lever-out tests were conducted. All tests were carried out under exact fit conditions. The closed-cell polyurethane (PU) foam, which was used as an artificial bone cavity, was characterized mechanically in order to preempt any potential impact on the test results. Results and conclusions: The pull-out forces as well as the lever moments of the examined cups differ significantly depending on the elementary cells used. The best results in pull-out forces and lever-out moments are shown by the press-fit cups with a combined structure. The results for the assessment of primary stability are related to the geometry used (unit cell), the dimensions of the unit cell, and the volume and porosity responsible for the press fit. Corresponding functional relationships could be identified. The findings show that the implementation of reduced cups in a press-fit design makes sense as part of the development work.

Additive manufacturing technique-designed metallic porous implants for clinical application in orthopedics

Traditional metallic scaffold prostheses, as vastly applied implants in clinical orthopedic operations, have achieved great success in rebuilding limb function. However, mismatch of bone defects and additional coating requirements limit the long-term survival of traditional prostheses. Recently, additive manufacturing (AM) has opened up unprecedented possibilities for producing complicated structures in prosthesis shapes and microporous surface designs of customized prostheses, which can solve the drawback of traditional prostheses mentioned above. This review presents the most commonly used metallic additive manufacturing techniques, the microporous structure design of metallic scaffolds, and novel applications of customized prostheses in the orthopedic field. Challenges and future perspectives on AM fabricated scaffolds are also summarized.

Influence of soft tissue in the assessment of the primary fixation of acetabular cup implants using impact analyses

BACKGROUND

The acetabular cup (AC) implant primary stability is an important determinant for the success of cementless hip surgery but it remains difficult to assess the AC implant fixation in the clinic. A method based on the analysis of the impact produced by an instrumented hammer on the ancillary has been developed by our group (Michel et al., 2016a). However, the soft tissue thickness present around the acetabulum may affect the impact response, which may hamper the robustness of the method. The aim of this study is to evaluate the influence of the soft tissue thickness (STT) on the acetabular cup implant primary fixation evaluation using impact analyses.

METHODS

To do so, different AC implants were inserted in five bovine bone samples. For each sample, different stability conditions were obtained by changing the cavity diameter. For each configuration, the AC implant was impacted 25 times with 10 and 30 mm of soft tissues positioned underneath the sample. The averaged indicator I_m was determined based on the amplitude of the signal for each configuration and each STT and the pull-out force was measured.

FINDINGS

The results show that the resonance frequency of the system increases when the value of the soft tissue thickness decreases. Moreover, an ANOVA analysis shows that there was no significant effect of the value of soft tissue thickness on the values of the indicator I_m (F = 2.33; p-value = 0.13).

INTERPRETATION

This study shows that soft tissue thickness does not appear to alter the prediction of the acetabular cup implant primary fixation obtained using the impact analysis approach, opening the path towards future clinical trials.

A novel approach to determine primary stability of acetabular press-fit cups

Today hip cups are used in a large variety of design variants and in increasing numbers of units. Their development is steadily progressing. In addition to conventional manufacturing methods for hip cups, additive methods, in particular, play an increasingly important role as development progresses. The present paper describes a modified cup model developed based on a commercially available press-fit cup (Allofit 54/JJ). The press-fit cup was designed in two variants and manufactured using selective laser melting (SLM). Variant 1 (Ti) was modeled on the Allofit cup using an adapted process technology. Variant 2 (Ti-S) was provided with a porous load bearing structure on its surface. In addition to the typical (complete) geometry, both variants were also manufactured and tested in a reduced shape where only the press-fit area was formed. To assess the primary stability of the press-fit cups in the artificial bone cavity, pull-out and lever-out tests were carried out. Exact fit conditions and two-millimeter press-fit were investigated. The closed-cell PU foam used as an artificial bone cavity was mechanically characterized to exclude any influence on the results of the investigation. The pull-out forces of the Ti-variant (complete-526 N, reduced-468 N) and the Ti-S variant (complete-548 N, reduced-526 N) as well as the lever-out moments of the Ti-variant (complete-10 Nm, reduced-9.8 Nm) and the Ti-S variant (complete-9 Nm, reduced-7.9 N) show no significant differences in the results between complete and reduced cups. The results show that the use of reduced cups in a press-fit design is possible within the scope of development work.

Risk factors for unsuccessful acetabular press-fit fixation at primary total hip arthroplasty

BACKGROUND

Surgeon at primary total hip arthroplasty sometimes cannot achieve sufficient cementless acetabular press-fit fixation and must resort to other fixation methods. Despite a predominant use of cementless cups, this issue is not fully clarified, therefore we performed a large retrospective study to: (1) identify risk factors related to patient or implant or surgeon for unsuccessful intraoperative press-fit; (2) check for correlation between surgeons' volume of operated cases and the press-fit success rate.

HYPOTHESIS

Unsuccessful intra-operative press-fit more often occurs in older female patients, particular implants, due to learning curve and low-volume surgeons.

MATERIALS AND METHODS

Retrospective observational cohort of prospectively collected intraoperative data (2009-2016) included all primary total hip arthroplasty patients with implant brands that offered acetabular press-fit fixation only. Press-fit was considered successful if acetabulum was of the same implant brand as the femoral component without additional screws or cement. Logistic regression models for unsuccessful acetabular press-fit included patients' gender/age/operated side, implant, surgeon, approach (posterior n=1206, direct-lateral n=871) and surgery date (i.e. learning curve).

RESULTS

In 2077 patients (mean 65.5 years, 1093 females, 1163 right hips), three different implant brands (973 ABG-II™-Stryker, 646 EcoFit™ Implantcast, 458 Procotyl™ L-Wright) were implanted by eight surgeons. Their unsuccessful press-fit fixation rates ranged from 3.5% to 23.7%. Older age (odds ratio 1.01 [95% CI: 0.99-1.02]), female gender (2.87 [95% CI: 2.11-3.91]), right side (1.44 [95% CI: 1.08-1.92]), surgery date (0.90 [95% CI: 1.08-1.92]) and particular implants were significant risk factors only in three surgeons with less successful surgical technique (higher rates of unsuccessful press-fit with Procotyl™-L and EcoFit™ [P=0.01]). Direct-lateral hip approach had a lower rate of unsuccessful press-fit than posterior hip approach (P<0.01), but there was no correlation between surgeons' volume and rate of successful press-fit (Spearman's rho=0.10, P=0.82). Subcohort of 961 patients with 5-7-years follow-up indicated higher early/late cup revision rates with unsuccessful press-fit.

DISCUSSION

Success of press-fit fixation depends entirely on the surgeon and surgical approach. With proper operative technique, the unsuccessful press-fit fixation rate should be below 5% and the impact of patients' characteristics or implants on press-fit fixation is then insignificant. Findings of huge variability in operative technique between surgeons of the presented study emphasize the need for surgeon-specific data stratification in arthroplasty studies and indicate the possibility of false attribution of clinically observed phenomena to patient-related factors in pooled data of large centers or hip arthroplasty registers.

LEVEL OF EVIDENCE

Level III, retrospective observational case control study.

Characterization of thick titanium plasma spray coatings on PEEK materials used for medical implants and the influence on the mechanical properties

Coating poly-ether-ether-ketone (PEEK) with rough and porous titanium plasma spray (TPS) coatings is a technique which is commonly used to enhance the osseointegrative properties of medical implants. However, the influence of the TPS coating on the PEEK mechanical properties has not been sufficiently evaluated to date. In this study, PEEK samples were coated with a thick TPS layer with grains of 90µm and 180µm diameter. The coating characteristics and the adhesive strength of the coatings on the samples were determined and compared to coatings on titanium samples. The influence of the coating process on the mechanical and chemical-physical properties of PEEK was also evaluated. All TPS coatings on PEEK and titanium fulfilled the manufacturer's requirements for thickness (200 ± 50µm), porosity (30 ± 10%) and roughness (90µm grain diameter coating: 25 ± 5µm and 180µm grain diameter coating: 45 ± 15µm) and were able to meet the demands required for adhesive strength (> 22MPa) and shear strength (> 20MPa). However, the mechanical properties i.e. yield stress, fracture strain, flexural modulus and flexural stress, of the PEEK samples were influenced by the coating process, while the chemical-physical properties were not altered.