Impaction Force Influences Taper-Trunnion Stability in Total Hip Arthroplasty

Influence of the use of an adhesive connection on the joint strength of modular hip endoprostheses

INTRODUCTION

Modular hip implants enables a more precise adaptation of the prosthesis to the patient's anatomy. However, they also carry the risk of increased revision rates due to micromotion at the taper junction. In order to minimize this risk, one potential solution is to establish an adhesive bond between the metal taper junctions. Load-stable bonding techniques, already successfully employed in dentistry for connecting materials such as metals and ceramics or different alloys, offer a promising approach. Nevertheless, the bond strength of tapered adhesive bonds in modular hip implants has not been investigated to date.

MATERIALS AND METHODS

Twenty-eight tapered junctions, consisting of a taper (female taper) and a trunnion (male taper) were turned using TiAl6V4 ELI (n = 16) and CoCr28Mo6 (n = 12). The process parameters cutting speed (vc = 50 m/min or 100 m/min) and feed (f = 0.1 mm, 0.05 mm or 0.2 mm) were varied for the trunnions. For each set of process parameters, one trunnion and one taper were additionally subjected to sandblasting. To investigate the effect of geometry, angular mismatch in the samples were measured. The taper pairs were bonded with a biocompatible adhesive, and push-out tests were subsequently performed.

RESULTS

The push-out forces generated from the taper connections where both tapers were sandblasted showed a mean push-out force of 5.70 kN. For the samples with only the trunnion sandblasted, the mean force was 0.58 kN, while for the samples with only taper sandblasted the mean push-out force was 1.32 kN. When neither of the tapers was sandblasted the mean push-out force was 0.91 kN. No significant effect of the process parameters on the push-out force was observed. Only the reduced valley depth Svk showed a slight correlation for the CoCr28Mo6 samples (R2 = 0.54). The taper pairs with taper mismatch (between trunnion and taper) greater than |0.1°| did not show lower push-out forces than the specimens with lower taper mismatch.

CONCLUSIONS

Sandblasted and adhesive-bonded tapered connections represent a viable suitable alternative for modular hip implant connections. Slight differences in taper geometry do not result in reduced push-out forces and are compensated by the adhesive. In mechanically joined tapers these differences can lead to higher wear rates. Further investigation under realistic test conditions is necessary to assess long-term suitability.

Effect of Femoral Head Material, Surgeon Experience, and Assembly Technique on Simulated Head-Neck Total Hip Arthroplasty Impaction Forces

BACKGROUND

There is no standard method for assembling the femoral head onto the femoral stem during total hip arthroplasty (THA). This study aimed to measure and record dynamic 3-dimensional (3D) THA head-neck assembly loads from residents, fellows, and attending surgeons, for metal and ceramic femoral heads.

METHODS

An instrumented apparatus measured dynamic 3D forces applied through the femoral stem taper in vitro for 31 surgeons (11 attendings, 14 residents, 6 fellows) using their preferred technique (ie, number of hits or mallet strikes). Outcome variables included peak axial force, peak resultant force, impulse of the resultant force, loading rate of the resultant force, and off-axis angle. They were compared between femoral head material, surgeon experience level, and the number of hits per trial.

RESULTS

Average peak axial force was 6.92 ± 2.11kN for all surgeons. No significant differences were found between femoral head material. Attendings applied forces more "on-axis" as compared to both residents and fellows. Nine surgeons assembled the head with 1 hit, 3 with 2 hits, 14 with 3 hits, 2 with 4 hits, and 3 with ≥5 hits. The first hit of multihit trials was significantly lower than single-hit trials for all outcome measures except the off-axis angle. The last hit of multihit trials had a significantly lower impulse of resultant force than single-hit trials.

CONCLUSION

Differences in applied 3D force-time curve dynamic characteristics were found between surgeon experience level and single and multihit trials. No significant differences were found between femoral head material.

The effect of trunnionosis on the risk of re-revision following femoral head exchange in hip arthroplasty

BACKGROUND

Fretting corrosion at modular junctions contributes to arthroplasty failure. Currently, no evidence-based guidelines are available regarding the acceptable level of trunnion corrosion that can occur in vivo. We aimed to examine the relationship between trunnion corrosion and risk of re-revision to assist surgeons with intraoperative decision making.

METHOD

Grading by 3 independent examiners of revised and re-revised head components was performed using a modified Goldberg corrosion scale. Samples were separated into low-grade (LG) and high-grade (HG) corrosion. Mechanical testing determined the relationship between corrosion severity and pull-off strength at the head-stem junction.

RESULTS

529 retrieved femoral heads were analysed. A positive association was detected between males and HG corrosion (OR 2.07; 95% CI, 1.45-2.94; p < 0.001). No difference between the survivorship of LG and HG heads was detected (p-value = 0.247). In the re-revised sample, the first implant had a time in situ that was on average 7.97 years longer (95% CI, 5.4-10.6) than that of the subsequent re-revised femoral head. Severe corrosion on the first head was associated with a 37.5 (95% CI, 4.00-1944) fold increase of HG on the subsequent head (p < 0.001). Femoral disassembly force had a positive correlation with stem taper corrosion grade (p = 0.001).

CONCLUSIONS

A well-fixed stem with corrosion may remain in situ.

Trunnions and Modularity in Total Hip Arthroplasty: A Historical Review With Current Clinical Implications

Trunnion in total hip arthroplasty refers to the interface between the neck of a femoral stem and the femoral head. Clinical complications arising from damage to this junction, whether it be due to mechanical wear, corrosion, or a combination, are referred to as mechanically assisted crevice corrosion (MACC), also commonly known as trunnionosis. With the use of modular hip prostheses, which help customize offset and leg length to an individual patient's anatomy, the incidence of MACC and revision due to MACC has increased in recent years. Although the cause of MACC is multifactorial, with patient factors and technique factors contributing to this condition, taper design and geometry, metallurgical properties of implants, and size mismatch of the bearing couple are some of the implant factors that have also been implicated in this clinical phenomenon. Understanding the history of taper design and geometry, the track record of older implants, and the rationale behind the development of current prostheses can help surgeons choose the right implants for their patients and accurately assess the pros and cons of new implants being introduced to the market each year.

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.

A novel method to improve femoral head and stem taper stability intraoperatively in total hip arthroplasty - a proof of concept study

BACKGROUND

Mechanically assisted crevice corrosion (MACC) has been associated with the compromised durability and fixation of modular total hip implants, adverse reaction of local tissue, and other undesirable clinical outcomes in total hip arthroplasty (THA). MACC is primarily caused by the relative motion between the femoral head and stem. To minimize the relative motion the taper connection between the two components must be strong enough. The current study addressed the following questions: (1) Does increasing the mass of the femoral stem improve the taper connection strength intraoperatively? (2) Does increasing the mass of the femoral stem reduce the risk of periprosthetic tissue damage intraoperatively?

HYPOTHESIS

Increasing the mass of the femoral stem improve the taper connection strength intraoperatively.

MATERIALS AND METHODS

During the experiment, femoral heads were impacted onto the stem tapers with and without an additional weight attached to the stem. The femoral heads were then pulled off to investigate the strength of the taper connection. The stem displacement and acceleration at impaction were also measured to evaluate the risk of periprosthetic tissue damage.

RESULTS

The results showed that the pull-off force was increased by 24% (p=0.011, n=6) when an additional weight was attached to the stem. The additional weight also reduced the maximum stem acceleration and maximum stem displacement by 37% (p<0.001, n=6) and 14% (p=0.094, n=6), respectively.

DISCUSSION

These findings suggest that the femoral head and stem taper connection strength can be significantly improved and the risk of periprosthetic tissue damage significantly reduced intraoperatively by attaching an additional weight to the stem to increase its mass.

LEVEL OF EVIDENCE

III, comparative in vitro mechanical investigation.

An Overview of the Stability and Fretting Corrosion of Microgrooved Necks in the Taper Junction of Hip Implants

Fretting corrosion at the head-neck interface of modular hip implants, scientifically termed trunnionosis/taperosis, may cause regional inflammation, metallosis, and adverse local tissue reactions. The severity of such a deleterious process depends on various design parameters. In this review, the influence of surface topography (in some cases, called microgrooves/ridges) on the overall performance of the microgrooved head-neck junctions is investigated. The methodologies together with the assumptions and simplifications, as well as the findings from both the experimental observations (retrieval and in vitro) and the numerical approaches used in previous studies, are presented and discussed. The performance of the microgrooved junctions is compared to those with a smooth surface finish in two main categories: stability and integrity; wear, corrosion, and material loss. Existing contradictions and disagreements among the reported results are reported and discussed in order to present a comprehensive picture of the microgrooved junctions. The current research needs and possible future research directions on the microgrooved junctions are also identified and presented.

Technical Considerations and Avoiding Complications in Total Hip Arthroplasty

Total hip arthroplasty (THA) is considered to be the surgical procedure of the 20th century. Current projections show that by 2030, primary THA is expected to grow by 171%, with revision THA expected to increase by 142% in the same time frame. Although complications are not common, when they occur, they can cause notable morbidity to the patient. Understanding the unique anatomy and needs of each patient will prepare the surgeon to avoid soft-tissue or bony injury, optimize prosthesis placement, and decrease the risk of infection. This article aims to highlight common causes of early revision THA and provide specific technical strategies to avoid these complications. Following a systematic approach to the primary THA and using these techniques will assist the surgeon in avoiding complications to revision hip arthroplasty.

Parametric analysis of the effect of impaction load on the stability of head-neck junction in total hip arthroplasty

BACKGROUND

Tribocorrosion at head-neck interface is one of the main causes leading to the failure of hip implants in total hip arthroplasty. Impaction load has been acknowledged as one of the key factors influencing the stability of the taper junction. It is understood that the magnitude of impaction force differs from the surgeon to surgeon in primary total hip arthroplasty or revision. Clinically, it is sufficient enough to keep the male and female tapers inseparable utilizing a low impaction, which seems to contradict previous researches. The objective of this study was to investigate the effect of impaction loads on the stability of taper junction during assembly and gaits.

METHODS

A finite element model with 12/14 taper and the taper mismatch of 4' was developed for investigation. The impaction force profiles were collected from surgeon as the inputs, and then the contact mechanics over one or multiple gaits was further analyzed and validated utilizing hip simulator test.

FINDINGS

Impaction force ranging from 200 to 2000 N could provide the same taper connection effect after the first gait due to the secondary seating. As for impaction loads of 3000 N and above, an increased impaction force would lead to the tighter taper connection.

INTERPRETATION

The effect of impaction load on the stability of head-neck junction is a piecewise function, indicating that the stability of taper junction is not affected by different impaction loads and tends to be consistent while its magnitude is below the threshold. Instead, the stability of taper junction is positively correlated with impaction force.

Stand up! Are normal weight-bearing forces sufficient for a 12/14 Morse taper locking in total hip arthroplasty?

BACKGROUND

The aim of this study was to investigate total hip arthroplasty (THA) Morse taper pull-off strengths after impaction prior to cyclical loading compared to cyclical loading alone. The practical relevance of the experiment is to provide a perspective on what may be clinically satisfactory taper assembly given the spectrum of head tapping patterns used by surgeons, as well as compare traditional impaction performed in standard THA with alternate methods of taper engagement such as 'in situ assembly' used in micro-invasive techniques.

METHODS

36 taper constructs utilising a combination of cobalt-chrome alloy and ceramic-titanium alloy junctions were investigated in vitro in wet and dry conditions with cyclical loading of the constructs. Taper disengagement strengths with and without impaction were compared. Secondary investigation of the surface roughness of the heads and tapers was also assessed.

RESULTS

An impaction to a wet taper resulted in a greater pull off force compared to a dry taper with a CoCr head and taper combination. Impacting the head and dryness of the taper did not affect pull off forces of a ceramic femoral head on titanium taper. Pulling a head off a taper significantly alters the head surface roughness.

CONCLUSION

Impaction of a taper does not provide any benefit over cyclical loading of a taper assembly alone for pull-off strength.

Effect of femoral head material on head-to-trunnion impaction motion and taper

The purpose of this study was to evaluate the effect of femoral head material on the impaction force, relative motion, and stability of the trunnion. There were three groups with different head materials (n = 5 per group)-CoCr Group: 36 mm CoCr heads, Ceramic Group: 36 mm ceramic heads, Ceramic + Sleeve Group: 36 mm ceramic heads with a titanium sleeve-that were all impacted twice and disengaged onto titanium alloy (Ti6al4V) trunnions in in vitro conditions. A high-speed camera system was utilized to characterize relative displacement behavior of the head-trunnion junction motion. The first impact force of Ceramic + Sleeve Group (14,241    SD, 935) was significantly lower than the first impact force in Ceramic Group (14,961 N, SD = 184). Ceramic + Sleeve Group had a lower magnitude bounce-back displacement following the first impact (17.7 μm, SD = 11), p < 0.05) compared to CoCr Group (298.8 μm, SD = 84) and Group 2 (196.5 μm, SD = 31). Ceramic + Sleeve Group sat further on the trunnion (cumulative final displacement, 366.8 μm, SD = 71, p < 0.001) compared to CoCr Group (142.5 μm, SD = 41.8) and Ceramic Group (183.8 μm, SD = 30). Ceramic + Sleeve Group demonstrated two distinct disengagement patterns-(a) the sleeve disengaged from the trunnion (pull-off force 6810 N), and (b) the femoral head disengaged from the sleeve (pull-off force 18,620 N), with large fluctuations in pull-off force. The presence of a titanium sleeve with a ceramic head resulted in significant differences in impaction force on the trunnion, motion and displacement, and unique mechanisms for disengagement. Further investigation is required to determine potential clinical impact.

Importance of surgical assembly technique on the engagement of 12/14 modular tapers

Fretting-corrosion at the modular taper junction in total hip replacements (THR), leading to implant failure, has been identified as a clinical concern and has received increased interest in recent years. There are many parameters thought to affect the performance of the taper junction, with the assembly process being one of the few consistently identified to have a direct impact. Despite this, the assembly process used by surgeons during THR surgery differs from a suggested 'ideal' process. For example, taper junctions of cutting tools should be pushed together rather than impacted, while ensuring as much concentricity as possible between the male and female taper and loading axis. This study devised six simple assembly methodologies to investigate how surgical variations affect the success of the compressive fit achieved at the taper interface compared to a controlled assembly method, designed to represent a more 'ideal' scenario. Key findings from this study suggest that a more successful and repeatable engagement can be achieved by quasi-statically loading the male and female taper concentrically with the loading axis. This was shown by a greater disassembly to assembly force ratio of 0.626 ± 0.07 when assembled using the more 'ideal' process, compared to 0.480 ± 0.05 when using a method closer to that used by a surgeon intraoperatively. Findings from this study can be used to help inform new surgical instrumentation and an improved surgical assembly method.

Finite Element Analysis of Transhumeral and Transtibial Percutaneous Osseointegrated Endoprosthesis Implantation

Cadaveric mechanical testing of a percutaneous osseointegration docking system (PODS) for osseointegration (OI) prosthetic limb attachment revealed that translation of the exact system from the humerus to the tibia may not be suitable. The PODS, designed specifically for the humerus achieved 1.4-4.8 times greater mechanical stability in the humerus than in the tibia despite morphology that indicated translational feasibility. To better understand this discrepancy, finite element analyses (FEAs) modeled the implantation of the PODS into the bones. Models from cadaveric humeri (n = 3) and tibia (n = 3) were constructed from CT scans, and virtual implantation preparation of an array of endoprosthesis sizes that made contact with the endosteal surface but did not penetrate the outer cortex was performed. Final impaction of the endoprosthesis was simulated using a displacement ramp function to press the endoprosthesis model into the bone. Impaction force and maximum first principal (circumferential) stress were recorded to estimate stability and assess fracture risk of the system. We hypothesized that the humerus and tibia would have different optimal PODS sizing criteria that maximized impaction force and minimized first principal stress. The optimal sizing for the humerus corresponded to implantation instructions, whereas for the tibia optimal sizing was three times larger than the guidelines indicated. This FEA examination of impaction force and stress distribution lead us to believe that the same endoprosthesis strategy for the humerus is not suitable for the tibia because of thin medial and lateral cortices that compromise implantation.

Intraoperative assembly of anatomical shoulder prosthesis frequently results in malalignment of the modular taper junction

Anatomical shoulder arthroplasties (ASA) may fail because of micromotion at the modular taper junction causing wear due to fretting. Sufficient taper strength can reduce micromotion and potential reasons for failure. However, there are no normative standards for a safe assembly process performed intraoperatively by the surgeon. The purpose of this study is to determine the effect of common intraoperative assembly strategies and to identify critical influencing factors on taper stability. ASA with standard and stemless humeral component in combination with concentric Al₂ O₃ heads and eccentric CoCr28Mo6 alloyed humeral heads were tested. Taper angles and surface roughness were determined. Force magnitudes and impact directions were recorded using a sensorized head impactor and a three-dimensional force measuring platform. Subsequently, the axial pull-off forces were measured and taper engagement areas were macroscopically evaluated. In comparison to standard stem tapers that were impacted with an assembly device, stemless tapers were impacted into the artificial bone with significantly lower forces. Taper strength correlates to maximum impact force and was higher for CoCr28Mo6 heads with a mean pull-off ratio of 0.56 than for Al₂ O₃ heads with 0.37. Interestingly, all tapers showed an asymmetric clamping behavior, due to tilting during impaction. This is caused by the variation of the resulting force vector and further promoted by humeral head eccentricity. Assembly technique markedly influences the force magnitude, impact direction, impulse, and consequently taper strength. The resulting force vector and head eccentricity were identified as potential risk factors for taper malalignment.

Pseudotumors in Total Joint Arthroplasty

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The true incidence of pseudotumors in association with total joint arthroplasty is underestimated.

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Pseudotumors occur with metal-on-metal, metal-on-polyethylene, and metal-on-ceramic articulations.

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Metal ion levels should not be the only factor in decision-making regarding revision surgery.

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Revision surgery is only indicated in symptomatic patients with clinical and radiographic findings and elevated metal ion levels.

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Revision to a non-metal articulation is strongly suggested.

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.

Taper corrosion: a complication of total hip arthroplasty

The focus on taper corrosion in modular hip arthroplasty increased around 2007 as a result of clinical problems with large-head metal-on-metal (MoM) bearings on standard stems. Corrosion problems with bi-modular primary hip stems focused attention on this issue even more.

Factors increasing the risk of taper corrosion were identified in laboratory and retrieval studies: stiffness of the stem neck, taper diameter and design, head diameter, offset, assembly force, head and stem material and loading.

The high variability of the occurrence of corrosion in the clinical application highlights its multi-factorial nature, identifying the implantation procedure and patient-related factors as important additional factors for taper corrosion.

Discontinuing the use of MoM has reduced the revisions due to metal-related pathologies dramatically from 49.7% (MoM > 32 mm), over 9.2% (MoM ⩽ 32 mm) to 0.8% (excluding all MoM).

Further reduction can be achieved by omitting less stiff Ti-alloys and large metal heads (36 mm and above) against polyethylene (PE).

Standardized taper assembly of smaller and ceramic heads will reduce the clinical occurrence of taper corrosion even further. If 36 mm heads are clinically indicated, only ceramic heads should be used.

Taper-related problems will not comprise a major clinical problem anymore if the mentioned factors are respected.

Cite this article: EFORT Open Rev 2020;5:776-784. DOI: 10.1302/2058-5241.5.200013

Evaluation of pull-off strength and seating displacement of sleeved ceramic revision heads in modular hip arthroplasty

Corrosion in revision total hip arthroplasty can be mitigated using a ceramic head on a well-fixed in situ stem, but concerns of their early failure because of any surface defects on in situ stem necessitates the use of a titanium sleeve, which furnishes a factory-finish surface. These sleeves are manufactured in different sizes allowing neck-length adjustment. The strength of the taper junction of non-sleeved primary heads is well-investigated, but the influence of an interposed titanium sleeve on achieving a secure taper lock is unclear. Therefore, this study aimed to investigate the pull-off strength and seating displacement of revision ceramic heads and titanium taper sleeves. Two different head diameters and two different taper adapter sleeve offset lengths were mated with trunnions at two different impaction forces. The seating displacement and pull-off force was recorded for each specimen. Profilometry of the grooved outer surfaces of the sleeve and trunnion was done before and after testing to analyze the change in surface roughness. The influence of head diameter, sleeve offset, and impaction force on seating displacement and pull-off force was analyzed using analysis of covariance. Pull-off forces for 6 kN assembly force were approximately three times those for 2 kN. The head diameter did not have a significant effect on the measured parameters. Compared with short offset length sleeves, extra-long increased seating displacement by 31% and reduced pull-off forces by 15%. While sleeves of different offset lengths permit control of neck length, surgeons must be careful of the impact of this choice on the stability of implant. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:1523-1528, 2020.

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.

What the Surgeon Can Do to Reduce the Risk of Trunnionosis in Hip Arthroplasty: Recommendations from the Literature

Trunnionosis, defined as wear and corrosion at the head–neck taper connection, is a cause of failure in hip arthroplasty. Trunnionosis is linked to a synergistic combination of factors related to the prosthesis, the patient, and the surgeon. This review presents analytical models that allow for the quantification of the impact of these factors, with the aim of providing practical recommendations to help surgeons minimize the occurrence of this failure mode. A tighter fit reduces micromotion and, consequently, fretting of the taper connection. The paramount parameters controlling the fixation force are the coefficient of friction and the impaction force. The influence of the head diameter, as well as of the diameter and angle of the taper, is comparatively small, but varus alignment of the taper and heads with longer necks are unfavourable under physiologic loads. The trunnion should be rinsed, cleaned, and dried carefully, while avoiding any contamination of the bore—the female counterpart within the head—prior to assembly. Biological debris, and even residual water, might critically reduce the fixation of the taper connection between the head and the neck. The impaction force applied to the components should correspond to at least two strong blows with a 500 g hammer, striking the head with an ad hoc impactor aligned with the axis of the taper. These strong blows should correspond to a minimum impaction force of 4000 N.