Head-neck Interface Research Articles

Corrosion at the head-neck interface of current designs of modular femoral components: essential questions and answers relating to corrosion in modular head-neck junctions.

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.

Trunnionosis in total hip arthroplasty: a review.

Trunnionosis is defined as wear of the femoral head–neck interface and has recently been acknowledged as a growing cause of total hip arthroplasty failure. Some studies have reported that it accounts for up to 3 % of all revisions. The exact cause of trunnionosis is currently unknown; however, postulated etiologies include modular junction wear, corrosion damage, and metal ion release. Additionally, implant design and trunnion geometries may contribute to the progression of component failure. In order to aid in our understanding of this phenomenon, our aim was to present the current literature on (1) the effect of femoral head size on trunnionosis, (2) the effect of trunnion design on trunnionosis, (3) localized biological reactions associated with trunnionosis, and (4) gross trunnion failures. It is hoped that this will encourage further research and interest aimed at minimizing this complication.

The resistance to torque of head fixation of stemmed femoral components

Event Abstract Back to Event The resistance to torque of head fixation of stemmed femoral components Arthur Paiva Santos1, Patricia Ortega Cubillos1 and Carlos Rodrigo Roesler1 1 Universidade Federal de Santa Cataria, Laboratório de Engenharia Biomecânica, Brazil Introduction: Total Hip Arthroplasty (THA) is one of the most frequent and successful procedures in orthopedics nowadays. Due to better flexibility to adjust specific patients characteristics, modular hip design in which femoral head is fitted on the stem neck has gained greater popularity. In theses cases, the clinical success depends on the rotational stability of the head and neck pair. Bad fitting ma impair the functional performance of the hip joint and promote fretting corrosion on the head-neck interface. According, there is published (ABNT NBR 14396-4)[1] and in development (ISO/DIS 7206-13)[2] technical standards to conduct laboratory testing for the determination of resistance to torque of head fixation of stemmed femoral components. The present study aims to determine resistance to torque of head fixation for hip prosthesis from four different worldwide manufacturers. Five head-neck assemblies were tested for each brand (n = 5), in a total of twenty assemblies. For all tests, it was used femoral stem with offset of 37.5 mm and cone size of 12/14. All the femoral heads had diameter of 28 mm. Materials and Methods: Torque tests were performed in a hydraulic biaxial universal testing machine (MTS model BIONIX, Eden Praire, USA) with a 15kN axial load transducer and 150 Nm torque transducer. The heads were manually positioned in the stem and the stems were immersed in acrylic resin for fixation. After this, the stem neck was positioned and fixed collinear to the machine load axis and an axial compression load of 500 N was applied to the head to fit it on the stem. The entire system was immersed in distilled water bath kept in 37°C ± 1°C. A propylene support was used to apply 1000 N of uniaxial static load at the head and maintain this load constant during the test. For the test a 0.5°/min rotation was applied to the head. The acquisition of torque, angle and load data was performed with frequency of 40 Hz. The torque strength was calculated through a parallel line of linear part of the graph made in a distance of 3 degrees from the origin. Results: The Figures 1 and 2 show graphs of torque vs. angle and the average value of torque strength measured for each manufacturer respectively. From the graphs (Figures 1 and 2) it may be observed basically two different rotational resistance levels. The designs of brands A and B promote torsional strength all above 25 Nm while for brands C and D it is around 15 Nm. Figure 1. Graphics of resistance to torque vs. angle for four different manufactures. Figure 2. Resistence to torque of head fixation of stemmed femoral components for four different manufacturers. Discussion: This demonstrates there is not a consensus between manufactures about what is the target value for the evaluated parameter. From a biomechanical point of view, Davy et al.,(1988)[3] measured maximum torsional moment of 22 Nm around a hip stem during daily tasks. Schneider et al.,(2001)[4] measured torsional moment around 10 Nm on femoral intramedullary nail during post-operative period. It is worth to note the greater repetibility of the torque vs. angle curves for the design of manufacturer D (Figures 1 and 2). It indicates a very precise manufacturing and finishing of the medical product with optimal matching. Conclusion: The repetibility of the torque vs. angle curves indicates a precision machining of the hip prosthesis, that generate a fit between head and the stemmed that should be persecuted by all hip implant manufacturers.

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

PurposeTrunnionosis 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. MethodsMechanical 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. ResultsLarge 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. ConclusionIn 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.

A novel analytical approach for determining the frictional moments and torques acting on modular femoral components in total hip replacements

A three dimensional analytical approach was developed to determine the frictional moment vector generated by the relative sliding of the head-cup bearing couple of a total hip replacement. The frictional moment projection onto the femoral neck was also determined over the loading cycle. Predicted frictional moments for nine combinations of bearing materials and diameters were in close agreement with existing in vitro data. The analytical method was then applied to simplified gait (lubrication conditions of dry and serum), ISO standard gait and physiological level gait loading cycles. ISO standard gait had a total contact force of about two fold of physiological level gait and there was a corresponding increase in the maximum frictional torque on neck from 0.66×BW%m to 0.88×BW%m. For the ISO standard gait, the maximum frictional torque occurred at the same instance of maximum frictional moment and the maximum contact force. In contrast, for the physiological level gait, the frictional torque did not occur at the same instance as the peak load. This suggests that the neck frictional torque is a function of other parameters, such as angle between neck axis and frictional moment vector, as well as the magnitude of the contact force and frictional moment. The developed methodology was able to predict the maximum magnitude and change of directions of moments and the variation of torque at the head neck interface. The data will be useful for experimental studies assessing the fretting behaviour of the head neck junction, by providing appropriate loading data.

Late Nontraumatic Dissociation of the Femoral Head and Trunnion in a Total Hip Arthroplasty.

Background. Modular total hip arthroplasties are increasingly popular because customisation allows optimal restoration of patient biomechanics. However, the introduction of component interfaces provides greater opportunities for failure. We present a case of late nontraumatic dissociation of the head-neck interface, more than 10 years after insertion. Case Description. A 58-year-old woman had a left metal-on-metal total hip arthroplasty in 2002 for hip dysplasia. Following an uneventful 10-year period, she presented to hospital in severe pain after standing from a seated position, and radiographs demonstrated complete dissociation of the modular femoral head from the stem, with the femoral head remaining in its cup. There was no prior trauma or infection. Mild wear and metallosis were present on the articulating surface between the femoral head and trunnion. Soft tissues were unaffected. Discussion and Conclusions. This is the latest occurrence reported to date for nontraumatic component failure in such an implant by more than 7 years. The majority of cases occur in the context of dislocation and attempted closed reduction. We analyse and discuss possible mechanisms for failure, aiming to raise awareness of this potential complication and encouraging utmost care in component handling and insertion, as well as the long term follow-up of such patients.

A Finite Element Investigation into the Effects of Head Size and Trunnion Design on the Micromotion at the Head-Neck Interface in THR

The modularity of femoral head and femoral stem provides many benefits to surgeons. However, case-reports have shown failure in large head Metal-on-Metal hip replacement due to trunnionosis. The exact causes of trunnionosis are not yet identified but the additional interface at the modular joint seems to be a contributing factor. In this study, a three dimensional non-linear finite element model was created to analyse the effects of head size and trunnion design on the micromotion at the head-neck interface. Four different metal head sizes and two trunnions designs and materials were used in the model. The femoral heads were assembled onto the trunnions with 7kN axial force and one cycle of gait load was applied to the head after assembly. The study showed that the micromotion was substantially increased in femoral head larger than 36mm. Trunnions material has greater effect on micromotion than trunnion design, particularly with the larger head sizes. The stability at the modular junction is important. Our findings suggest that there is a limit of assembly force to maintain enough stability on the joint; beyond this limit; the maximum micromotion will not be affected.

Research on the torsional fretting behavior of the head–neck interface of artificial hip joint

The torsional fretting behavior of the head–neck interface of artificial hip joint composed of Ti6Al4V and CoCrMo was investigated. The torsional fretting mechanisms were evaluated by experiments at various angular displacement amplitudes. The friction coefficient for the head–neck interface was calculated using contact mechanics. The head–neck interface model was established in accordance with the artificial hip joint head–neck size. The conical angle and diameter of the artificial hip joint head were optimized using data derived from the finite element method (FEM). The results indicate that torsional fretting changes from partial slip regime to slip regime with increasing angular displacement amplitude. Friction torque, friction dissipated energy, and the friction coefficient increase with increasing angular displacement amplitude and wear intensifies. Finite element simulation results demonstrate that self-locking occurs in the head–neck interface of artificial hip joint more readily at small conical angles. The head–neck interface is least susceptible to fretting when the conical angle is 5°42′30″. Relative displacement and contact stress can be reduced by appropriately increasing the conical angle within a small angle range and increasing the artificial hip joint head diameter, which can decrease fretting wear.

The Influence of Head Size on Corrosion and Fretting Behaviour at the Head-Neck Interface of Artificial Hip Joints

The primary goal of this study was to determine if head size affects corrosion and fretting behaviour at the head-neck taper interface of modular hip prostheses. Seventy-four implants were retrieved that featured either a 28mm or a 36mm head with a metal-on-polyethylene articulation. The bore of the heads and the neck of the stems were divided into eight regions each and graded by three observers for corrosion and fretting damage separately using modified criteria as reported in the literature. The 36mm head size featured a significant difference in the corrosion head scores (p=0.022) in comparison to the 28mm heads. This may be attributed to a greater torque acting along the taper interface due to activities of daily living.

Severe Metal-induced Osteolysis Many Years After Unipolar Hip Endoprosthesis

Modularity of the femoral head-neck junction provides increased intraoperative flexibility to the surgeon. Complications of this modularity include damage to the trunnion, with subsequent bone and/or soft tissue loss from adverse reactions to metal debris. We describe two cases of severe metal-induced osteolysis and soft tissue damage requiring revision 10 and 13 years following implantation of a unipolar endoprosthesis. Damage to the trunnion resulted in severe acetabular and trochanteric osteolysis and soft tissue loss requiring complex revision surgery. Several reports have shown the trunnion, the head-neck interface, and the neck-stem couple as the causes of this early failure secondary to metal ion release from mechanical fretting corrosion or from crevice corrosion at these modular interfaces. These reports have been in association with a total hip prosthesis rather than a unipolar endoprosthesis. Revision of a unipolar endoprosthesis is most commonly attributable to stem loosening or acetabular erosion from the large femoral head articulating on the host acetabular cartilage and not owing to failure of the trunnion. Trunnion damage resulting in a severe reaction to metal debris with acetabular osteolysis, erosion of the greater trochanter, and loss of the abductor mechanism can occur years after implantation of a cementless unipolar endoprosthesis. This raises questions regarding long-term safety of the modular interface of a contemporary cementless stem and a large-diameter unipolar head. We recommend long-term followup of patients with a unipolar endoprosthesis as early recognition and treatment are required to avoid a potentially complex revision.

Radiological evaluation of intertrochanteric fracture fixation by the proximal femoral nail

BackgroundSuccessful treatment of intertrochanteric femoral fractures was reportedly influenced by the position of the fixation devices, by reduction quality and by fracture type. MethodsThe records of 227 patients with intertrochanteric fractures treated by intramedullary hip screws were analysed retrospectively. The angle and distance from the femur head apex were transformed into Cartesian coordinates. Comparisons were performed between patients with no mechanical failure (207 patients, 90.7%), with cutouts (15 patients, 6.6%) and with secondary loss of reduction (5 patients, 2.2%). ResultsThe standard tip apex distance (TAD) measurement above 25mm did not predict failure (p=0.62). Mechanical failure rates increased from 4.8% to 34.4% when the centre of lag screw was not in the second quarter of the head–neck interface line (the so-called “safe zone”) (p=0.001). Lag screw insertion lower or higher than 11mm of the head apex line were associated with failure rates of 5.5% and 18.6%, respectively (p=0.004). Multivariate logistic regression showed that lag screw insertion not within the “safe-zone” was associated an Odds Ratio of 13.4 (95% CI 2.24–81) for mechanical failure (p=0.004). ConclusionsThe TAD scale focuses on length measurement and lacks the vector properties of multidirectional measurements. Vector analysis revealed that the caudal-cranial correct lag screw position is the most important factor in preventing mechanical failure.

Retrieval analysis and in vitro assessment of strength, durability, and distraction of a modular total hip replacement

We investigated a commercial Co-Cr-alloy head--Ti6Al4V alloy neck and Ti6Al4V stem modular total hip replacement. We assessed the distraction forces after in vitro cycling in bovine serum, fatigue durability, fretting corrosion damage, and load bearing capacity of new implants using fatigue-corrosion, pull-off, scanning electron microscopy, fatigue and compression investigations. In addition, we studied corrosion, fretting damage, and distraction forces on retrievals. For both retrievals and in vitro test samples, the neck-stem interface required the higher distraction force as compared with the head-neck interface. One of 12 retrievals showed strong fretting corrosion at the neck-stem interface which resulted in a high disassembly force of about 16 kN. For in vitro test samples, the neck-stem pull-off force initially increased during cycling and showed a maximum value of 5.704 kN at ∼100,000 cycles, which is equivalent to gait cycles performed in approximately 36 days. Overall, assembly force, initial component settling, and interface corrosion primarily determine the force required to distract the modular components. One million cycles fatigue failure of the neck can be expected at a maximum compression load of -6.5 kN. No component failure was observed during quasistatic compression; rather the neck deformed plastically and the ultimate compression load-bearing capacity was -13 kN.

Dissociation of modular total hip arthroplasty at the femoral head–neck interface after loosening of the acetabular shell following hip dislocation

Modular component dissociation is a potential problem of current modular total hip arthroplasty (THA) systems. We describe a case of dissociation of the modular THA at the femoral head–neck interface after loosening of the acetabular shell during closed reduction for posterior dislocation of THA. The causes of this dissociation and acetabular shell loosening are discussed. Successful treatment was provided with surgical revision of the acetabular and the femoral head components. The present case serves as a graphic reminder that the acetabular shell overhanging the acetabular bone must be avoided when implanting modular THA components.

The Structure of the Fusion Glycoprotein of Newcastle Disease Virus Suggests a Novel Paradigm for the Molecular Mechanism of Membrane Fusion

Background: Membrane fusion within the Paramyxoviridae family of viruses is mediated by a surface glycoprotein termed the “F”, or fusion, protein. Membrane fusion is assumed to involve a series of structural transitions of F from a metastable (prefusion) state to a highly stable (postfusion) state. No detail is available at the atomic level regarding the metastable form of these proteins or regarding the transitions accompanying fusion. Results: The three-dimensional structure of the fusion protein of Newcastle disease virus (NDV-F) has been determined. The trimeric NDV-F molecule is organized into head, neck, and stalk regions. The head is comprised of a highly twisted β domain and an additional immunoglobulin-like β domain. The neck is formed by the C-terminal extension of the heptad repeat region HR-A, capped by a four-helical bundle. The C terminus of HR-A is encased by a further helix HR-C and a 4-stranded β sheet. The stalk is formed by the remaining visible portion of HR-A and by polypeptide immediately N-terminal to the C-terminal heptad repeat region HR-B. An axial channel extends through the head and neck and is fenestrated by three large radial channels located approximately at the head–neck interface. Conclusion: We propose that prior to fusion activation, the hydrophobic fusion peptides in NDV-F are sequestered within the radial channels within the head, with the central HR-A coiled coil being only partly formed. Fusion activation then involves, inter alia, the assembly of a complete HR-A coiled coil, with the fusion peptides and transmembrane anchors being brought into close proximity. The structure of NDV-F is fundamentally different than that of influenza virus hemagglutinin, in that the central coiled coil is in the opposite orientation with respect to the viral membrane.

Dissociation of Modular Hip Arthroplasty Components After Dislocation

Modular hip arthroplasty systems, currently widely employed, offer the advantage of increased intraoperative flexibility in component selection with reduced inventory, as well as the disadvantage of modular component dissociation. Dissociation during closed reduction for dislocation is reported in three patients at three different interface levels: (1) fixed acetabular shell-polyethylene linear interface, (2) bipolar acetabular component-femoral head interface, and (3) femoral head-neck interface. Subsequent open reduction was required in each case. Although this potential disadvantage of modular hip systems does not outweigh the benefits, it does warrant that certain precautions be taken when implanting modular components. The acetabular linear should lie flush within the metallic shell after impaction. The femoral head should be firmly impacted onto the neck. Both should resist reasonable manual force of disassembly. Should a modular hip arthroplasty component dislocate, gentle reduction under general anesthesia and fluoroscopic control is warranted. Careful inspection of pre- and postreduction roentgenograms for signs of modular component dissociation is mandatory.