Deformation of press-fitted metallic resurfacing cups. Part 1: Experimental simulation

Vibratory insertion of press-fit acetabular components requires less force than a single blow technique

Aims

Periprosthetic fracture and implant loosening are two of the major reasons for revision surgery of cementless implants. Optimal implant fixation with minimal bone damage is challenging in this procedure. This pilot study investigates whether vibratory implant insertion is gentler compared to consecutive single blows for acetabular component implantation in a surrogate polyurethane (PU) model.

Methods

Acetabular components (cups) were implanted into 1 mm nominal under-sized cavities in PU foams (15 and 30 per cubic foot (PCF)) using a vibratory implant insertion device and an automated impaction device for single blows. The impaction force, remaining polar gap, and lever-out moment were measured and compared between the impaction methods.

Results

Impaction force was reduced by 89% and 53% for vibratory insertion in 15 and 30 PCF foams, respectively. Both methods positioned the component with polar gaps under 2 mm in 15 PCF foam. However, in 30 PCF foam, the vibratory insertion resulted in a clinically undesirable polar gap of over 2 mm. A higher lever-out moment was achieved with the consecutive single blow insertion by 42% in 15 PCF and 2.7 times higher in 30 PCF foam.

Conclusion

Vibratory implant insertion may lower periprosthetic fracture risk by reducing impaction forces, particularly in low-quality bone. Achieving implant seating using vibratory insertion requires adjustment of the nominal press-fit, especially in denser bone. Further preclinical testing on real bone tissue is necessary to assess whether its viscoelasticity in combination with an adjusted press-fit can compensate for the reduced primary stability after vibratory insertion observed in this study.

A novel primary stability test method for artificial acetabular shells considering vertical load during level walking and shell position

Uncemented acetabular shell primary stability is essential for optimal clinical outcomes. Push-out testing, rotation testing, and lever-out testing are major evaluation methods of primary stability between the shell and bone. However, these test methods do not consider shell loads during daily activity and shell installation angle. This study proposes a novel evaluation method of acetabular shell primary stability considering load during level walking and acetabular installation angles such as inclination and anteversion. To achieve this, a novel primary stability test apparatus was designed with a shell position of 40° acetabular inclination and 20° anteversion. The vertical load, corresponding to walking load, was set to 3 kN according to ISO 14242-1, which is the wear test standard for artificial hip joints. The vertical load was applied by an air cylinder controlled by a pressure-type electro-pneumatic proportional valve, with the vertical load value monitored by a load cell. Torque was measured when angular displacement was applied in the direction of extension during the application of vertical load. For comparison, we also measured torque using the traditional lever-out test. The novel primary stability test yielded significantly higher primary stabilities; 5.4 times greater than the lever-out test results. The novel primary stability test failure mode was more similar to the clinical failure than the traditional lever-out test. It is suggested that this novel primary stability test method, applying physiological walking loads and extension motions to the acetabular shell, better reflects in vivo primary stability than the traditional lever-out test.

Impaction procedure influences primary stability of acetabular press-fit components

The aim of the study was to investigate whether the primary stability of press-fit acetabular components can be improved by altering the impaction procedure. Three impaction procedures were used to implant acetabular components into human cadaveric acetabula using a powered impaction device. An impaction frequency of 1 Hz until complete component seating served as reference. Overimpaction was simulated by adding ten strokes after complete component seating. High-frequency implantation was performed at 6 Hz. The lever-out moment of the acetabular components was used as measure for primary stability. Permanent bone deformation was assessed by comparison of double micro-CT (µCT) measurements before and after impaction. Acetabular component deformation and impaction forces were recorded, and the extent of bone-implant contact was determined from 3D laser scans. Overimpaction reduced primary acetabular component stability (p = 0.038) but did not significantly increase strain release after implantation (p = 0.117) or plastic deformations (p = 0.193). Higher press-fits were associated with larger polar gaps for the 1 Hz reference impaction (p = 0.002, R² = 0.77), with a similar trend for overimpaction (p = 0.082, R² = 0.31). High-frequency impaction did not significantly increase primary stability (p = 0.170) at lower impaction forces (p = 0.001); it was associated with smaller plastic deformations (p = 0.035, R² = 0.34) and a trend for increased acetabular component relaxation between strokes (p = 0.112). Higher press-fit was not related to larger polar gaps for the 6 Hz impaction (p = 0.346). Overimpaction of press-fit acetabular components should be prevented since additional strokes can be associated with increased bone damage and reduced primary stability as shown in this study. High-frequency impaction at 6 Hz was shown to be beneficial compared with 1 Hz impaction. This benefit has to be confirmed in clinical studies.

Influence of acetabular cup thickness on seating and primary stability in total hip arthroplasty

Insufficient primary stability of acetabular hip cups is a complication resulting in early cup loosening. Available cup designs vary in terms of wall thickness, potentially affecting implant fixation. This study investigated the influence of different wall thicknesses on the implantation process and the resulting primary stability using excised human acetabula. Implantations were performed using a powered impaction device providing consistent energy with each stroke. Two different wall thicknesses were compared in terms of seating progress, polar gap remaining after implantation, bone-to-implant contact area, cup deflection, and lever out moment. Thin-walled cups showed higher lever out resistance (p < 0.001) and smaller polar gaps (p < 0.001) with larger bone contact toward the dome of the cup (p < 0.001) compared to thick-walled cups. Small seating steps at the end of the impaction process were observed if a high number of strokes were needed to seat the cup (p = 0.045). A high number of strokes led to a strain release of the cup during the final strokes (p = 0.003). This strain release is indicative for over-impaction of the cup associated with bone damage and reduced primary stability. Adequate cup seating can be achieved with thin-walled cups with lower energy input in comparison to thicker ones. Thin-walled cups showed improved primary stability and enable implantation with lower energy input, reducing the risk of over-impaction and bone damage. Additional strokes should be avoided as soon as no further seating progress has been observed.

Influence of outer geometry on primary stability for uncemented acetabular shells in developmental dysplasia of the hip

Excellent primary stability of uncemented acetabular shells is essential to obtain successful clinical outcomes. However, in the case of developmental dysplasia of the hip (DDH), aseptic loosening may be induced by instability due to a decrease of the contact area between the acetabular shell and host bone. The aim of this study was to assess the primary stability of two commercially-available acetabular shells, hemispherical and hemielliptical, in normal and DDH models. Synthetic bone was reamed using appropriate surgical reamers for each reaming condition (normal acetabular model). The normal acetabular model was also cut diagonally at 40° to create a dysplasia model. Stability of the acetabular components was evaluated by the lever-out test. In the normal acetabular model conditions, the maximum primary stabilities of hemispherical and hemielliptical shells were observed in the 1-mm under- and 1-mm over-reamed conditions, respectively, and the resulting stabilities were comparable. The lateral defect in the dysplasia model had an adverse effect on the primary stabilities of the two designs. The lever-out moment of the hemielliptical acetabular shell was 1.4 times greater than that of the hemispherical acetabular shell in the dysplasia model. The hemispherical shell is useful for the normal acetabular condition, and the hemielliptical shell for the severe dysplasia condition, in the context of primary 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.

Impaction technique influences implant stability in low-density bone model

Aims

Cementless acetabular components rely on press-fit fixation for initial stability. In certain cases, initial stability is more difficult to obtain (such as during revision). No current study evaluates how a surgeon’s impaction technique (mallet mass, mallet velocity, and number of strikes) may affect component fixation. This study seeks to answer the following research questions: 1) how does impaction technique affect a) bone strain generation and deterioration (and hence implant stability) and b) seating in different density bones?; and 2) can an impaction technique be recommended to minimize risk of implant loosening while ensuring seating of the acetabular component?

Methods

A custom drop tower was used to simulate surgical strikes seating acetabular components into synthetic bone. Strike velocity and drop mass were varied. Synthetic bone strain was measured using strain gauges and stability was assessed via push-out tests. Polar gap was measured using optical trackers.

Results

A phenomenon of strain deterioration was identified if an excessive number of strikes was used to seat a component. This effect was most pronounced in low-density bone at high strike velocities. Polar gap was reduced with increasing strike mass and velocity.

Conclusion

A high mallet mass with low strike velocity resulted in satisfactory implant stability and polar gap, while minimizing the risk of losing stability due to over-striking. Extreme caution not to over-strike must be exercised when using high velocity strikes in low-density bone for any mallet mass.

Cite this article: Bone Joint Res 2020;9(7):386–393.

Time-dependent Viscoelastic Response of Acetabular Bone and Implant Seating during Dynamic Implantation of Press-fit Cups

Deformation of an acetabular cup implant during cementless implantation is indicative of the radial compressive forces, and such of the initial implant fixation strength. Stress relaxation in the surrounding bone tissue following implantation could reduce the deformation of the cup and thus primary implant fixation. The aim of this study was therefore to determine the early shape change of the implanted cup immediately after implantation with different press-fit levels and whether recording the force during cup impaction can be used to estimate initial cup fixation. Cup implantations into porcine acetabulae (n=10) were performed using a drop tower. The force induced by the drop weight and cup seating after each impact was recorded. Deformation of the implanted cup was determined with strain gauges over a period of 10min. Lever-out torques were measured to assess the initial fixation strength. Stress relaxation in the bone caused a reduction in cup deformation of 13.52±4.06% after 1min and 29.34±5.11% after 10min. The fixation strength increased with a higher force magnitude during impaction (R_s²=0.810, p=0.037). Reduction of the radial compressive forces due to stress relaxation of the surrounding bone should be considered during press-fit cup implantation in order to compensate for the reduced fixation strength over time. In addition, recording the implantation force could help to estimate initial fixation strength.

Fixation Stability of Uncemented Acetabular Cups With Respect to Different Bone Defect Sizes

BACKGROUND

In total hip arthroplasty, acetabular press-fit cups require a proper bone stock for sufficient primary implant fixation. The presence of acetabular bone defects compromises the primary fixation stability of acetabular press-fit cups. The aim of the present study is to determine the fixation stability of a cementless acetabular cup regarding standardized bone defects in an experimental setup.

METHODS

An acetabular defect model was developed and transferred to a biomechanical cup-block model. The lack of superior cup coverage was divided into 4 stages of superior rim loss (33%, 50%, 67%, and 83%) in the anterior-posterior direction and into 4 stages of mediolateral wall absence (11%, 22%, 33%, and 50%). This resulted in 11 different defect cavities, which were compared to the intact cavity in push-in and lever-out tests of one press-fit cup design (56 mm outer diameter). Thereby, push-in force, lever-out moment, lever-out angle, and interface stiffness were determined.

RESULTS

The determined lever-out moments range from 15.53 ± 1.38 Nm (intact cavity) to 1.37 ± 0.54 Nm (83%/50% defect). Smaller defects (33%/11%, 33%/22%, and 50%/11%) reduce the lever-out moments by an average of 33.9% ± 2.8%.

CONCLUSION

The lack of mediolateral acetabular coverage of 50% was assessed as critical for cementless cup fixation, whereby the contact zone between implant and bone in the defect is lost. A lack of 20% to 30% mediolateral coverage appears to be acceptable for press-fit cup fixation in the presence of primary stability. A defect of 50%/50% was identified as the threshold for using additional fixation methods.

Effect of gap outside contact area on lubrication of metal-on-Metal total hip replacement

Ball-in-socket metal on metal (MOM) contacts were analysed using the Abaqus Finite Element package to simulate dry contact between the acetabular cup and the femoral head. Different cup thicknesses of 4, 6, 8, and 10 mm were considered using a polyurethane foam block support system. Elastohydrodynamic lubrication (EHL) analyses were developed for the contacts using three different approaches to specify the contact. These were (i) A simple model based on the radii of relative curvature, (ii) An equivalent contact model developed so that its dry contact area and maximum pressure replicated the values obtained from the FE analysis, and (iii) A modified version of (ii) that also ensured equivalence of the gap shape outside the contact area. Published in vivo information for the hip joint contact forces over the walking cycle was used to specify the operating conditions for the EHL analysis. The analysis method was found to be effective for all points of the walking cycle for cases where the cup thickness exceeded 5 mm and modelling approach (ii) was identified as satisfactory. For a cup thickness of 4 mm, membrane action began to emerge in the FE analyses so that such contacts behaved in a different way.

Influence of the Acetabular Cup Material on the Shell Deformation and Strain Distribution in the Adjacent Bone-A Finite Element Analysis

In total hip arthroplasty, excessive acetabular cup deformations and altered strain distribution in the adjacent bone are potential risk factors for implant loosening. Materials with reduced stiffness might alter the strain distribution less, whereas shell and liner deformations might increase. The purpose of our current computational study was to evaluate whether carbon fiber-reinforced poly-ether-ether-ketones with a Young´s modulus of 15 GPa (CFR-PEEK-15) and 23 GPa (CFR-PEEK-23) might be an alternative shell material compared to titanium in terms of shell and liner deformation, as well as strain distribution in the adjacent bone. Using a finite element analysis, the press-fit implantation of modular acetabular cups with shells made of titanium, CFR-PEEK-15 and CFR-PEEK-23 in a human hemi-pelvis model was simulated. Liners made of ceramic and polyethylene were simulated. Radial shell and liner deformations as well as strain distributions were analyzed. The shells made of CFR-PEEK-15 were deformed most (266.7 µm), followed by CFR-PEEK-23 (136.5 µm) and titanium (54.0 µm). Subsequently, the ceramic liners were radially deformed by up to 4.4 µm and the polyethylene liners up to 184.7 µm. The shell materials slightly influenced the strain distribution in the adjacent bone with CFR-PEEK, resulting in less strain in critical regions (<400 µm/m or >3000 µm/m) and more strain in bone building or sustaining regions (400 to 3000 µm/m), while the liner material only had a minor impact. The superior biomechanical properties of the acetabular shells made of CFR-PEEK could not be determined in our present study.

Titanium Acetabular Component Deformation under Cyclic Loading

Acetabular cup deformation may affect liner/cup congruency, clearance and/or osseointegration. It is unclear, whether deformation of the acetabular components occurs during load and to what extent. To evaluate this, revision multi-hole cups were implanted into six cadaver hemipelvises in two scenarios: without acetabular defect (ND); with a large acetabular defect (LD) that was treated with an augment. In the LD scenario, the cup and augment were attached to the bone and each other with screws. Subsequently, the implanted hemipelvises were loaded under a physiologic partial-weight-bearing modality. The deformation of the acetabular components was determined using a best-fit algorithm. The statistical evaluation involved repeated-measures ANOVA. The mean elastic distension of the ND cup was 292.9 µm (SD 12.2 µm); in the LD scenario, 43.7 µm (SD 11.2 µm); the mean maximal augment distension was 79.6 µm (SD 21.6 µm). A significant difference between the maximal distension of the cups in both scenarios was noted (F(1, 10) = 11.404; p = 0.007). No significant difference was noted between the compression of the ND and LD cups, nor between LD cups and LD augments. The LD cup displayed significantly lower elastic distension than the ND cup, most likely due to increased stiffness from the affixed augment and screw fixation.

Effect of impaction energy on dynamic bone strains, fixation strength, and seating of cementless acetabular cups

Seating a cementless acetabular cup via impaction is a balancing act; good cup fixation must be obtained to ensure adequate bone in-growth and cup apposition, while acetabular fracture must be avoided. Good impaction technique is essential to the success of hip arthroplasty. Yet little guidance exists in the literature to inform surgeons on "how hard" to hit. A drop rig and synthetic bone model were used to vary the energy of impaction strikes in low and high-density synthetic bone, while key parameters such as dynamic strain (quantifying fracture risk), implant fixation, and polar gap were measured. For high energy impaction (15 J) in low-density synthetic bone, a peak tensile strain was observed during impaction that was up to 3.4× as large as post-strike strain, indicating a high fracture risk. Diminishing returns were observed for pushout fixation with increasing energy. Eighty-five percent of the pushout fixation achieved using a 15 J impaction strike was attained by using a 7.5 J strike energy. Similarly, polar gap was only minimally reduced at high impaction energies. Therefore it is suggested that higher energy strikes increase fracture risk, but do not offer large improvements to fixation or implant-bone apposition. It may difficult be for surgeons to accurately deliver specific impaction energies, suggesting there is scope for operative tools to manage implant seating. © 2019 The Authors. Journal of Orthopaedic Research^® published by Wiley Periodicals, Inc. on behalf of Orthopaedic Research Society. J Orthop Res 37:2367-2375, 2019.

Comparison study of temperature and deformation changes in the femoral component of a novel ceramic-on-ceramic hip resurfacing bearing to a metal standard, using a cadaveric model

Hip resurfacing is an attractive alternative to total hip replacement preserving bone and reducing dislocation risk. Recent metal-on-metal designs have caused failure due to metal wear debris. Ceramic implants may mitigate this risk. Temperature increase in periprosthetic bone during cementation can lead to osteonecrosis, while deformation of the component can affect joint lubrication and may increase wear through clamping. Both processes may lead to implant loosening. This study quantifies the temperature and deformation change in a novel ceramic hip resurfacing femoral component compared to a metal standard during cemented implantation in a fresh frozen cadaveric model. Study design and methods Eight femora were prepared from four fresh frozen cadavers. One surgeon experienced in hip resurfacing surgery (J.H.) prepared the femora by reaming. Four ceramic and four metal implants of equal and varying size were cemented in place. Bone and surface temperatures were taken using a probe in the periprosthetic bone and an infrared laser thermometer, respectively. Deformation was measured using a micrometre. Measurements were taken before implantation and every 5-min intervals up to 30 min. The average bone-temperature increment was lower for ceramic heads than for metal heads. Although this difference was not statistically significant, the average bone temperature incremental change in small sizes (42 and 46 mm) was higher than in the large sizes (48 and 50 mm). Most metal heads sustained bearing diameter change that was still near its peak value 30 min after implantation, whereas the ceramic heads suffered a lower diameter change and most of the samples recovered their original diameter 30 min after implantation. Both implants behave similarly, however, a lower temperature rise in bone was observed with ceramic heads. This may lower the risk for thermal damage on periprosthetic bone. The ceramic heads deformed less during surgical implantation. This was not significant.

Deformation of acetabular press-fit cups: Influence of design and surgical factors

BACKGROUND

Deformation of acetabular cups when press-fitted into an undersized cavity is inevitable due to the inhomogeneous stiffness of acetabular bone. Thinner cups or screw holes might increase the risk of high cup deformation. The aim of this study was to examine the influence of cup design and liner assembly on the deformation response during cup implantation.

METHODS

Acetabular cups with different designs were implanted into polyurethane foam models simulating the anatomical situation with nominal press-fits of 1mm and without nominal press-fits (line-to-line). Deformations were determined using a tactile coordinate measuring machine. A 3D laser scanner was used to determine the contact conditions at the cup-cavity interface. Polyethylene and ceramic liners were assembled to the implanted cups and the influence of the insertion on the deformation response evaluated. Fixation strength of the cups was determined by push-out testing.

FINDINGS

Cup deformation increased with smaller wall thickness (P < 0.037) and screw holes (P < 0.001). Insertion of ceramic liners reduced the deformation (P < 0.001), whereas polyethylene liners adapted to the deformation of the implanted cups (P > 0.999). Thin-walled cups exhibited a higher fixation strength for similar implantation forces (P = 0.011).

INTERPRETATION

Thin-walled cups achieved higher fixation strengths and might be more bone-preserving. However, in combination with screw holes and high press-fit levels, wall thickness should be considered carefully to avoid excessive cup deformations leading to potential complications during liner assembly. Line-to-line insertion of thin-walled cups should be accompanied with a rough surface coating to minimize the loss of fixation strength due to the low press-fit fixation.

Comparing the cup deformation following implantation of a novel ceramic-on-ceramic hip resurfacing bearing to a metal standard in a cadaveric model

Hip resurfacing is an attractive alternative to total hip replacement preserving bone and reducing dislocation risk. Recent metal-on-metal designs have caused failure due to metal wear debris. Ceramic implants may mitigate this risk. Deformation of the acetabular cup can affect the lubrication, producing high friction torques between the femoral head and the cup that would increase wear and/or lead to cup loosening due to femoral head clamping. Our objective was to quantify the deformation of a novel monobloc ceramic hip resurfacing cup component compared to a metal standard, in a fresh frozen cadaveric model using a press-fit technique representative of standard surgical conditions. For this study eight acetabula were prepared from four fresh frozen cadavers. One surgeon with extensive experience in hip resurfacing surgery (J.H.) prepared the acetabulum by sequential reaming. The implants were then impacted into the acetabulum. Four ceramic and four metal implants were used of equal and varying size. Deformation was measured peri-implantation, and at 30 min, using an optical high-precision deformation sensor (GOM GmbH, Braunschweig, Germany). The maximum inscribed circle and the measurement of radial segment techniques were used. Deformation was greater in the metal implants (mean: 34-22mm) immediately after implantation. At 30 min after implantation, the deformation increased to 36mm in the metal and 26mm in the ceramic cup. Greater diameter changes were observed in larger cups. Metal and ceramic implants did not return to the initial diameter. We conclude the ceramic resurfacing acetabular implants undergo similar deformation to existing metal-on-metal implants. The deformation observed was significantly less in the ceramic component at 30 min on one measure. Less deformation may result in better surface conditions and wear characteristics. Deformation change did not resolve after 30 min for both implants.

In vitro examination of the primary stability of three press-fit acetabular cups under consideration of two different bearing couples

BACKROUND

For preclinical statements about the anchoring behavior of prostheses, the primary stability of the prosthesis is of special importance. It was the aim of this study to examine and compare the relevant relative micromotions of three different acetabulum prostheses by introducing three-dimensional torques.

METHODS

The cups were implanted under standard conditions into an anatomical artificial bone model. Three-dimensional torques were applied to the acetabular cups. Taking into account the resulting frictional moments of two different bearing couples, ceramic-on-ceramic and ceramic-on-polyethylene, the relative micromotions of the cups were recorded as maximum total micromotion, translational and rotational micromotion, and the primary stability values of the three cups were compared.

RESULTS

Relative micromotion of all cup models was always significantly smaller with the CoC bearing couples than with the CoP bearing couples (p < 0.001). The rotational micromotion was always lower (p < 0.001) than the translational micromotion, and the rotational as well as the translational micromotions were each always lower than the maximum total micromotion (p < 0.001, p < 0.010). The thinnest-walled cup system always showed the largest relative micromotions.

CONCLUSION

The results of our study can be interpreted as indicating that the low relative micromotions of all cups - irrespective of the use of CoC or CoP bearing couples - are within an acceptable range favoring secondary osseointegration of the implants. Furthermore, we were able to show that the cup wall thickness and the surface quality of the cup systems have an influence on the primary stability and the elastic deformability of the examined cup systems.

Femoral side-only revision options for the Birmingham resurfacing arthroplasty

BACKGROUND

The Birmingham Hip Resurfacing (BHR) system (Smith and Nephew) was developed as an alternative to conventional total joint replacement for younger, more active patients. Among other complications exists the risk for femoral component failure. The only marketed revision option for such a complication involves exchange of all components for a total replacement arthroplasty. This presents as a considerable and potentially unnecessary operative burden where revision of only the femoral prosthesis would suffice. We have analysed revision options for BHR in the context of periprosthetic femoral fractures with a stable acetabular component.

METHODS

Technical details of dual mobility hip systems available in Australia were collated and analysed to assess for potential 'off label' use with an existing BHR acetabular component. These data were then compared with the custom-made Smith and Nephew dual mobility implant with respect to clearance and sizing.

RESULTS

Two dual mobility articulation modalities from two companies were identified as appropriate for potential usage with four products analysed in detail. These two demonstrated acceptable sizing and clearance measurements.

CONCLUSION

Comparison between readily available dual mobility prostheses with custom-made implants showed off label dual mobility prosthetic use to be a viable alternative for femoral-only revisions with in situ BHR. Single component revision has several advantages which include: a less complex surgical procedure, shorter operative time, decreased blood loss and the expectation of resultant lower morbidity. Furthermore, this less complex revision surgery should give comparable results to that of primary total hip arthroplasty.

Acetabular Debonding: An Investigation of Porous Coating Delamination in Hip Resurfacing Arthroplasty

Background

To date, there have been no published investigations on the cause of acetabular debonding, a rare failure phenomenon in metal-on-metal hip resurfacing where the acetabular porous coating delaminates from the implant while remaining well fixed to the pelvic bone.

Purposes

This study aims to summarize the current understanding of acetabular debonding and to investigate the discrepancy in rate of debonding between two implant systems.

Patients and Methods

To elucidate potential causes of debonding, we retrospectively analyzed a single-surgeon cohort of 839 hip resurfacing cases. Specifically, we compared rate of debonding and manufacturing processes between two implant systems.

Results

Group 1 experienced significantly more cases of debonding than Group 2 cases (4.0% versus 0.0%, p value<0.0001). Implant manufacturing processes differed in surface coating, heat treatment, postmanufacturing treatment, and apex thickness. Debonded implants were more likely to have missed RAIL guidelines (p=0.04).

Conclusions

We identified implant system, postoperative time, and acetabular component placement as variables contributing to rate of debonding. We recommend minimizing acetabular inclination angle according to RAIL guidelines. Further, we evaluated manufacturing differences between the two implant systems but did not have access to proprietary data to identify the cause of debonding. Both implants met ASTM standards, yet only the Group 1 implant debonded. This suggests the second implant had greater fatigue shear strength. Because the Group 2 implant achieved a more durable interface that did not debond, we suggest the ASTM F1160 standard for fatigue shear strength be increased to that achieved by its manufacturer.

Level of Evidence II

A retrospective evaluation of prospectively collected data.

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.

Effect of bearing friction torques on the primary stability of press-fit acetabular cups: A novel in vitro method

Aseptic loosening is the main reason for revision of total hip arthroplasty, and relative micromotions between cementless acetabular cups and bone play an important role regarding their comparatively high loosening rate. Therefore, the aim of the present study was to analyze the influence of resulting frictional torques on the primary stability of press-fit acetabular cups subjected to two different bearing partners. A cementless press-fit cup was implanted in bone-like foam. Primary stability of the cup was analyzed by determining spatial total, translational, and rotational interface micromotions by means of an eddy current sensor measuring system. Torque transmission into the cup was realized by three synchronous servomotors considering resultant friction torques based on constant friction for ceramic-on-ceramic (CoC: μ = 0.044; max. resultant torque: 1.5 Nm) and for ceramic-on-polyethylene (CoP: μ = 0.063; max. resultant torque: 1.9 Nm) bearing partners. Rotational micromotion of CoC was 8.99 ± 0.85 µm and of CoP 13.39 ± 1.43 µm. Translational micromotion of CoC was 29.93 ± 1.44 μm and of CoP 39.91 ± 2.25 μm. Maximum total relative micromotions were 37.10 ± 1.07 μm for CoC and 51.64 ± 2.18 μm for CoP. Micromotions resulting from CoC were statistically lower than those resulting from CoP (p < 0.05). The described 3D-measuring set-up offers a novel in vitro method of measuring primary stability of acetabular cups. We can therefore conclude, that primary stability of acetabular cup systems can be observed using either the lower friction curve (CoC) or the higher friction curve (CoP). In future studies different cup designs or cup fixation mechanisms may be tested and compared in vitro and assessed prior to implantation. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:2745-2753, 2018.

Biomechanical behavior of modular acetabular cups made of poly-ether-ether-ketone: A finite element study

After total hip arthroplasty, stress-shielding is a potential risk factor for aseptic loosening of acetabular cups made of metals. This might be avoided by the use of acetabular cups made of implant materials with lower stiffness. The purpose of this numerical study was to determine whether a modular acetabular cup with a shell made of poly-ether-ether-ketone or poly-ether-ether-ketone reinforced with carbon fibers might be an alternative to conventional metallic shells. Therefore, the press-fit implantation of modular cups with shells made of different materials (Ti6Al4V, poly-ether-ether-ketone, and poly-ether-ether-ketone reinforced with carbon fibers) and varying liner materials (ceramics and ultra-high-molecular-weight polyethylene) into an artificial bone cavity was simulated using finite element analysis. The shell material had a major impact on the radial shell deformation determined at the rim of the shell, ranging from 17.9 µm for titanium over 92.2 µm for poly-ether-ether-ketone reinforced with carbon fibers up to 475.9 µm for poly-ether-ether-ketone. Larger radial liner deformations (up to 618.4 µm) occurred in combination with the shells made of poly-ether-ether-ketone compared to titanium and poly-ether-ether-ketone reinforced with carbon fibers. Hence, it can be stated that conventional poly-ether-ether-ketone is not a suitable shell material for modular acetabular cups. However, the radial shell deformation can be reduced if the poly-ether-ether-ketone reinforced with carbon fiber material is used, while deformation of ceramic liners is similar to the deformation in combination with titanium shells.

Early shape change behaviour of an uncemented contemporary hip cup: A cadaveric experiment replicating host bone behaviour through temperature control

Modular uncemented acetabular components are in common use. Fixation is dependent upon press-fit but the forces necessary to achieve initial stability of the construct at implantation may deform the shell and prevent optimal seating of the polyethylene liner insert. Previous work using single-time point measurements in uncontrolled ambient temperature poorly replicates the native state. A controlled study was performed to observe the time-dependent behaviour of an uncemented acetabular shell in the early phase after implantation into the human acetabulum at near physiological temperature. Using a previously validated cadaveric hip model at controlled near physiological temperature with standardised surgical technique, immediate and delayed shell geometry was determined. Eight custom made 3-mm-thick titanium alloy (TiAl6V4) shells were implanted into four cadavers (eight hips). Time-dependent shell deformation was determined using the previously validated ATOS Triple Scan III (ATOS) optical measurement system. The pattern of change in the shape of the surgically implanted shell was measured at three time points after insertion. We found a consistent pattern for quantitative and directional deformation of the shells. In addition, there was consistency for relaxation of the deformation with time. Immediate mean change in shell radius was 104 µm (standard deviation 32, range 67–153) relaxing to mean 96 µm (standard deviation 32, range 63–150) after 10 min and mean 92 µm (standard deviation 28, range 66–138) after 20 min. The clinical significance of this work is the finding of a time-dependent early deformation of acetabular titanium shells on insertion adjusted for near physiological temperature-controlled host bone.

Initial stability of a highly porous titanium cup in an acetabular bone defect model

BACKGROUNDS

The purpose of this study was to quantify the initial stability of a highly porous titanium cup using an acetabular bone defect model.

METHODS

The maximum torque of a highly porous titanium cup, with a pore size of 640 μm and porosity of 60%, was measured using rotational and lever-out torque testing and compared to that of a titanium-sprayed cup. The bone models were prepared using a polyurethane foam block and had three levels of bone coverage: 100, 70, and 50%.

RESULTS

The highly porous titanium cup demonstrated significantly higher maximum torque than the titanium-sprayed cups in the three levels of bone defects. On rotational torque testing, it was found to be 1.5, 1.3, and 1.3 times stronger than the titanium-sprayed cups with 100, 70 and 50% bone coverage, respectively. Furthermore, it was found to be 2.2, 2.3, and 1.5 times stronger on lever-out testing than the titanium-sprayed cup. No breakage in the porous layers was noted during the testing.

CONCLUSION

This study provides additional evidence of the initial stability of highly porous titanium cup, even in the presence of acetabular bone defects.

Influence of anisotropic bone properties on the biomechanical behavior of the acetabular cup implant: a multiscale finite element study

Although the biomechanical behavior of the acetabular cup (AC) implant is determinant for the surgical success, it remains difficult to be assessed due to the multiscale and anisotropic nature of bone tissue. The aim of the present study was to investigate the influence of the anisotropic properties of peri-implant trabecular bone tissue on the biomechanical behavior of the AC implant at the macroscopic scale. Thirteen bovine trabecular bone samples were imaged using micro-computed tomography (μCT) with a resolution of 18 μm. The anisotropic biomechanical properties of each sample were determined at the scale of the centimeter based on a dedicated method using asymptotic homogenization. The material properties obtained with this multiscale approach were used as input data in a 3D finite element model to simulate the macroscopic mechanical behavior of the AC implant under different loading conditions. The largest stress and strain magnitudes were found around the equatorial rim and in the polar area of the AC implant. All macroscopic stiffness quantities were significantly correlated (R² > 0.85, p < 6.5 e-6) with BV/TV (bone volume/total volume). Moreover, the maximum value of the von Mises stress field was significantly correlated with BV/TV (R² > 0.61, p < 1.6 e-3) and was always found at the bone-implant interface. However, the mean value of the microscopic stress (at the scale of the trabeculae) decrease as a function of BV/TV for vertical and torsional loading and do not depend on BV/TV for horizontal loading. These results highlight the importance of the anisotropic properties of bone tissue.

Non-invasive vibrometry-based diagnostic detection of acetabular cup loosening in total hip replacement (THR)

Total hip replacement is aimed at relieving pain and restoring function. Currently, imaging techniques are primarily used as a clinical diagnosis and follow-up method. However, these are unreliable for detecting early loosening, and this has led to the proposal of novel techniques such as vibrometry. The present study had two aims, namely, the validation of the outcomes of a previous work related to loosening detection, and the provision of a more realistic anatomical representation of the clinical scenario. The acetabular cup loosening conditions (secure, and 1 and 2 mm spherical loosening) considered were simulated using Sawbones composite bones. The excitation signal was introduced in the femoral lateral condyle region using a frequency range of 100-1500 Hz. Both the 1 and 2 mm spherical loosening conditions were successfully distinguished from the secure condition, with a favourable frequency range of 500-1500 Hz. The results of this study represent a key advance on previous research into vibrometric detection of acetabular loosening using geometrically realistic model, and demonstrate the clinical potential of this technique.

Shell design and reaming technique affect deformation in mobile-bearing total hip arthroplasty acetabular components

Press-fit acetabular components are susceptible to rim deformation. The inherent variability within acetabular reaming techniques may generate increased press-fit and, subsequently, additional component deformation. The purpose of this study was to analyze the insertion and deformation characteristics of acetabular components designed for dual-mobility systems based on component design, size, and reaming technique. Shell deformation was quantified in a validated worst-case scenario foam pinch model. Thin-walled, one-piece, and modular dual-mobility shells of varying size were implanted in under- and over-reamed cavities with insertion force measured and shell deformation assessed using digital image correlation. Increased shell size resulted in larger rim deformation in one-piece components, with a reduction in press-fit by 1 mm resulting in up to 48% reduction in insertion forces and between 23% and 51% reduction in shell deformation. Lower insertion forces and deformations were observed in modular components. Variability in acetabular reaming plays a significant role in the ease of implantation and component deformation in total hip arthroplasty. Modular components are less susceptible to deformation than thin-walled monoblock shells. Care should be taken to avoid excessive under-reaming, particularly in the scenario of large shell size and high-density patient bone stock.

Deformation of the Zurich cementless acetabular cup caused by implantation in a canine cadaver model

OBJECTIVE

To evaluate the change in geometry of the Zurich total hip arthroplasty (THA) acetabular component after implantation.

ANIMALS

Hemipelves from adult mix-breed dogs weighing between 20 and 25 kg.

METHODS

Digital image correlation imaging was performed prior to, immediately after, and 24 hours after impaction of Zurich THA acetabular component, and after removal of the cup from the specimen. Patterns of deformation were qualitatively described, and maximal deformations were compared between time points.

RESULTS

All cups deformed after implantation into the hemipelves by "pinching" in a cranial-caudal direction and dorsoventral expansion, resulting in an ellipsoid configuration to the peripheral rim. The mean ± SD maximum deformation at the rim immediately post-impaction was 0.202 ± 0.052 mm, or approximately 0.4 mm of diametrical deformation. Deformation did not change after the 24-hour saline bath. Impaction and subsequent extraction had a marginal effect on the original cup geometry, as maximum deformation at the rim after cup extraction was 0.074 ± 0.032 mm, relative to prior to impaction.

CONCLUSIONS

The original Zurich cup geometry is distorted as a consequence of the press-fit mechanism. Further studies are required to determine whether deformation induced by impaction has any association with polyethylene wear rates or other prosthesis-related complications.

Finite element model of the impaction of a press-fitted acetabular cup

Press-fit surgical procedures aim at providing primary stability to acetabular cup (AC) implants. Impact analysis constitutes a powerful approach to retrieve the AC implant insertion properties. The aim of this numerical study was to investigate the dynamic interaction occurring between the hammer, the ancillary and bone tissue during the impact and to assess the potential of impact analysis to retrieve AC implant insertion conditions. A dynamic two-dimensional axisymmetric model was developed to simulate the impaction of the AC implant into bone tissue assuming friction at the bone-implant interface and large deformations. Different values of interference fit (from 0.5 to 2 mm) and impact velocities (from 1 to 2 m.s^-1) were considered. For each configuration, the variation of the force applied between the hammer and the ancillary was analyzed and an indicator I was determined based on the impact momentum of the signal. The simulated results are compared to the experiments. The value of the polar gap decreases with the impact velocity and increases with the interference fit. The bone-implant contact area was significantly correlated with the resonance frequency (R ² = 0.94) and the indicator (R ² = 0.95). The results show the potential of impact analyses to retrieve the bone-implant contact properties.

Can physical joint simulators be used to anticipate clinical wear problems of new joint replacement implants prior to market release?

One of the most important mandates of physical joint simulators is to provide test results that allow the implant manufacturer to anticipate and perhaps avoid clinical wear problems with their new products. This is best done before market release. This study gives four steps to follow in conducting such wear simulator testing. Two major examples involving hip wear simulators are discussed in which attempts had been made to predict clinical wear performance prior to market release. The second one, involving the DePuy ASR implant systems, is chosen for more extensive treatment by making it an illustrative example to explore whether wear simulator testing can anticipate clinical wear problems. It is concluded that hip wear simulator testing did provide data in the academic literature that indicated some risk of clinical wear problems prior to market release of the ASR implant systems. This supports the idea that physical joint simulators have an important role in the pre-market testing of new joint replacement implants.

Acetabular shell deformation as a function of shell stiffness and bone strength

Press-fit acetabular shells used for hip replacement rely upon an interference fit with the bone to provide initial stability. This process may result in deformation of the shell. This study aimed to model shell deformation as a process of shell stiffness and bone strength. A cohort of 32 shells with two different wall thicknesses (3 and 4 mm) and 10 different shell sizes (44- to 62-mm outer diameter) were implanted into eight cadavers. Shell deformation was then measured in the cadavers using a previously validated ATOS Triple Scan III optical system. The shell-bone interface was then considered as a spring system according to Hooke's law and from this the force exerted on the shell by the bone was calculated using a combined stiffness consisting of the measured shell stiffness and a calculated bone stiffness. The median radial stiffness for the 3-mm wall thickness was 4192 N/mm (range, 2920-6257 N/mm), while for the 4-mm wall thickness the median was 9633 N/mm (range, 6875-14,341 N/mm). The median deformation was 48 µm (range, 3-187 µm), while the median force was 256 N (range, 26-916 N). No statistically significant correlation was found between shell stiffness and deformation. Deformation was also found to be not fully symmetric (centres 180° apart), with a median angle discrepancy of 11.5° between the two maximum positive points of deformation. Further work is still required to understand how the bone influences acetabular shell deformation.

The influence of metallic shell deformation on the contact mechanics of a ceramic-on-ceramic total hip arthroplasty

Total hip arthroplasty of ceramic-on-ceramic bearing combinations is increasingly used clinically. The majority of these implants are used with cementless fixation that a metal-backing shell is press-fitted into the pelvic bone. This usually results in the deformation of the metallic shell, which may also influence the ceramic liner deformation and consequently the contact mechanics between the liner and the femoral head under loading. The explicit dynamic finite element method was applied to model the implantation of a cementless ceramic-on-ceramic with a titanium shell and subsequently to investigate the effect of the metallic shell deformation on the contact mechanics. A total of three impacts were found to be necessary to seat the titanium alloy shell into the pelvic bone cavity with a 1 mm diameter interference and a 1.3 kg impactor at 4500 mm s(-1) velocity. The maximum deformation of the metallic shell was found to be 160 µm in the antero-superior and postero-inferior direction and 97 µm in the antero-inferior and postero-superior direction after the press-fit. The corresponding values were slightly reduced to 67 and 45 µm after the ceramic liner was inserted and then modified to 74 and 43 µm under loading, respectively. The maximum deformation and the maximum principal stress of the ceramic liner were 31 µm and 144 MPa (tensile stress), respectively, after it was inserted into the shell and further increased to 52 µm and 245 MPa under loading. This research highlights the importance of the press-fit of the metallic shell on the contact mechanics of the ceramic liner for ceramic-on-ceramic total hip arthroplasties and potential clinical performances.

In-vivo degradation of middle-term highly cross-linked and remelted polyethylene cups: Modification induced by creep, wear and oxidation

In this study Raman (RS) and Fourier Transform Infrared (FT-IR) spectroscopic techniques were exploited to study 11 retrieved liners made of remelted highly cross-linked polyethylene (HXLPE), with the intent to elucidate their in-vivo mechanical and chemical degradation. The retrievals had different follow-ups, ranging from a few months to 7 years of implantation time and belong to the first generation of highly cross-linked and remelted polyethylene clinically introduced in 1999, but still currently implanted. Raman assessments enabled to discriminate contributes of wear and creep on the total reduction of thickness in different locations of the cup. According to our results, although the most of the viscoelastic deformation occurred during the first year (bedding-in period), it progressed during the steady wear state up to 7 years with much lower but not negligible rate. Overall, the wear rate of this remelted HXLPE liner was low. Preliminary analysis on microtomed sections of the liners after in-vivo and in-vitro accelerated aging (ASTM F2003-02) enabled to obtain a phenomenological correlation between the oxidation index (OI) and the amount of orthorhombic phase fraction (αc), which can be easily non-destructively measured by RS. Profiles of αc obtained from different locations of the cups were used to judge the oxidative degradation of the 11 retrievals, considering also the ex-vivo time elapsed from the revision surgery to the spectroscopic experiments. Low but measurable level of oxidation was detected in all the short-term retrievals, while in the middle-term samples peaks of OI were observed in the subsurface (up to OI=4.5), presumably induced by the combined effect of mechanical stress, lipid absorption and prolonged ex-vivo shelf-aging in air.

The influence of the strength of bone on the deformation of acetabular shells: a laboratory experiment in cadavers

Concerns have been raised that deformation of acetabular shells may disrupt the assembly process of modular prostheses. In this study we aimed to examine the effect that the strength of bone has on the amount of deformation of the acetabular shell. The hypothesis was that stronger bone would result in greater deformation. A total of 17 acetabular shells were inserted into the acetabula of eight cadavers, and deformation was measured using an optical measuring system. Cores of bone from the femoral head were taken from each cadaver and compressed using a materials testing machine. The highest peak modulus and yield stress for each cadaver were used to represent the strength of the bone and compared with the values for the deformation and the surgeon's subjective assessment of the hardness of the bone. The mean deformation of the shell was 129 µm (3 to 340). No correlation was found between deformation and either the maximum peak modulus (r² = 0.011, t = 0.426, p = 0.676) or the yield stress (r² = 0.024, t = 0.614, p = 0.549) of the bone. Although no correlation was found between the strength of the bone and deformation, the values for the deformation observed could be sufficient to disrupt the assembly process of modular acetabular components.

A novel method to assess primary stability of press-fit acetabular cups

Initial stability is an essential prerequisite to achieve osseointegration of press-fit acetabular cups in total hip replacements. Most in vitro methods that assess cup stability do not reproduce physiological loading conditions and use simplified acetabular models with a spherical cavity. The aim of this study was to investigate the effect of bone density and acetabular geometry on cup stability using a novel method for measuring acetabular cup micromotion. A press-fit cup was inserted into Sawbones® foam blocks having different densities to simulate normal and osteoporotic bone variations and different acetabular geometries. The stability of the cup was assessed in two ways: (a) measurement of micromotion of the cup in 6 degrees of freedom under physiological loading and (b) uniaxial push-out tests. The results indicate that changes in bone substrate density and acetabular geometry affect the stability of press-fit acetabular cups. They also suggest that cups implanted into weaker, for example, osteoporotic, bone are subjected to higher levels of micromotion and are therefore more prone to loosening. The decrease in stability of the cup in the physiological model suggests that using simplified spherical cavities to model the acetabulum over-estimates the initial stability of press-fit cups. This novel testing method should provide the basis for a more representative protocol for future pre-clinical evaluation of new acetabular cup designs.

Validation of an optical system to measure acetabular shell deformation in cadavers

Deformation of the acetabular shell at the time of surgery can result in poor performance and early failure of the hip replacement. The study aim was to validate an ATOS III Triple Scan optical measurement system against a co-ordinate measuring machine using in vitro testing and to check repeatability under cadaver laboratory conditions. Two sizes of custom-made acetabular shells were deformed using a uniaxial/two-point loading frame and measured at different loads. Roundness measurements were performed using both the ATOS III Triple Scan optical system and a co-ordinate measuring machine and then compared. The repeatability was also tested by measuring shells pre- and post-insertion in a cadaver laboratory multiple times. The in vitro comparison with the co-ordinate measuring machine demonstrated a maximum difference of 5 µm at the rim and 9 µm at the measurement closest to the pole of the shell. Maximum repeatability was below 1 µm for the co-ordinate measuring machine and 3 µm for the ATOS III Triple Scan optical system. Repeatability was comparable between the pre-insertion (below 2 µm) and post-insertion (below 3 µm) measurements in the cadaver laboratory. This study supports the view that the ATOS III Triple Scan optical system fulfils the necessary requirements to accurately measure shell deformation in cadavers.

A fatigue assessment technique for modular and pre-stressed orthopaedic implants

Orthopaedic implants experience large cyclic loads, and pre-clinical analysis is conducted to ensure they can withstand millions of loading cycles. Acetabular cup developments aim to reduce wall thickness to conserve bone, and this produces high pre-stress in modular implants. As part of an implant development process, we propose a technique for preclinical fatigue strength assessment of modular implants which accounts for this mean stress, stress concentrating features and material processing. A modular cup's stress distributions were predicted computationally, under assembly and in vivo loads, and its cyclic residual stress and stress amplitude were calculated. For verification against damage initiation in low-cycle-fatigue (LCF), the peak stress was compared to the material's yield strength. For verification against failure in high-cycle-fatigue (HCF) each element's reserve factor was calculated using the conservative Soderberg infinite life criterion. Results demonstrated the importance of accounting for mean stress. The cup was predicted to experience high cyclic mean stress with low magnitude stress amplitude: a low cyclic load ratio (Rl=0.1) produced a high cyclic stress ratio (Rs=0.80). Furthermore the locations of highest cyclic mean stress and stress amplitude did not coincide. The minimum predicted reserve factor Nf was 1.96 (HCF) and 2.08 (LCF). If mean stress were neglected or if the stress ratio were assumed to equal the load ratio, the reserve factor would be considerably lower, potentially leading to over-engineering, reducing bone conservation. Fatigue strength evaluation is only one step in a broader development process, which should involve a series of verifications with the full range of normal and traumatic physiological loading scenarios, with representative boundary conditions and a representative environment. This study presents and justifies a fatigue analysis methodology which could be applied in early stage development to a variety of modular and pre-stressed prosthesis concepts, and is particularly relevant as implant development aims to maximise modularity and bone conservation.

A novel method for measuring acetabular cup deformation in cadavers

Press-fit acetabular shells are subject to compressive loading conditions upon insertion during surgery. These conditions may result in deformation of the shell, potentially adversely affecting the performance of any liner fitted within it. This technical note describes the method used to measure acetabular shell deformation in a cadaver study using an optical GOM ATOS Triple Scan system. Six custom-made titanium acetabular shells were scanned prior to and immediately following insertion into three cadavers (one shell per acetabulum). A three-dimensional colour map was created for each shell, and cross sections were taken at the rim to determine the maximum diametrical deformation. Values of between 30 and 150 µm were recorded, with the results suggesting a size-dependent response.

In vivo implant fixation of carbon fiber-reinforced PEEK hip prostheses in an ovine model

Carbon fiber-reinforced polyetheretherketone (CFR/PEEK) is theoretically suitable as a material for use in hip prostheses, offering excellent biocompatibility, mechanical properties, and the absence of metal ions. To evaluate in vivo fixation methods of CFR/PEEK hip prostheses in bone, we examined radiographic and histological results for cementless or cemented CFR/PEEK hip prostheses in an ovine model with implantation up to 52 weeks. CFR/PEEK cups and stems with rough-textured surfaces plus hydroxyapatite (HA) coatings for cementless fixation and CFR/PEEK cups and stems without HA coating for cement fixation were manufactured based on ovine computed tomography (CT) data. Unilateral total hip arthroplasty was performed using cementless or cemented CFR/PEEK hip prostheses. Five cementless cups and stems and six cemented cups and stems were evaluated. On the femoral side, all cementless stems demonstrated bony ongrowth fixation and all cemented stems demonstrated stable fixation without any gaps at both the bone-cement and cement-stem interfaces. All cementless cases and four of the six cemented cases showed minimal stress shielding. On the acetabular side, two of the five cementless cups demonstrated bony ongrowth fixation. Our results suggest that both cementless and cemented CFR/PEEK stems work well for fixation. Cup fixation may be difficult for both cementless and cemented types in this ovine model, but bone ongrowth fixation on the cup was first seen in two cementless cases. Cementless fixation can be achieved using HA-coated CFR/PEEK implants, even under load-bearing conditions.

High initial stability in porous titanium acetabular cups: a biomechanical study

Initial stability with limited micromotion in uncemented total hip arthroplasty acetabular components is essential for bony attachment and long-term biomechanical fixation. This study compared porous titanium fixation surfaces to clinically established, plasma-sprayed designs in terms of interface stability and required seating force. Porous plasma-sprayed modular and metal-on-metal (MOM) cups were compared to a modular, porous titanium designs. Cups were implanted into polyurethane blocks with1-mm interference fit and subsequently edge loaded to failure. Porous titanium cups exhibited 23% to 65% improvement in initial stability when compared to plasma-sprayed cup designs (P=.01): a clinically significant increase, based on experience and prior literature. The results of this study indicate increased interface stability in porous titanium-coated cups without significantly increasing the necessary force and energy required for full seating.

Acetabular cup design influences deformational response in total hip arthroplasty

BACKGROUND

Press-fit acetabular components are susceptible to deformation in an underreamed socket, with excessive deformation of metal-on-metal (MOM) components potentially leading to increased torsional friction and micromotion. Specifically, however, it remains unclear how cup diameter, design, and time from implantation affect shell deformation.

QUESTIONS/PURPOSES

We asked whether (1) changes in component geometry and material altered maximum shell deformation and (2) time-dependent deformational relaxation processes occurred.

METHODS

Diametral deformation was quantified after press-fit implantation of metal shells into a previously validated polyurethane model. Experimental groups (n = 6-8) consisted of 48-, 54-, 60-, and 66-mm MOM cups of 6-mm wall thickness, 58-mm cups of 10-mm wall thickness, and CoCrMo and Ti6Al4V 58-mm modular cups.

RESULTS

Greater cup diameter, thinner wall construction, and Ti6Al4V modular designs generated conditions for maximum shell deformation ranging from 0.047 to 0.267 mm. Relaxation (18%-32%) was observed 120 hours postimplantation in thin-walled and modular designs.

CONCLUSIONS

Our findings demonstrate a reduction of shell deformation over time and suggest, under physiologic loading, early component deformation varies with design.

CLINICAL RELEVANCE

Component deformation should be a design consideration regardless of bearing surface. Designs neglecting to adequately address deformational changes in vivo could be susceptible to diminished cup survival, increased wear, and premature revision.

Deformation of the Durom acetabular component and its impact on tribology in a cadaveric model--a simulator study

BACKGROUND

Recent studies have shown that the acetabular component frequently becomes deformed during press-fit insertion. The aim of this study was to explore the deformation of the Durom cup after implantation and to clarify the impact of deformation on wear and ion release of the Durom large head metal-on-metal (MOM) total hips in simulators.

METHODS

Six Durom cups impacted into reamed acetabula of fresh cadavers were used as the experimental group and another 6 size-paired intact Durom cups constituted the control group. All 12 Durom MOM total hips were put through a 3 million cycle (MC) wear test in simulators.

RESULTS

The 6 cups in the experimental group were all deformed, with a mean deformation of 41.78 ± 8.86 µm. The average volumetric wear rate in the experimental group and in the control group in the first million cycle was 6.65 ± 0.29 mm(3)/MC and 0.89 ± 0.04 mm(3)/MC (t = 48.43, p = 0.000). The ion levels of Cr and Co in the experimental group were also higher than those in the control group before 2.0 MC. However there was no difference in the ion levels between 2.0 and 3.0 MC.

CONCLUSIONS

This finding implies that the non-modular acetabular component of Durom total hip prosthesis is likely to become deformed during press-fit insertion, and that the deformation will result in increased volumetric wear and increased ion release.

CLINICAL RELEVANCE

This study was determined to explore the deformation of the Durom cup after implantation and to clarify the impact of deformation on wear and ion release of the prosthesis. Deformation of the cup after implantation increases the wear of MOM bearings and the resulting ion levels. The clinical use of the Durom large head prosthesis should be with great care.

Hip resurfacing with the Biomet Hybrid ReCap-Magnum system: 7-year results

The purpose of this study was to report our clinical outcome of a large series of metal-on-metal hip resurfacing arthroplasty (HRA) using the hybrid Biomet ReCap-Magnum system. This is a single-designer surgeon series with an average of 5 ± 1 years. Seven hundred forty consecutive hybrid HRAs were performed in 653 patients. Kaplan-Meier survivorship with any revision as an end point was 96.4% at 7 years. Twenty-five (3.4%) cases were revised: 8 due to acetabular component loosening, 6 due to femoral neck facture, 4 due to failure of femoral component fixation, 2 due to deep infection, 2 due to adverse wear, 1 due to psoas tendonitis, 1 due to recurrent dislocation, and 1 due to unexplained pain. Biomet ReCap and Magnum HRA components with hybrid fixation methods showed excellent survivorship for a minimally selected young patient cohort at 7 years.

The effect of cup orientation and coverage on contact mechanics and range of motion of metal-on-metal hip resurfacing arthroplasty

Implant malpositioning has been identified as a factor associated with clinical failures of metal-on-metal hip resurfacings (MoMHRs). This study investigated the effect of cup orientation and cup coverage on the contact mechanics (incidence of edge-loading) and range of motion (ROM) of MoMHR. Three generic MoMHRs with differing amounts of cup coverage were considered at various orientations. Contact area and contact pressure at the bearing surface were predicted for each design using finite element (FE) method. The ROM was determined based on the geometry overlap. Edge contact was found at lower angles of inclination (65°) for lower coverage cup designs; however, they also provided the greatest ROM. Conversely, cups with greater coverage did not exhibit edge contact until the cup was more steeply positioned (75°), however ROM was reduced. This study enables both sets of variable to be considered in the design of metal-on-metal bearings in hip.