Implant fracture of modular revision stems is a major complication after total hip arthroplasty revision (rTHA). Studies looking at specific modular designs report fracture rates of 0.3% to 0.66% whereas fractures of monobloc designs are only reported anecdotally. It is unclear whether the overall re-revision rate of modular designs is higher and if, whether stem fractures or other revision reasons are responsible for this elevation. All revisions within 5 years after implantation of a revision stems (n0=13,900; n5=2506) were analysed using Cox regression with design (modular: n=17, monobloc: n=27), BMI, Sex and Elixhauser Score as independent variables. One stage and two stage revisions were analysed separately (1-stage: modular n= 7,102; monobloc n= 4,542; 2-stage: 1,551 / 704). The revision volume of the hospitals was also considered (low: <20 revisions, medium: 21–50 revisions, high: >50 revisions). For the 1-stage revisions, the re-revision risk after 4 years was 14,3% [13.2%, 15.5%] for monobloc and 17.4% [16.40%, 18.40%] for modular stems (p< 0.001). Stem fracture was the reason for re-revision in 2.4% of the modular (fracture rate 0.42%) and 0.6% of the monobloc revisions. The difference in re-revision rates between the designs was mainly due to differences in dislocation and stem loosening. For the 2-stage revisions, the revision risks for either design were similar (21.7% [18,5%, 25.4%] vs. 23.0% [20.8%, 25.4%]; p=0.05). Patient characteristics influenced the comparison between the two designs in the 1-stage group but very little in the 2-stage group. Modular revision stem fractures only contribute very minor to re-revision risk. In 2-stage revisions, no difference in overall re-revision rates between designs was observed. This might indicate that the differences observed for 1-stage procedures are due to differences between the patient cohorts, not reflected by the parameters available or surgeon choice.
Despite higher rates of revision after total hip arthroplasty (THA) being reported for uncemented stems in patients aged > 75 years, they are frequently used in this age group. Increased mortality after cemented fixation is often used as a justification, but recent data do not confirm this association. The aim of this study was to investigate the influence of the design of the stem and the type of fixation on the rate of revision and immediate postoperative mortality, focusing on the age and sex of the patients. A total of 333,144 patients with primary osteoarthritis (OA) of the hip who underwent elective THA between November 2012 and September 2022, using uncemented acetabular components without reconstruction shells, from the German arthroplasty registry were included in the study. The revision rates three years postoperatively for four types of stem (uncemented, uncemented with collar, uncemented short, and cemented) were compared within four age groups: < 60 years (Young), between 61 and 70 years (Mid-I), between 71 and 80 years (Mid-II), and aged > 80 years (Old). A noninferiority analysis was performed on the most frequently used designs of stem.Aims
Methods
The early revision rate in elective Total Hip Arthroplasty (THA) three years after surgery in elderly patients over 80 years is significantly lower for cemented stems in the German Arthroplasty Register (EPRD): cemented 3,1% (3.0 – 3.2) vs. uncemented 4.2% (4.1 – 4.3; p < 0.001). However, the mortality rate in elderly patients is elevated for cemented fixation. This study presents a detailed analysis of the influence of stem type and fixation on revision and mortality rate in this patient cohort. Elective primary THA cases for primary Coxarthrosis using uncemented cups from the EPRD data base were analysed (n0= 37,183). Four stem type groups were compared: cementless, cementless with collar, cementless short, and cemented. Stems with at least 300 cases at risk three years after surgery were analysed individually. The reference stem was determined as the stem with the lowest revision rate and at least 1000 cases under surveillance 3 years after surgery (n3 = 28,637). The revision rate for cemented stems (2.5% [2.2–1.81] was lower than for uncemented (4.5% [4.2–4.9]; p<0.001) and uncemented short stems (4.2% [3.1–5.7]; p=0.002). The revision rate of uncemented collared stems (2.3% [1.5–3.6]) was similar to cemented stems (p=0.89) and lower than for uncemented stems (p=0.02). One year mortality showed no sig. differences between the groups (p>0.17): cemented 3.2% [2.9–3.6], uncemented 3.4% [3.1–3.7], uncemented short 3.5% [2.5–4.9], uncemented collar 2.0% [1.2–3.2]. “Cementless” and “cementless short” stems should not be used in patients over 80 years due to the higher revision risk. If cementing should be avoided, “cementless collared” stems seem to be a good alternative combined with a tendency for a lower one year mortality rate.
Special acetabular polyethylene (PE) liners are intended to increase the stability of the artificial hip joint, yet registry studies on them are limited. The pupose of this study was to investigate differences in revision rates for mechanical complications in primary cementless total hip arthroplasty (THA) with standard and special PE acetabular liners in patients with ostheoarthritis. Data from the German Arthroplasty Registry (EPRD) between 2012 until 2020 were analysed. Patients with diagnosed ostheoarthritis of the hip without relevant prior surgeries, who received a primary cementless THA with a ceramic/PE bearing articulation were included. Cumulative incidences of revision for mechanical complications for Standard and 4 special PE liners (Lipped, Increased Offset, Angulated, Angulated|Increased Offset) were determined using the Kaplan-Meier Estimator. Confounding factors were investigated with a Cox proportional-hazards model. In total 151.104 cases were included. 7-year unadjusted revision-free survival for mechanical complications compared to Standard liners (97.7%) was lower for Angulated (97.4%), Lipped (97.2%) and Angulated|Increased Offset liners (94.7%), but higher for Increased Offset liners (98.1%). Risk of revision for mechanical complications was not significantly different between Standard, Lipped and Angulated liners. Increased Offset liners (HR=0.68; 95% CI=0.5–0.92) reduced, while Angulated|Increased Offset liners (HR= 1.81; 95% CI=1.38–2.36) increased the risk. Higher age at admission and an Elixhauser comorbidity index greater zero increased the risk, whereas a larger liner share slightly reduced the risk. Only the use of Increased Offset liners reduced the risk of revision for mechanical complications compared to Standard liners — other special liners did not.
The influence of the surgical process on implant loosening and periprosthetic fractures (PPF) as major complications in uncemented total hip arthroplasty (THA) have rarely been studied due to the difficult quantification. Meanwhile registry analyses have clearly shown a decrease in complications with increasing experience. The goal of this study was to determine the extent of variability in THA stem implantation between highly experienced surgeons with respect to implant-size, -position, press-fit, contact area, primary stability and the effect of using a powered impaction tool. Primary hip stems were implanted in 16 cadaveric femur pairs by three experienced surgeons using manual and powered impaction. Quantitative CTs were taken before and after each process step and stem tilt, canal-fill-ratio, pressfit and contact area between bone and implant determined. 11 femur pairs were additionally tested for primary stability under cyclic loading conditions. Higher variations in press-fit and contact area between the surgeons for manual impactions compared to powered were observed. Stem tilt and implant sizing varied between surgeons but not between impaction methods. Larger stems exhibited less micromotion compared to smaller stems. Larger implants may increase PPF risk, while smaller implants reduce primary stability. The reduced variation for powered impactions indicates that appropriate measures may promote a more standardized process. The observed variations between the experienced surgeons may represent the acceptable range for this specific stem design. Variability in the implantation process warrants further investigations since certain deviations e.g. a stem tilt towards varus, might increase bone stresses and PPF risk.
Reaming of the acetabular cavity prior to cementless cup implantation aims to create a defined press-fit between implant and bone. The goal is to achieve full implant seating with the desired press-fit to reduce the risk of early cup loosening and the risk of excessive cup deformation. Current research concentrated on the spherical deviations of the reamed cavity compared to the reamer size, but the direct relationship between nominal press-fit, reamer geometry, cavity shape and bone-implant contact has not yet been investigated. The aim of this study was to determine the influence of the reaming process, the surface coating, and the implantation force on the achieved press-fit situation. Fresh-frozen porcine acetabulae (n = 20) were prepared and embedded. Hemispherical reamers were used and the last reaming step was performed using a vertical drilling machine to ensure a proper alignment of the cavity axis. A hand-guided 3D laser scanner was used (HandySCAN 700, Creaform) to determine the reamer geometry and the cavity shape. Press-fit cups with two different surface coatings (Ø44 mm, Porocoat/Gription, DePuy Synthes) were implanted using a drop tower. The Porocoat cup was implanted with impacts from lower drop heights (low implantation force) and press-fits of 1 mm and 2 mm. The Gription cup, exhibiting a rougher surface, was implanted with low and high implantation forces and a press-fit of 1 mm. Bone-implant contact was analysed by the registration of the cup and cavity surface models, scanned prior to implantation, to the scan of the implanted cup. The cup surface was divided in areas with and without contact to the surrounding cavity. Overhang indicates that there was no adjacent cavity surface surrounding the implanted cup. The transition between contact and a gap at the cup dome was defined as contact depth and used as indicator for the cup seating.INTRODUCTION
METHODS
Revision of total knee endoprostheses (TKA) is increasing in number and causes rising healthcare costs. For constrained prostheses, the use of intramedullar femoral stems is standard. However, there is a big variety of available stem types with regard to length, type of fixation (cemented vs. hybrid) and fixation area (diaphyseal vs. metaphyseal). The aim of this biomechanical study was to investigate the primary stability of revision TKA with different stem types and different femoral bone defects, to find out whether smaller or shorter stems may achieve sufficient stability while preserving bone for re-revision. 30 right human femora were collected, fresh frozen and divided in six groups, matching for age, gender, height, weight and bone density. In group 1–3 a bone defect of AORI type F2a (15mm medial) and in group 4–6 a defect of AORI type F3 (25mm on both sides) was created. In all six groups the same modular femoral surface component (Endo-Model-W, Waldemar Link) was used, combined with different stem types (100/ 160 mm cemented / uncemented / standard/ anatomical with / without cone). Additionally, one trial was set up, omitting the modular stem. The correct fit of the implants was confirmed by fluoroscopy. After embedding, specimens were mechanically loaded 10mm medially and parallel to the mechanical femoral axis with an axial force of 2700N and a torsional moment of 5.6Nm at a flexion angle of 15° with respect to the coupled tibial plateau according to in-vivo gait load for 10,000 cycles (1Hz) in a servohydraulic testing machine (Bionix, MTS). The relative movement between implant, cement and distal femur was recorded using a stereo video system (Aramis3D,gom). An axial pull-out test at 1mm/min was performed after dynamic loading.Introduction
Methods
The disadvantage of removing a well-fixed femoral stem are multiple (operating time, risk of fracture, bone and blood loss, recovery time and post-op complications. Ceramic heads with titanium adapter sleeves (e.g. BIOLOX®OPTION, Ceramtec) are a possibility for putting a new ceramic head on slightly damaged used tapers. ‘Intolerable’ taper damages even for this solution are qualitatively specified by the manufacturers. The aim of this study was to determine the fracture strength of ceramic heads with adapter sleeves on stem tapers with such defined damage patterns. Pristine stem tapers (Ti-6Al-4V, 12/14) were damaged to represent the four major stem taper damage patterns specified by the manufacturers:
‘Truncated’: Removal of 12.5% of the circumference along the entire length of the stem taper at a uniform depth of 0.5mm parallel to the taper slope. ‘Slanted’: Removal of 33.3% of the proximal diameter perimeter with decreasing damage down to 3.7mm from the proximal taper end. ‘Cut’: Removal of the proximal 25% (4mm) of the stem taper. ‘Scratched’: Stem tapers from a previous ceramic fracture test study with a variety of scratches and crushing around the upper taper edge from multiple ceramic head fractures. The ‘Control’ group consisted of three pristine tapers left undamaged. BIOLOX®OPTION heads (Ø 32mm, length M) with Ti adapter sleeves were assembled to the damaged stem tapers and subjected to ISO7206-10 ultimate compression strength testing. The forces required to fracture the head were high and caused complete destruction of the ceramic heads in all cases. The ‘Truncated’ group showed the lowest values (136kN ± 4.37kN; Fig. 3). Forces were higher and similar for the ‘Cut’ (170kN ± 8.89kN), ‘Control’ (171.8 ± 16.5kN) and ‘Slanted’ (173kN ± 21.9kN) groups, the ‘Scratched’ group showed slightly higher values (193kN ± 11.9kN). The Ti adapter sleeves were plastically deformed but did not fail catastrophically. The present study suggests that manufacturer's recommendations for removal of a well fixed femoral stem could be narrowed down to the ‘Truncated’ condition. Even this might not be necessary since the fracture load is still substantially higher than the ASTM standard requires. Surgeons should consider to keep stems with larger taper damages as previously thought and spare the patient from stem revision. The greatest reservation regarding adapter sleeves is the introduction of the new metal-on-metal interface between stem and sleeve, which could possibly facilitate fretting-corrosion, which is presently one of the major concerns for modular junctions (3). Clinically such problems have not been reported yet. Ongoing FE-simulations are performed to investigate whether micromotions between stem and head taper are altered by the investigated damages.
During revision surgery, the active electrode of an electrocautery device may get close to the implant, potentially provoking a flashover. Incidents have been reported, where in situ retained hip stems failed after isolated cup revision. Different sizes of discoloured areas, probably induced by electrocautery contact, were found at the starting point of the fracture. The effect of the flashover on the implant material is yet not fully understood. The aim of this study was to investigate the fatigue strength reduction of Ti-6Al-4V titanium alloy after electrocautery contact. 16 titanium rods (Ti-6Al-4V, extra low interstitial elements, according to DIN 17851, ⊘ 5 mm, 120 mm length) were stress-relief annealed (normal atmosphere, holding temperature 622 °C, holding time 2 h) and cooled in air. An implant specific surface roughness was achieved by chemical and electrolytic polishing (Ra = 0.307, Rz = 1.910). Dry (n = 6) and wet (n = 6, 5 µl phosphate buffered saline) flashovers were applied with a hand-held electrode of a high-frequency generator (Aesculap AG, GN 640, monopolar cut mode, output power 300 W, modelled patient resistance 500 Ω). The size of the generated discoloured area on the rod's surface - representative for the heat affected zone (HAZ) - was determined using laser microscopy (VK-150x, Keyence, Japan). Rods without flashover (n = 4) served as control. The fatigue strength of the rods was determined under dynamic (10 Hz, load ratio R = 0.1), force-controlled four-point bending (FGB Steinbach GmbH, Germany) with swelling load (numerical bending stress 852 MPa with a bending moment of 17.8 Nm) until failure of the rods. The applied bending stress was estimated using a finite-element-model of a hip stem during stumbling. Metallurgical cuts were made to analyse the microstructure.Introduction
Material and Methods
Precise determination of material loss is essential for failure analysis of retrieved hip cups. To determine wear, the measured geometry of the retrieval hast to be compared to its pristine geometry, which usually is not available. There are different approaches to generate reference geometries to approximate the pristine geometry that is commonly assumed as sphere. However, the geometry of press fit cup retrievals might not be spherical due to deformation caused by excessive press-fitting. The effect of three different reference geometries on the determined wear patterns and material loss of pristine and worn uncemented metal-on-metal hip cups was determined. The surfaces of two cups (ASR, DePuy, Leeds; one pristine, one a worn retrieval) were digitized using a coordinate measurement machine (CRYSTA-Apex S574, Mitutoyo; 3 µm accuracy). Both cups were measured undeformed and while being deformed between a clamp. Three different methods for generating reference geometries were investigated (PolyWorks|Inspector 2018, InnovMetric). Method 1: A sphere with the nominal internal cup dimensions was generated. Method 2: A sphere was fitted to the measured data points after removing those from worn areas (deviation > 3 µm is defined as wear) to eliminate the influence of manufacturing tolerances on the nominal diameter. Method 3: Measurements, which displayed visual deformation in the computed wear pattern based on the best fit sphere, were fitted with an ellipsoid. The direction of the deformation axes and the amount of deformation were used to scale the best fit ellipsoid. Linear wear was calculated from the distance of the respective reference geometry to the measured point cloud. Finally, material loss is defined as the difference in volume of the reference geometry and the measured geometry.INTRODUCTION
METHODS
Post-operative periprosthetic femoral fractures (PFF) are a devastating complication associated with high mortality and are costly. Few risk factors are modifiable apart from implant choice. The design features governing risk of PFF are unknown. We estimated the 90-day risk of revision for PFF associated with design features of cementless femoral stems and to investigate the effect of a collar on early PFF risk using a biomechanical in-vitro model. 337 647 primary THAs from the National Joint Registry (UK) were included in a multivariable survival and regression analysis to identify the adjusted hazard of PFF revision following primary THA using cementless stems. The effect of a collar in cementless THA on early PFF was evaluated in an in-vitro model using paired fresh frozen cadaveric femora.Background
Patients, materials and methods
Clinical symptoms arising from corrosion within taper junctions of modular total hip prostheses are of increasing concern [1]. In particular, bi-modular implant designs showed increased failure rates due to wear originating from the neck-stem junction [2]. In-vivo corrosion-related failure is less frequently observed for head-stem junctions [3]. It is hypothesized that fretting and crevice corrosion are associated with micromotions between the mating surfaces of a taper junction [4]. The aim of this study was to measure micromotion occurring within a head-stem junction of a conventional prosthesis and clarify by how much it is exceeded in a neck-stem junction of a bi-modular prosthesis that exhibited severe corrosion and early implant failure. The micromotions within two taper articulations were investigated: a head-stem taper (Corail, DePuy Synthes, Leeds, UK, Figure 1) and a neck-stem taper of a bi-modular THA prosthesis (Rejuvenate, Stryker, Kalamazoo, MI, USA). Both tapers were assembled with 2000 N. Loading at an angle of 50° to the taper axes (identical for both) in direction of the stem axis was incrementally increased from 0 N to 1900 N (n=3). Small windows (< 2.5 mm2) were cut through the female tapers by electric discharge machining, exposing the male taper surface for direct micromotion measurements by microscopic topographic measurements (Infinite Focus Microscope, Alicona Imaging GmbH, Austria). Subsequently, feature matching of the images from the differently loaded implants was applied (Matlab 2016b, The MathWorks Inc., Natick, MA, USA) to determine the local relative motion between the mating surfaces.Introduction
Material & Methods
The restoration of the anatomical hip rotation center (HRC) has a major influence on the longevity of hip prostheses. Deviations from the HRC of the anatomical joint after total hip arthroplasty (THA) can lead to increased hip joint forces, early wear or loosening of the implant. The contact conditions of acetabular press-fit cups after implantation, including the degree of press-fit, the existence of a polar gap and cup orientation, may affect the HRC restoration, and therefore implant stability. The aim of this study was to determine the influence of acetabular press-fit, polar gap and cup orientation on HRC restoration during THA. THAs were performed by an experienced orthopaedic surgeon in full cadaveric models simulating real patient surgery (n=7). Acetabular cups with a Porocoat™ (n=3) and Gription™ surface coating (n=4) were implanted (DePuy Synthes, Leeds, UK). Computed tomography (CT) scans prior to surgery, as well as after reaming and implantation of press-fit cups were used to calculate the HRC displacement. After aligning the pelves in the anterior pelvic plane, 3D reconstruction of the HRC at each stage was performed by fitting spheres to the femoral head, the reamed cavity and the inserted cup. 3D surface models of the cups were generated using a laser scanner and were registered to the CT images. The effective press-fit was calculated using the diameters of spheres, fitted to the cavity prior to cup insertion and to the outer cup coating. The polar gap was defined as the difference between the outer cup surface and the subchondral bone at the cup pole. Anteversion and abduction angles were calculated as difference between the cup planes and the sagittal and transverse plane, respectively.INTRODUCTION
METHODS
Loosening is a major cause for revision in uncemented hip prostheses due to insufficient primary stability. Primary stability after surgery is achieved through press-fit in an undersized cavity. Cavity preparation is performed either by extraction (removing bone) or compaction (crushing bone) broaching. Densification of trabecular bone has been shown to enhance primary stability in human femora; however, the effect of clinically used compaction and extraction broaches on human bone with varying bone mineral density (BMD) has not yet been quantified. The purpose of this study was to determine the influence of the broach design and BMD on the level of densification at the bone-cavity interface, stem seating, the bone-implant contact area and the press-fit achieved. Paired human femora (m/f=11/12, age=60±18 y) were scanned with quantitative computed tomography (QCT, Philips Brilliance 16) before broaching, with the final broach, after its removal and after stem implantation. Compaction broaching (n=4) was compared in an INTRODUCTION
METHODS
Taper corrosion in Total Hip Arthroplasty has surfaced as a clinically relevant problem and has recently also been reported for metal heads against polyethylene. Low neck stiffness is a critical contributing factor. Catastrophic taper failures have been reported for one particular stem design with a small V-40 taper made from a less stiff titanium-alloy. The purpose of this study was to identify factors involved in the failure process. 31 revised CoCr heads ranging from 32 to 44m diameter combined with TMZF-Titanium alloy stem with a V-40 taper (Accolade I) were analysed. Stems were only available for catastrophic failure cases with dis-association (n=8) or taper fracture (n=1). Clinical data were limited to time-in-situ, patient gender and age. Head material loss increased with time in situ (r²=0.49, p<0.001). Longer heads and material loss exceeding 15mm³ showed bottoming out and consecutive catastrophic stem taper failure. Heads with failed stem tapers were all 36mm diameter. The head starts rotating on the stem taper after bottoming out, causing major abrasive wear, ultimately resulting in catastrophic failure; it is surprising that these catastrophic cases did not exhibit clinical symptoms due to raised Co and Cr metal ions, which must have resulted from the large amount of CoCr lost from the female head taper. This would have attracted medical attention and prevented catastrophic failure by taper dis-association. Control exams of patients treated with the respective stem type in combination with large CoCr heads should include metal ion determination in blood or serum, even if no clinical symptoms are present, in order to detect taper corrosion before catastrophic failure occurs.
Total hip replacement is one of the most successful orthopaedic surgeries, not least because of the introduction of modular systems giving surgeons the flexibility to intraoperatively adapt the geometry of the artificial joint to the patient's anatomy. However, taper junctions of modular implants are at risk of fretting-induced postoperative complications such as corrosion, which can lead to adverse tissue reactions. Interface micro-motions are suspected to be a causal factor for mechanical loading-induced corrosion, which can require implant revision. The aim of this study was to determine the micro-motions at the stem-head taper interface during daily activities and the influence of specific material combinations. The ball heads (ø 32mm, 12/14, size L, CoCr or Al2O3) were quasi-statically assembled to the stems (Ti or CoCr, Metha, Aesculap AG, Germany, v=0.5 kN/s, F=6 kN, n=3 each, 10° adduction/ 9° flexion according to ISO 7206-4) and then loaded sinusoidally using a material testing machine (Mini Bionix II, MTS, USA, Figure 1). The peak forces represented different daily activities [Bergmann, 2010]: walking (2.3 kN), stair climbing (4.3 kN) and stumbling (5.3 kN). 2,000 loading cycles (f=1 Hz) were applied for each load level. Six eddy-current sensors, placed between stem and head, were used to determine the displacement (interface micro-motion and elastic deformation) between head and stem (Figure 1). A finite element model (FEM) based on CAD data was used to determine the elastic deformation of the prostheses for the experimentally tested activities (Abaqus, Simulia, USA). Tie-junctions at all interfaces prevented relative movements of the adjacent surfaces. The resultant translations at the centre of the ball head were determined using a coordinate transformation and a subsequent subtraction of the elastic deformation.Introduction
Materials & Methods
Modular acetabular liners are fixed in metal shells by a taper locking mechanism. Male tapers of the liner and female tapers of the metal shell have different taper angles resulting in an angular gap. Depending on the specific manufacturing tolerances varying angular gaps may result and, thus, different contact mechanics may be generated that could alter the stresses within the acetabular liner. Therefore, the aim of the current study was to experimentally determine stresses in a ceramic liner depending on different angular gaps under Two ceramic liners were instrumented at the outer contour with five strain gauge (SG) rosettes each (Fig.1). First, metal shells were axially seated in an asymmetric press-fit model with 0.5 mm under-reaming, then liners were assembled with a 2 kN axial load. SG5 was placed at the flat area of the liner, the other four were placed circumferentially in 90 degrees offset on the rear side. SG2 and SG4 were mounted opposite to each other in press-fit direction while SG1 and SG3 were placed in the non-supported direction. Three inclination angles (0°, 30°, 45°) were tested under Introduction
Materials and Methods
Failure of the neck-stem taper in one particular bi-modular primary hip stem due to corrosion and wear of the neck piece has been reported frequently1, and stems were recalled. A specific pattern of material loss on the CoCr neck-piece taper in the areas of highest stresses on the proximal medial male taper was observed in a retrieval study of 27 revised Rejuvenate implants revised after 3 to 38 month time in situ (Stryker, Kalamazoo, MI, USA) (Figure 1). One neck piece exhibited additionally wear marks at the distal end of the flat male neck taper indicating contact with the female taper of the stem. The purpose of this study was to understand the observed failure scenario of bottoming-out by investigating the stem taper morphologies. The geometry of taper contact surfaces was determined using a Coordinate Measurement Machine (BHN 805, Mitutoyo, Japan). An algorithm based on the individual unworn areas of the respective taper surfaces was applied to all retrievals. One retrieval is additionally investigated by infinite focus microscopy (G4, Alicona, Austria) in the main wear areas on the neck piece taper, and the bottom, facing each other inside the junction (surfaces of the distal end of the male and the bottom of the female taper).Introduction
Materials and Methods
Wear and corrosion between head and stem tapers of modular hip implants have recently been related to clinical failures, possibly due to high friction moments in poorly lubricated joints [1–2]. In-vivo measurements have revealed reversing joint friction moments in the hip during a gait cycle [3], which may foster relative motion between the modular components. Blood, soft tissue or bone debris at the taper interface during assembly can lead to decreased stability or increased stress concentrations due to non-uniform loading [4]. The purpose of this study is to investigate the influence of taper contamination and the assembly force on the seating characteristic of the head on the stem incorporating realistic reversing joint friction moments. Cobalt chrome heads (M-SPEC, 36mm, +1.5mm; n=5) were assembled on titanium femoral stems (Corail 12/14, both components Depuy Synthes; n=5) by quasistatic axial push-on forces (F=0.5kN, 1kN, 2kN). Heads were modified by milling a flat plane, to which the joint load was applied alternately to point A and point B for 20 cycles to provide reversing moments (heel-strike FA=1971N, MA=5.4Nm; toe-off FB=807N, MB=4.6Nm; Fig. 1). All 6 degrees of freedom of relative displacement between head and stem were determined in the unloaded state and after each loading cycle. A coordinate measurement machine (accuracy ±2µm) was used to determine the components positions. Pull-off forces were measured after the last loading cycle. Each taper was tested in pristine condition and then contaminated with a bone chip (1.7±0.2mg).Introduction
Methods
Dual modular hip prostheses were introduced to optimize the individual and intra-surgical adaptation of the implant design to the native anatomics und biomechanics of the hip. The downside of a modular implant design with an additional modular interface is the potential susceptibility to fretting, crevice corrosion and wear [1–2]. The purpose of this study was to characterize the metal ion release of a modular hip implant system with different modular junctions and material combinations in consideration of the corrosive physiological environment. One design of a dual modular hip prosthesis (Ti6Al4V, Metha®, Aesculap AG, Germany) with a high offset neck adapter (CoCrMo, CCD-angle of 130°, neutral antetorsion) and a monobloc prosthesis (stem size 4) of the same implant type were used to characterize the metal ion release of modular and non-modular hip implants. Stems were embedded in PMMA with 10° adduction and 9° flexion according to ISO 7206-6 and assembled with ceramic (Biolox® delta) or CoCrMo femoral heads (XL-offset) by three light impacts with a hammer. All implant options were tested in four different test fluids: Ringer's solution, bovine calf serum and iron chloride solution (FeCl3-concentration: 10 g/L and 114 g/L). Cyclic axial sinusoidal compressive load (Fmax = 3800 N, peak load level of walking based on in vivo force measurements [3]) was applied for 10 million cycles using a servohydraulic testing machine (MTS MiniBionix 370). The test frequency was continuously varied between 15 Hz (9900 cycles) followed by 1 Hz (100 cycles). The metal ion concentration (cobalt, chromium and titanium) of the test fluids were analysed using ICP-OES and ICP-MS at intervals of 0, 5·105, 2·106 and 10·106 cycles (measuring sensitivity < 1 µg/L).Introduction
Methods
