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. 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.Aims
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
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
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
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
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
Micromotions between stem and neck adapter depend on prosthesis design and material coupling. Based on the results of this study, the amount of micromotion seems to reflect the risk of fretting-induced fatigue in vivo. Bimodular hip prostheses were developed to allow surgeons an individual reconstruction of the hip joint by varying length, offset and anteversion in the operation theatre. Despite these advantages, the use of these systems led to a high rate of postoperative complications resulting in revision rates of up to 11% ten years after surgical intervention. During daily activities taper connections of modular hip implants are highly stressed regions and contain the potential of micromotions between adjacent components, fretting and corrosion. This might explain why an elevated number of fretting-induced neck fractures occurred in clinics. However, some bi-modular prostheses (e.g. Metha, Aesculap, Ti-Ti) are more often affected by those complications than others (e.g. H-Max M, Limacorporate, Ti-Ti or Metha, Ti-CoCr) implying that the design and the material coupling have an impact on this failure pattern. Therefore, the purpose of this study was to clarify whether clinical successful prostheses offer lower micromotions than those with an elevated number of in vivo fractures.Summary
Introduction
From a mechanical point of view, the clinical use of pedicle screws in the atlas is a promising alternative to lateral mass screws due to an increased biomechanical fixation. The most established surgical technique for posterior screw fixation in the atlas (C1) is realised by screw placement through the lateral mass [1]. This surgical placement may lead to extended bleeding from the paravertebral venous plexus as well as a violation of the axis (C2) nerve roots [1]. Using pedicle screws is an emerging technique which utilises the canal passing through the posterior arch enabling the use of longer screws with a greater contact area while avoiding the venous plexus and axis nerve roots. The aim of this Summary Statement
Introduction
Nucleotomy almost doubles the transmitted forces on the facet joints in human lumbar spine, regardless of the amount of removed nucleus pulposus. Low back pain involves the lumbar facet joints in 15% to 45% of the cases. The surgical intervention, nucleotomy, might also lead to painful facets with a high risk; however, its mechanism is yet to be fully understood. The aim of this study is to reveal how a small amount of nucleus removal can change the force transmission on the facets.Summary
Introduction
At the clinical CT image resolution level, there is no influence of the image voxel size on the derived finite element human cancellous bone models Computed tomography (CT)-based finite element (FE) models have been proved to provide a better prediction of vertebral strength than dual-energy x-ray absorptiometry [1]. FE models based on µCTs are able to provide the golden standard results [2], but due to the sample size restriction of the µCT and the XtremeCT machines, the clinical CT-based FE models is still the most promising tool for the in vivo prediction of vertebrae's strength. It has been found [3] that FE predicted Young's modulus of human cancellous bone increases as the image voxel size increases at the µCT resolution level [3]. However, it is still not clear whether the image voxel size in the clinical range has an impact on the predicted mechanical behavior of cancellous bone. This study is designed to answer this question.Summary
Introduction
Lumbar spinal specimens exhibited high fatigue strength. The cycles to failure are not only dependent on the maximum peak load, but also on the load offset or the amplitude, respectably. Spinal injury might be caused by whole body vibrations. The permitted exposure to vibration in the workplace is therefore limited. However, there is a lack in knowledge how external vibrations might cause internal damages. Numerical whole body models might provide the potential to estimate the dynamic spinal loading during different daily activities, but depends on knowledge about the corresponding fatigue strength. This study is aiming to determine the Summary
Introduction
