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
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
