Introduction. 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. Material & Methods. 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 mm. 2. ) 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. Results. Loading with 1900 N resulted in micromotions of 1.0 µm ± 0.1 µm at the head-stem taper (Figure 2). The stepwise loading showed the motion trajectory, suggesting toggling with the dominant displacement in axial direction and small transversal movements. Neck-stem micromotion was significantly higher (14.2 µm ± 1.7 µm, p < 0.001). The trajectory revealed a tilt of the neck in direction of the force. The male taper returned into its initial position after the load was removed, indicating a repetitive rocking motion within every load sequence. Discussion. The higher micromotion at the neck-stem taper junction is likely caused by the larger lever arm (20-fold) between load application and taper engagement. This can serve to explain the susceptibility of bi-modular prostheses to an elevated rate of problems due to fretting corrosion. Similar findings are speculated to apply for large-diameter heads, which showed high failure rates in clinical practice [5]. For any figures or tables, please contact the authors directly

Modifying Knee anatomy during mechanical Total Knee Arthroplasty (TKA) may impact ligament balance, patellar tracking and quadriceps function. Although well fixed, patients may report high levels (20%) of dissatisfaction. One theory is that putting the knee in neutral mechanical alignment may be responsible for these unsatisfactory results. Kinematic TKA has gained interest in recent years; it aims to resurface the knee joint and preservation of natural femoral flexion axis about which the tibia and patella articulate, recreating the native knee without the need for soft tissue relaease. That's being said, it remains the question of whether all patients are suitable for kinematic alignment. Some patients' anatomy may be inherently biomechanically inferior and recreating native anatomy in these patients may result in early implant failure. The senior author (PAV) has been performing Kinematic TKA since 2011, and has developed an algorithm in order to better predict which patient may benefit from this technique. Lower limb CT scans from 4884 consecutive patients scheduled for TKA arthroplasty were analysed. These exams were performed for patient-specific instrumentation production (My Knee®, Medacta, Switzerland). Multiple anatomical landmarks used to create accurate CT-based preoperative planning and determine the mechanical axis of bone for the femur and tibia and overall Hip-knee-Ankle (HKA). We wanted to test the safe range for kinematic TKA for the planned distal resection of the femur and tibia. Safe range algorithm was defined as the combination of the following criteria: – Independent tibial and femoral cuts within ± 5° of the bone neutral mechanical axis and HKA within ± 3°. The purpose of this study is to verify the applicability of the proposed safe range algorithm on a large sample of individual scheduled for TKA. The preoperative tibial mechanical angle average 2.9 degrees in varus, femoral mechanical angle averaged 2.7 degrees in valgus and overall HKA averaged of 0.1 in varus. There were 2475 (51%) knees out of 4884, with femur and tibia mechanical axis within ±5° and HKA within ±3° without need for bony corrections. After applying the algorithm, a total of 4062 cases (83%) were successfully been evaluated using the proposed protocol to reach a safe range of HKA ±3° with minimal correction. The remaining 822 cases (17%) could not be managed by the proposed algorithm because of their unusual anatomies and were dealt with individually. In this study, we tested a proposed algorithm to perform kinematic alignment TKA avoiding preservation/restoration of some extreme anatomies that might not be suitable for TKA long-term survivorship. A total of 4062 cases (83%) were successfully eligible for our proposed safe range algorithm for kinematic TKA. In conclusion, kinematically aligned TKA may be a promising option to improve normal knee function restoration and patient satisfaction. Until we have valuable data confirming the compatibility of all patients' pre arthritic anatomies with TKA long-term survivorship, we believe that kinematically alignment should be performed within some limits. Further studies with Radiostereometry or longer follow up might help determine if all patients' anatomies are suitable for Kinematic TKA

Background. Titanium, in particular Ti6Al4V, is the standard material used in cementless joint arthroplasty. Implants are subjected to cyclic loading where fracture is the reason for re-operation in 1.5–2.4% of all revisions in total hip arthroplasty. In order to strengthen critical regions, surface treatments such as shot peening may be applied. A superficial titanium oxide layer is naturally formed on the surface as a protective film at ambient conditions. However, as its thickness is only in the range of several nanometers, it is prone to be destroyed by high loads - as present at the surface during bending - leading to an ‘oxidative wear’ in a corrosive environment [1]. The present study aims to evaluate the shot peening treatment on Ti6Al4V regarding its potential for cyclically loaded parts under a dry and a corrosive testing medium. Materials and Methods. Hour-glass shaped titanium specimens (Ti6Al4V) with a minimal diameter of 10 mm have been subjected to an annealing treatment at 620°C for 10h to remove initial residual stresses introduced during machining. Subsequently, a high-intensity shot peening treatment with cut wire followed by a low-intensity cleaning process with glass beads have been performed (Metal Improvement, Germany). Arithmetic mean roughness R. a. of the treated surfaces was measured (Mahr Perthometer M2, Germany). Residual stress depth profiles prior to and after shot peening have been measured by a Fe-filtered Co-K(alpha) radiation (GE Measurement&Control, USA) and calculated using the sin. 2. (psi) method. Fatigue strength has been determined by two servo-hydraulic hydropulsers (Bosch Rexroth, Germany) at 10 Hz and a load ratio of R=0.1 either under dry conditions (8 specimens) or surrounded by a 0.9-% saline solution (6 specimens) (BBraun, Germany) (Fig. 1). Testing has been performed until fracture occurred or the total number of 10 × 10. 6. cycles has been reached. All fracture surfaces have been analyzed after testing using FEG-SEM (Zeiss LEO 1530 VP Gemini, Germany). Results. Surface roughness increased significantly (p<0.01) after shot peening treatment from R. a, annealed. = 0.24 μm (±0.09 μm) to R. a, peened. = 2.02 μm (±0.16μm). Residual stresses have been introduced during shot peening up to a depth of 200μm with a maximum of 870 MPa at the surface (Fig. 2, left). All specimens showed clear signs of fatigue fracture after failure. Regarding fatigue strength, no differences have been observed between testing in saline solution or a dry environment (Fig. 2, right). Discussion. Shot peening has shown to significantly increase fatigue strength of a Ti6Al4V alloy after testing up to 10 × 10. 6. cycles. Thus, it seems to be an appropriate treatment for highly loaded components in cementless joint arthroplasty. In this context, a corrosive environment around a cyclically loaded implant does not seem to have any influence on their long term mechanical behaviour. However, it still needs to be clarified to which extend shot peening might decrease the risk of an early implant failure due to micro-motion between assembled parts (fretting) [2]