Objectives. Screw plugs have been reported to increase the fatigue strength of stainless steel locking plates. The objective of this study was to examine and compare this effect between stainless steel and titanium locking plates. Methods. Custom-designed locking plates with identical structures were fabricated from stainless steel and a titanium alloy. Three types of plates were compared: type I unplugged plates; type II plugged plates with a 4 Nm torque; and type III plugged plates with a 12 Nm torque. The stiffness, yield strength, and fatigue strength of the plates were investigated through a four-point bending test. Failure analyses were performed subsequently. Results. For stainless steel, type II and type III plates had significantly higher fatigue strength than type I plates. For titanium, there were no significant differences between the fatigue strengths of the three types of plates. Failure analyses showed local plastic deformations at the threads of screw plugs in type II and type III stainless steel plates but not in titanium plates. Conclusion. The screw plugs could increase the fatigue strength of stainless steel plates but not of titanium plates. Therefore, leaving screw holes open around fracture sites is recommended in titanium plates. Cite this article: L-W. Hung, C-K. Chao, J-R. Huang, J. Lin. Screw head plugs increase the fatigue strength of stainless steel, but not of titanium, locking plates. Bone Joint Res 2018;7:629–635. DOI: 10.1302/2046-3758.712.BJR-2018-0083.R1

Introduction. 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. Material and Methods. 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. Results. The control rods failed at the pushers of the setup (median: 94,550, range: 194,000 cycles). The rods with flashover failed directly at the HAZ significantly earlier than the control rods (p = 0.018). The analysis of the microstructure showed a transformation of the equiaxed α+β microstructure to a bimodal state. The size of the HAZs were equal for the dry (median: 1.51 mm. 2. , range: 5.68 mm. 2. ) and wet flashovers (median: 0.92 mm. 2. , range: 2.50 mm. 2. , p = 0.792). The cycles to failure were smaller for the dry flashover (median: 22,650 cycles, range: 5,700) than the wet flashover but not reaching statistical significance (median: 32,200, range: 57,900; p = 0.052). No correlation between the dimension of the HAZs and the cycles to failure was found (dry: r. 2. = 0.019, p = 0.8; wet: r. 2. = 0.015, p = 0.721). Discussion. Flashovers induced by an electrocautery device reduce the fatigue strength of Ti-6Al-4V. Since no correlation between the size of the HAZs and the cycles to failure was found, every contact between electrocautery devices and metal implants should be avoided. For any figures or tables, please contact authors directly

Introduction. Laser marking of implants surfaces is necessary in order to provide traceability during revisions which will help identify product problems more quickly, better execute product recalls and improve patient safety. There are several methods of marking employed within the medical field such as chemical etching, electro pencil marking, mechanical imprinting, casting of markings, marking with vibratory type contact, ink jet, hot foil and screen printing. However, these methods have various drawbacks including marking durability or addition of potentially toxic chemical compounds. As a result laser marking has become the preferred identification process for orthopedic implants. Laser marking is known for its high visual quality, good reproducibility and precision. However there are concerns about the laser marking potential to affect fatigue life of a device. There is a limited number of research papers that studied the effect of laser marking on fatigue life of implants. The objective of the current study is to investigate the effects of laser marking on the fatigue life of titanium alloy material. Material and Methods. Two groups of four point bend specimens were used to investigate the effect of laser marking on the fatigue life. The first group comprised of the specimens without laser marking while the second group comprised of specimens with laser marking currently utilized for the implant surfaces. Prior to conducting the fatigue testing, a non-destructive X-ray diffraction (XRD) residual stress analysis was conducted to determine if the laser marking had introduced any residual stresses. Imaging analysis was also conducted to examine any potential surface damage on the test sample's surface. A servo-hydraulic test machine was used for the fatigue four point bend testing regime where the inner and outer spans were 30 mm and 90 mm respectively. All testing was conducted at a frequency of 10 Hz, a stress ratio R=0.1, and sine-wave loading in air. Testing was stopped at 10 Million cycles or at failure of the specimen. Results & Discussion. Figure 1 shows that laser marking process can create a fine network of surface cracks. Table 1 shows the results of residual stress measurements. Laser making introduced high tensile stresses on the components whereas “as machined” component without laser marking exhibited compressive stresses inherent due to machining. The result from the S-N curve testing is shown in Figure 2. The current laser marking components demonstrated 41% reduction in fatigue strength compared to non-laser marked specimens. The reduction in fatigue strength is due to the residual tensile stresses generated at the laser marking location which can lead to crack propagation from small micro fractures created during the surface melting process. Conclusion. This study has shown conclusively that laser marking of implants if located at high stress regions can lead to early fatigue failure. Based on the results from the study it is advisable to locate the laser markings at the region of lowest or compressive stress areas and when possible the laser marking process should be selected as to create the minimal damage to the surface. For figures/tables, please contact authors directly.

Introduction: Existing fatigue studies of ACL fixation have two disadvantages. There is no agreed standard protocol, making comparison of various studies difficult and average results are presented, disregarding data spread. This may be over-optimistic, because approximately half the fixations will not achieve the average level. The effect of data spread can be summarised using the one-sided 80/80 lower tolerance limit (LTL). This LTL indicates the strength that at least 80% of fixations will reach, with an 80% probability. It is commonly used in engineering.

We fatigue-tested a new resorbable composite screw (PLLA/tri-calcium phosphate) and a metal interference screw. We present average data and tolerance limits.

Methods: Porcine BPTB grafts (Ø=9mm) were fixed inside tibial tunnels (Ø=10mm) using composite or metal screws. Each screw was tested for static pull-out strength (n=6) and cyclic loading to failure at 330N and 415N (n=5 each level)

Means and standard deviations of pullout strength were compared. Log-log curves were fitted between force level and cycles to failure. LTLs were calculated.

Results: During static loading, all repairs failed by graft pullout or tissue failure. During cyclic loading, all except one graft fixed with composite screws failed by pullout. Grafts fixed with metal screws failed by bone fracture in 60% of the cases. A composite screw loaded at 300N would last on average 272 cycles or at least (LTL) 7 cycles. At 200N the average and LTL were 38,218 and 966 cycles. Corresponding values for the metal screw were 263 (mean) and 12 (LTL) at 300N; and 12,454 and 564 at 200N.

Discussion and Conclusions: Repairs with metal screw had higher pullout strength, but proved more prone to fatigue. Higher incidence of bone graft fracture in fatigue testing with metal screws suggests that their sharp threads act as stress risers. Fatigue testing of ACL reconstructions shows wide variation, due to several factors. Average levels are therefore over-optimistic and tolerance limits gives a better indication of screw performance. We suggest that tolerance limits should be reported in future studies.

Aims

The risk of mechanical failure of modular revision hip stems is frequently mentioned in the literature, but little is currently known about the actual clinical failure rates of this type of prosthesis. The current retrospective long-term analysis examines the distal and modular failure patterns of the Prevision hip stem from 18 years of clinical use. A design improvement of the modular taper was introduced in 2008, and the data could also be used to compare the original and the current design of the modular connection.

Revision surgeries for orthopaedic infections are done in two stages – one surgery to implant an antibiotic spacer to clear the infection and another to install a permanent implant. A permanent porous implant, that can be loaded with antibiotics and allow for single-stage revision surgery, will benefit patients and save healthcare resources. Gyroid structures can be constructed with high porosity, without stress concentrations that can develop in other period porous structures [1] [2]. The purpose of this research is to compare the resulting bone and prosthesis stress distributions when porous versus solid stems are implanted into three proximal humeri with varying bone densities, using finite element models (FEM). Porous humeral stems were constructed in a gyroid structure at porosities of 60%, 70%, and 80% using computer-aided design (CAD) software. These CAD models were analyzed using FEM (Abaqus) to look at the stress distributions within the proximal humerus and the stem components with loads and boundary conditions representing the arm actively maintained at 120˚ of flexion. The stem was assumed to be made of titanium (Ti6Al4V). Three different bone densities were investigated, representing a healthy, an osteopenic, and an osteoporotic humerus, with an average bone shape created using a statistical shape and density model (SSDM) based on 75 cadaveric shoulders (57 males and 18 females, 73 12 years) [3]. The Young's moduli (E) of the cortical and trabecular bones were defined on an element-by-element basis, with a minimum allowable E of 15 MPa. The Von Mises stress distributions in the bone and the stems were compared between different stem scenarios for each bone density model. A preliminary analysis shows an increase in stress values at the proximal-lateral region of the humerus when using the porous stems compared to the solid stem, which becomes more prominent as bone density decreases. With the exception of a few mesh dependent singularities, all three porous stems show stress distributions below the fatigue strength of Ti-6Al-4V (410 MPa) for this loading scenario when employed in the osteopenic and osteoporotic humeri [4]. The 80% porosity stem had a single strut exceeding the fatigue strength when employed in the healthy bone. The results of this study indicate that the more compliant nature of the porous stem geometries may allow for better load transmission through the proximal humeral bone, better matching the stress distributions of the intact bone and possibly mitigating stress-shielding effects. Importantly, this study also indicates that these porous stems have adequate strength for long-term use, as none were predicted to have catastrophic failure under the physiologically-relevant loads. Although these results are limited to a single boney geometry, it is based on the average shape of 75 shoulders and different bone densities are considered. Future work could leverage the shape model for probabilistic models that could explore the effect of stem porosity across a broader population. The development of these models are instrumental in determining if these structures are a viable solution to combatting orthopaedic implant infections

Introduction. The fatigue strength of ultrahigh molecular weight polyethylene (UHMWPE) in total joint implants is crucial to its long term success in high demand applications, such as in the knee, and is typically determined by measuring the crack propagation resistance in razor-notched specimens under cyclic load [1]. This only tells part of the story: that is, how well the material resists crack propagation once a crack is present. A second, equally important component of fatigue strength is how well the material resists crack formation. Previous studies cyclically loaded a cantilevered post until failure [2], postulating that the post would break very quickly after crack initiation. Parran et. al. proposed a novel method to measure the crack initiation time by holding a sample in constant tension until a crack was visually observed [3]. We hypothesize that the crack initiation times of various UHMWPEs will follow similar trends as the more omnipresent crack propagation resistance tests. Materials and Methods. The following UHMWPE formulations were tested: (i) virgin, (ii) gamma sterilized in vacuum, (iii) 91 kGy gamma irradiated, and (iv) 91 kGy gamma irradiated and subsequently melted. GUR1020 and GUR1050 bar stock of varying irradiation doses were machined into compact tension specimens [4] with a notch depth of 17 mm and a blunt notch root radius of 0.25 mm, mimicking a geometry of a joint replacement component. Specimens were held in constant tension until failure; 3 to 5 different loads between 1 kN and 2.25 kN (n=3 samples per load per material) were tested. A video camera was focused on the face of the notch and took a picture every 10 seconds. The photos were reviewed to manually determine the crack initiation time (Fig 1). The time it took for the sample to completely fail – that is, shear into two separate pieces – was also recorded. Results. For all materials tested, the crack initiation time (Fig 2a,b) and the time to failure (Fig 2c,d) decreased as the applied load increased. The crack initiation time increased for the gamma sterilized materials when compared to the virgin materials while the time to failure decreased. The highly crosslinked, 91 kGy materials had crack initiation times and times to failure that were less than that of the virgin material. Post irradiation melting greatly diminished the fatigue strength of the material, yielding the lowest crack initiation time and time to failure. Discussion. The test yielded results consistent with current knowledge: that is, high-dose irradiation yields a slight drop in fatigue strength, and post-irradiation melting greatly reduces strength. This test was simple to set up and run and can be a good tool to determine the relative fatigue strengths of UHMWPE formulations for orthopaedic applications. For figures/tables, please contact authors directly.

Fracture of contemporary femoral stems is a rare occurrence. Earlier THR stems failed due to design issues or post manufacturing heat treatments that weakened the core metal. Our group identified and analyzed 4 contemporary fractured femoral stems after revision surgery in which electrochemical welds contributed to the failure. All four stems were proximally porous coated titanium alloy components. All failures occurred in the neck region post revision surgery in an acetabular cup exchange. All were men and obese. The fractures occurred at an average of 3.6 years post THR redo (range, 1.0–6.5 years) and 8.3 years post index surgery (range, 5.5–12.0 years). To demonstrate the effect of electrocautery on retained femoral stems following revision surgery, we applied intermittent electrosurgical currents at three intensities (30, 60, 90 watts) to the polished neck surface of a titanium alloy stem under dry conditions. At all power settings, visible discoloration and damage to the polished neck surface was observed. The localized patterns and altered metal surface features exhibited were like the electrosurgically-induced damage priorly reported. The neck regions of all components studied displayed extensive mechanical and/or electrocautery damage in the area of fracture initiation. The use of mechanical instruments and electrocautery was documented to remove tissues in all 4 cases. The combination of mechanical and electrocautery damage to the femoral neck and stem served as an initiation point and stress riser for subsequent fractures. The electrocautery and mechanical damage across the fracture site observed occurred iatrogenically during revision surgery. The notch effect, particularly in titanium alloys, due to mechanical and/or electrocautery damage, further reduced the fatigue strength at the fractured femoral necks. While electrocautery and mechanical dissection is often required during revision THA, these failures highlight the need for caution during this step of the procedure in cases where the femoral stem is retained

Although 3D-printed porous dental implants may possess improved osseointegration potential, they must exhibit appropriate fatigue strength. Finite element analysis (FEA) has the potential to predict the fatigue life of implants and accelerate their development. This work aimed at developing and validating an FEA-based tool to predict the fatigue behavior of porous dental implants. Test samples mimicking dental implants were designed as 4.5 mm-diameter cylinders with a fully porous section around bone level. Three porosity levels (50%, 60% and 70%) and two unit cell types (Schwarz Primitive (SP) and Schwarz W (SW)) were combined to generate six designs that were split between calibration (60SP, 70SP, 60SW, 70SW) and validation (50SP, 50SW) sets. Twenty-eight samples per design were additively manufactured from titanium powder (Ti6Al4V). The samples were tested under bending compression loading (ISO 14801) monotonically (N=4/design) to determine ultimate load (F. ult. ) (Instron 5866) and cyclically at six load levels between 50% and 10% of F. ult. (N=4/design/load level) (DYNA5dent). Failure force results were fitted to F/F. ult. = a(N. f. ). b. (Eq1) with N. f. being the number of cycles to failure, to identify parameters a and b. The endurance limit (F. e. ) was evaluated at N. f. = 5M cycles. Finite element models were built to predict the yield load (F. yield. ) of each design. Combining a linear correlation between FEA-based F. yield. and experimental F. ult. with equation Eq1 enabled FEA-based prediction of F. e. . For all designs, F. e. was comprised between 10% (all four samples surviving) and 15% (at least one failure) of F. ult. The FEA-based tool predicted F. e. values of 11.7% and 12.0% of F. ult. for the validation sets of 50SP and 50SW, respectively. Thus, the developed FEA-based workflow could accurately predict endurance limit for different implant designs and therefore could be used in future to aid the development of novel porous implants. Acknowledgements: This study was funded by EU's Horizon 2020 grant No. 953128 (I-SMarD). We gratefully acknowledge the expert advice of Prof. Philippe Zysset

Summary. 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. Introduction. 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 in vitro fatigue strength of spinal specimens from donors of working age. Patients & Methods. Lumbar functional spinal units (L2/L3 and L4/L5) from midlife donors (45–65 yrs, n = 24) and young donors (20–45 yrs, n = 6) were collected and stored deep frozen. CT scans were obtained to determine the endplate area and the bone mineral density of the vertebrae. Their product is referred to as vertebral capacity (VC). Muscles were removed from the thawed specimens, but apart from the transversal ligaments, all ligaments and the intervertebral disc were left intact. During the experiments, the specimens were immersed in saline solution (37°C) containing antibiotics (PAA, Austria) to reduce biological degeneration. After preconditioning (2.5 h) the specimens were exposed to continuous sinusoidal axial compression (5Hz, <300,000 cycles). Distinct changes in the characteristic creep curve of specimens’ height indicated fatigue failure. Specimens of midlife donors were equally assigned to three groups with different peak-to-peak loads (NORM: 0–2 kN; HIGH: 0–3 kN; OFFSET: 1–3 kN), while specimens from young donors were solely assigned to the HIGH group, since a previous study [1] had shown that young specimens hardly failed for NORM loading conditions. Findings from that previous study (midlife, n = 6; young, n = 6) were merged to NORM for analyses. Results. Within the NORM group, specimens only failed within 300,000 cycles when VC was below 2,000 cm. 2. mg K. 2. HPO. 4. /ml (8 of 20). Within the HIGH group, endplate failure occurred frequently within the test duration (10 of 13; 1 excluded). For the OFFSET group, specimen failure was occasionally observed (4 of 7; 1 excluded). Exponential regression of cycles to failure dependent on VC showed significant correlations for the specimen loaded in the NORM and HIGH group (r. 2. NORM. = 0.57, p = 0.029; r. 2. HIGH. = 0.47, p = 0.029; r. 2. OFFSET. = 0.83, p = 0.091). Discussion/Conclusion. Specimens’ fatigue failure strength depends on load offset and amplitude. The group with higher loading amplitudes (HIGH: 1.5 kN) resisted fewer loading cycles than those with the smaller amplitude (OFFSET: 1 kN), even though the maximum peak was the same (3 kN). The exponential regression is conservative, since several specimens did not fail within the predicted loading cycles. Vertebral capacity might suitable predict the fatigue strength of specimens. Together with numerical modelling, these findings might promote the appraisal of occupational diseases and might help to determine the duty cycles for new implants. The funding of FIOSH, Germany is thankfully acknowledged (project F2059 and F2069)

Introduction. Porous scaffolds for bone ingrowth have numerous applications, including correcting deformities in the foot and ankle. Various materials and shapes may be selected for bridging an osteotomy in a corrective procedure. This research explores the performance of commercially pure Titanium (CPTi) and Tantalum (Ta) porous scaffold materials for use in foot and ankle applications under simplified compression loading. Methods. Finite element analysis was performed to evaluate von Mises stress in 3 porous implant designs: 1) a CPTi foot and ankle implant (Fig 1) 2) a similar Ta implant (wedge angle = 5°) and 3) a similar Ta implant with an increased wedge angle of 20°. Properties were assigned per reported material and density specifications. Clinically relevant axial compressive load of 2.5X BW (2154 N) was applied through fixtures which conform to ASTM F2077–11. Compressive yield and fatigue strength was evaluated per ASTM F2077–11 to compare CPTi performance in design 1 to the Ta performance of design 3. Results. FEA results indicate peak stresses at fixture contact locations. Similar designs (CPTi design 1 and Ta design 2) resulted in similar von Mises stresses (Fig 1). Increasing the wedge angle (Ta design 3) increased stress by 15%. The static compressive yield strength of CPTi design 1 (20,560 N) was similar to the Ta design 3 (20,902 N), with yield manifesting as barreling and crushing of the components (Fig 2a). However, the fatigue strength of CPTi (6,000 N) was 40% lower than the Ta design 3 (9,500 N) (Fig 3). In both cases fracture initiated from regions of highest stress predicted in FEA. Fracture progression was not instantaneous and was characterized by an accumulation of damage (Fig 2b–c) leading to gross component fracture and loss of implant integrity. Discussion. FEA is a useful tool to determine stress variations and can be used to identify worst case within a material: in this case, a larger implant wedge angle leads to higher stresses. Additionally, FEA accurately predicted fracture initiation location. However, material selection plays a large role in porous implant performance: although FEA predicted higher stresses in a Ta component with a greater wedge angle than a similar sized CPTi component, static compressive strengths were nearly identical, and the Ta component had 58% higher fatigue strength. When selecting a material or geometry for an implant application, both FEA and static testing allow for rapid evaluation of designs. However, caution should be used in interpreting the results: the ultimate performance of an implant in-vivo will depend on its ability to maintain integrity over a long period of time, and should be characterized by dynamic testing

BACKGROUND. This scientific work is a non-interventional, experimental and prospective comparative study of two very high-viscosity PMMA bone cements: DePuy CMW 2G and Palacos® fast R+G. Reference product: Palacos® R+G. Fast-setting PMMA bone cements are used in the endoprothetics of the patella and knee (in Australia) and are also used to cement an artificial acetabulum (in the UK). Are there any differences regarding the characteristics of the two fast-setting PMMA bone cements?. MATERIALS AND METHODS. All cements were mixed as specified by the manufacturer and analysed on the following parameters: handling properties (mixing, waiting, working and hardening phase), powder/liquid-ratio, mechanical properties (ISO 5833:2002 and DIN 53435), fatigue strength (ISO 16402) and elution profile. All tests were done in an acclimatised laboratory with temperatures set at 23.5°C ± 0.5°C and a humidity of >40%. Of two batch numbers, 11 units of each bone cement were tested. RESULTS AND DISCUSSION. The handling properties of the two tested PMMA bone cements Palacos® fast R+G and CMW 2G are highly similar (n=12). CMW 2G reaches the mixing and waiting phase approximately 20s later than Palacos® fast R+G. Palacos® fast R+G has a similar working, but a shorter hardening phase than CMW 2G. In addition, working with Palacos® fast R+G was advantageous due to its green dye. Palacos® fast R+G has a higher powder/liquid-ratio of 2.550. Due to the higher powder percentage, the cement has a shorter mixing and waiting phase than CMW 2G with a ratio of 2:1. Both analysed bone cements fulfil the quasi-static properties of ISO 5833:2002 and DIN 53435. Palacos® fast R+G was far superior in its ISO compressive strength (MPa) shown through one-way analysis of variance (ANOVA) (p<0.01) and independent two sample t-test (p<0.01) at 0.05 level of significance (n=20)(Fig. 1). CMW 2G has a higher quasi-static ISO bending strength (MPa) than Palacos® fast R+G, but the same test shows a much higher fatigue strength (ISO 16402) of Palacos® fast R+G (n=5) (Fig. 2). Palacos® R+G and Palacos® fast R+G show a similar elution profile (n=3), whereas CMW 2G shows a much lower antibiotic elution over time. CMW 2G releases approximately 1/3 of gentamicin per mould body after 24h. After day 3 and 5, CMW 2G has a significantly lower gentamicin release than Palacos® fast R+G (Fig. 3). Palacos® fast R+G has a higher gentamicin release, due to its hydrophilic polymer basis, which is identical to Palacos® R+G. CMW 2G contains pure PMMA and is therefore more hydrophobic than the other two tested cements. CONCLUSION. Handling with Palacos® fast R+G was advantageous due to its green dye. Because of the shorter handling phases of Palacos® fast R+G, it is superior as it minimises the length of surgeries. Mechanical properties according (ISO 5833:2002 and DIN 53435) were comparable. Palacos® fast R+G has a statistically significant higher ISO compressive strength (MPa). Palacos® fast R+G also showed higher fatigue strength (ISO 16402). Palacos® fast R+G was far superior in matters of gentamicin release over time

Objectives: The purpose of this biomechanical study was to compare the mechanical properties of locked nails and screws made from either stainless steel or titanium alloy. Methods: The specially designed locked nails and screws with the same structures were made from either stainless steel or titanium alloy. The structural factors investigated included inner diameter and root radius for locking screws and outer diameter and nail hole size for locked nails. The mechanical properties investigated included bending stiffness, strength, and fatigue life. Finite element models were used to simulate the mechanical tests and compute the stress concentration factors. Results: Increasing the root radius and the inner diameter could effectively increase the fatigue strength of the locking screws. Fatigue strength increased more in titanium than in stainless steel screws, especially when the inner diameter was increased. In contrast, the titanium locked nails were much weaker than their stainless steel counterparts. Finite element models could closely predict the results of the biomechanical tests with a correlation coefficient that ranged from −0.58 to −0.84 for screws and was −0.98 for nails. The stress concentration factors ranged from 1 to 1.81 for screws and from 3.06 to 4.17 for nails. Conclusions: With larger root radius and inner diameter, titanium locking screws could provide much stronger fatigue strength than stainless steel counterparts. However, titanium locked nails might lose their advantages of superior mechanical strength because of high notch sensitivity and this limitation should be a critical concern clinically. Finite element analyses could be reliably used in research and development of locked nails and locking screws

Objectives. Opening wedge high tibial osteotomy (HTO) is an established surgical procedure for the treatment of early-stage knee arthritis. Other than infection, the majority of complications are related to mechanical factors – in particular, stimulation of healing at the osteotomy site. This study used finite element (FE) analysis to investigate the effect of plate design and bridging span on interfragmentary movement (IFM) and the influence of fracture healing on plate stress and potential failure. Materials and Methods. A 10° opening wedge HTO was created in a composite tibia. Imaging and strain gauge data were used to create and validate FE models. Models of an intact tibia and a tibia implanted with a custom HTO plate using two different bridging spans were validated against experimental data. Physiological muscle forces and different stages of osteotomy gap healing simulating up to six weeks postoperatively were then incorporated. Predictions of plate stress and IFM for the custom plate were compared against predictions for an industry standard plate (TomoFix). Results. For both plate types, long spans increased IFM but did not substantially alter peak plate stress. The custom plate increased axial and shear IFM values by up to 24% and 47%, respectively, compared with the TomoFix. In all cases, a callus stiffness of 528 MPa was required to reduce plate stress below the fatigue strength of titanium alloy. Conclusion. We demonstrate that larger bridging spans in opening wedge HTO increase IFM without substantially increasing plate stress. The results indicate, however, that callus healing is required to prevent fatigue failure. Cite this article: A. R. MacLeod, G. Serrancoli, B. J. Fregly, A. D. Toms, H. S. Gill. The effect of plate design, bridging span, and fracture healing on the performance of high tibial osteotomy plates: An experimental and finite element study. Bone Joint Res 2018;7:639–649. DOI: 10.1302/2046-3758.712.BJR-2018-0035.R1

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]

Purpose: To perform a biomechanical comparison between an older established and a recently introduced technique, used in suturing semitendinosus quadrupled grafts. Methods: Flexor tendons were harvested from pigs giving a tendon of similar dimensions to semitendinosus. Specimens were prepared using an older established suturing method utilising a Bunnel ‘whip’ stitch (group A, 21 specimens), and a recently introduced(. 1. ) method where the tendon is sutured back on itself having an overlap of either 20mm or 30mm and forming a closed loop (group B, 40 specimens). In group A, a tibial fixation button was used and grafts were prepared as to have a common representative overall length. Consideration was given in mounting either end of these grafts in representative conditions. The lengths of Group B specimens were of comparable dimensions to group A, but were mounted by using custom-made grips incorporating roller bars. Tests were performed in a Dartec servohydraulic materials testing machine in fatigue and in single loading at various strain rates and by using physiological loading patterns and in physiological ambient conditions. Results: Group A specimens failed in a small load range of 200–250N and at the whipstitch, which snapped at the knot tied around the tibial button. Group B specimens failed either in the overlap region (for the shorter overlaps) or in mid-tendon substance (for longer overlaps). In general group A showed low fatigue strength and high unpredictability in its fatigue lifetime. Group B showed nearly 3 times as high fatigue strength and consistent predictable results throughout the range of loads used (200–600N). Conclusion: The new technique for suturing quadrupled semitendinosus grafts has been evaluated in tests under more physiological loading and ambient conditions. The technique significantly improves the fatigue life of the graft and should permit the goal of a more aggressive rehabilitation programme

Background: Paediatric spinal systems made from stainless steel are effective at correcting early onset scoliosis in a non-fusion technique. The use of similar systems manufactured from titanium is an attractive alternative as it would allow Magnetic Resonance Imaging of the patient with its recognised imaging advantages. Objective: We performed a prospective in vitro study to compare the mechanical performance of a current clinically used stainless steel construct with an identical proposed titanium alternative. Methods: Twelve spinal constructs of each material were constructed in a typical in vivo configuration using a corpectomy model in accordance with ASTM F1798 standard. Five samples for each metal were subjected to axial compressive static loading at a rate of 1mm/s until plastically deformed. Seven samples for each metal were then subject to varying compressive cyclic loads until a 5 million cycle run out was observed. From this data a fatigue S-N curve was generated. Results: The stiffness of each construct was then calculated and the results were statistically analysed. For steel and titanium we calculated 95% confidence intervals of 23.9 to 35.7 and 18.8 to 23.7 respectfully. Significance P(< 0.05). The fatigue strength to 5 million cycles was 179N and 150N for steel and titanium respectfully. Failure occured most commonly in the rods close to the transverse rod connector or the pedicle screw / polyethylene block interface. Conclusions: We conclude that with identical dimensions, the stainless steel constructs had a significantly higher Modulus of Elasticity than titanium. The fatigue strength for steel was also higher than titanium. The potential use of titanium as an alternative to stainless steel in paediatric spinal systems is still an attractive alternative. Given the results, we would suggest that further re-designing and testing be carried out before clinical release and then initially be reserved for selected patients with lower body weight or physical demands

Modular hip prostheses were introduced to optimize the intra-surgical adaptation of the implant design to the native anatomy 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. For testing hip implants with proximal femoral modularity according to ISO & ASTM, sodium chloride solutions are frequently used to determine the fatigue strength and durability of the stem-neck connection. The present study illustrate that the expansion of standard requirements of biomechanical testing is necessary to simulate metal ion release as well as fretting and crevice corrosion by using alternative test fluids. To assess the primary stability of tibial plateaus in vitro, different approaches had been undergone: cement penetration depth analysis, static tension or compression loading until interface failure. However, these test conditions do not reflect the in vivo physiologic loading modes, where the tibial plateau is predominantly subjected to combined compression and shear forces. The objectives were to evaluate the impact of the tibial keel & stem length on the primary stability of a posterior-stabilised tibial plateau under dynamic compression-shear loading conditions in human tibiae

Aims: The main objective of the research was to investigate alternative processes, respect to hydroxylapatite plasma spray coating, in order to obtain metallic bio-materials presenting good osteointegration ability. An innovative process consisting of mechanical and thermochemical treatments was tested and a surface and mechanical characterization performed on treated samples. Methods: The material investigated was the Ti-6Al-7Nb alloy. The surface modification process consists of grit blasting, passivation, alkali etching and thermal treatment performed in air or in vacuum. Crystallographic structure was investigated by XRD and TEM. Surface morphology and composition were assessed by SEM, EDS and AES analysis. Bioactivity was tested by soaking in standard SBF solution. Metal ion release measurements were performed by GFAA-ICP technique on withdrawn solution after soaking samples in SBF. Scratch and fatigue tests were performed as mechanical characterization of the material. Results: The alkali etching strongly modifies the surface morphology of titanium and its alloys producing a microporous layer and a drastic increment in surface wettability. The use of previous passivation treatment modifies the surface crystallographic structure, forms a graded interface between the surface and the substrate, enhances the surface layer adhesion and scratch resistance, increases the corrosion resistance of the material and causes a low metal ion release. The use of a vacuum atmosphere during heat treatment inhibits rutile formation and scratch tests evidenced low damage on it. During soaking in SBF the formation of a reaction layer and of precipitated crystals containing Ca and P was detected on the treated samples. The precipitate morphology resembles that of apatite. The fatigue strength was 260 MPa for the treated series, while it was 460 MPa in the case of the grit blasted series without any additional treatment and therefore significantly higher. Conclusions: It can be concluded that the surface of treated samples shows chemical, structural and morphological modifications. The passivation pre-treatment causes the formation of different crystallographic phases and of a smoother interface with the substrate. The treated samples evidenced a quite low metal ion release and interacted with SBF solution, showing a moderate bioactivity. The disadvantage of this process is the decrease in fatigue strength. This aspect suggests that when surface etching and modifications are performed with the aim of enhancing metal osteointegration ability, a careful investigation of their influence on the fatigue resistance must be performed

Introduction. Today TKR is considered one of the most successful operative procedures in orthopedic surgery. Nevertheless, failure rates of 2 – 10% depending on the length of the study and the design are still reported. This provides evidence for further development in knee arthroplasty. Particularly the oxide ceramics used now in THA show major advantages due to their excellent tribological properties, their significantly reduced third-body wear as well as their high corrosion resistance. A further advantage of ceramic materials is their potential use in patients with metal allergy. Metallic wear induces immunological reactions resulting in hypersensitivity, pain, osteolysis and implant loosening. The purpose of our study was to examine the safety of the tibial component of a novel all-ceramic TKR. Materials and Methods. We tested the tibial components of the primary knee implant BPK-S Integration Ceramic. Both the tibial and the femoral component consist of BIOLOX®delta ceramic The standards ISO 14879-1 and ASTM F1800-07 describe the test set-up for the experimental fatigue strength testing of tibial components from knee implants. We conducted the testing with a significantly increased maximum load of 5,300 N (900 N are required). A final burst strength test was carried out after the fatigue load testing in the same embedding and with the same test set-up. Results. No specimen failed during fatigue load testing. The subsequent post-fatigue burst strength testing showed a maximum strength against fracture of at least 9.7 kN for size 3 and at least 12.1 kN for size 6. Discussion. The good results of the strength testing of the tibial component of the BPK-S Integration Ceramic tibial plateau supported the good initial clinical outcome without any implant specific complications of this knee design. Further clinical studies have to show if this design fulfills the high expectations over long periods of time