Finite Element Simulation and Sensitivity Analysis of the Cohesive Parameters for Delamination Modeling in Power Electronics Packages.

Delamination is a critical failure mode in power electronics packages that can significantly impact their reliability and performance, due to the large amounts of electrical power managed by the most recent devices which induce remarkable thermomechanical loads. The finite element (FE) simulation of this phenomenon is very challenging for the identification of the appropriate modeling tools and their subsequent calibration. In this study, we present an advanced FE modeling approach for delamination, together with fundamental guidelines to calibrate it. Considering a reference power electronics package subjected to thermomechanical loads, FE simulations with a global-local approach are proposed, also including the implementation of a bi-linear cohesive zone model (CZM) to simulate the complex interfacial behavior between the different layers of the package. A parametric study and sensitivity analysis is presented, exploring the effects of individual CZM variables on the delamination behavior, identifying the most crucial ones and accurately describing their underlying functioning. Then, this work gives valuable insights and guidelines related to advanced and aware FE simulations of delamination in power electronics packages, useful for the design and optimization of these devices to mitigate their vulnerability to thermomechanical loads.

Study of the Electron Beam Melting Process Parameters' Influence on the Tensile Behavior of 3D Printed Ti6Al4V ELI Alloy in Static and Dynamic Conditions.

The Ti6Al4V alloy is widely adopted in many high-end applications in different fields, including the aerospace, biomechanics, and automotive sectors. Additive manufacturing extends its range of possible applications but also introduces variations in its mechanical performance, depending on the whole manufacturing process and the related control parameters. This work focuses on the detailed tensile stress-strain characterization at low and high strain rates of a Grade 23 Ti alloy manufactured by electron beam melting (EBM). In particular, the main aim is to study the effect of the variation of the EBM process parameters on the performance of the material and their consequent optimization in order to obtain the best printed material in terms of ductility and strength. The adopted optical experimental setups allow the semi-local scale analysis of the neck section which makes possible the accurate estimation of stress, strain, and strain rate, all over the post-necking range and up to the very incipient specimen failure. Among the EBM printing process parameters, the speed function was previously identified as the one mainly affecting the material performance at static rates. Therefore, two different parameter sets, corresponding to the standard value and to an optimized value of the speed function parameter, respectively, are tested here at dynamic rates of 1, 15, and 700 s-1, for assessing the effect of the speed function on the dynamic material response. The results show that the optimized parameter set has a better performance compared to the standard one in terms of strength and ductility. In particular, in both static and dynamic conditions, it presents an increase of the true stress-strain curve (about 5% on average) and an increase of the failure strain (about 11% on average). Moreover, in respect to the standard parameter set, the optimized one is also characterized by a huge increase of the amplification due to the strain rate (about 49% on average for the considered strain rates).

Cyclic fatigue and torsional resistance evaluation of Reciproc R25 instruments after simulated clinical use.

BACKGROUND: To assess the influence of multiple uses on the cyclic flexural fatigue and torsional resistance of Reciproc R25 (REC25) instruments. METHODS: Overall, 256 canals (two for each mandibular molar) were prepared using Reciproc R25 instruments. A total of 96 25-mm-long Reciproc R25 instruments were divided in six groups (N.=8) on the basis of different number of canals shaped: new instruments (control group), one, two, three, four, or six canals shaped during simulated clinical use for other groups respectively. Sixteen files were used for each of the six groups (eight instruments for cyclic fatigue test and eight ones for torsional test). Times to fracture (TtF) for cyclic fatigue and ultimate torsional strength and angle of rotation to fracture for torsional resistance were recorded. Data were statistically evaluated by the analysis of variance (ANOVA) with P<0.05. RESULTS: As for cyclic fatigue resistance, there was no significant difference between new REC25 and REC25 used in one, two and three canals respectively (P>0.05). REC25 used in four and six canals showed significant lower times to fracture when compared with other groups (P<0.05) with REC25 used in six canals showing the significant lowest times to fracture (P<0.05). Considering torsional resistance parameters, no significant difference was observed between the new and used instruments (P>0.05). CONCLUSIONS: REC25 files showed a significant decrease in cyclic fatigue resistance only when used in four or more molar canals while their torsional behavior was not affected by multiple uses.

Cutting efficiency of heat-treated nickel-titanium single-file systems at different incidence angles.

Cutting efficiency of Reciproc R25 (REC) and Reciproc blue R25 (REB) at different inclinations was evaluated. Sixty new files were tested at 90°, 70° and 45° of inclination in relation to the sample (n = 10), using a customised machine. All files were activated in reciprocation against standardised gypsum blocks for 120 s. Cutting efficiency was determined by measuring the block weight loss with an analytical balance and measuring the length of the block surface cut using a digital calliper. Data were statistically analysed (two-way ANOVA, Bonferroni t-test) with the significance level set at P < 0.05. There was no difference for REC among the tested angles. REB had no statistical difference between 90° and 70°; however, its cutting efficiency significantly increased at 45°. There was a significant difference between REC and REB at 45° only. Under these conditions, increased file inclination to 45° and blue heat treatment improved cutting efficiency of reciprocating files.

Cyclic fatigue resistance, torsional resistance, and metallurgical characteristics of M3 Rotary and M3 Pro Gold NiTi files.

OBJECTIVES: To evaluate the mechanical properties and metallurgical characteristics of the M3 Rotary and M3 Pro Gold files (United Dental). MATERIALS AND METHODS: One hundred and sixty new M3 Rotary and M3 Pro Gold files (sizes 20/0.04 and 25/0.04) were used. Torque and angle of rotation at failure (n = 20) were measured according to ISO 3630-1. Cyclic fatigue resistance was tested by measuring the number of cycles to failure in an artificial stainless steel canal (60° angle of curvature and a 5-mm radius). The metallurgical characteristics were investigated by differential scanning calorimetry. Data were analyzed using analysis of variance and the Student-Newman-Keuls test. RESULTS: Comparing the same size of the 2 different instruments, cyclic fatigue resistance was significantly higher in the M3 Pro Gold files than in the M3 Rotary files (p < 0.001). No significant difference was observed between the files in the maximum torque load, while a significantly higher angular rotation to fracture was observed for M3 Pro Gold (p < 0.05). In the DSC analysis, the M3 Pro Gold files showed one prominent peak on the heating curve and 2 prominent peaks on the cooling curve. In contrast, the M3 Rotary files showed 1 small peak on the heating curve and 1 small peak on the cooling curve. CONCLUSIONS: The M3 Pro Gold files showed greater flexibility and angular rotation than the M3 Rotary files, without decrement of their torque resistance. The superior flexibility of M3 Pro Gold files can be attributed to their martensite phase.

Implant Complications After Magnetically Controlled Growing Rods for Early Onset Scoliosis: A Multicenter Retrospective Review.

BACKGROUND: Traditional growing rods have a reported wound and implant complication rate as high as 58%. It is unclear whether the use of magnetically controlled growing rods (MCGR) will affect this rate. This study was performed to characterize surgical complications following MCGR in early onset scoliosis. METHODS: A multicenter retrospective review of MCGR cases was performed. Inclusion criteria were: (1) diagnosis of early onset scoliosis of any etiology; (2) 10 years and younger at time of index surgery; (3) preoperative major curve size >30 degrees; (4) preoperative thoracic spine height <22 cm. Complications were categorized as wound related and instrumentation related. Complications were also classified as early (<6 mo from index surgery) versus late (>6 mo). Distraction technique and interval of distraction was surgeon preference without standardization across sites. RESULTS: Fifty-four MCGR patients met inclusion criteria. There were 30 primary and 24 conversion procedures. Mean age at initial surgery was 7.3 years (range, 2.4 to 11 y), and mean duration of follow-up 19.4 months. Twenty-one (38.8%) of 54 patients had at least 1 complication. Fifteen (27.8%) had at least 1 revision surgery. Six (11.1%) had broken rods (2 to 4.5 and 4 to 5.5 mm rods); two 5.5 mm rods failed early (4 mo) and 4 late (mean=14.5 mo). Six (11.1%) patients experienced 1 episode of lack or loss of lengthening, of which 4 patients subsequently lengthened. Seven patients (13.0%) had either proximal or distal fixation-related complication at average of 8.4 months. Two patients (3.7%) had infections requiring incision and drainage; 1 early (2 wk) with wound drainage and 1 late (8 mo). The late case required removal of one of the dual rods. CONCLUSIONS: This study shows that compared with traditional growing rods, MCGR has a lower infection rate (3.7% vs. 11.1%). MCGR does not appear to prevent common implant-related complications such as rod or foundation failure. The long-term implication remains to be determined. LEVEL OF EVIDENCE: Level IV.

Magnetically Controlled Growing Rods for the Management of Early-onset Scoliosis: A Preliminary Report.

BACKGROUND: A new method for the management of early-onset scoliosis (EOS) has been recently introduced: it consists of a magnetically controlled growing rod (MCGR) that allows gradual outpatient distractions under control of an external remote device. The aim of the present study is to present a series of 10 patients with EOS managed with MCGR (Ellipse TM MAGEC System, Irvine, CA). METHODS: We implanted MCGR in 10 patients affected by EOS. Scoliosis and kyphosis angles, T1-T12 and T1-S1 length were evaluated preoperatively, postoperatively, and at the last follow-up. A visual analogue scale score was used to evaluate pain during outpatient rod distraction procedures. The mean follow-up is 27 months. All patients attended distractions of the magnetic rod through an external remote control every 3 months. The mean predicted distraction was 3 mm at each lengthening session. RESULTS: The mean Cobb angle value was 64.7±17.4 degrees (range, 45 to 100 degrees) preoperatively and 28.5±13.9 degrees (range, 15 to 59 degrees) at the latest follow-up. The mean T1-S1 length value was 27.1±5.4 cm (range, 16 to 34.8 cm) preoperatively and 32.8±4 cm (range, 26.5 to 39 cm) at the latest follow-up. The mean T1-T12 length value was 16.2±2.7 cm (range, 10 to 19 cm) preoperatively and 20.6±2.9 cm (range, 15.5 to 23.5 cm) at the latest follow-up.The average monthly T1-T12 height increase was 0.8 mm, whereas the average monthly T1-S1 increase was 0.9 mm.Two patients experienced a rod breakage and 1 patient had a pull-out of the apical hooks. CONCLUSIONS: Although implant-related complications could occur, as in all EOS growing rods procedures, MCGR can be effectively used in patients with EOS. This spinal instrumentation can overcome many of the complications related with the traditional growing rods implants. This procedure can be effectively used in outpatient settings, minimizing surgical scarring, surgical site infection, and psychological distress due to multiple surgeries needed in the traditional growing rods system, improving quality of life, and saving health care costs. LEVEL OF EVIDENCE: Level IV.

Torsional and Cyclic Fatigue Resistance of a New Nickel-Titanium Instrument Manufactured by Electrical Discharge Machining.

INTRODUCTION: The purpose of this study was to evaluate the torsional and cyclic fatigue resistance of the new Hyflex EDM OneFile (Coltene/Whaledent AG, Altstatten, Switzerland) manufactured by electrical discharge machining and compare the findings with the ones of Reciproc R25 (VDW, Munich, Germany) and WaveOne Primary (Dentsply Maillefer, Ballaigues, Switzerland). METHODS: One hundred-twenty new Hyflex EDM OneFile (#25/0.08), Reciproc R25, and WaveOne Primary files were used. Torque and angle of rotation at failure of new instruments (n = 20) were measured according to ISO 3630-1 for each brand. Cyclic fatigue resistance was tested measuring the number of cycles to failure in an artificial stainless steel canal with a 60° angle and a 3-mm radius of curvature. Data were analyzed using the analysis of variance test and the Student-Newman-Keuls test for multiple comparisons. The fracture surface of each fragment was examined with a scanning electron microscope. RESULTS: The cyclic fatigue of Hyflex EDM was significantly higher than the one of Reciproc R25 and WaveOne Primary (P < .05 and P < .001, respectively). Hyflex EDM showed a lower maximum torque load (P < .05) but a significantly higher angular rotation (P < .0001) to fracture than Reciproc R25 and WaveOne Primary. No significant difference was found comparing the maximum torque load, angular rotation, and cyclic fatigue of Reciproc R25 and WaveOne Primary (P > .05). CONCLUSIONS: The new Hyflex EDM instruments (controlled memory wire) have higher cyclic fatigue resistance and angle of rotation to fracture but lower torque to failure than Reciproc R25 and WaveOne Primary files (M-wire for both files).

The Universal Clamp hybrid system: a safe technique to correct deformity and restore kyphosis in adolescent idiopathic scoliosis.

PURPOSE: Adolescent idiopathic scoliosis (AIS) is a tridimensional deformity characterized by coronal and sagittal profiles changes. We present a series of 62 patients affected by AIS and treated by thoracic Universal Clamps and transpedicular lumbar screws hybrid system. METHODS: Mean age was 13 years. Average pre-operative Cobb angle was 63° ± 12°. Patients were divided into two groups depending on the kyphosis angle: lower than 45° (51 patients, 82.3 %, mean 21° ± 3°) and higher than 45° (11 patients, 17.7 %, mean 62° ± 6°). RESULTS: The average percentage of coronal correction was 70 ± 3 % (mean post-operative Cobb angle 19° ± 4°, P < 0.001). In patients with pre-operative physiological thoracic kyphosis-hypokyphosis, we observed an increase in the average value (32° ± 4°, P < 0.001), while in patients with pre-operative hyperkyphosis, mean decrease of thoracic kyphosis was 19° ± 3° (43° ± 4°, P < 0.001). CONCLUSIONS: This case-series study showed the efficacy and safety of Universal Clamp hybrid system in correcting coronal deformity and restoring physiological thoracic kyphosis in patients affected by AIS.

Sagittal profile control in patients affected by neurological scoliosis using Universal Clamps: a 4-year follow-up study.

PURPOSE: Patients affected by cerebral palsy often develop progressive scoliosis that can results in trunk instability with an impairment of both coronal and sagittal balance. The aim of this retrospective study was to demonstrate the ability of UC to control the sagittal profile in a consecutive series of patients affected by neurological scoliosis. METHODS: From 2006 to 2008, 84 patients (57 F, 27 M) affected by neurological scoliosis were treated surgically. Mean age was 14 years (range 10-17). The etiology was mainly cerebral palsy. The average pre-operative Cobb angle was 73° ± 16°. Patients were divided into three groups according to the pre-operative presence of: physiological kyphosis (mean 29° ± 8°), thoracic lordosis (mean 10° ± 6°) and hyperkyphosis (51° ± 8°). A posterior access was performed in all patients using thoracic UC associated with transpedicular lumbar screws and a conventional claw at the upper extremity of the construct. RESULTS: The average percentage of coronal correction was 72%. In all three groups, we observed a common trend toward maintaining or restoring the physiological values. Mean follow-up time was 36 months. At the 1-year follow-up, the mean loss of correction was 7° ± 2° in the coronal plane and 2° ± 1° in the sagittal plane with no other change thereafter. CONCLUSIONS: The hybrid construct using UC appears effective in neurological scoliosis treatment, providing a good correction of the deformity in both coronal and sagittal planes. In the present series, physiological thoracic kyphosis has been restored in all patients, providing better sitting tolerance in wheelchair-bound patients, and retaining standing and walking abilities in ambulatory patients.

Surgical treatment of neurological scoliosis using hybrid construct (lumbar transpedicular screws plus thoracic sublaminar acrylic loops).

In the nineties, most spinal surgeons supported the validity of segmental spine instrumentation, but this procedure has progressively been abandoned because difficult and with a high risk of neurological complications, in favor of the Cotrel-Dobousset (CD). The CD instrumentation is based on segmentation of curves, thus improving the angular correction and actuates sagittal profile. Sublaminar acrylic loops (Universal Clamp) shows the same resistance to stress as steel or titanium alloy sublaminar wires. The simple procedure and the tensioning of the strips allows re-tensioning and progressive correction. The increased contact area, improves corrective forces, thus reducing the risk of laminar fractures. The aim of this study was to verify the validity of this spinal fixation implant in the surgical treatment of a consecutive series of patients affected by neurologic scoliosis. The authors treated surgically 84 patients affected by neurologic scoliosis with an average age of 14 years (range 10-17). Universal Clamps associated with Socore TM spinal assembly, transpedicular lumbar screws and thoracic hooks at the upper end of the curve were used. The etiology of disease was cerebral palsy in 81 cases, Friedreich ataxia in two cases and Aicardi syndrome in one case. The average preoperative angular value was 73° ± 16°. It was implanted a mean of seven Clamps for each procedure (range 5-9). The average percentage of correction was 72%. Mean operative time was 240 ± 30 min with mean blood loss of 1200 ± 400 ml. No intra-operative complications occurred. Mean follow-up was 36 months. At one-year follow-up the mean loss of correction was 7° ± 2° with no re-intervention required. This is the first report on treatment of neurological scoliosis with this hybrid construct (lumbar screws, thoracic acrylic clamps, thoracic hooks at the upper end of the curve). In this group of patients the Universal Clamps technique appeared safe and effective and its mechanical performance is comparable to all-level screws construct. Furthermore, the kyphotic component can be better managed in case of thoracic lordosis. The most important aspect of this technique is a short operative time and low vascular and neurologic risks combined with a satisfying stability in the short-postoperative period. Nevertheless, it is important to value results on a long-term follow-up to analyze correction loss, pseudoarthrosis, and mechanical failure of the strips.

Linezolid therapy for pediatric thoracic spondylodiscitis due to Staphylococcus aureus sepsis.

We report the case of an immunocompetent child with spondylodiscitis as a result of staphylococcal sepsis, which was successfully treated with linezolid. The patient was admitted with fever and circumferential swelling in the paradorsal region, which was evident only in the flexed back position. A chest X-ray showed a pleural effusion with pneumonitis and dorsal kyphosis. Following the yield of Staphylococcus aureus from blood cultures, the initial therapy of ceftriaxone and amikacin was changed to vancomycin. However, the dorsal swelling increased further and imaging investigations showed destruction of the vertebral bodies D8-D10 and surrounding tissue swelling. Vancomycin was changed to linezolid, and the patient began to improve; a full recovery was made. Our case suggests that even if spondylodiscitis is rare in the pediatric age-group, particularly as a complication of staphylococcal sepsis, early diagnosis and prompt and appropriate therapy are important to prevent severe complications.

Evaluation and improvement of the efficiency of the Seidel humeral nail by numerical-experimental analysis of the bone-implant contact.

Seidel intramedullary humeral nail is locked distally by a spreading device and proximally by transverse cross locking screws. The main problems found in its use are: the loosening of the distal anchorage, even after X-ray verification of the correct expansion of the device; the formation of distal fractures, occurring even 1 month after the operation; the system's low stability. The problems noted can be ascribed to the behaviour of the spreading device for distal fixing. The present study, therefore, was directed at analysing the contact between the spreading mechanism and the medullary canal in relation to the geometric parameters of the mechanism itself. The main objective was to define the correct regulation of the expansion in the operating theatre, and to delimit the more appropriate conditions of use, allowing the surgeon to evaluate the suitability of this particular device for the type of fracture under treatment. Numerical and experimental techniques were used to perform an analysis of the implant behaviour. This investigation, referred to immediate post-operative condition, allowed to evaluate the stability to traction of the system, to define the typology and properties of the bone-implant contact zone and to quantify the stresses produced, all as a function of the parameter on which the surgeon intervenes in the operating theatre: the number of turns used to tighten the spreading screw. The results obtained confirm and explain the disadvantages associated with the distal expansion system: the bone-implant contact turns out to be inefficient, as revealed by the distribution of the pressure on the inner wall of the medullary canal. On the basis of the results, it is possible to define the optimal conditions of use of the nail, and to formulate a simple solution for the improvement of its performance.

A transversely isotropic, transversely homogeneous microstructural-statistical model of articular cartilage.

Articular cartilage is a multi-phasic, composite, fibre-reinforced material. Therefore, its mechanical properties are determined by the tissue microstructure. The presence of cells (chondrocytes) and collagen fibres within the proteoglycan matrix influences, at a local and a global level, the material symmetries. The volumetric concentration and shape of chondrocytes, and the volumetric concentration and spatial arrangement of collagen fibres have been observed to change as a function of depth in articular cartilage. In particular, collagen fibres are perpendicular to the bone-cartilage interface in the deep zone, their orientation is almost random in the middle zone, and they are parallel to the surface in the superficial zone. The aim of this work is to develop a model of elastic properties of articular cartilage based on its microstructure. In previous work, we addressed this problem based on Piola's notation for fourth-order tensors. Here, mathematical tools initially developed for transversely isotropic composite materials comprised of a statistical orientation of spheroidal inclusions are extended to articular cartilage, while taking into account the dependence of the elastic properties on cartilage depth. The resulting model is transversely isotropic and transversely homogeneous (TITH), the transverse plane being parallel to the bone-cartilage interface and the articular surface. Our results demonstrate that the axial elastic modulus decreases from the deep zone to the articular surface, a result that is in good agreement with experimental findings. Finite element simulations were carried out, in order to explore the TITH model's behaviour in articular cartilage compression tests. The force response, fluid flow and displacement fields obtained with the TITH model were compared with the classical linear elastic, isotropic, homogeneous (IH) model, showing that the IH model is unable to predict the non-uniform behaviour of the tissue. Based on considerations that the mechanical stability of the tissue depends on its topological and microstructural properties, our long-term goal is to clearly understand the stability conditions in topological terms, and the relationship with the growth and remodelling mechanisms in the healthy and diseased tissue.

Effect of fluid boundary conditions on joint contact mechanics and applications to the modeling of osteoarthritic joints.

The long-term goal of our research is to understand the mechanism of osteoarthritis (OA) initiation and progress through experimental and theoretical approaches. In previous theoretical models, joint contact mechanics was implemented without consideration of the fluid boundary conditions and with constant permeability. The primary purpose of this study was to investigate the effect of fluid boundary conditions at the articular surfaces on the contact mechanics, in terms of load sharing and fluid flow properties using variable permeability. The tested conditions included totally sealed surfaces, open surfaces, and open surfaces with variable permeability. While the sealed surface model failed to predict relaxation times and load sharing properly, the class of open surface models (open surfaces with constant permeability, and surfaces with variable permeability) gave good agreement with experiments, in terms of relaxation time and load sharing between the solid and the fluid phase. In particular, the variable permeability model was judged to be the most realistic of the three models, from a biological and physical point of view. This model was then used to simulate joint contact in the early and late stages of OA. In the early stages of OA, the model predicted a decrease in peak contact pressure and an increase in contact area, while in the late stages of OA, peak pressures were increased and contact areas were decreased compared to normal. These findings agree well with experimental observations.

A method to estimate the elastic properties of the extracellular matrix of articular cartilage.

In this work we propose a method to estimate the elastic properties of the extracellular matrix of articular cartilage, once the elastic properties of the chondrocytes and the whole tissue are known. The influence of the elastic properties of the tissue and the cell concentration on the estimated elastic properties of the matrix are investigated.