Effects of merged holes, partial thread removal, and offset holes on fatigue strengths of titanium locking plates.

BACKGROUND: This study investigated the effects of screw hole merging, thread removal, and screw hole offset on the mechanical properties of locking plates. METHODS: Finite element models were used to develop the optimal design of the merged holes. Four titanium locking plates with different hole designs were analyzed. Type I had threaded round holes. Type II had merged holes. Type III had merged holes with partial thread removal. Type IV had threaded offset holes. Mechanical experiments similar to finite element analyses were conducted and compared. Screw bending tests were used to assess the screw holding power. FINDINGS: Finite element analyses showed the optimal merging distance between two round screw holes was 3.5 mm with 2/3 circumferences in each hole. The stresses of types II and III were respectively 6.42% and 7.33%, lower than that of type I. The stress of type IV was 1.66% higher than that of type I. In the mechanical tests, the fatigue lives of types II and III were respectively 3.86 and 7.16 times higher than that of type I. The fatigue life of type IV was 37% lower than that of type I. The differences in the bending strengths of screws were insignificant. INTERPRETATION: Merging holes could mitigate screw hole stress and increase the fatigue lives of the plates significantly. Partial thread removal could further improve the fatigue life. Merging holes and thread removal did not decrease the screw holding power significantly. The fatigue lives were significantly decreased in plates with offset holes.

The Effect of the Energy Release Rate on the Local Damage Evolution in TRIP Steel Composite Reinforced with Zirconia Particles.

In this study, the effect of the energy release rate on the transformation-induced plasticity (TRIP) steel composite reinforced with 5 vol% ceramic particles is determined using the crystal plasticity simulation of the coupled brittle-ductile damage model and validated by experimental results. A miniature dog bone tensile sample is subjected to an interrupted in situ quasi-static tensile test up to a true strain of 20.3%. Using the commercial digital image correlation program VEDDAC and the image processing method in MATLAB, the test data are utilized to monitor the progress of local microstrain and damage. The impact of the energy release rate of ceramic particles is investigated by simulation using a coupled crystal plasticity-dislocation density model with ductile-brittle criteria for the corresponding phases. It can be shown that the local deformations predicted by the numerical simulation and the experimental data are qualitatively comparable. The damage pixel of the experiment, smaller Ecr (1.0 × 108), and larger Ecr (1.2 × 108) cases of energy release rates are 4.9%, 4.3%, and 5.1%, respectively. Furthermore, on a global strain of 20.3%, the relative error between simulation and experimental validation of smaller Ecr (1.0 × 108) and larger Ecr (1.2 × 108) cases is 12.2% and 4%, respectively.

Half-threaded holes markedly increase the fatigue life of locking plates without compromising screw stability.

AIMS: To determine whether half-threaded screw holes in a new titanium locking plate design can substantially decrease the notch effects of the threads and increase the plate fatigue life. METHODS: Three types (I to III) of titanium locking plates were fabricated to simulate plates used in the femur, tibia, and forearm. Two copies of each were fabricated using full- and half-threaded screw holes (called A and B, respectively). The mechanical strengths of the plates were evaluated according to the American Society for Testing and Materials (ASTM) F382-14, and the screw stability was assessed by measuring the screw removal torque and bending strength. RESULTS: The B plates had fatigue lives 11- to 16-times higher than those of the A plates. Before cyclic loading, the screw removal torques were all higher than the insertion torques. However, after cyclic loading, the removal torques were similar to or slightly lower than the insertion torques (0% to 17.3%), although those of the B plates were higher than those of the A plates for all except the type III plates (101%, 109.8%, and 93.8% for types I, II, and III, respectively). The bending strengths of the screws were not significantly different between the A and B plates for any of the types. CONCLUSION: Removing half of the threads from the screw holes markedly increased the fatigue life of the locking plates while preserving the tightness of the screw heads and the bending strength of the locking screws. However, future work is necessary to determine the relationship between the notch sensitivity properties and titanium plate design.Cite this article: Bone Joint Res 2020;9(10):645-652.

Contradictory working length effects in locked plating of the distal and middle femoral fractures-a biomechanical study.

BACKGROUND: Working length have been reported to affect the plate stress and fixation stiffness. However, the results of previous studies have been controversial. The present study was to determine working length effects on different locations of femoral bone gap. METHODS: Five composite femurs with wide bone gaps at five levels (G1, 2, 3, 5, and 7), were fixed with locking plates. G1-3, G5 and G7 represented gaps at distal femur, distal-middle femur and middle femur respectively. Strain gauges were applied near the screw holes. The plate-bone constructs were loaded through a hemicylinder on the femoral head with total constraints at the distal femur. The micro-strains, axial stiffness and interfragmentary motions were recorded. Then the locking screws were removed one by one and the tests were re-run. The working length effects were compared and correlated. FINDINGS: In distal femurs (G1-3), long working length was negatively correlated with the highest strains (r = -0.97, -0.95 and - 0.95, p < 0.01) and axial stiffness (r = -1, -0.96 and -0.99, p < 0.01). In distal-middle femurs (G5), as the working length increased, the highest strain decreased initially and then increased (r = 0.81, p = 0.026) and the axial stiffness decreased (r = -0.98, p < 0.01). In middle femurs (G7), the highest strain and gap motions were much higher than that in the other groups and not significantly correlated with the working length change. INTERPRETATION: Long working length could reduce the highest plate strain in distal femurs, but had no significant effects in middle femurs. The working length effects were markedly affected by the loading and boundary conditions.

Modification of the screw hole structures to improve the fatigue strength of locking plates.

BACKGROUND: The fatigue fracture of locking plates can substantially threaten fracture treatment results. In the present study, three measures for modifying the screw hole structures of plates were implemented to improve their fatigue strength. MATERIALS: Custom-made identical titanium locking plates, except the screw hole configurations, were tested using four-point bending load. The three measures were partial removal of screw threads on the tension side of the plates, reduction of screw hole size, and modification of the thread radii. There were six types of plates: control (Type I), half of the threads removed (Type II) or one-third of the threads (Type III), smaller screw holes (Type IV), and increase of the thread root radii (Type V) or crest radii (Type VI). FINDINGS: Compared with the control, Types II and III significantly improved the fatigue strength (14.5 and 10.1 times, respectively). Decreasing the size of the screw hole (Type IV) also yielded a higher fatigue strength (17.6%). Type VI significantly improved the fatigue strength (9.8 times). However, Type V decreased the fatigue strength (14%). For cyclic stiffness, Type IV was significantly higher than other types statistically. Failure analyses showed typical fatigue fracture in all plates and the cracks were always initiated at the thread crest. INTERPRETATIONS: The fatigue strength of titanium locking plates can be significantly improved by structural changes in the screw holes. Removing the threads of the plates and increasing the crest radii of the threads were more effective measures than decreasing the size of the screw holes.

Improving socket design to prevent difficult removal of locking screws.

INTRODUCTION: Reports of driver slippage leading to difficult locking screw removals have increased since the adoption of titanium for screw fabrication; the use of titanium is known to cause cross-threading and cold welding. Such problems occur most frequently in screws with hex sockets, and may cause serious surgical complications. This study aimed to improve screw socket design to prevent slippage and difficult screw removal. MATERIALS AND METHODS: Three types of small sockets (hex, Torx, and cruciate) and six types of large sockets (hex, Torx, Octatorx, Torx+ I, Torx+ II, and Torx+ III) with screw head diameters of 5.5 mm were manufactured from titanium, and corresponding screwdrivers were manufactured from stainless steel. The screw heads and drivers were mounted on a material testing machine, and torsional tests were conducted to simulate screw usage in clinical settings at two insertion depths: 1 and 2 mm. Ten specimens were tested from each design, and the maximum torque and failure patterns were recorded and compared. RESULTS: For small sockets in 2 mm conditions, the hex with the largest driver core had the highest torque, followed by Torx and cruciate. In these tests, the drivers were twisted off in all specimens. However, under the 1 mm condition, the hex slipped and the torque decreased markedly. Overall, torque was higher for large sockets than for small sockets. The Octatorx, with a large core and simultaneous deformation of the driver and socket lobes, had the highest torque at almost twice that of the small hex. The hex had the lowest torque, a result of slippage in both the 1 and 2 mm conditions. Torx plus designs, with more designed degrees of freedom, were able to maintain a higher driving angle and larger core for higher torque. CONCLUSIONS: The hex design showed slipping tendencies with a marked decrease in torque, especially under conditions with inadequate driver engagement. Large sockets allowed for substantial increases in torque. The Torx, Octatorx, and Torx plus designs displayed better performance than the hexes. Improvements to the socket design could effectively prevent slippage and solve difficult screw removal problems.

Notch sensitivity jeopardizes titanium locking plate fatigue strength.

INTRODUCTION: Notch sensitivity may compromise titanium-alloy plate fatigue strength. However, no studies providing head-to-head comparisons of stainless-steel or titanium-alloy locking plates exist. MATERIALS AND METHODS: Custom-designed identically structured locking plates were made from stainless steel (F138 and F1314) or titanium alloy. Three screw-hole designs were compared: threaded screw-holes with angle edges (type I); threaded screw-holes with chamfered edges (type II); and non-threaded screw-holes with chamfered edges (type III). The plates' bending stiffness, bending strength, and fatigue life, were investigated. The stress concentration at the screw threads was assessed using finite element analyses (FEA). RESULTS: The titanium plates had higher bending strength than the F1314 and F138 plates (2.95:1.56:1) in static loading tests. For all metals, the type-III plate fatigue life was highest, followed by type-II and type-I. The type-III titanium plates had longer fatigue lives than their F138 counterparts, but the type-I and type-II titanium plates had significantly shorter fatigue lives. All F1314 plate types had longer fatigue lives than the type-III titanium plates. The FEA showed minimal stress difference (0.4%) between types II and III, but the stress for types II and III was lower (11.9% and 12.4%) than that for type I. CONCLUSIONS: The screw threads did not cause stress concentration in the locking plates in FEA, but may have jeopardized the fatigue strength, especially in the notch-sensitive titanium plates. Improvement to the locking plate design is necessary.

A Strip Cell in Pyroelectric Devices.

The pyroelectric effect affords the opportunity to convert temporal temperature fluctuations into usable electrical energy in order to develop abundantly available waste heat. A strip pyroelectric cell, used to enhance temperature variation rates by lateral temperature gradients and to reduce cell capacitance to further promote the induced voltage, is described as a means of improving pyroelectric energy transformation. A precision dicing saw was successfully applied in fabricating the pyroelectric cell with a strip form. The strip pyroelectric cell with a high-narrow cross section is able to greatly absorb thermal energy via the side walls of the strips, thereby inducing lateral temperature gradients and increasing temperature variation rates in a thicker pyroelectric cell. Both simulation and experimentation show that the strip pyroelectric cell improves the electrical outputs of pyroelectric cells and enhances the efficiency of pyroelectric harvesters. The strip-type pyroelectric cell has a larger temperature variation when compared to the trenched electrode and the original type, by about 1.9 and 2.4 times, respectively. The measured electrical output of the strip type demonstrates a conspicuous increase in stored energy as compared to the trenched electrode and the original type, by of about 15.6 and 19.8 times, respectively.

Study on Pyroelectric Harvesters with Various Geometry.

Pyroelectric harvesters convert time-dependent temperature variations into electric current. The appropriate geometry of the pyroelectric cells, coupled with the optimal period of temperature fluctuations, is key to driving the optimal load resistance, which enhances the performance of pyroelectric harvesters. The induced charge increases when the thickness of the pyroelectric cells decreases. Moreover, the induced charge is extremely reduced for the thinner pyroelectric cell when not used for the optimal period. The maximum harvested power is achieved when a 100 µm-thick PZT (Lead zirconate titanate) cell is used to drive the optimal load resistance of about 40 MΩ. Moreover, the harvested power is greatly reduced when the working resistance diverges even slightly from the optimal load resistance. The stored voltage generated from the 75 µm-thick PZT cell is less than that from the 400 µm-thick PZT cell for a period longer than 64 s. Although the thinner PZT cell is advantageous in that it enhances the efficiency of the pyroelectric harvester, the much thinner 75 µm-thick PZT cell and the divergence from the optimal period further diminish the performance of the pyroelectric cell. Therefore, the designers of pyroelectric harvesters need to consider the coupling effect between the geometry of the pyroelectric cells and the optimal period of temperature fluctuations to drive the optimal load resistance.

Biomechanical evaluation of bending strength of spinal pedicle screws, including cylindrical, conical, dual core and double dual core designs using numerical simulations and mechanical tests.

Pedicle screws are used for treating several types of spinal injuries. Although several commercial versions are presently available, they are mostly either fully cylindrical or fully conical. In this study, the bending strengths of seven types of commercial pedicle screws and a newly designed double dual core screw were evaluated by finite element analyses and biomechanical tests. All the screws had an outer diameter of 7 mm, and the biomechanical test consisted of a cantilever bending test in which a vertical point load was applied using a level arm of 45 mm. The boundary and loading conditions of the biomechanical tests were applied to the model used for the finite element analyses. The results showed that only the conical screws with fixed outer diameter and the new double dual core screw could withstand 1,000,000 cycles of a 50-500 N cyclic load. The new screw, however, exhibited lower stiffness than the conical screw, indicating that it could afford patients more flexible movements. Moreover, the new screw produced a level of stability comparable to that of the conical screw, and it was also significantly stronger than the other screws. The finite element analysis further revealed that the point of maximum tensile stress in the screw model was comparable to the point at which fracture occurred during the fatigue test.

Comparison study of the pullout strength of conventional spinal pedicle screws and a novel design in full and backed-out insertions using mechanical tests.

Recently, new pedicle screw designs have been developed. However, these designs' performances are still unclear, especially when backed out after insertion. The objective of this study was to investigate the performances of different screw designs when backed out from full insertion. Seven conventional designs of the pedicle screw and one novel design were inserted into polyurethane foam (0.32 g/cm(3)). All screws were first fully inserted (43 mm) and were backed out 360°. Axial pullout tests were performed and the reaction force was measured. The results showed that the conical screw of type 1 with a small inner diameter provided the highest pullout strength in both full insertion and backed-out insertion (2401.85 and 2169.82 N, respectively). However, this screw's pullout strength significantly decreased (9.7%) when backed out from full insertion. There was no significant difference between the conical screw of type 1 with a small inner diameter and double duo core screw (p > 0.01) in backed-out insertion. The cylindrical screw with a small diameter, dual inner core screw and double dual core screw also provided good results in both full insertion (2115.44, 2182.99 and 2226.93 N, respectively) and backed-out conditions (2065.80, 2014.28 and 1941.29 N, respectively). The increased pullout strength of the conical design could be due to the effect of bone compaction. However, the screw exhibited less consistent pullout strength when backed out when compared with the other designs. The conical screw should be inserted to the precise position without turning back, especially in osteoporosis patients. The dual inner core screw and double dual core screw could provide greater stability in both conditions. Care should be taken when using both the cylindrical screw with a small thread depth and the dual outer core screw.

Multiobjective optimization design of spinal pedicle screws using neural networks and genetic algorithm: mathematical models and mechanical validation.

Short-segment instrumentation for spine fractures is threatened by relatively high failure rates. Failure of the spinal pedicle screws including breakage and loosening may jeopardize the fixation integrity and lead to treatment failure. Two important design objectives, bending strength and pullout strength, may conflict with each other and warrant a multiobjective optimization study. In the present study using the three-dimensional finite element (FE) analytical results based on an L25 orthogonal array, bending and pullout objective functions were developed by an artificial neural network (ANN) algorithm, and the trade-off solutions known as Pareto optima were explored by a genetic algorithm (GA). The results showed that the knee solutions of the Pareto fronts with both high bending and pullout strength ranged from 92% to 94% of their maxima, respectively. In mechanical validation, the results of mathematical analyses were closely related to those of experimental tests with a correlation coefficient of -0.91 for bending and 0.93 for pullout (P < 0.01 for both). The optimal design had significantly higher fatigue life (P < 0.01) and comparable pullout strength as compared with commercial screws. Multiobjective optimization study of spinal pedicle screws using the hybrid of ANN and GA could achieve an ideal with high bending and pullout performances simultaneously.

Biomechanical influence of pin placement and elbow angle on joint distraction and hinge alignment for an arthrodiatasis elbow-pin-fixator construct.

Stiffness and contracture of the periarticular tissues are common complications of a post-traumatic elbow. Arthrodiatasis is a surgical technique that uses an external fixator for initial immobilization and subsequent distraction. The two prerequisites for an ideal arthrodiatasis are concentric distraction (avoiding bony contact) and hinge alignment (reducing internal stress). This study used the finite element (FE) method to clarify the relationship between these two prerequisites and the initial conditions (pin placement, elbow angle, and distraction mode). A total of 12 variations of the initial conditions were symmetrically arranged to evaluate their biomechanical influence on concentric distraction and hinge alignment. The humeroulnar surface was hypothesized to be ideally distracted orthogonal to the line joining the tips of the olecranon and the coronoid. The eccentric separation of the humeroulnar surfaces is a response to the non-orthogonality of the distracting force and joining line. Pin placement significantly affects the effective moment arm of the fixing pins to distract the bridged elbow. Both elbow angle and distraction mode directly alter the direction of the distracting force at the elbow center. In general, the hinges misalignment occurs in a direction opposite to the distraction-activated site. After joint distraction, the elastic deflection of the fixing pins inevitably makes both elbow and fixator hinges to misalign. This indicates that both joint distraction and hinge alignment are the interactive mechanisms. The humeroulnar separation is more concentric in the situation of the 120 degrees humeral distraction by using stiffer pins with convergent placement. Even so, the eccentric displacement of the elbow hinge is a crucial consideration in the initial placement of the guiding pin to compensate for hinge misalignment.

High-frequency resonant characteristics of triple-layered piezoceramic bimorphs determined using experimental measurements and theoretical analysis.

This is an experimental, theoretical, and numerical investigation of vibration characteristics in high-frequency resonance, which are studied for parallel- and series-type piezoelectric bimorphs. In the experimental measurements, the full-field optical technique known as electronic speckle pattern interferometry (ESPI) is used to measure the transverse (out-of-plane) and planar (in-plane) resonant frequencies and corresponding mode shapes for piezoelectric bimorphs. In addition, in-plane resonant frequencies are obtained from impedance analysis and the response curves of the frequency spectra show different vibration characteristics of the piezoelectric bimorphs with different electrical connections. Piezoelectric bimorphs with normal connections have three-dimensional coupled vibration characteristics and the out-of-plane vibration dominates the motion. However, only in-plane vibration motions can be excited in the high-frequency range for abnormal connections, and the resonant characteristics are similar to the single-layered piezoelectric plate. The triple-layered piezoelectric bimorphs with abnormal connection are also analyzed using theoretical analysis. The resonant frequencies, mode shapes, and normalized displacements are calculated based on the analytical solution. The experimental results and the theoretical analysis are in good agreement with the numerical calculations using the finite element method. From the discussion of the results for the parallel- and series-type piezoelectric bimorphs with normal and abnormal connections, the vibration characteristics at high frequencies are completely analyzed in this study.

Comparison of multiple linear regression and artificial neural network in developing the objective functions of the orthopaedic screws.

Optimizing the orthopaedic screws can greatly improve their biomechanical performances. However, a methodical design optimization approach requires a long time to search the best design. Thus, the surrogate objective functions of the orthopaedic screws should be accurately developed. To our knowledge, there is no study to evaluate the strengths and limitations of the surrogate methods in developing the objective functions of the orthopaedic screws. Three-dimensional finite element models for both the tibial locking screws and the spinal pedicle screws were constructed and analyzed. Then, the learning data were prepared according to the arrangement of the Taguchi orthogonal array, and the verification data were selected with use of a randomized selection. Finally, the surrogate objective functions were developed by using either the multiple linear regression or the artificial neural network. The applicability and accuracy of those surrogate methods were evaluated and discussed. The multiple linear regression method could successfully construct the objective function of the tibial locking screws, but it failed to develop the objective function of the spinal pedicle screws. The artificial neural network method showed a greater capacity of prediction in developing the objective functions for the tibial locking screws and the spinal pedicle screws than the multiple linear regression method. The artificial neural network method may be a useful option for developing the objective functions of the orthopaedic screws with a greater structural complexity. The surrogate objective functions of the orthopaedic screws could effectively decrease the time and effort required for the design optimization process.

A neurogenetic approach to a multiobjective design optimization of spinal pedicle screws.

A pedicle screw fixation has been widely used to treat spinal diseases. Clinical reports have shown that the weakest part of the spinal fixator is the pedicle screw. However, previous studies have only focused on either screw breakage or screw loosening. There have been no studies that have addressed the multiobjective design optimization of the pedicle screws. The multiobjective optimization methodology was applied and it consisted of finite element method, Taguchi method, artificial neural networks, and genetic algorithms. Three-dimensional finite element models for both the bending strength and the pullout strength of the pedicle screw were first developed and arranged on an L(25) orthogonal array. Then, artificial neural networks were used to create two objective functions. Finally, the optimum solutions of the pedicle screws were obtained by genetic algorithms. The results showed that the optimum designs had higher bending and pullout strengths compared with commercially available screws. The optimum designs of pedicle screw revealed excellent biomechanical performances. The neurogenetic approach has effectively decreased the time and effort required for searching for the optimal designs of pedicle screws and has directly provided the selection information to surgeons.

Notch sensitivity of titanium causing contradictory effects on locked nails and screws.

The purpose of this biomechanical study was to compare the mechanical properties of specially designed locked nails and screws with the same structures and 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 yield load, cyclic bending stiffness, and fatigue life. Finite element models were used to simulate the mechanical tests and compute the stress concentration factors. Increasing the root radius and the inner diameter could effectively increase the fatigue life of the locking screws. The fatigue life of titanium screws was higher (by 1.4- to >6-fold) than that of stainless steel screws, especially when the inner diameter was increased. In contrast, the fatigue life of titanium locked nails was lower (by about 1/4 to 1/3) than that of their stainless steel counterparts. Finite element models could closely predict the results of the biomechanical tests with a Pearson 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 1.97 for screws and from 3.89 to 4.99 for nails. The present study suggested that with larger root radius and inner diameter, titanium locking screws could provide much higher fatigue life 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 taken seriously during the design process.

Biomechanical influences of pin placement and elbow angle on hinge alignment and joint distraction of bridged elbow-pin-fixator construct.

Joint distraction and mobilization with a hinged external fixator preserves elbow stability and mobility. However, the alignment of both elbow and fixator hinges was the initial prerequisite of the arthrodiatasis technique. The main goal of this study was to numerically evaluate the kinematic influence of the device, surgery, and joint factors on hinge alignment. The kinetic effects of the pins placement and elbow angle on concentric distraction and mobilization were also discussed. A unilaterally hinged elbow-fixator system with a 14 links and 10 degrees-of-freedom was instrumented into a humeroulnar model. The Denavit-Hartenberg method with the homogeneous transformation matrixes was applied to perform kinematic analysis of the linkage system. The predicted results revealed that the concurrence of hinge alignment (i.e., kinematic) and concentric distraction (i.e., kinetic) necessitates two telescopic tubes orthogonal to the elbow hinge. The degrees-of-freedom arrangement of the fixator articulators plays a significant role in hinge alignment. After joint distraction, two hinges might be misaligned due to the difference in the structural rigidity of the pins, fixator, and stiffened elbow. Furthermore, those two prerequisite are interactive and sensitive to elbow angle, fixator design, and pin placement of the bridged elbow-pin-fixator construct. In addition, the ideally concentric distraction might occur only at an elbow angle of 120 degrees owing to the ulnar anatomy. Meticulous planning is necessary for such highly technically demanding surgery.

Biomechanical comparisons of different posterior instrumentation constructs after two-level ALIF: a finite element study.

Anterior lumbar interbody fusion (ALIF) with cylindrical cages and supplemental posterior fixation has been widely used for internal disc derangement. However, most researchers have focused on single-level ALIF. Therefore, the biomechanical performance of various fixation constructs after two-level ALIF is not well characterized. This research used three-dimensional finite element models (FEM) with a nonlinear contact analysis to evaluate the initial biomechanical behavior of five types of fixation devices after two-level ALIF (L3/L4, L4/L5) under six loading conditions. These fixation constructs included a three-level pedicle screw and rod, a two-level translaminar facet screw, a two-level transfacet pedicle screw, a bisegmental pedicle screw and rod, and a bisegmental pedicle screw and rod with cross-linking. The FEM's developed in this study demonstrate that, compared to the other four types of posterior fixation constructs analyzed, the three-level pedicle screw and rod provide the best biomechanical stability. Both two-level facet screw fixation constructs showed unfavorable loading in lateral bending. For the construct of the three-level pedicle screw and rod, the middle-segment pedicle screw should not be omitted even though a cross-link is used. The two-level ALIF models with cages and posterior fixation constructs that we developed can be used to evaluate the initial biomechanical performance of a wide variety of posterior fixation devices prior to surgery.

Fabrication of optical filters based on polymer asymmetric Bragg couplers.

In this work, we successfully developed a process to fabricate dual-channel polymeric waveguide filters based on an asymmetric Bragg coupler (ABC) using holographic interference techniques, soft lithography, and micro molding. At the cross- and self-reflection Bragg wavelengths, the transmission dips of approximately -16.4 and -11.5 dB relative to the 3 dB background insertion loss and the 3 dB transmission bandwidths of approximately 0.6 and 0.5 nm were obtained from an ABC-based filter. The transmission spectrum overlaps when the effective index difference between two single waveguides is less than 0.002.