Screw Orientation Research Articles

Observations of the influence of threading dislocations on the recombination enhanced partial dislocation glide in 4H-silicon carbide epitaxial layers

Electron-hole recombination enhanced glide of Shockley partial dislocations bounding expanding stacking faults and their interactions with threading dislocations have been studied in 4H- silicon carbide epitaxial layers. The mobile silicon-core Shockley partial dislocations bounding the stacking faults are observed to cut through threading edge dislocations, leaving no trailing dislocation segments in their wake. When the Shockley partial dislocations interact with threading screw dislocations, 30° partial dislocation dipoles are initially deposited in their wake. These partial dislocation dipoles quickly and spontaneously snap into screw orientation whereupon they cross slip and annihilate, leaving a prismatic stacking fault on the (21¯1¯0) plane.

ANGLED SCREW HOLES FOR ANTERIOR POSTS AND A FRAME-POSITIONING DEVICE FOR GAMMA KNIFE RADIOSURGERY

Gamma knife radiosurgery requires frame positioning so that the treatment target is as close to the center of the frame and as low as possible to cover all of the posterior fossa contents. In this study, we report the use of two devices developed by the senior author (MWM) that facilitate these two crucial objectives in the treatment of intracranial targets using the gamma knife. Custom front posts with threaded screw holes drilled at 5-, 10-, and 15-degree angles were created by the manufacturer at our request. A U-shaped metal device for frame positioning was designed in-house and fits into the holes at the 100-mm mark on the lateral sides of the Leksell stereotactic frame base. This allowed the positioning device to snap securely into the frame for use in positioning. The positioning device was constructed so that the lowest possible frame position would be achieved with each frame application, while avoiding collisions with the magnetic resonance imaging localizer box. Angled front posts allowed for pin contacts with the cranium anterior and/or superior to the superior temporal line despite a lateral or posterior position of the frame. This avoided penetration of the temporalis muscle and reduced discomfort for patients. The U-shaped metal device was used in place of the Velcro straps or ear bars routinely used for frame positioning in which the distance from the frame base to the top of the head must always be measured to avoid collisions with the localizer box. During the past 2 years, these devices have been used on a daily basis, achieving the desired results. In many cases, their use has avoided the need for frame repositioning and rescanning for targets that cannot be reached because of inexact frame positioning. A new design with angled screw holes in the front posts used for gamma knife radiosurgery allows surgeons to avoid penetration of the temporalis muscle and to maintain a perpendicular orientation of the fixation screw to the outer table of the cranium. They may also prevent mechanical creep caused by the obliquity of pin contact with the cranium and resulting loss of torque. We also present a simple device that may be useful in frame positioning. The device ensures a frame position as low as possible without the need for measurement at the time of frame positioning.

Interaction between Recombination Enhanced Dislocation Glide Process Activated Basal Stacking Faults and Threading Dislocations in 4H-Silicon Carbide Epitaxial Layers

AbstractElectron-hole recombination enhanced glide of Shockley partial dislocations bounding expanding stacking faults and their interactions with threading dislocations in 4H silicon carbide epitaxial layers have been studied using synchrotron white beam X-ray topography and in situ electroluminescence. The mobile silicon-core Shockley partial dislocations bounding the stacking faults are able to cut through threading edge dislocations leaving no trailing dislocation segments in their wake. However, when the Shockley partial dislocations interact with threading screw dislocations, trailing 30o partial dislocation dipoles are initially deposited in their wake due to the pinning effect of the threading screw dislocations. These dipoles spontaneously snap into their screw orientation, regardless the normally immobile carbon-core Shockley partial dislocation components in the dipoles. They subsequently cross slip and annihilate, leaving a prismatic stacking fault in (2-1-10) plane with the displacement vector 1/3[01-10].

Intramedullary Nail Fixation with Posterior-to-Anterior Compared to Transverse Distal Screw Placement for Tibiotalocalcaneal Arthrodesis: A Biomechanical Investigation

Biomechanical studies on retrograde intramedullary fixation for tibiotalocalcaneal fusion have been reported, but no studies have investigated dorsiflexion stiffness, load-to-failure, fatigue endurance, and plastic deformation using different distal screw orientations. Also, no studies have examined the effect of bone density on different distal screw orientations while using a fatigue loading mode. Eight matched pairs of cadaver legs were used. In one leg from each pair an intramedullary nail was inserted with lateral-to-medial distal screws and in the other with posterior-to-anterior screws. These samples underwent dorsiflexion fatigue testing with determination of initial and final stiffness, load-to-failure, and degree of plastic deformation at failure. DEXA scanning was done of each cadaver specimen to determine bone mineral density. Statistical analysis was performed using the Student t-test and a Pearson correlation. Significance level was set at p < 0.05. The specimens with posterior-to-anterior screws had a significantly higher fatigue endurance load-to-failure (1130.0 +/- 362.0 N compared to 801.0 +/- 227 N, p = 0.01). They also had significantly higher final stiffness (203.1 +/- 23.1 N/mm compared to 146.6 +/- 46.2 N/mm, p = 0.05) and lower plastic deformation (2.4 +/- 1.5 mm compared to 3.8 +/- 2.3 mm, p = 0.04). There was a statistically significant correlation between bone mineral density and the difference in construct deformation with posterior-to-anterior and lateral-to-medial screw orientation (r = 0.76, p = 0.03). In this biomechanical investigation of tibiotalocalcaneal arthrodesis with intramedullary nail fixation, posterior-to-anterior distal screw orientation provided more stable fixation than lateral-to-medial screw orientation.

Computer simulation of reactions between an edge dislocation and glissile self-interstitial clusters in iron

Clusters of self-interstitial atoms (SIAs) are formed in metals by high-energy displacement cascades, often in the form of small dislocation loops with a perfect Burgers vector, b. Atomic-scale computer simulation is used here to investigate their reaction with an edge dislocation gliding in α-iron under stress for the situation where b is inclined to the dislocation slip plane. The b of small loops (37 SIAs here) changes spontaneously and the interstitials are absorbed as a pair of superjogs. The line glides forward at critical stress τc when one or more vacancies are created and the jogs adopt a glissile form. A large loop (331 SIAs here) reacts spontaneously with the dislocation to form a segment with b = ⟨100 ⟩, which is sessile on the dislocation slip plane, and as applied stress increases the dislocation side arms are pulled into screw orientation. At low temperature (100 K), the ⟨100⟩ segment remains sessile and the dislocation eventually breaks free when the screw dipole arms cross-slip and annihilate. At 300 K and above, the segment can glide across the loop and transform it into a pair of superjogs, which become glissile at τc. Small loops are weaker obstacles than voids with a similar number of vacancies, large loops are stronger. Irrespective of size, the interaction processes leading to superjogs are efficient for absorption of SIA clusters from slip bands, an effect observed in flow localization.

Comparison of Initial Compression Force Across the Subtalar Joint by Two Different Screw Fixation Techniques

The purpose of this study was to compare the magnitude of compressive force across the posterior facet of the subtalar joint between 2 different screw orientations in a simulated subtalar fusion model fixated with cannulated 7.3-mm screws. Eight paired fresh, frozen cadaver feet were used. Before testing, the bone mineral density of the calcanei and tali was measured with dual energy x-ray absorptiometry to ensure comparable bone mineral density. The paired cadaver calcanei and tali were noted to have less than .05 g/cm(2) difference in bone mineral density. The paired specimens were each randomly assigned to 1 of 2 methods of fixation across the subtalar joint: a plantar approach, where the screw was oriented vertically from the plantar aspect of the calcaneus, distal to the tuberosity; and a posterior approach, where the screw was oriented obliquely from the posterior calcaneal tuberosity. A load-sensing washer was placed under the cancellous screw head before insertion of the screw to measure the magnitude of the compression force across the simulated fusion site. The mean compression force for the posterior-to-anterior approach was 581 N, with a standard deviation of 242 N. The mean force for the plantar-to-superior approach was 604 N, with a standard deviation of 276 N. The compression force obtained by the 2 approaches was not significantly different (P = .74). The plantar-to-superior fixation technique represents an alternative subtalar arthrodesis technique to the posterior-to-anterior approach, potentially allowing one to revise a previously failed arthrodesis when the standard fixation techniques cannot be applied.

Does Screw Configuration Affect Subtrochanteric Fracture after Femoral Neck Fixation?

A subtrochanteric femur fracture after cannulated screw fixation of a femoral neck fracture is a devastating complication. We hypothesized that an apex-distal screw orientation would tolerate higher loads to subtrochanteric fracture. Human cadaveric femora were instrumented with three cannulated screws in either an apex-distal or an apex-proximal configuration. Specimens were loaded along the mechanical axis to failure creating a subtrochanteric femur fracture. Ultimate load to failure and the effect of bone density on load to failure were compared between groups. There was a greater load to failure in the apex-distal group compared with the apex-proximal group. The mean force to fracture in the apex-distal group (11,330 N; standard deviation = 3151 N) was greater than the mean force to fracture in the apex-proximal group (7795 N; standard deviation = 3194 N). Previous investigations have shown improved femoral neck fixation with an apex-distal configuration, but none has examined the relationship between screw orientation and subtrochanteric fractures. Our observations support the use of an apex-distal configuration for cannulated screw fixation of femoral neck fractures.

Plantar-to-Dorsal Compared to Dorsal-to-Plantar Screw Fixation for Proximal Chevron Osteotomy: A Biomechanical Analysis

A change in screw orientation in fixing the chevron proximal first metatarsal osteotomy was noted anecdotally to improve fixation strength. The authors hypothesized that plantar-to-dorsal screw orientation would be more stable than the conventional dorsal-to-plantar screw orientation for fixation of the chevron osteotomy. The purpose of this study was to determine if the load-to-failure and stiffness of the chevron type proximal first metatarsal osteotomy stabilized using plantar-to-dorsal screw fixation were greater than with the more conventional dorsal-to-plantar screw fixation method. One foot from each of eight matched cadaver pairs was randomly assigned to one of two groups: 1) fixation with a dorsal-to-plantar lag screw or 2) fixation with a plantar-to-dorsal lag screw. A proximal chevron osteotomy was then created using standard technique and the metatarsal was fixed according to previously established method. The bone was potted in polyester resin, and the construct was fitted into a materials testing system machine in which load was applied to the plantar aspect of the metatarsal until failure. The two groups were compared using a two-tailed Student t test. The average load-to-failure and stiffness of the chevron osteotomy fixed with the plantar-to-dorsal lag screw were significantly greater (p < 0.05) than the group fixed with more conventional dorsal-to-plantar lag screws. Plantar-to-dorsal screw orientation was more stable than the conventional dorsal-to-plantar screw orientation for fixation of the proximal chevron osteotomy. Plantar-to-dorsal screw orientation should be considered when using the chevron proximal first metatarsal osteotomy.

Creep behaviour of a cast TiAl-based alloy for industrial applications

The creep behaviour of a cast TiAl-based alloy with nominal chemical composition Ti–46Al–2W–0.5Si (at.%) was investigated. Constant load tensile creep tests were performed in the temperature range 973–1073 K and at applied stresses ranging from 200 to 390 MPa. The minimum creep rate is found to depend strongly on the applied stress and temperature. The power law stress exponent n is determined to be 7.3 and true activation energy for creep Q is calculated to be 405 kJ/mol. The initial microstructure of the alloy is unstable during creep exposure. The transformation of the α 2(Ti 3Al)-phase to the γ(TiAl)-phase, needle-like B2 particles and fine Ti 5Si 3 precipitates and particle coarsening are observed. Ordinary dislocations in the γ-matrix dominate the deformation microstructures at creep strains lower than 1.5%. The dislocations are elongated in the screw orientation and form local cusps, which are frequently associated with the jogs on the screw segments of dislocations. Fine B2 and Ti 5Si 3 precipitates act as effective obstacles to dislocation motion. The kinetics of the creep deformation within the studied temperature range and applied stresses is proposed to be controlled by non-conservative motion of dislocations.

Pedicle Screw Placement in the Thoracic Spine

A cadaveric study comparing image guidance technology to fluoroscopic guidance as a means of pedicle screw placement in the thoracic spine, using a unique starting point for screw placement. To assess accuracy of thoracic pedicle screw placement using image guidance versus fluoroscopic guidance for screw insertion. While use of pedicle screws in the thoracic spine has been increasing, its adoption has been slower than for the lumbar spine, reflecting concern regarding possible vascular or spinal cord injury due to screw malplacement. Given these risks, efforts to improve the accuracy of thoracic pedicle screw placement remain appropriate. Stereotactic guidance has been applied in other aspects of spinal surgery to improve the accuracy of instrumentation placement. Pedicle screws were placed in the thoracic spines of eight cadavers, using either a stereotactic guidance or a manual, fluoroscopically guided technique. A slightly more superior and lateral starting point from prior descriptions was used. Each cadaver was instrumented with pedicle screws in the upper thoracic (T1-T2), middle thoracic (T4-T7), and lower thoracic (T9-T10) regions. In the upper and middle thoracic regions, screws with a 4.0-mm shank diameter were used while in the lower thoracic region a shank diameter of 4.5 mm was used. Postinstrumentation CT scans, followed by anatomic dissections, were used to evaluate screw exit rates and orientation relative to the pedicle axis. Exit rates for the two techniques and the effect of vertebral level on exit rate were compared using a chi analysis. The effect of pedicle diameter was tested using a Pearson correlation coefficient. No significant differences in the overall exit rates or orientation were found between the two techniques. There were significant differences in exit rates between the middle (47%), compared with the upper (9%) and lower (16%) thoracic regions, respectively (P < 0.001). A significant correlation between pedicle diameter and exit rate was also found (P < 0.0001). Our study showed no significant differences in the overall exit rates between the two techniques. Image guidance may increase confidence of surgeons with limited experience in thoracic pedicle screw placement. Successful placement of screws within the pedicle varies with the anatomic diameter of the pedicle itself. Concerns regarding accuracy of screw placement should be greatest in the middle thoracic vertebrae (T4-T7), where pedicle diameters are smallest and proximity of the great vessels is nearest.

Oblique screws at the plate ends increase the fixation strength in synthetic bone test medium.

To test the hypothesis that oblique screws at the ends of a plate provide increased strength of fixation as compared to standard screw insertion. Biomechanical laboratory study in synthetic bone test medium. Narrow 4.5-mm stainless steel low-contoured dynamic compression plates were anchored with cortical screws to blocks of polyurethane foam. The fixation strength in cantilever bending (gap closing mode) and torsion was quantified using a material testing system. Different constructs were tested to investigate the effect of the screw orientation at the end of the plate (straight versus oblique at 30 degrees), the plate, and bridging length as well as the number of screws. An oblique screw at the plate end produced an increased strength of fixation in all tests; however, the difference was more significant in shorter plates and in constructs with no screw omission adjacent to the fracture site. Both longer plates and increased bridging length produced a significantly stronger construct able to withstand higher compression loads. Under torsional loading, the fixation strength was mainly dependent on the number of screws. The current data suggest that when using a conventional plating technique, plate length is the most important factor in withstanding forces in cantilever bending. With regard to resisting torsional load, the number of screws is the most important factor. Furthermore, oblique screws at the ends of a plate increase fixation strength.

The locking of 35° 〈1 1 0〉 dislocations in NiAl

Using high-resolution TEM, we observed the core structure of 〈1 1 0〉 dislocations in NiAl locked in 35° directions. These observations showed no evidence of climb decomposition. We conclude that the dislocations are not locked by climb decomposition but by the threefold core spreading of the large screw component of 35° 〈1 1 0〉 dislocations. In NiAl, 〈1 1 0〉 slip is more difficult than 〈1 1 1〉 slip because 〈1 1 0〉 dislocations are locked along two directions, whereas 〈1 1 1〉 dislocations are locked along the screw orientation only.

Analysis of screw pullout strength: A function of screw orientation in subtalar joint arthrodesis

The purpose of this cadaveric study was to compare the pullout strength and failure load of 2 different screw orientations for subtalar arthrodesis. Twenty-six specimens from 13 cadaver donors (1 left and 1 right each) were used. A 7.3- x 65-mm long-thread profile (32-mm length) cannulated screw inserted into the calcaneus from dorsal to plantar (calcaneal specimens) was compared with a 7.3- x 65-mm short-thread profile (16-mm length) cannulated screw inserted into the talus from plantar to dorsal (talar specimens). A torque screwdriver with a calibrated electronic vernier scale measured the torque of screw insertion. Screw pullout strength and load failure were measured by using a servohydraulic materials testing machine. Distraction was applied along the axis of the screw at a displacement rate of 25 mm/min. The peak torque of insertion in all calcaneal specimens was reached on initial insertion through the dorsal subchondral bone plate of the calcaneus; in talar specimens, it was reached as the screw threads were completely buried into the talus. A significant difference (P = .00647) was found between the mean torque of insertion for the calcaneal (1.50 Nm) and talar specimens (1.30 Nm). A comparison of pullout forces showed a significantly stronger mean failure load for calcaneal specimens (P = .000085). The mean failure load for paired calcaneal specimens was 1782 N compared with a mean 1245 N for talar specimens. Although the pullout force was clearly greater in the dorsal-to-plantar screw application, the pullout force in the plantar-to-dorsal orientation was also considerable.

The onset and blocking of slip in NiAl

NiAl single crystals that are well annealed and oriented along the [0 0 1] orientation to suppress <1 0 0> dislocations have been used to study the onset of slip. Under a constant stress, slip is initiated abruptly with increasing temperature. The slip vector is identified to be <0 1 1> by slip line and transmission electron microscopy (TEM) observations. There is a rate-independent slip initiation temperature that decreases approximately linearly with stress. TEM observations show that the <0 1 1> slip is blocked by dislocation locking along both of the two <1 1 1> directions in the {0 1 1} glide plane. The locking is explained by the non-planar splitting of the screw component along the three-fold line directions. The blocking of <0 1 1> slip by dislocation locking long two line directions is in contrast to <1 1 1> dislocations in NiAl that are locked only along the screw orientation, and to <1 0 0> dislocations that are not locked at all. This property explains why <0 1 1> slip is more difficult than <1 1 1> and <0 0 1> slips, the general absence of <0 1 1> dislocations in annealed crystals, and the need for these dislocations to be nucleated in [0 0 1]-oriented NiAl. Possible mechanisms for the onset of <0 1 1> slip in [0 0 1] NiAl are discussed and compared.

Finite element analysis of the initial stability of ankle arthrodesis with internal fixation: flat cut versus intact joint contours

Objective. Qualitative comparison of the initial stability provided by two joint preparation techniques and various screw configurations in ankle arthrodesis, using the finite element method. Design. A three-dimensional model of a healthy ankle was developed from computed tomography images. Two groups of models were built, one with the joint contours resected to produce flat surfaces, and the second with the joint contours preserved. In each case, a variety of screw orientations were examined. Background. Despite the improved results of ankle arthrodesis, failure rates due to non-union are still reported. The initial stability of the arthrodesis construct seems important in the final outcome of the fusion. Methods. Non-linear contact finite element analyses were performed in the arthrodesis constructs subjected to internal/external torsion and dorsiflexion. Micromotions at the bone-to-bone interface were calculated for frictionless and Coulomb friction contact, and compared for the two joint preparation techniques and screw configurations. Results. Overall lower peak micromotions were predicted when preserving the joint contours both in torsion and dorsiflexion. For both preparation techniques, the lowest micromotions tended to occur with the screws inserted at 30° with respect to the long axis of the tibia, crossing above the fusion site. Inclusion of friction in the models caused a general decrease on the magnitude of the micromotions as compared to the frictionless case, but did not affect the ranking of the models. Conclusions. The finite element method can be used as a qualitative tool to study the initial stability of ankle arthrodesis, overcoming the difficulties of measuring bone-to-bone interface micromotions experimentally. Better initial stability was predicted for ankle arthrodesis when the joint contours were preserved rather than resected. Crossing the screws above the fusion site at a steeper angle also tended to increase the stability at the fusion site. Relevance Finite element analyses can help during the pre-operative planning of ankle arthrodesis. When bone density is not compromised, preserving the joint contour and inserting the screws at less than 45° to the long axis of the tibia, crossing over the arthrodesis site, may offer better initial stability.

A comparison of bicortical and intramedullary screw fixations of jones' fractures

Two different fixations for treatment of Jones' fracture were tested in bone models and cadaveric specimens to determine the differences in the stability of the constructs. A bicortical 3.5-mm cannulated cortical screw and an intramedullary 4.0-mm partially threaded cancellous screw were tested using physiologic loads with an Instron 8500 servohydraulic tensiometer (Instron Corporation, Canton, MA). In bone models, the bicortical construct (n = 5, 87+/-23 N) showed superior fixation strength (p = .0009) when compared to the intramedullary screw fixation (n = 5, 25+/-13 N). Cadaveric testing showed similar statistical significance (p = .0124) with the bicortical construct (n = 5, 152+/-71 N) having greater load resistance than the intramedullary screw fixation (n = 4, 29+/-20 N). In bone models, the bicortical constructs (23+/-9 N/mm) showed over twice the elastic modulus than the intramedullary screw fixations (9+/-4 N/mm) with statistical significance (p = .0115). The elastic modulus in the cadaveric group showed a similar pattern between the bicortical (19+/-17 N/mm) and intramedullary (9+/-6 N/mm) screw constructs. Analysis of the bicortical screw failure patterns revealed that screw orientation had a critical impact on fixation stability. The more distal the exit site of the bicortical screw was from the fracture site, the greater the load needed to displace the fixation.

MRI SUSCEPTIBILITY ARTEFACTS RELATED TO SCAPHOID SCREWS: THE EFFECT OF SCREW TYPE, SCREW ORIENTATION AND IMAGING PARAMETERS

Metal implants produce susceptibility artefacts in magnetic resonance imaging. We have explored the effects of scaphoid screw characteristics and orientation on MR susceptibility artefact. Titanium alloy, smallness and longitudinal alignment with the z-axis of the main magnetic field reduce the size of the susceptibility artefact.

Application of a modified jogged-screw model for creep of TiAl and α-Ti alloys

Stress exponents for creep, in the range of 5, are typically associated with dislocation creep processes, normally associated with a strong tendency for subgrain formation. In this article, we will demonstrate that there are several important alloy systems that have similar stress dependence and, yet, lack this tendency for subgrain formation. Specifically, dislocations in the intermetallic compound γ-TiAl and the hexagonal close-packed (hcp) α phase of the commercial Ti alloy, Ti-6242, tend to be homogeneously distributed with a tendency for alignment along screw orientation. In both alloy systems, the screw dislocations exhibit a large density of pinning points, which detailed transmission electron microscopy (TEM) investigation indicate are locations of tall jogs. These observations suggest that the jogged-screw model for creep should be appropriate after suitable modification for the presence of these tall jogs. This modified jogged-screw (MJS) model is presented, together with a discussion of the assumptions made, and the results of this model are shown to compare favorably with experiment for both alloy systems. The possible criteria for the formation of tall jogs are also described, and the potential application of this modified model to other alloy systems is discussed.

Atomistic Studies of Dislocation Glide in γ-TiAl

ABSTRACTComputer simulation of the core structure and glide of ordinary 1/2&lt;110] dislocations and &lt;101] superdislocations in L10 TiAl has been performed using the recently constructed BondOrder Potentials. This description of atomic interactions includes explicitly, within the tight-binding approximation, the most important aspects of the directional bonding, namely d-d, p-p and d-p bonds. The ordinary dislocation in the screw orientation was found to have a non-planar core and, therefore, high Peierls stress. The superdislocation was found to possess in the screw orientation either a planar (glissile) or a non-planar (sessile) core structure. However, the glissile core transforms into the sessile one for certain orientations of the applied stress. This implies a strong asymmetry of the yield stress and the break down of the Schmid law when the plastic flow is mediated by superdislocations. At the same time, this may explain the orientation dependence of the dislocation substructure observed in the single-phase γ-TiAl by electron microscopy.

Refined Prediction and Observation of Dislocation Structures in Low Σ Symmetric Grain Boundaries

The grain boundary dislocation structures of three Fe–4at.% Si bicrystals with an additional misorientation of the order of 2° were studied by transmission electron microscopy (TEM), one Σ5 [001] (310) and two Σ9 [110] (1\(\overline 1\)4). It was found that in the Σ5 case two Burgers vectors of the structure are not simple basic complete displacement shift lattice (DSC) vectors and are larger than any one of them. This enabled formation of the preferred grain boundary dislocation (GBD) structure with a hexagonal net composed of three screw dislocations and an independent set of parallel edge dislocations. In the Σ9 case the GBD structure comprises three apparently independent sets of parallel dislocations. The Burgers vectors of two sets could be established as basic DSC vectors in screw and edge orientation, respectively, with respect to the GB plane. For the third set formed by separated groups of four dislocations no corresponding basic DSC vector exists. This case needs further investigation. The previously published table of predicted GBD structures for symmetrical coincidence boundaries up to Σ11 was refined according to the new results.