Mutations in the Receptor Binding Domain of Severe Acute Respiratory Coronavirus-2 Omicron Variant Spike Protein Significantly Stabilizes Its Conformation.

The Omicron variant and its sub-lineages are the only current circulating SARS-CoV-2 viruses worldwide. In this study, the conformational stability of the isolated Receptor Binding Domain (RBD) of Omicron's spike protein is examined in detail. The parent Omicron lineage has over ten mutations in the ACE2 binding region of the RBD that are specifically associated with its ß hairpin loop domain. It is demonstrated through biophysical molecular computations that the mutations in the ß hairpin loop domain significantly increase the intra-protein interaction energies of intra-loop and loop-RBD interactions. The interaction energy increases include the formation of new hydrogen bonds in the ß hairpin loop domain that help stabilize this critical ACE2 binding region. Our results also agree with recent experiments on the stability of Omicron's core ß barrel domain, outside of its loop domain, and help demonstrate the overall conformational stability of the Omicron RBD. It is further shown here through dynamic simulations that the unbound state of the Omicron RBD remains closely aligned with the bound state configuration, which was not observed for the wild-type RBD. Overall, these studies demonstrate the significantly increased conformational stability of Omicron over its wild-type configuration and raise a number of questions on whether conformational stability could be a positive selection feature of SARS-CoV-2 viral mutational changes.

A biomechanical comparison of three fixation methods for unstable femoral neck fractures with medial calcar defect.

BACKGROUND: Unstable femoral neck fractures with medial calcar defects are difficult to manage. The optimal fixation methods for these fractures have been a subject of ongoing debate among orthopedic surgeons. In this study, three different fixation techniques for vertical, medial defected femoral neck fractures were compared. METHODS: In this study, a biomechanical analysis was conducted to compare three fixation methods: cannulated screws (Group 1), cannulated screws combined with a medial buttress plate (Group 2), and intramedullary nails (Group 3). Synthetic composite bone models representing vertical collum femoris fractures with medial calcar defects were used. Each group consisted of seven specimens, and, to maintain consistency, a single surgeon performed the surgical procedure. Biomechanical testing involved subjecting the specimens to axial loading until failure, and the load to failure, stiffness, and displacement values were recorded. Normality was tested using the Shapiro-Wilk test. One-way ANOVA and Tukey's HSD post hoc test were used for comparisons. RESULTS: The difference in the load to failure values was statistically significant among the groups, with Group 2 exhibiting the highest load to failure value, followed by Group 3 and Group 1. Stiffness values were significantly higher in Group 2 than in the other groups. Displacement values were not significantly different between the groups. Fracture and displacement patterns at the point of failure varied across the groups. CONCLUSION: The results of this study indicate that fixation with a medial buttress plate in combination with cannulated screws provides additional biomechanical stability for vertical femoral neck fractures with medial calcar defects. Intramedullary nail fixation also demonstrated durable stability in these fractures. These findings can be used to better understand current management strategies for these challenging fractures to promote the identification of better evidence-based recommendations.

Novel screw fixation placement configuration for the treatment of Pauwels type III femoral neck fractures: a finite element analysis.

Verticality of transcervical hip fractures in young patients is usually connected with typically high-energy fractures which are known as Pauwels type III. Artificial femoral head replacement surgery is mostly not considered for treating femoral neck fractures in such patients. The commonly used devices for the fixation of vertical femoral neck fractures are multiple screws or a sliding hip screw with or without an antirotation screw. Size, location and length of the screws are the most effective parameters in terms of the structural performance of internal fixation implants, but the optimal configuration of the screws is necessary to be investigated to direct the clinical practice. The aim of this study is to compare the biomechanical stability of the standard inverted triangle configuration with the various newly proposed x-crossed screw configurations. FEA simulations carried out in this study demonstrated that using an x-crossed-right assembly in treating Pauwels type III femoral neck fractures satisfies the biomechanical stability in terms of maximum von Mises stresses and maximum femoral head displacement. However, in terms of maximum relative neck fracture displacement, the x-crossed-right assembly would not entirely suffice the desired biomechanical stability. Therefore, using an x-crossed screw assembly in treating femoral neck fractures would provide the needed biomechanical stability.

Anatomical and Biomechanical Stability of Single/Double Screw-Cancellous Bone Fixations of Regan-Morry Type III Ulnar Coronoid Fractures in Adults: CT Measurement and Finite Element Analysis.

OBJECTIVE: At present, it is still uncertain whether single screw has the same stability as double screws in the treatment of ulnar coronal process basal fracture (Regan-Morry type III). So, we aimed to compare the pull-out force and anti-rotation torque of anterior single/double screw-cancellous bone fixation (aSSBF, aDSBF) in this fracture, and further study the influencing factors on anatomical and biomechanical stability of smart screw internal fixations. METHODS: A total of 63 adult volunteers with no history of elbow injury underwent elbow CT scanning with associated three-dimensional reconstruction that enabled the measurements of bone density and fixed length of the proximal ulna and coronoid. The models of coronal process basal fracture, aSSBF and aDSBF, were developed and validated. Using the finite element model test, the sensitivity analysis of pull-out force and rotational torque was carried out. RESULTS: The pull-out force of aSSBF model was positively correlated with the density of the cancellous bone and linearly related to the fixed depth of the screw. The load pattern of pull-out force of aDSBF model was similar to that of aSSBF model. The ultimate torque of aDSBF model was higher than that of aSSBF model, but the load pattern of ultimate torque of both models was similar to each other when the fracture reset was satisfactory, and the screw nut attaches closely to coronoid process. Moreover, with enhancement of initial pre-tightening force, the increase of ultimate torque of both models was small. CONCLUSIONS: In addition to three pull-out stability factors of smart screw fixations, fracture surface fitting degree and nut fitting degree are the other two important anatomical and biomechanical stability factors of smart screw fixations both for rotational stability. When all pull-out stability and rotational stability factors meet reasonable conditions simultaneously, single or double screw fixation methods are stable for the treatments of ulnar coronoid basal fractures.

Comparison of the biomechanical stability of transverse and oblique screw trajectories in retrograde intramedullary nailing of supracondylar femur fractures.

BACKGROUND: The goal was to determine the effect of addition of oblique trajectory distal interlock screws to a retrograde intramedullary femoral nail on implant stability (stiffness), cycles to failure and mode of failure. The hypothesis was that addition of oblique screws would increase implant stability and number of loading cycles to failure. METHODS: Eight matched pairs were tested; one femur implanted with a femoral nail with only transverse distal interlock screws and the other with transverse and oblique interlock screws. Axial compressive load was applied to the femoral head and the gluteal tendon was tensioned vertically to simulate standing or at 45° to the sagittal plane to simulate stair climbing. Loads were cycled to increasing amplitude until failure of fixation (10 mm displacement or 10° rotation). FINDINGS: In simulated standing, oblique screw specimen had greater sagittal bending (bowing) than transverse only specimen. Transverse (axial) plane motion was higher in simulated stair climbing in oblique screw specimen. Oblique screw specimen had higher sagittal plane translation at 600 N of load. At 300 N, oblique screw specimen had lower internal-external rotation than transverse only specimen. A larger number of cycles to failure were observed in four oblique screw of seven paired specimen. Failure (10 mm or 10 degrees of motion) was only achieved during simulated stair climbing. INTERPRETATION: Our hypothesis that adding oblique screws improves fixation was rejected. Activities of daily living other than standing may constitute a challenge to fracture fixation; fixation failure occurred at lower loads in simulated stair climbing than standing.

Biomechanical comparison of elbow stability constructs.

BACKGROUND: Despite surgical stabilization of complex elbow trauma, additional fixation to maintain joint congruity and stability may be required. Multiple biomechanical constructs include a static external fixator (SEF), a hinged external fixator (HEF), an internal joint stabilizer (IJS), and a hinged elbow orthosis (HEO). The optimal adjunct fixation to surgical reduction is yet to be determined. METHODS: Eight matched cadaveric upper extremities were tested in a biomechanical model. Anteroposterior stress radiographs were obtained of the elbow in full supination at 0° and 45° of elbow flexion with the weight of the hand serving as a varus load as the baseline. A 360° capsuloligamentous soft-tissue release was performed around the elbow. The biomechanical constructs were applied in the same sequential order: SEF, HEF, IJS, and HEO. For each construct, 0 kg (0-lb) and 2.3 kg (5-lb) of weight were applied to the distal arm. At both weights, radiographs were obtained with the elbow at 0° and 45° of flexion, with subsequent measurement of displacement, congruence at the ulnohumeral joint, and the ulnohumeral opening angle. Statistical analysis was performed to quantify the strength and stability of each construct. RESULTS: Compared with the control group at 0° with and without 2.3 kg (5-lb) of varus force and at 45° with and without 2.3 kg (5-lb) of varus force, no difference was noted in the medial ulnohumeral joint space, lateral ulnohumeral joint space, or ulnohumeral opening angle between the SEF, HEF, and IJS. The gap change after exertion of a 2.3-kg (5-lb) force between the control condition and application of each construct demonstrated no difference between the SEF, HEF, and IJS. Comparison among destabilized elbows showed no significant difference between the SEF, HEF, and IJS. The HEO catastrophically failed in each position at 0 kg (0-lb) of weight. CONCLUSION: The SEF, HEF, and IJS are neither superior nor inferior at maintaining elbow congruity with the weight of the arm and 2.3 kg (5-lb) of varus stress. The HEO did not provide additional stability to the unstable elbow.

Biomechanical Analysis of Crossed Pinning Construct in Supracondylar Fracture of Humerus: Does the Point of Crossing Matter?

Introduction This appears to be the first biomechanical study that compares the stability of various locations of the crossing points in crossed pinning Kirschner wiring (K-wire) construct in treating pediatric supracondylar humerus fracture (SCHF). Additionally, this study compared the biomechanical stability between crossed pinning K-wire construct and the three-lateral divergent K-wire construct. Methods For the study purpose, 30 synthetic humerus bones were osteotomised at mid-olecranon fossa, anatomically reduced, and pinned using two 1.6-millimeter K-wires in five different constructs. A total of six samples were prepared for each construct and tested for extension, flexion, valgus, varus, internal rotation, and external rotation forces. Results As for crossed pinning K-wire construct, the center crossing point emerged as the stiffest construct in both linear and rotational forces, in comparison to the lateral crossing point, superior crossing, and medial crossing point Conclusion Based on this analysis, it is highly recommended that, if the crossed pinning construct is selected to treat supracondylar humerus fracture, the surgeon should aim for center crossing point as it is the most stable construct. Nevertheless, if lateral and superior crossing points are obtained during the initial attempt of fixation, the fixation may be accepted without revising the K-wire as the stability of these two constructs are comparable and portrayed no significant difference when compared to that of the center crossing point. Additionally, it is essential to avoid the medial crossing point as it is significantly less stable in terms of rotational force when compared to the center crossing point.

Posterior unilateral exposure and stability reconstruction with pedicle and lamina screw fixation for the cervical dumbbell tumorectomy: a case report and biomechanical study.

PURPOSE: Cervical dumbbell tumor is usually removed via a posterior approach and may require the spinal fixation sometimes. However, the present surgical methods involved either more trauma or a higher risk of instability of the cervical spine. A new technique of unilateral exposure and stability reconstruction with pedicle and lamina screws fixation for posterior cervical dumbbell tumorectomy was described and compared with conventional techniques. METHODS: Posterior unilateral exposure, hemi-laminectomy and facetectomy were performed in one patient with the cervical dumbbell tumor between C3 and C4. The stability was reconstructed by the unilateral pedicle and lamina screws fixation (UPLS), and a strip of shaped allograft bone was also implanted between the superior and inferior lateral mass. Biomechanical stability test of this new technique was investigated using seven fresh-frozen human cervical spine specimens (C4-C7) and compared with unilateral pedicle screw (UPS) and bilateral pedicle screw fixation (BPS) techniques. A continuous pure moment of ± 2.0 Nm was applied to the specimen in flexion, extension, lateral bending and axial rotation. RESULTS: The cervical dumbbell tumor was removed completely, and bone fusion with continuous bone trabecula was maintained in the patient on the final follow-up examination at 18 months postoperatively. Biomechanical stability tests revealed that the range of motion of the UPLS fixation plus graft bone implant was the same as the BPS fixation in flexion (1.8°vs. 1.5°, p = 0.58) and extension (2.3°vs. 2.2°, p = 0.73), but significantly bigger in lateral bending (3.9° vs. 1.0°, p < 0.001) and axial rotation (6.8° vs. 3.8°, p = 0.002), which were significantly smaller than the UPS fixation in all directions (all p < 0.001). CONCLUSIONS: For the treatment of cervical dumbbell tumor, posterior unilateral exposure and stability reconstruction with pedicle and lamina screws fixation following hemi-laminectomy and facetectomy appear to be a more stable and lesser trauma technique. LEVEL OF EVIDENCE: Diagnostic: individual cross-sectional studies with consistently applied reference standard and blinding.

Osseodensification drilling vs conventional manual instrumentation technique for posterior lumbar fixation: Ex-vivo mechanical and histomorphological analysis in an ovine model.

Lumbar fusion is a procedure associated with several indications, but screw failure remains a major complication, with an incidence ranging 10% to 50%. Several solutions have been proposed, ranging from more efficient screw geometry to enhance bone quality, conversely, drilling instrumentation have not been thoroughly explored. The conventional instrumentation (regular [R]) techniques render the bony spicules excavated impractical, while additive techniques (osseodensification [OD]) compact them against the osteotomy walls and predispose them as nucleating surfaces/sites for new bone. This work presents a case-controlled split model for in vivo/ex vivo comparison of R vs OD osteotomy instrumentation in posterior lumbar fixation in an ovine model to determine feasibility and potential advantages of the OD drilling technique in terms of mechanical and histomorphology outcomes. Eight pedicle screws measuring 4.5 mm × 45 mm were installed in each lumbar spine of eight adult sheep (four per side). The left side underwent R instrumentation, while the right underwent OD drilling. The animals were killed at 6- and 12-week and the vertebrae removed. Pullout strength and non-decalcified histologic analysis were performed. Significant mechanical stability differences were observed between OD and R groups at 6- (387 N vs 292 N) and 12-week (312 N vs 212 N) time points. Morphometric analysis did not detect significant differences in bone area fraction occupancy between R and OD groups, while it is to note that OD showed increased presence of bone spiculae. Mechanical pullout testing demonstrated that OD drilling provided higher degrees of implant anchoring as a function of time, whereas a significant reduction was observed for the R group.

Tensioning device increases biomechanical stability of tuberosity fixation technique with cerclage sutures in reverse shoulder arthroplasty for fracture.

BACKGROUND: Complex proximal humeral fractures in elderly patients are increasingly treated with primary reverse total shoulder arthroplasty. Many surgeons use cerclage sutures for tuberosity fixation in reverse total shoulder arthroplasty for proximal humeral fractures. In this study, we hypothesized that sutures fixated with a tensioning device would achieve higher initial fixation stability of the tuberosities compared with manually knotted cerclage sutures in a biomechanical model. METHODS: A 4-part fracture was created in 7-paired human cadaver proximal humeri. The tuberosities were reduced anatomically and fixed with 3 cerclage sutures in a standardized technique. Tightening was performed either manually (n = 7) or with a cerclage tensioning device with 50 Newton meter (N m) (n = 7). The humeri were placed in a custom-made test setup enabling internal and external rotation. Cyclic loading with gradually increasing load was applied with a material testing machine starting with 20 N m and increasing by 5 N m after each 100th cycle until failure (>15° rotation of the tuberosities). Motion of the tuberosities was measured with a 3-dimensional camera system. RESULTS: Overall, the knot group reached 1040 ± 152 cycles, and the device group reached 1820 ± 719 cycles (P = .035). Major fragment motion was detected in the humeral shaft axis and in the distal divergence of the tuberosities. After 900 cycles, the knot group showed increased rotation of both lesser and greater tuberosities in all 3 axes around the humeral shaft compared with the device group. CONCLUSION: Biomechanical stability of the reattached tuberosities is significantly increased, and rotational movement of the tuberosities is decreased after tightening of the applied cerclage sutures with a tensioning device compared with manual knotting. However, transferability of these promising biomechanical results and their clinical relevance have to be verified with clinical studies.

Effect of haptic geometry in C-loop intraocular lenses on optical quality.

The biomechanical stability of intraocular lenses (IOLs) must achieve high-quality optical performance and clinical outcomes after cataract surgery. For this reason, the quality and performance features of the IOLs should be previously analysed following the Standard ISO 11979-2 and ISO 11979-3. The ISO 11979-3 tries to reproduce the behaviour of the IOL in the capsular bag by compressing the lens between two clamps. With this test, it has been demonstrated that the haptic design is a crucial factor to obtain biomechanical stability. Hence, the main goal of this study was to design an aberration-free aspheric IOL and to study the influence of haptic geometry on the optical quality. For that purpose, 5 hydrophobic IOLs with different haptic design were manufactured and their biomechanical stability was compared experimentally and numerically. The IOLs were classified as stiff and flexible designs depending on their haptic geometry. The biomechanical response was measured by means of the compression force, the axial displacement, the angle of contact or contact area, the decentration, the tilt and the strain energy. The results suggest that in vitro and in silico compression tests present similar responses for the IOLs analysed. Furthermore, the flexible IOL designs presented better biomechanical stability than stiff designs. These results were correlated with the optical performance, where the optical quality decreases with worst biomechanical stability. This numerical methodology provides an indisputable advance regarding IOL designs, leading to reduce costs by exploring a feasible space of solutions during the product design process and prior to manufacturing.

Biomechanical evaluation of four different posterior instrumentation techniques for single-level transforaminal lumbar interbody fusion: a finite element analysis.

This study aims to investigate the fixation strength of unilateral cortical bone trajectory screw fixation (UCBT) and UCBT with contralateral translaminar facet screw fixation (UCBT-TFS) by repeating the verification of three finite element models. Three healthy female models of the lumbosacral spine were constructed. For each of them, four transforaminal lumbar interbody fusion (TLIF) models with the following instruments were created: bilateral traditional trajectory pedicle screw fixation (TT), bilateral cortical bone trajectory screw fixation (CBT), UCBT, and UCBT-TFS. A 150-N compressive load with 10 N/m moments was applied to simulate flexion, extension, lateral bending, and axial rotation. The range of motion (ROM), the stress of the cages, and the stress of the posterior fixations were compared. TT and UCBT-TFS had a similar low ROM compared to the intact models, and CBT showed a higher ROM in lateral bending. UCBT resulted in the highest ROM under all loading conditions, especially in lateral bending (116% and 170% greater than TT in left bending and right bending). UCBT induced a significant increase in the peak stress of cages and instruments, followed by CBT and UCBT-TFS, and the lowest mean values were observed for TT. Among the four different fixation techniques, TT offered the highest fixation strength and lowest implant stress, followed by UCBT-TFS and CBT, while UCBT was the least stable and resulted in increased stress of the screws and cages. UCBT-TFS improved biomechanical stability and appeared to be a less invasive alternative in well-selected patients with single-level TLIF.

Customised Selection of the Haptic Design in C-Loop Intraocular Lenses Based on Deep Learning.

In order to increase the probability of having a successful cataract post-surgery, the customisation of the haptic design of the intraocular lens (IOL) according to the characteristics of the patient is recommended. In this study, we present two prediction models based on deep neural networks (DNNs). One is capable of predicting the biomechanical stability of any C-loop IOL, whereas the other can predict the haptic design that fits a desired biomechanical response, enabling the selection of the optimal IOL as a function of the IOL diameter compression. The data used to feed the networks has been obtained from a validated finite element model in which multitude of geometries are tested according to the ISO 11979-3 compression test, a standard for the mechanical properties of the IOLs. The biomechanical response model provides a very high accurate response (Pearson's r = 0.995), whilst the IOL haptic design model shows that several IOL designs can provide the same biomechanical response (Pearson's r = 0.992). This study might help manufacturers and ophthalmologists both analyse any IOL design and select the best IOL for each patient. In order to facilitate its application, a graphical user interface (GUI) was created to show the potential of deep learning methods in cataract surgery.

Mechanical Behavior of Elastic Self-Locking Nails for Intramedullary Fracture Fixation: A Numerical Analysis of Innovative Nail Designs.

Intramedullary nails constitute a viable alternative to extramedullary fixation devices; their use is growing in recent years, especially with reference to self-locking nails. Different designs are available, and it is not trivial to foresee the respective in vivo performances and to provide clinical indications in relation to the type of bone and fracture. In this work a numerical methodology was set up and validated in order to compare the mechanical behavior of two new nailing device concepts with one already used in clinic. In detail, three different nails were studied: (1) the Marchetti-Vicenzi's nail (MV1), (2) a revised concept of this device (MV2), and (3) a new Terzini-Putame's nail (TP) concept. Firstly, the mechanical behavior of the MV1 device was assessed through experimental loading tests employing a 3D-printed component aimed at reproducing the bone geometry inside which the device is implanted. In the next step, the respective numerical model was created, based on a multibody approach including flexible parts, and this model was validated against the previously obtained experimental results. Finally, numerical models of the MV2 and TP concepts were implemented and compared with the MV1 nail, focusing the attention on the response of all devices to compression, tension, bending, and torsion. A stability index (SI) was defined to quantify the mechanical stability provided to the nail-bone assembly by the elastic self-locking mechanism for the various loading conditions. In addition, results in terms of nail-bone assembly stiffness, computed from force/moment vs. displacement/rotation curves, were presented and discussed. Findings revealed that numerical models were able to provide good estimates of load vs. displacement curves. The TP nail concept proved to be able to generate a significantly higher SI (27 N for MV1 vs. 380 N for TP) and a greater stiffening action (up to a stiffness difference for bending load that ranges from 370 Nmm/° for MV1 to 1,532 Nmm/° for TP) than the other two devices which showed similar performances. On the whole, a demonstration was given of information which can be obtained from numerical simulations of expandable fixation devices.

Achievable pin spanning angulation in anterosuperior pelvic external fixation.

INTRODUCTION: Pelvic external fixation using anterosuperior pins provides a quick method of stabilization without necessitating fluoroscopic guidance. Various locations, depths, and inclinations have been cited for external fixator pins; however, the existing literature lacks clear indications for the angular difference between pins. Thus, we aimed to determine the greatest degree of sagittal pin spanning angulation (SPSA) between two iliac crest pins and how intraosseous depth (ID) affects these angulations. MATERIALS AND METHODS: A newly developed computer algorithm produced cross sections of 3D pelvic reconstructions in the sagittal plane in 5° increments. Computer-generated pins with IDs of 60, 75, and 90 mm were positioned in 5° increments transversely. Pins were assessed for cortical containment to define values for SPSA and transverse pin spanning angulation (TPSA). RESULTS: A bimodal distribution revealed varying degrees of insertion frequency and SPSA, cranially and caudally. The caudal distribution exhibited greater cortical containment with larger values for SPSA and TPSA. The highest insertion frequency (85.7%) and largest SPSA (155°) were observed for the 60-mm ID. Increasing ID resulted in further bony penetration and smaller values for SPSA and TPSA. CONCLUSIONS: Expanding the degree of SPSA between inserted pins in anterosuperior pelvic external fixation can be challenging due to the thinning of the iliac wing, which affords a narrow corridor for intraosseous pin containment. An ID of 60 mm allows larger degrees of SPSA while maintaining higher rates of cortical pin containment when compared to pins with greater IDs.

Syndesmotic malreduction may decrease fixation stability: a biomechanical study.

BACKGROUND: This study aims to investigate the malreduction of syndesmosis and its effects on stability. METHODS: The biomechanical tests, including the three-dimensional (3D) displacement of the syndesmotic incisura, fibular rotation angle, and torque resistance, were performed on six cadaver legs. These specimens were first tested intact (intact group), then cut all the syndesmotic ligaments and fixed in anatomical position (anatomical model group) and test again. After that, syndesmosis was fixed in 1 cm malreduction (anterior and posterior displacement group) to do the same test. RESULTS: In internal or external load, there were significant differences in torque resistance and fibular rotation angle (internal t = 2.412, P = 0.036; external t = 2.412, P = 0.039) between the intact and post-malreduction groups. In internal rotation load, there were significant differences in sagittal displacement between the intact and post-malreduction groups (P = 0.011), and between the anatomical and post-malreduction groups (P = 0.020). In external rotation load, significant differences existed between the intact and ant-malreduction group (P = 0.034) in sagittal (anterior-posterior) displacement. Significant differences also existed between the intact and post-malreduction groups (P = 0.013), and between the anatomical and post-malreduction groups (P = 0.038) in coronal (medial-lateral) displacement. CONCLUSIONS: Malreduction in different conditions does affect the stability of the syndesmotic fixation. The result of the study may reveal the biomechanical mechanism of poor clinical outcome in syndesmosis malreduction patients and pathological displacement patterns of the ankle under syndesmotic malreduction conditions. LEVEL OF EVIDENCE: III.

Systematic Study on the Biomechanical Stability of C-Loop Intraocular Lenses: Approach to an Optimal Design of the Haptics.

To study the main design parameters that affect the mechanical stability of C-loop intraocular lenses, leading to an optimal design that minimizes the axial displacement, tilt and rotation. A total of 144 geometrical variations were studied on a 1-piece, non-angulated, C-loop hydrophobic acrylate intraocular lens. The study was performed in a finite element modeling simulation. The suitable set of variations was determined using a mixed-factorial analysis, allowing to analyse the impact of the different designs on the mechanical stability of the lens (compression force, axial displacement, tilt and rotation). The design parameters under study were: the length, width, thickness and opening angle of the haptic, the haptic-optic junction and the start of the haptic curvature. The compression (or reaction) force is affected by the haptic width, the haptic-optic junction, and the interaction between both. The axial displacement is mainly affected by the width and thickness of the haptic, and the size of the haptic-optic junction as well. The tilt is affected by the haptic thickness and the interaction between the haptic curvature and the haptic-optic junction. The rotation is affected by the start of the haptic curvature, the haptic-optic junction and the haptic width. The haptic-optic juntion is one of the most influential parameters affecting the four responses studied of the C-Loop IOL. The smaller the haptic-optic juntion, the better biomechanical stability.

Effects of transverse connector on reduction and fixation of atlantoaxial dislocation and basilar invagination using posterior C1-C2 screw-rod technique.

BACKGROUND CONTEXT: The mechanical strength provided by internal fixation is crucial for maintaining reduction and facilitating bony fusion. Though satisfactory results with the C1-C2 technique have been acquired in most clinical reports, the related problems of fusion delay and pseudarthrosis still exist. To increase the chance of bony fusion, a transverse connector (TC) is frequently used to augment torsional stiffness of thoracolumbar screw/rod constructs. Nevertheless, the clinical implication of TC in the management of atlantoaxial dislocation (AAD) and basilar invagination (BI) remains largely unknown. PURPOSE: To evaluate the effects of TC application on C1-C2 screw-rod constructs based on consecutive adult patients with AAD and BI in a single institution over a 10-year period. STUDY DESIGN: A retrospective study. PATIENT SAMPLE: Patients with AAD and BI, who were treated with posterior C1-C2 screw-rod technique with or without TC usage from June 2007 to June 2017 at a single institution. OUTCOME MEASURES: The radiological measurements included the anterior atlantodental interval (AADI), posterior atlantodental interval (PADI), height of odontoid process above Chamberlain line, and cervicomedullary angle (CMA). Patients' neurologic status was evaluated with the Japanese Orthopaedic Association (JOA) score. Fusion status was evaluated at different follow-up periods. METHODS: We compared the difference of clinical, radiological, and surgical outcomes between the TC and NTC groups postoperatively. RESULTS: In total, there were 149 consecutive patients in the TC group and 168 patients in the NTC group. On average, 1.2 TCs per patient were used in the TC group. No significant differences were identified for operative time and blood loss between groups. There was also no statistical difference in the radiological measurements of AADI, PADI, Chamberlain line, and CMA between the TC and NTC groups preoperatively and postoperatively. A significantly higher JOA score was obtained in the TC group than that in the NTC group postoperatively. The fusion rates were higher in the TC group than those in the NCT group at the early stage postoperatively (3 and 6 months; p<.01). CONCLUSIONS: Use of TCs seems to improve bony fusion and neurologic outcomes in the treatment of AAD and BI with C1-C2 screw-rod technique.

The effect of the mandibular plane angle on fracture line stability: An ex vivo experimental study.

BACKGROUND/AIMS: Mandibular angle fractures fixated with plate osteosynthesis techniques have to withstand the effects of muscle attachments. Individual variations in the craniofacial morphology may alter the biomechanical resistance of the bone-plate construct. The aim of the present study was to determine the influence of variations in the mandibular plane angle (MPa) on the biomechanical stability of sheep mandibular angle fractures (MAFs). MATERIALS AND METHODS: Sixty sheep hemi-mandibles were used. The mandibles were positioned on a test jig that simulated low (15°, group L), normal (25°, group N), and high (35°, group H) MPa. Unfavorable MAFs were created with thin diamond cutting disks. One four-hole, 9.0-mm-spacing, standard titanium miniplate of 2.0 mm thickness and 5.0-mm-long screws were inserted at the superior border of the alveolar bone in monoplanar orientation. Specimens were then subjected to vertical loads between 10 N and 150 N in a universal testing machine. The displacement values at each 10 N force increment and the load magnitude at which 3.0 mm displacement limit was reached were recorded. RESULTS: Starting from 40 N, the displacement values at each 10 N increment in the H group were significantly higher than those of the L and N groups until 150 N (P < 0.05). The force magnitude required to reach 3.0 mm of displacement in the H group was significantly lower than that for the L and N groups (P < 0.05 for each). CONCLUSIONS: The one-miniplate monoplanar fixation technique used in sheep MAF with high MPa is more likely to offer lower biomechanical resistance to the vertical forces applied over the molar region than do the normal and low MPa.

Biomechanical Stability Before and After Graft Fusion with Unilateral and Bilateral Pedicle Screw Fixation: Finite Element Study.

BACKGROUND: Minimally invasive transformational lumbar interbody fusion (MI-TLIF) with unilateral pedicle screw (UPS) fixation was controversial. The aim of this study was to compare the stability between UPS and bilateral pedicle screw (BPS) fixation before and after graft fusion. METHODS: An L3-L5 finite element model was modified to simulate L4/5 MI-TLIF. Five different statuses of posterior instrumentation were simulated: UPS fixation or BPS fixation before and after graft fusion and removal of posterior instrumentation after graft fusion. Range of motion and Von Mises stress were evaluated for intact and instrumentation models in all loading planes. RESULTS: Range of motion of the L4/5 segment with UPS fixation was 2.1, 1.3, and 1.7 times greater than those with BPS fixation before fusion in flexion-extension, lateral bending, and axial rotation, respectively, while it was 1.3, 1.1, and 1.4 times greater after fusion. The peak Von Mises stresses on posterior instrumentations with UPS fixation ranged from 1.0 to 1.7 times greater than those in BPS fixation before fusion, while it ranged from 1.0 to 1.4 times greater after fusion. The peak Von Mises stresses on intervertebral graft with UPS fixation ranged from 1.9 to 3.5 times greater than those with BPS fixation before fusion, while it ranged from 0.9 to 1.2 times greater after fusion. CONCLUSIONS: Fusion of graft improved the fixation effect of posterior instrumentation system. Unilateral pedicle screw fixation could provide similar biomechanical stability to bilateral pedicle screw fixation in 1-level MI-TLIF after fusion.