Characterizing Strain Rate-Dependent Mechanical Properties for Bovine Cortical Bones.

Comprehensive knowledge of strain rate-dependent viscoelastic properties of bony materials is necessary to understand the mechanisms of bone fracture under impact loading conditions (e.g., falls, traffic accidents, and military environments). Although the mechanical properties of bones have been studied for several decades, the high strain rate data and corresponding material parameters of the rate-dependent constitutive models are still limited. In this study, split Hopkinson pressure bar technique was used to test bovine cortical bones, to obtain the rate-dependent stress-strain curves in two directions (along and perpendicular to the bone fibers). A constitutive relationship comprising two terms was then applied to identify the material constants with strain rate effect and viscoelastic properties. In this model, the linear elasticity was combined with nonlinear viscoelasticity components to describe the overall nonlinear strain rate dependence. The presented data give strong experimental evidence and basis for further development of numerical biomechanical models to simulate human cortical bone fracture.

[Three-dimensional Finite Element Analysis to T-shaped Fracture of Pelvis in Sitting Position].

We developed a three-dimensional finite element model of the pelvis. According to Letournel methods, we established a pelvis model of T-shaped fracture with its three different fixation systems, i. e. double column reconstruction plates, anterior column plate combined with posterior column screws and anterior column plate combined with quadrilateral area screws. It was found that the pelvic model was effective and could be used to simulate the mechanical behavior of the pelvis. Three fixation systems had great therapeutic effect on the T-shaped fracture. All fixation systems could increase the stiffness of the model, decrease the stress concentration level and decrease the displacement difference along the fracture line. The quadrilateral area screws, which were drilled into cortical bone, could generate beneficial effect on the T-type fracture. Therefore, the third fixation system mentioned above (i. e. the anterior column plate combined with quadrilateral area screws) has the best biomechanical stability to the T-type fracture.

[Finite element analysis of five internal fixation modes in treatment of Day type â ¡crescent fracture dislocation of pelvis].

Objective: To compare the biomechanical differences among the five internal fixation modes in treatment of Day type Ⅱ crescent fracture dislocation of pelvis (CFDP), and find an internal fixation mode which was the most consistent with mechanical principles. Methods: Based on the pelvic CT data of a healthy adult male volunteer, a Day type Ⅱ CFDP finite element model was established by using Mimics 17.0, ANSYS 12.0-ICEM, Abaqus 2020, and SolidWorks 2012 softwares. After verifying the validity of the finite element model by comparing the anatomical parameters with the three-dimensional reconstruction model and the mechanical validity verification, the fracture and dislocated joint of models were fixed with S 1 sacroiliac screw combined with 1 LC-Ⅱ screw (S 1+LC-Ⅱ group), S 1 sacroiliac screw combined with 2 LC-Ⅱ screws (S 1+2LC-Ⅱ group), S 1 sacroiliac screw combined with 2 posterior iliac screws (S 1+2PIS group), S 1 and S 2 sacroiliac screws combined with 1 LC-Ⅱ screw (S 1+S 2+LC-Ⅱ group), S 2-alar-iliac (S 2AI) screw combined with 1 LC-Ⅱ screw (S 2AI+LC-Ⅱ group), respectively. After each internal fixation model was loaded with a force of 600 N in the standing position, the maximum displacement of the crescent fracture fragments, the maximum stress of the internal fixation (the maximum stress of the screw at the ilium fracture and the maximum stress of the screw at the sacroiliac joint), sacroiliac joint displacement, and bone stress distribution around internal fixation were observed in 5 groups. Results: The finite element model in this study has been verified to be effective. After loading 600 N stress, there was a certain displacement of the crescent fracture of pelvis in each internal fixation model, among which the S 1+LC-Ⅱ group was the largest, the S 1+2LC-Ⅱ group and the S 1+2PIS group were the smallest. The maximum stress of the internal fixation mainly concentrated at the sacroiliac joint and the fracture line of crescent fracture. The maximum stress of the screw at the sacroiliac joint was the largest in the S 1+LC-Ⅱ group and the smallest in the S 2AI+LC-Ⅱ group. The maximum stress of the screw at the ilium fracture was the largest in the S 1+2PIS group and the smallest in the S 1+2LC-Ⅱ group. The displacement of the sacroiliac joint was the largest in the S 1+LC-Ⅱ group and the smallest in the S 1+S 2+LC-Ⅱ group. In each internal fixation model, the maximum stress around the sacroiliac screws concentrated on the contact surface between the screw and the cortical bone, the maximum stress around the screws at the iliac bone concentrated on the cancellous bone of the fracture line, and the maximum stress around the S 2AI screw concentrated on the cancellous bone on the iliac side. The maximum bone stress around the screws at the sacroiliac joint was the largest in the S 1+LC-Ⅱ group and the smallest in the S 2AI+LC-Ⅱ group. The maximum bone stress around the screws at the ilium was the largest in the S 1+2PIS group and the smallest in the S 1+LC-Ⅱ group. Conclusion: For the treatment of Day type Ⅱ CFDP, it is recommended to choose S 1 sacroiliac screw combined with 1 LC-Ⅱ screw for internal fixation, which can achieve a firm fixation effect without increasing the number of screws.

Underbody blast effect on the pelvis and lumbar spine: A computational study.

Explosion from an anti-tank landmine under a military vehicle, known as underbody blast (UBB), may cause severe injury or even death for the occupants inside the vehicle. Severity and patterns of lower extremity, pelvis and lumbar spine injuries subjected to UBB have been found highly related to loading conditions, i.e. the vertical acceleration pulse. A computational human model has been developed and successfully simulated the tibia fracture under UBB in the previous study. In the present study, it was further improved by building a detailed lumbar spine and pelvis model with high biofidelity. The newly developed pelvis and lumbar spine were validated against component level test data in the literature. Then, the whole body model was validated with the published cadaver sled test data. Using the validated whole body model, parametric studies were conducted by adjusting the peak acceleration and time duration of pulses produced in the UBB to investigate the effect of waveform on the injury response. The critical values of these two parameters for pelvis and lumbar spine fracture were determined, and the relationship between injury pattern and loading conditions was established.

Biomechanical analysis of the fixation systems for anterior column and posterior hemi-transverse acetabular fractures.

OBJECTIVE: The aim of this study was to evaluate the biomechanical properties of common fixation systems for complex acetabular fractures. METHODS: A finite element (FE) pelvic model with anterior column and posterior hemi-transverse acetabular fractures was created. Three common fixation systems were used to fix the posterior wall acetabular fractures: 1. Anterior column plate combined with posterior column screws (group I), 2. Anterior column plate combined with quadrilateral area screws (group II) and 3. Double-column plates (group III). And 600 N, representing the body weight, was loaded on the upper surface of the sacrum to simulate the double-limb stance. The amounts of total and relative displacements were compared between the groups. RESULTS: The total amount of displacement was 2.76 mm in group II, 2.81 mm in group III, and 2.83 mm in group I. The amount of relative displacement was 0.0078 mm in group II, 0.0093 mm in group III and 0.014 mm in group I. CONCLUSION: Our results suggested that all fixation systems enhance biomechanical stability significantly. Anterior column plate combined with quadrilateral area screws has quite comparable results to double column plates, they were superior to anterior column plate combined with posterior screws.

Biomechanical comparison of fixation systems in posterior wall fracture of acetabular by finite element analysis.

BACKGROUND: The use of reconstruction plates and lag screws has been recommended for fractures to the posterior wall of the acetabulum. However, little information about the rigidity of recommended forms of fracture fixation is available. This study aimed to evaluate the biomechanical difference among the fixation systems. METHODS: A posterior wall fracture, which is represented by softer elements with lower elastic modulus, was created along an arc of 40-90° about the acetabular rim. Three different fixation systems: screws alone, reconstruction plate with screws, reconstruction plate with T-shaped plates were used to fix the posterior wall fractures to the acetabulum. RESULTS: All three fixation system can be used to obtain good functional outcomes. The reconstruction plate with T-shaped plates was beneficial to increasing the effective stiffness, decreasing the stress concentration and enhancing the rigidity of fracture fixation. So this fixation system served an ideal result in the analysis. CONCLUSION: Theoretically, the reconstruction plate with T-shaped plates system may reduce many of the risks and limitations compared to the other fixation systems. This fixation system may result in a clinical benefit.

Biomechanical Role of the C1 Lateral Mass Screws in Occipitoatlantoaxial Fixation: A Finite Element Analysis.

STUDY DESIGN: Finite element analysis. OBJECTIVE: To determine and compare the construct stability of occipitoatlantoaxial (C0-C1-C2) fixation provided by occipital plate, rod, and screw fixation with or without C1 lateral mass screw (C1LMS). SUMMARY OF BACKGROUND DATA: Occipitoatlantoaxial fixation techniques use C2 pedicle screw (C2PS) with and without C1LMS that are then incorporated into occipital plate fixation points using occipital screw. There has, however, been no consensus about the standard occiput to C2 fixation in literature and few reports exist about the effects of additional intervening rigid C1LMS on the biomechanics. The role of biomechanics of the addition of C1LMS in occipitoatlantoaxial fixation for fusion is not known. METHODS: A nonlinear finite element model (FEM) of the intact upper cervical spine had been developed and validated. Then an FEM of an unstable model treated with occipital plate combined with C2PS and C1LMS fixation (C1LMS + C2PS + plate), was compared to that with C2PS fixation (C2PS + plate). Vertical load of 50 N was applied on the C0, to simulate head weight and 1.5 Nm torque was applied to the C0 to simulate flexion, extension, lateral bending, and axial rotation. RESULTS: Compared with C2PS + plate, the C1LMS + C2PS + plate reduced the range of motion of C0-C2 segment by 3.0%, 35.4%, 29.2%, and 56.9% in flexion, extension, lateral bending, and axial rotation, respectively, and it also led to lower occipital screw and superior rod stresses in all loading conditions. CONCLUSION: The addition of supplemental C1LMS to occiput-C2 fixation not only enhances greater stability, especially during axial rotation, but also has the capability of distributing the stress evenly and reduces the risk of construct failure because of occipital screw pullout and rod fracture. Therefore, this method may be important to elderly patients with osteopenia or osteoporosis and it may promote a high occipitoatlantoaxial fusion rate. LEVEL OF EVIDENCE: N/A.

The Influence of Pelvic Ramus Fracture on the Stability of Fixed Pelvic Complex Fracture.

This study aims to evaluate the biomechanical mechanism of pelvic ring injury for the stability of pelvis using the finite element (FE) method. Complex pelvic fracture (i.e., anterior column with posterior hemitransverse lesion) combined with pelvic ramus fracture was used to evaluate the biomechanics stability of the pelvis. Three FE fracture models (i.e., Dynamic Anterior Plate-Screw System for Quadrilateral Area (DAPSQ) for complex pelvic fracture with intact pubic ramus, DAPSQ for complex pelvic fracture with pubic ramus fracture, and DAPSQ for complex pelvic fracture with fixed pubic ramus fracture) were established to explore the biomechanics stability of the pelvis. The pubic ramus fracture leads to an unsymmetrical situation and an unstable situation of the pelvis. The fixed pubic ramus fracture did well in reducing the stress levels of the pelvic bone and fixation system, as well as displacement difference in the pubic symphysis, and it could change the unstable situation back to a certain extent. The pelvic ring integrity was the prerequisite of the pelvic stability and should be in a stable condition when the complex fracture is treated.

Biomechanical Analysis of the Fixation System for T-Shaped Acetabular Fracture.

This study aims to evaluate the biomechanical mechanism of fixation systems in the most frequent T-shaped acetabular fracture using finite element method. The treatment of acetabular fractures was based on extensive clinical experience. Three commonly accepted rigid fixation methods (double column reconstruction plates (P × 2), anterior column plate combined with posterior column screws (P + PS), and anterior column plate combined with quadrilateral area screws (P + QS)) were chosen for evaluation. On the basis of the finite element model, the biomechanics of these fixation systems were assessed through effective stiffness levels, stress distributions, force transfers, and displacements along the fracture lines. All three fixation systems can be used to obtain effective functional outcomes. The third fixation system (P + QS) was the optimal method for T-shaped acetabular fracture. This fixation system may reduce many of the risks and limitations associated with other fixation systems.