Finite element model development of a child pelvis with optimization-based material identification

Finite Element Analysis of Various Osteotomies Used in the Treatment of Developmental Hip Dysplasia in Children

Late-discovered developmental hip dysplasia deformities often necessitate complex surgical treatments and meticulous preoperative planning. The selection of osteotomies is contingent upon the patient's age and the specific structural deformity of the hip. In our anatomical hip model, derived from the data of a 12-year-old patient, we performed virtual osteotomies that are commonly recommended for such cases. We precisely constructed geometric models for various osteotomies, including the Dega, Pemberton, Tönnis, Ganz, Chiari pelvic, and Pauwels femoral osteotomies. We employed Autodesk Inventor for the finite element analysis of the hip joint and the corrective osteotomies. In comparing one-stage osteotomies, we noted that the Dega and Ganz pelvic osteotomies, especially when combined with the Pauwels femoral osteotomy, yielded the most favorable outcomes. These combinations led to enhanced femoral head coverage and reduced intra-articular pressure. Furthermore, we calculated the femoral head-to-acetabulum volume ratio for both the Dega and Pauwels osteotomies. The encouraging results we obtained advocate for the integration of finite element analysis in virtual osteotomies of the pelvis and femur as a preoperative tool in the management of developmental hip dysplasia.

Finite Element Analysis of Normal and Dysplastic Hip Joints in Children

From a surgical point of view, quantification cannot always be achieved in the developmental deformity in hip joints, but finite element analysis can be a helpful tool to compare normal joint architecture with a dysplastic counterpart. CT scans from the normal right hip of an 8-year-old boy and the dysplastic left hip of a 12-year-old girl were used to construct our geometric models. In a three-dimensional model construction, distinctions were made between the cortical bone, trabecular bone, cartilage, and contact nonlinearities of the hip joint. The mathematical model incorporated the consideration of the linear elastic and isotropic properties of bony tissue in children, separately for the cortical bone, trabecular bone, and articular cartilage. Hexahedral elements were used in Autodesk Inventor software version 2022 ("Ren") for finite element analysis of the two hips in the boundary conditions of the single-leg stance. In the normal hip joint on the cartilaginous surfaces of the acetabulum, we found a kidney-shaped stress distribution in a 471,672 mm2 area. The measured contact pressure values were between 3.0 and 4.3 MPa. In the dysplastic pediatric hip joint on a patch of 205,272 mm2 contact area, the contact pressure values reached 8.5 MPa. Furthermore, the acetabulum/femur head volume ratio was 20% higher in the dysplastic hip joint. We believe that the knowledge gained from the normal and dysplastic pediatric hip joints can be used to develop surgical treatment methods and quantify and compare the efficiency of different surgical treatments used in children with hip dysplasia.

Evaluation of the short-term curative effect of closed reduction in the treatment of developmental dysplasia of the hip based on three-dimensional magnetic resonance imaging finite element analysis

Background

Based on the Digital Imaging and Communications in Medicine (DICOM) data of three-dimensional magnetic resonance imaging (3D-MRI), finite element models of the hip joints of children with developmental dysplasia of the hip were established. The primary objectives included simulation and analysis of the finite element model pre- and post-closed reduction under different stances and loads, and evaluation of the size and distribution of von Mises stress in the acetabulum and femoral head pre- and post-operation and the short-term effects.

Methods

Acetabular index measurements of both the unaffected and affected sides were conducted, alongside International Hip Dysplasia Institute (IHDI) classification of the affected hip. Establishing the finite element model of both the affected and unaffected hips was based on the 3D-T1WI sequence DICOM data, using Mimics, 3-matic, and Ansys software, before and after closed reduction surgery. The size and distribution data of von Mises stress on the affected side of the acetabulum and femoral head were collected pre- and post-operation.

Results

The study indicated that the increasing acetabular index of the affected hip was directly proportional to the increasing severity based on IHDI classification (P < 0.05). Preoperative IHDI classification significantly correlated with the von Mises stress (r = 0.560–0.569, 0.562–0.564, P < 0.05). Under different stances and load conditions, the von Mises stress on the affected side post-operation was lower than that noted pre-operation (P < 0.01), while that on the acetabulum increased proportionally to the load. Although the magnitude and distribution of von Mises stress on the affected side of the acetabulum were similar to those on the healthy side post-operation, there were statistical differences between the two (P < 0.01). The von Mises stress of the lateral column of the femoral head post-operation was significantly lower than that noted pre-operation (P < 0.01). While the high-stress points of the lateral column disappeared post-operation, the von Mises stress was evenly distributed in the femoral head.

Conclusions

The 3D-MRI finite element could provide the von Mises stress value and distribution characteristics of the acetabulum and femoral head pre- and post-operation. Closed reduction can, therefore, improve the size and distribution of von Mises stress on the affected acetabulum and femoral head.

Prediction of fracture initiation and propagation in pelvic bones

The objective is developing an XFEM model that is capable of predicting different types of fracture in the pelvic bone under various loading conditions. Previously published mechanical and failure characteristics of cortical and cancellous tissues were implemented and assigned to an intact pelvic bone with specified cortical and cancellous tissues. Various loading conditions, including combined load directions, were applied to the acetabulum to model different types of fracture (e.g., anterior/posterior wall fracture and transverse fracture) in the pelvic bone. The predicated types of fracture and the maximum force at fracture were compared to those acquired from previously published experimental tests. Anterior/posterior wall fracture and transverse fracture were the most common types of fractures determined in the simulations. The XFEM simulations were able to predict similar fractures to those reported in the experimental tests. The maximum fracture force in the XFEM model was found to be 18.6 kN compared to 8.85 kN reported in the previous experimental tests. The results revealed that different types of fracture in the pelvic bones can be caused by the various loading conditions in unstable high-rate impact loads. Using proper mechanical and failure behaviors of cortical and cancellous tissues, XFEM modeling of pelvic bone is capable of predicting bone fracture. In future work, the XFEM models of cancellous and cortical tissues can be assigned to other bones in human body skeleton so that the failure mechanism in such bones can be investigated.

Effect of anthropometry scaling on the response of the piper child scalable human body model subject to pelvic impact

The Open Source PIPER child scalable human body model was publicly released in April 2017 (www.piper-project.org) along with frontal and side impact validation conditions. The objective of this paper is to investigate the effect of anthropometry scaling on the response of the model in side pelvic impact. Three setups from two published studies were used: (1) a lateral drop test (2) a greater trochanter impact with a rigid pendulum (3) a pelvis side impact with a flat surface. The first study used scaling assumption developed for crash test dummy design (setups 1 and 2) and the second performed tests on post mortem human surrogates. The baseline 6 years old child model was scaled using a model morphing methodology to match the stature and weight of the surrogates used in the two published studies. Overall, the main trends observed in the three setups can be approached using the baseline model. Although the model morphing did not account for specific skeletal dimensions, it reduced some of the discrepancies between model response and reference for the drop test and flat plate impact. However, it had little effect on the pendulum test. In that case, the model response was in the corridor at low speed but above at higher speeds. Possible reasons for this difference should be further investigated.

EFFECT OF MATERIAL MODEL SELECTION ON LATERAL IMPACT SIMULATIONS OF PELVIC COMPLEX

Material mechanical behavior plays an important role in pelvic complex simulation under lateral impact. Aiming to investigate effects of material model selection on the responses of lateral impact simulations, a seating pelvic complex model was constructed. The model was subjected to a series of impacts at velocity of 3–10[Formula: see text]m/s, and two material models were, respectively, assigned to the pelvic bone to evaluate the accuracy of the simulation. The results showed that the pelvic response and fracture pattern with plastic–elastic material model agreed well with the literature, while linear elastic material model was dissatisfied factory, especially the pelvic response at low velocity deviated from most cadaveric test data. In addition, drastic change of arterial pressure was responsible for hemorrhages associated with pelvic fracture. Ligament loading sequence verified that the posterior pelvic ring bore the greatest amount of load during the impact. Based on the above findings, we concluded that a plastic–elastic with strain rate effect material model can improve the simulation accuracy of pelvic complex under lateral impact, and pelvic fracture pattern may help to estimate the parameters’ selection in impact simulation.

Modular hemipelvic endoprosthesis with a sacral hook: a finite element study

Background

A novel hemipelvic endoprosthesis with a sacral hook was introduced previously, and its clinical outcome with midterm follow-up showed decreased prosthesis-related complications, especially decreased rate of aseptic loosening. The aim of present study was to evaluate the role of a sacral hook in prosthesis stability and the biomechanical properties of this hemipelvic endoprosthesis.

Methods

A three-dimensional model of the postoperative pelvis was developed using computed tomography (CT) images. A force of 500 N was applied, and the distribution of stress and displacement was evaluated. Comparisons were performed to explore the role of the sacral hook in prosthesis stability. Prosthesis improvement was simulated to reduce unexpected breakage of the pubic connection plate.

Results

In the reconstructed hemipelvis, stress distributions were concentrated on the superior area of the acetabulum, sacral connection component, and sacral hook. A maximum stress of 250 MPa was observed at the root of the sacral connection component. The sacral hook reduced the maximum stress and displacement by 14.1% and 32.5%, respectively, when the prosthesis was well fixed and by 10.0% and 42.1%, respectively, when aseptic loosening occurred. Increasing the thickness of the pubic connection plate from 2 to 3.5 mm reduced the maximum stress by 32.0% and 15.8%, respectively.

Conclusion

A hemipelvic endoprosthesis with a sacral hook fulfills the biomechanical demands of the hemipelvis and is safe under static conditions. The sacral hook is important for prosthesis stability. Increasing the thickness of the pubic connection plate can reduce the maximum stress and risk of fatigue breakage.

Prediction of mechanical behavior of cartilaginous infant hips in pavlik harness: A subject-specific simulation study on normal and dysplastic hips

In dysplastic infant hips undergoing abduction harness treatment, cartilage contact pressure is believed to have a role in therapeutic cartilage remodeling and also in the complication of femoral head avascular necrosis. To improve our understanding of the role of contact pressure in the remodeling and the complication, we modeled cartilage contact pressure in cartilaginous infant hips undergoing Pavlik harness treatment. In subject-specific finite element modeling, we simulated contact pressure of normal and dysplastic hips in Pavlik harness at 90° flexion and gravity-induced abduction angles of 40°, 60° and 80°. We demonstrated that morphologies of acetabulum and femoral head both affected contact pressure distributions. The simulations showed that in Pavlik harness, contact pressure was mainly distributed along anterior and posterior acetabulum, leaving the acetabular roof only lightly loaded (normal hip) or unloaded (dysplastic hip). From a mechanobiological perspective, these conditions may contribute to therapeutic remodeling of the joint in Pavlik harness. Furthermore, contact pressure increased with the angle of abduction, until at the extreme abduction angle (80°), the lateral femoral head also contacted the posterior acetabular edge. Contact pressure in this area could contribute to femoral head avascular necrosis by reducing flow in femoral head blood vessels. The contact pressure we simulated can plausibly account for both the therapeutic effects and main adverse effect of abduction harness treatment for developmental dysplasia of the hip. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

A Cortical Thickness Mapping Method for the Coxal Bone Using Morphing

As human body finite element models become more integrated with the design of safety countermeasures and regulations, novel models need to be developed that reflect the variation in the population's anthropometry. However, these new models may be missing information which will need to be translated from existing models. During the development of a 5th percentile female occupant model (F05), cortical thickness information of the coxal bone was unavailable due to resolution limits in the computed tomography (CT) scans. In this study, a method for transferring cortical thickness information from a source to a target model with entirely different geometry and architecture is presented. The source and target models were the Global Human Body Models Consortium (GHBMC) 50th percentile male (M50) and F05 coxal bones, respectively. To project the coxal bone cortical thickness from the M50 to the F05, the M50 model was first morphed using a Kriging method with 132 optimized control points to the F05 anthropometry. This technique was found to be accurate with a mean nodal discrepancy of 1.27 mm between the F05 and morphed M50 (mM50) coxal bones. Cortical thickness at each F05 node was determined by taking the average cortical thickness of every mM50 node, non-linearly weighted by its distance to the F05 nodes. The non-linear weighting coefficient, β, had a large effect on the accuracy and smoothness of the projected cortical bone thickness. The optimal projection had β = 4 and was defined when the tradeoff between projection accuracy and smoothness was equal. Finally, a quasi-static pelvis compression was simulated to examine to effect of β. As β, increased from 0 to 4, the failure force decreased by ~100 N, whereas the failure displacement increased by 0.9 mm. Results from quasi-static compression tests of the F05 pelvis were comparable to experimental results. This method could be applied to other anatomical regions where cortical thickness variation is important, such as the femur and ribs and is not limited to GHBMC-family models. Furthermore, this process will aid the development of subject-specific finite element models where accurate cortical bone thickness measurements cannot be obtained.

Dynamic response and material sensitivity analysis of pelvic complex numerical model under side impact

BACKGROUND

The surrogate design and clinical diagnostic suggest that the pelvic dynamic response should be the basis of bone fracture mechanism study under side impact. Pelvic response indicators are the impact force, compression (C), viscous criterion (VC), bone stress, and bone strain. However, no evaluation of these indicators has been conducted.

OBJECTIVE

To evaluate pelvic response indicators under side impact.

METHODS

A sitting pelvic finite element (FE) complex model comprising bone, artery, ligaments, and soft tissue was constructed. The dynamic response of the model under side impact with initial velocity of 3 m/s was investigated and material sensitivity analysis was complemented by changing bone elastic modulus.

RESULTS

The pelvic FE model could predict response under side impact. Specifically, the indicators such as artery pressure and strain, together with the ligaments axial force and strain were provided. The sensitivity analysis showed the impact force, bone stress, and axial force were sensitive to the elastic modulus, whereas, C, VC, bone strain, and artery pressure were not.

CONCLUSIONS

The sitting FE model in this study can predict pelvic dynamic response, and C, VC, bone strain and artery pressure are proposed for pelvic tolerance instead of impact force under side impact.

Dynamic Simulation of Biomechanical Behaviour of the Pelvis in the Lateral Impact Loads

The objective of this study was to develop and validate a novel 3D dynamic model of a pelvic side-impactor system. The biomechanical responses of a pelvic flexible model (having .mnf file suffix) under the lateral impact load for predicting the bone fracture mechanism are investigated as well. The 3D solid model of the side-impactor system was imported into MSC/ADAMS software for analyzing the dynamic model, and the pelvic flexible model was extracted from the CT images of a Chinese female volunteer. The flexible model of the pelvis system was developed considering a wide range of mechanical properties in the bone complex and soft tissue to achieve a realistic biomechanical response during a lateral impact. Good agreements were achieved between the dynamic simulations and the experimental results of pelvic side impacts, in terms of the biomechanical criteria. The dynamic model of impactor system could be employed to investigate the hip protector effectiveness, improving the vehicle safety, and biomechanical response of the other human organs.

Hip Joint Contact Pressure Distribution During Pavlik Harness Treatment of an Infant Hip: A Patient-Specific Finite Element Model

Developmental dysplasia of the hip (DDH) in infants under 6 months of age is typically treated by the Pavlik harness (PH). During successful PH treatment, a subluxed/dislocated hip is spontaneously reduced into the acetabulum, and DDH undergoes self-correction. PH treatment may fail due to avascular necrosis (AVN) of the femoral head. An improved understanding of mechanical factors accounting for the success/failure of PH treatment may arise from investigating articular cartilage contact pressure (CCP) within a hip during treatment. In this study, CCP in a cartilaginous infant hip was investigated through patient-specific finite element (FE) modeling. We simulated CCP of the hip equilibrated at 90 deg flexion at abduction angles of 40 deg, 60 deg, and 80 deg. We found that CCP was predominantly distributed on the anterior and posterior acetabulum, leaving the superior acetabulum (mainly superolateral) unloaded. From a mechanobiological perspective, hypothesizing that excessive pressure inhibits growth, our results qualitatively predicted increased obliquity and deepening of the acetabulum under such CCP distribution. This is the desired and observed therapeutic effect in successful PH treatment. The results also demonstrated increase in CCP as abduction increased. In particular, the simulation predicted large magnitude and concentrated CCP on the posterior wall of the acetabulum and the adjacent lateral femoral head at extreme abduction (80 deg). This CCP on lateral femoral head may reduce blood flow in femoral head vessels and contribute to AVN. Hence, this study provides insight into biomechanical factors potentially responsible for PH treatment success and complications.

DYNAMIC RESPONSE OF PELVIC COMPLEX FINITE ELEMENT STUDY AND VALIDATION UNDER SIDE IMPACT

Objective: To investigate and validate dynamic response of the pelvis, a finite element model of seated pelvic complex comprising of bone, ligaments, abdominal artery and soft tissue was developed and concurrently, a cadaver experiment was set up. Materials and Methods: Based on supine scanned CT images, we first developed an FE pelvic complex model and modified it to construct a seated pelvic model by anteriorly rotating the proximal femur to 90[Formula: see text]. For the cadaver experiment, a customized pelvic impact apparatus was designed and optical devices, strain gauges and pressure detectors were used to measure the pelvic response. Results: The results of the FE analysis and the cadaver tests were congruent in terms of impact force and fracture sites. Dynamic arterial response to the lateral impact showed hemodynamic instability that was displayed in pressure variation. The response of ligaments indicated that the posterior ligaments of pelvic ring experienced a larger amount of load. Conclusion: FE results provided the impact of ligaments and arteries besides impact force, compression (C) and viscous criterion (VC). Accordingly, the cadaver experiment measured arterial pressure, impact force, bone strain and compression. The compatibility between the FE and cadaver analyses demonstrates the high bio-fidelity of our pelvic complex model.

Finite element analysis of the valgus knee joint of an obese child

Background

Knee valgus and varus morbidity is at the second top place in children lower limb deformity diseases. It may cause abnormal stress distribution. The magnitude and location of contact forces on tibia plateau during gait cycle have been indicated as markers for risk of osteoarthritis. So far, few studies reported the contact stress and force distribution on tibial plateau of valgus knee of children.

Methods

To estimate the contact stresses and forces on tibial plateau of an 8-year old obese boy with valgus knee and a 7-year old healthy boy, three-dimensional (3D) finite element (FE) models of their left knee joints were developed. The valgus knee model has 36,897 nodes and 1,65,106 elements, and the normal knee model has 78,278 nodes and 1,18,756 elements. Paired t test was used for the comparison between the results from the 3D FE analysis method and the results from traditional kinematic measurement methods.

Results

The p value of paired t test is 0.12. Maximum stresses shifted to lateral plateau in knee valgus children while maximum stresses were on medial plateau in normal knee child at the first peak of vertical GRF of stance phase. The locations of contact centers on medial plateau changed 3.38 mm more than that on lateral plateau, while the locations of contact centers on medial plateau changed 1.22 mm less than that on lateral plateau for healthy child from the first peak to second peak of vertical GRF of stance phase.

Conclusions

The paired t test result shows that there is no significant difference between the two methods. The results of FE analysis method suggest that knee valgus malalignment could be the reason for abnormal knee load that may cause knee problems in obese children with valgus knee in the long-term. This study may help to understand biomechanical mechanism of valgus knees of obese children.

A novel combined hemipelvic endoprosthesis for peri-acetabular tumours involving sacroiliac joint: a finite element study

PURPOSE

Our aim was to introduce a novel combined hemipelvic endoprosthesis for pelvic reconstruction after Enneking type I/II/IV resection and to evaluate the biomechanical properties of the endoprosthesis using finite element analysis.

METHODS

A three-dimensional finite element model of the postoperative pelvis was developed based on computed tomography (CT) images of the patient with the best post-operative limb function. A force of 400 N was applied along the longitudinal axis of the normal and post-operative pelvis for two positions: standing on two feet and sitting. Stress-distribution analysis was performed in both positions, and results were compared. Prosthesis improvements were simulated by intervertebral fusion and extra screw fixation.

RESULTS

In the normal pelvis, stress distributions were mostly concentrated on the superior area of the acetabulum, arcuate line, sacroiliac joint and sacral midline in both static conditions, and peak stresses of 1.52 MPa and 4.53 MPa were observed at the superior area of the greater sciatic notch and ischial tuberosity, respectively. For the reconstructed hemipelvis, stress distributions were concentrated on the connecting rods of the acetabular component and the proximal segment of the pedicle rods, and peak stresses of 252 MPa and 213 MPa were observed on the proximal pedicle rods of the fourth lumbar vertebra for standing and sitting, respectively. Interbody fusion of the fourth and fifth lumbar vertebrae and extra screw fixation to the sacrum decreased the peak stresses by 33.0 % and 18.3 % while standing and by 10.8 % and 6.6 % while sitting.

CONCLUSION

Reconstruction with combined hemipelvic endoprosthesis after types I/II/IV resection of the pelvis fulfilled physiological and biomechanical demands of the hemipelvis and yielded good biomechanical characteristics.

Finite element study of human pelvis model in side impact for Chinese adult occupants

OBJECTIVE

The occupant's pelvis is very vulnerable to side collision in road accidents. Finite element (FE) studies on pelvic injury help to design occupant protection devices to improve vehicle safety. This study was aimed to develop a highly biofidelic pelvis model of Chinese adults and assess its sensitivity to variations in pelvis cortical bone thickness, bone material properties, and loading conditions.

METHODS

In this study, 4 different FE models of the pelvis were developed from the computed tomography (CT) data of a volunteer representing the 50th percentile Chinese male. Two of them were meshed using entirely hexahedral elements with variable and constant cortical thickness distribution (the V-Hex and C-Hex models), and the others were modeled with hexahedral elements for cancellous bone and variable or constant thickness shell elements for cortical bone (the V-HS and C-HS models). In model developments, the semi-automatic multiblock meshing approach was employed to maintain the pelvis geometric curvature and generate a high-quality hexahedral mesh. Then, several simulations with postmortem human subjects (PMHS) tests were performed to obtain the most accurate model in predicting pelvic injury. Based on the most accurate model, sensitivity studies were conducted to analyze the effects of the cortex thickness, Young's modulus of the cortical and cancellous bone, impactor velocity, and impactor with or without padding on the biomechanical responses and injuries of pelvis.

RESULTS

The results indicate that the models with variable cortical bone thickness can give more accurate predictions than those with constant cortical thickness. Both the V-Hex and V-HS models are favorable for simulating pelvic response and injury, but the simulation results of the V-Hex model agree with the tests better. The sensitivity study shows that pelvic response is more sensitive to alterations in the Young's modulus of cortical bone than cancellous bone. Compared to failure displacement, peak force is more sensitive to the cortical bone thickness. However, displacement is more sensitive to the Young's modulus of cancellous bone than peak force. The padding attached on the impactor plays a significant role in absorbing the impact energy and alleviating pelvic injury.

CONCLUSIONS

The all-hex meshing method with variable cortical bone thickness has the highest accuracy but is time-consuming. The cortical bone plays a determining role in resisting pelvic fracture. Peak impact force appears to be a reasonable injury predictor for pelvic injury assessment. Some appropriate energy absorbers installed in the car door can significantly reduce pelvic injury and will be beneficial for occupant protection.

BIOMECHANICAL RESPONSE AND INJURY OF OCCUPANT'S PELVIS IN SIDE IMPACTS: EFFECTS OF THE FEMORAL HEAD AND LOADING CONDITIONS

The occupant's pelvis is most susceptible to injuries in side collision accidents. To further investigate the pelvis biomechanical responses and injury mechanisms in side impacts, a biofidelic pelvis finite element (FE) model was created. In contrast to previous studies, the model was based directly on the CT data of a volunteer representing the 50th percentile Chinese male. Both cortical and cancellous bone were modeled with hexahedral elements. Through model validations against Post Mortem Human Subjects (PMHS) tests, the pelvis responses and injuries under side impacts were analyzed. Meanwhile, additional simulations were carried out utilizing the validated model to study the effects of the femoral head, impactor pad and impactor velocity on pelvic injuries. The results indicated that the most frequent injury type of the pelvis is pubic rami fracture, followed by fractures of the femoral head, greater trochanter and acetabulum. In validation against the test of Guillemot et al., the critical load of pelvic fracture was 3.8 kN. In validation against the tests of Beason et al., the peak impact force under unpadded load and padded load was 4.3 kN and 3.1 kN, respectively, while the (VC)max was 0.25 m/s and 0.16 m/s, respectively. Peak impact force appears to be a reasonable criterion to assess pelvic injury. Moreover, the femoral head and impactor pad play an important role in absorbing impact energy, distributing impact load, and alleviating pelvic injury.

3-D finite element analysis of the influence of synovial condition in sacroiliac joint on the load transmission in human pelvic system

The anterior part of the sacroiliac joint (SIJ) is a synovial joint, with little gliding and rotary movement between the contact surfaces of SIJ during locomotion. Due to its complex structure, especially when considering the surrounding ligaments, it is difficult to construct an accurate three-dimensional (3-D) finite element model for the human pelvis. Most of the pelvic models in the previous studies were simplified with either SIJ fusing together or without the sacral bone. However, the influence of those simplifications on the load transmission in human pelvis has not been studied, so the reliability of those studies remains unclear. In this study, two 3-D pelvic models were constructed: an SIJ fusing model and an SIJ contacting model. In the SIJ fusing model, the SIJ interfaces were fused together. In the SIJ contacting model, the SIJ interfaces were just in contact with each other without fusion. Compared with the SIJ contacting model, the SIJ fusing model have smaller movements in the SIJ. The stress distribution area in the SIJ fusing model on sacroiliac cartilages was also different. Those differences contributed to the decline of tensile force in the SIJ surrounding ligaments and the re-distribution of stress in the pelvic bones. In addition, the SIJ fusing model was far less sensitive to the increase in modulus of the sacroiliac cartilages, and decrease in stiffness of the ligaments surrounding the SIJ. The presence of synovia in the SIJ had greater influence on the load transmission in the human pelvic system. Therefore, the effect of the presence of synovia should not be neglected when the biomechanical behavior of human pelvis is being studied, especially for those studies related to clinical applications.

Childhood obesity as a risk factor for bone fracture: a mechanistic study

OBJECTIVE

To investigate the risk of bone fracture sustained by obese children exposed to falls. The bone fracture risk of obese children would be greater than that of their nonobese counterparts was hypothesized.

DESIGN AND METHODS

Finite element-based computational models for children that reflected various levels of obesity by varying body mass and the thickness of the subcutaneous adipose tissue layer was developed. The models took account of both the momentum effect of variation of body mass and the cushion effect of variation of soft tissue thickness and examined these two contradictory effects on pelvic bone fracture risk through a set of sideways fall simulations with a range of impact speeds.

RESULTS

The critical impact speed that yielded pelvic bone fracture decreased as the levels of obesity increased, which meant that the momentum effect of a greater body mass took precedence over the cushion effect of the soft tissue layer.

CONCLUSIONS

The result suggests that obese children have a greater risk of pelvic bone fracture than do their nonobese counterparts in sideways falls. A further implication is that current child safety devices, systems, and regulations will need to be revisited as the prevalence of child obesity increases.

The influence of head diameter and wall thickness on deformations of metallic acetabular press-fit cups and UHMWPE liners: a finite element analysis

BACKGROUND

To increase the range of motion of total hip endoprostheses, prosthetic heads need to be enlarged, which implies that the cup and/or liner thickness must decrease. This may have negative effects on the wear rate, because the acetabular cups and liners could deform during press-fit implantation and hip joint loading. We compared the metal cup and polyethylene liner deformations that occurred when different wall thicknesses were used in order to evaluate the resulting changes in the clearance of the articulating region.

METHODS

A parametric finite element model utilized three cup and liner wall thicknesses to analyze cup and liner deformations after press-fit implantation into the pelvic bone. The resultant hip joint force during heel strike was applied while the femur was fixed, accounting for physiological muscle forces. The deformation behavior of the liner under joint loading was therefore assessed as a function of the head diameter and the resulting clearance.

RESULTS

Press-fit implantation showed diametral cup deformations of 0.096, 0.034, and 0.014 mm for cup wall thicknesses of 3, 5, and 7 mm, respectively. The largest deformations (average 0.084 ± 0.003 mm) of liners with thicknesses of 4, 6, and 8 mm occurred with the smallest cup wall thickness (3 mm). The smallest liner deformation (0.011 mm) was obtained with largest cup and liner wall thicknesses. Under joint loading, liner deformations in thin-walled acetabular cups (3 mm) reduced the initial clearance by about 50 %.

CONCLUSION

Acetabular press-fit cups with wall thicknesses of ≤5 mm should only be used in combination with polyethylene liners >6 mm thick in order to minimize the reduction in clearance.

Finite element analysis of the pelvis after modular hemipelvic endoprosthesis reconstruction

PURPOSE

The aim of this study was to investigate the biomechanics of the pelvis reconstructed with a modular hemipelvic prosthesis using finite element (FE) analysis.

METHODS

A three-dimensional FE model of the postoperative pelvis was developed and input into the Abaqus FEA software version 6.7.1. Mesh refinement tests were then performed and a force of 500 N was applied at the lamina terminalis of the fifth lumbar vertebra along the longitudinal axis of the normal pelvis and the postoperative pelvis for three positions: sitting, standing on two feet, and standing on the foot of the affected side. Stress distribution analysis was performed between the normal pelvis and postoperative pelvis at these three static positions.

RESULTS

In the normal pelvis, stress distribution was concentrated on the superior area of the acetabulum, arcuate line, sacroiliac joint, sacral midline and, in particular, the superior area of the greater sciatic notch. In the affected postoperative hemipelvis, stress distribution was concentrated on the proximal area of the pubic plate, the top of the acetabular cup, the connection between the CS-fixator and acetabular cup and the fixation between the prosthesis and sacroiliac joint.

CONCLUSIONS

Stress distribution of the postoperative pelvis was similar to the normal pelvis at three different static positions. Reconstruction with a modular hemipelvic prosthesis yielded good biomechanical characteristics.

Development of a 10-year-old paediatric thorax finite element model validated against cardiopulmonary resuscitation data

Thoracic injury in the paediatric population is a relatively common cause of severe injury and has an accompanying high mortality rate. However, no anatomically accurate, complex paediatric chest finite element (FE) component model is available for a 10-year old in the published literature. In this study, a 10-year-old thorax FE model was developed based on internal and external geometries segmented from medical images. The model was then validated against published data measured during cardiopulmonary resuscitation performed on paediatric subjects.

The Effect of Boundary Condition on the Biomechanics of a Human Pelvic Joint Under an Axial Compressive Load: A Three-Dimensional Finite Element Model

The finite element (FE) model of the pelvic joint is helpful for clinical diagnosis and treatment of pelvic injuries. However, the effect of an FE model boundary condition on the biomechanical behavior of a pelvic joint has not been well studied. The objective of this study was to study the effect of boundary condition on the pelvic biomechanics predictions. A 3D FE model of a pelvis using subject-specific estimates of intact bone structures, main ligaments and bone material anisotropy by computed tomography (CT) gray value was developed and validated by bone surface strains obtained from rosette strain gauges in an in vitro pelvic experiment. Then three FE pelvic models were constructed to analyze the effect of boundary condition, corresponding to an intact pelvic joint, a pelvic joint without sacroiliac ligaments and a pelvic joint without proximal femurs, respectively. Vertical load was applied to the same pelvis with a fixed prosthetic femoral stem and the same load was simulated in the FE model. A strong correlation coefficient (R2=0.9657) was calculated, which indicated a strong correlation between the FE analysis and experimental results. The effect of boundary condition changes on the biomechanical response depended on the anatomical location and structure of the pelvic joint. It was found that acetabulum fixed in all directions with the femur removed can increase the stress distribution on the acetabular inner plate (approximately double the original values) and decrease that on the superior of pubis (from 7 MPa to 0.6 MPa). Taking sacrum and ilium as a whole, instead of sacroiliac and iliolumber ligaments, can influence the stress distribution on ilium and pubis bone vastly. These findings suggest pelvic biomechanics is very dependent on the boundary condition in the FE model.

Patient-Specific Geometry Modeling and Mesh Generation for Simulating Obstructive Sleep Apnea Syndrome Cases by Maxillomandibular Advancement

The objective of this paper is the reconstruction of upper airway geometric models as hybrid meshes from clinically used Computed Tomography (CT) data sets in order to understand the dynamics and behaviors of the pre- and postoperative upper airway systems of Obstructive Sleep Apnea Syndrome (OSAS) patients by viscous Computational Fluid Dynamics (CFD) simulations. The selection criteria for OSAS cases studied are discussed because two reasonable pre- and postoperative upper airway models for CFD simulations may not be created for every case without a special protocol for CT scanning. The geometry extraction and manipulation methods are presented with technical barriers that must be overcome so that they can be used along with computational simulation software as a daily clinical evaluation tool. Eight cases are presented in this paper, and each case consists of pre- and postoperative configurations. The results of computational simulations of two cases are included in this paper as demonstration.