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 (R(2)=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.     

A biomechanical model on muscle forces in the transfer of spinal load to the pelvis and legs.

Based on musculoskeletal anatomy of the lower back, abdominal wall, pelvis and upper legs, a biomechanical model has been developed on forces in the load transfer through the pelvis. The aim of this model is to obtain a tool for analyzing the relations between forces in muscles, ligaments and joints in the transfer of gravitational and external load from the upper body via the sacroiliac joints to the legs in normal situations and pathology. The study of the relation between muscle coordination patterns and forces in pelvic structures, in particular the sacroiliac joints, is relevant for a better understanding of the aetiology of low back pain and pelvic pain. The model comprises 94 muscle parts, 6 ligaments and 6 joints. It enables the calculation of forces in pelvic structures in various postures. The calculations are based on a linear/non-linear optimization scheme. To gain a better understanding of the function of individual muscles and ligaments, deviant properties of these structures can be preset. The model is validated by comparing calculations with EMG data from the literature. For agonistic muscles, good agreement is found between model calculations and EMG data. Antagonistic muscle activity is underestimated by the model. Imposed activity of modelled antagonistic muscles has a minor effect on the mutual proportions of agonistic muscle activities. Simulation of asymmetric muscle weakness shows higher activity of especially abdominal muscles.     

人工半骨盆假体置换联合腰椎椎弓根螺钉固定术后生物力学的有限元分析

目的:建立人工半骨盆假体置换与联合腰椎椎弓根螺钉固定后的三维有限元模型,评价腰骶段生物力学改变后半骨盆假体力学结构的特点。方法:采用CT薄层扫描采集原始数据,分别建立正常骨盆、半骨盆假体置换术后以及半骨盆假体置换联合腰椎椎弓根螺钉固定术后骨盆的三维有限元模型,分别在第4腰椎上终板平面施以500 N的垂直纵向载荷,分析不同骨盆模型的应力分布特点。结果:与正常骨盆有限元模型相比,半骨盆假体置换术后健侧骨盆应力分布以骶髂关节、髋臼窝及耻骨为主,置换侧半骨盆假体以耻骨连接棒、髋臼杯及髂骨座为主,最大应力出现在耻骨连接棒,应力峰值为65.62 MPa。联合腰椎椎弓根螺钉固定后健侧应力相对减小,置换侧髂骨固定座与骶骨固定处应力相对减小,应力分布以腰椎椎弓根钉棒、耻骨连接棒及髋臼杯为主,最大应力出现在椎弓根螺钉,应力峰值为107 MPa。结论:半骨盆假体置换联合腰椎椎弓根螺钉固定后钉棒分担了半骨盆置换后健侧骨盆及置换侧髂骨固定座与骶骨固定处附近的部分应力,缓解应力集中现象,降低术后骨盆破坏风险,一定程度上增加了半骨盆置换后骨盆的稳定性。     

Measurements of cross-sectional areas in selected human wrist-joint ligaments.

The cross-sectional area of a particular ligament is an important characteristic in order to establish the biomechanical properties of this ligament. Calculations of the cross-sectional areas of the ligaments of the wrist joint are made from two three-dimensional models. It is discussed that differences between the presented and the scarcely published data on cross-sectional areas are the result of different divisions of the wrist-joint ligamentous complex into separate ligaments.     

New aspects of the morphology and function of the human hip joint ligaments.

The capsular ligaments of the human hip joint were submitted to exact morphological analysis, and they proved to be multiple and numerous. We have described various ligamentous systems and their interconnections, and have suggested new terminologies and systematics. The ligaments were subjected to functional analysis by means of measuring strips to determine the positions in which the ligaments are taut. The ligament systems were all found to serve a restrictive function, and various parts of the apparatus restricted all possible movements in the hip joint. Extension is restricted by the medial iliofemoral complex, abduction by the pubofemoral ligament, and adduction by the posterior coxal ligaments and by the superior ischiofemoral ligament. Flexion is restricted by the inferior ischiofemoral ligaments, inward rotation by the superior ischiofemoral ligament, and outward rotation by the lateral iliofemoral complex. Only the ligament of the femoral head is unable to exert a restricting function, despite reaching a state of tension in extreme adduction.     

Biomechanical response of the pubic symphysis in lateral pelvic impacts: a finite element study

Automotive side impacts are a leading cause of injuries to the pubic symphysis, yet the mechanisms of those injuries have not been clearly established. Previous mechanical testing of isolated symphyses revealed increased joint laxity following drop tower lateral impacts to isolated pelvic bone structures, which suggested that the joints were damaged by excessive stresses and/or deformations during the impact tests. In the present study, a finite element (FE) model of a female pelvis including a previously validated symphysis sub-model was developed from computed tomography data. The full pelvis model was validated against measured force-time impact responses from drop tower experiments and then used to study the biomechanical response of the symphysis during the experimental impacts. The FE models predicted that the joint underwent a combination of lateral compression, posterior bending, anterior/posterior and superior/inferior shear that exceeded normal physiological levels prior to the onset of bony fractures. Large strains occurred concurrently within the pubic ligaments. Removal of the contralateral constraints to better approximate the boundary conditions of a seated motor vehicle occupant reduced cortical stresses and deformations of the pubic symphysis; however, ligament strains, compressive and shear stresses in the interpubic disc, as well as posterior bending of the joint structure remained as potential sources of joint damage during automotive side impacts.     

Biomechanical model study of pelvic belt influence on muscle and ligament forces

Many patients with low back and/or pelvic girdle pain feel relief after application of a pelvic belt. External compression might unload painful ligaments and joints, but the exact mechanical effect on pelvic structures, especially in (active) upright position, is still unknown. In the present study, a static three-dimensional (3-D) pelvic model was used to simulate compression at the level of anterior superior iliac spine and the greater trochanter. The model optimised forces in 100 muscles, 8 ligaments and 8 joints in upright trunk, pelvis and upper legs using a criterion of minimising maximum muscle stress. Initially, abdominal muscles, sacrotuberal ligaments and vertical sacroiliac joints (SIJ) shear forces mainly balanced a trunk weight of 500N in upright position. Application of 50N medial compression force at the anterior superior iliac spine (equivalent to 25N belt tension force) deactivated some dorsal hip muscles and reduced the maximum muscle stress by 37%. Increasing the compression up to 100N reduced the vertical SIJ shear force by 10% and increased SIJ compression force with 52%. Shifting the medial compression force of 100N in steps of 10N to the greater trochanter did not change the muscle activation pattern but further increased SIJ compression force by 40% compared to coxal compression. Moreover, the passive ligament forces were distributed over the sacrotuberal, the sacrospinal and the posterior ligaments. The findings support the cause-related designing of new pelvic belts to unload painful pelvic ligaments or muscles in upright posture.     

3D finite element simulation of the opening movement of the mandible in healthy and pathologic situations

One of the essential causes of disk disorders is the pathologic change in the ligamentous attachments of the disk-condyle complex. In this paper, the response of the soft components of a human temporomandibular joint during mouth opening in healthy and two pathologic situations was studied. A three-dimensional finite element model of this joint comprising the bone components, the articular disk, and the temporomandibular ligaments was developed from a set of medical images. A fiber reinforced porohyperelastic model was used to simulate the behavior of the articular disk, taking into account the orientation of the fibers in each zone of this cartilage component. The condylar movements during jaw opening were introduced as the loading condition in the analysis. In the healthy joint, it was obtained that the highest stresses were located at the lateral part of the intermediate zone of the disk. In this case, the collateral ligaments were subject to high loads, since they are responsible of the attachment of the disk to the condyle during the movement of the mandible. Additionally, two pathologic situations were simulated: damage of the retrodiscal tissue and disruption of the lateral discal ligament. In both cases, the highest stresses moved to the posterior part of the disk since it was displaced in the anterior-medial direction. In conclusion, in the healthy joint, the highest stresses were located in the lateral zone of the disk where perforations are found most often in the clinical experience. On the other hand, the results obtained in the damaged joints suggested that the disruption of the disk attachments may cause an anterior displacement of the disk and instability of the joint.     

Vibration testing of a fresh-frozen human pelvis: the role of the pelvic ligaments

A biodynamic model of the human pelvis is being developed in the frame of a research project on low back pain. In order to validate such model, the dynamic behaviour of the human pelvis needs to be investigated. In this study, a human fresh-frozen specimen comprising the three bones of the pelvic girdle and its ligamentous system has been used to perform vibration testing. In such test the response of the system to vibrations is measured at various points on the structure for frequencies between 10 and 340 Hz. The vibration testing is performed a first time on the specimen with intact ligamentous system. The measurements are taken two more times after subsequent bilateral resection of both the sacrotuberous and the sacrospinous ligaments first, and the iliolumbar ligaments afterwards. A comparison between the system response obtained in the three configurations provides information on the role of the resected ligaments in the dynamics of the system, thus on their relevance in the model. Results indicate that the sacrospinous, the sacrotuberous and the iliolumbar ligaments do not play a role in the pelvis dynamics as measured in this study, and will therefore not be represented in the biodynamic model.     

Control exerted by ligaments

The function of the ligaments as local controllers, independent of the central nervous system, in maintaining the integrity of the joint is demonstrated by modelling the human knee in the sagittal plane, and studying its anterior-posterior motion. In addition to the ligaments, the model includes the characteristic geometry of the joint surface and some muscle groups. The connecting reaction forces at the point of contact between the tibia and the femur are considered to be constraint forces due to three different surface motions--gliding, rolling and combined gliding and rolling. It is demonstrated that the ligamentous structure maintains these holonomic and nonholonomic constraints that describe the joint motion, and that stability of the knee joint is provided mainly by ligaments. Muscular structures further stabilize and contribute to joint movement. Computer simulation of rolling movement of the knee is presented to illustrate the importance of the ligaments for joint integrity and stability.     

髋骨术后仿真生物力学分析与临床研究

目的:探讨坐耻骨肿瘤切除术后髋骨生物力学变化及其与恢复期并发症产生的关系,指导术后骨盆重建.方法:构建单侧完整髋骨及坐耻骨肿瘤切除术后髋骨有限元模型,在相同约束和负载条件下,计算髋骨相同部位位移、应力及应变值,比较与分析模拟手术前后力学变化,结合临床评价意义.结果:手术前后髋骨节点位移、应力及应变分布区域相似,手术前后骶髂关节节点位移及应变、髋臼顶节点应力及应变有显著性差异;手术前后骶髂关节节点应力、坐骨大切迹应力及应变无显著性差异.结论:坐耻骨肿瘤切除术后主要影响同侧骶髂关节、髋臼顶的生物力学状态,恢复期并发症发生的原因由骶髂关节、髋臼顶生物力学改变及耻骨联合与耻骨上支连接与稳定作用的消失所致,三者相互作用,相互影响.     

Biomechanics of knee ligaments: injury, healing, and repair

Knee ligament injuries are common, particularly in sports and sports related activities. Rupture of these ligaments upsets the balance between knee mobility and stability, resulting in abnormal knee kinematics and damage to other tissues in and around the joint that lead to morbidity and pain. During the past three decades, significant advances have been made in characterizing the biomechanical and biochemical properties of knee ligaments as an individual component as well as their contribution to joint function. Further, significant knowledge on the healing process and replacement of ligaments after rupture have helped to evaluate the effectiveness of various treatment procedures. This review paper provides an overview of the current biological and biomechanical knowledge on normal knee ligaments, as well as ligament healing and reconstruction following injury. Further, it deals with new and exciting functional tissue engineering approaches (ex. growth factors, gene transfer and gene therapy, cell therapy, mechanical factors, and the use of scaffolding materials) aimed at improving the healing of ligaments as well as the interface between a replacement graft and bone. In addition, it explores the anatomical, biological and functional perspectives of current reconstruction procedures. Through the utilization of robotics technology and computational modeling, there is a better understanding of the kinematics of the knee and the in situ forces in knee ligaments and replacement grafts. The research summarized here is multidisciplinary and cutting edge that will ultimately help improve the treatment of ligament injuries. The material presented should serve as an inspiration to future investigators.     

A new reconstruction of Sts 14 pelvis (Australopithecus africanus) from computed tomography and three-dimensional modeling techniques

The purpose of this study is to propose a new reconstruction of the australopithecine Sts 14 pelvis from original fossils. Digital models created from CT images allow us to perform mirroring operations, select valid regions after digital interposition, and reassemble parts. The key-element of the reconstruction is the sacroiliac joint, restored from right and left articular surfaces, which places of the pubic symphysis close to the sagittal plane. The complete pelvis is obtained by 3D model mirroring of hip-bone and sacrum. The present reconstruction of the Sts 14 pelvis is consistent with Schmid's (1983) [Folia Primatol. 40, 283-306, 1983] and Häusler and Schmid's A.L. 288-1 [J. Hum. Evol. 29, 363-383, 1995] pelvic reconstructions by illustrating a relatively platypelloid shape of the pelvic cavity and laterally inclined iliac blades. The pelvic morphology suggests that australopithecines had a less posteriorly tilted sacrum in erect posture than modern humans. As compared with Lovejoy's [Am. J. Phys. Anthropol. Suppl. 50, 460, 1979] A.L. 288-1 pelvic reconstruction, the less transversely flattened shape of the Sts 14 pelvic cavity led to obstetrical mechanics characterized as in humans by ante-ischiatic birth and a curved trajectory. We deduce a human-like movement of rotation and flexion of the fetal skull in the Sts 14 pelvic cavity.     

The dynamic flexion/extension properties of the lumbar spine in vitro using a novel pendulum system

The biomechanical properties of the ligamentous cadaver spine have been previously examined using a variety of experimental testing protocols. Ongoing technical challenges in the biomechanical testing of the spine include the application of physiologic compressive loads and the application of dynamic bending moments while allowing unconstrained three-dimensional motion. The purpose of this study was to report the development of a novel pendulum apparatus that addressed these challenges and to determine the effects of various axial compressive loads on the dynamic biomechanical properties of the lumbar functional spinal unit (FSU). Lumbar FSUs were tested in flexion and extension under five axial compressive loads chosen to represent physiologic loading conditions. After an initial rotation, the FSUs behaved as a dynamic, underdamped vibrating elastic system. Bending stiffness and coefficient of damping increased significantly as the compressive pendulum load increased. The apparatus described herein is a relatively simple approach to determining the dynamic bending properties of the FSU, and potentially disc arthroplasty devices. It is capable of applying physiologic compressive loads at dynamic rates without constraining the kinematics of the joints, crucial requirements for testing FSUs in vitro.     

Investigation of impact loading rate effects on the ligamentous cervical spinal load-partitioning using finite element model of functional spinal unit C2–C3

The cervical spine functions as a complex mechanism that responds to sudden loading in a unique manner, due to intricate structural features and kinematics. The spinal load-sharing under pure compression and sagittal flexion/extension at two different impact rates were compared using a bio-fidelic finite element (FE) model of the ligamentous cervical functional spinal unit (FSU) C2–C3. This model was developed using a comprehensive and realistic geometry of spinal components and material laws that include strain rate dependency, bone fracture, and ligament failure. The range of motion, contact pressure in facet joints, failure forces in ligaments were compared to experimental findings. The model demonstrated that resistance of spinal components to impact load is dependent on loading rate and direction. For the loads applied, stress increased with loading rate in all spinal components, and was concentrated in the outer intervertebral disc (IVD), regions of ligaments to bone attachment, and in the cancellous bone of the facet joints. The highest stress in ligaments was found in capsular ligament (CL) in all cases. Intradiscal pressure (IDP) in the nucleus was affected by loading rate change. It increased under compression/flexion but decreased under extension. Contact pressure in the facet joints showed less variation under compression, but increased significantly under flexion/extension particularly under extension. Cancellous bone of the facet joints region was the only component fractured and fracture occurred under extension at both rates. The cervical ligaments were the primary load-bearing component followed by the IVD, endplates and cancellous bone; however, the latter was the most vulnerable to extension as it fractured at low energy impact.     

Stress analysis of the standing foot following surgical plantar fascia release

Plantar fascia release is a surgical alternative for patients who suffer chronic heel pain due to plantar fasciitis and are unaffected by conservative treatment. A computational (finite element) model for analysis of the structural behavior of the human foot during standing was utilized to investigate the biomechanical effects of releasing the plantar fascia. The model integrates a system of five planar structures in the directions of the foot rays. It was built according to accurate geometric data of MRI, and includes linear and non-linear elements that represent bony, cartilaginous, ligamentous and fatty tissues. The model was successfully validated by comparing its resultant ground reactions with foot-ground pressure measurements and its predicted displacements with those observed in radiological tests. Simulation of plantar fascia release (partial or total) was accomplished by gradually removing parts of the fascia in the model. The results showed that total fascia release causes extensive arch deformation during standing, which is greater than normal deformation by more than 2.5mm. Tension stresses carried by the long plantar ligaments increased significantly, and may exceed the normal average stress by more than 200%. Since the contribution of the plantar fascia to the foot's load-bearing ability is of major importance, its release must be very carefully considered, and the present model may be used to help surgeons decide upon the desired degree of release.