Won Man Park

Comparison of eight published static finite element models of the intact lumbar spine: Predictive power of models improves when combined together

Finite element (FE) model studies have made important contributions to our understanding of functional biomechanics of the lumbar spine. However, if a model is used to answer clinical and biomechanical questions over a certain population, their inherently large inter-subject variability has to be considered. Current FE model studies, however, generally account only for a single distinct spinal geometry with one set of material properties. This raises questions concerning their predictive power, their range of results and on their agreement with in vitro and in vivo values. Eight well-established FE models of the lumbar spine (L1-5) of different research centers around the globe were subjected to pure and combined loading modes and compared to in vitro and in vivo measurements for intervertebral rotations, disc pressures and facet joint forces. Under pure moment loading, the predicted L1-5 rotations of almost all models fell within the reported in vitro ranges, and their median values differed on average by only 2° for flexion-extension, 1° for lateral bending and 5° for axial rotation. Predicted median facet joint forces and disc pressures were also in good agreement with published median in vitro values. However, the ranges of predictions were larger and exceeded those reported in vitro, especially for the facet joint forces. For all combined loading modes, except for flexion, predicted median segmental intervertebral rotations and disc pressures were in good agreement with measured in vivo values. In light of high inter-subject variability, the generalization of results of a single model to a population remains a concern. This study demonstrated that the pooled median of individual model results, similar to a probabilistic approach, can be used as an improved predictive tool in order to estimate the response of the lumbar spine.

Effects of degenerated intervertebral discs on intersegmental rotations, intradiscal pressures, and facet joint forces of the whole lumbar spine

The effects of intervertebral disc (IVD) degeneration on biomechanics of the lumbar spine were analyzed. Finite element models of the lumbar spine with various degrees of IVD degeneration at the L4-L5 functional spinal unit (FSU) were developed and validated. With progression of degeneration, intersegmental rotation at the degenerated FSU decreased in flexion-extension and left-right lateral bending, intradiscal pressure at the adjacent FSUs increased in flexion and lateral bending, and facet joint forces at the degenerated FSU increased in lateral bending and axial rotation. These results could provide fundamental information for understanding the mechanism of injuries caused by IVD degeneration.

Biomechanical comparison of instrumentation techniques in treatment of thoracolumbar burst fractures: a finite element analysis

BackgroundThere are several surgical techniques currently employed to treat thoracolumbar burst fractures, including anterior fixation, posterior fixation, or combined anterior-posterior fixation. Biomechanical analysis of the various types of surgical techniques is therefore critical to enable selection of the appropriate surgical method for successful spinal fusion. However, the effects of the various spinal fusion techniques on spinal stiffness have not been clearly defined, and the strengths and weaknesses of each fusion technique are still controversial.MethodsThe biomechanical effects of increasing the number of anterior rods and removing the mid-column in anterior fixation, posterior fixation, and combined anterior-posterior fixation on spinal stiffness in thoracolumbar burst fractures was investigated. Finite element analysis was used to investigate the effects of the three fusion methods on spine biomechanics because of its ability to control for variables related to the material and experimental environment.ResultsThe stiffness of the fused spinal junction highly correlates with the selection of an additional posterior fixation. The mid-column decompression showed a significant change in stiffness, although the effect of decompression was much less than that with the application of posterior fixation and the anterior rod number. In addition, two-rod anterior fixation without additional posterior fixation is able to provide enough spinal stability; and one-rod anterior fixation with posterior fixation yields better results in regard to preventing excessive motion and ensuring spinal stability.ConclusionsThe present study shows that careful consideration is necessary when choosing the anterior rod number and applying posterior fixation and mid-column decompression during surgical treatment of thoracolumbar burst fractures.

In vivo Loads in the Lumbar L3-4 Disc during a Weight Lifting Extension

BACKGROUND Knowledge of in vivo human lumbar loading is critical for understanding the lumbar function and for improving surgical treatments of lumbar pathology. Although numerous experimental measurements and computational simulations have been reported, non-invasive determination of in vivo spinal disc loads is still a challenge in biomedical engineering. The object of the study is to investigate the in vivo human lumbar disc loads using a subject-specific and kinematic driven finite element approach. METHODS Three dimensional lumbar spine models of three living subjects were created using MR images. Finite element model of the L3-4 disc was built for each subject. The endplate kinematics of the L3-4 segment of each subject during a dynamic weight lifting extension was determined using a dual fluoroscopic imaging technique. The endplate kinematics was used as displacement boundary conditions to calculate the in-vivo disc forces and moments during the weight lifting activity. FINDINGS During the weight lifting extension, the L3-4 disc experienced maximum shear load of about 230 N or 0.34 bodyweight at the flexion position and maximum compressive load of 1500 N or 2.28 bodyweight at the upright position. The disc experienced a primary flexion-extension moment during the motion which reached a maximum of 4.2 Nm at upright position with stretched arms holding the weight. INTERPRETATION This study provided quantitative data on in vivo disc loading that could help understand intrinsic biomechanics of the spine and improve surgical treatment of pathological discs using fusion or arthroplasty techniques.

Stress analysis in a pedicle screw fixation system with flexible rods in the lumbar spine

Abstract Breakage of screws has been one of the most common complications in spinal fixation systems. However, no studies have examined the breakage risk of pedicle screw fixation systems that use flexible rods, even though flexible rods are currently being used for dynamic stabilization. In this study, the risk of breakage of screws for the rods with various flexibilities in pedicle screw fixation systems is investigated by calculating the von Mises stress as a breakage risk factor using finite element analysis. Three-dimensional finite element models of the lumbar spine with posterior one-level spinal fixations at L4—L5 using four types of rod (a straight rod, a 4 mm spring rod, a 3 mm spring rod, and a 2 mm spring rod) were developed. The von Mises stresses in both the pedicle screws and the rods were analysed under flexion, extension, lateral bending, and torsion moments of 10 N m with a follower load of 400 N. The maximum von Mises stress, which was concentrated on the neck region of the pedicle screw, decreased as the flexibility of the rod increased. However, the ratio of the maximum stress in the rod to the yield stress increased substantially when a highly flexible rod was used. Thus, the level of rod flexibility should be considered carefully when using flexible rods for dynamic stabilization because the intersegmental motion facilitated by the flexible rod results in rod breakage.

Biomechanical evaluation of double bundle augmentation of posterior cruciate ligament using finite element analysis

BACKGROUND Posterior cruciate ligament injuries commonly occur during sports activities or motor vehicle accidents. However, there is no previous comparison study of single bundle reconstruction, double bundle reconstruction, and double bundle augmentation with respect to biomechanical characteristics such as stability and ligament stress. METHODS A three-dimensional finite element model of a lower extremity including femur, tibia, cartilage, meniscus, collagen fibers, and four major ligaments was developed and validated. In addition to the intact, posterior cruciate ligament injured, single bundle reconstruction, double bundle reconstruction, and double bundle augmentation models were developed. Then, the posterior and rotational tibial translations as well as the ligament stresses were predicted for 89 N posterior force and 3 Nm internal torque, respectively, in the normal (no secondary deficiency) and the secondary deficiency cases using finite element analysis. FINDINGS The posterior stability and ligament stresses following double bundle augmentation were superior to those of single and double bundle reconstructions, especially after secondary deficiency in the reconstructed grafts, despite little difference in posterior stability between double bundle reconstruction and augmentation in the normal case. Similarly, the double bundle augmentation had the greatest rotational stability while there was little advantage in ligament stress compared to those of the other reconstruction method. INTERPRETATION Double bundle augmentation has advantages with regard to posterior and rotational stabilities as well as ligament stress in comparison with other reconstruction methods, especially following secondary deficiencies in the reconstructed grafts.

A Biomechanical Analysis of an Artificial Disc With a Shock-absorbing Core Property by Using Whole-cervical Spine Finite Element Analysis.

Study Design. A biomechanical comparison among the intact C2 to C7 segments, the C5 to C6 segments implanted with fusion cage, and three different artificial disc replacements (ADRs) by finite element (FE) model creation reflecting the entire cervical spine below C2. Objective. The aim of this study was to analyze the biomechanical changes in subaxial cervical spine after ADR and to verify the efficacy of a new mobile core artificial disc Baguera C that is designed to absorb shock. Summary of Background Data. Scarce references could be found and compared regarding the cervical ADR devices’ biomechanical differences that are consequently related to their different clinical results. Methods. One fusion device (CJ cage system, WINNOVA) and three different cervical artificial discs (Prodisc-C Nova (DePuy Synthes), Discocerv (Scient’x/Alphatec), Baguera C (Spineart)) were inserted at C5-6 disc space inside the FE model and analyzed. Hybrid loading conditions, under bending moments of 1 Nm along flexion, extension, lateral bending, and axial rotation with a compressive force of 50 N along the follower loading direction, were used in this study. Biomechanical behaviors such as segmental mobility, facet joint forces, and possible wear debris phenomenon inside the core were investigated. Results. The segmental motions as well as facet joint forces were exaggerated after ADR regardless of type of the devices. The Baguera C mimicked the intact cervical spine regarding the location of the center of rotation only during the flexion moment. It also showed a relatively wider distribution of the contact area and significantly lower contact pressure distribution on the core than the other two devices. A “lift off” phenomenon was noted for other two devices according to the specific loading condition. Conclusion. The mobile core artificial disc Baguera C can be considered biomechanically superior to other devices by demonstrating no “lift off” phenomenon, and significantly lower contact pressure distribution on core. Level of Evidence: N/A

Effect of mechanical loading on heterotopic ossification in cervical total disc replacement: a three-dimensional finite element analysis

The development of heterotopic ossification (HO) is considered one of the major complications following cervical total disc replacement (TDR). Even though previous studies have identified clinical and biomechanical conditions that may stimulate HO, the mechanism of HO formation has not been fully elucidated. The objective of this study is to investigate whether mechanical loading is a biomechanical condition that plays a substantial role to decide the HO formation. A finite element model of TDR on the C5–C6 was developed, and HO formation was predicted by simulating a bone adaptation process under various physiological mechanical loadings. The distributions of strain energy on vertebrae were assessed after HO formation. For the compressive force, most of the HO formation occurred on the vertebral endplates uncovered by the implant footplate which was similar to the Type 1 HO. For the anteriorly directed shear force, the HO was predominantly formed in the anterior parts of both the upper and lower vertebrae as the Type 2 HO. For both the flexion and extension moments, the HO shapes were similar to those for the shear force. The total strain energy was reduced after HO formation for all loading conditions. Two distinct types of HO were predicted based on mechanically induced bone adaptation processes, and our findings were consistent with those of previous clinical studies. HO formation might have a role in compensating for the non-uniform strain energy distribution which is one of the mechanical parameters related to the bone remodeling after cervical TDR.

Stress concentration near pin holes associated with fracture risk after computer navigated total knee arthroplasty

During computer navigated total knee arthroplasty, pin holes are drilled in the femur and tibia to allow the placement of navigation trackers, and fractures associated with these pin holes have recently been reported. We hypothesized that an increase in stress around the pin holes is one of the most relevant factors contributing to the fracture. In this study, we used finite element analysis to investigate the stresses around femoral pin holes with respect to the mode of pin penetration, the diameter of the pin holes, and the degree of osteoporosis. Our results indicate that increases in pin hole diameter and reduction in bone strength as a result of osteoporosis intensify the stresses around the pin holes, especially in cases of transcortical pin penetration.

Effect of centers of rotation on spinal loads and muscle forces in total disk replacement of lumbar spine

The placement of artificial disks can alter the center of rotation and kinematic pattern; therefore, forces in the spine during the motion will be affected as a result. The relationship between the location of joint center of artificial disks and forces in the spinal components is not investigated. A musculoskeletal model of the spine was developed, and three location cases of center of rotation were investigated varying 5 mm anteriorly and posteriorly from the default center. Resultant joint forces, ligament forces, facet forces, and muscle forces for each case were predicted during sagittal motion. No considerable difference was observed for joint force (maximum 14%). Anterior shift of center of rotation induced the most ligament forces (200 N) and facet forces (130 N) among the three cases. Posterior and anterior shifts of centers of rotation from the default location caused considerable changes in muscle forces, respectively: 108% and 70% of increase in multifidi muscle and 157% and 187% of increase in short segmental muscle. This study showed that the centers of rotation due to the design and the surgical placement of artificial disk can affect the kinetic results in the spine.