Jasper Johan Homminga

Osteoporosis changes the amount of vertebral trabecular bone at risk of fracture but not the vertebral load distribution

Study Design. A finite-element study to investigate the amount of trabecular bone at risk of fracture and the distribution of load between trabecular core and cortical shell, for healthy, osteopenic, and osteoporotic vertebrae. Objectives. To determine differences between healthy, osteopenic, and osteoporotic vertebrae with regard to the risk of fracture and the load distribution. Summary of Background Data. The literature contains no reports on the effects of osteopenia and osteoporosis on load distribution in vertebral bodies, nor any reports on the amount of trabecular bone at risk of fracture. Methods. Computed tomography data of vertebral bodies were used to construct patient-specific finite-element models. These models were then used in finite-element analyses to determine the physiologic stresses and strains in the vertebrae. Results. For all three classes of vertebrae the contribution of the trabecular core to the total load transfer decreased from about 70% near the endplates to about 50% in the midtransverse region. The amount of trabecular bone that is at risk of fracture was about 1% for healthy vertebrae, about 3% for osteopenic vertebrae, and about 16% for osteoporotic vertebrae. Conclusions. Our finite-element models indicated that neither osteopenia nor osteoporosis had any effect on the contribution of the trabecular core to the total load placed on the vertebra. The trabecular core carried about half the load. Our finite-element models indicated that osteoporosis had a significant effect on the amount of trabecular bone at risk of fracture, which increased from about 1% in healthy vertebrae to about 16% for osteoporotic vertebrae.

Influence of Interpersonal Geometrical Variation on Spinal Motion Segment Stiffness Implications for Patient-Specific Modeling

Study Design. A validated finite element model of an L3–L4 motion segment is used to analyze the effects of interpersonal differences in geometry on spinal stiffness. Objective. The objective of this study is to determine which of the interpersonal variations of the geometry of the spine have a large effect on spinal stiffness. This will improve patient-specific modeling. Summary of Background Data. The parameters that define the geometry of a motion segment are vertebral height, disc height, endplate width, endplate depth, spinous process length, transverse process width, nucleus size, lordosis angle, facet area, facet orientation, and the cross-sectional areas of the ligaments. All these parameters differ between patients. The influence of each parameter on spinal stiffness is largely unknown and such knowledge would greatly help in patient-specific modeling of the spine. Methods. The range of interpersonal variation of each of the geometric parameters was set at mean ± 2SD (covering 95% of the population). Subsequently, we determined the effect of each of these ranges on the bending stiffness in flexion, extension, axial rotation, and lateral bending. Results. Disc height had the largest influence; a maximal disc height reduced the spinal stiffness to 75–86% of the mean motion segment stiffness, and a minimal disc height increased the spinal stiffness to 154–226% of the mean motion segment stiffness. Lordosis angle, transversal and longitudinal facet angle, endplate depth, and area of the capsular ligament also had a substantial influence (>5%) on the stiffness, but considerable less than the influence of the disc height. Ligament areas, nucleus size, spinous process length, and length of processes are of negligible effect (<2%) on the stiffness. Conclusion. The disc height should be accurately determined in patients to estimate the spinal stiffness. Ligament areas, nucleus size, spinous process length, and transverse process width do not need patient-specific modeling.

The effect of three-dimensional geometrical changes during adolescent growth on the biomechanics of a spinal motion segment.

During adolescent growth, vertebrae and intervertebral discs undergo various geometrical changes. Although such changes in geometry are well known, their effects on spinal stiffness remains poorly understood. However, this understanding is essential in the treatment of spinal abnormalities during growth, such as scoliosis. A finite element model of an L3-L4 motion segment was developed, validated and applied to study the quantitative effects of changing geometry during adolescent growth on spinal stiffness in flexion, extension, lateral bending and axial rotation. Height, width and depth of the vertebrae and intervertebral disc were varied, as were the width of the transverse processes, the length of the spinous process, the size of the nucleus, facet joint areas and ligament size. These variations were based on average growth data for girls, as reported in literature. Overall, adolescent growth increases the stiffness with 36% (lateral bending and extension) to 44% (flexion). Two thirds of this increase occurs between 10 and 14 years of age and the last third between 14 years of age and maturity. Although the height is the largest geometrical change during adolescent growth, its effect on the biomechanics is small. The depth increase of the disc and vertebrae significantly affects the stiffness in all directions, while the width increase mainly affects the lateral bending stiffness. Hence, when analysing the biomechanics of the growing adolescent spine (for instance in scoliosis research), the inclusion of depth and width changes, in addition to the usually implemented height change, is essential.

The fracture risk of adjacent vertebrae is increased by the changed loading direction after a wedge fracture

Study Design. In vitro biomechanical study. Objective. To measure the effect that off-axis vertebral loading has on the stiffness and failure load of vertebrae. Summary of Background Data. Adjacent level vertebral fractures not only are common in patients who received a vertebroplasty treatment but also occur in patients with conservatively treated wedge fractures. The wedge-like deformity, which is present in both groups, changes the spinal alignment. The load of vertebrae adjacent to the fractured vertebra will change from perpendicular to the endplate to a more shearing, off-axis, load. This change may induce a higher fracture risk for vertebrae adjacent to wedge-like deformed vertebrae. Methods. Twenty vertebrae, harvested from one osteopenic cadaver spine and three osteoporotic cadaver spines, were loaded until failure. The vertebrae were loaded either perpendicular to the upper endplate, representing vertebrae in a spine without wedge fractures (0° group, n = 10), or at an angle of 20°, representing vertebrae adjacent to a wedge fracture (20° groups, n = 10). Vertebral failure load and stiffness were the most important outcome measures. Results. The failure load was significantly higher (P = 0.028) when tested at 0° (2854 N, SD = 622 N), compared with vertebrae tested at 20° (2162 N, SD = 670 N). Vertebrae were also significantly stiffer (P < 0.001) when tested at 0° (4017 N/mm, SD = 970 N/mm) than those tested at 20° (2478 N/mm, SD = 453 N/mm). Conclusion. The failure load of osteopenic/osteoporotic vertebrae was 24% lower under off-axis loads (20°) than under axial loads (0°). This study may lead to a better understanding of the etiology of adjacent vertebral fractures after wedge-like deformities and demonstrates the importance of height reconstruction of wedge fractures in order to normalize the loading conditions on adjacent vertebrae.

Can vertebral density changes be explained by intervertebral disc degeneration

One of the major problems facing the elderly spine is the occurrence of vertebral fractures due to low bone mass. Although typically attributed to osteoporosis, disc degeneration has also been suggested to play a role in vertebral fractures. Existing bone adaptation theories and simulations may explain the biomechanical pathway from a degenerated disc to an increased fracture risk. A finite element model of a lumbar segment was created and calibrated. Subsequently the disc properties were varied to represent either a healthy or degenerated disc and the resulting bone adaptation was simulated. Disc degeneration resulted in a shift of load from the nucleus to the annulus. The resulting bone adaptation led to a dramatically reduced density of the trabecular core and to an increased density in the vertebral walls. Degeneration of just the nucleus, and in particular the dehydration of the nucleus, resulted in most of this bone density change. Additional annulus degeneration had much less of an effect on the density values. The density decrease in the trabecular core as seen in this study matches clinical observations. Therefore, bone remodeling theories can assists in explaining the potential synergistic effects of disc degeneration and osteoporotis in the occurrence of vertebral fractures.

Prophylactic vertebroplasty can decrease the fracture risk of adjacent vertebrae: An in vitro cadaveric study

Adjacent level vertebral fractures are common in patients with osteoporotic wedge fractures, but can theoretically be prevented with prophylactic vertebroplasty. Previous tests on prophylactic vertebroplasties have been performed under axial loading, while in vivo changes in spinal alignment likely cause off-axis loads. In this study we determined whether prophylactic vertebroplasty can also reduce the fracture risk under off-axis loads. In a previous study, we tested vertebral bodies that were loaded axially or 20° off-axis representing vertebrae in an unfractured spine or vertebrae adjacent to a wedge fracture, respectively. In the current study, vertebral failure load and stiffness of our previously tested vertebral bodies were compared to those of a new group of vertebral bodies that were filled with bone cement and then loaded 20° off-axis. These vertebral bodies represented adjacent-level vertebrae with prophylactic bone cement filling. Prophylactic augmentation resulted in failure loads that were comparable to those of the 0° group, and 32% greater than the failure loads of the 20° group. The stiffness of the prophylacticly augmented vertebrae was 21% lower than that of the 0° group, but 27% higher than that of the 20° group. We conclude that prophylactic augmentation can decrease the fracture risk in a malaligned, osteoporotic vertebra. Whether this is enough to actually prevent additional vertebral fractures in vivo remains subject of further study.

Toward a more realistic prediction of peri-prosthetic micromotions

The finite element (FE) method has become a common tool to evaluate peri‐prosthetic micromotions in cementless total hip arthroplasty. Often, only the peak joint load and a selected number of muscle loads are applied to determine micromotions. Furthermore, the applied external constraints are simplified (diaphyseal fixation), resulting in a non‐physiological situation. In this study, a scaled musculoskeletal model was used to extract a full set of muscle and hip joint loads occurring during a walking cycle. These loads were applied incrementally to an FE model to analyze micromotions. The relation between micromotions and external loads was investigated, and how micromotions during a full loading cycle compared to those calculated when applying a peak load only. Finally, the effect of external constraints was analyzed (full model vs. diaphyseal fixation and reduced number of muscle loads). Relatively large micromotions were found during the swing phase when the hip joint forces were relatively low. Maximal micromotions, however, did concur with the peak hip joint force. Applying only a peak joint force resulted in peak micromotions similar to those found when full walking cycle loads were applied. The magnitude and direction of the micromotions depended on the applied muscle loads, but not on external constraints.

Posteriorly directed shear loads and disc degeneration affect the torsional stiffness of spinal motion segments; a biomechanical modeling study

Study Design. Finite element study. Objective. To analyze the effects of posterior shear loads, disc degeneration, and the combination of both on spinal torsion stiffness. Summary of Background Data. Scoliosis is a 3-dimensional deformity of the spine that presents itself mainly in adolescent girls and elderly patients. Our concept of its etiopathogenesis is that an excess of posteriorly directed shear loads, relative to the bodys intrinsic stabilizing mechanisms, induces a torsional instability of the spine, making it vulnerable to scoliosis. Our hypothesis for the elderly spine is that disc degeneration compromises the stabilizing mechanisms. Methods. In an adult lumbar motion segment model, the disc properties were varied to simulate different aspects of disc degeneration. These models were then loaded with a pure torsion moment in combination with either a shear load in posterior direction, no shear, or a shear load in anterior direction. Results. Posteriorly directed shear loads reduced torsion stiffness, anteriorly directed shear loads increased torsion stiffness. These effects were mainly caused by a later (respectively earlier) onset of facet joint contact. Disc degeneration cases with a decreased disc height that leads to slackness of the annular fibers and ligaments caused a significantly decreased torsional stiffness. The combination of this stage with posterior shear loading reduced the torsion stiffness to less than half the stiffness of a healthy disc without shear loads. The end stage of disc degeneration increased torsion stiffness again. Conclusion. The combination of a decreased disc height, that leads to slack annular fibers and ligaments, and posterior shear loads very significantly affects torsional stiffness: reduced to less than half the stiffness of a healthy disc without shear loads. Disc degeneration, thus, indeed compromises the stabilizing mechanisms of the elderly spine. A combination with posteriorly directed shear loads could then make it vulnerable to scoliosis. Level of Evidence: N/A

Does bone cement in percutaneous vertebroplasty act as a stress riser

Study Design. An in vitro cadaveric study. Objective. To determine whether percutaneous vertebroplasty (PVP) with a clinically relevant amount of bone cement is capable of causing stress peaks in adjacent-level vertebrae. Summary of Background Data. It is often suggested that PVP of a primary spinal fracture causes stress peaks in adjacent vertebrae, thereby leading to additional fractures. The in vitro studies that demonstrated this relationship, however, use bigger volumes of bone cement used clinically. Methods. Ten fresh-frozen vertebrae were loaded until failure, while registering force and displacement as well as the pressure under the lower endplate. After failure, the vertebrae were augmented with clinically relevant amounts of bone cement and then again loaded until failure. The force, displacement, and pressure under the lower endplate were again registered. Results. Stress peaks were not related to the location of the injected bone cement. Both failure load and stiffness were significantly lower after augmentation. Conclusion. On the basis of our findings, we conclude that vertebral augmentation with clinically relevant amounts of bone cement does not lead to stress peaks under the endplate. It is therefore unlikely that PVP, in itself, causes detrimental stresses in the adjacent vertebrae, leading to new vertebral fractures. Level of Evidence: N/A

Twente spine model: A complete and coherent dataset for musculo-skeletal modeling of the thoracic and cervical regions of the human spine.

Musculo-skeletal modeling could play a key role in advancing our understanding of the healthy and pathological spine, but the credibility of such models are strictly dependent on the accuracy of the anatomical data incorporated. In this study, we present a complete and coherent musculo-skeletal dataset for the thoracic and cervical regions of the human spine, obtained through detailed dissection of an embalmed male cadaver. We divided the muscles into a number of muscle-tendon elements, digitized their attachments at the bones, and measured morphological muscle parameters. In total, 225 muscle elements were measured over 39 muscles. For every muscle element, we provide the coordinates of its attachments, fiber length, tendon length, sarcomere length, optimal fiber length, pennation angle, mass, and physiological cross-sectional area together with the skeletal geometry of the cadaver. Results were consistent with similar anatomical studies. Furthermore, we report new data for several muscles such as rotatores, multifidus, levatores costarum, spinalis, semispinalis, subcostales, transversus thoracis, and intercostales muscles. This dataset complements our previous study where we presented a consistent dataset for the lumbar region of the spine (Bayoglu et al., 2017). Therefore, when used together, these datasets enable a complete and coherent dataset for the entire spine. The complete dataset will be used to develop a musculo-skeletal model for the entire human spine to study clinical and ergonomic applications.