The Simulation of Muscles Forces Increases the Stresses in Lumbar Fixation Implants with Respect to Pure Moment Loading

Simplified loading conditions such as pure moments are frequently used to compare different instrumentation techniques to treat spine disorders. The purpose of this study was to determine if the use of realistic loading conditions such as muscle forces can alter the stresses in the implants with respect to pure moment loading. A musculoskeletal model and a finite element model sharing the same anatomy were built and validated against in vitro data, and coupled in order to drive the finite element model with muscle forces calculated by the musculoskeletal one for a prescribed motion. Intact conditions as well as a L1-L5 posterior fixation with pedicle screws and rods were simulated in flexion-extension and lateral bending. The hardware stresses calculated with the finite element model with instrumentation under simplified and realistic loading conditions were compared. The ROM under simplified loading conditions showed good agreement with in vitro data. As expected, the ROMs between the two types of loading conditions showed relatively small differences. Realistic loading conditions increased the stresses in the pedicle screws and in the posterior rods with respect to simplified loading conditions; an increase of hardware stresses up to 40 MPa in extension for the posterior rods and 57 MPa in flexion for the pedicle screws were observed with respect to simplified loading conditions. This conclusion can be critical for the literature since it means that previous models which used pure moments may have underestimated the stresses in the implants in flexion-extension and in lateral bending.

Motion preservation surgery for scoliosis with a vertebral body tethering system: a biomechanical study

Purpose There is a paucity of studies on new vertebral body tethering (VBT) surgical constructs especially regarding their potentially motion-preserving ability. This study analyses their effects on the ROM of the spine. Methods Human spines (T10-L3) were tested under pure moment in four different conditions: (1) native, (2) instrumented with one tether continuously connected in all vertebrae from T10 to L3, (3) additional instrumented with a second tether continuously connected in all vertebrae from T11 to L3, and (4) instrumented with one tether and one titanium rod (hybrid) attached to T12, L1 and L2. The instrumentation was inserted in the left lateral side. The intersegmental ROM was evaluated using a magnetic tracking system, and the medians were analysed. Please check and confirm the author names and initials are correct. Also, kindly confirm the details in the metadata are correct. The mentioned information is correct Results Compared to the native spine, the instrumented spine presented a reduction of less than 13% in global ROM considering flexion–extension and axial rotation. For left lateral bending, the median global ROM of the native spine (100%) significantly reduced to 74.6%, 66.4%, and 68.1% after testing one tether, two tethers and the hybrid construction, respectively. In these cases, the L1-L2 ROM was reduced to 68.3%, 58.5%, and 38.3%, respectively. In right lateral bending, the normalized global ROM of the spine with one tether, two tethers and the hybrid construction was 58.9%, 54.0%, and 56.6%, respectively. Considering the same order, the normalized L1-L2 ROM was 64.3%, 49.9%, and 35.3%, respectively. Conclusion The investigated VBT techniques preserved global ROM of the spine in flexion–extension and axial rotation while reduced the ROM in lateral bending.

Biomechanics of a laterally placed sacroiliac joint fusion device supplemental to S2 alar-iliac fixation in a long-segment adult spinal deformity construct: a cadaveric study of stability and strain distribution

OBJECTIVE S2 alar-iliac (S2AI) screw fixation effectively enhances stability in long-segment constructs. Although S2AI fixation provides a single transarticular sacroiliac joint fixation (SIJF) point, additional fixation points may provide greater stability and attenuate screw and rod strain. The objectives of this study were to evaluate changes in stability and pedicle screw and rod strain with extended distal S2AI fixation and with supplemental bilateral integration of two sacroiliac joint fusion devices implanted using a traditional minimally invasive surgical approach. METHODS Eight L1–pelvis human cadaveric specimens underwent pure moment (7.5 Nm) and compression (400 N) tests under 4 conditions: 1) intact (pure moment loading only); 2) L2–S1 pedicle screw and rod with L5–S1 interbody fusion; 3) added S2AI screws; and 4) added bilateral laterally placed SIJF. Range of motion (ROM), rod strain, and screw-bending moment (S1 and S2AI) were analyzed. RESULTS Compared with S1 fixation, S2AI fixation significantly reduced L5–S1 ROM in right lateral bending by 50% (0.11°, p = 0.049) and in compression by 39% (0.22°, p = 0.003). Compared with fixation ending at S1, extending fixation with S2AI significantly decreased sacroiliac joint ROM by 52% (0.28°, p = 0.02) in flexion, by 65% (0.48°, p = 0.04) in extension, by 59% (0.76°, p = 0.02) in combined flexion-extension, and by 36% (0.09°, p = 0.02) in left axial rotation. The addition of S2AI screws reduced S1 screw-bending moment during flexion (0.106 Nm [43%], p = 0.046). With S2AI fixation, posterior L5–S1 primary rod strain increased by 124% (159 μE, p = 0.002) in flexion, by 149% (285 μE, p = 0.02) in left axial rotation, and by 99% (254 μE, p = 0.04) in right axial rotation. Compared with S2AI fixation, the addition of SIJF reduced L5–S1 strain during right axial rotation by 6% (28 μE, p = 0.04) and increased L5–S1 strain in extension by 6% (28 μE, p = 0.02). CONCLUSIONS Long-segment constructs ending with S2AI screws created a more stable construct than those ending with S1 screws, reducing lumbosacral and sacroiliac joint motion and S1 screw-bending moment in flexion. These benefits, however, were paired with increased rod strain at the lumbosacral junction. The addition of SIJF to constructs ending at S2AI did not significantly change SI joint ROM or S1 screw bending and reduced S2AI screw bending in compression. SIJF further decreased L5–S1 rod strain in axial rotation and increased it in extension.

CONSTRAINT QUALIFICATIONS IN PARTIAL IDENTIFICATION

The literature on stochastic programming typically restricts attention to problems that fulfill constraint qualifications. The literature on estimation and inference under partial identification frequently restricts the geometry of identified sets with diverse high-level assumptions. These superficially appear to be different approaches to closely related problems. We extensively analyze their relation. Among other things, we show that for partial identification through pure moment inequalities, numerous assumptions from the literature essentially coincide with the Mangasarian–Fromowitz constraint qualification. This clarifies the relation between well-known contributions, including within econometrics, and elucidates stringency, as well as ease of verification, of some high-level assumptions in seminal papers.

Finite Element Investigation of Moment Effect on Fretting Fatigue

Fretting fatigue is a degrading process which is responsible for considerable amount of mechanical structure failure every year. In the present study, a finite element model is proposed to show the effect of a bending moment on a flat surface under fretting loading. The results show that the bending moment has a major effect on the friction stress distribution on the surface of the two solids under contact. Finite element analysis predicts an increased damage effect on the surface of solids when a load is applied as a pure moment. The results predict elevation in the relative slip between the surfaces after applying the bending moment.

Effect of Passive Support of the Spinal Muscles on the Biomechanics of a Lumbar Finite Element Model

Finite element (FE) modeling of the passive ligamentous spine is widely used to assess various biomechanical behaviors. Currently, FE models that incorporate the vertebrae, ligaments, and the personalized geometry of the bony spine may be used in conjunction with external loads from the muscles. However, while the muscles place a load (moment) on the spine and support it simultaneously, the effect of the passive support from the adjacent spinal muscles has not been considered. This study thus aims to investigate the effect of passive support from the psoas major, quadratus lumborum, and erector muscles on the range of motion (RoM) and intradiscal pressure (IDP) of the lumbar spine. Various L2-sacrum spinal models that differed only in their muscle properties were constructed and loaded with a pure moment (2.5–15.0 Nm) alone or combined with a compressive (440 or 1000 N) follower load. The RoM and IDP of the model that excluded the effect of muscles closely matched previous FE results under the corresponding load conditions. When the muscles (40–160 kPa) were included in the FE model, the RoM at L2 was reduced by up to 6.57% under a pure moment (10 Nm). The IDP was reduced by up to 6.45% under flexion and 6.84% under extension. It was also found that the erector muscles had a greater effect than the psoas major and quadratus muscles.

A new in vitro spine test rig to track multiple vertebral motions under physiological conditions

In vitro pure moment spine tests are commonly used to analyse surgical implants in cadaveric models. Most of the tests are performed at room temperature. However, some new dynamic instrumentation devices and soft tissues show temperature-dependent material properties. Therefore, the aim of this study is to develop a new test rig, which allows applying pure moments on lumbar spine specimens in a vapour-filled chamber at body temperature. As no direct sight is given in the vapour-filled closed chamber, a magnetic tracking (MT) system with implantable receivers was used. Four human cadaveric lumbar spines (L2–L5) were tested in a vapour atmosphere at body temperature with a native and rigid instrumented group. In conclusion, the experimental set-up allows vertebral motion tracking of multiple functional spinal units (FSUs) in a moisture environment at body temperature.

Changes in foraminal area with anterior decompression versus keyhole foraminotomy in the cervical spine: a biomechanical investigation

OBJECTIVEAnterior cervical discectomy and fusion (ACDF) with or without partial uncovertebral joint resection (UVR) and posterior keyhole foraminotomy are established operative procedures to treat cervical disc degeneration and radiculopathy. Studies have demonstrated reliable results with each procedure, but none have compared the change in neuroforaminal area between indirect and direct decompression techniques. The purpose of this study was to determine which cervical decompression method most consistently increases neuroforaminal area and how that area is affected by neck position.METHODSEight human cervical functional spinal units (4 each of C5–6 and C6–7) underwent sequential decompression. Each level received the following surgical treatment: bilateral foraminotomy, ACDF, ACDF + partial UVR, and foraminotomy + ACDF. Multidirectional pure moment flexibility testing combined with 3D C-arm imaging was performed after each procedure to measure the minimum cross-sectional area of each foramen in 3 different neck positions: neutral, flexion, and extension.RESULTSNeuroforaminal area increased significantly with foraminotomy versus intact in all positions. These area measurements did not change in the ACDF group through flexion-extension. A significant decrease in area was observed for ACDF in extension (40 mm2) versus neutral (55 mm2). Foraminotomy + ACDF did not significantly increase area compared with foraminotomy in any position. The UVR procedure did not produce any changes in area through flexion-extension.CONCLUSIONSAll procedures increased neuroforaminal area. Foraminotomy and foraminotomy + ACDF produced the greatest increase in area and also maintained the area in extension more than anterior-only procedures. The UVR procedure did not significantly alter the area compared with ACDF alone. With a stable cervical spine, foraminotomy may be preferable to directly decompress the neuroforamen; however, ACDF continues to play an important role for indirect decompression and decompression of more centrally located herniated discs. These findings pertain to bony stenosis of the neuroforamen and may not apply to soft disc herniation. The key points of this study are as follows. Both ACDF and foraminotomy increase the foraminal space. Foraminotomy was most successful in maintaining these increases during neck motion. Partial UVR was not a significant improvement over ACDF alone. Foraminotomy may be more efficient at decompressing the neuroforamen. Results should be taken into consideration only with stable spines.

The Qwerty Phenomenon: Its Relevance in a World with Creative Destruction

Does quality always win? Looking at the critical drivers of success in and efficiency of high-tech markets, two contrasting perspectives exist in the academic sector. One camp argues that the higher quality of a product or service exerts a major influence on its market success. Consequently, an inferior market player should not persist. The opposite group emphasises the importance of network effects, which can lead to lock-ins in inferior situations or being stuck in a bad equilibria accordingly, also known as the QWERTY phenomenon. In this paper, we investigate this debate. We demonstrate that the missing consideration of the status quo bias in previous studies leads to the rejection of the QWERTY phenomenon, which means that independent of the quality offered by a business or service the pure moment of who reaches the customer first, establishes a status quo from which it is hardly possible to escape. We give several examples with inferior market leaders. We suggest that this phenomenon causes only temporary harm, and lock-ins could be overcome by Schumpeterian creative destruction. Therefore, we claim that even if lock-ins exist, they pose no problems as innovative market participants have the opportunity to introduce new business models.

Biomechanical Evaluation of Lumbar Decompression Adjacent to Instrumented Segments

BACKGROUND: Multilevel lumbar stenosis, in which 1 level requires stabilization due to spondylolisthesis, is routinely treated with multilevel open laminectomy and fusion. We hypothesized that a minimally invasive (MI) decompression is biomechanically superior to open laminectomy and may allow decompression of the level adjacent the spondylolisthesis without additional fusion. OBJECTIVE: To study the mechanical effect of various decompression procedures adjacent to instrumented segments in cadaver lumbar spines. METHODS: Conditions tested were (1) L4-L5 instrumentation, (2) L3-L4 MI decompression, (3) addition of partial facetectomy at L3-L4, and (4) addition of laminectomy at L3-L4. Flexibility tests were performed for range of motion (ROM) analysis by applying nonconstraining, pure moment loading during flexion-extension, lateral bending, and axial rotation. Compression flexion tests were performed for motion distribution analysis. RESULTS: After instrumentation, MI decompression increased flexion-extension ROM at L3-L4 by 13% (P = .03) and axial rotation by 23% (P = .003). Partial facetectomy further increased axial rotation by 15% (P = .03). After laminectomy, flexion-extension ROM further increased by 12% (P = .05), a 38% increase from baseline, and axial rotation by 17% (P = .02), a 58% increase from baseline. MI decompression yielded no significant increase in segmental contribution of motion at L3-L4, in contrast to partial facetectomy and laminectomy (<.05). CONCLUSION: MI tubular decompression is biomechanically superior to open laminectomy adjacent to instrumented segments. These results lend support to the concept that in patients in whom a multilevel MI decompression is performed, the fusion might be limited to the segments with actual instability.