Biomechanical Evaluation of a Lumbar Interspinous Spacer
Biomechanical Evaluation of a Lumbar Interspinous Spacer

Author: Avanthi Chikka

Publisher:

Published: 2011

Total Pages: 127

ISBN-13:

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Lumbar interspinous spacers have recently become popular as an alternative treatment for low back pain. These devices are primarily used to treat spinal stenosis and facet arthritis and are intended to unload the facet joints, restore foraminal height and provide stability mainly in extension while allowing normal range of motion in other loading modes at the index level and also at the adjacent segments. The goals of this study were three fold: (i) to evaluate the biomechanical stability provided by the Superion Interspinous Spacer (ISS) (Vertiflex®, San Clemente, CA) implanted in the lumbar spine; (ii) to study the effect of transection of supraspinous ligament (SSL) on the lumbar spine implanted with ISS; and (iii) to investigate the effect of graded facetectomies (50%, 75%, 100% facetectomies for both unilateral and bilateral cases) following the placement of ISS in the lumbar spine. This study is basically divided into two parts: an in vitro investigation and finite element analysis. The in vitro biomechanical study was conducted on six human lumbar motion segments (3 L2-L3 and 3 L4-L5) in the following test sequence: (i) intact, (ii) implanted (SSL intact), (iii) SSL dilated longitudinally at the center (with ISS), (iv) 50% resection of SSL (with ISS), (v) 100% resection of SSL (with ISS) (vi) injured (ISS removed). A finite element (FE) analysis was performed for the same test cases and also to investigate the effect of graded facetectomies using an experimentally validated 3D L3-S1 model. A bending moment of 10 Nm was applied in all loading modes (flexion-extension, right/left lateral bending, and right/left axial rotation) while a bending moment of 10 Nm with 400N compressive follower load was applied only in flexion-extension. Range of motion (ROM) was recorded for each of the test constructs. In addition to ROM, intradiscal pressure (IDP), facet loads and stress contour plots for these test constructs were obtained from the FE model. Repeated measures one way ANOVA was used to perform statistical analysis to determine the statistically significant differences between different test constructs for the in vitro data. The in vitro results showed that the mean ROM was significantly (p0.05) reduced in extension post-implantation. There was a minimal decrease in mean ROM in flexion as well, but it was not significant (p0.05). ROM in lateral bending and axial rotation were not affected. Also, there was no significant difference in ROM in any of the loading modes for the instrumented cases with and without SSL. The FE results were in agreement with the in vitro results except in flexion. Unlike the in vitro results, the flexion ROM increased slightly with progressive transection of SSL. From the FE data, it was observed that there was a significant reduction in IDP and facet loads in extension following the placement of ISS. However, there was no considerable difference in IDP and facet loads for the implanted cases with partial or complete transection of SSL. ROM, IDP and facet loads were not affected at the adjacent levels in any of the loading modes for any of the test constructs. It was also observed that there was an effect of graded facetectomies (50%, 75% and 100%) on the instrumented spine with a significant increase in ROM in flexion in case of both unilateral and bilateral facetectomies. The ISS provided an increased stability in extension while preserving motion comparable to intact in the other loading modes. Also, it can be inferred that SSL plays an insignificant role in segmental stability in extension, lateral bending and axial rotation and has nominal effect in flexion. In addition, the results suggest that ISS may not be used in combination with graded facetectomies for both unilateral and bilateral cases.

Biomechanical Evaluation of the Aspen Lumber Interspinous Fusion Device
Biomechanical Evaluation of the Aspen Lumber Interspinous Fusion Device

Author: Manoj Kumar Kodigudla

Publisher:

Published: 2011

Total Pages: 142

ISBN-13:

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Degenerative disc disease (DDD) and spondylolisthesis are the most common spinal disorders that lead to low back pain in the lumbar region. The treatment options for low back pain may range from conservative treatment to fusing the vertebrae by surgical methods. Although there are many fusion devices available to treat low back pain, interspinous fusion device (Aspen) is an alternative to other fusion devices. The device consists of two plates (wide plate, lock plate) and a set screw, and can be implanted using minimally invasive surgical procedure. The purpose of the current study was to evaluate the stability and loosening of the Aspen device under complex cyclic loading with resection of interspinous and supraspinous ligaments. An in vitro study of motion analysis and cyclic loading on six lumbar motion segments was conducted. The range of motion of Aspen implanted segments was assessed before and after cyclic loading, to evaluate the stability provided and loosening of the implant. A three dimensional, non linear and validated finite element model of L3-S1 lumbar spine was also used for additional biomechanical analysis. The in vitro study suggested that Aspen interspinous device stabilized the segments after resecting the interspinous and supraspinous ligaments. Aspen reduced motion across the segment than intact but more significantly in flexion and extension (p

Stability Imparted by a Posterior Lumbar Interbody Fusion Cage Following Surgery
Stability Imparted by a Posterior Lumbar Interbody Fusion Cage Following Surgery

Author: Vadapalli Sasidhar

Publisher:

Published: 2004

Total Pages: 264

ISBN-13:

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In order to promote solid fusion across a decompressed spinal segment, inter-body spacers/cages are used with and without posterior instrumentation to provide an initial "rigid" fixation of the segment. Inter-body spacers (cages) of various shapes (e.g., rectangular, cylindrical) and materials are currently available on the market. Important factors affecting the biomechanics of the fused segment are (i) cage shape and placement, (ii) cage material property (iii) surgical approach used-posterior vs. antero lateral (iv) cage with additional instrumentation. The objective of this study is to address change in the stability and stress patterns associated with the various factors described above. A cadaveric study using established protocols and a finite element (FE) study were conducted. For the cadaveric study, nine fresh ligamentous lumbar spine specimens (L1-S2) were radiographed out of which six specimens were prepared for testing by fixing a base to the sacrum and a loading frame to the top-most vertebra. Each specimen was subjected to pure moment (6 Nm in steps of 1.5 Nm) in six loading modes: flexion, extension, right and left lateral bending, and right and left axial rotation. The load-displacement data was collected in a sequential manner for the following cases: 1) intact spine, 2) insertion of rectangular cages (Vertebral spacer PR, Synthes, Inc.), 3) fixation with posterior instrumentation, 4) fatiguing the instrumented spine. The relative motion of L4 with respect to L5 was calculated for all these cases. A validated three-dimensional, nonlinear FE model of lumbar spine from L3-L5 was used. The model was modified to simulate the bilateral placement of cages alone. Contact surfaces were defined between the cages and the endplates to simulate the bone-implant interface. The cages were placed using posterior approach and left antero lateral approach to see the effect of the surgical approach on the stability of the segment. In the FE model with cage placed using posterior approach, posterior instrumentation was added. For this model the material property of the cage was changed form PEEK to titanium to study the change in load sharing and stresses on the endplates. For all the models a 6Nm moment was applied and all the six loading cases were simulated. The relative motion of L4 with respect to L5 was calculated, stresses in the implants and endplates were studied. Results from the in vitro study indicate that the stability of the spine decreased after the stand alone placement of bilateral cage when compared to the intact for all the loading cases except in flexion. However, no statistically significant difference was seen in the stability between intact and stand alone cage placement. After stabilization with posterior fixation using the pedicle screw rod system, the stability increased in all loading cases. There was no significant change in stability after fatiguing. The FE model predictions for the bilateral cage alone and with additional instrumentation placed at L4-L5 disc space were within 1 SD of the cadaveric data in all loading modes. There was no change in stability offered by stand alone cage placement using antero lateral approach and posterior approach. For the cage made of titanium peak Von mises stress in the endplates were twice of that for cage made of PEEK. Cages placed laterally from the mid-sagittal plane provide better stability in bending when compared to medially placed cages.

Biomechanical Evaluation of Lumbar Interbody Fusion Surgeries with Varying Interbody Device Shapes, Material Properties, and Supplemental Fixation
Biomechanical Evaluation of Lumbar Interbody Fusion Surgeries with Varying Interbody Device Shapes, Material Properties, and Supplemental Fixation

Author: Sushil P. Sudershan

Publisher:

Published: 2017

Total Pages: 130

ISBN-13:

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Low back pain (LBP) is one of the most commonly reported problems in the United States. The most common causes of LBP are injury or overuse, pressure on neural tissue from different pathologies (disc herniation, stenosis, degenerative disc disease, etc.). Interbody fusion is a well-established treatment option for patients with LBP. For each patient, the pathology and the surgeon's preference determine the surgical approach. The implants are usually manufactured from either PEEK or Titanium in a variety of shapes and sizes. Usually, the spacers are supplemented with supplemental fixation for motion restriction. However, the initial surgery studies showed success for standalone scenarios without any major complications. Ideally, a standalone surgery that provided stability while fusion occurred, could help decrease operation time and cost. To this date, a large scale study that compared various surgical approaches, implant footprints and materials, and addition of supplemental fixation has not been conducted. To conduct this parametric study, the experiment was separated into an in-vitro and in-silico study. The in-vitro study will allow us to record ROM data that can be used in conjunction with the in-silico model to determine parameters such as stress, strain, and load on the endplate. The in-vitro study showed that a standalone PLIF surgical approach may be a viable option but the TLIF standalone case does not successfully restrict the motion to less than intact or stabilize the motion segment with peek cages. However, for both surgical approaches the pedicle screw fixation successfully restricted motion or stabilized the motion segment. The in-silico study showed that the ALIF and LLIF surgical approaches may be viable options for standalone scenarios but require further investigation. Similar, to the in-vitro study, pedicle screw greatly restricted the motion segment for all surgical approaches regardless of implant size or material property. The study showed that an increase in implant footprint resulted in higher overall motion restriction for all surgical approaches. The various simulated supplemental fixation stabilized the motion for all loading conditions. For the anterior and lateral approaches, lumbar plates provide an additional means of stabilization instead of pedicle screw fixation. The study also observed that a larger implant footprint also shifted the endplate stresses to the periphery, which is composed of stronger bone. Overall, PEEK cages produced lower endplate stresses than titanium cages which may help reduce subsidence incidence. Supplemental fixation always reduced the endplate stresses compared to standalone scenarios except for some cases in the TLIF surgery. The TLIF surgery also showed higher endplate shear load when compared with other surgeries.

Biomechanical Evaluation of Posterior Dynamic Stabilization Systems in Lumbar Spine
Biomechanical Evaluation of Posterior Dynamic Stabilization Systems in Lumbar Spine

Author: Bharath K. Parepalli

Publisher:

Published: 2009

Total Pages: 204

ISBN-13:

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Fusion has been the gold standard treatment for treating the disc degeneration. Fusion surgeries restrict the motion at the implanted level there by imposing additional load at the adjacent levels. Many clinical studies have showed that adjacent segment degeneration was observed in patients over time. In order to overcome problems with fusion devices, dynamic stabilization systems are being used to treat disc degeneration related problems. These implants restore intersegmental motion across the implanted level with minimal effects on the adjacent levels. In vitro cadaveric testing was conducted on seven harvested sheep spines using established protocols. Axient was implanted in the spines 3 months prior to sacrificing. Main aim of this testing is to see if the performance is altered by the presence of surrounding muscle tissue. The specimens were prepared and tested under load control protocol. All six loading modes were tested by applying a pure moment of 10Nm (in steps of 2.5Nm) and angular displacement was calculated for the following cases: 1) Intact spine + Axient with surrounding muscle tissue, 2) Intact spine + Axient with muscle tissue removed, 3) Intact spine (with implant removed). Relative motion of L4 vertebra with respect to L5 was calculated. Statistical analysis was performed (on the implanted level data) to see if there is a statistical significance between cases 1 and 2. Biomechanical testing was also performed on 4 human cadavers to observe the trend with Axient compared to FE results. A validated 3-D non linear finite element model of the L3-S1 lumbar spine was used to evaluate biomechanics of various dynamic stabilization systems in comparison with traditional rigid rod system. The model was modified at L4-L5 level to simulate three different dynamic stabilization systems (DSFM-1, DSFM-2 and Axient, Innovative Spinal Technologies Inc., Mansfield, MA). Grade I was simulated at L4-L5 level. Follower preload of 400N and a 10Nm bending moment was applied to simulate physiological flexion, extension, lateral bending and axial rotation. Range of motion (ROM), intra discal pressure (IDP) and facet loads were calculated for all the models. Implant with better performance was then compared with fusion system in both grade I and grade II degenerated spines. In vitro results showed that there is no significant difference in the performance of the Axient with and without surrounding muscle tissue in terms of range of motion. Coming to FE results, Axient performed better over the other two implants (DSFM-1 and DSFM-2). Axient device was able to restore the motion at the implanted level compared to fusion device. Higher motions were observed at the adjacent level (L5-S1) with fusion device compared to intact and injured models. Both devices were able to stabilize the diseased spine and unload the treated disc.

Biomechanical Evaluation of Disc Annular Repair Technology in Human Lumbar Spine
Biomechanical Evaluation of Disc Annular Repair Technology in Human Lumbar Spine

Author: Sarath C. Koruprolu

Publisher:

Published: 2014

Total Pages: 85

ISBN-13:

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The availability of pre-operative data on the biomechanical stability of an annular repair device may influence the clinical management of lumbar spine surgery. Having the knowledge of the performance of various annular repair devices can assist in the selection of better choices for treatment. Numerous studies investigated the effect of various implants such as artificial nucleus replacement, repair or annular tears, novel annulus repair devices with full or partial discectomy etc. However, the area of nucleus and annulus repair technology still needs to be further researched upon to devise an alternative disc replacement device that would reduce pain, minimize the restriction of range of motion and decrease the degenerative effects on the adjacent segments. As a first step towards achieving this goal, an implant must be tested under appropriate biomechanical protocols to study its stability during complex physiological motion that is encountered clinically. This study determined the biomechanical performance of a novel annular repair device in an in-vitro in the human cadaveric lumbar spine. The test criterion was to evaluate implant migration during complex cyclic loading, study its effects on the range of motion of the functional spinal segment and intradiscal pressures. The overall stability of the device is studied under extreme physiological impact loading and the finite element analysis of the construct is conducted and compared to the in vitro data. Six human cadaveric lumbar functional spine unit specimens (L2-Sacrum