The Injection of Gels Through an Intact Annulus Maintains Biomechanical Performance without Extrusion Risk.

For autologous-disc-derived chondrocyte transplantation (ADCT) a transglutaminase crosslinked gelatine gel and an albumin hyaluronic acid gel, crosslinked with bis-thio-polyethylene glycol, were injected through a syringe into a degenerated intervertebral disc, where they solidified in situ. This biomechanical in vitro study with lumbar bovine motion segments evaluated disc height changes, motion characteristics in a quasi-static spine loading simulators, and the potential extrusion risk of these biomaterials in a complex dynamic multi-axial loading set-up with 100,000 loading cycles. After the injection and formation of the gel in the center of the nucleus, the disc height increase was about 0.3 mm. During cyclic testing, a gradual decrease in height could be detected due to viscoelastic effects and fluid loss. No gel extrusion could be observed for all specimens during the entire test procedure. A macroscopic inspection after dissections showed an accumulation of the solidified gel in the center of the nucleus. The results demonstrate that the injection of in situ solidifying gels through the intact annulus allows for the stable maintenance of the injected gel at the target location, with high potential for use as a suitable scaffold to anchor therapeutically applied cells for disc regeneration within the treated nucleus pulposus.

Endplate Trauma During Implant Insertion Affects the Expulsion Risk of Anterior Lumbar Interbody Fusion Devices.

Background Anterior cage migration in anterior lumbar interbody fusion is a serious complication. To address this risk, cage designs are now available with integrated screw or blade fixation or specially designed surface geometries with large teeth or ridges. However, the implantation technique itself has not yet been addressed as a potential risk factor for cage migration. This study aimed to investigate whether a cage that is implantable without gouging the vertebral endplates has improved resistance to anterior migration. Methodology A novel three-piece modular cage was inserted between two vertebral body replacements (polyurethane (PU) foam grade 15 pcf) in two ways. In group 1 (modular), the cage was inserted in a wedge within a wedge fashion according to the manufacturer's instructions such that damage to the PU foam was minimized. In group 2 (mono-bloc), the modular cage was inserted pre-assembled as a one-piece, mono-bloc device. This insertion method required impaction and increased the potential of gouging the PU surfaces. Then, an axial preload was applied to the PU test blocks to simulate the preload on the spine in vivo and an anteriorly direct expulsion force was applied to the cages. Results The mean expulsion yield load in the test group with modular implantation was 392 ± 19 N compared to 287 ± 16 N in the test group where the mono-bloc implants were inserted and endplate gouging occurred. This difference was statistically significant (p < 0.05). Thus, the onset of cage migration occurred at significantly higher loads in the test group with modular insertion without endplate gouging compared to one-piece impaction with gouging taking place. In contrast, the stiffness and the ultimate load were similar in both test groups (p > 0.05). Conclusions This study showed that the cage insertion technique may have a significant effect on the cage migration risk. Prevention of endplate gouging during cage implantation has the potential to improve the primary stability of the cage.

How does free rod-sliding affect the posterior instrumentation for a dynamic stabilization using a bovine calf model?

STUDY DESIGN: A biomechanical cadaveric study in lumbar calf spine. OBJECTIVE: Evaluation of the effects of selected degrees of freedom (df) on the dynamic stabilization of the spine in terms of segmental range of motion (RoM), center of rotation (CoR), and implant loadings. SUMMARY OF BACKGROUND DATA: For dorsal stabilization, rigid implant systems are becoming increasingly complemented by numerous dynamic systems based on pedicle screws and varying df. However, it is still unclear which df is most suitable to accomplish a physiologically related dynamic stabilization, and which loadings are induced to the implants. Human and calf specimens are reported to show certain similarities in their biomechanics. Young healthy calf specimens are not degenerated and show less interindividual differences than elderly human specimens. However, the existing differences between species limit the conclusions in a preclinical setting. METHODS: Six calf specimens from level L3-L4 were analyzed in flexion and extension with a 6-df robotic spine simulator. A clinical functional radiological examination tool was used and parameters such as RoM, CoR, and implant loadings were determined for 6 configurations: (1) intact, (2) defect, (3) rigid fixation, (4) free craniocaudal (CC) rod-sliding, (5) free polyaxiality, and (6) combined free rod-sliding and free polyaxiality. The location of the CoR was determined relative to vertebral body dimensions. A CoR repositioning was defined as sufficient when its median differed less than 5% of the vertebral body dimensions. RESULTS: Free rod-sliding in the CC direction restored the CoR from the defect back to the intact condition. The RoM could be significantly reduced to approximately 1/2 of the intact condition. Compared with the rigid condition, the implant bending moments increased from 0.3/-0.8 Nm (flexion/extension) to 1.3/-1.2 Nm for the free CC rod-sliding condition. CONCLUSION: Free CC rod-sliding restores the intact conditions of the tested kinematic parameters most suitably and at the same time reduces the RoM. Stabilization toward the intact condition could decrease the risk of stress shielding and the progress of segment degeneration. LEVEL OF EVIDENCE: N/A.

Can prevention of a reherniation be investigated? Establishment of a herniation model and experiments with an anular closure device.

STUDY DESIGN: Biomechanical in vitro study. OBJECTIVE: To establish a reliable in vitro herniation model with human cadaver spines that enables evaluation of anular closure devices. SUMMARY OF BACKGROUND DATA: Biomechanically, it is desirable to close anulus defects after disc herniation to preserve as much nucleus as possible. Multiple anular closure options exist to prevent reherniation. A reliable test procedure is needed to evaluate the efficacy and reliability of these implants. METHODS: Two groups of human lumbar segments (n = 6 per group) were tested under cyclic loading until herniation occurred or 100,000 load cycles were applied. One group contained moderate/severe degenerated discs. A second group had mild degenerated discs. Intradiscal pressure was measured in the intact state to confirm disc quality.If herniation occurred, the extruded material was reinserted into the disc and the anulus defect was treated with the Barricaid anular closure device (Intrinsic Therapeutics, Inc., Woburn, MA). Disc height and 3-dimensional flexibility of the specimens in the intact, defect, and implanted states were measured under pure moments in each principal motion plane. Afterwards, provocation of reherniation was attempted with additional 100,000 load cycles. RESULTS: Likelihood of herniation was strongly linked to disc degeneration and supported by the magnitude of intradiscal pressure. In moderate/severe degenerated discs, only 1 herniation was created. In mild degenerated discs, herniations were reliably created in all specimens. Using this worst-case model, herniation caused a significant reduction of disc height, which was nearly restored with the implant. In no case was reherniation or implant migration visible after 100,000 load cycles after Barricaid implantation. CONCLUSION: We established a human herniation model that reliably produced nucleus extrusion during cyclic loading by selecting specimens with low disc degeneration. The Barricaid seems to prevent nucleus from reherniating. The reliability of this method suggests the opportunity to investigate other anulus closure devices and nucleus replacement techniques critically.

Posterior motion preserving implants evaluated by means of intervertebral disc bulging and annular fiber strains.

BACKGROUND: The aims of motion preserving implants are to ensure sufficient stability to the spine, to release facet joints by also allowing a physiological loading to the intervertebral disc. The aim of this study was to assess disc load contribution by means of annular fiber strains and disc bulging of intact and stiffened segments. This was compared to the segments treated with various motion preserving implants. METHODS: A laser scanning device was used to obtain three-dimensional disc bulging and annular fiber strains of six lumbar intervertebral discs (L2-3). Specimens were loaded with 500N or 7.5Nm moments in a spine tester. Each specimen was treated with four different implants; DSS™, internal fixator, Coflex™, and TOPS™. FINDINGS: In axial compression, all implants performed in a similar way. In flexion, the Coflex decreased range of motion by 13%, whereas bulging and fiber strains were similar to intact. The DSS stabilized segments by 54% compared to intact. TOPS showed a slight decrease in fiber strains (5%) with a range of motion similar to intact. The rigid fixator allowed strains up to 2%. In lateral bending, TOPS yielded range of motion values similar to intact, but maximum fiber strains doubled from 6.5% (intact) to 13.8%. Coflex showed range of motion, bulging and strain values similar to intact. The DSS and the rigid fixator reduced these values. The implants produced only minor changes in axial rotation. INTERPRETATION: This study introduces an in vitro method, which was employed to evaluate spinal implants other than standard biomechanical methods. We could demonstrate that dynamic stabilization methods are able to keep fiber strains and disc bulging in a physiological range.

A method to perform spinal motion analysis from functional X-ray images.

Identifying spinal instability is an important aim for proper surgical treatment. Analysis of functional X-ray images delivers measurements of the range of motion (RoM) and the center of rotation (CoR). In today's practice, CoR determination is often omitted, due to the lack of accurate methods. The aim of this work was to investigate the accuracy of a new analysis software (FXA™) based on an in vitro experiment. Six bovine spinal specimens (L3-4) were mounted in a robot (KR125, Kuka). CoRs were predefined by locking the robot actuator tool center point to the estimated position of the physiologic CoR and taking a baseline X-ray. Specimens were deflected to various RoM(preset) flexion/extension angles about the CoR(preset). Lateral functional radiographs were acquired and specimen movements were recorded using an optical motion tracking system (Optotrak Certus). RoM and CoR errors were calculated from presets for both methods. Prior to the experiment, the FXA™ software was verified with artificially generated images. For the artificial images, FXA™ yielded a mean RoM-error of 0.01 ± 0.03° (bias ± standard deviation). In the experiment, RoM-error of the FXA™-software (deviation from presets) was 0.04 ± 0.13°, and 0.10 ± 0.16° for the Optotrak, respectively. Both correlated with 0.998 (p < 0.001). For RoM < 1.0°, FXA™ determined CoR positions with a bias>20mm. This bias progressively decreased from RoM = 1° (bias = 6.0mm) to RoM = 9° (bias<1.5mm). Under the assumption that CoR location variances <5mm are clinically irrelevant on the lumbar spine, the FXA™ method can accurately determine CoRs for RoMs > 1°. Utilizing FXA™, polysegmental RoMs, CoRs and implant migration measurements could be performed in daily practice.

Influence of low glucose supply on the regulation of gene expression by nucleus pulposus cells and their responsiveness to mechanical loading.

OBJECT: Environmental alterations resulting in a decrease in the nutrient supply have been associated with intervertebral disc (IVD) degeneration, particularly of the nucleus pulposus (NP). The goal of the present study was to examine the hypothesis that glucose deprivation alters the metabolism of NP cells and their responsiveness to mechanical loading. A possible interaction of glucose supply and hydrostatic pressure (HP) with gene expression by NP cells has not been investigated. METHODS: The influence of glucose supply (physiological concentration: 5 mM, reduction: 0 or 0.5 mM) and cyclic HP loading (2.5 MPa, 0.1 Hz, 30 minutes) on bovine and human NP cell matrix turnover was analyzed by quantitative real-time reverse transcriptase­polymerase chain reaction. Glucose-dependent effects on cell viability were determined by trypan blue exclusion. A glycosaminoglycan (GAG) assay was performed to determine nutritional effects on the protein level. RESULTS: Glucose reduction resulted in significant downregulations (p < 0.05) of aggrecan, collagen-I, and collagen-II gene expression by bovine NP cells. Exemplary human donors also displayed a similar trend for aggrecan and collagen-II, whereas matrix metalloproteinases (MMPs) tended to be upregulated under glucose deprivation. After HP loading, human NP cells showed individual upregulations of collagen-I and collagen-II expression, while MMP expression tended to be downregulated under glucose reduction relative to a normal glucose supply. Cell viability decreased with glucose deprivation. The GAG content was similar in all groups at Day 1, whereas at Day 3 there was a significant increase under physiological conditions. CONCLUSIONS: Glucose deprivation strongly affected NP cell metabolism. The effects of an altered glucose supply on gene expression were more pronounced than the mechanically induced effects. Data in this study demonstrate that the glucose environment is more critical for disc cell metabolism than mechanical loads. In individual human donors, however, adequate mechanical stimuli might have a beneficial effect on matrix turnover during IVD degeneration.

Vertebral trabecular main direction can be determined from clinical CT datasets using the gradient structure tensor and not the inertia tensor--a case study.

Osteoporosis is a wide spread disease with one-third of all women beyond their menopause and a fifth of men above the age of 50 years suffering from it. Patient specific finite element models would be a great improvement for the diagnosis of vertebral fracture risk. Different material models have been proposed, which incorporate information about the anisotropy of trabecular bone in addition to bone mineral density (BMD) using a second rank structure tensor. Two alternative structure measurement methods, gradient structure tensor (GST) and inertia tensor (IT), were investigated. Structure was determined from in situ scans. This was compared to structure computed with the mean intercept length (MIL) tensor from microCT scans at the same locations. GST delivered information comparable to MIL regarding the structural main direction even at normal dose standard clinical settings (median of the scalar products of up to approximately 0.98). IT was not comparable to MIL (median approximately 0.4). Neither of the alternatives could determine eigenvalues comparable to these determined from MIL (p>0.5). In conclusion, this study could show that the measurement of the structural main direction is possible for in situ scans in a clinical setting. It was shown that the method of choice to determine trabecular main direction in situ is GST. Knowing the main direction a transverse isotropic fabric tensor can be constructed.

Dependency of disc degeneration on shear and tensile strains between annular fiber layers for complex loads.

BACKGROUND: One of the first signs of disc degeneration is the formation of circumferential tears within the annulus fibrosus. It is assumed that high shear and tensile strains between the lamellae mainly cause the initiation of these failures. However, it is not known which load application and which degree of disc degeneration could lead to the highest strains and therefore, might induce the formation of tears. Therefore, the aim of this finite element (FE) study was, to find load combinations that would yield highest shear and tensile strains in differently degenerated discs. MATERIALS AND METHODS: A three-dimensional FE-model of a motion segment L4-5 was utilized in different degrees of disc degeneration (healthy, mild, moderate, and severe). The degenerated models consider the reduction of disc height, endplate curvatures, the osteophyte formation, the increase of nucleus compressibility, and the decrease of fiber and ligament stiffness. An axial compression load of 500 N together with moments of 7.5 Nm in single and combined load directions were simulated. RESULTS: High strains for the healthy and degenerated discs were predicted for load combinations, particularly for the combination of lateral bending plus flexion or extension. The maximum strains were located in the postero-lateral region of the disc. In comparison to the healthy disc, the maximum strains increased slightly for the mildly and moderately degenerated disc. Strains decreased strongly for the severely degenerated disc. With progressive degeneration, the size of the region of maximum strains diminished and the location transferred from the inner annulus to the adjacent bony endplates. CONCLUSIONS: The results could be a possible explanation for the initiation of circumferential tears. The mildly degenerated disc model, which represents early stages of life, suggests that circumferential tears could primarily occur at these stages, especially for the load combinations of lateral bending plus axial rotation and lateral bending plus flexion.

Prospective design delineation and subsequent in vitro evaluation of a new posterior dynamic stabilization system.

STUDY DESIGN: Finite element and in vitro study. OBJECTIVE: Finite element calculations to delineate a dynamic fixator and confirmation with an in vitro experiment. SUMMARY AND BACKGROUND DATA: In the last few years, there was a paradigm shift from rigid to dynamic fixation of spinal segments. However, some so-called dynamic implants like the Dynesys performed still stiffer than anticipated. The aim of this study was to optimize a dynamic stabilization system. METHODS: The development steps of this implant design can be summarized in a development loop. First, a finite element model of an intact human L4-L5 segment was used to delineate implant stiffness parameters for the implant, in consideration of clinical concerns and safety aspects. These data were used in a second step, leading to the final implant design. This development process was completed with an appropriate in vitro experiment. The optimal axial and bending stiffness were computed to reduce the spinal motion by 30%. For the validation process, in vitro tests were performed on 6 human lumbar spinal segments L2-L3 with a median age of 52. The model and the specimens were loaded with pure unconstrained moments of 7.5 Nm in flexion, extension, lateral bending, and axial rotation. RESULTS: This study demonstrated the advantages of employing a finite element model for the implant parameter delineation. It was possible to prospectively outline the needed stiffness parameters for a desired spinal range of motion achievement. CONCLUSION: In summary, FEM may accelerate the development and the realization of a new implant design.

Which axial and bending stiffnesses of posterior implants are required to design a flexible lumbar stabilization system?

Dynamic stabilization devices have been introduced to clinics as an alternative to rigid fixation. The stiffness of these devices varies widely, whereas the optimal stiffness, achieving a predefined stabilization of the spine, is unknown. This study was focused on the determination of stiffness values for posterior stabilization devices achieving a flexible, semi-flexible or rigid connection between two vertebrae. An extensively validated finite element model of a lumbar spinal segment L4-5 with an implanted posterior fixation device was used in this study. The model was exposed to pure moments of 7.5 and 20Nm around the three principal anatomical directions, simulating flexion, extension, lateral bending and axial rotation. In parametrical studies, the influence of the axial and bending fixator stiffness on the spinal range of motion was investigated. In order to examine the validity of the computed results, an in-vitro study was carried out. In this, the influence of two posterior stabilization devices (DSS and rigidly internal fixator) on the segmental stabilization was investigated. The finite element (FE)-model predicted that each load direction caused a pairing of stiffness relations between axial and bending stiffness. In flexion and extension, however, the bending stiffness had a neglectable effect on the segmental stabilization, compared to the axial stiffness. In contrast, lateral bending and axial rotation were influenced by both stiffness parameters. Except in axial rotation, the model predictions were in a good agreement with the determined in-vitro data. In axial rotation, the FE-model predicted a stiffer segmental behavior than it was determined in the in-vitro study. It is usually expected that high stiffness values are required for a posterior stabilization device to stiffen a spinal segment. We found that already small stiffness values were sufficient to cause a stiffening. Using these data, it may possible to develop implants for certain clinical indications.

Interaction between finite helical axes and facet joint forces under combined loading.

STUDY DESIGN: Finite element study. OBJECTIVE: To investigate the interaction between the finite helical axis and facet joint loads under combined loading. SUMMARY OF BACKGROUND DATA: Finite helical axes (FHA) in a functional spinal unit can indicate mechanical disorders and are relevant for the development of new arthoplasty techniques. The facet joints protect the intervertebral discs from excessive movements. The relationship between the FHAs and facet joint forces is not well-understood, because previous studies have separated both, spinal motion and facet forces. METHODS: A finite element model of a lumbar spinal segment L4-L5 was used to simulate axial compression load of 500 N together with moments starting from 0 to 7.5 Nm in single anatomic main planes. Load combinations of 7.5 Nm were generated by changing the load direction in steps of 15 degrees between each pair of the 3 anatomic mainplanes. RESULTS: For single axes loading, the FHAs were found to be in the center of the disc under small moments, independently from load directions. The facet joints were only slightly loaded. Higher moments increased the forces in facet joints up to 105 N in axial rotation, followed by extension (50 N) and lateral bending (36 N). Combined moments did not essentially increase the facet forces compared with the same moment applied in an anatomic main direction. High facet forces might have directed the FHAs to migrate posteriorly, especially for axial rotation. This situation resulted in FHAs outside the disc toward the compressed facet joint. CONCLUSION: For clinical practice, patients immediately after the operation, or patients with facet joint arthritis should reduce or avoid axial rotation alone or in combination with other load applications, especially axial rotation plus lateral bending or flexion.

Biomechanical evaluation of conventional anulus fibrosus closure methods required for nucleus replacement. Laboratory investigation.

OBJECT: Nucleus replacement implants became increasingly attractive as an alternative to fusion, discectomy, or total disc replacement. The goals of nucleus replacement are to restore disc height and flexibility and to preserve the anatomy. However, implant extrusions have been reported and are the major concern. In this study the authors investigated different conventional surgical methods for anulus closure: suture alone, and fibrin glue and cyanoacrylate glue, alone and with suture. METHODS: The in vitro testing was conducted using 30 lumbar spinal segments obtained from calves. In each specimen, an incision was made; the nucleus was removed and subsequently replaced by a collagen matrix. The incisions were treated with anulus closure methods in 5 groups of animals. Flexibility was assessed in a spine tester. Subsequently, specimens were exposed to cyclic fatigue loading by using a hydraulic loading frame. Specimens were excentrically loaded in sine waveform up to a maximum of 100,000 cycles with 4-24 Nm at 5 Hz while being rotated at 360 degrees/minute. RESULTS: Removal of the nucleus caused a significant loss of stability. The segmental stability could be restored after the implantation. Fatigue testing indicated that suturing was able to sustain 3400 cycles. Fibrin glue failed earlier than cyanoacrylate glue. Both combinations (suture with glue) provided longer stability to the anulus closure. CONCLUSIONS: The results suggested that closing the anulus incision with suture or fibrin glue alone might not be appropriate. The authors found that the best method was cyanoacrylate glue with suture. Although this method provided the longest duration of closure, it could not sustain the maximum number of fatigue cycles. Conventional methods could improve the outcome compared with using no closure. Nonetheless, the authors' findings highlight the demand for an appropriate anulus reconstruction method or device with good long-term reliability.

A method to obtain surface strains of soft tissues using a laser scanning device.

A three-dimensional laser scanning device was developed allowing surface digitization of musculoskeletal and soft tissue structures under different loads. Image-processing algorithms were formulated for image registration. These were used to determine displacement mapping and then surface strains. Various validation experiments were performed. Accuracy was obtained on a test cylinder after rigid rotation and on a silicon cylinder compressed in four loading steps. The system accuracy (including the scanning and the data evaluation) was +/-0.10% strain in vertical and +/-0.16% strain in shear and circumferential direction for the rigid rotation exhibiting the zero-strain situation. Silicon cylinder compression showed that the accuracy was best for small strains, whereas strains >5% evoked a slight underestimation increasing further with higher strains (error of 0.54% for 7.22% vertical strain). It was possible to increase the accuracy by performing the strain measurements via sub-steps. This had a remaining error of 0.41% for 7.22% vertical strain. A further experiment was carried out in order to acquire the surface strain of a human lumbar intervertebral disc while it was forced to flexion and extension. This study introduced a laser-based scanning method to obtain soft tissue surface strains. It is important to know the strain distribution of musculoskeletal structures and soft tissues. This could help to better understand the mechanical loading of biological structures e.g. the processes in fracture healing. These data could also be used to assist in the validation process for finite-element models.

Stepwise reduction of functional spinal structures increase disc bulge and surface strains.

Previous studies postulated that an axial compression of lumbar intervertebral discs causes a complex strain pattern on the annulus. This pattern is not fully understood, since most studies measured only the uniaxial ultimate tensile strain of the annulus. The aim of this study was to investigate surface strains and their relation to disc bulging. This work was extended to study some defects that are relevant for the intermediate process of finite element modeling. Six specimens (L2-3) with a median age of 51 years were utilized for this in vitro study. Specimens were loaded with pure moments (2.5-7.5Nm) in the principal directions. The anatomy was subsequently reduced in three steps: (1) ligamentous and bony posterior structures, (2) anterior and posterior ligaments and (3) nucleus. Measured were ranges of motion, three-dimensional disc bulging and surface strains of the outer annulus. Lateral bending showed the largest axial strains (9.7%) for intact specimens, which increased to 15.1% after the removal of posterior structures. Disc bulging was largest in flexion with 1.56mm, which increased to 2.06mm after step (1). Defect (2) caused that flexion yielded the largest axial strains with 22.6% and 2.17mm of bulging. We could also determine a constriction effect of these ligaments. Nucleotomy did not essentially increase anterior disc bulging in flexion, but inward disc bulging increased by 0.55mm, in extension. Due to the increase in the complexity of finite element models, it is difficult to obtain data from the literature for validation purposes. This study presents new data, which assist in the development of such models.

The relation between intervertebral disc bulging and annular fiber associated strains for simple and complex loading.

Mechanical failure of the annulus fibrosus is mostly indicated by tears, fissures, protrusions or disc prolapses. Some of these annulus failures can be caused by a high intradiscal pressure. This has an effect on disc bulging. However, it is not fully understood how disc bulging is related to disc loading. Therefore, the aim of this study was to investigate the annular fiber strains and disc bulging under simple and complex spinal loads. A novel laser scanner was used to image surfaces of six L2-3 segments. Specimens were loaded with 500 N or 7.5 Nm in a spine tester while acquiring surface maps. Loading was applied in the three principal main directions and four combined directions. Disc bulging and tissue surface strains in annulus collagen fiber directions were computed. Two conditions were measured; intact and defect (vertebral body-disc-body units). Axial compression resulted in 2.7% fiber associated strains in intact segments and the defect increased strains up to 6.7%. Disc bulging increased from 0.7 mm to 0.87 mm. Flexion produced 7.2% fiber associated strains and 1.63 mm bulge going up to 17.5% and 2.21 mm after the defect. Highest fiber associated strains were found for the combination of axial rotation plus lateral bending with 24.6% and with a maximal bulging of 1.14 mm. It was found that there is no tight relationship between fiber associated strains and disc bulging. This was especially seen for the load combinations. Highest fiber associated strains were found to be located in small posterolateral regions. Fiber associated strains had a much higher magnitude than previously reported fiber associated strains. The results showed that combined loading is most likely to produce higher associated fiber strains compared to single axis loading.

A new laser scanning technique for imaging intervertebral disc displacement and its application to modeling nucleotomy.

BACKGROUND: Nucleotomy is a standard procedure for treating disc prolapse. It can reduce intervertebral disc height, flattening and displacing the disc, which could lead to a painful narrowing of the foramina due to nerve root compression. The purpose of this study was to investigate the disc displacement of a complete spinal segment with and without nucleotomy. We hypothesized that a nucleotomy under a certain load combination might amplify disc displacement. METHODS: A laser scanner was developed for recording three-dimensional disc displacement of six loaded L4-5 specimens for three conditions: intact, disc with vertebral bodies and subsequent nucleotomy. Specimens were exposed to pure moments of 7.5 N m in the three principal anatomical directions. Disc displacement was obtained at maximal deflection. A finite element model was validated and subsequently utilized to determine disc displacement. The task of the finite element model was to provide supplemental data for the posterolateral region, which could not be measured from intact specimens. FINDINGS: Disc displacement measurements of intact specimens were limited to the anterior part of discs, whereas the finite element model was able to provide the missing data of the dorsal disc region. The simulation of load combinations showed that the highest disc displacement was 1.9 mm at the lateral or posterolateral region. The nucleotomy increased the disc displacement up to 2.1mm, whereas the displacement zenith migrated posterolaterally. INTERPRETATION: These results could be a possible explanation for disadvantages of nucleotomy as a treatment. With the methodology presented here, we would be able to assess the performance of nucleus implants by determining the disc displacement map. This could also give us appropriate information of the annular deformation, which is needed for the development of motion preserving implants.

The relation between the instantaneous center of rotation and facet joint forces - A finite element analysis.

BACKGROUND: The instantaneous center of rotation in a functional spinal unit is an indicator for mechanical disorders and is relevant for the development of motion preserving techniques. In addition to the intervertebral disc, the facet joints also play a major role for load transmission through the spine, providing stability to it. The relationship between the rotation center and facet joint forces is not fully understood, since previous studies have separated both; spinal motion and facet forces. METHODS: A finite element model of a L4-5 lumbar spinal segment was exposed to an axial compression preload of 500 N. Pure unconstrained moments of 7.5 Nm were additionally applied in the three anatomical main planes. The instantaneous center of rotation and the facet joint forces were investigated. FINDINGS: For small moments, the center of rotation was found to be almost in the center of the disc, no matter what motion direction. With an increasing flexion moment, the center of rotation moved anteriorly. The facet joints remained unloaded in flexion. With proceeding extension movement, the center of rotation moved posteriorly. The facet forces increased up to 50 N. In lateral bending, with increasing moment the center of rotation migrated posteriorly in the ipsilateral side of the disc. The forces in the facet joints rose to 36 N. In axial rotation, the center of rotation migrated towards the compressed facet joint with increasing moment. Axial rotation yielded the maximum facet forces with 105 N. INTERPRETATION: The determination of the rotation center is highly sensible against measurement resolution obtained during in vivo and in vitro studies. This finite element method can be used to complement the knowledge of the rotation center location from former experimental findings.

Creep associated changes in intervertebral disc bulging obtained with a laser scanning device.

BACKGROUND: Lumbar disc bulging has been determined with different methods in the past. Reported methods of bulging assessment were limited to a direct physical contact, were two-dimensional and were time-consuming. Assessing the three-dimensional contour of a biological object under load would imply that the tissue would creep and therefore changes its contour. For that purpose, we were interested how fast the contour has to be assessed and how creeping would counteract on the intradiscal pressure and disc height. METHODS: For that purpose, a laser based three-dimensional contour scanner was developed. This scanner was especially designed to be mounted in a spine tester. For 15 min a static compression of 500 N was applied to seven human lumbar segments having all ligaments, facets and arches removed. Disc height, intradiscal pressure and disc contour were time dependently measured. FINDINGS: Load application reduced the disc height by 1.14 mm. The further decrease showed a typical creep behavior whereas the intradiscal pressure slightly but significantly decreased from 0.49 to 0.48 MPa. Cross-sectional disc contours showed that bulging was largest anterolateral followed by the anterior region. The creeping also increased the disc circumference. This effect varied region dependently having a maximum of 0.1 mm posterolateral. INTERPRETATION: Results suggest that geometries of biological tissues should be obtained within one minute avoiding superimposing creep effects. This new method might be used to evaluate disc injuries, degeneration and disc treatments. Measuring disc contours under different loads and conditions yields the outer annular strain distribution. This is a prerequisite for the development of cell seeded and tissue engineered implants.

Intradiscal pressure, shear strain, and fiber strain in the intervertebral disc under combined loading.

STUDY DESIGN: Finite element study. OBJECTIVE: To investigate intradiscal pressure, shear strain between anulus and adjacent endplates, and fiber strain in the anulus under pure and combined moments. SUMMARY OF BACKGROUND DATA: Concerning anulus failures such as fissures and disc prolapses, the mechanical response of the intervertebral disc during combined load situations is still not well understood. METHODS: A 3-dimensional, nonlinear finite element model of a lumbar spinal segment L4-L5 was used. Pure unconstraint moments of 7.5 Nm in all anatomic planes with and without an axial preload of 500 N were applied to the upper vertebral body. The load direction was incrementally changed with an angle of 15 degrees between the 3 anatomic planes to realize not only moments in the principle motion planes but also moment combinations. RESULTS: Intradiscal pressure was highest in flexion and lowest in lateral bending. Load combinations did not increase the pressure. A combination of lateral bending plus flexion or lateral bending plus extension strongly increased the maximum shear strains. Lateral bending plus axial rotation yielded the highest increase in fiber strains, followed by axial rotation plus flexion or axial rotation plus extension. The highest shear and fiber strains were both located posterolaterally. An additional axial preload tended to increase the pressure, the shear, and fiber strains essentially for all load scenarios. CONCLUSIONS: Combined moments seem to lead to higher stresses in the disc, especially posterolaterally. This region might be more susceptible to disc failure and prolapses. These results may help clinicians better understand the mechanical causes of disc prolapses and may also be valuable in developing preventive clinical strategies and postoperative treatments.