Ya-Wen Kuo

Rheology of intervertebral disc: an ex vivo study on the effect of loading history, loading magnitude, fatigue loading, and disc degeneration.

Study Design. An ex vivo biomechanical study on the rheological properties of healthy porcine and degenerated human intervertebral disc. Objective. To quantify the effect of loading history, loading magnitude, fatigue loading, and degeneration on disc rheology. Summary of Background Data. Disc rheological parameters, i.e., the aggregate modulus (HA) and hydraulic permeability (k) regulate the mechanical and biologic function of disc. The knowledge of effects of loading condition and degeneration on disc rheology can be beneficial for the design of new disc/nucleus implants or therapy. Methods. The following 4 phases of experiments were conducted to find the changes of disc rheological properties: (1) Effect of loading history during 1-hour creep (640 N) and relaxation (20% strain) test. (2) Effect of loading magnitude (420 N vs. 640 N) during the creep test. (3) Effect of fatigue loading (420 N, 5 Hz for 0.5, 1, and 2 hours) on the creep loading behavior. (4) Difference of healthy porcine and degenerated human discs during creep loading. The experimental data were fitted with linear biphasic model. Results. The aggregate modulus increased but hydraulic permeability decreased during creep loading. The aggregate modulus decreased but the hydraulic permeability did not change significantly during relaxation loading. The higher creep loading increased the aggregate modulus but decreased the hydraulic permeability. The fatigue loading did not change the aggregate modulus significantly, but decreased hydraulic permeability. Comparing the degenerated human disc to the healthy porcine disc, the aggregate modulus was higher but the hydraulic permeability was lower. Conclusion. The external loading and degeneration induce disc structural changes, e.g., the disc water content and interstitial matrix porosity, hence affect the disc rheological properties. The increase of aggregate modulus may be due to the reduction of disc hydration level, whereas the decrease of hydraulic permeability may be because of the shrinkage of disc matrix pores.

Mechanism of fractures of adjacent and augmented vertebrae following simulated vertebroplasty

Percutaneous vertebroplasty (VP) is a minimally invasive procedure that is used to treat osteoporosis-induced vertebral compression fractures (OVCFs). Frequently observed complications are fractures of adjacent and augmented vertebrae. In the present work, mechanisms for these fractures are presented. Fresh 4-level osteoporotic thoracic motion segments were tested. Both ends of the specimen were mounted. The lower level of the free vertebra was compressively fractured and followed by an injection of a 3.5 mL of a PMMA bone cement. Three steps of fatigue loading (5 Hz for 5 h) were incrementally and vertically applied on the specimens from 650 N to 950 N to 1150 N. Specimens of intact, compressively fractured, cement augmented and post-fatigued loading were radiographed for the measurement of deformations of the vertebra, the canal, and the foramen. At the end of fatigue loading, the vertebrae were sliced for micro morphologic analysis. The largest height loss after fatigue loading was at the posterior region of the augmented vertebra. In the augmented vertebra, fissures were found along the bone-cement interface. These fissures split the cement and the trabeculae and propagated into the vertebrae and the endplates. The compactness ratio of the trabeculae region of the adjacent cranial vertebra was higher than that for intact and adjacent caudal ones. We attribute the fracture of the augmented vertebra, following simulated VP, to the initiation of fissures along the cement-bone interface, which, in turn, may be due to uneven deformation of the vertebra. Fracture of the adjacent cranial vertebra is attributed to collapse of its trabeculae.

Spinal Traction Promotes Molecular Transportation in a Simulated Degenerative Intervertebral Disc Model

Study Design. Biomechanical experiment using an in situ porcine model. Objective. To find the effect of traction treatment on annulus microstructure, molecular convection, and cell viability of degraded discs. Summary of Background Data. Spinal traction is a conservative treatment for disc disorders. The recognized biomechanical benefits include disc height recovery, foramen enlargement, and intradiscal pressure reduction. However, the influence of traction treatment on annulus microstructure, molecular transportation, and cell viability of degraded discs has not been fully investigated. Methods. A total of 48 thoracic discs were dissected from 8 porcine spines (140 kg, 6-month old) within 4 hours after killing them and then divided into 3 groups: intact, degraded without traction, and degraded with traction. Each disc was incubated in a whole-organ culture system and subjected to diurnal loadings for 7 days. Except for the intact group, discs were degraded with 0.5 mL of trypsin on day 1 and a 5-hour fatigue loading on day 2. From day 4 to day 6, half of the degraded discs received a 30-minute traction treatment per day (traction force: 20 kg; loading: unloading = 30 s: 10 s). By the end of the incubation, the discs were inspected for disc height loss, annulus microstructure, molecular (fluorescein sodium) intensity, and cell viability. Results. Collagen fibers were crimped and delaminated, whereas the pores were occluded in the annulus fibrosus of the degraded discs. Molecular transportation and cell viability of the discs decreased after matrix degradation. With traction treatment, straightened collagen fibers increased within the degraded annulus fibrosus, and the annulus pores were less occluded. Both molecular transportation and cell viability increased, but not to the intact level. Conclusion. Traction treatment is effective in enhancing nutrition supply and promoting disc cell proliferation of the degraded discs. Level of Evidence: N/A

Rheological and dynamic integrity of simulated degenerated disc and consequences after cross-linker augmentation.

Study Design. An in situ study using whole-organ culture system. Objective. To study the effect of disc degeneration at different stages on its rheological and dynamic properties and to investigate the efficacy of exogenous cross-linking therapy. Summary of Background Data. Disc degeneration can involve protein denaturation or microdefects to the discs collagen fiber network. A disc degeneration model using whole-organ culture technique can be effectively used for the screening of treatments of degenerated discs. Exogenous cross-linking therapy has been shown to enhance the mechanical properties of the disc by cross-linking collagen. However, the efficacy of this therapy on the degenerated disc is unclear. Methods. A total of 40 porcine thoracic discs were assigned to 5 groups: intact discs, moderately degenerated discs, moderately degenerated discs with cross-linker augmentation, severely degenerated discs, and severely degenerated discs with cross-linker augmentation. The disc degeneration was simulated by trypsin digestion and mechanical fatigue loading. Rheological properties, dynamic properties, water content, and histological analysis were conducted after a 7-day incubation. Results. The mechanical properties of moderate degenerated discs significantly decrease both in rheological and dynamic properties, and laminate structure disorganization was observed. Mechanical defects of severely degenerated discs resulted in disc height loss, an increase in the aggregate modulus and stiffness modulus, and a decrease in the damping coefficient, hydraulic permeability, and water content. Cross-linker augmentation significantly recovered mechanical properties of moderately degenerated discs and restored the water content compared with the intact disc. However, the augmentation did not fully repair the severely degenerated discs. Conclusion. Trypsin-induced extracellular matrix damage resulted in a change of the discs biomechanics. Cross-linker augmentation recovers the rheological and dynamic properties of moderately degenerated discs but not of the severely degenerated discs. The genipin cross-linker may be able to improve the proteoglycan depletion effect in the nucleus pulposus but may not be effective to restore the structural damage in the collagen molecule of the anulus fibrosus. Level of Evidence: N/A

Interference in intradiscal pressure measurement using a needle‐type pressure transducer

Abstract The invasive needle‐type pressure transducer is widely used in the measurement of intradiscal pressure (IDP). However, the protocol for measuring IDP is not a standard procedure. Most of the in vitro studies employ an anterior insertion of the transducer while most of the in vivo studies employ a lateral insertion. The interference between transducer and disc needs to be understood before comparing the results of the two different protocols. Sixteen porcine one‐motion segments (T9–10, T11–12) were used. The transducers were inserted into the disc from either the anterior site (N = 9) or the lateral site (N = 7). All specimens had axial compressive force applied on eight locations along the mid‐sagittal line on the top of the cast to simulate flexion, neutral, and extension bending moments. The effects of bending moment and alignment of pressure transducers on the IDP were analyzed. Both the alignment of the transducer and bending moment affect the measurement of IDP. The IDP measured by an anterior‐inserted‐transducer (ant‐IDP) declined gradually from flexion through neutral to extension. The IDP measured by a lateral‐inserted‐transducer (lat‐IDP) during flexion and extension is higher than the IDP measured during the neutral condition. The ant‐IDP is higher than the lat‐IDP during flexion and neutral, but lower during extension. The ant‐IDP may be overestimated, compared to lat‐IDP, during flexion and neutral but underestimated during extension. We suggest that lateral insertion of transducer may be a better choice for both in vivo and in vitro IDP measurement.

STRAIN ANALYSIS OF PARS INTERARTICULARIS AND ITS IMPLICATIONS IN THE SPONDYLOLYSIS REHABILITATION STRATEGIES

The rehabilitation and exercise strategy for spondylolysis is controversial. Both lordotic and anti-lordotic braces, and both flexion and extension oriented motions were advised for spondylolysis rehabilitation. However, the biomechanical outcomes of the two strategies were less understood. The objective of current study is to evaluate the effect of postural sway on the strain of pars interarticularis, and to find the mechanical mechanism of lordotic and anti-lordotic rehabilitation strategies for spondylolysis. Two-motion-segment porcine cervical spine model (C3-C5, C6-T1) was used to simulate human lumbar spine. The loadings induced by anterior, neutral, posterior, anterolateral, lateral and posterolateral sways were simulated by applying vertical eccentric pointed loadings. Two uni-axial strain gages were applied on the dorsal surface of bilateral pars interarticularis of middle vertebrae. It was revealed that the pars strains induced by posterior, lateral, posterolateral sways were significantly greater than those induced by anterior, neutral and anterolateral sways. The ipsilateral pars strains were higher than the contralateral pars strains during lateral and posterolateral sways. However, pars strains of ipsilateral and contralateral side were similar during anterolateral sway. These results proved that the postural sway affects the strain of pars interarticularis. The anti-lordotic braces and exercise are likely to reduce the strain of pars interarticularis.

Effect of Degeneration on Fluid-Solid Interaction within Intervertebral Disk Under Cyclic Loading - A Meta-Model Analysis of Finite Element Simulations.

The risk of low back pain resulted from cyclic loadings is greater than that resulted from prolonged static postures. Disk degeneration results in degradation of disk solid structures and decrease of water contents, which is caused by activation of matrix digestive enzymes. The mechanical responses resulted from internal solid–fluid interactions of degenerative disks to cyclic loadings are not well studied yet. The fluid–solid interactions in disks can be evaluated by mathematical models, especially the poroelastic finite element (FE) models. We developed a robust disk poroelastic FE model to analyze the effect of degeneration on solid–fluid interactions within disk subjected to cyclic loadings at different loading frequencies. A backward analysis combined with in vitro experiments was used to find the elastic modulus and hydraulic permeability of intact and enzyme-induced degenerated porcine disks. The results showed that the averaged peak-to-peak disk deformations during the in vitro cyclic tests were well fitted with limited FE simulations and a quadratic response surface regression for both disk groups. The results showed that higher loading frequency increased the intradiscal pressure, decreased the total fluid loss, and slightly increased the maximum axial stress within solid matrix. Enzyme-induced degeneration decreased the intradiscal pressure and total fluid loss, and barely changed the maximum axial stress within solid matrix. The increase of intradiscal pressure and total fluid loss with loading frequency was less sensitive after the frequency elevated to 0.1 Hz for the enzyme-induced degenerated disk. Based on this study, it is found that enzyme-induced degeneration decreases energy attenuation capability of disk, but less change the strength of disk.

The compensation mechanism of cervical muscle dysfunction on spinal stability – an in vitro study using porcine model

Abstract Muscle injury/impairment results in decreased muscle forces. Decreased muscle forces can be compensated by synergic muscle forces. However, the effects of muscle dysfunction and compensation on spinal stability are not clear yet. Eight porcine cervical spine specimens (C2‐T1) were tested by the spine flexibility testing apparatus. The apparatus was equipped with muscle force replication of three paired cervical muscles. The simulations of muscle recruitment included; no muscle recruitment, normal muscle recruitment, muscle dysfunction without compensation, and muscle dysfunction with two compensation strategies: the minimized muscle forces and the minimized axial forces. The spinal stability was examined by the neutral zone (NZ) and range of motion (ROM), which was the sagittal motion of specimen applied with external moment at 0.5 and 2 Nm respectively. Initial positions of specimens were also recorded. NZ and ROM were largest in the no muscle test, and smallest in the muscle dysfunction without compensation test. NZ and ROM of muscle dysfunction with minimal axial force compensation were larger than those with minimal muscle force compensation. This study concluded that: (1) the muscle dysfunction without compensation constrains spinal motion; (2) impaired muscle with compensations cannot stabilize cervical spine efficiently as normal muscles does; and (3) compensation strategy of minimal muscle forces provides better spinal stability than that of minimal axial forces.

Recovering the mechanical properties of denatured intervertebral discs through Platelet-Rich Plasma therapy.

Degenerative disc disease is one of the most common causes of low back pain instigating huge socioeconomic costs and posing an immense burden on healthcare systems worldwide. New therapeutic approaches to damaged intervertebral discs are therefore of great interest. Platelet-Rich Plasma (PRP) has been proposed for the repair and regeneration of degenerated discs, but there remains a knowledge gap regarding its effectiveness and influence on disc material properties. The objective of this study was to investigate and quantify the material properties of intact, denatured, and PRP treated discs. A systematic methodology was established in the process, where ex-vivo experiments were conducted and material properties were extracted using an inverse finite element approach. The results showed that PRP is able to recover the mechanical properties of denatured discs, thereby providing a promising effective therapeutic modality.