Introduction: In vivo measurements of intradiscal stresses are difficult. McNally measured stress profiles in human discs. It is unclear why some exhibit stress peaks in posterior annulus while others do not. Therefore finite element (FE) models are useful to improve the knowledge of stress distribution in the disc. We compared experimental and numerical stress in discs under axial loading, in non degenerated and degenerated disc.

Methods: The FE disc model resembles one fourth of a full disc. The annulus contains both matrix and fibers, while the nucleus only consists of matrix. Similar load profiles were applied and model predictions of matrix stress were compared to experiments (stress profilometry).

Results: Both experimental data and numerical simulations exhibit a peak of axial stress in posterior annulus and lower peaks in anterior annulus. Simulating a “normal” disc results in a uniform matrix stress profile from posterior to anterior. By reducing the fixed charged density (FCD) to 50% in both nucleus and annulus, stress profiles become non-uniform. Stresses in the nucleus decrease. Axial annulus stresses exhibit peaks on anterior and posterior side. Stress peaks increase when FCD decrease under the same loading.

Discussion: The size of the peaks computationally depends on the FCD in discs. Decreasing the FCD shows development of stress peaks in the annulus. A uniform stiffness is seen in nucleus region, but not in annulus. The hydrostatic pressure, due to the FCD, is not high enough to evenly distribute the load over the whole disc. The posterior stress peaks may explain why hernia develops particularly in the posterior annulus.

Introduction: Dynesys is a novel, dynamic stabilization system designed for the treatment of degenerative conditions of the lumbar spine that present with unstable motion segments. This system uses pedicle screws with a modular spacer mounted on a stabilising cord, which controls movement of the instrumented segment in all planes. The purpose of this study was to investigate changes in the biomechanic response of the intervertebral disc (IVD) under normal, flexed and extended loading conditions before and after Dynesys is applied. The IVDs of both the instrumented (bridged) and the adjacent (floating) segment were studied.

Methods: Twelve L3-5 cadaveric segments were dissected and compressed to 1kN in 6° flexion, neutral and 4° extension. The test was done without spacers and with spacers measured to +2mm, neutral and −2mm, where neutral equates to the normal distance between the pedicle screws without an applied load. The stress distribution in the mid-sagittal and posterolateral diameters of both the bridged and floating discs was measured using a miniature pressure transducer. This resulted in greater than 300 stress profiles per specimen. Disc movement and segment motion during loading were recorded using ultrasound imaging and infra-red reflection respectively.

Results: Without stabilization, stress peaks observed in the anterior annulus increased by more than 85% as the specimen was loaded from 4° extension to 6°flexion. With the application of Dynesys, these anterior stress peaks were reduced across the bridged segment. This was most pronounced in 6° flexion where anterior stress peaks of greater than 1 MPa were reduced by 100% in the bridged segment in more than 90% of specimens.

Conclusions: The degree of flexion or extension of the specimen during loading influences the peak stresses generated in the annulus. Dynesys has the potential to relieve peak stresses in the anterior annulus which is most pronounced when the specimen is loaded in flexion.

Intervertebral disc function and dysfunction is governed by its structural architecture of concentric layers of highly ordered collagen fibres. This architecture is important at the mm scale for overall mechanical performance of the disc; and at the micron scale for mechano-transduction signalling pathways of the disc cells that are responsible for matrix maintenance and therefore disc health. To understand such mechanical behaviour 3-dimensional collagen fibre architecture must be quantified in intact intervertebral discs. Conventional imaging modalities lack either the spatial resolution (e.g. x-ray diffraction) or penetration (e.g. optical, electron or confocal laser microscopy) to yield mechanically important information. Preliminary studies of scanning acoustic microscopy (SAM) at 50 MHz visualises alternating layers of fibre texture, however exactly what is being imaged requires both explanation and validation. Three-dimensional SAM data sets obtained from intact discs were compared to polarised-light and scanning electron micrographs of individual layers of fibres, peeled by micro-dissection from discs. The dimensions of the structural features were measured and recorded. Optical and electron microscopy revealed that each layer consisted of highly oriented collagen fibres of diameter 5 μm with regularly spaced splits between fibres with a spacing of approximately 20–30 μm. The SAM data sets showed layers with a uniform highly oriented fibre texture that reversed between adjacent layers. Resolution of the texture was limited by the acoustic system to approximately 30 μm. It is clear that SAM at 50 MHz cannot resolve and therefore image individual collagen fibres. However, the regular defects in the fibre layers can be visualised and convey complete information about local collagen fibre architecture. SAM therefore provides an effective way of quantifying the fibrous structure of intact, hydrated, unfixed intervertebral discs.

Study Design: Cadaveric study on the effects of Dynesys.

Summary of Background Data: Dynesys is a novel form of soft stabilization that utilises pedicle screws and modular spacers mounted on a stabilising cord to control movement of the instrumented segment in all planes. In this way it provides a biomechanical alternative with greater physiological function than spinal fusion and may prevent the penalties of “overworking” adjacent levels.

Objective: The biomechanical response of both the instrumented and adjacent intervertebral discs (IVD) is investigated under compressive loading in flexion and extension. The effects of varying spacer heights on intradiscal pressure distribution are also reported.

Methods: Twelve L3-5 cadaveric lumbar segments were compressed to 1 kN in 6° flexion, neutral and 4° extension. The stress distribution in the mid-sagittal and posterolateral diameters of both the bridged and adjacent discs was measured by withdrawing a miniature pressure transducer across the IVD. Dynesys was applied across a single level and +2mm, neutral and −2mm spacer configurations tested in each position of loading. Over 2500 stress profiles were collected and the data obtained from measurements with and without application of Dynesys was analysed.

Results: In the absence of instrumentation stress peaks in the anterior annulus increased with a greater degree of specimen flexion. In 0° to 6° flexion, Dynesys eliminated the anterior stress peaks observed in the instrumented disc in 80% of specimens tested. In the +2mm to −2mm spacer range tested, posterior stress peaks were generally seen to increase with decreasing spacer height. Little effect is seen with the application of Dynesys to a non-degenerate disc. Preliminary analysis of the data suggests that stress distribution through the adjacent disc appears largely unchanged with instrumentation of the inferior segment.

Conclusions: Dynesys has the potential to relieve peak stresses in the anterior annulus seen particularly in positions of flexion. Spacer size influences the generation of peak stresses seen within the posterior annulus. Initial observations indicate that the IVD of the adjacent motion segment is not biomechanically prejudiced following the application of Dynesys.

Introduction: Dynesys is a novel, dynamic stabilization system designed for the treatment of degenerative conditions of the lumbar spine that present with unstable motion segments. This system uses pedicle screws with a modular spacer mounted on a stabilising cord, which controls movement of the instrumented segment in all planes. The purpose of this study was to investigate changes in the biomechanic response of the intervertebral disc (IVD) under normal, flexed and extended loading conditions before and after Dynesys is applied. The IVDs of both the instrumented (bridged) and the adjacent (floating) segment were studied.

Methods: Eight L3–5 cadaveric segments were dissected and compressed to 1kN in 6° flexion, neutral and 4° extension. The test was done without spacers and with spacers measured to +2mm, neutral and −2mm, where neutral equates to the normal distance between the pedicle screws without an applied load. The stress distribution in the mid-sagittal and postero-lateral diameters of both the bridged and floating discs was measured using a miniature pressure transducer. This resulted in greater than 300 stress profiles per specimen. Disc movement and segment motion during loading were recorded using ultrasound imaging and infrared reflection respectively.

Results: Without stabilization, stress peaks observed in the anterior annulus increased by more than 85% as the specimen was loaded from 4° extension to 6°flexion. With the application of Dynesys, these anterior stress peaks were reduced across the bridged segment. This was most pronounced in 6° flexion where anterior stress peaks of greater than 1 MPa were reduced by 100% in the bridged segment in more than 90% of specimens.

Conclusions: The degree of flexion or extension of the specimen during loading influences the peak stresses generated in the annulus. Dynesys has the potential to relieve peak stresses in the anterior annulus which is most pronounced when the specimen is loaded in flexion.

Introduction: Cadaveric intervertebral discs (IVD) must perform consistently and repeatably with time and cyclic loading if the results from long experimental protocols are to be considered valid. Experiment design should take into account the potential for changes in the biomechanical properties of the intervertebral disc. Changes in the pressure distribution and stress profiles across the IVD along with variation in movement of the anterior annulus during a load cycle give a good indication as to the biomechanic status of the IVD. The purpose of this study was to assess the biomechanic response of the IVD to repeated cyclic loading, in normal, flexed and extended positions over a prolonged period.

Methods: Ten multisegment cadaveric lumbar spine specimens (L3-5 or L1-3) were dissected and compressed to 1kN in 6° flexion, neutral and 4° extension. The anterior annulus was imaged during loading using ultrasound. The stress distribution along the mid-sagittal and antero-postero-lateral (APL) diameters of both discs was measured by withdrawing a miniature pressure transducer from posterior to anterior across the IVD during loading. Stress profilometry and ultrasound imaging was performed over a two day period.

Results: Ultrasound imaging provides an easy method for observing disc movement during compressive loading of a multi-segment specimen through positions of extension and flexion. Anterior disc bulging increased by more than 150% as the specimen is loaded from 4° of extension to 6° flexion. Repeated passes of the pressure transducer across both the mid-sagittal and APL diameter of the discs produced repeatable stress profiles. Similarly, ultrasound imaging of the anterior annulus showed comparable disc movement after cyclic loading.

Conclusions: Preliminary results suggest that the biomechanical behaviour of the IVDs of a multi-segment specimen do not change significantly following prolonged testing and multiple cyclic loading.