Introduction

Cauda equina syndrome represents the constellation of symptoms and signs resulting from compression of lumbosacral nerve routes. Combined with subjective neurological findings, a reduction in anal tone is an important sign deeming further imaging necessary. Our main objective was to investigate the validity of DRE for assessment of anal tone.

Background

Spondylolysis (SL) of the lower lumbar spine is frequently associated with spina bifida occulta (SBO). There has not been any study that has demonstrated biomechanical or genetic predispositions to explain the coexistence of these two pathologies.

Introduction

Lumbar spondylolysis is a fatigue fracture of the pars interarticularis and correlates with Spina Bifida Oculta (SBO) in 67%.

Introduction: Loads acting on scoliotic spines are thought to be asymmetrical and involved in progression of the scoliotic deformity. Abnormal loading patterns could lead to changes in bone and disc cell and activity and hence to vertebral body and disc wedging. At present however there are no direct measurements of intradiscal stresses or pressures in scoliotic spines.

Methods: Stress profilometry was used to measure horizontal and vertical stresses at 5mm intervals across 25 intervertebral discs of 7 scoliotic patients during anterior reconstructive surgery. Identical horizontal and vertical stresses for at least two consecutive readings defined a region of hydrostatic pressure. Results were compared with similar stress profiles measured during surgery across 10 discs of 4 spines with no lateral curvature and with data from the literature.

Results: Profiles across scoliotic discs were very different from those measured across normal discs of a similar age. Hydrostatic pressure regions were only seen in 16/25 discs, extended only over a short distance and were displaced towards the convexity. Mean pressures were significantly greater (0.24MPa) than those measured in other anaesthetised patients (< 0.06 MPa). A stress peak in the concave annulus was a common feature (13/25) in scoliotic discs. In 21/25 discs, stresses in the concave annulus were greater than in the convex annulus, indicating asymmetric loading in these anaesthetised, recumbent patients.

Conclusions: Intradiscal pressures and stresses in scoliotic discs are abnormal even in the absence of significant applied load. Disc cells respond to changes in pressure, hydration and deformation by altering matrix synthesis and turnover in vivo and in vitro. Hence, whatever the cause of the abnormal pressures and stresses in the scoliotic discs, if present during daily life, these could lead to disc matrix changes and especially if asymmetrical, to disc wedging and progression of the scoliotic deformity.

Work supported by Fondation Cotrel

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: Age-related hormonal changes and inactivity lead to systemic bone loss and osteoporotic fractures. However, it is not clear why the vertebral body should be affected so often, or why its anterior region should characteristically sustain a “wedge” deformity. We hypothesise that intervertebral disc degeneration in elderly spines leads to altered spinal load-sharing in such a manner that the anterior region of the vertebral body becomes vulnerable to injury.

Methods: Forty thoraco-lumbar “motion segments”, consisting of two vertebrae and the intervening disc and ligaments, were obtained from cadaver spines aged 62–94 yrs. Volumetric bone mineral density (BMD) was measured for various regions of each vertebra using a Lunar Piximus DXA scanner. The distribution of the applied compressive force (1.5 kN) between the anterior and posterior halves of the vertebral body was calculated by pulling a needle-mounted pressure transducer along the sagittal midline of the adjacent disc. Pressure measurements were integrated over area to give force. Anterior and posterior disc forces were subtracted from the applied 1.5 kN to indicate loading of the neural arch. Measurements were repeated with the specimens positioned to simulate various postures in life. The strength of each motion segment was determined by compressing it to failure while positioned in a forward stooped posture. Disc and vertebral morphology were assessed from radiographs, and from digital photographs of tissue sections.

Results: Load-bearing by the neural arch in erect posture increased in the presence of intervertebral disc degeneration, and was inversely proportional to the average height of the disc (P< 0.01). High neural arch load-bearing was associated with relatively low BMD in the anterior vertebral body (P< 0.01), and with low compressive strength (P< 0.0001). BMD in the anterior region of the vertebral body was the best univariate predictor of compressive strength (R² = 0.78). Stepwise multiple linear regression showed that 86% of the variance in compressive strength could be explained by the following: anterior vertebral body BMD, vertebral body X-sectional area, and neural arch load-bearing (% of applied load). Forcing age, gender and spinal level into the model did little to improve the prediction.

Discussion and Conclusions: Results strongly support our hypothesis. Evidently, intervertebral disc degeneration and narrowing cause the neural arch to “stress shield” the anterior vertebral body whenever the spine is held erect. This leads to reduced BMD in the anterior vertebral body, weakening the spine when it is loaded in a stooped posture. The small age-dependence of results can be attributed to the relatively narrow age range of specimens tested. Vertebral fracture risk can best be assessed from BMD measured in the anterior half of the vertebral body.

Introduction: Intra-Discal Electrothermal Therapy (IDET) has been used to treat chronic discogenic low back pain. Proposed mechanisms of action include denervation of the posterior annulus and collagen denaturation. Previous authors have reported on changes in internal disc mechanics following IDET including reduction in stress concentrations possibly leading to a more even distribution of load across the end-plate¹. A novel intradiscal decompression catheter has been developed to reduce local disc bulging in cases of contained prolapse. This new catheter is inserted percutaneously into a disc and advanced under radiographic control into a postero-lateral position targeting the herniation. The decompression catheter uses more focused heating and higher temperatures than previous devices and is intended to provide a local decompression of the disc through a thermally-mediated reduction in nuclear volume. The purpose of this study was to investigate changes in internal stress profiles following use of the new catheter.

Methods: Five cadaveric lumbar ‘motion segments’ were dissected from two spines (age 64–84 yrs). Each segment was compressed, normally to 1 kN, while a miniature pressure transducer was withdrawn from posterior to anterior across the mid-sagittal diameter of the disc producing a baseline stress profile. A decompression catheter was inserted into the disc and its position confirmed with plain radiography. The temperature of the catheter was increased to 90°c over a period of 14 minutes. Stress profiles were then repeated.

Results: Stress profiles in three of the five segments showed changes consistent with degenerative change. In these discs stress profiles following ‘treatment’ showed up to a 35% reduction in the magnitude of stress peaks in the posterior annulus. There was very little change in the distribution of stress in the two non-degenerate discs. Stress in the nucleus appeared unchanged in all discs.

Conclusions: Treatment of degenerate discs with the decompression catheter lead to a measurable alteration annular stress peaks that have been associated with degenerative disc disease, while non-degenerate discs were unaffected. These preliminary findings of an ongoing study suggest that the novel decompression catheter has a biomechanical effect in certain classes of disc.

Bone microhardness has been successfully correlated with important functional parameters such as mineralisation and stiffness. It provides a means of examining the mechanical competence of bone at a micron scale, averaging the effect of osteonal lamellae but sensitive to variation in mineral content within a bone, and, with careful selection of indentation site, able to obtain material characteristics separate from any effects of porosity. However, the effect of bone’s viscoelasticity on such measurements has been largely ignored. This preliminary study investigates the post-indentation size change of Vickers indentations on wet bone. 4 axial slices of bovine femur were harvested from the same shaft, and polished. Each sample was subjected to 4 sets of 10 Vickers indentations with a load of 50 g and holding period of 15 s. The indentation size was measured immediately after the load was removed, and then again at intervals for a period up to 24 hours after the indentation was made. To avoid dehydration, the bone stood in water during the indentation testing and during measurement, and between each measurement period it was fully immersed in water. Measured hardness significantly decreased with time, by approximately 30% in total. The rate of post-indentation recovery is difficult to analyse since the driving force of residual strain decreases as recovery takes place. However a simple exponential fit to the variation of HV with time in the form of H = H(final).(1−exp(−kt)) + H(initial) suggests that the size of the indentation tends towards a constant size between 5 and 24 hours after indentation. Thus we conclude that care should be taken when making “early” measurements given the rapid rate of change in indentation size. Caution should also be employed when interpreting such data.

Cortical porosity is a useful evaluator of bone since it is sensitive to changes in bone turnover. The aim of this study was to evaluate cortical bone porosity of human vertebrae samples using Scanning Acoustic Microscopy (SAM). Currently the common techniques used to determine bone porosity are histomorphometry or scanning electronmicrosopy images. Both methods require extensive preparation of the bone samples. SAM represents a new technique with the great advantage of minimal sample interference since the bone is imaged in water, or saturated, and requires just one flat surface which is scanned (but not contacted) by the transducer. 46 specimens between the ages of 64–90 years were randomly selected and ground before SAM imaging of was carried out using a 400 MHz transducer. For each sample posterior and anterior sections of the cortical bone were scanned several times, and the porosity measured using Scion image software to process the images. It was possible to image the entire anterior or posterior cortex in a single image with 4 mm spatial resolution. Measured porosity was in the region 5 % – 21 %, and showed a significant increase with age for the female specimens but no age dependence in the male specimens. At low porosity (< 6 %) vertebral compressive strength was uncorrelated with porosity. However, at higher porosities strength was highly correlated with porosity. (As would be expected, strength decreased with increasing porosity). High frequency SAM has potential for future bone characterisation, particularly where it is desirable to correlate local measurements of material properties such as nanohardness or microhardness, with microstructure.

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.

Introduction: Whilst conventional Magnetic Resonance Imaging (MRI) is universally used as the method of choice for examining the boundaries of the intervertebral disc clinically it gives little information about the internal structure of the disc. This is largely due to the fact that the normal resolution of such devices (typically 1mm in plane and 3mm out of plane) is just too large to resolve structures and pathologies of interest.

Aim: This work aims to describe the appearance of normal and pathological discs when imaged using a high resolution system. It then tests the hypothesis that a degeneration grading scale based upon such observations corresponds well with the graded appearance of the sectioned disc.

Method: 13 lumbar discs from 7 non-chondrodystrophoid dogs (age 2–10 yr, mean 5.7 yr) were employed in this study. They were imaged using a small bore 0.5T research imaging system using a T2* weighted pulse sequence (TR=500ms, TE=17ms), a 60mm field of view, 1 mm slice thickness, in plane resolution was 230 μm. A grading scale based on the standard visual scale was developed for grading these images.

Results: The outer and middle annulus had a strongly banded appearance with adjacent lamellae having high and low intensities (in spite of there similar chemical composition). The inner annulus (and frequently all the posterior annulus) had a uniform high intensity appearance as did the nucleus. Frequently, there has a well defined dark boundary between the annulus and nucleus. Increasing degeneration lead to disorder of the annulus structure and non-uniformity in the nucleus. Statistical comparison of the visual and MRI grading scales were extremely good (α=0.90–0.95) except for the posterior annulus (α=0.26).

Conclusion: Many features of the MRI appearance of discs at high resolution, such as the banded structure of the annulus, were not expected and must be due to some subtle physical processes. Care must therefore be taken with the interpretation of such images, in particular to assessment of hydration. Grading of high resolution images corresponded well to the ‘gold standard’ of visual appearance on sectioning. However, this scale is totally different to that used to grade discs using conventional clinical MRI.

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.

Scoliosis is a disease characterised by vertebral rotation, lateral curvature and changes in sagittal profile. The role of mechanical forces in producing this deformity is not clear. It is thought that abnormal loading deforms the disc, which becomes permanently wedged. Modelling and in vitro studies suggest that such deformations should increase intradiscal pressure. Intradiscal pressure has been measured previously in a variety of clinical environments. The aim of this study is to measure pressure profiles across scoliotic discs to provide further information on the role of mechanical forces in scoliosis.

Pressure readings were obtained in consented patients with ethical approval using a needle-mounted sterilised pressure transducer (Gaeltec, Dunvegan, Isle of Skye) calibrated as described previously. The transducer needle was introduced into the disc of an anaesthetised patient during routine anterior scoliosis surgery and pressure profiles measured. Signals were collected, amplified and analysed using Power-lab and a laptop computer.

Pressure profiles across 10 human scoliotic discs from 3 patients have been measured to date. Pressures varied from 0.1 to 1.2 MPa.

Annular pressures showed high pressure, non-isotropic regions on the concave but not convex side of these discs.

Nuclear pressures recorded from the discs of these scoliotic patients were higher than those recorded previously in non-scoliotic recumbent individuals.

Osteoporotic vertebral fractures are normally attributed to weakening of the vertebral body. However, the compressive strength of the spine also depends on the manner in which the intervertebral disc presses on the vertebral body, and on load-bearing by the neural arch. We present preliminary results from a large-scale investigation into the relative importance of these three influences on vertebral compressive strength.

Lumbar motion segments from elderly cadavers were subjected to 1.5 kN of compressive loading while the distribution of compressive stress was measured along the antero-posterior diameter of the intervertebral disc, using a miniature pressure-transducer. The overall compressive force on the disc, obtained by integrating the stress profile ( 1), was subtracted from the 1.5 kN applied load to give the force resisted by the neural arch. Stress profilometry was performed with each motion segment positioned to simulate the erect standing posture, and a forward stooping posture. Vertebral strength was measured by compressing the motion segments to failure in the forward stooping posture. In life, the spine is usually compressed most severely in this posture.

A univariate analysis of results from the first 9 motion segments (aged 72–92 yrs) showed that vertebral strength increased from 2.0 kN to 4.6 kN as the compressive force resisted by the neural arch in erect postures decreased from 1.1 kN to 0.4 kN (r² = 0.42, p = 0.05). Updated results from this on-going study will be presented at the meeting.

Preliminary results suggest that habitual load-bearing by the neural arch in erect postures can lead to progressive weakening of the vertebral body, which is effectively “stress-shielded” by the neural arch. This weakening is exposed when the spine is loaded severely in a forward stooped posture, when it has a reduced compressive strength. This mechanism could explain some features of osteoporotic vertebral fractures in old people.