Posterior lumber interbody fusion (PLIF) has the theoretical advantage of optimising foraminal decompression, improving sagittal alignment and providing a more consistent fusion mass in adult patients with isthmic spondylolisthesis (IS) compared to posterolateral fusion (PLF). Previous studies with only short-term follow-up have not shown a difference between fusion techniques.

An observational cohort study was performed of a single surgeon's patients treating IS over a ten year period (52 patients), using either PLF (21 pts) or PLIF (31pts). Preoperative and 12-month data were collected prospectively, and long-term follow-up was by mailed questionnaire. Preoperative patient characteristics between the two groups were not significantly different. Average follow-up was 7 years, 10 months, and 81% of questionnaires were returned. Outcome measures were Roland Morris Disability Questionnaire (RMDQ), Low Back Outcome Score (LBOS), SF-12v2 and SF-6D R2. The SF-6D R2 is a “whole of health” measure.

PLIF provided better short- and long-term results than PLF. The PLIF group had significantly better LBOS scores in the long term, and non-significantly better RMDQ scores in the long term. As measured by RMDQ Minimum Clinically Important Difference (MCID) short term set at 4, RMDQ MCID set at 8, the LBOS MCID set at 7.5 points and by SF-12v2 physical component score (PCS), PLIF patients performed better than PLF patients. When analysing single level fusions alone, the difference is more pronounced, with PCS, mental component scores and SF-6D R2 all being significantly better in the PLIF group rather than the PLF group.

This paper strongly supports the use of PLIF to obtain equivalent or superior clinical outcomes when compared to PLF for spinal fusion for lumbar isthmic spondylolisthesis. The results of this study are the first to report to such long-term follow-up comparing these two procedures.

The spinal motion segment relies critically on there being a mechanically robust integration between the compliant disc tissues and the rigid vertebral bone. Achieving such integration represents a major structural challenge. This study explores in detail the microstructural mechanisms involved in both the nucleus-endplate and annulus-endplate regions.

Vertebra-nucleus-vertebra samples were obtained from mature ovine lumbar motion segments and subjected to a novel ring-severing technique designed to eliminate the strain-limiting influence of any remaining annular elements. These samples were loaded in tension and then chemically fixed in order to preserve the stretched fibre arrangement, and then decalcified. Annulus-vertebra samples were similarly treated but without any loading prior to fixation. Differential interference contrast optical microscopy was then used to image at high resolution cryosectioned slices of the still integrated disc-vertebral endplate regions while maintained in their fully hydrated state.

Structural continuity across the nucleus-endplate junction was sufficient for the samples to support, on average, 20 N before tensile failure occurred. Microscopic examination revealed fibres inserting into the endplates and extending continuously from vertebra to vertebra in the central nuclear region. While the fibres in the nucleus possess a significant level of structural integration with the endplates their role is not primarily a tensile one: rather, in combination with their convoluted geometry, they confer on the nucleus a form of ‘tethered’ mobility. This permits a high degree of shape change in the nucleus during normal disc function in which hydrostatic loading plays an essential role. The annular fibre bundles on entering the endplate are shown to subdivide into sub-bundles to form a 3-D multi-leaf morphology with each leaf separated by cartilaginous endplate matrix. This branched morphology increases the interface area between bundle and matrix in proportion to the number of sub-bundles formed.

Our study challenges previously published views on nucleus-endplate relationships. We also show that the robust integration of the annular fibres in the endplate is achieved via a branched morphology exploiting a mechanism of shear-stress transfer, with the anchorage strength optimised over a relatively short endplate insertion depth.

FAI has been implicated in the progression of osteoarthritis (OA) and early detection may allow for treatment, which can slow or halt progression. FAI is a difficult condition to image and there is little objective evidence about imaging accuracy. We aim to measure the accuracy of five imaging modalities.

Three blinded observers retrospectively reviewed five different modalities from two age and sex matched groups: A patient group referred to the outpatient clinic with a clinical diagnosis of FAI and a control group who had had CT scans of the pelvis for suspected trauma, where the Pelvic scan had been reported as showing no injuries.

The imaging modalities were: Standard x-ray; Antero-Posterior, Lateral; Condition-specific x-ray projections; Dunn view, lateral internal rotation; Standard Computer Tomography (CT) multiplanar reconstruction (MPR); axial, sagittal and coronal; Condition-specific CT MPR; angled axial, angled coronal; 3D modelling; and surface rendered dynamic.

We found marked variations in the sensitivity, specificity, Positive Predictive Value (PPV) and Negative Predictiive Value (NPV) for each of the following imaging modalities: Standard X-ray; Sensitivity 51.9; Specificity; 57.1; PPV; 40; NPV; 68.3 Special X-rays; Sensitivity; 66.7; Specificity; 57.1; PPV; 46.1; NPV; 75.7. Standard CT MPR; Sensitivity; 40.7; Specificity; 75.5; PPV; 47.8; NPV; 69.8 Special CT MPR; Sensitivity; 48.1; Specificity; 57.1; PPV; 46.4; NPV; 70.8 Dynamic 3D CT models; Sensitivity; 55.6; Specificity; 69.3; PPV; 42.8; and NPV; 71.8.

The Dynamic 3D CT models (where the observer can manipulate the model in real time three dimension to control the perspective) proved to be the most accurate, closely followed by the special X-Ray views, which were also the most sensitive. The Standard CT MPRs were the most specific but had a low sensitivity.

This is the first study to measure sensitivity, specificity and PPV and NPV for these imaging modalities in FAI. We recommend the use of condition-specific X-Ray views as well as 3D CT Models for optimal imaging accuracy in this condition. Standard X-Ray views and CTs proved less useful.

There is evidence that various anatomical structures have altered morphology with ageing, and anecdotal evidence of changing lumbar spinous process (LSP) morphology with age. This study aims to clarify the influence of age on LSP morphology, and on lumbar spine alignment.

200 CT scans of the abdomen were reformatted with bone windows allowing precise measurement of LSP dimensions and lumbar lordosis. Observers were blinded to patient demographics. Inter-observer reliability was confirmed.

The smallest LSP is at L5. The male LSP is on average 2-3mm higher and 1mm wider than the female LSP. LSP height increases significantly with age at every level in the lumbar spine (P<10^-5 at L2). The LSPs increase in height by 2-5mm between 20-85 years of age (P<10^-6), which was as much as 31% at L5 (P<10^-8). Width increases proportionally more, by 3-4mm or greater than 50% at each lumbar level (P<10^-11). Lumbar lordosis decreases in relation to increasing LSP height (P<10^-4) but is independent of increasing LSP width (P=0.2).

The height and width of the spinous processes increases with age. Increases in spinous process height are related to a loss of lumbar lordosis and may contribute to sagittal plane imbalance.

Auckland City Hospital, Auckland, New Zealand.

To show that the spinous processes (SPs) increase in size with age.

To investigate the incidence of SP abutment, relationship to disc degeneration and age related kyphosis.

Describe patterns of SP neoarticulation in relation to back pain and intersegmental axial rotation and deformity.

We reviewed 200 Abdominal CTs, CT myelograms and 100 standing x-rays (age 18-90 years).

We measured SP size, interspinous gap, patterns of neoarticulation, disc height, lumbar lordosis and axial rotation.

We compared symptomatic and asymptomatic groups.

A 30-50% increase in SP size coupled combined with a loss of disc height leads to increasing rates of SP abutment after the age of 35 years. 30% of people over the age of 60 years have SP abutment.

There is a 15 degree increase in standing lumbar kyphosis with age.

Four patterns of SP neoarticulation are seen. Degenerative changes in the SP articulation increase by more than 80% in a symptomatic cohort.

Oblique SP articulation is 2.5 times more likely in symptomatic individuals and associated with a rotational intersegmental deformity.

Ageing is accompanied by SP enlargement and abutment, contributing to a loss of lumbar lordosis.

Patterns of neoarticulation and degeneration appear associated with back pain and rotational deformity.

Background: The Lumbar Spinous Processes (LSP) have an important anatomical and biomechanical function. They also influence access to the spinal canal for neural decompressive surgical procedures. There is evidence that various anatomical structures have altered morphology with ageing, and there is anecdotal evidence of changing LSP morphology with age. This study aims to clarify the influence of age on LSP morphology, and on lumbar spine alignment.

Method: 200 CT scans of the abdomen were reformatted with bone windows allowing precise measurement of LSP dimensions, and Lumbar Lordosis. Observers were blinded to patient demographics. Inter-observer reliability was confirmed.

Results: The smallest LSP is at L5. The male LSP is on average 2–3mm higher and 1mm wider than the female LSP. LSP height increases significantly with age at every level in the lumbar spine (P< 10-5 at L2). The LSPs increase in height by 2–5mm between 20–85 years of age (P< 10-6), which was as much as 31% at L5 (P< 10-8). Width increases proportionally more, by 3–4mm or greater than 50% at each lumbar level (P< 10-11). Lumbar lordosis decreases in relation to increasing LSP height (P< 10-4) but is independent of increasing LSP width (P=0.2).

Conclusions: The height and width of the spinous processes increases with age. Increases in spinous process height are related to a loss of lumbar lordosis and may contribute to sagittal plane imbalance.

Conflicts of Interest: None

Source of Funding: None

Intervertebral disc herniation and internal disc disruption are both thought to be primarily mechanically based pathologies. Although several studies have previously disrupted discs in vitro, none have examined the resulting disruptions microscopically.

The technique of nuclear pressurization was used to mechanically disrupt ovine lumbar motion segments. A hollow injection screw was inserted longitudinally through the inferior vertebra of each motion segment, so that the injection screw’s tip was located in the centre of the nucleus. Through this screw, a radio-opaque gel was gradually injected into each segment’s nucleus until failure occurred, marked by a large drop in nuclear pressure, or focal change to the disc’s periphery. Following mechanical testing, the internal failure characteristics of each motion segment were assessed using micro-CT and microscopy. During nuclear pressurization, motion segments were held in one of four postures:

0° flexion,

Group I (0° flexion; n=12): Discs failed at a mean nuclear pressure of 13.2±2.1MPa. In most cases failure occurred in a diffuse manner via sequential circumferential tears within the posterior annulus. Group II (7° flexion; n=17): Discs failed at a mean nuclear pressure of 11.2±2.5MPa. Compared to the Group I discs, 7° flexion led to the creation of radial tears extending through the central posterior disc wall. Two types of radial tear occurred: mid-axial and annular-endplate. Mid-axial radial tears were confined to the annulus. Annular-endplate radial tears incorporated both annular and endplate failure; endplate failure in these tears always occurred adjacent to the mid-annulus at the cartilaginous/vertebral endplate junction. Group III (10° flexion; n=17): Discs failed at a mean nuclear pressure of 9.8±2.6MPa. Compared to the Group II discs, 3° of additional flexion increased the proportion of annular-endplate radial tears. Group IV (7° flexion + 2° axial rotation; n=25): Discs failed at a mean nuclear pressure of 7.9±2.4MPa. Compared to the Group II discs, the addition of 2° axial rotation significantly decreased the nuclear pressure at which discs failed, and reduced the occurrence of mid-axial radial tears.

Postures that reduced the disc wall’s ability to withstand high nuclear pressures were associated with an increase in the proportion of disc failures that incorporated tears of the cartilaginous endplates, specifically at the cartilaginous/vertebral endplate junction adjacent to the mid-annulus. The robustness of this junction appears to be intimately linked to the robustness of the disc wall.

There is controversy whether or not the midline structures (spinous processes, inter and supraspinous ligaments) should be preserved or sacrificed (MLP vs MLS) during decompression in the treatment of lumbar spinal stenosis(LSS). MLP operations are popular as they preserve the posterior tension resisting structures. Equally it is increasingly recognized that the facet joints(FJ), partially resected during decompressive procedures for LSS, have importance resisting postoperative spondylolisthesis and instability. This study was performed to examine the effects of MLP or MLS upon FJ morphology.

MRI scans from 7 patients with LSS (L2/3 to L5/ S1), and 4 patients without LSS (L3/4 to L5/S1) were examined and subjected to theoretical decompression with operative plans that performed decompression via a 10mm corridor that either preserved the midline structures via a parasagittal/laminomy (MLP), or sacrificed the midline structures providing an angled corridor for decompression from the opposite side of the table (MLS). The lateral margin of the decompression was the medial border of the pedicle. Cross sectional area (CSA) was determined for the FJ before and after decompression with both MLP and MLS using Image J cross sectional area analysis.

The cross sectional area of the facet joints prior to surgery was 287 mm2 at L3/4, 275 at L4/5, and 284 at L5/S1 in non-LSS pts. In LSS patients the values were 257, 267, 328, and 319mm2 at the levels L2/3 to L5/S1 respectively. MLS reduced the FJ CSA by 6, 4, and 0 % respectively in the non LSS pts (L3/4 to L5/S1), and 14, 9, 11, and 6% in the LSS pts (L2/3 to L5/S1). MLP reduced the FJ CSA by 34, 25 and 17% in non LSS pts (L3/4 to L5/S1), and 57, 43, 39 and 29% in the LSS pts (L2/3 to L5/S1). The differences between the MLP and MLS reductions of FJ CSA were highly significant. Greater relative reductions were seen with MLP when the CSA of the inferior articular process was examined.

This study demonstrates that preservation of the midline significantly reduces the CSA of the facet joint and in particular the IAP. Biomechanical and clinical evidence suggests that the FJs have a greater role in stability of the motion segment in the lumbar spine, and this study suggests that well-intentioned determination to preserve the midline structures may have a deleterious effect upon the anatomical structures responsible for stability of the lumbar spine.

Most researchers have employed conventional histological and related methods to investigate the complex architecture of the IVD. Recognizing the inherent limitations of these methods we have pioneered new microstructural and micromechanical techniques that have greatly enhanced our understanding of the 3-D architecture of the IVD. Using sectioning planes that take full account of the oblique fibre angles in the annular wall, combined with specialized optical imaging techniques that provide high resolution structural images of fully hydrated thick sections we have described new levels of structural complexity that are clearly implicated in the biomechanical function of this highly complex connective tissue organ.

The primary regions of structural interest are the annulus, the annular-endplate junction and the nucleus-end-plate junction. Within the complex multilayered annular wall we have identified a system of collagen-rich bridging structures that both integrate proximate oblique and counter-oblique layers as well as providing long-range radial continuity across many layers. We argue that this system has an important biomechanical role of lashing alternate ‘like’ layers together whilst providing for some freedom of fibre angle change between immediately adjacent layers coursing in counter oblique directions. Thus, under the deformations generated by direct compressive, bulging, flexion and minor rotational forces, the structural integrity of the annulus is maintained.

We have also clarified important features of both annular/endplate and nucleus/endplate structural integration. Our very recent structural studies of the lumbar motion segment suggest that the current models of disc/endplate integration require substantial revision. This presentation will describe new experimental evidence in support of a more appropriate model of structural integration.

The Lumbar Spinous Processes (LSP) have an important anatomical and biomechanical function protecting the neural structures in the spinal canal, and as an anchor for the inter and supraspinous ligaments, and the inter-segmental paraspinal muscles. They also influence access to the spinal canal for neural decompressive surgical procedures. More recently the LSPs have attracted increased interest as a site for surgical device attachment in an attempt to both decrease the symptoms of spinal stenosis, and as a site for intersegmental stabilization without formal fusion. There is evidence that various anatomical structures have altered morphology with ageing, and there is anecdotal evidence of changing LSP morphology with age. This study aims to clarify the influence of age on LSP morphology, and on lumbar spine alignment.

200 CT scans of the abdomen were reformatted with bone windows in sagittal and coronal planes allowing precise measurement of LSP dimensions, and Lumbar Lordosis. Observers were blinded to patient demographics. Inter-observer reliability was examined. Data was analysed by an independent statistician.

The smallest LSP is at L5. The male LSP is on average 2–3mm higher and 1mm wider than the female LSP. LSP height increases significantly with age at every level in the lumbar spine (p< 10-5 at L2). The LSPs increase in height by 2–5mm between 20–85 years of age (p< 10-6), which was as much as 31% at L5 (p< 10-8). Width increases proportionally more, by 3–4mm or greater than 50% at each lumbar level (p< 10–11). Lumbar lordosis decreases in relation to increasing LSP height (p< 10-4) but is independent of increasing LSP width (p=0.2).

This study demonstrates that the dimensions of the LSP change with age. Increases in LSP height occur with age. More impressive increases in LSP width occur with advancing age. This study suggests that loss of lumbar lordosis is correlated with changing LSP morphology.

The increased width of the LSP with age influences access to the spinal canal, particularly if midline-preserving approaches are attempted in the ageing population. There is increased bone volume for bone grafting procedures with increasing age. The reduced distance between LSPs with age may influence design of implants that stabilize this region of the spine, and occur not only as a result of disc space narrowing, but also as a consequence of increased LSP dimensions.

The detailed anatomy of interconnectivity of intervertebral disc annular fibre layers remains unclear and a structural survey of interlammellar connectivity is required to understand this anatomy and mechanical behavior. The subsequent failure modes of the annulus under hydrostatic loading require definition to understand genesis of annular tears and disc herniation.

Interlamellar Connectivity. We imaged anterior annular sections from ovine lumbar discs. Using differential interference contrast microscopy we were able to reconstruct a three-dimensional image of the interconnecting bridging network between layers. Annular Disruption. The nuclei of ovine lumbar discs were gradually pressurised to failure by injecting a viscous radio-opaque gel via their inferior vertebrae. Investigation of the resulting annular disruption was carried out using micro-computed tomography and DIC microscopy. This allowed analysis of annular failure patterns and herniation, with analysis of the pathway of nuclear movement during prolapse in relation to annular fibre separation within and between fibre layers.

Interlamellar Connectivity. A high level of connectivity between apparently disparate bridging elements was revealed. The extended form of the bridging network is that of occasional substantial radial connections spanning many lamellae with a subsidiary fine branching network. The fibrous bridging network is highly integrated with the lamellar architecture via a collagen-based system of interconnectivity. In particular this bridging network appears to have a major role in anchoring leading edges of incomplete annular lamellae. Annular Disruption and Disc Herniation. Gel extrusion from the posterior annulus was the most common mode of disc failure. Unlike other regions of the annular wall, the posterior region was unable to distribute hydrostatic pressures circumferentially. In each extrusion case, severe disruption to the posterior annulus was observed. While intralamellar disruption occurred in the mid annulus, interlamellar disrupt ion occurred in the outer posterior annulus. Radial ruptures between lamellae always propagated in the mid-axial plane.

The interlamellar architecture of the annulus is far more complex than has previously been recognised and this paper further defines the microanatomy of the disc wall. The hydrostatic pressure failure mode of the posterior annulus mirrors clinic al sites of annular tear and disc prolapsed in the neutral loading position.

Lumbar Total Disc Replacement (LTDR) is an alternative to fusion for the surgical management of discogenic back pain not responding to conservative therapy. Theoretical advantages include preservation of motion and possible reduction in adjacent segment degeneration. The aim is to review the early results of LTDR in an Auckland private practice.

A prospective study was carried out on 32 consecutive patients treated with LTDR. Discogenic back pain was confirmed with plain radiographs, MRI and CT lumbar discogram. The Charité TDR was used in 19 patients and A-MAV in 13. Follow-up was to a minimum of two years. Patients completed Modified Roland Questionnaires (MRQ), Visual Analogue Pain Scores (VAPS) and Lower Back Outcome Scores (LBOS) pre and post-operatively. Patients’ notes and radiographs were reviewed.

All outcomes measures improved significantly. Mean MRQ scores improved from 14.2 to 6.4 and 4.8 (at one and two years respectively). Mean VAPS improved from 5.3 to 2.5 and 1.7. Mean LBOS improved from 33 to 52 and 57. 84% had radiographic evidence of motion at the level of the prosthesis. There were four major complications. A retroperitoneal haematoma, an incisional hernia, a ureteric stenosis and subsidence of the prosthesis each occurred in separate patients. These all occurred in the first ten patients of the study and were related to the approach. There were no major thrombo-embolic or infective complications and no revision surgery was required.

LTDR is a safe and effective surgical treatment of discogenic back pain. There is however, a steep learning curve with regard to the approach and a higher risk of complications during this time. Initial results are encouraging, but longer term follow-up needs to be performed.

Post Traumatic Fixed Thoraco-Lumbar Spinal Deformity may result in pain, regional and or global spinal deformity and neural compromise. Treatment is demanding as osteotomy is required in either anterior alone or both anterior and posterior spinal columns with concomitant reconstruction. This paper reviews 15 years experience with these cases.

A retrospective review of 21 patients operated on over 15 years was conducted. Patients were grouped based on original thoraco-lumbar injury pattern – Type A, B and C. Osteotomies and reconstruction were performed from both anterior and posterior approaches dependent upon the pathology. Clinical and radiological follow up for all patients was a minimum of one year. Analysis of outcomes was performed in relation to the clinical and radiological success. Complications were recorded.

Sixteen patients had two-column involvement and five had only the anterior column affected. Initial injury patterns were – Type A–9, Type B–4, and Type C–8. Approaches were anterior in six (five in Type A injuries), posterior and anterior in 11 (five two-stage and six three-stage operations), and posterior only in four (one pedicle subtraction osteotomy, one vertebral column resection, one posterior reduction of a dislocation, and one case abandoned after the posterior procedure). Anterior reconstruction was performed with structural iliac crest (two), titanium mesh cages (14) and expanding corpectomy cages (three). All 14 cases requiring posterior stabilisation were treated with pedicle screw based systems. The global assessment of outcome was individualised to the original indication – mechanical pain, deformity, and or symptomatic spinal stenosis. Success (good or excellent outcome) was achieved in 16 cases. Failure (fair or poor outcome) occur red in three completed cases. These three cases had chronic pain (two major, one minor). Two patients had incomplete assessment – one dying of MI in recovery after a technically successful procedure – and one developing deep infection with abandonment of the later stages (see above). There was one non-union. There were no neurological complications.

Delayed treatment of late posttraumatic deformity is challenging however good results are achievable with attention to the specific clinical and biomechanical requirements of each case. Technical failure occurred with inadequately radical intervention on one occasion. Major chronic thoracotomy pain occurred in one otherwise technically successful reconstruction.

Introduction: The basic architecture of the annulus fibrosus has long been established; successive lamellae containing parallel collagen fibers cross obliquely as you move through the annular wall, with the lamellae anchored in the endplates to form a multi-ply structure. Less is known of the interactions between fiber populations in the multi-laminate annulus fibrosus. Their significant contribution to the material behaviour was highlighted in Elliot and Setton’s 2001 attempt to build a material model based on experimental measurements of properties of the annulus. Recent research has confirmed a localized rather than a homogeneous or dispersed mode of interconnectivity between lamellae. Whilst clearly indicating localized bridging structures these studies have allowed only a glimpse of how these bridging elements fit within the overall lamellar architecture. The aim of this investigation was to analyse the interlamellar interconnectivity in its full 3-dimensional form and in complete segments of the annular wall.

Methods: Anterior segments of ovine lumbar discs in two age groups were sectioned along the oblique fiber angle. A 3-dimensional picture of the translamellar bridging network (TLBN) is developed using structural information obtained from fully hydrated unstained serial sections imaged by differential interference contrast optics.

Results: A high level of connectivity between apparently disparate bridging elements was revealed. The extended form of the bridging network is that of occasional substantial radial connections spanning many lamellae with a subsidiary fine branching network. The fibrous bridging network is highly integrated with the lamellar architecture via a collagen-based system of interconnectivity.

Discussion: This study demonstrates a far greater complexity to the interlamellar architecture of the disc annulus than has previously been recognised. Our findings are clearly relevant to disc biomechanics. Significant degrading of the TLBN may result in annular weakening leading potentially to disc failure. Most importantly this work opens the way to a much clearer understanding of the micro-anatomy of the disc wall.

Introduction: Compound mechanical loadings have been used to re-create clinically relevant annular disruptions in vitro. However, the role that individual loading parameters play in disrupting the lumbar disc’s annulus remains unclear. Using the recently described technique of nuclear inflation, the role that elevated nuclear pressures play in disrupting the lumbar intervertebral disc’s annulus fibrosus was investigated.

Methods: The nuclei of 12 ovine lumbar motion segments, posterior elements removed, were gradually pressurized by injecting a viscous radio-opaque gel via an injection screw fitted axially through their inferior vertebrae. Pressurization was conducted until catastrophic failure of the disc occurred. Investigation of the resulting annular disruption was carried out in tandem using micro-computed tomography and differential interference contrast microscopy.

Results: 3 of the 12 motion segments tested were excluded from the results due to improper placement of the injection screw, resulting in pressurization of the inferior vertebra rather than the nucleus. Mean failure pressure of the remaining 9 motion segments was 14.1 ± 3.9 MPa. Peak rates of pressurization ranged from 0.1–0.4MPa/s. Gel extrusion from the posterior annulus occurred in 7 discs and was the most common mode of failure. Unlike other aspects of the annular wall, the posterior region was unable to distribute hydrostatic pressures circumferentially. In each extrusion case, sever disruption to the posterior annulus occurred. While intralamellar disruption occurred in the mid annulus, interlamellar disruption occurred in the outer posterior annulus. Radial ruptures between lamellae always occurred in the mid-axial plane.

Discussion: With respect to the annular wall, the posterior region is most susceptible to failure in the presence of high nuclear pressure, even when loaded in the neutral position. The limited ability of the injected gel to cross the posterior-posterolateral boundaries, effectively concentrating hydrostatic stress within the posterior annulus, indicates that the laminate architecture along these radial lines is of mechanical significance. Within the outer posterior annulus, the prominence of inter-lamellar rather than intralamellar disruption indicates weak interlamellar cohesion. This suggests that nuclear material migrating down a radial fissure may easily track circumferentially within an interlamellar space upon reaching the inner lamellae of the outer annulus. This may explain why the majority of herniations are limited to protrusions contained within the outer annular wall.. The tendency for annular fibres to rupture in the mid-axial plane when loaded hydrostatically suggests that for a radial fissure or herniation to occur at the annular-endplate junction, a compounding bending or torsional load is required.

Introduction Spondylolysis and isthmic spondylolisthesis (IS) have both a familial and mechanical aetiology, yet the phenotypic expression of the familial aetiology is unknown except for the observation of spinal bifida occulta. Other posterior element abnormalities are unrecognised, and any facet joint orientation (FJO) abnormality at the effected level has been ignored because of presumed previous mechanical defunctioning by the pars defect. The recognition of multilevel sagittal FJO in L4/5 degenerative spondylolisthesis (DS), begs the question whether more proximal segment examination may reveal FJ variations in IS.

Methods MRI scans were used to measure orientation of the FJ at L3/4, L4/5, and L5/S1 in 30 individuals with normal scans, and 30 patients with IS. The angular measurement recorded is in relation to the coronal plane. Repeated measures assessment confirmed method validity.

Results Mean measurement of axial FJO at L3/4 and L4/5 was 51.1 and 42.5deg in normal subjects, and 45.2 and 35.0deg in IS. The more coronal angulation at the levels above a pars defect in IS was highly statistically significant (p=0.0006 & p=0.00002). At L5/S1 orientations were the same (39deg).

Discussion Relative coronal FJO in the lumbar spine may be the phenotypic expression of the congenital aetiology of IS. The mechanism of effect may be increased stress concentration at the pars between or below coronally oriented FJs. These more coronal FJOs in IS also explain:- the common observation of retrolisthesis at L4/5 above IS when the L4/5 disc degenerates, lateral overhang of the L4/5 FJ to the L5 pedicle entry-point above an IS, and the exceptionally uncommon combination of DS at L4/5 and IS at L5/S1 when both disorders are independently common. This latter observation can be further explained by the generalization that DS occurs in those individual with sagittal lumbar facets, and that IS occurs in those with more coronal FJs.

Introduction The management of cervical spine facet fractures, dislocations and subluxations in the literature is controversial. Many implants have been tested biomechanically and clinically. The overall biomechanical evidence points to greater stability with posterior constructs, however anterior surgery has practical advantages in terms of less dissection and local trauma than the posterior approach. The aim of this audit was to assess radiological results of facet joint fracture dislocations treated between January 2000 and August 2004. The audit was designed to examine the hypothesis that anterior fixation is inferior to posterior or combined anterior and posterior fixation.

Methods The clinical notes and radiological images of patients who present with a uni- or bifacet fracture dislocation during the study period were retrospectively reviewed. There were 21 patients treated during this period. 4 patients had incomplete radiological follow-up and were excluded. 12 Patients underwent anterior procedures, 3 posterior and 2 combined. Radiological follow-up included analysis of post-operative and final follow up x-rays. Failures were defined as evidence of nonunion, failure of metal ware, persisting kyphosis greater than 11 degrees or change in translation greater than 4 mm. Complications noted were 2 superficial infections, 1 psuedarthrosis 1 aspiration pneumonia, 1 ileus.

Results Overall 1 patient receiving anterior surgery developed a pseudarthrosis. This patient went on to develop fusion with posterior wiring and graft. Two patients developed wound infections following posterior wiring. All patients developed radiological fusion. Statistically there was no difference in radiological failure between anterior, posterior or combined anterior and posterior fusion.

Discussion There is insufficient evidence to reject the null hypothesis, anterior plating is inferior to posterior wiring or combined anterior and posterior procedures, and neither can the alternative be accepted. Better biomechanical results have been reported for posterior instrumentations and some authors have reported high rates of radiological failure with anterior fixation. However the anterior approach is associated with fewer complications in the literature6. The complicated nature of the facet fracture and the accompanying ligament injuries require patients to be assessed on an individual basis and treated as such.

Introduction Understanding how annular failure might occur following increased nuclear pressurisation requires an experimental approach that avoids artefactual injury to the annulus but reveals structural disruption resulting directly from the pressurisation event. The aim of this study was to investigate the fundamental mechanisms by which both intra and inter-lamellar relationships are disrupted by nuclear pressurisation, with the development of a model that might accurately reproduce mechanisms of intervertebral disc injury secondary to events causing raised intradiscal pressure.

Methods Bovine motion segments were subjected to internal pressurisation using a novel “through vertebra” method. Intra and inter-lamellar sections were deliberately chosen so as to expose systematic patterns of structural disruption resulting from the pressurisation event. This micro-disruption was investigated using a novel method which combined microtensile manipulation and simultaneous differential contrast imaging of the fully hydrated unstained sections.

Results The inner annulus was most severely disrupted. The middle regions developed a series of regular clefts along axes of weakness within the in-plane arrays of fibres in each lamella with a slight oblique passage radially away from the centre. These annular clefts separated the pre-existing transverse or side-to-side interconnections within the longitudinal fibre arrays. Progression to the peripheral lamellae occurred when the clefts crossed lamellae with associated inter-lamellar junction separation, with progressively lesser degrees of disruption further from the central area of pressurisation.

Discussion This study demonstrates that raised intradiscal pressure creates a consistent pattern of annular failure, which may preceed clinically relevant disc lesions, and specifically annular lesions. These findings offer a possible explanation for (a) annular weakening that alters the ability of the nucleus to maintain hydration after load, (b) the initiation of paths for annular tear development, (c) pathways that may expand to allow disc prolapse and (d) pathways for ingrowth of inflammatory and neural tissue mediating disc pain.

Introduction Central placement of a total disc arthroplasty (TDA) in the coronal plane will result in equivalent facet joint loading, less tendency for lateral core migration, optimum kinematics, and better outcomes. This study was performed to determine which of the radiographic markers – the vertebral body, the pedicles, or the spinous process – provides the most accurate guide to the coronal midline, so to optimise coronal TDA. The coronal midline was defined as the perpendicular bisector of a line drawn between the midpoints of the two facet joints.

Methods Axial CT images were reconstructed from 35 abdominal CT’s to show the relevant anatomy at L4, L5, and S1. Measurements were taken comparing the consistency of the midpoints of the vertebral body, the pedicles, and the spinous processes, in relation to the coronal midline.

Results The mean distance from the coronal midline to the vertebral body midpoint was 0.55mm (SD 0.45), to the interpedicular midpoint was 0.19mm (SD 0.40), and to the spinous process midpoint was and 1.30mm (SD 1.30). 16% of the distances from the coronal midline to the spinous process midpoint were greater than or equal to 3mm, compared with 0% of the distances to the interpedicular midpoint or the vertebral body midpoint. The interpedicular midpoint was significantly closer to the coronal midline than the spinous process midpoint or the vertebral body midpoint at all levels (p< 0.001).

Discussion The interpedicular midpoint is the most accurate guide to the coronal midline. We recommend this landmark be used in preference to the spinous processes or the vertebral body midpoint when placing the implant in TDA. The close location of the interpedicular midpoint to the implant, compared with the more posteriorly located spinous process, means the likelihood of parallax error, by rotation of the patient or the C arm, is reduced using the interpedicular midpoint.

Introduction Compressive loads applied to the disc are translated into an internal hydrostatic pressure in the nucleus which is resisted by the annulus. The anisotropic, inhomogeneous, multiply, collagenous architecture of the annulus reflects the complex pattern of mainly tensile stresses developed in this region of the disc during normal function. While many previous investigators have analysed the tensile behaviour of the annulus there still remains much to be learned about the fundamental structural relationships within the disc wall and upon which normal function depends. There is also much to be learned about how alterations in these relationships might lead to disc malfunction. Both intra and inter-lamellar structural relationships will be fundamental to the maintenance of annular wall strength. The aim of this study was to use high resolution ‘live’ imaging to explore the fundamental structural relationships governing the elasticity, intrinsic strength and rupture behaviour of intra-lamellar sections.

Methods In-plane intra-lamellar sections of nominal thickness 70–90μm were cut from the outer lamellae of bovine discs using a sledging microtome. Using a micro-mechanical technique in combination with simultaneous high resolution differential interference contrast optical microscopy (DIC) structural responses both along and transverse to the primary direction of the mono-array of collagen fibres were studied.

Results and Discussion Stretching along the primary alignment direction revealed a biomechanical response consistent with the behaviour of an array of strong fibres whose strength is governed primarily by the strength of embedding in the vertebral endplates rather than from inter-fibre cohesion along their length. The mono-aligned array, even when lacerated, is highly resistant to any further tearing across the alignment direction. Although not visible in the relaxed mono-arrays, transverse stretching revealed a highly complex set of interconnecting structures embodying a series of hierarchical relationships not previously revealed. It is suggested that these structures might play an important role in the containment under pressure of the nuclear contents. The dramatic differences in rupture behaviour observed along versus across the primary fibre direction are consistent with known clinical consequences arising from varying degrees of annular wall damage, and might also explain various types of disc herniation. The lamellar architecture of the healthy disc revealed by this ‘live’ tissue investigation provides an important reference framework for exploring structural changes associated with disc trauma and degeneration.