Introduction. Sustained loading on the intervertebral disc leads to loss of disc height. The generally accepted explanation for this is that the disc loses height due to an unbalance between the external load on the disc and the osmotic pressure in the disc. Consequently, water is expelled from the disc until the osmotic attraction reaches an equilibrium with the pressure applied. In this study, we compared the time course of loss of disc height with loss of pressure in the nucleus. We expected to see a similar time course of disc height and intra-discal pressure. Methods. Fifteen caprine lumbar discs were tested in a saline bath. Of each motion segment both vertebral bodies were cut-off close to the endplate. After a preload of 6 hours at 10N, an axial compressive load of 150N was applied to the discs for 18 hours by an Instron testing device. An 1.33mm pressure needle was inserted in the nucleus to measure hydrostatic pressure. Both change of disc height and change of nucleus pressure were measured at 2 samples/s. A double Kelvin–Voigt model was fitted to estimate the time constants of both hydrostatic pressure and disc height loss. The model comprises two time constants: the first modelling a fast change, the second a slow change. A paired t-test was used to compare the time constants of both the pressure and the disc height. Results. The slow time constant of the change of disc height (3.4 hours, sd=0.8) was larger (p=0.0006) than the slow time constant of the change of nucleus pressure (2.4 hours, sd=0.8). The difference between the time constants of the fast regime (0.33 hours resp 0.27 hours) was not significant (p=0.27). Discussion. In contrast to our expectations, we found a difference between the slow time-dependent behavior of the change of disc height and nucleus pressure. The discs reached an equilibrium between internal and external pressure well before the change of disc height came to a stop. This indicates that the change of disc height depends on more variables than mechanical equilibrium alone. Likely, viscoelastic properties of the annulus fibrosis have an important role as well. If confirmed, regenerative therapies should not only focus on restoring nucleus pulposus pressure to regain disc height, but also consider annulus properties

Stand-alone anterior lumbar interbody fusion (ALIF) provides the opportunity to avoid supplemental posterior fixation. This may reduce morbidity and complication rate, which is of special interest in patients with reduced bone mineral density (BMD). This study aims to assess immediate biomechanical stability and radiographic outcome of a stand-alone ALIF device with integrated screws in specimens of low BMD. Eight human cadaveric spines (L4-sacrum) were instrumented with SynFix-LR™ (DePuy Synthes) at L5/S1. Quantitative computed tomography was used to measure BMD of L5 in AMIRA. Threshold values proposed by the American Society of Radiology 80 and 120 mg CaHa/mL were used to differentiate between Osteoporosis, Osteopenia, and normal BMD. Segmental lordosis, anterior and posterior disc height were analysed on pre- and postoperative radiographs (Fig 1). Specimens were tested intact and following instrumentation using a flexibility protocol consisting of three loading cycles to ±7.5 Nm in flexion-extension, lateral bending, and axial rotation. The ranges of motion (ROM) of the index level were assessed using an optoelectronic system. BMD ranged 58–181mg CaHA/mL. Comparison of pre- and postoperative radiographs revealed significant increase of L5/S1 segmental lordosis (mean 14.6°, SD 5.1, p < 0.001) and anterior disc height (mean 5.8mm, SD 1.8, p < 0.001), but not posterior disc height. ROM of 6 specimens was reduced compared to the intact state. Two specimens showed destructive failure in extension. Mean decrease was most distinct in axial rotation up to 83% followed by flexion-extension. ALIF device with integrated screws at L5/S1 significantly increases segmental lordosis and anterior disc height without correlation to BMD. Primary stability in the immediate postoperative situation is mostly warranted in axial rotation. The risk of failure might be increased in extension for some patients with reduced lumbar BMD, therefore additional posterior stabilization could be considered. For any figures or tables, please contact the authors directly

Using deep learning and image processing technology, a standardized automatic quantitative analysis systerm of lumbar disc degeneration based on T2MRI is proposed to help doctors evaluate the prognosis of intervertebral disc (IVD) degeneration. A semantic segmentation network BianqueNet with self-attention mechanism skip connection module and deep feature extraction module is proposed to achieve high-precision segmentation of intervertebral disc related areas. A quantitative method is proposed to calculate the signal intensity difference (SI) in IVD, average disc height (DH), disc height index (DHI), and disc height-to-diameter ratio (DHR). According to the correlation analysis results of the degeneration characteristic parameters of IVDs, 1051 MRI images from four hospitals were collected to establish the quantitative ranges for these IVD parameters in larger population around China. The average dice coefficients of the proposed segmentation network for vertebral bodies and intervertebral discs are 97.04% and 94.76%, respectively. The designed parameters of intervertebral disc degeneration have a significant negative correlation with the Modified Pfirrmann Grade. This procedure is suitable for different MRI centers and different resolution of lumbar spine T2MRI (ICC=.874~.958). Among them, the standard of intervertebral disc signal intensity degeneration has excellent reliability according to the modified Pfirrmann Grade (macroF1=90.63%~92.02%). we developed a fully automated deep learning-based lumbar spine segmentation network, which demonstrated strong versatility and high reliability to assist residents on IVD degeneration grading by means of IVD degeneration quantitation

Daytime spinal loading is twice as long as night time rest, but diurnal disc height changes due to fluid flow are balanced. A direction-dependent permeability of the endplates, favouring inflow over outflow, has been proposed to explain this; however, fluid also flows through the annulus fibrosus. This study investigates the poro-elastic behaviour of entire intervertebral discs in the context of diurnal fluid flow. Caprine discs were preloaded in saline for 24 hours under different levels of static load. Under sustained load, we modulated the disc's swelling pressure by replacing saline for demi-water and back again to saline, both for 24h intervals. We measured the disc height creep and used stretched exponential models to determine the respective time constants. Reduction of culture medium osmolality induced an increase in disc height, and the subsequent restoration induced a decrease in disc height. Creep varied with the mechanical load applied. No direction-dependent resistance to fluid flow was observed. In addition, time constants for mechanical preloading were much shorter than for osmotic loading, suggesting that outflow is faster than inflow. However, a time constant does not describe the actual rate of fluid flow: close to equilibrium fluid flow is slower than far from equilibrium. As time constants for mechanical loading are shorter and daytime loading twice as long, the system is closer to the loading equilibrium than to the unloading equilibrium. Therefore, paradoxically, fluid inflow is faster during the night than fluid outflow during the day

Mechanical loading is important to maintain the homeostasis of the intervertebral disc (IVD) under physiological conditions but can also accelerate cell death and tissue breakdown in a degenerative state. Bioreactor loaded whole organ cultures are instrumental for investigating the effects of the mechanical environment on the IVD integrity and for preclinical testing of new therapies under simulated physiological conditions. Thereby the loading parameters that determine the beneficial or detrimental reactions largely depend on the IVD model and its preparation. Within this symposium we are discussing the use of bovine caudal IVD culture models to reproduce tissue inflammation or matrix degradation with or without bioreactor controlled mechanical loading. Furthermore, the outcome parameters that define the degenerative state of the whole IVD model will be outlined. Besides the disc height, matrix integrity, cell viability and phenotype expression, the tissue secretome can provide indications about potential interactions of the IVD with other cell types such as neurons. Finally, a novel multiaxial bioreactor setup capable of mimicking the six degrees-of-freedom loading environment of IVDs will be introduced that further advances the relevance of preclinical ex-vivo testing

The purpose of this study was to develop a novel, minimally invasive therapy for nucleus pulposus augmentation without the need for major surgical incision. Two optimum patented self-assembling peptides based on natural amino acids were mixed with glycosaminoglycans (GAGs) to form reversible, tunable hydrogels that mimic the vital biological osmotic pumping action and aid in swelling pressure of the intervertebral disc (IVD). Separate peptide and GAG solutions can be switched from fluid to gel upon mixing inside the body. The gels were analysed using a series of complementary techniques (FTIR, TEM & rheometry) to determine their cross-length scale structure and properties. Approaches to developing a clinical product were then developed including the incorporation of a fluorescent probe and a CT contrast agents to aid visualization of the gels, and a semi-automatic syringe driver rig, incorporating a pressure sensor, for the delivery of the solutions into the intervertebral discs. The efficacy of the procedure in restoring disc height and biomechanics was examined using chemically degenerated bovine caudal samples. It was found the presence of the GAGs stabilized the peptides forming stiffer gels, even upon injection through a long (∼10cm) small gauge needle. The injected gels were easily visualized post injection by microCT and by eye during dissection under visible and UV light. It was also noted that following injection, the disc height of the degenerated samples was restored to a similar level of that observed for native discs. A hydrogel has been developed that is injected through a narrow bore needle using a semi-automatic delivery rig and forms a self-assembled gel in situ which has shown to restore the disc height. Further tests are now underway to examine their biomechanical performance across more physiological time periods

Introduction. Intervertebral disc degeneration has been associated with low back pain (LBP) which is a major cause of long-term disability worldwide. Observed mechanical and biological modifications have been related to decreased water content. Clinical traction protocols as part of LBP management have shown positive outcomes. However, the underlying mechanical and biological processes are still unknown. The study purpose was to evaluate the impact of unloading through traction on the mechanobiology of healthy bovine tail discs in culture. Method. We loaded bovine tail discs (n=3/group) 2h/day at 0.2Hz for 3 days, either in dynamic compression (-0.01MPa to -0.2MPa) or in dynamic traction (-0.01MPa to 0.024MPa). In between the dynamic loading sessions, we subjected the discs to static compression loading (-0.048MPa). We assessed biomechanical and biological parameters. Result. Over the 3 days of loading, disc height decreased upon dynamic compression loading but increased upon unloading. The neutral zone was restored for all samples at the end of the dynamic unloading. Upon dynamic compression, the stiffness increased over time while the hysteresis decreased. Upon dynamic unloading, sulfated glycosaminoglycan (sGAG) release in the medium was lower at the endpoint. In the outer annulus fibrosus (AFo), we saw a higher water/sGAG of at least 30%. In the nucleus pulposus, COL2 mRNA was expressed more highly upon dynamic unloading while MMP3, iNOS and TRPV4 expression levels were lower. In the AFo of the unloading group, COL2 expression was higher but COL1 was lower. Conclusion. The biomechanical and biological results consistently indicate that dynamic unloading of healthy bovine discs in culture facilitates water uptake and promotes an anti-catabolic response which reflects a function optimization of the disc. This work combines biomechanical and biological results and opens the door to evidence-based improvement of regenerative protocols for degenerated discs and conservative LBP management. This study is funded by AO Foundation and AO Spine

Introduction. Low back pain (LBP) is a worldwide leading cause of disability. This preclinical study evaluated the safety of a combined advanced therapy medicinal product developed during the European iPSpine project (#825925) consisting of mesendoderm progenitor cells (MEPC), derived from human induced pluripotent stem cells, in combination with a synthetic poly(N-isopropylacrylamide) hydrogel (NPgel) in an ovine intervertebral disc degeneration (IDD) model. Method. IDD was induced through nucleotomy in 4 adult sheep, 5 lumbar discs each (n=20). After 5 weeks, 3 alternating discs were treated with NPgel (n=6) or NPgel+MEPC (n=6). Before sacrifice, animals were subjected to: MRI of lumbar spines (disc height and Pfirmann grading); blood sampling (hematological, biochemical, metabolic and lymphocyte/monocytes immunological). After 3 months the sheep were sacrificed. The spines were processed for: macroscopic morphology (Thompson grading), microscopic morphology (Histological grading), and glycosaminoglycan content (GAG, DMMB Assay). Furthermore, at sacrifice biodistribution of human MEPC was assessed by Alu-sequences quantification (qPCR) from three tissue samples of heart, liver, spleen, brain, lungs, and kidneys, and PBMCs collected to assess activation of systemic immune cells. To each evaluation, appropriate statistical analysis was applied. Result. Flow cytometry showed no induction of systemic activation of T cells or monocytes. Alu quantification did not give detection of any cells in any organ. Disc height index was slightly increased in discs treated with NPgel+MEPC. Pfirmann's and Thompson's classification showed that treatment with NPgel or NPgel+MEPC gave no adverse reactions. Histological grading showed similar degeneration in vertebrae treated with NPgel+MEPC or with NPgel alone. The amount of GAG was significantly increased in the nucleus pulposus following treatment with NPgel+MEPC compared to NPgel alone, in which a decrease was observed compared to untreated discs in both nucleus pulposus and annulus fibrosus. Conclusion. This study showed the safety of both NPgel+MEPC and NPgel treatments

The main load on the disc is a compression load. In humans this leads to a 16hrs loading phase followed by 8hrs of rest. Loads due to daily activities are superimposed on this diurnal pattern. The mechanical effect of the diurnal loading part is a slow, time dependent, change of disc height. This time dependent deformation can be described by a four parameter model (Double Kelvin-Voigt). This model describes the mechanical behaviour in a slow and a fast regime. In the present research we describe the changes during the loading phase with a constant load or constant deformation. We expect these changes to be dependent on disc size. Ten motion segments (L2L3 and L4L5) of rabbits, rats and pigs were tested in a saline bath. The posterior part of the motion segment was removed. Both outer endplates of the motion segment were embedded in bone cement and connected to the loading device. The maximum load was half of the body weight (bw). Protocol for rat and rabbit: Step1: preload (5%bw, 4hrs) Step2: Creep test (load 50%bw, 4hrs) Step3: preload (5%bw, 4hrs) Step4: Stress relaxation test (the deformation at 50%bw was maintained for 4hrs.). Protocol porcine: Due to the large disc size of the porcine samples duration of each test phase was increased to 12hrs. The applied load and the change of disc height was measured at 2/s. The time dependent mathematical model (Matlab) consists of two spring-damper combinations: the first modelling a fast mechanical change, the second a slow mechanical change. Both the time dependent behaviour of the creep experiment and of the stress relaxation experiment were determined. The influence of disc size was expressed in terms of volume, periphery, disc height, cross sectional area, wet area and ratio volume vs wet area. We found a large difference of time constants between the creep experiment and the stress relaxation experiment. In both, the time constants increased with disc size for the slow regime but decreased with disc size for the fast regime. Time constants of the slow regime (hrs) vs fast regime (hrs):. rat: 0.65 (slow creep)/0.18 (slow relaxation) vs 0.09 (fast creep)/0.03 (fast relaxation),. rabbit: 0.91 (slow creep)/0.38 (slow relaxation) vs 0.06 (fast creep)/0.01 (fast relaxation),. pig: 1.32 (slow creep)/0.40 (slow relaxation) vs 0.03 (fast creep)/0.01 (fast relaxation). The relation between time constants and disc height was almost linear (R2=0.98). We found a relation between mechanical behavior and disc size. The time constants of both the fast and the slow regimes changed with disc size. Animal discs can be used as a model for human discs under sustained loading but the results need to be corrected for the disc size. The difference between creep and stress relaxation could be attributed to the nonlinear spring constant of the disc. An increasing disc size leads to a larger time constant of the slow regime in a Kelvin-Voigt model but to a smaller time constant in the fast regime of the model

Osteoarthritis (OA) is a painful and disabling chronic condition that constitutes a major challenge to health care worldwide. There is currently no cure for OA and the analgesic pharmaceuticals available do not offer adequate and sustained pain relief, often being associated with significant undesirable side effects. Another disease associated with degenerating joints is Intervertebral disc degeneration (IVDD) which is a leading cause of chronic back pain and loss of function. It is characterized by the loss of extracellular matrix, specifically proteoglycan and collagen, tissue dehydration, fissure development and loss of disc height, inflammation, endplate sclerosis, cell death and hyperinnervation of nociceptive nerve fibers. The adult human IVD seems incapable of intrinsic repair and there are currently no proven treatments to prevent, stop or even retard disc degeneration. Fusion is currently the most common surgical treatment of symptomatic disc disease. However, radiographic follow-up studies have revealed that many patients develop adjacent segment disc degeneration due to altered spine biomechanics. The development of safe and efficacious disease modifying OA drugs (DMOADs) that treat pain and inflammation in joints will improve our ability to control the disease. I addition, a biologic treatment of IVDD is desirable. This presentation will provide an overview of recent advances and future prospects of a multimodal biologic treatment of OA, and IVDD. We will focus on Link N, a naturally occurring peptide representing the N terminal region of link protein and the first 1–8 residues of Link N (short Link N, sLN) responsible for the biologic therapy in question

The intervertebral disc faces high compressive forces during daily activities. Axial compression induces creeping fluid loss and reduction in disc height. With degeneration, disc fluids and height are progressively lost, altering biomechanics. It is assumed that this loss of fluids is caused by a drop in osmolality in the disc due to proteoglycan depletion. Here we investigate the isolated effect of a reduction in osmosis on the biomechanical properties of the intervertebral disc. Continuous diurnal loading was applied to healthy caprine intervertebral discs in a loaded disc culture system for a total of 6 days. We increased testing bath osmolality with two doses of polyethylene-glycol (PEG), thereby reducing the osmotic gradient between the disc and the surrounding fluid. This way we could study the isolated effect of reduced osmosis on axial creep, without damaging the disc. We evaluated: daily creep and recovery, recovery time-constants and compressive stiffness. Additionally, we investigated water content. There was a strong dose-dependent effect of PEG concentration on water content and axial creep behaviour: disc height, amplitude and rate of creep and recovery were all significantly reduced. Axial compressive stiffness of the disc was not affected. Reduction of water content and amplitude of creep and recovery showed similarity to degenerative disc biomechanics. However, the time-constants increased, indicating that the hydraulic permeability was reduced, in contrast to what happens with degeneration. This suggests that besides the osmotic gradient, the permeability of the tissues determines healthy intervertebral disc biomechanics

Although success has been achieved with implantation of bone marrow mesenchymal stem cells (bMSCs) in degenerative discs, its full potential may not be achieved if the harsh environment of the degenerative disc remains. Axial distraction has been shown to increase hydration and nutrition. Combining both therapies may have a synergistic effect in reversing degenerative disc disease. In order to evaluate the effect of bMSC implantation, axial distraction and combination therapy in stimulating regeneration and retarding degeneration in degenerative discs, we first induced disc degeneration by axial loading in a rabbit model. The rabbits in the intervention groups performed better with respect to disc height, morphological grading, histological scoring and average dead cell count. The groups with distraction performed better than those without on all criteria except the average dead cell count. Our findings suggest that bMSC implantation and distraction stimulate regenerative changes in degenerative discs in a rabbit model

In a study on ten fresh human cadavers we examined the change in the height of the intervertebral disc space, the angle of lordosis and the geometry of the facet joints after insertion of intervertebral total disc replacements. SB III Charité prostheses were inserted at L3-4, L4-5, and L5-S1. The changes studied were measured using computer navigation sofware applied to CT scans before and after instrumentation. After disc replacement the mean lumbar disc height was doubled (p < 0.001). The mean angle of lordosis and the facet joint space increased by a statistically significant extent (p < 0.005 and p = 0.006, respectively). By contrast, the mean facet joint overlap was significantly reduced (p < 0.001). Our study indicates that the increase in the intervertebral disc height after disc replacement changes the geometry at the facet joints. This may have clinical relevance

Purpose of the Study. Assessment of long term results of Chemonucleolysis vs. surgery for soft disc herniation. Material and Methods. From 1982 to 1985, 100 patients with symptomatic disc herniation were randomly allocated to receive either Chemonucleolysis(48) or surgery(52) after a three months trial of conservative treatment. Ten of the chemonucleolysis had surgery. At 10–13 year follow up, Sixty one of the original 100 patients responded, (32 chemonucleolysis; 29 surgery). At 25–27 year Forty four patients responded (24 chemonucleolysis; 20 surgery). Clinical outcome at one year was according to a self-assessment questionnaire: Completely, better improved, the same, or worse, and at the later follow up periods, according to the Macnab criteria. The results of two groups of patients were compared using Chi square and T test for independent samples. The disc height of the affected disc was measured from the lateral lumbar spine radiograph taken pre-operatively, and at the later two time points, and compared to a normal adjacent disc expressed as a percentage. Results. The intent to treat clinical outcome results showed a high success rate at one year (93% chemonucleolysis; 96% surgery completely better or improved); and also at the later two time points with no significant difference between the groups. The radiological data showed no difference in disc height between the groups at any of the time points with some reduction over time with aging. Conclusion. There is no difference between the groups in either clinical or radiological outcome over time. Chymopapain, presently not widely available, should be made available again. No conflict of interest. No funding obtained. This abstract has not been previously published in whole or substantial part nor has it been presented previously at a national meeting

Intervertebral disc (IVD) degeneration is one of the major causes of back pain. A number of emerging treatments for the condition have failed during clinical trial due to the lack of robust biomechanical testing during product development. The aim of this work was to develop improved in-vitro testing methods to enable new therapeutic approaches to be examined pre-clinically. It forms part of a wider programme of research to develop a minimally invasive nucleus augmentation procedure using self-assembling hydrogels. Previous static testing on extracted IVDs have shown large inter-specimen variation in the measured stiffness when specimen hydration and fluid flow were not well controlled. In this work, a method of normalising the hydration state of IVDs prior-to and during compressive testing was developed. Excised adult bovine IVDs underwent water-pik treatment and a 24-hour agitated bath in monosodium citrate solution to maximise fluid mobility. Specimens were submerged in a saline bath and held under constant pressure for 24 hours, after which the rate of change of displacement was low. Specimens were then cyclically loaded, from which the normalised specimen stiffness was determined. A degenerate disc model was developed with the use of enzymatic degeneration, allowing specimens to be tested sequentially in a healthy, degenerate, and then treated state. Self-assembling peptide-GAG hydrogels were tested using the developed method and the effect of treatment on stiffness and disc height were assessed. Compared to previous static tests, the improved method reduced the variation in the normalised specimen stiffness. In addition, statistically significant differences were seen before and after enzymatic degradation to simulate degeneration, thus providing controls against which to evaluate treatments. The augmentation of the nucleus with the hydrogel intervention reduces the stiffness of the degenerate disc towards that of the healthy disc. This method is now being used to further investigate nucleus augmentation devices

Long term, secondary implant fixation of Total Disc Replacements (TDR) can be enhanced by hydroxyapatite or similar osseo-conductive coatings. These coatings are routinely applied to metal substrates. The objective of this in vivo study was to investigate the early stability and subsequent bone response adjacent to an all polymer TDR implant over a period of six months in an animal model. Six skeletally mature male baboons (Papio annubis) were followed for a period of 6 months. Using a transperitoneal exposure, a custom-sized Cadisc L device was implanted into the disc space one level above the lumbo-sacral junction in all subjects. Radiographs of the lumbar spine were acquired prior to surgery, and post-operatively at intervals up to 6 months to assess implant stability. Flourochrome markers (which contain molecules that bind to mineralization fronts) were injected at specified intervals in order to investigate bone remodeling with time. Animals were humanely euthanized six months after index surgery. Test and control specimens were retrieved, fixed and subjected to histological processing to assess the bone-implant-bone interface. Fluorescence microscopy and confocal scanning laser microscopy were utilized with BioQuant image analysis to determine the bone mineral apposition rates and gross morphology. Radiographic evaluation revealed no loss of disc height at the operative level or adjacent levels. No evidence of subsidence or significant migration of the implant up to 6 months. Heterotopic ossification was observed to varying degrees at the operated level. Histology revealed the implant primary fixation features embedded within the adjacent vertebral endplates. Flourochrome distribution revealed active bone remodeling occurring adjacent to the polymeric end-plate with no evidence of adverse biological responses. Mineral apposition rates of between 0.7 and 1.7 microns / day are in keeping with literature values for hydroxyapatite coated implants in cancellous sites of various species. Radiographic assessment demonstrates that the Cadisc L implant remains stable in vivo with no evidence of subsidence or significant migration. Histological analysis suggests the primary fixation features are engaged, and in close apposition with the adjacent vertebral bone. Flourochrome markers provide evidence of a positive bone remodelling response in the presence of the implant

Summary Statement. Creep behaviour can only be quantified accurately when the testing time exceeds the estimated time constant of the creep process. The new parameters obtained in this paper can be used to describe normal behaviour up to 24 hrs. Background. Diurnal loading on the human spine consists of 16hrs loading and 8hrs rest. After an initial load increase, due to rising in the morning, an axial loading is maintained throughout the day. As a consequence subsidence of the intervertebral disc (IVD) occurs during the day while disc height recovers during the night. This behaviour is time dependent (non-linear). In literature different constitutive equations have been used to describe creep. A stretched exponential (Kolraush-Wilson-Watts, KWW) and a double Voight (DV) model have both been used to quantify the creep behaviour. Using these models, time constants and the deformation at equilibrium are estimated. It is unsure whether these different approaches yield to valid predictions. In this study we compared the validity of different equations for the prediction of creep behavior. Materials and Methods. IVDs (T9-T12) were obtained from 5 human spines. IVD's with osteophytes and/or disc narrowing were excluded from the test. The transverse area of each disc was measured and used to calculate the required compression load. IVDs were preloaded at 0.1MPa for 12hrs, compressed (0.8MPa, 24hrs) and finally unloaded (0.1MPa, 24hrs). Tests were performed in a saline bath. A KWW model and a DV model were fitted to the measured creep data (least squares method). Model parameters, e.g. the time constant and maximum deformation, were calculated for a test duration of 4, 8, 12, 16, 20 and 24hrs. Results. 4hours loading: KWW model: Time constant = 70hrs. Deformation = 3.0 mm. DV model: Time constant = 5hrs, Deformation = 1.7 mm. 24hours loading: KWW model: Time constant = 17hrs. Deformation = 3.2 mm. DV model: Time constant =12.5hrs, Deformation = 2.1 mm. Discussion. Both models described the measured data well but the model parameters were highly sensitive to test duration. For both models the estimated time constant varied with test duration. When extrapolating the measured data beyond test duration, the DV model under-estimated and the KWW model over-estimated creep behaviour. The 24hrs experiment was still too short for an accurate determination of the parameters. The upper and lower limits of the parameters can be estimated using a KWW and Voight model. Conclusions. Creep behaviour can only be quantified accurately when test duration exceeds the estimated time constant of the creep process. All reported time constants in current literature are based upon experiments that lack sufficient test duration. The new parameters obtained in this paper can be used to describe normal behaviour up to 24 hrs., but are not suitable for extrapolation beyond the test duration

We obtained intervertebral discs with cartilage endplates and underlying cancellous bone at operation from patients with degenerative disc disease and then used immunohistochemical techniques to localise the nerves and nerve endings in the specimens. We used antibodies for the ubiquitous neuronal protein gene product 9.5 (PGP 9.5). Immunoreactivity to neuropeptide Y was used to identify autonomic nerves and calcitonin gene-related peptide (CGRP) and substance P to identify sensory nerves. Blood vessels were identified by immunoreactivity with platelet-endothelial cell-adhesion molecule (CD31; PECAM). In a control group with no known history of chronic back pain, nerve fibres immunoreactive to PGP 9.5 and neuropeptide Y were most closely related to blood vessels, with occasional substance P and CGRP immunoreactivity. In patients with severe back pain and markedly reduced disc height, proliferation of blood vessels and accompanying nerve fibres was observed in the endplate region and underlying vertebral bodies. Many of these nerves were immunoreactive to substance P or CGRP, and in addition, substance P- and CGRP-immunoreactive nociceptors were seen unrelated to blood vessels. Quantification by image analysis showed a marked increase in CGRP-containing sensory nerve fibres compared with normal control subjects. We speculate that a chemotactic response to products of disc breakdown is responsible for the proliferation of vascularity and CGRP-containing sensory nerves found in the endplate region and vertebral body adjacent to degenerate discs. The neuropeptides substance P and CGRP have potent vasodilatory as well as pain-transmitting effects. The increase in sensory nerve endings suggests increase in blood flow, perhaps as an attempt to augment the nutrition of the degenerate disc. The increase in the density of sensory nerves, and the presence of endplate cartilage defects, strongly suggest that the endplates and vertebral bodies are sources of pain; this may explain the severe pain on movement experienced by some patients with degenerative disc disease