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Apparent diffusion in nucleus pulposus is associated with pain and mobility improvements after spinal mobilization for acute low back pain

Pain perception, trunk mobility in flexion, extension, and lateral flexion, and apparent diffusion coefficient (ADC) within nucleus pulposus of all lumbar discs were collected before and after posterior-to-anterior mobilization in 16 adults with acute low back pain. ADC was computed from diffusion maps and 3 specific portions of the nucleus pulposus were investigated: anterior (ADCant ), middle (ADCmid), and posterior (ADCpost ), and their mean as ADCall , a summary measure of ADC within nucleus pulposus. Pain ratings were significantly reduced after mobilization, and mobility of the trunk was significantly increased. Concomitantly, a significant increase in ADCall values was observed. The greatest ADCall changes were observed at the L3-L4 and L4-L5 levels and were mainly explained by changes in ADCant and ADCpost . The simultaneous reduction in pain and increase of water diffusion within nucleus pulposus has has been previously observed in subjects with chronic conditions and exists in the acute phase of the disease. Since the largest changes in ADC were observed at the periphery of the nucleus pulposus, and taken together with pain decrease, our results suggest that increased peripheral random motion of water molecules is implicated in the modulation of the intervertebral disc nociceptive response.


Introduction
Among all musculoskeletal pain conditions, the prevalence and burden from low back pain (LBP) [ICD-10-CM, code M54.5] is very high throughout the world: out of the 291 conditions studied in the Global Burden of Disease 2010 study, LBP ranked highest in terms of disability and sixth in terms of overall burden 1 . Spinal mobilization is a very common approach for LBP, and when a spinal mobilization is correctly performed by a trained orthopaedic manual physical therapist (OMPT), the intervention has low risk of injury and may result in immediate detectable improvements in pain and larger articular amplitudes. However, despite the widespread use of lumbar joint mobilization, the physiological responses of lumbar anatomical structures are still largely unknown. Recent advances in magnetic resonance imaging (MRI) of the musculoskeletal have nevertheless allow to observe the movement of water within and between tissues in vivo, and is called diffusion-weighted (DW) MRI. This emerging imaging technology is particularly sensitive to small changes in fluid flow and has a great potential for studying the influence of physical therapy interventions such as manual therapy, exercise, and physical agents on musculoskeletal structures 2 . Based on the comparison between DW images and non-DW images using the same MRI sequence, it is possible to reconstruct the mapping of the diffusion and to calculate an apparent diffusion coefficient (ADC) within intervertebral disc (IVD) [3][4][5][6] . Interestingly, DW MRI of the IVD has been successfully used for some years by Beattie and his colleagues 7-10 and allowed to link the decreasing pain reported by subjects with chronic LBP following single session of lumbar posterior-to-anterior (PA) pressures from L5 to L1 levels associated to McKenzie prone press-ups 11 , to the increase in ADC values in the lumbar IVD 9 or high-velocity, short-amplitude thrust at L5-S1 level 10 . From a physiological point of view, diffusion of water within IVD has been suggested as one mechanism of analgesia following manual mobilization/ manipulation 2 , but the complete mechanism is still unknown.
Despite the exciting and innovative natures of the studies that explored simultaneously ADC in IVD and pain changes after spinal mobilization/ manipulation in LBP patients, different methodological choices may have influenced the results complete the data collection, a second MRI scan, identical to the first, was carried out on the subject, within an hour after the spinal mobilization. After the second scan, pain ratings and trunk mobility tests were again performed by the two investigators.
Total time of the procedure was around 90 min, including 2 × 12 min for MRI, and 45 min for physical examination (pain ratings and trunk mobility tests) and questionnaires (subjects were in sitting posture during around 35 min).

Physical examination and PA mobilization
The physical examination was done by the principal investigator (T.P.), a certified OMPT, with more than 30 years of experience. It consisted of a complete orthopaedic manual therapy physical examination, inspired by Maitland's physical examination 22 , and aimed to collect information, first subjective (interrogation) and then objective (physical assets), to confirm the origin of the lumbar pain symptoms of the subject. It also allowed the OMPT to reassess the subject after the spinal mobilization. During trunk mobility tests (T F, T E, T LF l , and T LF r ), a centimetric measure of major fingertip-to-floor distance was made before and after mobilization.
For PA mobilizations, the OMPT chose: the location of force application on the spinous process(es), the components of the movements and the grades (rhythm and amplitude) varying with his feelings and the evolution of the patient's pain 22,23 , and duration of mobilizations, as during treatment at own office. Total duration of the mobilizations was timed, and primary (more than half the total mobilization time) and secondary (less than half the time) locations of the applied forces on spinous processes were gathered.

MRI acquisition
Two lumbar MRI scans were realized for each patient, one before and one after spinal mobilization. All sessions were conducted at the same time of the day (6:00-8:00 PM) to control the diurnal variations of the fluid content in IVDs.
The procedure used for image acquisition is similar to the one described by Beattie et al. 7 . All images were obtained using a 1.5 Tesla MRI scanner (MAGNETOM Symphony, Siemens AG, Munich, Germany) at the nuclear magnetic resonance department of Grand Hôpital de Charleroi (Site of "Notre-Dame", Charleroi, Belgium). Multi-element spine coils were used for the T2-weighted and DW images. An abdominal coil was also used for the DW images. Subjects entered the scanner head first, with the hips and knees flexed to approximately 30 degrees. Spin echo techniques were used to obtain T2-weighted sagittal and axial views using the parameters described in Table 1. DW image parameters are also summarized in Table 1. For each slice, DW imaging was obtained by applying diffusion gradients in 3 orthogonal directions and the mean ADC was constructed on the basis of averages of signal intensity from 3 directional DW images 7 . The diffusion-weighting b-factor was 400 s mm −2 , regarded as the best combination of diffusion weighting and signal intensity 7,8,10,24 .
A 3-level modified version 7 of the grading system initially developed by Pfirrmann et al. 25 was used to identify the presence and extend of IVD degeneration. Intensity (brightness) and homogeneity of the T2 signal in the nuclear region of midsagittal images was estimated for all IVDs. Hyperintense, homogenous, bright-white NP, with a clear distinction between the AF and NP was graded as 1 (normal); inhomogenous, gray NP, that can be distinguished from the AF as 2 (intermediate); and inhomogenous, gray or black NP, that can not be distinguished from the AF as 3 (hypointense). Each of the T2-weighted images of all subjects were evaluated independently by one of the investigators (R.F.) and a radiologist, with more than 30 years of experience in the field of musculoskeletal system, to classify the IVDs and consensus between the 2 examiners was used to address any disagreements in classification 9 .

Image analysis
Diffusion sequences were acquired to quantify the micro-movements of water molecules within the IVD of the lumbar spine. ADC was computed and provides the image of the mobility of water molecules. Maps of the mean ADC were calculated on-line by the MRI scanner with the standard software. After the images were obtained, the files were saved and transferred to a remote workstation for analysis.
The interpretation of the images and the calculation of the ADC were achieved by the radiologist and one investigator (R.F.). ADC measurements were conducted for each IVD in the sagittal medial (Figure 1c), and right and left parasagittal planes (Figures 1b and 1d, respectively). The adequate position of the 3 section planes used for ADC measurements were verified by linking them to a T2-weighted cross section passing through the IVD (Figure 1a).
The adequate position of half-height of each IVD of the lumbar spine on the ADC map was determined using a T2-weighted cross section passing through the IVD.

Statistical analyses
All statistical procedures were performed with SigmaPlot software (Version 11.0, Systat Software, San Jose, CA). A one-way RM ANOVA was used to compare the VAS results between before and after the mobilization. A two-way RM ANOVA was used to compare the centimetric data results for bending tests of flexion, extension and left and right lateral flexions and ADC results in IVDs between before and after the mobilization.
All data are presented as means and SD and were checked for normality (Shapiro-Wilk) and equal variance tests. A two-way (level × treatment) RM ANOVA with a post hoc Holm-Sidak method for pairwise multiple comparisons was performed and used to examine the effect of the mobilization by PA pressures. The effect size (η 2 ) was calculated as the sums of the squares for the effect of interest (level, treatment and level × treatment) divided by the total sums of the squares. The significance level α was set at 0.05 for all analyses and post hoc statistical power was calculated (SigmaPlot, Version 11.0, Systat Software, San Jose, CA).
To determine whether ADC all correlates with clinical data of pain (VAS) and trunk mobility (T F, T E, T LF l and T LF r ), a PCA was performed with R software (FactoMineR and factoextra packages).
Intra-rater reliability of ADC measures realized between 2 sessions by R.F. investigator was determined at one year interval for the paired measures at randomly selected IVD levels of 3 randomly selected subjects. The values obtained were exactly the same, showing perfect intra-rater reliability and therefore not requiring the calculation of an intraclass correlation coefficient (ICC).
Test-restest (relative) reliability of ADC measures between 2 MRI scans for one LBP subject (male, 33 years, 183 cm, 93 kg, pain duration: one week) was estimated using an ICC calculated using R software (irr package), based on a single rater/measurement, absolute-agreement, two-way random effects model (ICC(2,1), see Shrout and Fleiss 26 ). The subject was sitting on an chair during 35 min between the 2 measures, and do not receive the lumbar mobilization intervention. Good to excellent relative reliability results were observed with ICC ranging from 0.86 to 0.98.
Within-subject variability, or absolute reliability, attributable to repeated measures between 2 MRI scans, was assessed by the standard error of measurement percent change (SEM % ) calculated as (SEM/Mean) × 100, where SEM is the standard error of measurement and Mean is the mean of all observations from the 2 scans. SEM was calculated as SD × √ 1 − ICC 7 , where SD is the standard deviation of the pooled measures of the 2 scans. SEM % results ranged from 2.1 to 4.7.

Clinical data
Mean±SD total duration of PA mobilizations was 639±102 s. Primary locations of PA mobilizations were applied at L 1 (n=1), L 3 (n=3), L 4 (n=7), and L 5 (n=5) levels, and secondary locations were only applied on 3 subjects at T 11 (n=1), L 1 (n=1), and L 5 (n=1) levels. All subjects had a DN4 score <4, indicating the absence of neuropathic pain. Median (Q1-Q3) QDSA-T was 22 (18.5-26.5), QDSA-S was 13.5 (9.75-16.25), and QDSA-A was 10 (5.75-11.5). VAS and OAS pain ratings were significantly reduced after mobilization with a very large effect size ( Table 2). A mean±SD reduction on VAS of 3.4±1.7 on 10 (62±25%) was observed. Mobility of the trunk, assessed by T F, T E, T LF l , and T LF r , was significantly increased with medium to large effect sizes ( Table 2). A mean reduction of major fingertip-to-floor distance of 6 cm was observed for T F, 5 cm for T E, 4 cm for T LF l , and 5 cm for T LF r .

Diffusion of water within discs
Mean ADC values before and after intervention, for the 9 ROIs at the 5 anatomical levels for anterior, middle, and posterior portions of IVDs along the sagittal medial, and parasagittal left and right planes are presented in Figure 2.
A significant mean increase in ADC all values was observed after mobilization, with difference of means between 82.1 (change of 5.9%) and 160.7 × 10 −6 mm 2 s −1 (13.2%) (Tables 2 and 3). Similar significant results were observed in the anterior (ADC ant between 99.2 (8.8%) and 205.5 × 10 −6 mm 2 s −1 (20%)), middle (ADC mid between 71.1 (5%) and 151.8 × 10 −6 mm 2 s −1 (16%)), and posterior portions of the IVD (ADC post between 76.1 (6.0%) and 159.8 × 10 −6 mm 2 s −1 (20.1%)). Significant differences in ADC all , ADC ant , ADC mid , and ADC post were observed at all anatomical levels, except L 5 -S 1 ( Table 3). In addition, no significant difference was observed in ADC mid at L 2 -L 3 ( Table 3). The greatest ADC all changes were observed at the L 3 -L 4 and L 4 -L 5 levels and were mainly explained by changes in ADC ant and ADC post (Table 3).   (Figures 3b to 3d). The main contribution of variables to dimension 1 were ∆T LF l , ∆T E, and ∆T LF r . Dimension 2 was mainly explained by ∆VAS and ∆T F, and dimension 3 by anatomical level, ∆ADC all , and ∆T F. ∆VAS was negatively correlated with ∆T F (Figs. 3b and 3d) and ∆ADC all with anatomical level (Figs. 3c and 3d).

Discussion
The rationale for studying an acute LBP population was based on previous research findings that subjects with longer than 2-month symptoms durations did not respond as well to a manual therapy mobilization 9 . Second, even if MRI is a technique capable of providing information both on the morphology of the IVD and on its molecular composition, it is desirable to direct research effort toward characterizing changes that are linked directly to clinical symptoms 29 .
Our results support previous findings of a simultaneous pain reduction and increase of ADC in the NP of chronic LBP subjects after PA lumbar mobilization 9 but provide new data concerning the acute phase of disease, and trunk mobility in an older population with higher pain intensity levels. Beattie et al. 9 were the first to explore the short-term effect of oscillating PA pressures to the lumbar spinous processes followed by prone press-ups exercises in chronic LBP subjects on pain intensity and water diffusion within NP of IVD. They observed two subgroups: "within-session responders" and "not-within-session responders", based on a reduction of pain of at least 2/10 within-session or not. No attempt was made to separate our sample into "within-session responders" and "not-within-session responders" since its small size and that only 4 subjects show a pain reduction of less than 2/10, sometimes combined with a large increase in ADC values.
Mean age of our population was 46 years with a pain intensity at baseline of 5.4/10 on VAS. The mean age of the population studied by Beattie et al. 9 was 26 years with an average pain intensity on a typical day of 3.7/10 on the 11-point numeric rating  scale. The difference in pain intensity between the two studies could not be explained by gender differences, since 9/12 (75%) subjects in the "within-session responders" group of Beattie's study were female and 11/16 (69%) in ours. On the other hand, a difference in body mass index (BMI) could explain it, since higher values are associated with higher pain intensity levels in patients with LBP 30,31 . A mean lower value of 21.0 kg m −2 was observed in "within-session responders" of Beattie's study compared to 26.6 in ours. The 62% mean reduction in pain following PA mobilization is higher than that reported in previous investigations, with a mean decrease ranging between 33 and 41%, when mobilization was applied: on the most painful lumbar level, at a random lumbar level, or even at painful lumbar level and all other lumbar levels [32][33][34] . A potential explanation of this difference may the lower homogeneity of the patient's groups of previous investigations that include LBP subjects with too long pain symptoms duration: up to 3 months 34 , more than 6 months 32 , and even up to 60 months 33 .
Normal IVD is considered as a poorly innervated organ, since its innervation is restricted to the outer layers and consists of small nerve fibers and some large fibers forming mechanoreceptors. Nerve fibers accompany the blood vessels or arrive via independent ways: branches of sinuvertebral nerve, nerve branches from the ventral rami of spinal nerves, or gray rami communicantes 35 . IVD could also receive nerve branches from the anterior and posterior longitudinal ligaments 35 . In contrast, in degenerative IVD, Coppes et al. 36 demonstrated a more important and profound innervation compared to normal discs. Furthermore, nociceptive properties of at least some of these nerves are strongly suggested by their immunoreactivity for substance P. These observations are used to defend the hypothesis of the existence of discogenic pain, in degenerative IVDs. By definition, discogenic pain is a pain due to a mechanical or chemical irritation of nerves innervating the IVD. Based on our results and those of Beattie and colleagues 7-10 , we believe that the simultaneous reduction in pain observed in patients and increase of the water diffusion within IVD is not an epiphenomenon linked to mobilization, and that, on the contrary, these two 15.3% 13.4% 11.8% physiological events would be intimately related, directly or indirectly. It is not inconsistent to speculate that an increased water diffusion would lead to a re-expansion of the IVD and therefore reduce the mechanical stresses on the large mechanoreceptors nerve fibers. Furthermore, increasing the speed of the water and blood flow in the IVD could decrease local inflammatory process and thus the pain. On one side, it is accepted that onset of the disc degeneration process start to occur in the third decade of life, with dehydration of the NP and changes in the molecular structures of its components 37 . On the other side, a link exists between water diffusion in NP, estimated by ADC, and visual degeneration of lumbar IVD, using Pfirrmann's grading system 38 . Surprisingly, a reduction in ADC values of 4% was observed between normal and moderately degenerated discs but severely degenerated discs showed 5% larger ADC values than normal discs, presumably due to free water in cracks and fissures in the degenerated NP of those discs 38 . After a spinal thrust, LBP subjects with fewer lumbar degenerated discs showed better increased in ADC values than those with many 10 . Here, the majority of IVD graded as moderately degenerated for more cranial anatomical levels and as severely degenerated for more caudal levels, and ADC changes were higher at more cranial levels compared to caudal, with non significant changes at L 5 -S 1 .
To our knowledge, changes in trunk mobility have never been studied concurrently with changes in pain and water diffusion within NP. Even if the assessment of trunk mobility is a strong point of our protocol, a potential bias is that the investigator that assess trunk mobility was not blinded to if PA had been performed or not. Using a principal component analysis (PCA), several novel and important observations were made about the relationships between changes in pain, trunk mobility and water diffusion. First, a negative correlation between changes in pain and changes in trunk flexion was observed, but not with changes in extension and lateral flexions. Second, a negative correlation between changes in water diffusion and lumbar anatomic levels was observed. In line with previous findings, the mobility of trunk in extension [39][40][41] and in flexion 40 improved significantly after PA mobilization. However, some studies failed to report significant increase in trunk extension 32,33 and flexion 39 . We show a significant increase of 29.9±23% for trunk flexion, 8.1±8% for trunk extension, 9.9±8% for left lateral trunk flexion, and 8.9±8% for right lateral trunk flexion. The significant mean change of 9 cm we observed for major fingertip-to-floor distance during trunk flexion after PA mobilization in our acute population, was greater than the significant mean change of 2.7 cm reported by Goodsell et al. 33 in chronic subjects. In contrast to the non-significant mean change of 0.3 and 0.12 cm for right and left lateral trunk flexions reported by Samir et al. 42 in chronic subjects after PA mobilization, we observed a significant mean change of 5 and 4 cm. Our results suggest that trunk mobility improvements after PA mobilizations could be more important in acute subjects than chronic. However, fingertip-to-floor method measures total forward, backward, and lateral bending movements, including movement of the spine, hips, and pelvis. This method does not allow to specify at which level mobility changes occur.
Although the use of DW MRI in humans has mainly been applied to the central nervous system and in particular the brain, more recently, this method has become increasingly successful in the musculoskeletal system and has led to a broadening of knowledge both in diagnosis and intervention, using the ADC. ADC values were determined in 80 lumbar IVDs, from L 1 -L 2 to L 5 -S 1 levels. An increase in ADC all of 7.2% was observed for L 1 -L 2 ; 5.9% for L 2 -L 3 ; 13.2% for L 3 -L 4 ; 16.0% for L 4 -L 5 and 4.1% for L 5 -S 1 . Beattie et al. 9 observed a mean ADC increase of 4.2% within L 5 -S 1 IVD in 'immediate responder' group (n=10) after PA mobilization. At all anatomical levels, change in ADC all values were greater than SEM % of 2.1 observed on one subject after 10 minutes of prone lying, which is compatible to the SEM values reported by Beattie et al. 8 on 24 subjects after 10 minutes of prone lying and ranging from -3.5 to 3.4%. Therefore, ADC all changes observed after PA mobilization must be considered as real changes linked to mobilization and not to measurement errors. Even if has been long established that the IVD is one of the largest avascular anatomical structure in the body 29 , it nevertheless remains a living structure that requires convection and diffusion mechanisms to ensure nutrition. Diffusion is defined as the movement of matter driven by a concentration gradient and convection is described as the bulk movement of fluids 43 . It is generally believed that diffusion is the main transport mechanism for small solutes with convection playing a more important role in the transport of larger solutes 43 . DW images provide a characterization of water transport under the combined influence of diffusion and convection. An increase of diffusion/convection in the NP is thought to be beneficial, while decreased diffusion/convection has been linked with degeneration. Diffusion of water within the IVD is influenced by pressure gradients and chemical forces acting on it, as well as structural barriers such as a nuclear "cleft". Pressure gradients within IVD could be influenced by externally applied forces, such as those generated by manual therapy techniques 10, 44,45 . We hypothesize that diffusion of water could be related to opening-closure mechanism of IVD. This mechanism has been observed in vivo by Kulig et al. 46 , when applying a PA pressure at the lumbar spine. A pressure applied at a given vertebral level results in an extension movement (opening) at this level and on the upper level, and on the contrary a movement of flexion (closure) on the lower level.
Correlations were previously described between anatomical levels and ADC values but findings were inconsistent. Some studies show that ADC values increase significantly with more caudal IVDs 3, 24 , decrease significantly with more caudal IVDs 47 , or even are not significantly correlated with IVD levels 38 . In a more recent study 12 , the influence of age on these relationships was observed, with ADC mean values for young subjects (<45 years) increasing from L 1 -L 2 to L 2 -L 3 / L 3 -L 4 levels and decreasing to more caudal levels, and decreasing continuously for elderly subjects (>45 years). Furthermore, static traction was associated with an increase in diffusion of water within the L 5 -S 1 IVDs of middle-age individuals, but not in young adults, suggesting age-related differences in the diffusion response 48 . Here, PCA results show that ADC all values tend to decrease with more caudal IVDs.
Today, there is a paucity of research that describes the physiologic events associated with analgesia following intervention for LBP 10 . Since ADC is a measure of the magnitude of random (Brownian) diffusion motion of water molecules, it provides information about the physiologic state of the NP. Previous studies estimate ADC of NP with only one ROI. Here, ADC all was estimated from the mean of anterior, middle, and posterior portions of the NP, which were themselves estimated based on the mean of 3 ROIs (sagittal medial, and left and right parasagittal planes). We believe that our method is more representative of a physiological/ physiopathological process of the entire NP than measures based on a single ROI analysed in the mid-sagittal scan, since pathologically relevant disc measurements may be observed in parasagittal or other planes 49 .
Greatest changes in ADC all were observed at L 3 -L 4 and L 4 -L 5 levels, and are mainly explained by changes in ADC ant and ADC post . Note that PA mobilizations were applied between L 3 and L 5 in 15 subjects on 16. Since ADC ant and ADC post were greater than ADC mid changes, and taken together with pain decrease, our results suggest that increased peripheral random motion of water molecules in nucleus pulposus is implicated in the modulation of the IVD nociceptive response. This observation is all the more important since nerve fibres have been identified in the NP of degenerated IVDs 50 , which may still be more likely to be able to generate an efficient reduction of pain than healthy IVDs that are usually thought to be innervated only in the annular part. Therefore, it would be interesting to study the influence of these mobilizations, both in nucleus pulposus and annulus fibrosus, according to the 3 orthogonal directions of space (x,y,z) rather than using an average value of ADC. Pure water, for the purposes of diffusion is said to be isotropic; this means that the molecules are equally likely to diffuse in any direction. In a biological tissue like the NP, there may be a preferential diffusion direction, along collagen fibers, and diffusion is said anisotropic. Our methodology does not allow to study the anisotropic character of water diffusion within NP. This latter has already been observed previously within lumbar IVDs on healthy young adults 3 , with ADC z (diffusion perpendicular to the end-plate) values higher than ADC x and ADC y (diffusion in the disc plane). Very recently, a promising T2-weighted MRI method based on the location of the signal intensity weighted centroid, i.e. the arithmetic mean of the signal intensity of all pixels in a ROI, was developed as a biomarker for investigating fluid displacement within the disc 51 . It would be interesting to apply this method to our images.
From L 1 -L 2 to L 5 -S 1 IVD levels, the mean NP length in the sagittal plane is comprised between 19.3±2.9 and 21.6±3.1 mm, and height between 5.5±1.1 and 8.6±1.3 mm 52 . The method used here was based on the use of ROIs always having a circular surface area of 0.2 cm 2 , either a diameter of 5 mm, resulting in a total surface area of 15 mm long (anterior, middle, and posterior ROIs) by 5 mm high in the sagittal and the two parasagittal planes. Elliptical surfaces of varying dimensions, ranging from 40 and 80 mm 2 , have been used by others 24,47,53 , forcing the observers to place the large axis in the ventro-dorsal direction and the small axis in the cranio-caudal direction. The risk of using surfaces up to 80 mm 2 is to include the most internal part of the AF in the calculation of the ADC. This study was limited by the absence of a T1-weighted MRI sequence in order to estimate vertebral endplate signal changes and classify it according to their levels of degeneration 54 . Indeed, there is strong evidence that vertebral endplate structural changes are associated with non-specific LBP but it may be present in individuals without LBP 55 . Since the main and most important pathway for diffusion into the NP occurs from capillaries in the vertebral body via diffusion through the cartilaginous endplate 56 , another limitation is the lack of evaluation of vertebral endplate morphology. As described by Lakshmanan et al. 57 , concavity of the lumbar endplates is symmetrical in the frontal plane but shape shows considerable variability in the sagittal plane (flat, oblong or ex-centric), with inferior endplate shape becoming more ex-centric, i.e. location of the concavity apex in the posterior half of endplate (54-60% endplate diameter), from L 3 to L 5 levels. At these levels, significant ADC changes were observed within NP, corresponding approximately to the center or apex of the endplate, suggesting that the mechanical stimuli induced by PA mobilization may have a direct influence on vertebral endplates. By the way, permeability across the cartilage end plate is greater in the central portion, adjacent to the NP, than at the periphery, near the AF 58 . Finally, no attempt was made to assess subject's functional disability; the Oswestry Disability Index 59 , considered as the gold standard for measuring degree of disability and estimating quality of life in a subject with LBP, could have been realized to complete the clinical picture of our sample.