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Axial Loaded MRI of the Lumbar Spine

A. SAIFUDDIN*,†, S. BLEASE*, E. MACSWEENEY*

*Medtel Open MRI Centres, London and †The Royal National Orthopaedic Hospital NHS Trust,Stanmore, Middlesex, UK

Received: 18 February 2003 Revised: 25 April 2003 Accepted: 2 May 2003

Magnetic resonance imaging is established as the technique of choice for assessment of degenerativedisorders of the lumbar spine. However, it is routinely performed with the patient supine and the hipsand knees flexed. The absence of axial loading and lumbar extension results in a maximization ofspinal canal dimensions, which may in some cases, result in failure to demonstrate nerve rootcompression. Attempts have been made to image the lumbar spine in a more physiological state,either by imaging with flexion–extension, in the erect position or by using axial loading. This articlereviews the literature relating to the above techniques. Saifuddin, A. et al. (2003). Clinical Radiology58: 661–671.

q 2003 The Royal College of Radiologists. Published by Elsevier Science Ltd. All rights reserved.

Key words: lumbar spine MRI, erect, flexion, extension, axial loading.

INTRODUCTION

Magnetic resonance imaging (MRI) is established as the

technique of choice for assessment of degenerative disorders of

the lumbar spine [1] and its role in the diagnosis of lumbar

spinal stenosis has recently been reviewed [2–4]. However, it is

routinely performed with the patient supine and the hips and

knees flexed, resulting in relative lumbar flexion. With this

technique, the absence of axial loading and lumbar extension

results in a maximization of spinal canal dimensions, which

may result in failure to demonstrate nerve root compression.

The positional nature of lumbar spinal stenosis has been clearly

demonstrated with functional CT myelography [5]. Attempts

have been made to assess the lumbar spine in a more

“physiological” state, either by imaging with flexion/extension

[6,7], in the erect seated position [8–12], or using axial loading

[13–19]. This article reviews the literature relating to the above

techniques, with emphasis on axial loaded imaging using an

MRI compatible compression device (Dynawell, Dynamed AB,

Stockholm, Sweden) in a low field open scanner.

CADAVER STUDIES

The effect of applied stresses and positional changes relatedto the lumbar spine has been extensively reported in cadaverstudies. Changes in central canal, lateral recess and inter-vertebral foraminal dimensions in different states of loadingand position have been reported [20–23]. Nowicki et al. [20]assessed the effect of pure axial loading on the lumbar spine andfound that compression resulted in reduction of central canal,lateral recess and foraminal dimensions, but did not increasethe degree of nerve root displacement or compression. Inufusaet al. [21] assessed changes in central canal and foraminaldimensions between flexed, neutral and extended positions withthe addition of axial compression. In the extended position,significant reduction of central canal area, mid-sagittaldiameter and sub-articular diameter was seen, whereas theopposite was identified in the flexed position. Extensiondecreased all foraminal dimensions, whereas flexion increasedall foraminal dimensions. Foraminal cross-sectional area wasdecreased in extension by an average of 15%, and increased inflexion by an average of 12% compared with the neutralposition. Extension also results in an increase in the incidenceof nerve root contact or compression by bulging intervertebraldisc or ligamentum flavum. The incidence of root compressionalso increases with increasing disc degeneration [22]. Fujiwaraet al. [23] further assessed the effects of lateral bending andaxial rotation on foraminal dimensions and root compression,

0009-9260/03/$30.00/0 q 2003 The Royal College of Radiologists. Published by Elsevier Science Ltd. All rights reserved.

Guarantor and correspondent: Dr A Saifuddin, Consultant Radiologist,The Royal National Orthopaedic Hospital NHS Trust, Brockley Hill,Stanmore, Middlesex HA7 4LP, UK. Tel: þ44-20-8909-5443; Fax: þ44-20-8954-0281; E-mail: [email protected]

Clinical Radiology (2003) 58: 661–671doi:10.1016/S0009-9260(03)00215-0, available online at www.sciencedirect.com

and also the effects of extension and flexion on morphology ofthe disc and ligamentum flavum. The flexed position isassociated with greatest foraminal cross-sectional area, aspreviously reported, and with the least bulging of the disc andthickness of the ligamentum flavum. The opposite is seen in theextended position. Axial rotation reduces foraminal width andarea on the side of rotation, while increasing foraminal heightand area on the opposite side. Similarly, lateral bending reducesforaminal dimensions and area on the side of bending, whileincreasing the same parameters on the opposite side.

EFFECT OF LOADING ON THENORMAL/ASYMPTOMATIC LUMBAR SPINE

Intervertebral Disc

Beattie et al. [6] investigated changes in the position of thenucleus pulposus between flexion and extension in 20asymptomatic volunteers using sagittal T2-weighted MRimages. In the extended position, the distance from theposterior margin of the nucleus of normal discs from a linedrawn between the posterior margins of the adjacent vertebralbodies increased compared with the flexed position. No change

Fig. 1 – Dynawell compression device, comprising a vest, which isconnected to a foot-plate (large arrow) by two harnesses (small arrow).Compression is applied by loading the patient to 25% of body weight oneach of the two dials on the foot-plate and maintained for 5 min beforere-imaging.

Fig. 2 – Change in lumbar lordosis. (a) Sagittal T2-weighted fast spin-echo sequence without axial loading. The lordosis between L1 and S1 is measured at59.18. (b) Sagittal T2-weighted fast spin-echo sequence with axial loading. The lordosis between L1 and S1 is measured at 72.68.

CLINICAL RADIOLOGY662

Fig. 3 – Changes in segmental lordosis at L3/4 and L5/S1. (a) Sagittal T2-weighted fast spin-echo sequence without axial loading. The lordosismeasured at L3/4 is 6.28 and at L5/S1 is 34.08. (b) Sagittal T2-weighted fastspin-echo sequence with axial loading. The lordosis measured at L3/4 isnow 13.28 and at L5/S1 is 31.08.

Fig. 4 – Change in lumbar spine vertical height. (a) Sagittal T2-weightedfast spin-echo sequence without axial loading. The height between L1 andS1 is measured at 201 mm. (b) Sagittal T2-weighted fast spin-echosequence with axial loading. The height between L1 and S1 is measured at196 mm.

AXIAL LOADED MRI OF THE LUMBAR SPINE 663

was seen in the distance of the anterior disc margin from thefront of the vertebral bodies. These findings suggest that theanteroposterior dimensions of the nucleus decrease withincrease in lumbar lordosis. Such changes were not identifiedin degenerate discs.

Chung et al. [7] found no significant change in the shapeor dimensions of the L3/4 and L4/5 discs when comparingflexion, extension and rotation with the patient supine.However, comparison between flexion and extension MRI inthe erect position has demonstrated changes in discmorphology, with 27% of discs overall showing an increasein posterior annular bulging in the erect extended position.This percentage increased to 40% for degenerate discs [8].

Disc heights are affected by axial compression. Thestandardized technique for axially loaded MRI is to imagethe spine as for conventional MRI, with the patient supine andhips and knees flexed. This is followed by repeat imaging withthe hips and knees extended, a small roll under the lumbar spineto produce slight lordosis and then compression to 50% of thepatients body weight for 5 min using the Dynawell Com-pression device (Fig. 1) [16,17]. Using this technique, Kimuraet al. [15] assessed changes in lumbar disc height duringcompression in eight asymptomatic young adults, finding asignificant reduction of disc height only at the L4/5 level.

Spinal Alignment

Axial compression results in complex changes in spinalalignment, which can vary between motion segments.Kimura et al. [15] measured changes in intervertebralangles between the non-loaded position and during axialloading. Overall, there was a mild increase in lumbarlordosis between L1 and S1 (Fig. 2), which was consistentwith changes that can be seen between supine and erectlumbar spine radiography. However, at individual disclevels, a significant increase in lordosis was seen only atL3/4, whereas axial loading actually resulted in a significantreduction in lordosis at L5/S1 (Fig. 3) and no change inangle at L4/5.

Morphological changes of lumbar spine sagittal alignmentfollowing axial compression were also investigated byWisleder et al. [18]. These authors assessed changes in lumbarrotation, bending, compression and disc translation. In allcases, the lumbar spine underwent a reduction in height (Fig.4), which averaged approximately 4 mm. This was mainly dueto bending of the spine (increased lordosis). The L2–L4 levelsunderwent extension, whereas L5 became more flexed. The L2/3 to L4/5 discs translated forward, whereas the lumbosacraldisc moved backwards. Posterior rotation of the sacrum wasalso seen (Fig. 5).

Central Canal and Intervertebral Forminae

Several studies have investigated positional changes incentral canal dimensions. Chung et al. [7] identified reductionof spinal canal sagittal dimensions and dural sac cross-sectionalarea (DCSA) in healthy subjects imaged in the supine position

Fig. 5 – Posterior rotation of the sacrum. (a) Sagittal T2-weighted fast spin-echo sequence without axial loading. The sacral–horizontal angle ismeasured at 42.68. (b) Sagittal T2-weighted fast spin-echo sequence withaxial loading. The sacrum has rotated posteriorly with the sacral–horizontalangle reduced to 31.28. Mild flexion of L5 is also evident.


with extension and rotation. These positions also produced

increased thickness of the ligamentum flavum. Reduction in

central canal dimensions is also identified following imaging in

the erect extended position, especially at degenerate disc levels

[8]. Similar findings were recorded by Schmid et al. [10] who

noted mean DCSA of 268 mm2 in the upright flexed position,

which reduced to 224 mm2 in the upright extended position.

Axial loading with extension in the supine position also

produces reductions in DCSA, which are most commonly seen

at L4/5 and increase in frequency with increasing age (Fig. 6)

[16].

As with central canal dimensions, foraminal dimensions are

also reduced in the supine extension and the erect extended

positions as compared with the flexed position [7,10]. The

effect of axial loading on foraminal dimensions has not been

studied in vivo.

Fig. 6 – Reduction of DCSA at normal L4/5 disc level. (a) Axial T1-weighted spin-echo sequence without axial loading. The DCSA is measuredat 313 mm2. (b) Axial T1-weighted spin-echo sequence with axial loading.The DCSA is measured at 241 mm2.

Fig. 7 – Accentuation of disc herniation. Broad-based L5/S1 central andright paracentral disc prolapse with compression of the right S1 nerve root.(a) Axial T1-weighted spin-echo sequence without axial loading. Theanteroposterior dimension of the disc prolapse is measured at 9.22 mm.Measurement is made from a line joining the base of the pedicles to theposterior margin of the disc prolapse. (b) Axial T1-weighted spin-echosequence with axial loading. The anteroposterior dimension of the discprolapse is measured at 11.54 mm. Note also the increased compression ofthe right S1 nerve root and thecal sac.


Fig. 8 – Change in lumbar spine central canal dimensions. Elderly female patient with a history of neurogenic claudication. (a) Sagittal T1-weighted spin-echosequence without axial loading. A moderate degree of central stenosis is demonstrated at the L2/3 level but cerebrospinal fluid (CSF) is still identified ventral tothe cauda equina. (b) Sagittal T1-weighted spin-echo sequence with axial loading. Critical stenosis has developed at L2/3, mainly due to posterior impressionon the theca from the fat pad. (c) Axial T2-weighted fast spin-echo sequence without axial loading. Plentiful CSF is seen ventral to the cauda eqiuna. (d) AxialT2-weighted spin-echo sequence with axial loading. There is now complete loss of CSF around the cauda equina, indicating the presence of occult central canalstenosis.


EFFECT OF LOADING ON THESYMPTOMATIC/DEGENERATE

LUMBAR SPINE

Changes in Disc Morphology

As mentioned previously, Zamani et al. [8] demonstrated an

Fig. 9 – Change in lumbar spine lateral recess dimensions. (a) Axial T2-weighted fast spin-echo sequence without axial loading. (b) Axial T2-weighted spin-echo sequence with axial loading. A marked reduction inlateral recess anteroposterior dimension is demonstrated.

Fig. 10 – Development of occult facet ganglion. (a) Axial T2-weighted fastspin-echo sequence without axial loading. Fluid is seen in the left L4/5 facetjoint. (b) Axial T1-weighted spin-echo sequence with axial loading. Asmall facet ganglion has developed deep to the ligamentum flavum and iscontributing to the central stenosis.


increase in disc bulge in 40% of degenerate lumbar discs whenimaged in the erect–extended position. Increase in the gradingof disc herniation has been reported in six of 76 imaged disclevels between supine and erect–extended position MRI [11].Axial-loaded MRI has been reported to increase the size of discherniations in four of 19 patients (Fig. 7) [17]. However, Choy[24] identified that imaging the lumbar spine in compressionresulted in accentuation of disc herniation in 50% of patients, aswell as reproducing symptoms at the time of imaging.

Changes in Canal Dimensions

Sagittal lumbar canal dimensions between erect–flexed anderect-extended MRI have been correlated with sagittal canaldimensions at erect flexion–extension myelography [9]. A highcorrelation has been demonstrated between the MRI andmyelography measurements with a small, but statisticallysignificant, positional dependence on measurements, canaldimensions being least in the erect–extended position, asexpected. The same authors found significant positionalchanges in foraminal dimensions to be uncommon. Possiblyof more relevance is the identification of changes in therelationship of the disc margin to the adjacent nerve root withchanges in body position. This was investigated by Weishauptet al. [11], who looked at the effect of supine, erect–flexed anderect–extended MRI on nerve root contact, displacement andcompression in 30 patients with chronic low back or leg pain.Nerve root contact with disc was most commonly seen in theerect–flexed position, whereas the incidence of nerve rootdeviation increased in the erect–extended position. Actualnerve root compression only developed in a single case.

The effect on the lumbar spine of axial compression hasconcentrated on the assessment of changes in DCSA [13,14,17]. Willen et al. [17] assessed the effects of axial compressionin three patient groups, those with chronic low back pain,neurogenic claudication or sciatica. They determined theadditional valuable information (AVI) obtained on com-pression studies. This was defined as a greater than 15 mm2

reduction in DCSA to levels below 75 mm2 (the borderlineDCSA, below which spinal stenosis is considered to be presentbased on intradural pressure measurements), accentuation ofdisc herniation, lateral recess or formainal stenosis or thedevelopment of an intraspinal synovial cyst. AVI was obtainedin 69% of patients with neurogenic claudication (Fig. 8), 14%of patients with sciatica but none of the patients with purely lowback pain. However, the latter finding is not surprising as thecriteria assessed are more related to nerve root compressionrather than features that may be associated with back pain. Thepercentage of cases with claudication or sciatica in which AVIwas obtained increases if only patients with a relative degree ofstenosis are included (this being a DCSA below 130 mm2).Accentuation of lateral recess stenosis (Fig. 9), with associatedunilateral or bilateral nerve root compression was seen at 42sites in 35 patients, whereas an occult synovial cyst (Fig. 10)was demonstrated in a single case. The cause of reduced centraland lateral recess dimensions is related to changes in themorphology of the posterior disc margin (Fig. 11), increase inthickness of the ligamentum flavum (Fig. 12) and also changesin shape of the posterior epidural fat pad, which actually

Fig. 11 – Accentuation of degenerate disc bulges. (a) Sagittal T2-weightedfast spin-echo sequence without axial loading. The L3/4, L4/5 and L5/S1discs are degenerate with a mild disc bulges at all three levels. (b) SagittalT2-weighted fast spin-echo sequence with axial loading. The anteroposter-ior disc measurements have increased at all three levels. Note alsodevelopment of minor retrolisthesis at L3/4.


Fig. 12 – Increase in thickness of the ligamentum flavum. (a) Axial T2-weighted fast spin-echo sequence without axial loading. The thickness ofthe right and left ligamentum flavum is measured at 3.83 and 4.06 mm,respectively. (b) Axial T2-weighted spin-echo sequence with axial loading.The thickness of the right and left ligamentum flavum increases to 5.86 and7.88 mm, respectively.

Fig. 13 – Changes in morphology of the posterior epidural fat pad. (a)Axial T1-weighted spin-echo sequence without axial loading. The ventralsurface of the fat pad is concave. (b) Axial T1-weighted spin-echo sequencewith axial loading. The ventral surface of the fat pad is now straight.


develops a flattened or convex ventral surface that cancompress the thecal sac (Fig. 13). However, the exactcontribution of each of these factors has not been determined.

Although the effect of axial compression on the foramen hasnot been formally studied, our initial experience indicates thatoccult nerve root compression can be identified (Fig. 14).

CONCLUSIONS

It is clear from the combination of cadaveric and in vivoMRI studies that imaging the lumbar spine in the erect–extension position, or with axial compression can identifyoccult nerve root compression not identified on conventionalsupine imaging. Although such techniques may increase thesensitivity of MRI for identification of lumbar nerve rootcompression, further studies are required to determine the effecton specificity, before the true management value of thetechniques can be determined.

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Fig. 14 – Dynamic foraminal stenosis. Elderly man with right L5 rootsymptoms only on standing. (a) Sagittal T1-weighted spin-echo sequencethrough the right L5/S1 intervertebral foramen without axial loading. Fat isseen inferior and posterior to the exiting L5 nerve root, which is notcompressed. (b) Sagittal T1-weighted spin-echo sequence through the rightL5/S1 intervertebral foramen with axial loading. Fat has been effacedposterior to the L5 nerve root, which is now compressed, with flattening ofits inferior margin.


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24 Choy DS. Magnetic resonance imaging of the lumbosacral spine undercompression. J Clin Laser Med Surg, 1997;15:71–73.


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