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Year : 2006  |  Volume : 16  |  Issue : 4  |  Page : 643-649
Sacroiliitis in routine Mri for low back ache

Dept of Radiodiagnosis, Command Hospital (Northern Command), Udhampur, India

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Date of Submission06-Aug-2004
Date of Acceptance20-Oct-2006


Low backache is a common clinical presentation in medical practice and a common referral at most MRI centres. The presentation of lesions in low backache involving various lumbar spinal structures as well as sacroiliac joint often overlap and are clinically indistinguishable, necessitating evaluation by MRI. The detection and characterisation of sacroiliac lesions on MR imaging has been well established, but its presence during evaluation of low back cases has not been analysed and well documented. During routine MR imaging of patients with low back pain, the assessment of sacroiliac joint lesions is often neglected. The aim of this study was to determine the presence of sacroiliitis in MRI of cases with low back pain and analyse its prevalence and morphological patterns.

Keywords: MRI Lumbar Spine; Low Backache; Sacroiliitis

How to cite this article:
Sreedhar C M, Sree Ram M N, Alam A, Indrajit I K, Shanmuganandan K. Sacroiliitis in routine Mri for low back ache. Indian J Radiol Imaging 2006;16:643-9

How to cite this URL:
Sreedhar C M, Sree Ram M N, Alam A, Indrajit I K, Shanmuganandan K. Sacroiliitis in routine Mri for low back ache. Indian J Radiol Imaging [serial online] 2006 [cited 2021 Feb 25];16:643-9. Available from:

   Introduction Top

Low back pain is a common referral in routine MRI practice. The clinical presentation of low back ache is an outcome of many etiologies involving various structures. Often the clinical presentation of intervertebral disc, facet joint, nerve root, and sacroiliac joint diseases overlap and are indistinguishable. The commonest cause for low back pain is mechanical disorders, which is contributory to almost 90% of all cases. The patients of this group have no definable cause of their back pain, and is attributed to muscle or ligamentous "strain or injury". 10% of the remaining adults with back pain have symptoms related to systemic illness, like cancer, inflammatory back disease or infection. Sacroiliac joint arthritis accounts for a significant number of low back cases. This study was carried to determine the incidence of sacroiliac joint involvement in patients with low back ache. To achieve this, routine MRI of lumbar spine was augmented in our institute with a single additional 'fat suppressed' sequence of the lumbo-sacral region in the coronal plane, to identify sacroiliac involvement.

   Material and Methods Top


Seven hundred and fifty patients with an age range 7 to 89 years with low back ache underwent MR imaging during a period from Apr 2002 to Jul 2004. All subjects were evaluated in detail by clinical and spine examination prior to MRI lumbosacral region. The eligibility criteria included a) the chief complaint was low back pain; b) back pain with mechanical or inflammatory cause; c) back pain associated with trauma, canal stenosis, caries spine, referred pain to hip and foot; d) back pain associated with a failed back surgery; e) there was no contraindications to MR imaging (e.g. pacemaker, aneurysmal clips, foreign body in globe etc). We intentionally included a wide group of etiologies, to determine the prevalence of sacroiliitis in a cohort referred for MRI lumbar spine. The patient particulars and the salient clinical findings analysed in this study are displayed in Table 1. A detailed history was elicited from each patient including principal symptoms and signs, systemic examination, physical examination. Attention was given to HLA status, ESR and CRP values in relevant cases.

MRI Protocol

MR imaging was performed at our institute, using a 1.5 T (Magnetom Symphony with Quantum gradients [maximum gradient amplitude, 30 mT/m; slew rate, 125 mT/m/sec]; Siemens Medical Systems, Erlangen, Germany) with use of a spine phased-array coil. MR imaging of the lumbar spine was performed with a sagittal T1 and T2 weighted sequence, transverse T1 and T2 weighted sequence followed by a MR myelogram in three planes. This was followed by T2 STIR images acquired in oblique coronal plane, with selection of plane parallel to the anterior sacrum, to identify and evaluate sacroiliitis. The STIR protocol used was performed using the following measurements : TR:4410ms; TE:69ms; TI:160ms; FOV read:380; FOV phase:86.3; Slice thickness:4 mm; Slices:13 ;Flip angle:1600; Base resolution 320, Phase resolution 70 and Averages:4. The acquisition time of this additional fat suppressed sequence was 3 min 55 sec.

Data Collection and Results

Seven hundred and fifty patients with an age range 7 to 89 years with low back ache underwent MR imaging during a period from Apr 2002 to Jul 2004. The study comprised of two groups, principally based on final diagnosis with MRI; namely: those with and without MRI evidence of sacroiliitis. Out of this total, 692 cases had no MRI evidence of sacroiliitis, but demonstrated a varying combination of degenerative disc disease. However, with the use of a fat suppressed sequence of the lumbosacral region during the evaluation of lumbar spine, 59 patients (representing 7.86 %), displayed MRI features of sacroiliitis. Amongst these 42 patients were males (71.19%), with the remaining being females (28.81).

The age group pattern is given at Table 2, which reveals maximum number of sacroiliitis cases in this study belonging to the age bracket of 20 to 24, while in MRI scans with degenerative changes but having no evidence of sacroiliitis most frequently occurring in the 35-39 age brackets. The clinical diagnosis at the time of scan in the group of sacroiliitis included low back pain, sacroiliitis, post LCS and PIVD surgery, low backache associated with pain buttock, sciatica, hip and foot.

The patterns of involvement evaluated by MRI in the 59 cases of sacroiliitis are given in Table 3. The following were the prevalent and more frequent findings: lesions at both SI joints in 29 cases (49.15%), lesions at iliac aspect of the sacroiliac joint in 29 cases (49.15%), marrow edema in all cases (100%), articular erosions in 24 cases (40.67%) and normal joint space in 52 cases (67.79%). Table 4 displays the final diagnosis by MRI in both the groups: those with and those without MRI evidence of sacroiliitis. In the sacroiliitis group, pure sacroiliac joint lesions were present in 25 cases, representing 3.32 % of the entire study cases and 42.37 % within the group. There were 17,3 and 14 cases of sacroiliitis with disc desiccation, with disc bulge / herniation and with all respectively [Figure - 1]. In the second group i.e. with no MRI evidence of sacroiliitis, degenerative disc disease represented the single largest finding representing 89.95 % of the entire study cases and 92.19 % within the group.

   Discussion Top

The true prevalence of sacroiliac joint lesions is unknown in most population groups, either with or without low backache. Similarly, the presence of sacroiliitis on MRI has not been determined in low backache cases attributable to either mechanical or inflammatory or both these broad etiological groups. This study analyses the presence of sacroiliitis in MRI of lumbar spine referred for low backache, signifying the importance of this neglected entity in correct assessment of low backache. Low back pain is a common referral in routine MRI practice. Clearly, it is one of the most common symptoms evaluated and treated by practitioners. It has been observed that in any 12-month period in USA, 15% to 20% of the population has an episode of lumbosacral pain with back symptoms occurring in 50% of working age adults yearly [1]. Low back pain is associated with a wide range of clinical disorders. The commonest group is mechanical disorders, which occurs in more than 90% of all episodes of back pain. 10% of the remaining patients with back pain have symptoms related to systemic illness, like cancer, inflammatory back disease or infection [2].

The incidence of sacroiliac joint disease in patients with low-back pain has been earlier evaluated by using CT scans of lumbosacral spine [3]. Sacroiliac joint arthritis was identified by presence of subchondral sclerosis, osteophytosis, or cartilage loss. Of the 64 patients, 48 cases (75%) had CT evidence of osteoarthritis at the sacroiliac joint. The authors conclude that there is a relatively high incidence of SI joint arthritis in patients undergoing evaluation for low-back pain [3]. However, there are no similar or large cohort studies available in literature using MRI to determine sacroiliac joint disease in patients with low-back pain. In this study with the use of a fat suppressed sequence during the evaluation of lumbar spine, 59 patients (representing 7.86 %), displayed MRI features of sacroiliitis.

Sacroiliac joint is a unique joint in the human body with differences in type and thickness of articular cartilage between different regions of the sacral and iliac articular surfaces [4]. Light microscopy and immunohistochemistry has shown significant differences between the iliac and sacral articular cartilages as described in a recent study by Kampen [5]. The sacral cartilage is thick, has low cell density, and rests upon a thin bone end-plate supported by porous, cancellous bone. In comparison, the iliac cartilage is thin, has high cell density, resting on thicker subchondral bone end-plates, supported by twice as dense subchondral cancellous bone [6]. The thickness at sacral side and iliac side in adults is 4 mm and 1-2 mm respectively. The spongiosa trabeculae at sacral subchondral bone are inserted at right angles, implying a perpendicular load on the articular facet, unlike the iliac side where there is no definite alignment of subchondral spongiosa. Moreover, blood vessels penetrate subchondral bone plate at both the iliac and sacral surfaces, coursing closely on the overlying articular cartilage, which causes the high incidence of rheumatoid and inflammatory diseases at sacroiliac joint. In this study of 59 MRI proven sacroiliitis, the more frequent findings included : lesions at both SI joints in 29 cases (49.15%), lesions at iliac aspect of the sacroiliac joint in 29 cases (49.15%), marrow edema in all cases (100%), articular erosions in 24 cases (40.67%) and normal joint space in 52 cases (67.79%).

MRI of normal sacroiliac joint reveals an intermediate signal of the cartilage of the synovial compartment on T1 and T2 images limited by the signal void of bone cortex. On T1FS and Fast STIR images the cartilage has an intermediate to high signal. The marrow on T1, T2 and T1FS images has a homogeneous intermediate signal. Fat suppressed images are extremely useful in imaging of cases with sacroiliitis. Fat suppression causes rescaling of signal intensities, categorises cartilage as the brightest structure [7]. This additive effect along with the darkened appearance of fat in adjacent soft tissues and sacral, iliac and lumbar marrow, renders improved visualization of structures and increases the conspicuity of lesion, thereby improving their pickup rate. There are two fat suppressed sequences that are available: T1-weighted with fat suppression (T1FS) and fast short tau inversion recovery (Fast STIR) sequences. These are superior to T1 and T2 images, in demonstrating the changes of sacroiliitis [8]. [Figure - 2],[Figure - 3]

There are few normal variants, which merits consideration during MRI evaluation of sacroiliac joint. Partial volume artifact between the synovial and ligamentous compartments can be misinterpreted as erosions. There is normally a region of high signal at the immediate subchondral marrow, on Fast STIR images, which can be mistaken for early sacroiliitis. A patchy distribution of fat within the bone marrow as the sole finding should not be considered as an indicator for sacroiliitis. [8].

Vogler et al evaluated the CT appearances of sacroiliac joints in asymptomatic patients, to define the normal joint appearances and differentiate it from early CT signs of sacroiliitis [9]. In their study, they correlated findings in asymptomatic and sacroiliitis groups, and categorized them into two groups. CT findings that were grouped as poor CT indicators of sacroiliitis, by virtue of its frequent occurrence in the asymptomatic population included nonuniform iliac sclerosis (83%), focal joint space narrowing in patients over the age of 30 (74%), and ill-defined areas of subchondral sclerosis, particularly on the iliac side (67%). Conversely, the good CT indicators of sacroiliitis were those that occurred infrequently in the asymptomatic population, and comprised increased sacral subchondral sclerosis in subjects under the age of 40 (11%), bilateral or unilateral uniform joint space of less than 2 mm (2% or 0%, respectively) and erosions (2%) . A similar study using MRI is currently not available in published literature.

The value of MRI in the diagnosis of sacroiliitis has been well established. MRI accurately delineates the cardinal features of sacroiliitis, like changes in joint space width and symmetry, presence of erosions, subchondral edema, sclerosis, cysts and ankylosis [10]. Furthermore, MRI plays a useful role in patients with early disease, by its superior ability to directly image changes in articular cartilage [11]. Comparative studies between MRI and CT in the evaluation of patients with suspected sacroiliitis have further shown that the sensitivity and specificity of MR for the detection of cortical erosions and subchondral sclerosis when compared to CT images were 100 and 94.3%, respectively [12],[13].

MRI offers valuable information on the lesions affecting the various structures of the sacroiliac joint in sacroiliitis. Synovial pannus tissue appears low signal intensity on T1 and high signal on T2 , spared cartilage exhibits persisting low-signal foci within synovial compartment, bone erosions shows high signal regions at the bone periphery and subchondral bone sclerosis displays markedly hypointense subchondral bone and infectious sacroiliitis shows diffuse high signal intensity in the bone marrow [10].

Seronegative and HLA B27-associated spondyloarthropathy (SpA) is a heterogeneous group of related inflammatory joint disease that share typical clinical features and a unique association with the major histocompatibility complex class I molecule HLA-B27. It comprises of five clinically defined subcategories: ankylosing spondylitis (AS), psoriatic arthritis (PsA), reactive arthritis (ReA), inflammatory bowel disease-associated arthritis and undifferentiated spondyloarthropathy (uSpA). Sacroiliitis is the key symptom of all spondyloarthropathies and is often associated with inflammatory back pain [14],[15]. In this study, there were 12 (10 males and 2 female) patients in the group of 59 patients with sacroiliitis, with HLA results. While five of these had findings of sacroiliitis only, seven had associated findings of degenerative disc disease variably distributed.

The diagnostic value of MRI in the detection of early sacroiliitis has been described by Oostveen et al in their study [16]. The role of MRI in analyze the type and frequency of abnormalities of the sacroiliac joint (SIJ) in early seronegative spondyloarthropathy (SpA) has been further established. It identifies reliably joint erosions in early SpA and allows differentiation between active and chronic sacroiliitis [17]. European Spondyloarthropathy Study Group criteria are one of the many standards available for spondyloarthropathy (SpA). In a study by Brandt, aimed at determining the frequency of sacroiliitis and spondyloarthropathy while assessing the significance of HLA B27 measurements for diagnosis in early disease, it was found that HLA B27 positivity in inflammatory back pain patients with MRI-proven sacroiliitis positively predicts SpA. Undifferentiated SpA was the frequent SpA subset. The study also showed the presence during long term follow up of HLA B27-negative non-SpA patients with moderate unilateral sacroiliitis whom were classified as undifferentiated sacroiliitis [18][Figure - 4].

Few limitations were encountered during the conduct of this study. Of the 59 patients with sacroiliitis, HLA results were available in 13, 12 of which were positive, representing 20.3%. The results of the remaining 46 patients (77.7%) were not available. The patients referred were not from a single clinical specialist source within the hospital, but from multiple specialties like orthopedics, pediatrics, rheumatology, neurology, neurosurgery and general medicine. Gadolinium was used only in 5 of the 59 cases that were referred. The patient eligibility criteria for inclusion was wide with low back pain encompassing idiopathic, mechanical, inflammatory, infective, post operative causes and not merely to mechanical or inflammatory group. This was done intentionally to determine the prevalence of sacroiliitis in a large cohort referred for MRI lumbar spine. The coexistence of sacroiliitis in many of the above etiologies has not been fully documented and its prevalence not yet clearly determined. All the above limitations stemmed from a common underlying factor: this study highlights and encourages the use of a 'fat suppressed' coronal sequence of the sacroiliac joint, as a screening tool, during the evaluation of the lumbar spine.

The value of using a 'fat suppressed' coronal sequence of the sacroiliac joint, is that it identifies sacroiliitis reliably by the presence of subchondral edema, erosions and sclerosis even when there is no joint space narrowing. Once sacroiliitis is identified on MRI, the patient is evaluated clinically, assessed for HLA status, ESR and CRP for activity and subject to a periodic follow up, wherein intravenous gadolinium is used to characterise the disease activity.

In the final analysis, the use of a 'fat suppressed' coronal sequence during the evaluation of the lumbar spine is a simple and convenient screening strategy. It reliably identifies sacroiliitis, which is a demonstrable cause of low backache, but often neglected or missed.

   Conclusion Top

Low back pain is one of the commonest referral in MRI scan centres. Sacroiliitis, which is a demonstrable cause of low backache, is often missed clinically or neglected during routine MRI scanning of the lumbar region. Sacroiliac joint lesions accounts for a small but significant number of low back cases as evident form this study. This study highlights the diagnostic value and utility of adding a single 'fat suppressed' sequence of the lumbo-sacral region in the coronal plane. This adds marginally to the scan time but increases the yield of identifying incidental or manifest sacroiliac involvement in all cases referred for MRI for low back ache.

   References Top

1.Bigos S, Bowyer O, Braen G, et al. Acute low back problems in adults. Clinical practice guideline no. 14. AHCPR publication no. 95-0642. Rockville (MD): Agency for Health Care Policy and Research, Public Health Service, US Department of Health and Human Services; 1994.   Back to cited text no. 1    
2.Nachemson A: The lumbar spine: an orthopedic challenge. Spine 1976;1:59-71.  Back to cited text no. 2    
3.Hodge JC, Bessette B: The incidence of sacroiliac joint disease in patients with low-back pain. Can Assoc Radiol J. 1999;50:321-323.   Back to cited text no. 3    
4.Salsabili N, Valojerdy MR, Hogg DA.: Variations in thickness of articular cartilage in the human sacroiliac joint. Clin Anat. 1995;8:388-390.   Back to cited text no. 4    
5.Kampen WU, Tillmann B.: Age-related changes in the articular cartilage of human sacroiliac joint. Anat Embryol 1998;198:505-513.   Back to cited text no. 5  [PUBMED]  [FULLTEXT]
6.McLauchlan GJ, Gardner DL.: Sacral and iliac articular cartilage thickness and cellularity: relationship to subchondral bone end-plate thickness and cancellous bone density. Rheumatology. 2002;41:375-380.   Back to cited text no. 6    
7.Wittram C, Whitehouse GH, Bucknall RC: Fat suppressed contrast enhanced MR Imaging in the assessment of sacroiliitis. Clin Radiol. 1996;51:554-558  Back to cited text no. 7    
8.Wittram C, Whitehouse GH. : Normal variation in the magnetic resonance imaging appearances of the sacroiliac joints: pitfalls in the diagnosis of sacroiliitis. Clin Radiol. 1995;50:371-376   Back to cited text no. 8    
9.Vogler JB 3rd, Brown WH, Helms CA, Genant HK.: The normal sacroiliac joint: a CT study of asymptomatic patients. Radiology. 1984;151:433-437.   Back to cited text no. 9    
10.Murphey MD, Wetzel LH, Bramble JM, Levine E, Simpson KM, Lindsley HB. Sacroiliitis: MR imaging findings. Radiology. 1991;180:239-244.   Back to cited text no. 10    
11.Docherty P, Mitchell MJ, MacMillan L, Mosher D, Barnes DC et al.: Magnetic resonance imaging in the detection of sacroiliitis. J Rheumatol. 1992;19:393-401.   Back to cited text no. 11    
12.Wittram C, Whitehouse GH, Williams JW, Bucknall RC.: A comparison of MR and CT in suspected sacroiliitis. J Comput Assist Tomogr. 1996;20:68-72.   Back to cited text no. 12    
13.Yu W, Feng F, Dion E, Yang H, Jiang M, Genant HK.: Comparison of radiography, computed tomography and magnetic resonance imaging in the detection of sacroiliitis accompanying ankylosing spondylitis. Skeletal Radiol. 1998;27:311-320.   Back to cited text no. 13    
14.Bollow M, Braun J, Hamm B: Sacroiliitis: the key symptom of spondylathropathies. 1. The clinical aspects : Rofo Fortschr Geb Rontgenstr Neuen Bildgeb Verfahr. 1997;166:95-100.   Back to cited text no. 14    
15.Braun J, Sieper J.: The sacroiliac joint in the spondyloarthropathies. Curr Opin Rheumatol. 1996;8:275-287.   Back to cited text no. 15    
16.Oostveen J, Prevo R, den Boer J, van de Laar M : Early detection of sacroiliitis on magnetic resonance imaging and subsequent development of sacroiliitis on plain radiography. A prospective, longitudinal study. J Rheumatol. 1999;26:1953-1958   Back to cited text no. 16    
17.Puhakka KB, Jurik AG, Egund N, Schiottz-Christensen B et al: Imaging of sacroiliitis in early seronegative spondylarthropathy. Assessment of abnormalities by MR in comparison with radiography and CT. Acta Radiol. 2003;44:218-229.   Back to cited text no. 17    
18.Brandt J, Bollow M, Haberle J, Rudwaleit M, Eggens U, Distler A, Sieper J, Braun J : Studying patients with inflammatory back pain and arthritis of the lower limbs clinically and by magnetic resonance imaging: many, but not all patients with sacroiliitis have spondyloarthropathy. Rheumatology. 1999;38:831-836.   Back to cited text no. 18    

Correspondence Address:
C M Sreedhar
Dept of Radiodiagnosis , Command Hospital (Northern Command), Udhampur
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0971-3026.32288

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  [Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4]

  [Table - 1], [Table - 2], [Table - 3], [Table - 4]

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