Since the first reported association between LSTVs and low back pain by Bertolotti et al[12
] in 1917, there has been considerable debate and controversy among authors regarding the clinical relevance of the above entity. It has been suggested that articulation between a LSTV and the sacrum protects the intervening intervertebral disc by limiting motion across the pseudoarthrosis and transmitting stress to superior lumbar segments, thereby resulting in its early degeneration. In subjects with LSTV, low back pain can originate from the pseudoarthrosis, resultant instability and degeneration of the level cephalad to the LSTV, compression of the nerve root from hypertrophied transverse process, and the contralateral facet joint, when the articulation is unilateral[8
]. Clinicians who consider LSTV as a cause of low back pain and physicians who are involved with interventional procedures on the lumbar spine would benefit from MRI reports which describe the presence of this anatomical variation and provide accurate numbering of the lumbar vertebrae. MRI of the lumbar spine lacks the panoramic perspective of radiography and the spatial resolution of computed tomography, and unless the radiologist actively interrogates for this entity, the detection of LSTV may be missed.
In our experience, subjects with LSTV frequently demonstrate exaggerated lumbar lordotic curvature and lack of sharp angulations at the lumbosacral junction on the mid-sagittal MR image. Hence, we employed two simple angle measurements, the A-angle and the B-angle, which quantify the lumbosacral angulation and sacral inclination on sagittal MRI images, respectively. The A-angle is identical in measurement to the lumbosacral angle measured on lateral radiographs of the spine, increased values of which have been previously reported to be associated with low back pain[13
]. The B-angle was defined by the superior sacral surface and the superior endplate of L3 vertebra, as we have observed that, in most cases, the main curvature of the lumbar spine extends along the above levels.
In agreement with our observations, both angles were significantly increased in subjects with LSTV as compared to controls (Figure ). The two angles exhibited similar levels of sensitivity; however, the A-angle demonstrated higher specificity. Since most radiologists start the evaluation of lumbar spine MRIs from the sagittal images, it is suggested that a measurement of the A-angle or B-angle or at least a gross assessment of the sacral inclination and lumbar lordosis should be performed before proceeding to the detailed study assessment. An A-angle of greater than 40°, a B-angle of greater than 36°, a large inclination of the sacrum grossly with respect to axis of the scan table, or an exaggerated lumbar lordosis should alert the radiologist for the possible presence of an LSTV. Compared to the A-angle, the B-angle may be more difficult to measure (e.g., the forming lines may intersect out of the image) and is also harder to assess grossly by visual inspection. Of course, exaggerated lumbar lordosis may also result from significant degenerative disc disease, age or larger degrees of listhesis. The results of this study suggest that this finding should be carefully scrutinized in young subjects to avoid missing an LSTV.
Previous studies have reported several predictive similar signs which can suggest the presence of LSTV in lumbar spine MRIs. O’Driscoll et al[11
] described two types of intervertebral discs, which present at the lumbosacral junction in subjects with LSTV. The “type I” disc is smaller than the superior one, maintains its high T2 signal intensity, has no intranuclear cleft, and extends along the entire anteroposterior space without any fusion between the adjacent endplates, and has been considered indicative of an LSTV. However, the specificity of the above findings is questionable, as in cases of pseudoarthrosis the transitional disc may appear normal. A “type II” disc is a rudimentary disc which is smaller than the transitional one, maintains its signal intensity, has no intranuclear cleft, and is associated with fusion of the anterior endplates, both of which appear concave[11
]. Again, these signs are non-specific since a residual disc may exist at the S1-S2 level in up to 58% of cases, leading to over-diagnosis of LSTV[1
]. On axial images, the identification of the iliolumbar ligaments, arising from the L5 vertebra, enables precise detection of LSTV and accurate numbering of the lumbar vertebrae[9
]; however, in cases with segmentation anomalies of the thoracolumbar spine, the recognition of L5 vertebra may be incorrect[8
]. Another study demonstrated that LSTVs commonly assume a “squared” appearance in lateral radiographs, with the ratio of the anteroposterior diameter of the superior endplate to the inferior endplate being less than 1.37[14
By design, retrospective case-control studies are susceptible to selection bias; therefore a limitation of our study is that the studied material is not representative of the general population. Another limitation is the absence of correlation with radiographic findings, and the identification of the transitional anatomy being exclusively based on MRI. As a result, from the four classical types of LSTVs described by Castelvi et al[6
], we were able to identify the LSTVs which exhibited pseudoarthrosis or fusion with the sacrum (types II , III and IV), but not those with dysplastic transverse processes, measuring at 19 mm in craniocaudal dimension (type I). Therefore, we cannot exclude the possibility that one or more cases were erroneously included in the study or the control group. However, the intent of the study was to predict LSTV on MR imaging alone, as these simple measurements may help MRI readers in their routine practice when radiographs have not been performed, or are not available at the time of read out. Future prospective studies in larger populations may re-evaluate the predictive values of the angles we present in this paper.