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Lumbosacral Spine Plain Radiographs


Introduction

The lumbosacral spine consists of five large vertebrae that make up the lumbar spine and five fused vertebrae that make up one single bone which articulate on each side of pelvis called the sacrum. Overall, spinal balance is determined by a mixture of kyphosis and lordosis throughout the spine. Lumbar lordosis is an important factor in overall sagittal balance, which contributes to bipedalism posture and upright gait. Approximately 40–60 degrees of lordosis is present in the lumbar spine. Although there is inherent variability between patients, there is an increase in lumbar lordosis from the upper to lower segments with approximately two-thirds of lumbar lordosis occurring at the L4-S1 levels. The balance, alignment, and motion of the spine are determined by lumbopelvic alignment and pelvic compensation for sagittal alignment in the thoracic and lumbar spine.

Ultimately lumbopelvic parameters have an important impact on the pathogenesis of lumbar degenerative conditions and spinal deformity. The relationship between the lumbosacral spine and the pelvic morphology stresses the importance of plain radiographs in overall spinal alignment. The present chapter discusses the importance of plain radiographs in determining the relationship between the lumbosacral spine and the pelvis with an emphasis on clinical relevance and techniques when performing these measurements.

Specific Radiographic Parameters

During examination of the lumbosacral spine, indications for full-length or isolated posteroanterior and lateral radiographs may differ upon the presenting complaints. Standing full-length 36-inch posteroanterior and lateral radiographs are the baseline imaging studies to evaluate sagittal balance, which has been described in depth in a previous chapter. The hips and knees should be extended during standing radiographs to negate any compensatory mechanisms. The use of posteroanterior projection decreases patient radiation exposure by 41% in comparison with the use of anteroposterior projection. If a practitioner chooses to obtain isolated standing lumbosacral radiographs, the femoral heads should be included to adequately assess the relationship between lumbosacral and pelvic parameters. All measurements are made using lateral radiographs unless otherwise specified.

General Lumbar Spine

Coronal balance

When evaluating overall spinal balance, coronal alignment should be assessed with full-length 36-inch standing posteroanterior radiographs with the hips and knees extended. The presence of deformity, pelvic obliquity, and leg length discrepancy can be noted on posteroanterior radiographs. If a leg length discrepancy greater than 2 cm is already known, then a leg lift can be placed under the shorter leg during radiographic evaluation.

Coronal regional alignment

Coronal lumbar and lumbosacral curves are defined as having an apex from L1 to L5 and L5 to S1, respectively, as measured by the Cobb method from the end vertabrae. The cephalad end vertebra is the first vertebra in the cephalad direction whose superior endplate is tilted maximally toward the concavity of the curve. The caudad end vertebra is the first vertebra in the caudad direction whose inferior endplate is tilted maximally toward the concavity of the curve. Apical vertebral translation (AVT) is defined as the horizontal distance measured from the C7 plumb line to the center of the central sacral vertical line (CSVL) for lumbar and lumbosacral curves. The central sacral vertical line is defined as a vertical line drawn through the center of the S1 endplate. Coronal spinal balance is defined from the center of C7 and the midpoint of the thoracic trunk to the sacrum. Coronal imbalance of greater than 4 cm has been correlated with deterioration in pain and function scores in unoperated patients but not in patients with surgery. This suggests that sagittal correction may be more important than coronal correction and will be discussed later in this chapter.

Coronal pelvic alignment

Coronal pelvic alignment can be defined by leg length discrepancy and pelvic obliquity. Pelvic obliquity is measured by the angle created by a horizontal line and a line tangential to the top of the sacral ala. Pelvic obliquity can be seen as a compensatory mechanism from lumbar or lumbosacral deformity, leg length deformity, or both. Leg length discrepancy is defined as the vertical distance measured between horizontal lines drawn tangential to the top of each femoral head. It is important to note the etiology of the pelvic obliquity as compensatory lumbar or lumbosacral curves can be mistaken for true deformity in the setting of an undiagnosed leg length discrepancy greater than 2 cm.

Spinopelvic Parameters

Overall sagittal balance and the relationship between the lumbosacral spine and the pelvis are evaluated from the lateral radiograph. Key components of spinopelvic parameters include lumbar lordosis (LL), pelvic incidence (PI), pelvic tilt (PT), and sacral slope (SS). The influence of the cervical spine on the overall sagittal balance is worth mentioning here and is measured by the C7–S1 sagittal vertical axis (SVA) ( Fig. 1 ).

  • C7–S1 sagittal vertical axis

    • Draw a vertical plumb line centered at the midline of the C7 vertebral body to the posterior-superior corner of the S1 endplate

    • Measure the horizontal distance from the vertical plumb line

    • SVA has a (+) value when anterior to the sacral reference point and a (−) value when posterior to the sacral reference point.

Fig. 1, Sagittal vertical axis.

There is an association between the loss of lordosis and an anterior shift of the SVA. Spinopelvic harmony exists when the SVA < 50 mm. Restoration of the SVA achieves level gaze and has been found to lead to better Health Related Quality of Life (HRQoL) scores.

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