Spinal alignment

Introduction

Global spinal alignment has received much attention in the recent literature. Although often thought of as an issue unique to adult and pediatric spinal deformity, alignment in the sagittal plane is critical in all aspects of modern spine surgery (see Chapter 1 ). Often used interchangeably, spinal balance and spinal alignment are codependent; closely related yet fundamentally different concepts [ ]. Spinal alignment refers to the static radiographic measure of the cervical, thoracic, lumbar spine, and pelvis into both the coronal and sagittal planes [ ]. In contrast, spinal balance is the dynamic process through which humans work to maintain a horizontal gaze using neuromuscular control and compensatory mechanisms such as knee flexion, hip extension, and pelvic retroversion [ ].

History

Although it is commonly assumed to be a “modern” concept in spinal surgery, the effect of spinal deformity on standing balance was described as early as 1935 by Bohler [ ]. In his text on orthopedic trauma, he described the kyphotic deformity associated with vertebral body fractures and the compensatory mechanisms of pelvic retroversion, hip extension, and knee flexion required to maintain an upright posture.

It was not until the 1970s when French investigators [ , ] had a resurgent interest in dynamic balance as a concept related to spinal deformity surgery. Notably, Jean Dubousset described a “cone of economy” within which an upright posture could be maintained with minimal physical effort [ ]. The elucidation of clinically relevant radiographic pelvic parameters by Duval-Beupère was instrumental in quantifying the modern description of pelvic alignment in the sagittal plane [ ]. This work was expanded by Jackson [ ] with the evaluation of pelvic retroversion, lumbar lordosis, and sagittal alignment in patients with spondylolisthesis, confirming the importance of the pelvis in the role of global sagittal balance. Glassman [ ] showed that positive sagittal balance is poorly tolerated in patients; with the severity of symptoms increasing with worsening positive sagittal imbalance. In addition, Kuntz [ ] reported that the occiput sagittal balance is maintained within a very narrow range over the pelvis showing the importance of the occipital cervical axis in global sagittal balance. More recently, Schwab and Lafage [ ] showed that pelvic incidence matches lumbar lordosis and that increasing pelvic retroversion correlates with worse patient-reported outcomes.

Roussouly [ ] and Labelle [ ] showed that increasing pelvic incidence was associated with the risk of the development of spondylolisthesis. Although there may be a correlation between increasing pelvic incidence and severity of spondylolisthesis, studies (reviewed in Ref. [ ]) have failed to provide evidence that pelvic incidence can be used to predict the occurrence of spondylolisthesis.

Basic radiographic parameters—thoracic and lumbar spine

The alignment of the cervical, thoracic and lumbar spine in both the coronal and sagittal plane is best determined by using standing full-length 36-in. films that include all areas of the spine plus the pelvis and femoral heads (see Chapter 5 ). It is generally accepted that a Cobb angle of 10 degrees or less in the coronal plane is normal. Sagittal Cobb measurements taken from T1 to T12 in the thoracic spine have a normal range of 20–40 degrees. Alternatively, T4 may be used as the caudal end vertebra as the overlap of the humeral heads can make visualization of T1 difficult. Lumbar sagittal Cobb angles for the lumbar spine are measured from the inferior endplate of T12 to the top of the sacrum, and also range from 20 to 40 degrees.

The sagittal vertical axis (SVA) is used to determine the amount of displacement of the spine in relation to the sacrum. Vendatam et al. [ ] demonstrated that the mean SVA shifts forward from −5.6 cm in adolescents to −3.2 cm in the middle-aged and elderly population. A strong correlation exists between a positive shift in the SVA, loss of lumbar lordosis, and the onset of back pain and fatigue. Baek et al. [ ] demonstrated that in patients with osteoporosis, the most important risk factors for new vertebral compression fractures after treatment of an initial fracture by vertebroplasty are the degree of osteoporosis and the altered biomechanics of the spine due to spinopelvic malalignment With the onset of digital radiography, standardized, high-quality images including the entire spine can be obtained more reliably [ ]. Specialized low-dose, high-sensitivity radiography (EOS imaging) helps in visualizing the entire spine and lower extremities to aid in the assessment of global sagittal and coronal standing alignment and pelvic parameters [ ].