Key points
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Global spinal balance is equally important as it is a dynamic process that includes the spine along with compensatory mechanisms consisting of the hips, and ankles.
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Current commonly used spinal measurements include cervical lordosis, thoracic kyphosis, and lumbar lordosis, while the three most commonly used pelvic indices include the pelvic incidence, sacral slope, and pelvic tilt, where the pelvic incidence is constant in adults and has some racial variances.
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Aging and surgical disturbances in the alignment of the thoracic kyphosis and lumbar lordosis generally result in the development of positive sagittal balance where the primary lower extremities compensatory mechanisms come into play in an attempt to restore balance.
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Loss of normal spinal alignment results in the alteration of the two nonconstant pelvic indices; decreased sacral slope and pelvic tilt (retroversion) as the hips move toward maximum anatomical extension in an effort to maintain adequate global sagittal balance followed by knee flexion.
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Management options to address spinal disorders should account for spinal alignment, which can impact treatment approach and outcomes.
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 [ ].
The contribution of pelvic anatomy to global alignment
The pelvis has been described as the “cornerstone” on which spinal alignment is based. Dubousset emphasized the importance of the pelvis and how it could be thought of as a single specialized vertebra that supports the rest of the spine [ ]. This was based on the observation that increasing the angle of declination of the sacrum increased the lordosis in the lumbar spine. From this, During et al. [ ] defined the pelvic-sacral angle, which is a line through the center of the hip joints and the center of the upper surface of the sacrum, and a tangent line through the upper vertices of the sacrum. Modern terminology is clearly rooted in the work of Duval-Beaupere [ ] that further defined these concepts by introducing the compliment to the pelvisacral angle, the “Pelvic Incidence.” This work led to the acceptance of three closely related anatomical parameters that describe the influence of the pelvis on spinal alignment ( Fig. 16.1 ).
- 1.
Pelvic incidence is defined as the angle between a perpendicular dropped from a line tangent to the sacral endplate and a line from the center of the sacral endplate to the center of the femoral heads. This parameter changes with growth in childhood then becomes a constant in adults, a measure specific to the individual than can only change with a pelvic osteotomy or trauma. This constancy is due in large part to the relatively fixed position of the sacroiliac joints within the pelvis.
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Pelvic tilt is defined as the angle between a line from the center of the femoral heads to the center of the sacral endplate, and a vertical from the center of the femoral head. Pelvic tilt is a dynamic parameter that can be conceptualized as the pelvis rotating around the fixed femoral heads. It is the parameter that defines the compensation within the hip joints that occurs with any alteration of the global spinal alignment. As the spine loses lordosis with aging, the hips will gradually extend within their anatomically defined limits to correct this imbalance eventually reaching maximum extension. Less effective compensatory mechanisms such as knee flexion and ankle plantar flexion may then provide the additional correction.
- 3.
Sacral slope is the angle defined between a horizontal line and a line along the sacral endplate. It is a dynamic parameter that has a direct correlation with the loss of lumbar lordosis, closely tied to the tendency of pelvic tilt to increase with age and simply reflects the top of the sacrum becoming more parallel to the floor as the pelvis rotates posteriorly.
Cervical alignment contribution to global spinal balance
The cervical spine, like the pelvis, can be impacted by changes in alignment as it serves as a terminal compensation mechanism for maintaining a horizontal gaze. The cervical spine normally has a lordosis. However recent work by Le Huec [ ] and Faline [ ] has shown there is a significant prevalence of cervical kyphosis in a healthy population. Patients with Adolescent Idiopathic Scoliosis have also been shown to compensate for thoracic hypokyphosis with cervical kyphosis to maintain a horizontal gaze [ ]. Measurement of the cervical spine has traditionally utilized parameters such as the cervical sagittal vertical axis (cSVA), which is defined as the sagittal offset of a plumb line dropped from the center of C2 to the posterior superior aspect of C7. Radiographically, cervical lordosis has been measured as cervical curvature , which is defined as the angle between the endplates of C2-C7 [ ].
Thoracic and lumbar contribution to global spinal balance
While any significant deviation of the thoracic and lumbar spine from the central sacral vertebral line in the coronal plane is deemed pathologic, the range of normal of the sagittal plane of the thoracic and lumbar spine is wide. Lordosis of the lumbar spine is directly related to the position of the sacrum (sacral slope) and is dependent on the pelvic anatomy. As the sacral slope flattens, two reciprocal changes in the lower arc of lordosis can occur: the apex of the lordosis moves inferiorly or the radius of curvature increases as the lordosis involves more vertebral levels ( Fig. 16.2 ). However, the sacral slope is position dependent, and pelvic retroversion can influence this measurement.
