Summary of Key Points
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Spinal deformity is defined as a deviation from normal spinal alignment. Standard deviations of normal parameters suggest a wide variation in normal alignment and should be kept in mind when assessing spinal deformities.
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The proper clinical and radiographic measurement of regional and global curvatures of the spine is essential in defining a spinal deformity. Accurate assessment and measurement further allow for standardization for future study and outcomes to further advance surgical treatment of spinal deformities.
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The critically important characteristics of a spinal deformity include patient age, spinal abnormality or underlying cause including neurologic compromise (e.g., radiculopathy, myelopathy), deformity curve characteristics such as location, pattern, magnitude, and flexibility, pelvic alignment, and global spinal alignment.
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Multiple classification schemes have been proposed to advance the treatment of deformity with each scheme highlighting certain critical characteristics of a spinal deformity. These schemes each have unique strengths and shortcomings but nonetheless serve as important tools for studying and understanding deformity.
The spine is composed of regions with distinct alignment and biomechanical properties that contribute to global alignment. Although regional spinal curves vary widely from the occiput to the pelvis in asymptomatic individuals, global spinal alignment is maintained in a much narrower range for maintenance of horizontal gaze and balance of the spine over the pelvis and femoral heads. Spinal deformity is defined as a deviation from normal spinal alignment. Because the human condition is in part defined by the ability to comfortably stand upright and because the treatment of many patients with spinal disorders is directed at restoring this condition, spinal deformity needs to be defined in relation to neutral upright spinal alignment (NUSA) in asymptomatic individuals. NUSA in asymptomatic individuals is defined as standing with the knees and hips comfortably extended, the shoulders neutral or flexed, the neck neutral, and the gaze horizontal. Analysis of spinal alignment involves both clinical and radiographic evaluation. Although there are a myriad of angles and displacements for measuring spinal alignment, our subsequent analysis offers a systematic approach to analyzing regional and global spinal alignment.
Clinical and Radiographic Evaluation of Deformity
To evaluate a spinal deformity, it is necessary to do the following
- 1.
Perform clinical measurements (facilitated with photographs) in a neutral upright position (standing with the knees and hips comfortably extended, the shoulders and neck neutral) and a forward bend position (standing with feet together, the knees comfortably extended, the hips and spine flexed, and the arms dependent with fingers and palms opposed).
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Measure occipitocervical and cervical angles and displacements on standard standing anteroposterior and lateral cervical spine radiographs in a neutral upright position (standing with the knees and hips comfortably extended, the shoulders and neck neutral).
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Measure thoracic, lumbar, sacral, and pelvic angles and displacements, including spinal balance, on standard standing anteroposterior and lateral long cassette radiographs in a neutral upright standing position (standing with the knees and hips comfortably extended, the shoulders neutral or flexed [flexed for lateral radiographs], and the neck neutral).
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Obtain side-bending (supine) and flexion-extension (standing) radiographs when appropriate for evaluating the flexibility of a deformity curve.
All upright imaging is performed barefoot. In patients with increased or decreased thoracic/lumbar vertebrae, the anomalous vertebrae are included in the appropriate alignment-biomechanical zone. A leg length discrepancy of less than 2 cm is ignored unless the discrepancy significantly contributes to the spinal deformity. When the leg length discrepancy is greater than 2 cm, an appropriately thick lift is placed under the shorter leg.
Coronal Alignment Angles and Displacements
By convention, coronal angles have a positive value. Scoliotic curves are named for the convexity to the right or left. Coronal angulation of the head, shoulders, or pelvis is named for the elevated side: right is right up and left is left up. Schematic illustrations of representative clinical and radiographic measuring techniques for the coronal spinal alignment angles and displacements are detailed in Figures 144-1 and 144-2 .
Regional Spinal Alignment
The shoulder tilt angle is defined as the angle subtended by a horizontal reference line and a line drawn through the right and left coracoid processes. Trunk asymmetry (distortions of the torso) is measured using a scoliometer with the patient in a forward bend position (standing with feet together, the knees comfortably extended, the hips and spine flexed, and the arms dependent with fingers and palms opposed). The angle of trunk inclination is the angle between a horizontal reference line and the plane across the back at the greatest elevation of a rib prominence or lumbar prominence. In contrast to radiographic measurements, the shoulder tilt angle and angle of trunk inclination are clinical measurements of the effect of regional spinal deformity on trunk symmetry.
Occipitocervical (O-C2) curves are defined as having an apex from the occiput to C2; a coronal occipital reference line and the caudal end vertebrae are defined for measuring the Cobb angle.
Cervical coronal curves are defined as having an apex from the C2-3 disc to the C6-7 disc and measured by the Cobb method from the end vertebrae.
Cervicothoracic junction angles are defined from C7 to T1. Cervicothoracic coronal curves are defined as having an apex from C7 to T1 and measured by the Cobb method from the end vertebrae.
Proximal thoracic (T1-2 disc to T5 disc), main thoracic (T5-6 disc to T11-12 disc), thoracolumbar (T12-L1), lumbar (L1-2 disc to L4-5 disc), and lumbosacral (L5-S1) coronal curves are defined as having an apex in the above regions or zones and measured by the Cobb method from the end vertebrae.
The end vertebrae for all coronal curves are defined as the most rostral and caudal vertebrae that maximally tilt into the concavity of the curve. The end vertebrae define the ends of the scoliotic curve. The rostral end vertebra is the first vertebra in the rostral direction from a curve apex whose superior surface is tilted maximally toward the concavity of the curve. The caudal end vertebra is the first vertebra in the caudal direction from a curve apex whose caudal surface is tilted maximally toward the concavity of the curve. The apical vertebra or disc of a curve is defined as the most horizontal and laterally deviated vertebra or disc of the curve. Apical vertebral translation is defined as the horizontal distance measured from the C7 plumb line to the center of the apical vertebral body or disc for proximal thoracic and main thoracic curves and from the central sacral vertical line (CSVL) to the center of the apical vertebral body or disc for thoracolumbar and lumbar curves. The CSVL is defined as a vertical reference line drawn through the center of the S1 end plate. Apical vertebral rotation (AVR) is defined by the Nash-Moe classification system. (Because AVR is defined on anteroposterior radiographs, AVR is included with the coronal alignment.) Lateral olisthesis is defined by a modified Meyerding classification system. For lumbosacral coronal curves, the apical vertebra or disc is defined from L5 to S1; the rostral end vertebra and a horizontal reference line are defined for measuring the Cobb angle (on supine side-bending radiographs, the horizontal reference line may be reconstructed from the standing radiographs).
Pelvic Alignment
Pelvic alignment and morphology are defined by the pelvic obliquity and leg length discrepancy. Pelvic obliquity is defined most frequently as the angle subtended by a horizontal reference line and a line drawn tangential to the top of the crests of the ilium or the base of the sulci of the S1 ala. Pelvic obliquity may result from an intrinsic sacropelvic deformity, leg length discrepancy, or a combination of both. Leg length discrepancy is defined as the vertical distance measured between horizontal lines drawn tangential to the top of the right and left femoral heads.
Global Spinal Alignment
Head tilt is defined by the interpupillary angle (IPA). The IPA is defined as the angle subtended by a horizontal reference line and the interpupillary line. The interpupillary line is defined by a line drawn though the center of the right and left pupils. In contrast to radiographic measurements, the IPA is a clinical measurement of total coronal deformity of the spine and the effect on horizontal gaze.
Coronal spinal balance is defined from the center of C7 and the midpoint of the thoracic trunk to the sacrum. The C7-S1 coronal vertical axis (CVA) is defined as the horizontal distance measured from a vertical plumb line centered in the middle of the C7 vertebral body to the CSVL. The C7-S1 CVA has a positive value when the vertical plumb line is right of the CSVL and a negative value when the vertical plumb line is left of the CSVL. The thoracic trunk–S1 coronal vertical axis (TT-S1 CVA; also known as thoracic trunk shift) is defined as the horizontal distance measured from a vertical plumb line centered at the midpoint of the thorax to the CSVL. The TT-S1 CVA is measured at the midpoint between the rib cage on the left and the rib cage on the right at the level of the main thoracic apical vertebra; if there is no main thoracic apical vertebra, the TT-S1 CVA is measured at the level of T9. The TT-S1 CVA has a positive value when the vertical plumb line is right of the CSVL and a negative value when the vertical plumb line is left of the CSVL.
Sagittal Alignment Angles and Displacements
By convention, kyphosis has a positive value and lordosis a negative value. Schematic illustrations of representative clinical and radiographic measuring techniques for the sagittal and coronal spinal alignment angles and displacements are detailed in Figures 144-3 and 144-4 .
Regional Spinal Alignment
Occipitocervical junction angles are defined from the occiput to C2. The occiput-C2 angle is defined as the angle subtended by the McGregor line and a line drawn parallel to the inferior end plate of C2. The McGregor line is drawn from the dorsal rostral aspect of the hard palate to the most caudal point on the midline of the occipital curve. The C1-2 angle is defined as the angle subtended by a line drawn parallel to the inferior aspect of C1 and a line drawn parallel to the inferior end plate of C2.
Cervical lordosis angles are defined from C2 to C7. The C2-7 angle is defined as the angle subtended by a line drawn parallel to the dorsal border of the C2 vertebral body and a line drawn parallel to the dorsal border of the C7 vertebral body.
Cervicothoracic junction angles are defined from C6 to T2, as measured using the Cobb method. The C6-T2 angle is measured from the superior end plate of C6 to the inferior end plate of T2.
Thoracic kyphosis angles are defined from T1 to T12, as measured using the Cobb method. Total thoracic kyphosis is measured from the superior end plate of T1 to the inferior end plate of T12. The proximal thoracic kyphosis is measured from the superior end plate of T1 to the inferior end plate of T5. The main thoracic kyphosis is measured from the superior end plate of T4 to the inferior end plate of T12.
Thoracolumbar junction angles are defined from T10 to L2, as measured using the Cobb method. The T10-L2 angle is measured from the superior end plate of T10 to the inferior end plate of L2.
Lumbosacral lordosis angles are defined from T12-L1 to S1, as measured using the Cobb method. Total lumbosacral lordosis is measured from either the caudal end plate of T12 or the rostral end plate of L1 to the rostral end plate of S1. Lumbar lordosis is measured from the rostral end plate of L1 to the caudal end plate of L5.
Lumbosacral junctional angles are measured from L4 to S1, using the Cobb method. The L4-S1 angle is measured from the rostral end plate of L4 to the superior end plate of S1. The L4-5 angle is measured from the rostral end plate of L4 to the rostral end plate of L5. The L5-S1 angle is measured from the superior end plate of L5 to the rostral end plate of S1.
Ventral and dorsal olisthesis are defined by a modified Meyerding classification system.
Pelvic Alignment
Pelvic morphology and rotation are defined by the pelvic incidence, pelvic tilt, and sacral slope. Pelvic incidence (PI) is a constant value unaffected by body posture. The PI is defined as an angle subtended by a line drawn from the hip axis to the midpoint of the sacral end plate and a line perpendicular to the center of the sacral end plate. The hip axis (HA) is defined as the midpoint between the approximate centers of both femoral heads. As PI increases, lumbosacral lordosis must increase to maintain balanced sagittal global spinal alignment. In contrast to the PI, the sacral slope (SS) and pelvic tilt (PT) are posturally dependent values and change with rotation of the pelvis on the hip axis. SS is defined as the angle subtended by a horizontal reference line and the sacral end plate. PT is defined as the angle subtended by a vertical reference line through the HA and a line drawn from the midpoint of the sacral end plate to the HA. PT has a positive value when the midpoint of the sacrum is dorsal to the vertical reference line and a negative value when the midpoint of the sacrum is ventral to the vertical reference line. Geometrically, these pelvic angles produce the following equation: PI = SS + PT. The pelvis rotates on the HA to help maintain balanced sagittal global spinal alignment.
Global Spinal Alignment
Chin-brow to vertical angle is defined as the angle subtended by a vertical reference line and a line drawn parallel to the chin and brow with the neck in neutral or fixed position and the knees and hips extended. In contrast to the radiographic measurements, the chin-brow to vertical angle is a clinical measurement of the total sagittal deformity of the spine and the effect on horizontal gaze.
Sagittal spinal balance is defined from C7, T1, and T9 to the sacrum or HA. The C7-S1 sagittal vertical axis (SVA) is defined as the horizontal distance measured from a vertical plumb line centered in the middle of the C7 vertebral body to the dorsal rostral corner of the S1 end plate. The C7-S1 SVA has a positive value when the vertical plumb line is ventral to the sacral reference point and a negative value when the vertical plumb line is dorsal to the sacral reference point. The T1-HA sagittal tilt angle (STA) is defined as the angle subtended by a vertical reference line through the HA and a line drawn from the midpoint of the T1 vertebral body to the HA. The T9-HA STA is defined as the angle subtended by a vertical reference line through the HA and a line drawn from the midpoint of the T9 vertebral body to the HA. The T1-HA STA and T9-HA STA have a positive value when the T1 or T9 midpoint is ventral to the HA vertical reference line and a negative value when the T1 or T9 midpoint is dorsal to the HA vertical reference line.
Defining Spinal Deformity
Deformity is defined as a deviation from the normal shape or size. The eight critically important characteristics of a spinal deformity include patient age; spinal abnormality, including neurologic compromise (e.g., radiculopathy, myelopathy); deformity curve location; pattern; magnitude; flexibility; pelvic alignment; and global spinal alignment. Spinal deformity may be the primary or a secondary spinal disorder. The deformity may be idiopathic or secondary to known spinal abnormality (e.g., neuromuscular, degenerative, osteoporotic, infectious, traumatic). Spinal deformity may occur in a single plane or in a combination of three planes: coronal, sagittal, and axial. The three basic types of spinal deformity include scoliosis, kyphosis, and lordosis. Each may occur singly or in combination. In combination, coronal and sagittal deformity produces scoliokyphosis and scoliolordosis. Because the human condition is in part defined by the ability to comfortably stand upright and because treatment of many patients with spinal disorders is directed at restoring this condition, spinal deformity needs to be defined in relation to NUSA from the occiput to the pelvis in asymptomatic individuals.
Regional alignment is measured for spinal regions with distinct alignment and biomechanical properties: occipitocervical (OC), cervical (C), cervicothoracic (CT), proximal thoracic (PT), main thoracic (MT), thoracolumbar (TL), lumbar (L), lumbosacral (LS). Spinal deformity is defined by one major structural deformity curve and minor structural deformity curves. Structural curves are defined by their location, magnitude, and flexibility. Deformity major and minor structural deformity curves are classified as scoliotic, kyphotic, lordotic, scoliokyphotic, or scoliolordotic. The major and minor structural curves form a pattern further defining the spinal deformity. The deformity is then finally defined by pelvic alignment and global spinal alignment.
Classification Systems for Thoracic-Lumbar Spinal Deformity
Classification of deformity serves multiple functions. In classifying deformity, a common terminology is established for systematic characterization, allowing clear and concise communication among care providers and more uniform reporting in research. Classification systems can provide a guide for treatment, as in King’s initial attempt to guide selection of thoracic fusion levels for thoracic scoliosis. Closely related to this factor, homogenous cohorts can be compared for outcomes and most beneficial interventions. Natural history studies are also aided by classification systems, enhancing the understanding of spinal pathology. Multiple classification systems are discussed subsequently. For a full discussion on the particulars of stratification within each system, the reader is directed to the original publications referenced.
As previously stated, eight critically important characteristics of a spinal deformity should be considered in classification. Simpler classification systems are easier for the physician to use in clinical practice but often incorporate fewer of the critically important spinal deformity characteristics. Although more complicated classification systems incorporate more of the critically important spinal deformity characteristics, these systems are often more complicated for the physician to incorporate into clinical practice.
King and colleagues, in 1983, established the first formal classification system for adolescent idiopathic scoliosis that gained widespread use among spine surgeons ( Table 144-1 ). Of the eight critically important characteristics of a spinal deformity, the King classification system is limited to adolescent idiopathic scoliosis and only evaluates scoliotic curves in the coronal plane. The classification system focuses on thoracic curves and combined thoracic-lumbar double curves. Scoliotic deformity curve location, pattern, magnitude, and flexibility are included. Pelvic alignment and global spinal alignment are not included. A significant force in the development of this initial spinal deformity classification was the intention to define levels of fusion. This systematic approach served as a baseline from which to begin a more scientific understanding of deformity. The widespread application of the King classification system eventually led specialists to recognize its shortcomings. Classification information was based only on coronal images. The neglect of sagittal and axial alignment resulted in correction of spinal deformity in the coronal plane, often ignoring the sagittal and axial plane and producing sagittal deformity, namely the flatback syndrome. At the time of the King classification system development, Harrington rods were the primary instrumentation device and had limited ability to correct or control sagittal curves. Newer three-dimensional segmental instrumentation techniques, including hooks and pedicle screws, came to highlight the three-dimensional aspect of spinal deformity correction that was not fully addressed by the King classification. The King classification has poor applicability to three-dimensional correction of spinal deformity.
| Group | Criteria |
|---|---|
| Type I | S-shaped curve in which both thoracic curve and lumbar curve cross midline Lumbar curve larger than thoracic curve on standing radiograph Flexibility index a negative value (thoracic curve greater than or equal to lumbar curve on standing radiograph, but more flexible on side-bending view) |
| Type II | S-shaped curve in which thoracic curve and lumbar curve cross midline Thoracic curve greater than or equal to lumbar curve Flexibility index ≥ 0 |
| Type III | Thoracic curve in which lumbar curve does not cross midline (so-called overhang) |
| Type IV | Long thoracic curve in which L5 is centered over sacrum but L4 tilts into long thoracic curve |
| Type V | Double thoracic curve with T1 tilted into convexity of upper curve Upper curve structural on side-bending view |
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