*2.4.3. SVA*

Sagittal vertical axis is used to measure the degree of forward or backward angulation of a patient's posture. SVA is one of the easiest radiological parameters to measure, since the

**Figure 2.** Lumbar lordosis measurement: From inferior end plate of T12 and superior end place of sacral. This patient has a lumbar lordosis of 56.2°.

femoral heads do not have to be visualized. For this reason, the authors have found it to be a clinically relevant and useful measure to incorporate into clinical practice. The steps to measure SVA on a standard lateral view radiograph are as follows: (1) Identify the center of C7—inferior end plate, and draw a line straight down perpendicular to the bottom of the film. (2) Draw a vertical line from the posterior-superior corner of the sacrum. (3) Measure the distance between lines 1 and 2 [21]. A positive number indicates that C7 is in front of sacrum. On a clinical examination, the patient's head is likely to be in front of the torso as well as his trunk in a more forward flexed position. A negative number indicates that a C7 is behind the sacrum. This type of posture is often called swayback. Clinically, the patient likely stands with their pelvis more in front than their head (**Figure 3A** and **B**).

*2.4.2. Lumbar lordosis*

has a lumbar lordosis of 56.2°.

*2.4.3. SVA*

rior end plate of S1 (**Figure 2**).

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Lumbar lordosis is measured by the angulation from the inferior angle of T12 and the supe-

Sagittal vertical axis is used to measure the degree of forward or backward angulation of a patient's posture. SVA is one of the easiest radiological parameters to measure, since the

**Figure 2.** Lumbar lordosis measurement: From inferior end plate of T12 and superior end place of sacral. This patient

**Figure 3.** SVA—Sagittal vertical axis: The steps to measure SVA are as follows: (1) Identify the center of C7—inferior end plate, and draw a line straight down perpendicular to the bottom of the film. (2) Draw a vertical line from the posteriorsuperior corner of the sacrum. (3) Measure the distance between lines 1 and 2. (A) This individual has a (+) SVA of 169.98 mm. (B) This indivdual has a (-) SVA of 62.6mm.

#### *2.4.4. Pelvic tilt*

Radiological pelvic tilt is a sagittal measurement that can be assessed on a lateral radiograph. Refer to **Figure 4** for specifics. The following are the measurement steps: (1) Draw a line from the midpoint of the sacral end plate running perpendicular down to the bottom of the X-ray. (2) Draw a line from the center of the femoral heads to the center of the sacrum. (3) The angle between these two lines is the pelvic tilt. Also, pelvic tilt + sacral slope (SS) = pelvic incidence (**Figure 4**) [21].

#### *2.4.5. Sacral slope*

The last radiological measure discussed here is the least discussed as it is often difficult to measure. However, its importance is vital to understanding the relationship of the other parameters. Refer to **Figure 5** for specifics. The steps to measurement are as follows: (1) Draw a line along the superior sacral end plate. (2) Draw a line from the anterior superior edge parallel to the bottom of the X-ray. (3) This angle is the sacral slope. Pelvic incidence = sacral slope + pelvic tilt [22].

### **2.5. Inter-relationships between spinopelvic parameters**

A significant chain of interdependence exists between pelvic and spinal parameters. Pelvic incidence, as previously stated, is an independent and anatomic parameter that determines pelvic orientation and the optimal size of lumbar lordosis [19]. In practice, the PI of an individual is correlated together with his or her sacral slope. In this section, both SS and PI will be described in relation to LL and sagittal balance as well as the consequences of mismatch between the pelvic parameters and LL for the adult individual with spinal deformity.

**Figure 4.** Pelvic tilt. (1) Draw a line from the midpoint of the sacral end plate running perpendicular down to the bottom of the X-ray. (2) Draw a line from the center of the femoral heads to the center of the sacrum. (3) The angle between these two lines is the pelvic tilt. This individual's pelvic tilt measures 17°.

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**Figure 5.** Sacral slope: (1) Draw a line along the superior sacral end plate. (2) Draw a line from the anterior superior edge parallel to the bottom of the X-ray. (3) This angle is the sacral slope. This individual's sacral slope measures 24°.

#### *2.5.1. Influence of sacral slope on global sagittal alignment*

*2.4.4. Pelvic tilt*

120 Innovations in Spinal Deformities and Postural Disorders

(**Figure 4**) [21].

*2.4.5. Sacral slope*

slope + pelvic tilt [22].

**2.5. Inter-relationships between spinopelvic parameters**

two lines is the pelvic tilt. This individual's pelvic tilt measures 17°.

Radiological pelvic tilt is a sagittal measurement that can be assessed on a lateral radiograph. Refer to **Figure 4** for specifics. The following are the measurement steps: (1) Draw a line from the midpoint of the sacral end plate running perpendicular down to the bottom of the X-ray. (2) Draw a line from the center of the femoral heads to the center of the sacrum. (3) The angle between these two lines is the pelvic tilt. Also, pelvic tilt + sacral slope (SS) = pelvic incidence

The last radiological measure discussed here is the least discussed as it is often difficult to measure. However, its importance is vital to understanding the relationship of the other parameters. Refer to **Figure 5** for specifics. The steps to measurement are as follows: (1) Draw a line along the superior sacral end plate. (2) Draw a line from the anterior superior edge parallel to the bottom of the X-ray. (3) This angle is the sacral slope. Pelvic incidence = sacral

A significant chain of interdependence exists between pelvic and spinal parameters. Pelvic incidence, as previously stated, is an independent and anatomic parameter that determines pelvic orientation and the optimal size of lumbar lordosis [19]. In practice, the PI of an individual is correlated together with his or her sacral slope. In this section, both SS and PI will be described in relation to LL and sagittal balance as well as the consequences of mismatch

**Figure 4.** Pelvic tilt. (1) Draw a line from the midpoint of the sacral end plate running perpendicular down to the bottom of the X-ray. (2) Draw a line from the center of the femoral heads to the center of the sacrum. (3) The angle between these

between the pelvic parameters and LL for the adult individual with spinal deformity.

In a well-balanced spine, the SS is between 35 and 45° and the LL has an apex at L3–L4 [23] (**Figure 6**). In an individual with a low SS angle (<35° ), a regional hypolordotic lumbar deformity with a compensatory hypo-kyphosis or normal thoracic and lumbar apex at L5 may be observed [23]. *Regional deformity* is defined as sagittal kyphotic misalignment that affects a limited number of segments of the spine (i.e., the lumbar spine, the thoracic spine, the thoracolumbar junction (TLJ), or the lower lumbar spine). *Compensatory mechanisms* are changes in the sagittal alignment of spinal or non-spinal segments, different from those involved in regional deformity, to restore the

**Figure 6.** Drawings of sacral slope and sagittal spinal alignments. Left: Sacral slope (SS) < 35°, apex of lumbar lordosis (LL) at middle L5, the spine is hypolordotic and relatively normal kyphotic; middle: 35° < SS < 45°, apex of LL at middle L3–L4, the spine is well balanced; right: SS > 45°, apex of LL at base L3, the spine is hyperlordotic and hyper-kyphotic.

alignment of the gravity line or the horizontal gaze. Compensatory mechanisms need active muscle contraction by the subject [24, 25]. When the SS is high (>45° ), a regional hyper-lordosis lumbar deformity along with compensatory thoracic hyper-kyphosis may commonly be observed [23].
