Sagittal alignment

Why it matters

Standing upright costs little muscular effort when the trunk's centre of gravity sits over the hips. Pelvic tilt and the spinal parameters, sacral slope, lumbar lordosis and thoracic kyphosis, adapt to position it so that it is supported by the femoral heads with minimal muscular effort ^[1].

What it is

Seen from the side, the adult column has four curves: cervical and lumbar curves that are concave backward, and thoracic and sacrococcygeal curves kept from the fetus ^[2]. The pelvis is part of the same chain ^[1]:

• Pelvic incidence is a morphological angle, fixed for a given person, with a mean value of 50 to 55 °^[1].
• Sacral slope is a functional angle that changes with position: higher standing than seated ^[1].
• Pelvic tilt describes the pelvis rotating around the axis through the two femoral heads, forward or backward ^[1].

Pelvic incidence, pelvic tilt and sacral slope^[1]

$$\mathrm{PI} = \mathrm{PT} + \mathrm{SS}$$

Pelvic incidence is a fixed property of a person’s pelvis. It is shared between two angles that change with posture: pelvic tilt, how far the pelvis is rotated back, and sacral slope, how steep the top of the sacrum is. When the pelvis tilts back, the sacral slope falls by the same amount.

Variables of Pelvic incidence, pelvic tilt and sacral slope

Symbol	Meaning	Unit
$\mathrm{PT}$	pelvic tilt	°
$\mathrm{SS}$	sacral slope	°
$\mathrm{PI}$	pelvic incidence	°

Valid when

• Angles measured on a lateral radiograph that shows the femoral heads and the top of the sacrum
• Pelvic incidence is constant for an adult; tilt and slope change with posture, sitting or standing

Worked example^[1]

A pelvic tilt of 15° and a sacral slope of 40° add up to an incidence of 55°.

pt = 15 °, ss = 40 ° → pi = 55 °

pelvic tilt (°)sacral slope (°)

pelvic incidence: 55 °

How it works

In standing, the pelvis tilts forward as a whole and the top of S1 makes an angle of about 35 to 45 °^[1] with the horizontal ^[1]. Seated, the pelvis tilts back and the sacral slope falls, to 20 to 25 °^[1] on average ^[1]. People with a high pelvic incidence have a greater theoretical lumbar lordosis ^[1].

When it does not behave

When alignment is lost, the body compensates by rotating the pelvis, changing the lumbar lordosis and flexing the knees and ankles, at the cost of more energy ^[3]. With fixed imbalance, compensation can include pelvic retroversion and hip and knee flexion ^[3].

How we see and measure it

The angles are measured on lateral radiographs; the sideways curve of scoliosis is measured with the Cobb angle ^{[1, 4]}.

Common misconceptions

Check yourself

Pelvic tilt is 20° and sacral slope 35°. What is the pelvic incidence?

Their sum, because incidence equals tilt plus slope: fifty-five degrees ^[1].

What happens to sacral slope when you sit?

It decreases as the pelvis tilts back ^[1].

Read next

• Alignment and load lab, which measures these angles on the site's one 3D model.
• The vertebral column.