Sagittal alignment

Normal alignment

is due to the normal cervical lordosis, thoracic kyphosis and lumbar lordosis.

Vertical axis runs from centre of C2 → anterior border of T7 → middle of T12/L1 disc → Post of L3 vertebral body → crosses postero-superior corner of sacrum

Sagittal parameters that changes with age

Cervical spine alignment

First measure cervical lordosis (CL), the local measurement of cervical deformity

Second measure the cervical spine in the sagittal plane, the global measurement of cervical deformity

LL, PI, TK, CL relationships

Essentially: A large PI → Large LL → Large TK → Large CL

LL is a result of TK and PI

Because PI is a fixed parameter and TK has little flexibility.

CL was correlated with TK showing that as TK increases, CL also increases.

A change in CL is not large enough to maintain the head over the pelvis, but it does provide adequate maintenance of horizontal gaze.

CL was also found to correlate with SVA, PT

An illustration of the relationship between TIA, T1 slope, and CL.
A small TIA yields a low T1 slope and therefore less CL is required to balance the head over the thoracic inlet and trunk.
Conversely, a large TIA yields a greater T1 slope so that a greater magnitude of CL is required to balance the head over the thoracic inlet and trunk.
CL indicates cervical lordosis; TIA, thoracic inlet angle.

Compensatory mechanism

Basic measurements

Skull base measurements

See Basilar invagination

Chamberlain’s line (number 2)

extends from the hard palate to the opisthion (back of the foramen magnum)
On xray

the tip of the dens should be =<3 mm above this line.

If Dens 6mm above this line = definitely pathology

On CT and MRI the normal odontoid tip is 1.4mm(± 2.4) below the line
Seldom used as opisthion is hard to see on Xray and may be invaginated

McGregor’s Slope (MGS)

Measurement

is the angle between the posterior edge of the hard palate ↔ the caudal aspect of the occiput
McGregor’s line (number 1)

extends from the hard palate to the most caudal point of the occipital curve
dental tip should by less than 5 mm above this line.
McGregor has modified chamberlain's line for when the opisthion cannot be identified.
On CT and MRI the normal odontoid tip is 0.8mm (± 2.4) above the line

A surrogate for CBVA, which can be diffcult to visualize on standing radiographs.

Clinical significance

MGS < 2.16 degrees has been suggested as a goal for for CSD correction,
MGS of < 0 has been proposed as modier for CSD classication.
A high MGS is pathologic, as MGS ≥ 20 is an inclusion criteria for cervical deformity in ISSG studies.
Passias 2021 modifier

Low: >-9° and <0°
Moderate: -12° to -9° or 0° to 19°
Severe: <-12° or >19°

McRae’s line (Number 3)

from the basion (front of the foramen magnum) to the opisthion.

Dens should not advance above this line .

McRae’s line is also used to diagnose Chiari 1 malformations where the cerebellar tonsils, which should normally be above this line, are seen to descend more than 3mm in children and 5 mm in adults.

The mean position of the odontoid tip below the line is

5mm (± 1.8mm SD) on CT
4.6mm (± 2.6mm SD) on MRI

Wackenheim’s clivus-canal line (WCCL):

the odontoid should be tangential to or below the line that extends the course of the clivus (the clivus baseline).

If the clivus is concave or convex, this baseline is drawn to connect the basion to the base of the posterior clinoids on the clivus26

Klaus index (Number 4)

distance between tip of dens and the tuberculum–cruciate line between tuberculum (T) and internal occipital protuberance (IP).

This measures depth of the posterior fossa

A normal height is 40-41 mm. A decreased Klaus height index is seen in basilar invagination.

Fischgold’s Digastric line:

Line joining the digastric notches.
The normal distance from this line to the middle of the atlantooccipital joint is 10mm (decreased in BI).
No part of the odontoid should be above this line.
More accurate than the bimastoid line (FBML)

Fischgold’s Bimastoid line:

Line joining the tips of mastoid processes
The odontoid tip averages 2mm above this line (range: 3mm below to 10mm above) and this line should cross the atlantooccipital joint

the tip of the odontoid process projects normally not more than 10 mm above this line

Clival angle:

Normal clival angle (top) measured by the NTB angle of Welcker joining the nasion (N), tuberculum (T) and basion (B).

The angle should be < 130°.

Platybasia (middle) is marked by an increased NTB angle. This raises the basion and forces the foramen magnum plane (dotted line) to tilt upwards. (skull base angle>143°)

The same upward tilt of this plane also occurs with a short clivus (lower)

Cervicomedullary angle (CMA):

The angle between a line drawn through the long axis of the medulla on a sagittal MRI and a line drawn through the cervical spinal cord.

Normal CMA is 135–170°.

A CMA< 135° correlates with signs of

Cervicomedullary compression
Myelopathy
C2 radiculopathy

Disease

Chairi 1 related basilar invagination <125 deg

Clival axial angle

Bollo et al. identified radiological parameters that increased the likelihood of occipito-cervical fixation being required.

descent of the obex as well as the tonsils, below the plane of the foramen magnum (Chiari 1.5)
basilar invagination,
A clivo-axial angle < 125 degrees

Angle determined from the intersection of a line along the slope of the clivus (Wackenheims line) and a line drawn from the posteroinferior point of the C2 vertebral body up along the odontoid (posterior spinal line).

Ranawat’s line

Perpendicular distance between the centre of the sclerotic ring of C2 and a line drawn along the axis of the C1 vertebra.

Normally 17 mm; a distance less than 13 mm would suggest basilar invagination

Rheumatoid Cervical Spondylitis - Spine - Orthobullets

Grabb Oakes Measurement

Chiari 1

Midline MRI T2 image used

Line drawn from Basion to the inferior(bottom), posterior aspect of the C2 bone.

Perpendicular line drawn to the dura

Yellow line is the Grab oakes measurement

A normal Grabb Oakes Measurement is 9mm or less

Cervical Lordosis (CL)

Normal

Range 20-40°

Mean total CL is approximately −40° (neg; Lordosis), with, on average, the occiput-C1 segment being kyphotic.

The largest percentage (75%–80%) of cervical standing lordosis is at C1–C2

COG of the head sits almost directly above the centers of the C1 and C2 vertebral bodies there is a positive correlation with CL and increasing age

Only 6° (15%) of lordosis occurs at the lowest 3 cervical levels (C4–C7).

Not all flat cervical curvature or a kyphotic cervical spine is abnormal

C2 Slope

Passias 2021 modifiers

Low: <33°
Moderate: 33° to 49°
Severe: >49°

Clinical significance

Surrogate for T1-CL Mismatch: It acts as a simplified, single-value measure reflecting the degree of T1 slope-cervical lordosis (T1-CL) mismatch.
Correlation with HQROLs:

A greater C2 slope (specifically 17–38 degrees) is associated with worse health-related quality of life (HQROLs).

Classification Modifier:

It has been proposed as a novel modifier to the Ames classification system for CSD.

Surgical Target for Correction:

A goal correction of C2 slope to less than 10 degrees has been proposed as a surgical target.

Prediction and Prevention of DJK:

Failure to correct C2 slope to less than 10 degrees is a radiographic predictor of distal junctional kyphosis (DJK).
Achieving this target is associated with lower rates of DJK.
Some surgical approaches prioritise correcting C2 slope to less than 10 degrees to achieve optimal clinical outcomes.

Benzel: when positioning head for CVJ fusion make sure C2 slope is 90 deg to the floor.

Insight into Deformity Drivers:

It provides insight into the primary "driver" of the cervical deformity; for example, a low C2 slope with high T1 slope suggests a thoracolumbar issue, while the opposite indicates a primary cervical pathology.

Measured using

Cobb angles (most common clinically, despite potential over/underestimation)

Measures the lordosis from either C1–C7 or C2–C7

First line is drawn either parallel to the inferior endplate of C2 OR extending from the anterior tubercle of C1 to the posterior margin of the spinous process
Second line drawn parallel to the inferior endplate of C7

Cobb C1–C7 angle overestimates the CL
Cobb C2–C7 angle underestimates the CL

Harrison posterior tangent method

Drawing parallel lines to the posterior surfaces of all cervical vertebral bodies from C2–C7 and then sums the segmental angles for an overall cervical curvature angle/2
Harrison method may provide the best estimate of lordosis versus Cobb

Jackson physiological stress lines

Drawing a parallel lines to the posterior surface of the C7 and C2 vertebral bodies and measuring the angle between them

C2–T3 SVA Angle

Measures both cervical and upper-thoracic alignment,

Unlike the traditional C2-7 SVA

Higher values corresponding to more lordosis.

Clinical significance

Passias 2021

Low: >-25°
Moderate: -35° to -25°
Severe: <-35°

C2-T3 angle <−25 degrees is associated with more severe myelopathy per the modi ed Japanese Orthopaedic Association (mJOA) scale.
A high C2-T3 angle may signify compensatory cervical lordosis in the setting of insufficient sagittal correction and may predict DJK.

Thoracic kyphosis

Average 35°

Normal range 20-50°

Measured from T2-T12

By itself does not tell you the distribution of the magnitude of the curvature of the rest of the spine

T1 Slope

The T1 slope will determine the amount of subaxial lordosis required to maintain the COG of the head in a balanced position

Varies depending on Global spinal alignment as measured by

SVA AND
Inherent upper thoracic kyphosis (TK).

In patients with scoliosis, the T1 slope has been shown to correlate directly with SVA measured from the C2 odontoid plumb line to provide a measure of overall sagittal alignment.

Kim 2013: High preoperative T1 slope were more likely to have postoperative (Laminoplasty for cervical myelopathy) kyphotic changes at 2-years

Tamai 2018: Not easily visualised on xray so use C7 slope as surrogate

As only 18% of the T1 slope is visible

T1 Slope - C2-C7 lordorsis

Similar to PI - LL

Score	TS-CL
0	< 15°
1	15°–20°
2	> 20°

T1 slope minus Cervical Lordosis (TS-CL)

Low: <26°
Moderate: 26° to 45°
Severe: >45°
A postoperative T1-CL > 20 degrees correlates with greater postoperative cSVA, indicating higher risk of persistent deformity

C2–C7 Lordosis

Low: >3°
Moderate: -21° to 3°
Severe: <-21°

Lumbar lordosis

Average 60°

Normal range 20-80°

Measured from superior endplate of L1-S1

75% of lumbar lordosis occurs between L4 and S1

47% occurring at L5/S1

by itself does not tell you the distribution of the magnitude of the curvature of the rest of the spine

L4-S1 lordosis

Used to measure lumbar distribution index (L4-S1 lordosis/L1-S1 lordosis × 100).

to quantify how much of the lumbar lordosis is low in the whole lumbar lordosis

L4-S1 lordosis / total lordosis

Ideally 50-80% of the lumbar lordosis is located in the L4-S1 level

(which is around 34 deg as the pelvic incidence is on average around 50 deg)

Correlation between pelvic incidence and lumbar lordosis

Old patients have OA in hips and cant modify their Pelvic tilt to compensate for their positive sagittal balance

Common degenerative conditions, such as lumbar disc disease, will result in loss of anterior spinal height, while the height of the posterior elements remains unchanged.

This leads to loss of lumbar lordosis and a sagittal plane deformity.
Lumbar spinal fusion and decompression without restoration of the normal lumbar lordosis may improve a patient’s radicular leg pain, but may leave them with a fixed sagittal deformity and a suboptimal surgical outcome.

Presenti 2018:

The Amount of Proximal Lumbar Lordosis Is Related to Pelvic Incidence
PI influences only the proximal part of the lordosis, but not the distal part in an asymptomatic adult population

Lumbar sacral kyphosis

By Dubousset

Used in high grade spondylolisthesis due to the fact that the S1 endplate is a dome and not a flat surface

between L5 ? superior endplate and S1 dorsum

Relative spinopelvic parameters

C2 tilt and C2 pelvic angle (PA)

Goal C2 Tilt ~ 0

as C2 is normally is right on top the hips and knees

C2 PA increases then C2 tilt will be anterior to hips and knees.

One of the best single measuring tool because

it is angular
it looks at the whole spine deformity

Cervical sagittal vertical axis (SVA)

The gravity line (COG SVA)

Measurements

Global

Center of gravity (COG) of the head plumb line ↔ posterior superior corner of sacrum

Head COG: On lateral radiographs, anterior portion of the external auditory canal

Regional

Center of gravity (COG) of the head plumb line ↔ posterior superior corner of C7

C2 SVA

Measurement

Global

Centroid of C2 (or odontoid) plumb line ↔ posterior superior corner of sacrum

Regional

C2-7 SVA

Centroid of C2 (or odontoid) plumb line ↔ posterior superior corner of C7
Tang 2015: C2–7 SVA threshold of 4 cm was found to correlate with moderate disability based on the NDI

The T1 slope is a predictor of C2–C7 SVA.

Has been directly correlated with HRQOL (SF36)

Larger C2 SVA poorer HRQOL
Increasing C2 SVA (>40 mm) is correlated with worse outcomes as assessed by the Neck Disability Index (NDI).

Normal Cervical SVA Values in Asymptomatic Adults

Odontoid marker at C7	15.6 ± 11.2 mm
Odontoid marker at sacrum	13.2 ± 29.5 mm

C7 SVA

Distance from a C7 centroid plumb line ↔ posterior superior corner of sacrum

Not useful for alignment

Does not consider compensation

May be useful to track an individual over time

This is used since we cannot easily measure the centre of gravity directly, and so substitute the relative positions of the centre of the C7 vertebral body and the posterior limit of the S1 endplate.

Patients whose overall spinal balance lies outside this range are said to have decompensated sagittal deformity → this group has been shown to have lower quality of life scores.

Negative sagittal balance

the axis is posterior to the sacrum and occurs in patients with lumbar hyperlordosis

Positive sagittal balance

The axis is anterior to the sacrum and occurs in patients with hip flexion contracture or flat-back syndrome
Highly correlates with quality of life outcomes

T1 pelvic angle

T1 pelvic angle = T1 spinopelvic inclination (T1SPi) + pelvic tilt (PT)

Pros

accounts for both global mal-alignment and compensation through pelvic retroversion.
correlates with health-related quality of life in patients with adult spinal deformity.
unlike sagittal vertical axis, it does not vary on the basis of the extent of pelvic retroversion or patient support in standing.

Can be useful as a preoperative planning tool, with a target T1 pelvic angle of <14°

this is however not always true as T1 pelvic angle varies with pelvic tilt

Measurement

Angle between

Line from the femoral head axis ↔ centroid of T1 AND
Line from the femoral head axis ↔ middle of the S1 superior end plate

Protopsaltis 2014.pdf

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Vertebral pelvic angles

An expansion of the T1 pelvic angle.

Measurement

Angle formed between lines drawn from

The center of the femoral head to the center of the vertebral bodies at (TO)

The midpoint of the superior S1 endplate.

Describe the shape and the position of the hips

L1 PA = 50% of PI - 20

A rule of thumb

T4 PA in general close proximity to L1

T4 ventral to L1 if low PI
T4 dorsal to L1 if high PI

Global sagittal axis

Aka Global tilt
Angle between the line from midst of C7 to midpoint of femoral condyles and the line from this point to the mid point of S1

Odontoid-hip axis:

Angle between the vertical and the highest point of the dens

Normal values: -5° to +2°

If above +2°,predictive of proximal junctional kyphosis (PJK)

Spinopelvic parameters

Pelvic incidence (PI)

Sacral pedestal

=Pelvic Tilt + Sacral slope

Patients with a high PI tend to have a high lumbar lordosis.

How deep is your pelvis

Women will have deeper pelvis = bigger bum!

Normal value 55 ± 10.6°

How to draw

Line1: centre of S1 endplate to centre of femoral head
Line2: perpendicular to S1 endplate

This is fixed after skeletal maturity

PI increases with growth of the pelvis.
Once skeletal growth is complete, the PI is fixed, and can only be changed by fractures or osteotomies to the pelvis or sacrum.

Clinical

Correlates with severity of disease
PI has direct correlation with the Meyerding–Newman grade
PI and LL must be within 10 deg
A PT < 25° and a PI within 9° of the lumbar lordosis are radiographic outcomes that statistically correlate with positive outcomes after spinal deformity surgery.

Restoring a harmonious sagittal profile should be the surgical goal.

Some patients are naturally more ‘S shaped’, while others have a flatter profile.

Presenti 2018:

The Amount of Proximal Lumbar Lordosis Is Related to Pelvic Incidence
PI influences only the proximal part of the lordosis, but not the distal part in an asymptomatic adult population

Sacral slope (SS)

The slope of the S1 endplate relative to the horizontal plane

Normal value 39 ± 8°

Sacral slope = pelvic incidence - pelvic tilt

How to draw:

Line1: parallel to the S1 endplate
Line2: horizontal one (edge of film)

Foundation of the Roussouly classification

Pelvic tilt (PT)

Pelvic tilt = pelvic incidence - sacral slope

Pelvic tilt refers to the anteversion and retroversion of the pelvis

How to draw

Line1: Center of S1 to center of femoral head
Line2: vertical line (edge of film)

We can tolerate pelvic tilt for at most 20 deg

The angle from the femoral heads to the midpoint of the S1 endplate relative to the vertical plane

Normal value 13 ± 6°

A PT less than 25° is one of the desirable outcomes after spinal deformity surgery.

Cervical parameters

Horizontal Gaze: Chin-brow vertical angle (CBVA)

Measurements

Positioning

Patient standing with hips and knees extended
Neck is in a neutral or fixed position.

Photos

Angle

patient’s chin to brow
vertical line

Xrays

Angle

A line from Chin to forehead
A vertical line

CBVA of 10° has been described as an optimal target.

overcorrection of CBVA had a negative impact,

Significantly impacts activities of daily living.

Correction considering CBVA has been linked to positive postoperative outcomes:

Improved gaze
Ambulation
Activities of daily living

Normal CBVA has not been characterised, but postoperative values of +10 ° to −10 ° have been well tolerated in patients

Clinical analysis of global and cervical deformity using the chin–brow vertical angle (CBVA) and assess- ment of hip extension angle in standing position with knees extended (left and middle). Patients are asked to flex at the knees to achieve horizontal visual axis and the knee flexion angle is assessed

Chin–sternal distance (CSD)

Measurement

Distance between the mentum (bony point of the chin) ↔ the sternal notch (the top of the breastbone).
Measurement taken with the head in full extension and mouth closed,

Can help predict the likelihood of a difficult laryngoscopy during intubation.

Occiput-to-posterior wall distance.

Measurement:

Measurement taken with the individual stands with their heels, buttocks, and upper back touching the wall,
Distance between the back of their head (occiput) ↔ the wall

Normal Range:

0 - 2 cm

Abnormal Range:

>2cm may indicate thoracic hyperkyphosis

Clinical Significance:

Presence of postural abnormalities

Reduced walking speed,
Increased risk of fall

Musculoskeletal problems such as back pain, neck pain, and headaches

Whole body Kyphosis Angle (WBKA)

Min 2007

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Measured on photographs

A horizontal line was drawn at the level of the umbilicus. A reference point was marked at the junction of the posterior and middle third of this line. From this point a vertical line is drawn.
The WBKA is the sum of the angles from the tragus of the ear to this vertical line (tragus angle) and from the midpoint of the knee to this vertical line (knee angle) at the reference point.

a=tragus angle, b=knee angle, the sum of a and b constitutes WBKA. c=CBVA, d=thigh flexion angle.

Cervico-thoracic parameters

Neck tilt (NT)

Measurement

Angle between 2 lines both originating from the upper end of the sternum

A vertical line
Connecting to the center of the T1 endplate

Thoracic inlet angle (TIA)

TIA is related to T1 slope (angle between horizontal plane and T1 endplate)
Measurement

Angle between

a line originating from the center of the T1 endplate perpendicular to the T1 endplate
a line from the center of the T1 endplate and the upper end of the sternum.

Similar to Pelvic Tilt (PI)
TIA = T1 slope + NT

similar to PI=SS+PT

A relationship exists where TIA influences cranial offset and craniocervical alignment, suggesting it can guide deformity correction.
A small TIA creates a small T1 slope therefore a small CL angle needed to maintain the physiological neck tilting
The TIA and T1 slope may be used as parameters to evaluate sagittal balance, predict physiological alignment, and guide deformity correction of the cervical spine.

Evidence of importance of sagittal balance

Glassman et al 2005

352 patients with (+) sagittal balance reviewed
Severity of symptoms increase in a linear fashion with progressive positive sagittal balance
All health-related quality of life (HRQOL) measures worsen as C7 plumbline (C7PL) deviates more (SVA)
Relative kyphosis is very poorly tolerated in lumbar spine

Lafage 2009

PT correlates with HRQOL in the setting of adult deformity

High values of PT express compensatory pelvic retroversion for sagittal spinal malalignment.

T1-SPI correlation with HRQOL measures and outperforms SVA. This parameter carries the advantage of being an angular measurement which avoids the error inherent in measuring offsets in noncalibrated radiographs.

lafage2009.pdf

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