(**a**) Thoracic vertebra. Transverse section. Fourth month of intrauterine life. The ossification centers are now in place in the future vertebral body and in the posterior arcs. (**b**) Three successive and overlapping phases: Mesenchyme, Cartilage, and Ossification. According to Tsou. Embryology of congenital kyphosis. Clin Orthop 1977, 128, 18–25 (with his permission)

The growth cartilage roles are well distributed; the programming is hierarchical.

the decisive role of the future notochord

the redistribution of sclerotomes… crucial time

the closing of the posterior arc which is linked to the closure of the tube.

The sequences are linked to an immutable rhythm.

The failed closure of the neural stem (which normally closes around the 4th week of intrauterine life) determines the persistent opening of the posterior arch. All of these multiple, complex processes are contemporary and can be associated. *This explains why a vertebral malformation can be a polymalformation (eg, spina bifida).*

The vertebral body is a vascular sponge.

## The Fetal Period: The Strongest of All Growth Is the Intra-Uterine Period

The appearance of cartilage is rapidly followed by ossification fronts that progress in the posterior arcs as well as the center of the vertebra.

Annual growth rate vertex–coccyx: 44 cm!

The vertex–coccyx distance.

The neurocentral cartilage has a bidirectional activity.

The work of ossification progresses extremely slowly. It begins in the 2nd month of intrauterine life but only ends around the 18th year, and for some, at the 25th year!

## Vertebral Curves Are Not Primitive But Acquired

In the fetus *the vertex–coccyx distance* is 10 cm at 3 months and 25 cm at 4 months— the growth is very important between the 3rd and 4th months. At the 5th month, the vertex–coccyx distance is 30 cm and it will be 35 cm at birth (Fig. 6).

## At Birth, 30% of the Spine Is Ossified

neurological maturation

sitting height whose evolution directly reflects vertebral growth.

At birth, the vertebrae have three centers of ossification: one for the central anterior part, one for each posterior arc. The length at the end of growth will have almost tripled.

About 30% of the spine is ossified at birth. There is no significant difference between the vertebrae. The vertebral body of a thoracic vertebra is about 7 mm in height. The sitting size measures 35 cm.

*Evolution of spinal curvatures*

*Growth is a change of proportions*

## The First Five Years of Life Are Decisive: Living Growth

the establishment of the osseous medulla, which is adjusted relative to its container (cortex);

the constitution of cervical, thoracic, and lumbar curvatures which are dependent on verticalization.

Once erect, around the age of 1 year, the cervical curvature and lumbar curvature settle, they are dependent on neuromotor development and neurological maturation which is cephalocaudal.

After 5 years, the lower limbs grow 3.5 cm per year.

During the first year of life, the vertebral body may present as two ossification nuclei, but these small occurrences in ossification will unify rapidly. On the other hand, the coronal cleft, which is visible sometimes at birth, will disappear during the first year. This radiographic image can be visualized until the age of 4, and it must be considered as a variation of endochondral ossification.

From 1 to 5 years, the medullary systemization is refined.

Once the closure of neurocentral synchondrosis is completed, then commences the closure of the posterior arc, then only one can really speak of the posterior arc and vertebral body, whereas the denomination which precedes these actions is the following one: neural arc for the posterior part and central point for the future vertebral body.

Growth during this period remains very strong. The sitting height gain is 12 cm in the first year of life; the sitting height then increases from 35 to 47 cm. From 1 to 5 years the sitting height gain is 15 cm; sitting height increases from 47 to 62 cm.

The sitting height at the end of growth represents 52% of the standing height.

Sitting height has increased by 27 cm in 5 years! The remaining growth of sitting height after 5 years is 30 cm. In other words, within a few centimeters the spine grows as much during the first 5 years of life as during all of the rest of growth. It is necessary to put in perspective this increase of growth of the first 5 years of the life (+27 cm) compared to the puberty period where the sitting height grows 12–14 cm.

This growth spurt in the first 5 years of life is even stronger than during the pubertal period.

## Growth Between 5 Years and the Beginning of the Puberty

From 5 to 10 years, the growth of the trunk, and therefore of the spine, slows down as the sitting height will increase by about 10 cm. The annual growth rate of the trunk is 2 cm. The T1-LS segment grows 1.2 cm per year. *The annual elongation of the T1-L5 spine should theoretically not exceed 1.2 cm per year*. We must exploit this soft growth, especially when we manage an infantile scoliosis.

## Puberty, a Decisive Turn: New Acceleration

*The pubertal peak begins at 11 years of bone age in the girl*. The remaining growth in sitting height is about 12 cm, including 2 years of strong growth where the sitting height will increase by 7 cm and 3 years of soft growth where the sitting height will increase by only 5 cm. Growth slows down after the closing of the elbow growth cartilages.

*The pubertal peak in the boy starts later, at 13 years of bone age*. The remaining growth on the sitting size is 13 cm: a sharp growth for 2 years, 13–15 years of bone age, with a sitting height gain of 8 cm and a gentle growth at 15–18 years, where the sitting height gain is 5 cm.

Growth slows down significantly after the closure of the elbow growth cartilages.

All these figures are valuable because sitting height is a reflection of macrocrine growth, and this macrocrine growth is the product of microcurrents that occur mainly in the spine.

*Between the ages of 10 and 17, the vertebra has almost doubled in size*. It is at the time of puberty that the secondary ossification nuclei appear (especially at the level of the posterior arc) and the marginal vertebral apophysis that comes to sit above and below the vertebral body (Fig. 8).

## Each Level of the Spine: A Different Growth

## The Cervical Spine

The cervical spine represents 22% of C1-S1 segments. The volume of the cervical spine is 9 cm^{3} at birth. It is 110 cm^{3} at the end of growth. It has been multiplied by 12.

It measures 3.7 cm at birth and 12–13 cm at the end of growth. It has almost quadrupled in length. It represents approximately 15% of the sitting height at the end of growth and as mentioned, 22% of the C1S1 segment. The cervical cord is comfortable in the medullary canal. It occupies in the adult a surface of 80 mm^{2}, while the average surface of the vertebral canal is of the order of 376 mm^{2}.

It is necessary to differentiate the high cervical spine C1-C2 and the low cervical spine.

### Central Spinal Canal at the End of Growth

the superior cervical spine: C1–C2.

the inferior (subaxial) cervical spine: C3–C4–C5–C6–C7.

### Cervical Spine Height

Dimensions | |||
---|---|---|---|

Newborn |
6 years |
10 years |
15 years |

3.7 cm |
7.5 cm |
10 cm |
13 cm |

From the age of 5 years the medullary canal reaches 95% of its area.

### The Superior Cervical Spine

### The Growth of the Atlas (Figs. 27, 28, and 29)

The atlas has two lateral ossification nuclei that become the lateral masses.

After the age of 8, there is no visible growth cartilage.

There is a third nucleus of ossification which is anterior and which also contributes to the formation of the posterior arc. This ossification nucleus does not appear sometimes before the age of 1 year. It can be bifid but it is not pathological.

The ossification of the atlas may be incomplete resulting from a pseudo spina bifida. It should not be confused with a fracture.

### The Growth of the Axis Is Even More Complex

Ossification of the odontoid appears very early in the 5th month of intrauterine life. Two ossification centers appear and merge at birth. The top of the odontoid ossifies around the age of 6 years.

This terminal ossification fuses with the rest of the body of the odontoid at the age of 12 years.

The non-fusion of this center of ossification with the body of the odontoid creates the conditions of the odontoid bone whose pathogenesis is not very precise. Perhaps congenital? Traumatic?

The body of the odontoid is separated from the body of the axis by a growth cartilage. But this growth cartilage does not have a very elaborate structure. The body of the odontoid fuses with the body of the axis around the age of 6. In children, before the age of 6, an injury of the odontoid can therefore lead to detachment of this growth cartilage.