2 Development of the Spine

10.1055/b-0035-124587

2 Development of the Spine

Alain Dimeglio and Federico Canavese

Only a perfect knowledge of normal growth parameters allows a thorough understanding of the pathologic changes induced in a growing organism by an early onset spinal deformity. As the spinal deformity progresses, not only is spinal growth affected; by a “domino effect,” the size and shape of the thoracic cage are modified as well. This distortion of the thorax interferes with lung development. Over time, the spine disorder changes in nature from a mainly orthopedic issue to a severe, systemic pediatric disease causing thoracic insufficiency syndrome, cor pulmonale, and, in the most severe cases, death.

The growing spine is a mosaic of growth plates characterized by changes in rhythm. During growth, complex phenomena follow one another in very rapid succession. These events are well synchronized to maintain harmonious limb and spine relationships, as growth in the various body segments does not occur simultaneously in the same magnitude or at the same rate. The slightest error or modification can lead to a malformation or deformity, with negative effects on standing and sitting height; the shape, volume, and circumference of the thoracic cage; and lung development. ¹ ^, ²

2.1 Clinical Examination and Biometric Measurements

Growth holds the basics, and any surgical strategy should be adjusted according to the ratio between remaining growth and elapsed growth (Table 2.1).

**Table 2.1** Growth is a change in proportion
Developmental stage	Sitting height		Lower extremities
Developmental stage	Head	Trunk	Lower extremities
Fetus (early pregnancy)	50%	32%	18%
Fetus (late pregnancy)	35%	40%	25%
Newborn	25%	40%	35%
Infant	23%	37%	40%
Child	20%	35%	45%
Pre-Adolescent	18%	34%	48%
Adult	13%	40%	47%
Note: The ratio of the sitting height to the length of the lower extremities varies with age; it is 4.5 during early pregnancy, 3 during late pregnancy, 1.9 at birth, 1.3 during childhood, and 1 at skeletal maturity. ¹ ^, ² ^, ³ ^, ⁴

A height gauge, scales, a metric tape, and a bone age atlas are required at the time of consultation. All growths are synchronized, but each one has its own rhythm (Fig. 2.1). A thorough analysis of the standing and sitting height, arm span, weight, thoracic perimeter, T1-S1 spinal segment length, and respiratory function helps the surgeon to plan the best treatment at the right time. Therefore, these measurements should be repeated and carefully recorded at regular intervals to provide a real-time image of growth and charts that facilitate decision making. A clinical examination every 4 to 6 months allows the clinician to assess the growth velocity of the child and the different body segments easily. ¹ ^, ²

**Fig. 2.1** All growths are synchronized, but each one has its own rhythm. Growths do not progress at the same pace. At 5 years of age, the increases in weight and thoracic volume remain limited relative to the other growth indicators. At 5 years of age, children have reached 26% of their final weight but between 66% and 70% of their final sitting height.

2.1.1 Standing Height

The gain in standing height is approximately 25 cm during the first year of life and about 12.5 cm during the second year. Between the ages of 2 and 3 years, the gain in standing height is approximately 9 cm annually, and between the ages of 3 and 4 years, the gain in standing height is approximately 7 cm annually. At 5 years of age, the standing height increases by 5 to 5.5 cm each year in both boys and girls. At the beginning of puberty, the average remaining growth in the standing height is about 22.5 cm for boys (13%) and 20.5 cm for girls (11%).

Growth velocity is the best indicator of the beginning of puberty, on which so many decisions rest. The first sign of puberty is an increase in the rate of growth in the standing height to more than 0.5 cm per month or more than 6 to 7 cm per year. ¹ ^, ² ^, ³ ^, ⁴ Growth charts show that a standing height growth velocity of more than 6 cm per year in girls and more than 7 cm per year in boys is evidence that the patient is within his or her greatest growth spurt. ¹ ^, ² ^, ³ ^, ⁴ This rapid and significant increase in the growth rate is called peak height velocity or acceleration phase. During this phase, the average remaining growth in the standing height is about 16.5 cm for boys and 15 cm for girls. The first 2 years of puberty, characterized by significant growth, are followed by 3 years of a gradual and steady reduction in the growth rate, the so-called deceleration phase, during which the average remaining growth in the standing height is about 6 cm for boys and 5 cm for girls. ¹ ^, ² ^, ³

In 77% of boys, the first physical sign of puberty is testicular growth, which occurs on average 3.5 years before adult height is attained. In 93% of girls, the first physical sign of puberty occurs about 2 years before menarche, and final height is usually achieved 2.5 to 3 years after menarche. ¹ ^, ² ^, ³ ^, ⁴

The standing height is a global marker comprising two components: sitting height and subischial height. Because the trunk and the subischial region often grow at different rates and at different times, the standing height does not always exactly correlate with a loss of trunk height in children with severe spinal deformities. ¹ ^, ² ^, ³ ^, ⁴

2.1.2 Sitting Height

The sitting height correlates strictly with trunk height and is on average about 34 cm at birth; it is 88 cm at the end of growth for girls and 92 cm at the end of growth for boys. In children with severe spinal deformities, the loss of sitting height is related to the severity of the deformity. For this reason, it is important to monitor changes in sitting height rather than in standing height in children with progressive spinal deformities. During the first 3 years of life, or in a child with a neurologic disorder or a collapsing spine, it is recommended to measure the sitting height with the child in a supine position.

Growth is a succession of acceleration and deceleration phases comprising three periods. The first period is from birth to age 5 and is characterized by a gain in sitting height of 27 cm, with a gain of 12 cm occurring during the first year of life. The second period is from age 5 to 10 years and is a quiescent phase in which sitting height increases by 2.5 cm per year. The third period is characterized by a gain in sitting height of about 12 cm and corresponds to puberty. ¹ ^, ² ^, ³ During peak height velocity or acceleration phase, ⁴ the average remaining growth in the sitting height is about 12.5 cm for boys and about 11.5 cm for girls (Fig. 2.2, Fig. 2.3, Fig. 2.4). The average remaining growth in the sitting height during the deceleration phase is about 4 cm for boys and 3.5 cm for girls. ¹ ^, ² ^, ³ ^, ⁴

**Fig. 2.2** Percentage of remaining sitting height growth in boys (*gray*) and girls (*white*). Sitting height growth is completed 2 years earlier in girls than in boys on average.

**Fig. 2.3** Sitting height gain in boys. Values are expressed in centimeters and are average values. ¹ ^, ² ^, ³ In boys, the pubertal spurt occurs between 13 and 15 years of age.

**Fig. 2.4** Sitting height gain in girls. Values are expressed in centimeters and are average values. ¹ ^, ² ^, ³ In girls, the pubertal spurt occurs between 11 and 13 years of age.

2.1.3 Weight

Weight is a useful parameter for evaluating growth and increases 20-fold from birth to skeletal maturity. At 5 years of age, weight is approximately 20 kg; it is 30 kg by 10 years and reaches 60 kg or more by 16 years. In particular, weight usually doubles during the pubertal spurt, and each year of puberty corresponds to a weight increase of about 5 kg. This information should be kept in mind when a child is treated with a brace. Moreover, in a patient whose weight is 10% or more above normal, a scoliosis brace may be less effective than in a patient whose weight is within 10% above normal. ¹ ^, ² ^, ³ ^, ⁴

Growth energy requirements during the first 3 years of life are enormous and much greater than those of adults: 110 calories vs. 40 calories per kilogram per day; 2 g vs. 1 g of proteins per kilogram per day; and 150 mL vs. 5 mL of water per kilogram per day. Moreover, skeletal mineralization alone requires the storage of 1 kg of calcium between birth and adulthood.

Because most children with severe spinal deformities or neurologic impairment have a low body mass index (BMI) and thus are at a higher risk of surgical morbidity, weight is a valuable indicator. In selected cases, a hypercaloric nutritive protocol should be initiated before surgery. Children with pulmonary insufficiency characteristically have poor nutrition because the energy expenditure for the extra work of breathing approaches the nutritional gain derived from eating. It has been demonstrated that in about two-thirds of children with severe spinal deformities and thoracic insufficiency syndrome treated with expansion thoracoplasty surgery, the nutritional status improves significantly postoperatively.

Of note, the BMI can be misleading. A 9-year-old child with severe scoliosis, a standing height of 110 cm, and a weight of 12 kg can have a BMI that is somewhat reassuring. However, this information does not reflect reality because the standing height is similar to that of a child of 1 to 5 years of age and the weight comparable to that of a 2-year-old child. In children with low weight, the pubertal spurt changes are moderate because the weight must be at least 40 kg for the spurt to be normal. ¹ ^, ² ^, ³

The average birth weight is approximately 3 kg, which means that the blood volume is about 0.3 L. Weight plays an essential role in surgical planning, and the surgeon’s margin for maneuver is very narrow. Any weight loss, however slight, after spinal surgery before 5 years of age can have serious consequences. A 1-kg postoperative weight loss in an 18-kg patient represents about 6% of the total body weight, and therefore there is a major difference between operating on a 40-kg child and a 20-kg child. For this reason, children with spinal deformities who weigh less than 20 kg should be differentiated from those weighing more than 20 kg. Of note, weight gain in children after spinal surgery is a good indicator that the clinical situation is under control. ¹ ^, ² ^, ³

2.1.4 Arm Span

The arm span is an indirect measurement to evaluate standing height and can be used to assess predicted height in nonambulatory children with neuromuscular disorders, cerebral palsy, or myelomeningocele.

The arm span and the standing height have an almost perfect linear correlation. The standing height corresponds to approximately 97% of the arm span, with a small gender difference; the ratio of the arm span to the total standing height is greater in boys than it is in girls. This relationship persists throughout puberty and into adulthood. In 77% of healthy children, the arm span will be 0 to 5 cm greater than the standing height; in 22%, it will be 5 to 10 cm greater; and in 1% it will be greater by 10 cm or more. As a rule of thumb, the arm span divided by 2 is very close to the sitting height, and the arm span divided by 4 is close to the length of the T1-S1 spinal segment. ¹ ^, ² ^, ³

2.2 Spine and Thoracic Cage

The action of the spinal growth plates can be defined by four words: harmony, interaction, synchronization, and hierarchy. Symmetric and harmonious growth characterizes normal spines, although spinal growth itself is the product of more than 130 growth plates working at different paces. The development of the spine consists of a complex series of events involving multiple metabolic processes, genes, and signaling pathways.

The first 5 years of life are critical. In severe scoliosis, growth becomes asymmetric as a result of growth plate disorganization. Complex spinal deformities alter the growth of the spinal cartilage; as a result, the vertebral bodies become progressively distorted and can perpetuate the disorder by altering thoracic cage and lung growth (Fig. 2.5). Therefore, many scoliotic deformities can become growth plate disorders over time. ² ^, ⁵

**Fig. 2.5** Effects of spinal deformity on chest size and shape. Computed tomographic scan (transverse section) shows severe scoliosis with chest cage deformity. As a result of severe spinal deformities, the apical portion of the deformity penetrates inside the thoracic cage (endothoracic hump). The “spinal penetration index” measures the amount of endothoracic hump. ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵

Because of the complexity of the three-dimensional organization of the growth plates, no surgical implant is capable of controlling them in all spatial planes. Attempts have been made with staples, growing rods, vertebral tethering bands, rib expanders, trolley constructs, and apical arthrodesis. However, the results remain controversial.

The crankshaft phenomenon is the progression of a spinal deformity during which the anterior portion of the spine continues to grow while the posterior portion is blocked by arthrodesis. Growth modulation devices should restore normal growth plate development and should control the crankshaft phenomenon. ¹ ^, ² ^, ³ ^, ⁴ Moreover, growth modulation devices should follow the pace of spinal and thoracic growth by providing about 2.5 cm of sitting height and 2.5 kg of weight per year between ages 5 and 10, and 4 to 5 cm of sitting height and 5 kg of weight per year during the pubertal spurt. ¹ ^, ² ^, ³ ^, ⁴

2.2.1 Ossification of the Spine

Ossification of the vertebral bodies starts at the third month of intrauterine life. It begins in the thoracolumbar area and radiates from there both proximally and caudally.

Within the cervical spine, ossification centers appear first in the neural arches and then in the vertebral bodies. Ossification starts in the lower cervical spine and develops proximally.

Moreover, ossification centers in the anterior and posterior portions of the vertebrae do not grow at the same pace. In the thoracic region, posterior growth is faster than anterior growth, resulting in the progressive development of thoracic kyphosis. On the other hand, in the lumbar area, posterior growth is slower than anterior growth, resulting in lumbar lordosis. ¹ ^, ² ^, ³ ^, ⁴