Biomechanics of the Spine During Growth



Fig. 22.1
Growing cartilage of a vertebra. (a) Vertebral plateau. (b) Bipolar cartilage = neurocenral cartilage. (c) Posterior elements apophyseal cartilage. P periosteum, R ring apophysis





Vertebral Growth Plates


Each vertebra has many growth plate levels, but these are harmoniously located and coordinated, so that growth is realized in 3D and is perfectly harmonious.

Each vertebral plateau has its own growth plate, so one superior, one inferior, for each vertebra, giving vertebral growth in length with the rate of 1.2 mm/vertebra/year for lumbar region and 0.9–1 mm for thoracic region.

The neurocentral cartilage is a very important one that allows the junction between anterior body part with the posterior elements. It is a bipolar cartilage giving AP growth as well horizontal growth. They are located vertically, one right, one left, from the neural canal between the two plateau. The junction between vertebral plateau, cartilage and neurocentral cartilage, it is really an unique region when 3D growth is perfectly determined.

They will close between 8 and 12 years even more. It is clear that the unilateral closure of these cartilage lead to horizontal deformity giving typically a scoliosis.

The growth around posterior arch has a proper growth really linked to the neural tube evolution. Secondary ossification centers appears at the tip of spinous process transverse, facets joints and work under the influence of ligaments and muscles insertions. It is important to notice that pars inter articularis is already ossified at birth.

Generally fusion of spinous process on the middle line occurs at age 1, first in the thoracic area, then progress to lumbar and sacrum. Reversely ossification is active for atlas at age 2 and axis at age 4.

The size of the spinal canal remodel through construction – destruction process and has reached adult age size at age 5. This is an essential point for pathology allowing to perform for example circumferencial fusion on a small child without fear [1–3].

Finally as soon as age 10 appears at the superior and inferior plateau border a ring apophysis that ossify slowly according an apophysis front to back and fuse with the vertebral body between 18 and 25 years old.


Vertebral Growth Over Time


Figure 22.2: As it very well represented in the excellent book of Alain Dimeglio [4], spinal growth is exponencial from 0 to 5 years old. At that age, we must remember that 70 % of sitting height as achieved in girls and 66 % in boys. The gain is 27 cm for upper segment in 5 years (12 cm for the first year, 5 cm for second year, 4 cm for third year and 3 cm for 4th and 5th years). Between 5 and 10 years old, growth is more quiet 2–3 cm/year. At 10 years old 80 % of final height is acquired. Then growth spurt occurs with a 4.5 cm/year of growth in mean for the upper segment, then a deceleration more slow and variable from one patient to the other. Notice that the amplitude of such growth spurt is higher in boys (2 cm/year more) than in girls. But growth spurt is always delayed 2 years in boys according chronological age.

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Fig. 22.2
Growth in time (speed). TH total height, US upper segment height (sitting height), LL lower limbs height

We must remember also that at the time of growth spurt, the speed for each vertebra is 1.2 mm for thoracic and 1.6 mm for lumbar between 13 and 16 years for example.

Another item about T1 S1 size is important for biomechanics : Thoracic segment represent 2/3 and lumbar segment 1/3. But T1 S1 represent 50 % of sitting height. It is why it is important to know the maturation of bone and cartilage for spinal growth. Bone age of the hand is less helpful than Risser sign that remain an important marker; linked with the closure of triradiate cartilage (corresponding more or less to Risser +), linked and correlated to closure of the cartilage of the elbow, but also correlated to sexual maturation (pubic hair, breast in girls, testis volume and beard in boys). All this give an idea of the status of the patient at the time of examination allowing to approach the real vertebral age because none of these sign is absolute by itself, large variation may exist from one individual to another and if we multiply the signs we can reach more close measurement.

Finally it is very important to remember thoracic cage growth very much related to spine : From new born to adulthood, thoracic cage volume is multiply by 14 and 50 % of the final volume is achieved by age 10.

Notice that growth of the thorax in volume continue to increase during 2 years after end of spinal growth.


Biomechanical Consequences


We have many growth tables, charts, to help to control development of the spine and the thorax according age.

This help a lot to decide for example what is the good time to perform fusion when deformity and it is important to recognize the loss of height we can accept by doing so fusion, especially if it is extended to the thoracic area. For example, if we fuse the entire thoracic area in a boy of 10 years old, we remove 6 cm of thoracic height and if we do the same at the lumbar area, we remove 3.6 cm.

A typical consequence for biomechanics of the spinal growth in space as well in time is the crankshaft phenomenon (Fig. 22.3). That is the phenomenon complicating some treatment in the pediatric spine. It is seen when isolated posterior fusion is realized on a scoliotic immature spine. In spite of a solid posterior fusion, we can observe a progressive bending of the fusion mass because of the continuation of the anterior growth. This is explain clearly biomechanically because the growth plate on the front are deviated laterally but continue to grow and make progression of the deformity until growth is completed. It is why the treatment come really from biomechanics : it is a preventive anterior epiphysiodesis done at the same time as the posterior fusion and on the same levels [5–7].

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Fig. 22.3
Crankshaft phenomenon. A solid posterior fusion on a growing scoliotic spine play a role of posterior tether on a distorted element and the continuation of anterior growth (arrow) increase the rotational deformity. Prevention is of course anterior convex epiphysiodesis (at the opposite side of the posterior fusion)


Importance of the Soft Tissues Component of the Spinal Organ


Even if soft tissues (discs, ligaments, muscles) are almost half part of the anatomic structures of the spinal organ, their study is less developed than the one on the bone and cartilage side.

Nevertheless their quality appears very important about flexibility or stiffness even in some case laxity and instability.

The definition of instability is still controversial and involve as the bone and cartilage part coming from the joints facets and disc space structures as well as the soft tissue part coming from disc space structures, capsules, ligaments and muscles. The quality of these tissues depends mainly of genetic aspects of their basic molecular structures, especially connective tissues, collagen, as well as elastic tissues.

From early childhood, one can recognize the elastic component of the tissues with hyperextension of the thumb or hyperlaxity of the joints. This may contrast to joint stiffness with a decrease of normal motion. It is the same on muscles displacement and range of motion, everything is individual.

It is clear that with age maturation of the soft tissues occurs but are much less studied and known than bone and cartilage maturation. For example hypermobility Is more frequent in early age allowing from time to time hypermotion, making discussion with instability like C2 C3 flexion extension test where we can find such over alignment considered physiological.

On the other hand one can have large amount of displacement in severe neck injury in infants giving spinal cord injury without any bone fracture.

Of course the relative higher weight of the head in an infant related to the entire body can play a role in these problem. But it is also sure that with time this hypermobility decrease unless a real pathological connective tissue pattern remain such as in Ehlers Danlos or Marfan disease.

The biomechanical importance of the stability of the spine and the permanent mixture between bone and cartilage and soft tissue conditions is well demonstrated by the consequence of laminectomy in growing child. A biomechanical study has been made in spinal columns of neonate and children by measuring intradiscal pressure of the specimen submitted to constant load in the normal specimen first then after removal one by one of the posterior structures. Starting with only removal of the interspinous ligament the pressure start to increase then one side ligamentum flavum then both side then one facet joint then second facet. This demonstrate clearly that the pressure inside the disc space was increasing constantly as we produce increasing posterior destruction of the stabilizing elements with subsequent increase in kyphosis (Fig. 22.4) [8–11].

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Fig. 22.4
Evolution of kyphosis after experimental removal of the successive elements of the posterior arch in a spinal specimen. α sagittal angle, F force applied (D’après H. Robert. J Biophys Med Nucl. 1984;8(4):243–9)

This experiment corroborate perfectly with our observation in pathology in children with instability and kyphosis increasing also constantly. Finally the hyper pressure on the disc space transmit to the growth plate of the bodies and delay growth potential because of the hyper pressure and change completely the aspect of the body becoming cuneiform with decrease of anterior growth and progressing kyphosis.

Prevention by laminoplasty instead of laminectomy when possible consisting to replace the posterior elements removed “en bloc” after the neurosurgical intra spinal work associated with proper immobilization giving proper healing prevent kyphosis and secondary growth disorders.

The importance of the soft tissue maturation is also well demonstrated by a group of scoliotic curves where non surgical treatment was started after end of most vertebral growth done (Risser +++ or ++++) and continued over 1 year after completion of bone maturation. In the group of patients in spite of complete bone maturation improvement of the curve was maintained after removal of the brace : so the only explanation can come from maturation and stiffness that occurred on the soft tissues of such scoliotic spines [12].

The importance of soft tissue factor in the growing spine with a combination of ligamentous, muscular, and nervous components simultaneously injured during the treatment of some tumoral disease of the chest in paraspinal area like neuroblastoma is also easy to demonstrate.

In a child when such removal on intercostal space including from time to time one or two ribs and subsequent intercostal neurovascular bundle is done, the result is invariably a spinal deformity convex toward the side of the injury.

It is not a question of bone, but only a question of soft tissue defect giving slight instability on the ligament linking 2 or 3 spinal segments and localized difference on bone pressure of the growth plate secondary to paralysis of posterior muscles leading to deformity. Even if radiation therapy is given with the disastrous effect well known on the growth plate, the deformity is still convex to the side of the soft tissue injury. Reversely for Wilms tumor in a child treated with surgery and radiation, the deformity is secondary to asymetrical radiation because there is no involvement of paraspinal muscles and the effect of asymetrical radiation on growth plate give a deformity concave to the side of the injury. These two examples demonstrate well biomechanics of the bone as well as biomechanics of soft tissue on a growing spine [5, 10].



Setting Up of the Erect Posture and Its Consequences



Getting the Erect Posture in Humans


As we know at birth the sagittal spine shape looks like a gentle C arcuature [13]. When the child start to be on his prone position the heavy head start to be lifted up and the cervical lordosis develops. The next step is walking on the upper limbs and knees (“crawling four feet”) and cervical lordosis increase as well as lumbar lordosis initiates. Then when he stops, he starts to lift up the upper limbs and raise his back on the knee flexed, but hip extended and so lumbar lordosis progress to be established. Then he stand up with proper cervical lordosis, thoracic kyphosis and lumbar lordosis. The erect posture is so acquired and will develop all along childhood and adolescence to the adult age.


Maturation of the Central Nervous System


It is during that phase of setting up the erect posture that the maturation of the nervous system take place. At birth it is well known that the nervous system is not completely developed and still immature especially about the myelinisation of the central nervous area.

The coordination and achievement of balance continue to extend during a large part of childhood. The postural balance integrate sensori stimuli coming from eyes, ears, vestibular and proprioception is established with proper automatic function coming from the motor reaction of muscles surrounding joints themselves getting information from receptors inside the capsules or tendons.

We still don’t known exactly what are the neurohormonal transmitters working on the postural balance at the level of the relationship between right and left brain and the function of hypothalamus.

This is a way of research developing around the etiopathogeny of idiopathic scoliosis with the experiment done with the pineal gland, neurotransmitters function, as well in bipedal animal where scoliosis exist and can be reproduced and quadrupedal animals where scoliosis don’t exist and cannot be reproduced experimentally with neurological lesion done distant from the spine itself [14].

The maturation of this nervous system play probably a great role in the idiopathic scoliosis and may explain why some real infantile severe curves can regress completely with a pure non surgical treatment (brace or cast for example) and some even spontaneously, even with some remnent of the deformity with wedged vertebrae included in a straight spine. And also explain in case of persistent immaturity of the nervous system why some other malignant cases cannot correct with the same kind of treatment.


Biomechanical Consequences of the Erect Posture in Children and Adolescents


Of course the concepts recognized and developed here for the growing spine by the author are easily adaptable for adult spine.


Static and Dynamic 3D Balance


If we consider the erect posture in a human (as in a young or an older child or an adult), we know that in a standing position, both feet are pushing on the ground and delineate a surface ellipsoidal called the polygon of sustentation (Fig. 22.5). If you consider the same human at any age seated, you have the same polygon of sustentation now looking like a frame done with both thighs and ischial tuberosities.

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Fig. 22.5
Standing posture planes of reference and polygon of sustentation. (1) Polygon of sustentation. (2) Orthogonal gravity line. R sagittal plane, H horizontal plane, C coronal plane

Either standing or sitting from the center of this polygon of sustentation, if you draw the orthogonal line, you have all your body including the head aligned harmoniously within this line realizing such gravity line with a patient in a good 3 dimensional balance. Anatomically on the sagittal plane this line goes from tragus to the center of this polygon and the spine is harmoniously aligned with cervical lordosis, thoracic kyphosis, lumbar lordosis, sufficient pelvic tilt, hips extended and knee extended with the ankle joints at more or less 90° from the ground. This is defined like a balanced spine where the spinal curvatures balance themselves in opposite direction to achieve harmony and function with the head projecting inside this polygon of sustentation.

When the patient is seated, it is the same with the head entirely balanced to the center of the sitting frame and the spinal curves adapting to the proper pelvic tilt necessary to compensate the absence of lumbar lordosis or the kyphotic lumbar deformity. This balance is defined static when no motion at all is observed but in human there are constant motion within this surface of polygon in a 3D manner.


Cephalic and Pelvic Vertebra Concept


Resulting from the previous 3D balance, we consider the entire head with its own mass and weight (# 4.5 or 5 Kg) like the first vertebra (Figs. 22.6 and 22.7). This weight lying at the top of the spinal construct is almost always moving and realize like a gyroscope to maintain the balance [13, 15, 16].

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Fig. 22.6
Coming from the study of pelvic obliquity and scoliosis, the entire pelvis must be considered like one unique vertebra


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Fig. 22.7
The entire head must be considered as the cephalic vertebra and the pelvis as the pelvic vertebra acting as an intercalary bone to achieve balance

Simultaneously the entire pelvis (because of the minimal movement inside the sacro-iliac joint less than 1.5° motion unless pregnancy where it can reach 3.5° of motion) can be considered like one unique vertebra, the last of the spine and like an intercalary bone between trunk and lower limbs.

Doing so when a global thoracic kyphotic deformity occurs, the pelvic vertebra can compensate by anterior tilt realizing a compensation, lordosis at the lumbar level, etc. This pelvis vertebra has 6° of freedom for each hip joint and the same for lumbo-sacral junction.

It is interesting to notice here various work about this pelvic vertebra because it has been proven that the proper anatomy of this pelvic vertebra play a great role in the amount of lordosis necessary to achieve balance, especially in the sagittal plane. The angle between the center of femoral head and the orthogonal projection to the center of S1 superior plateau determine the Incidence. The variation of this angle is observed generally between various patients with no more than 12–15° variation. It is also remarkably stable during life and very few or almost no change after age 5. This angle determine the amount of lordosis necessary to get a good sagittal balance. The incidence represent in reality the positioning of the SI joint in the space regarding the one of the femoral heads. This explain the wide variation we can get between male and female and morphotype of patients (thin and tall or short and wide).


Concept of the Conus of Economical Consumption and Economical Function


If we consider the human body in a standing (or sitting) posture, the feet are located within the polygon of sustentation, the body under the influence of muscle function can move in a conical fashion without moving the feet (Fig. 22.8). The maximum variable amplitude are at the pelvic and the head levels, we can determine a maximum cone when muscles are working at their maximum of excursion and strength to maintain balance. But also we can determine a smooth cone when muscles are working at their minimum in some cases almost nothing to maintain balance mainly achieved by the passive balanced stretching of discs, ligaments and soft tissue structures with the minimum of muscle action. This is obtained when the body is well balanced within this smooth cone, the muscle function is economial. When the body is out of balance, we must anticipate a permanent curd costly muscle function [13, 15, 16].
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Aug 2, 2017 | Posted by in ORTHOPEDIC | Comments Off on Biomechanics of the Spine During Growth

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