Normal development of the spine, from birth, includes the progressive
longitudinal and axial growth of an osseous spinal column that allows a
neuroprotective mechanical axis for all motion throughout life. Growth of the
spinal column may become eccentric during life and can result in spinal
deformity.
In the developing spinal column, there exist four general features that
permit the successful growth of the human spine:
Osseous development is essential for the provision of a mechanical
axis for the appendicular skeleton as well as an axis for muscular
development. This axial skeleton has features that are unique to all mammalian
bipeds.Characteristic curves in the osseous spinal column need to develop
once the child begins to sit upright and eventually walk. Lordotic cervical
and lumbar segments begin to develop, and the development of kyphotic thoracic
and sacral segments permits balance in the sagittal plane.Continuous symmetric growth of the spinal column must occur while
the child develops. Both longitudinal (vertical) and latitudinal (axial)
growth must occur, and an eccentric growth in either direction can result in a
spinal deformity.Neural element protection remains highly important throughout
development and in adult life. The developing osseous spinal column must also
permit safe exit of all nerve roots. The spinal cord, as well as the osseous
column around it, continues to lengthen. A variation in growth rates can
occur, which may result in neurologic deficits and spinal column deformity
(e.g., tethering of the cord and syringomyelia).
Osseous development is essential for the provision of a mechanical
axis for the appendicular skeleton as well as an axis for muscular
development. This axial skeleton has features that are unique to all mammalian
bipeds.
Characteristic curves in the osseous spinal column need to develop
once the child begins to sit upright and eventually walk. Lordotic cervical
and lumbar segments begin to develop, and the development of kyphotic thoracic
and sacral segments permits balance in the sagittal plane.
Continuous symmetric growth of the spinal column must occur while
the child develops. Both longitudinal (vertical) and latitudinal (axial)
growth must occur, and an eccentric growth in either direction can result in a
spinal deformity.
Neural element protection remains highly important throughout
development and in adult life. The developing osseous spinal column must also
permit safe exit of all nerve roots. The spinal cord, as well as the osseous
column around it, continues to lengthen. A variation in growth rates can
occur, which may result in neurologic deficits and spinal column deformity
(e.g., tethering of the cord and syringomyelia).
All typical vertebrae, except for the atypical C1 (atlas) and C2 (axis),
ossify from three ossification centers. There is a gradual union of the
primary ossification centers of the anterior centrum and posterior arches
(Fig. 1). Most of the anterior
portions of the neural arch contribute to vertebral body formation.
C1 (atlas) growth is unique; it appears that the anterior primary
ossification center is present in fewer than 20% of all
neonates6. The
anterior ossification center usually develops between nine and twelve months
after birth. Two cartilaginous gaps (neurocentral synchondroses) are formed as
the anterior growth center expands laterally toward each of the paired
posterior ossification centers. The anterior ossification center is termed the
intercentraxium-16.
The posterior synchondrosis and the anterior neurocentral synchondroses are
fused by the time a child is four or five years old, and the diameter of the
C1 canal reaches adult proportions at approximately five to six years of
age.
C2 (axis) growth is also unique and involves the development of
the dens (odontoid process). Dens ossification is separated from the primary
ossification of the vertebral centrum by the cartilaginous region called the
dentocentral synchondrosis. The dentocentral synchondrosis is caudad to the
C1-C2 articulations, and it usually closes when a child is between five and
seven years of age.
The dens behaves like a long bone in that it has a cartilaginous epiphysis
at each end (the chondrum terminale and the dentocentral synchondrosis).
Eventually, a secondary ossification center forms at each end (the os
terminale and the intercentraxium-2). The C2 centrum has two epiphyses and
secondary ossification centers (superior and inferior). The inferior epiphysis
of the dens and the superior epiphysis of the centrum fuse before birth to
form the dentocentral synchondrosis.
The growth plate of the chondrum terminale contributes to the length of the
dens, and failure of such growth may lead to a hypoplastic dens. A smaller
amount of longitudinal growth occurs within the dentocentral synchondrosis and
contributes to both vertebral body height and dens length. A small amount of
growth also occurs through the end-plate epiphysis. The chondrum terminale at
the superior end of the dens forms a secondary ossification center at
approximately eight to ten years of age. This is termed the os terminale.
Fusion of the os terminale then occurs at approximately ten to thirteen years
of age7.
In general, the primary ossification center of each vertebral body does not
contribute to extensive cortical bone formation as it does in long bones. The
process of ossification within the centrum is similar to that of tertiary
ossification. Most mammals have a well-formed physis and epiphysis superiorly
and inferiorly, yet humans do not. For example, the giraffe has a very
well-developed growth plate and secondary epiphyseal ossification center at
each end of the centrum. This permits the longitudinal growth of the vertebral
body that is required for the giraffe to have such a long neck with only seven
cervical vertebrae (Fig.
2).
Longitudinal growth mostly occurs at the chondro-epiphyseal portions of the
end plates. These areas also allow circumferential expansion of the vertebral
centrum. The posterior elements also demonstrate longitudinal growth. An
anterior growth plate exists at each neurocentral synchondrosis. A posterior
growth plate at the spinous process synchondrosis allows the laminar and
pedicular regions to lengthen. Posterior element growth ceases at five to
eight years of age, after the synchondroses close. Anterior column growth
continues until sixteen to eighteen years of age, and asymmetric closure may
lead to scoliosis.
Latitudinal growth is the result of a combination of perichondral and
periosteal apposition, while longitudinal growth is due to endochondral
ossification13-15.Vertebral
growth plates enlarge via the addition of cells at the periphery. Interstitial
growth in the transverse growth plate appears to be related to the enlargement
of the epiphyseal ossification center.
The cytoarchitecture of the vertebral physis is typical of a slow-growing
physis, compared with the more rapidly growing physis of a long bone. The
vertebral physis has a limited height compared with that of long bones. The
perichondral ossification zone of Ranvier is much less evident in a growing
vertebra. Vertebrae grow through a typical endochondral ossification sequence,
with the initial production of primary spongiosa. Remodeling occurs throughout
the growth process, until skeletal maturity.
The term "ring apophysis" applies to the thickened periphery of
the end plates. This is the result of the ellipsoid enlargement of both the
intervertebral disc and the primary ossification center. The central epiphysis
thins, and the circumferential portions remain
thick16,17.
"Apophysis" is a misnomer. These regions undergo compression and
shear, rather than tension, as with true apophyses.
There exists a variable rate of ossification of the vertebral end plates,
with irregularities appearing on the surface of the end plates. When a child
is between eleven and fourteen years of age, a focus of ossification develops
around the end-plate epiphyses, and by twelve to fifteen years of age, these
epiphyses complete ossification to form the radiographic "ring"
analogous to secondary ossification centers in long bones.
"End-plate undulations" are also thought to be the result of
biomechanical stresses placed on the physis, especially shear stresses from
walking and running (normal
development)9,10,18,19.
Cessation of growth is marked by the fusion of the secondary ring
ossification center and the associated metaphyseal region of the vertebra.
Fusion takes place earlier in girls than in boys, which suggests that there
may be an estrogenic control of fusion.
Congenital Abnormalities That Can Become Progressive Deformities
After Birth
Hydromyelia and Syringomyelia
Hydromyelia is a structural variation in which the neural canal is
retained. It may be nonpathologic. Syringomyelia is also a dilation of the
retained neural tube within the cord. Most lesions are asymptomatic, although
some can lead to scoliosis and various degrees of neurologic deficit. Both
hydromyelia and syringomyelia can be diagnosed with use of magnetic resonance
imaging.
Congenital Scoliosis
This deformity is a combination of failure of formation and segmentation of
spinal elements that occurs sometime in the first five weeks after gestation.
The clinical progression of curves associated with congenital deformities is
variable. Congenital scoliosis may be associated with renal, cardiac, and
gastrointestinal defects.
Congenital Kyphosis
This deformity is often not evident at birth, although it can progress
rapidly once the child becomes upright. It is related to a hemivertebra
located within the posterior aspect of the centrum. Surgical intervention is
almost always required to limit progression of the spinal deformity.
Klippel-Feil Syndrome
This condition is mostly related to variable degrees of fusion of anterior
and posterior segments of the cervical spine. It may be undetected until late
childhood, and it may be associated with spina bifida and Sprengel deformity
of the shoulder.
Lumbosacral Agenesis
This disorder is of unknown etiology and clinically ranges from mild
radiographic dysplasia to complete absence of the lumbar or sacral spine with
associated spinal deformities. Sirenomelia is an extreme manifestation of this
disorder and includes conjoining of the lower extremities.
Spinal Dysraphism
In this condition, there is failure of fusion of posterior midline
vertebral structures. A range of presentations exists, from spina bifida
occulta, to meningocele, myelomeningocele, and myeloschisis. The associated
neurologic deficit is highly variable.
Tethered Cord
This anomaly occurs when the filum terminale fails to detach from the
sacrum during fetal or early postnatal development. Surgical release is
required when it is associated with a neurologic deficit, or when it is
identified with magnetic resonance imaging before correction of a spinal
deformity.
Diastematomyelia
An osseous or cartilaginous mass may extend from the vertebral body into
the spinal canal to divide the cord or even tether it. Often, excessive widths
between the pedicles suggest the abnormality, and confirmation is with
magnetic resonance imaging. A neurologic deficit may develop, perhaps because
of the tethering effect, at approximately two years of age or later.
Trauma and/or Force-Related Developmental Anomalies
Spinal Osteochondrosis and Scheuermann Disease
The effect of vertebral vessels penetrating the cartilaginous end plates
may cause weakness, and may result in either the nucleus pulposis penetrating
the end plate, or perhaps failure of ossification of the cartilage end plate.
Irregularity of the end plate develops and anterior vertebral wedging may
occur, which can lead to an exaggerated thoracic kyphosis.
Os Odontoideum
Os odontoideum is a pseudarthrosis of the dens following fracture and
includes progressive ossification of the chondrum terminale, which has intact
ligaments and therefore a blood supply. The condition may lead to instability
at C1-C2, which may require surgical stabilization.
End-Plate Fractures
The cartilage end plate and its attachment to the osseous end plate is a
relatively weak area. It may separate as a result of injury, much like a
growth-plate injury in a long bone. The separation may result in a posterior
migration of cartilage into the neural canal in the lumbar spine, a disorder
that may mimic disc herniation as seen in the adult population.
Spondylolysis (Spondylolisthesis)
This pars intra-articularis defect is not related to a congenital failure
of fusion of the posterior primary ossification centers. The bipedal gait of
the developing upright human and the repetitive hyperextension loading of the
pars area can lead to plastic deformation and bone stress effects and may
result in a separation and/or fracture of the pars intra-articularis that may
lead to spondylolisthesis.
Cervical Hypermobility
Pseudosubluxation may occur in the cervical spine as the result of the more
horizontally aligned facet joints. The C2-C3 and C3-C4 levels are more often
involved, and a lateral radiograph of the posterior laminar line of the
cervical spine may be helpful in defining any abnormal translation.
Dysplasias Causing Abnormal Spinal Growth
Achondroplasia
This condition is associated with a relatively narrow neural canal in which
symptoms of stenosis can develop during development. Surgical decompression is
often necessary in the lumbar spine for relief of stenosis symptoms.
Spondyloepiphyseal Dysplasia
This condition results from a defect in type-II collagen and leaves the
child with short stature and a variable degree of involvement of the spinal
column. The vertebrae have a reduced height, and the end plates are irregular.
The second cervical vertebra may appear as an os odontoideum or may undergo
hypoplasia or aplasia. Instability and subsequent stenosis at this level may
require surgical stabilization.
Mucopolysaccharidoses
Conditions such as Hurler syndrome (mucopolysaccharidosis I), and Morquio
syndrome (mucopolysaccharidosis IV) are among almost a dozen conditions that
are included in the group of mucopolysaccharidoses-related disorders.
Scoliosis and kyphosis-type deformities can develop in these children, and all
of these patients have relative degrees of osteoporosis.
Down Syndrome
This disorder is the most common chromosomal disorder in humans and occurs
once in every 700 births. The condition relates to trisomy of chromosome 21.
The spinal manifestations relate to instability at C1-C2 and also, to a
variable degree, in the subaxial spinal region. Surgical stabilization of
these unstable vertebrae may be required.
Idiopathic
Idiopathic Scoliosis
This condition in adolescence is due to a structural lateral curvature of
the spine that becomes evident in the late juvenile or adolescent period in an
otherwise normal child. If the lateral curve magnitude (>40° on
Cobb-angle measurement) and rotational deformity become too excessive,
surgical correction may be required.
Tumors Causing Abnormal Spinal Growth
Chordoma
This tumor is thought to arise from the altered growth of notochordal cells
that persist in the nucleus pulposus into early childhood. Most are found in
the region of the clivus at the base of the skull or in the sacrococcygeal
region.
Canal Tumors Such as a Neurofibroma
Neurofibromatosis type I can result in large central plexiform neurofibroma
tumors that are associated with spinal deformities such as scoliosis and
kyphosis. These tumors result in changes in the osseous architecture of the
vertebrae, including the pedicles and the associated neural foramina.
?