Radiographic Analysis of the Sagittal Alignment and Balance of the Spine in Parents who have a Child with Adolescent Idiopathic Scoliosis

In spite of the many anatomical similarities of the spine between all
vertebrates, true *idiopathic* scoliosis does not occur in primates other than
man. A unique aspect of man compared to other vertebrates, quadruped as well as
bipeds, is the way we ambulate in a fully upright posture, with extended hips
and knees, placing the upper body*s centre of gravity consistently over the
pelvis. All other vertebrates, even bipedal animals, have flexion contractures
of the hips and knees, and little or no pelvic or lumbar lordosis. This unique
fully erect position of the human spine results in different loading of the
vertebral column.

In all vertebrates, including humans, the spine is predominantly loaded by
axial compression, which is mainly carried by the anterior column that consists
of the vertebral bodies and intervertebral discs. However, due to the more
erect human position, shear loads are differently orientated in the human
spine. In a more horizontally (quadruped or bipedal) positioned spine shear
loads are mainly ventrally directed, whereas in the more vertical human
position, certain parts of the spine have been proven to be subjected to
dorsally directed shear loads, as was shown in work by the investigators.

The anatomy of all spines, with broad discs and vertebral end plates, as well
as the posterior location of the facet joints and the posterior pull of the
predominantly posteriorly located muscles and spinal ligaments, is well
designed to counteract both axial as well as ventrally directed shear loads.
However, these anatomical stabilizing mechanisms are deficient in the case of
dorsally directed shear loads, possibly leading to rotational instability.

Castelein et al. proposed two hypotheses regarding dorsal shear forces:
(1) Dorsal shear forces lead to rotational instability;
(2) Dorsal shear forces are the resultant vector of gravity and muscle
activity, and act on certain parts of the spine depending on its orientation in
space. These forces increase in magnitude with increasing backward tilt
(declination) and with reduced thoracic kyphosis.
Indeed, a recent in vitro study on porcine and human spinal segments did show
that dorsal shear loads lead to an impairment of rotational stability of the
spine.

Although the specific cause of adolescent idiopathic scoliosis (AIS) has not
been established, the role of genetic or hereditary factors in its development
is widely accepted. Wynne-Davies performed a study that strongly supported a
hereditary component. She examined 180 patients with AIS, and found in 25% that
other members in the family were affected.8 A following population study found
rates of affected first, second, and third-degree relatives of respectively
15.8%, 2.4%, and 1.4%. Furthermore, studies done on twins have shown a higher
concordance rate for AIS in monozygotic twins than dizygotic twins, with a
concordance rate of 73% versus 36% respectivably.
Despite these convincing arguments regarding the familial nature of AIS, the
particular mode of inheritability remains unclear.

Our hypothesis is that certain spinal configurations predispose more than
others for dorsal shear forces, and that these spinal configurations are
inherited.
Therefore, we suspect to find a significantly different sagittal spinal
alignment in the parents of children with AIS as compared to the normal
population.

To investigate whether parents of children with adolescent idiopathic scoliosis
have a significantly different neutral upright sagittal alignment compared to
adult volunteers whose offspring does not have adolescent idiopathic scoliosis.

Retrospective case-controled study

The level of radiation is categorized as IIa (0,1-1,0 mSv) according to IRCP 62
(* Radiological Protection in Biomedical Research*). Which implies that the
level of societal benefit should be intermediate. *To justify risk in category
IIa, the benefit will probably be related to increases in knowledge leading to
health benefit.*

Participating in this study is not a phycical or psychological burden.
Furthermore, participating will only take approximately 30 minutes.