The Fetal Skeletal System - HD

Introduction

My name is Dr. Carol Benson.
I work at Brigham and Women's Hospital
and Harvard Medical School in Boston.
I will be discussing the ultrasound evaluation
of the fetal skeletal system.

Ultrasound Assessment of the Fetal Skeletal System

Ultrasound is used to assess the fetal skeletal system.
We can evaluate the extremities, the spine,
and the calvarium.
Now the long bones develop during the first trimester
and they begin to ossify so
that they're visible on ultrasound by the end
of the first trimester.
By the late third trimester,
the secondary ossification centers are visible in the distal
femur, proximal tibia and proximal humerus.

Femur View

This is a view of a 41 week femur.
You can see that the calipers have been placed at either end
of the ossified portion of the femur
representing the diaphysis,
but you can see that in fact the femur is longer than that
with the epiphysis at one end,
the ossification center visible in the epiphysis
and then the femoral head
and neck at the other end of the bone.

Fetal Spine Evaluation

We can look at the fetal spine.
These are two 3D renderings
of the entire fetal spine on two different fetuses.
Notice how you can see the matching ossification centers all
the way up and down the spine.
You can also see the ribs forming the thorax on either side.

Evaluation of Extremities

Now we evaluate the extremities during our second
and third trimester ultrasound.
We look at the size of the long bones to look
for skeletal dysplasia.
We look for the presence of each of the extremities
to exclude an absent limb or amniotic band syndrome.
We evaluate the bones of the forearm to look
for radial hypoplasia.
We evaluate the hand position to exclude clenched fists
and the foot position to exclude club foot
or rocker bottom foot.

Femur Measurement

This is a view of a femoral measurement.
You can see that the calipers have been placed at either end
of the ossified portion of the femur,
and we use this to assess the size of the femur
for gestational age.

Upper Extremity View

This is the view of the upper extremity
of a fetus showing the humerus and both bones in the forearm.
The humerus is slightly longer and the radius slightly shorter.
And here we see a 2D
and a 3D image of the hand showing the normal five fingers.
Here you can see in fist form here you can see
with the hand open
and here we see images of the foot.
We have the gray scale image
of the foot showing the five toes of the foot,
and here we have the foot and ankle on this fetus
and the crossed feet and ankles on the other fetus.

Assessing Femur Length

We assess the femur length and compare its size to norms.
For gestational age, the femur length should be equal
to the mean plus or minus standard deviations
within those norms.
If the femur length falls for two
to four standard deviations below the mean, most
of those fetuses are growth restricted
and do not have a skeletal dysplasia.
But when the femur length falls more than four standard
deviations below the mean,
that usually represents a skeletal dysplasia.
And in most of those cases,
the bones will also appear abnormal by on ultrasound.

Evaluating Short Long Bones

So what do we do when the long bones measure too short?
Well, we wanna exclude a skeletal dysplasia.
We wanna determine if it's a dysostosis or a malformation
or a deformation.
This can happen, for example, with amniotic band syndrome
or from a restricted uterine environment.
When we suspect a skeletal dysplasia,
we wanna assess the degree of shortening
of all the long bones
and typically they will be more than four standard
deviations below the mean for gestational age.
Then we wanna look at the distribution
of which bones are involved.
Do does it involve the extremities, the spine,
the calvarium, and the ribs?
And this helps us determine what type
of skeletal dysplasia it is.
We wanna look at other bony abnormalities to look
for mineralization, fractures or bowing.
And it's also important to look for polydactyly.

Lethal Skeletal Dysplasias

Lethal skeletal dysplasias are those dysplasias which affect
the fetus such that the neonate cannot survive
after birth due to respiratory failure.
And typically ultrasound makes the diagnosis of this type
of dysplasia during the second trimester the kinds
of dysplasias that could possibly fall in this category,
some of them are listed here, thanatophoric dysplasia,
osteogenesis imperfecta, type two agenesis congenital
hypophosphatasia, short rib polydactyly syndrome.

Thanatophoric Dysplasia

Thanatophoric dysplasia is the most common lethal
skeletal dysplasia.
It's characterized by severe shortening
of the proximal long bones called micromelia.
The long bones are bowed.
The thorax is typically very narrow with short ribs
and the vertebral bodies are flattened.
The skull has an abnormal clover leaf shape
and often there's macrocephaly,
which is when the temporal lobe has excess gyration
and excess fissures.

So here is a fetus with thanatophoric dysplasia.
Notice the cloverleaf shape to the cranium.
You can see protruding in the frontal region as well
as protruding in the region of the temporal lobes.
When we look at the thorax
and abdomen of this fetus, we see
how narrow the thorax is anterior posteriorly compared
to the width of the abdomen
below it from anterior to posterior.
This is a fetus with thanatophoric dysplasia.
We look at the transverse view of the fetal thorax
and see how small it is, how very short the ribs are,
how the heart takes up most
of the area on a cross-sectional view of the chest.
On the 3D view.
With bony windows, we see how short the the ribs are
and how narrow the chest is.
And on the 3D surface rendering, we can see
how small the chest is compared
to the abdomen and the fetal head.
The long bones in this in this dysplasia are
often very abnormal.
Here we can see the left femur is bowed.
The right femur is also bowed.
The right humerus is bowed and the right tibia
and fibula are extremely short for gestational age.

Osteogenesis Imperfecta Type II

With osteogenesis imperfecta type two,
this is an autosomal recessive syndrome that is lethal,
that is the fetus cannot survive after birth.
The ultrasound findings in a fetus with this type
of osteogenesis imperfecta include fractures
and deformities of the long bones
and ribs, poor mineralization particularly of the skull
and a soft cranium.
While this is autosomal recessive, the other forms
of osteogenesis imperfecta, namely types one, three
and four are autosomal dominant and non-lethal.

So here's a fetus with osteogenesis imperfecta.
Type two, notice the fracture deformity of the right femur
with its angulation at a former fracture site.
Then when we look at a coronal view of the fetus, we can see
how narrow the thorax is compared
to the width of the abdomen.
This narrow thorax prevents the development of normal lungs.
Looking at another fetus
with osteogenesis imperfect type two,
we see a very short left humerus with bowing.
We can see when we look at the skull
that there is almost no ossification in the skull so
that we can see intracranial contents very clearly.
In addition, with gentle compression from the ultrasound
transducer, we see flattening of the skull at that site
because the cranium is so soft.

Osteogenesis Imperfecta Types I and IV

Osteogenesis imperfecta is type one
and four may also be diagnosed in utero.
Typically in these cases there's lagging
of growth of the long bones.
By the time the fetus is the fetus reaches
the third trimester.
These fetuses may also have mild fracture deformities
with bowing of the long bones or angulation.
They may have poor mineralization
and their skull may be somewhat soft.

This fetus has osteogenesis imperfecta type one.
Notice that the humerus shows bowing instead
of a nice straight diaphysis.
And when we look at the forearm, we see there's bowing
and angulation of the distal ulna and the distal radius,
and we also see bowing of the right tibia.
Looking a little more closely at this fetus, we see
that the other femur also has bowing.
And when we look at the cranium, we do see some ossification
of the cranium, unlike the fetus
who had osteogenesis imperfecta type two.
But in this case, still
with gentle pressure from the transducer,
we see flattening of the cranium.
So this skull is still soft.

This fetus also has osteogenesis imperfecta type four.
The femur on one side is normal length
with a normal straight diaphysis or shaft.
But when we look at the contralateral femur, we say it's,
we see it's foreshortened to two centimeters
and there's an angulation deformity due
to a previous fracture And this is just a magnified view of
that fracture deformity of one of the femurs.

Achondrogenesis

Achondrogenesis is another recessive,
autosomal recessive lethal skeletal dysplasia.
These fetuses have severely diminished
or absent ossification, marked shortening
of their limbs, absent vertebral body ossification as well
as normal cranial calvarial ossification.

So this fetus has achondrogenesis.
When we look at the thorax, we see it's small
with the heart occupying most
of the area on this cross-sectional view.
When we look at a longitudinal view of the fetus,
we see the large head, the very narrow thorax front to back,
and then it looks like the abdomen protrudes.
And that's only because relative
to the thorax it's quite large.
Looking at the long bones, we see an angulation deformity
of the radius angulation deformity of the femur
and marked shortening of the tibia and fibula with bowing.

This fetus also has achondrogenesis.
Notice how small the thorax is compared to the abdomen.
You can see it on the sagittal view of the fetus,
but you can also see it on the 3D image
of the fetus showing a small thorax and protruding abdomen.
And here you can see the measurement of the thorax compared
to the abdomen from anterior posterior is
markedly different.

Non-Lethal Skeletal Dysplasias

The non-lethal skeletal dysplasias ultrasound typically does
not diagnose in the second trimester,
but rather these are picked up more often in the third
trimester and these types of dysplasias are not as severe.
Infants can survive after delivery.
Some a couple are listed here.
Heterozygous achondroplasia osteogenesis imperfecta
as type one and four asphyxiating thoracic dystrophy.

Arthrogryposis

Another type of skeletal deformity is something
called arthrogryposis, which is not in and itself a diagnosis,
but rather it's a description of a fetus
who has multiple joint contractures.
And this can develop from a variety of etiologies
can develop from limitation to motion in utero such
as from oligohydramnios or a multiple gestation
or a bicornuate uterus.
It can also develop from abnormal nerve function,
abnormal musculature
or defective connective tissue with arthrogryposis.

The findings on ultrasound include multiple contractures,
intrauterine growth restriction.
Often there's polyhydramnios
and there may be hydrops due to lack
of movement of the fetus.

This fetus with arthrogryposis has abnormal posturing
of the lower extremities.
You can see fixed hyperextension at the knees.
You can see that the feet are both turned in
and persistently flexed.
When we look at each leg on its own, we can see
that there are club feet at the end of each of these
legs due to the contractures.

This is a different fetus,
but another one with arthrogryposis.
Notice the abnormal posturing of the hand here,
the right hand in this 3D view.
And notice the abnormal posturing of the foot.
It was persistently pointed forward
And here you can see what that foot looked like at birth.

This fetus has Larsen syndrome, which is another cause
of arthrogryposis.
This is a rare genetic syndrome characterized
by contractures.
Notice the hand and its abnormal posturing
and the foot with a rocker bottom and abnormal posturing
and how the leg is hyper extended up over the body.
The 3D view tells everything.
You can see how the legs are abnormally
extended up over the fetal abdomen with hyper extension
of the knees, abnormal motion at the hips
and abnormal positioning of the feet at the end of the legs.

This fetus had persistently clenched hands secondary
to in utero exposure to cytomegalovirus.
Notice in the head we see ventriculomegaly
and intracranial calcifications
and these hands remain flexed throughout the entire time
of the ultrasound.
The feet were also abnormal.
There were bilateral club feet,
but not the typical club feet.
They were turned upward, not just turned inward
and had unusual positioning of the feet.
Here they are 3D ultrasound prenatally.
Here they are immediately
after birth showing the same abnormal positioning
of both of the feet.

Upper Extremities Abnormalities

Now we also look at the upper extremities closely.
We look for abnormalities of the hands and the forearms.
Abnormalities may be inherently present due to malformation
or dysplasia from the beginning of development,
or they may develop
after the later on in gestation affected
by external factors such as uterine constraints
or amniotic bands that cause deformation
or disruption during development from a teratogen, one
of the most common abnormalities of the forearm
and arm are radial ray anomalies.
These are associated with VACTERL syndrome.
So if you do see a hypoplastic or
or agenesis of the radius,
do look carefully at the rest of the fetus
because it may have one of these other syndromes
that are listed here.
It also can be associated with trisomy 13 or trisomy 18.

This abnormality of the thumb was part of the VACTERL syndrome,
which is a an anomaly characterized by anomalies
of other parts of the body, including the heart
and the kidneys, as well
as the vertebral column and the radius.
So here we see abnormal attachment of the thumb to the hand.
When we look at the heart, we can see
that the aorta is overriding and widened.
This baby had a tetralogy of Fallot
and there was a two vessel umbilical cord.

This is another fetus with the VACTERL syndrome.
This one had a hypoplastic radius.
You can see that the radius is foreshortened
and the ulna is bent around that short radius.
Here we can see we've measured
or pointed out the small abnormal radius here.
This fetus also had a cardiac defect,
double outlet right ventricle, which we see here in the
parallel outflow tracks from the
transposition of the great vessels.

This is a fetus with trisomy 18 who had absence
of the radius on both sides.
Notice that the humerus appears normal,
the ulna is foreshortened, and then the hand is turned
and rotated inward to lie on top of the forearm.
And the same was true on the other side.
This is a magnified three-dimensional view of the bones
of the upper extremity showing the humerus, the ulna
and the hand turned over to collapse on top
of the forearm.

Limb Reduction Defects

Now limb reduction defects are those defects
where there's some part of the extremity missing.
The terminal transverse deletions are those
that are missing from one point onward.
These may be seen such as when there's an absent hand.
Very often these are isolated anomalies that are sporadic.
They're usually unilateral.
They may result from amniotic band syndrome
or from a vascular accident.
But sometimes these limb reduction defects result from
syndromes such as an orofacial digital syndrome like Poland
syndrome or again from amniotic bands.

This fetus is missing a hand notice.
On the gray scale ultrasound, we have the ulna
and the radius, and then just a little nubbin on the end
with the 3D, we can show the surface rendering
of this upper extremity to show the parents
what the upper extremity will look like.
And you can see that there's just a nub
where you would expect the hand to be.
We can take that 3D volume
and look more closely at the bony structures inside it.
This is that upper extremity.
We can see that the humerus, ulna and radius are intact
and that they're at least four carpal bones, which means
that the surgeons postnatally will be able
to create a wrist out of this small nub.
And that exists. This fetus is missing part
of the forearm and the hand on the left side.
Notice the right arm and hand are normal,
but the left arm has a normal upper arm
but only half of the forearm
and no hand at all On bony windows we can see exactly
how much of the radius and ulna are still present
before the truncation partway down the forearm.

Polydactyly

Polydactyly is when there's an extra finger or a toe.
Polydactyly in and
of itself does not cause problems postnatally,
but it is often associated
with other syndromes or anomalies.
And therefore we need to look closely at the
fetus when we see this.
Finding the skeletal dysplasias associated
with polydactyly or short rib polydactyly, chondrodysplasia,
ellipsoidal dysplasia, asphyxiating thoracic dystrophy.
We can also see polydactyly
with trisomy 13 Meckel Gruber syndrome
and sometimes polydactyly is just an autosomal dominant
trait with no other syndrome or anomaly.
And this is usually when it's post axial that is next
to the little finger or little toe.

This is a fetus who has polydactyly
with Meckel Gruber syndrome.
Notice on the clip through the hand the extra digit next
to the little finger and then when we look at the foot,
we can see that there are six digits across the
top of the foot.

This fetus also has polydactyly
with the extra digit sitting in front of the little finger.
Here we see a thumb and four normal fingers.
Here we see it here and then in front
of the little finger is an extra digit.
On the clip you can see it kind of protruding
to the side a little bit of that little finger.
Sometimes it just sits in front of that little finger.

And this fetus has a duplicated thumb.
You can see on the clip
that the thumb is too wide with too many bones.
And on the 3D you can see how wide the thumb is
with extra bones here and here.

Syndactyly

Syndactyly is a term for a claw shaped
or cleft through the middle of the hand
or the foot, typically characterized by missing digits,
sometimes with syndactyly of the extra digits remaining.
This can be associated with genetic syndromes such
as split hand foot malformation, Silver Russell
or Cornelia de Lange.

Here's a fetus missing the middle finger
with a cleft right down the middle of the hand,
you can see the cleft on the still image.
You can see the cleft on the clip.
But when we do the 3D, we actually get the bony window
and clearly see the thumb, the index finger and the little
and ring finger with a cleft running down the middle,
missing that middle finger.

This fetus has cleft hands.
Here you can see a cleft, or sorry, cleft foot.
Here you can see a cleft in the middle
of the foot here on the view of the bottom
of the foot we see a large cleft.
And here on the 3D we see that defect right in the middle
of the foot When fingers

Clinodactyly and Overlapping Digits

overlap and don't sit in their normal position.
It's called clinodactyly or overlapping digits.
And this is due to deviation
or deflection of the axis of the fingers,
often curving inwards.
So the fifth finger may curve towards the fourth finger
and this may be seen with trisomy 21
or you may have overlapping digits with trisomy 13 or 18.

This fetus has an overlapping finger just on one hand.
So the left hand was completely normal,
but the right hand had persistent overlap
of the second over the third digit.
So this is abnormal and should not be like that.
But this was an isolated anomaly that was not associated
with trisomy 18.

But this fetus does have trisomy 18.
Notice all of the overlapping digits.
The little finger in the index finger both overlap the
fingers adjacent to them as we see here and here.
And if we look here, we can get a closer look at
that overlapping index finger over the middle finger.

And this fetus has very short fingers. This is called brachydactyly.
Notice how easy it is to recognize
how short they are on the 3D view.

Clubfoot

Clubfoot can occur as an isolated abnormality
or maybe a component of a genetic syndrome
or may result from environmental pressures in utero such
as severe oligohydramnios or uterine anomaly.
The ultrasound findings are characteristic.
We see the bones of the foot lying in parallel to the bones
of the lower leg.

So here we have a 13 week fetus where we see the bones
of the lower leg and the bones
of the feet in the same plane.
We suspected a club foot,
but this is a very early gestation to make that diagnosis.
And so when she came back at 21 weeks gestation,
we now could confirm the diagnosis.
We now see the clubfoot on 2D ultrasound.
The bones of the feet lying in parallel with the bones
of the lower leg and seen even better on the 3D ultrasound
with the surface rendering showing the turning in of
that foot and the bony reconstruction showing the bones of
that foot lying in parallel with the bones of the lower leg.

Sandal Gap Deformity

Another foot anomaly we sometimes see
as a sandal gap deformity.
This is when there's abnormal prominence between of space
between the first and second toes
and it's can be seen
with some chromosomal abnormalities such
as DiGeorge syndrome with 22q deletion or trisomy 18.

So here we have three 3D images from a fetus
with the sandal gap deformity.
Notice the prominence of the space between the first
and second toes here and here.
Here we have both feet.
So you can see it's affecting both sides.
Here's just a slightly different angle of that view.

Proximal Focal Femoral Deficiency

We also look at the long bones.
An anomaly we pick up could be something called proximal
focal femoral deficiency when there's shortening of one
of the femurs but the other femur is normal.
Now there may also be other skeletal anomalies typically on
the same side as the abnormal femur.

So here we see a normal femur on the left
with a nice normal shaft and here we see bowing
and shortening of the right femur.
And on the 3D we can see how nice
and normal the left femur was
and how very abnormal the right one was with curving
and bowing and shortening.

This is that fetus after birth.
You can see that on the normal side we have the femoral neck
and head and the greater trochanter.
And on the abnormal side we're missing the greater
trochanter and the femoral neck.
And so it's foreshortened
and bent as we see here on the x-ray
and the compared to the normal left side,
just like we noted in utero

Amniotic Band Syndrome

With amniotic band syndrome,
there's early rupture of the amnion.
Fibrous strands then entrap
and adhere to the fetus leading to limb amputations
or deformities and other disruption
of the fetus including craniofacial defects, facial clefts,
abdominal wall defects.
And we can even have ectopia cordis.

Now on ultrasound we'll see a variety of deformities.
They're all different. It doesn't follow a a true syndrome
in that no two cases are the same.
We may see deformities of the skeleton, the cranium,
and face the abdominal wall.
We may have a clue as to the reason
for the abnormality if we can see the adherent bands
extending from the edge of the gestational sac to the fetus.
Now these should not be mixed
or they should not be misdiagnosed as amniotic sheets.

So here's a fetus with amniotic band syndrome
where the amniotic band is entangling one of the hands.
Notice this hand is very abnormal when the fetus moves you
can see a band traveling with it
as it moves away from the wall of the uterus.
And on 3D we can see how malformed
that hand is missing some fingers as well.

And this fetus has amniotic band syndrome.
The band must have encircled the leg
but not caused an amputation.
So we see an indentation all the way
around this leg here you see it in still image,
but this fetus did have an abnormal
foreshortened right foot.

Spinal Abnormalities

Part of the assessment
of the fetal skeleton involves looking at
spinal abnormalities.
We can see meningomyelocele, hemivertebrae, scoliosis,
diastematomyelia, myelocoele or sacral agenesis or agenesis
and sacrococcygeal teratomas.

Meningomyelocele

When a meningomyelocele is present there's disruption
of the posterior elements of the spine as well
as the skin over the spine.
Normally the spine has three ossification centers
that make a C shape When there's a meningomyelocele,
the posterior elements are splayed laterally.
There's a defect in the skin and a mass
or dorsal sac protrudes from the spinal canal out
behind the fetus.

As we see here, here are images from a fetus
with a meningomyelocele.
On a transverse view of the lower spine,
we can see a cystic structure protruding posteriorly.
This represents the dorsal sac.
On the sagittal view, again, we see the dorsal sac,
but we also see disruption of the bony elements
of the spine in the distal spinal region.
On the 3D view of the same fetus, we see widening
of the spinal canal distally due to the meningomyelocele
with splaying of the posterior elements.
On the 3D, we can identify the level of the meningomyelocele.
This one begins at L five S one.

This is another fetus with a meningomyelocele.
This was a much more subtle spinal defect on ultrasound,
but we had a clue that there was a meningomyelocele due
to the changes in the fetal head.
On an axial view, at the level of the ventricles,
we see the lemon shaped contour to the fetal head
with concavity and flattening of the frontal bones
and we see the banana shaped
to the cerebellum in the posterior fossa due
to the Chiari two malformation doing a 3D ultrasound
of the spine, we saw no abnormality.
We don't see splaying of the posterior ossification centers,
but a careful clip
through the distal spine in this fetus showed us
that spinal defect with absence of the skin
and slight splaying of the posterior elements.
This was an open defect, so it's the absence of the skin
and the protrusion of the posterior elements at that level.

Meningocele

A meningocele is a less severe form of spina bifida.
If there is a meningocele, you will have a spinal defect,
but only fluid in membranes will protrude from
the spinal canal.
No nerve roots will be involved.
These are often skin covered and
therefore the maternal serum alpha fetoprotein
will be normal.

On ultrasound, we will still see splaying
of the posterior elements
and we'll then see cystic mass protruding posteriorly
from the fetus.

So here's an example.
At 19 weeks gestation, we see when we look at the fetal head
that the posterior fossa is normal.
There is no Chiari two malformation.
There is no effacement of the cisterna magna.
When we look at the spine on longitudinal view,
it can look normal in some views,
but on a direct sagittal view we see disruption
of the posterior elements with a cystic mass protruding
posteriorly from that area.
And on transverse view we see splaying
of the posterior elements with a bit of protrusion
of cystic, a cystic lesion from the spinal canal.

This is another meningocele we can see
that protruding from the back
of the fetus is the cystic mass.
It does travel into the spinal cord
but only by a narrow stalk.
Here you can see the path of that meningocele
and its narrow stalk out of the spine.
This was just a meningocele notice that the posterior fossa
of the fetus is completely normal
with no Chiari two malformation.

Hemivertebrae

Now hemivertebrae
or when there's segmentation abnormalities of the spine with
where there might be two posterior elements on one side
and one on the other, these are associated
with a variety of syndromes.
On ultrasound, we'll see a kink in the spine with mismatch
of the posterior ossification centers.

This fetus has hemivertebrae in the distal lumbar spine
you can see that there's a kink in the lumbar spine on
longitudinal view
and if you count the posterior ossification centers,
you see there's a mismatch with an extra one located here
after we've matched the other pairs.
When we do the 3D rendering with the bony windows
and reconstruction,
you can see this is the hemivertebrae here.
There is one ossification center on one side
and there are two on the other side
with a single vertebral body.
So this is a hemivertebrae
with the extra one on one side.

This fetus also has a hemivertebrae.
You can see up here at L one
that there are two ossification centers on one
side and one on the other.
Slightly different views here and here.
This fetus also happens to have 13 ribs as you can see here.

Diastematomyelia

Now an unusual anomaly
of the spine is something called diastematomyelia.
This is when there's a bony or a cartilaginous
or fibrous spur that bisects the spinal canal
and the spinal cord.
This can cause a tethered cord if it's not treated shortly
after birth on ultrasound, we'll see widening
of the spinal canal
and in some cases a neural tube defect associated
with the diastematomyelia.

So here's a fetus with diastematomyelia
and a meningomyelocele.
You can see marked splaying of the posterior elements,
but you can also see a bony spur between
the posterior elements.
This was representing the spur
that splits the spinal canal here on 3D.
You can see the entire thing with splaying
of the posterior elements and that extra bony spur.
And here's what it looks like after birth.
Notice the marked splaying
that we see on the x-ray almost identical to
what we saw prenatally

Sacral Agenesis

Sacral agenesis is a complication found in fetuses
of diabetic mothers with poor glucose control.
It's characterized by hypoplasia or absence of two
or more of the sacral vertebrae
on a longitudinal view of the spine.
When we follow the spine down, we see
that we're missing some of the ossification centers here
and there's disorganization of the ones just
above the region of agenesis.
And when we look on the x-ray
after birth, it looks just like the prenatal findings
with disruption of the ossification centers above
and then missing ones below.

This fetus has four lumbar vertebrae
and no sacral vertebrae.
L one, L two, L three, L four,
and L four has a triangular shape
and then it converges on the two iliac crests
because there's a complete absence of the sacrum.
This was the prenatal view. Here's the postnatal view.
Looks just like the prenatal view, lumbar 1, 2, 3,
triangular shaped four
and the iliac crests

Cranial Anomalies

Evaluation of the skeletal system also should entail looking
at the cranium cranial anomalies can be found
and can be associated with other abnormalities
of the fetal skeletal system.

One of the most common cranial anomalies is
craniosynostosis, which is early closure of the sutures.
This can come in several forms like trigonocephaly
with trisomy 13
or a cloverleaf skull with thanatophoric dysplasia.
Another cranial anomaly is the lemon sign.
This is a complication of the Chiari two malformation
and we can have a strawberry skull with trisomy 18.

With craniosynostosis, there's premature closure of one
or more cranial sutures.
This occurs more often in males than females.
The complications lead to abnormal head shape,
abnormal facies, neurologic deficits,
and even sometimes hearing loss.
The prenatal diagnosis cannot be made in the second
trimester and often not even in the third trimester.

This is a case of craniosynostosis
that we did diagnose in utero at 34 weeks gestation.
We noticed that the fetus had a pointed skull instead
of the normal oval shape to the frontal portion
of the cranium.

Here we see on two different views
with trigonocephaly, there's craniosynostosis
as a result of premature fusion of the metopic suture.
That's the one anterior midline towards the forehead
and it's this type of craniosynostosis that's associated
with trisomy 13.

So here's a fetus with trisomy 13 and trigonocephaly.
Notice the pointing of the front part of the fetal cranium.
As we see here, this fetus also had semilobar holoprosencephaly, so there was fusion
of the lateral ventricles across the midline anteriorly.

A clover leaf skull is what we'll see with things such
as thanatophoric dysplasia.
It's from craniosynostosis, causing a trilobe shape
and a very prominent forehead.

This is a fetus with thanatophoric dysplasia
that has something of a cloverleaf skull.
We can see a little bit of bulging in the region
of the temporal lobes
and a little bit of bulging anteriorly here on the axial
view, but a little more prominent bulging
of the forehead on the profile view of the fetus.

Now a lemon shaped cranium is one that's often seen
with the Chiari two malformation that's seen
with a meningomyelocele.
It's characterized by flattening
or concavity of the frontal bones leading
to a lemon shaped skull.

You can see on this fetus who has a meningomyelocele here
and a protruding dorsal sac
that the cranium has the shape of a lemon with flattening
of the frontal bones and a pointed anterior portion
of the cranium.
This fetus also has ventriculomegaly.

The strawberry shaped cranium is associated with trisomy 18.
Here is a fetus who has a strawberry skull.
There's flattening posteriorly
and then almost a triangular shape to the front portion
of the cranium leading to the shape of a strawberry.
And this is commonly seen with trisomy 18.

This is another fetus with trisomy 18.
Notice the strawberry shape to the skull,
flattening on the sides
and flattening posteriorly,
giving it almost a triangular shape.

This is a 3D of the same fetus showing the very abnormal
posturing of the hands characteristic of trisomy 18.

Conclusion

This is a picture of a fetus telling me to stop.
I've come to the end of my discussion of ultrasound,
of fetal skeletal abnormalities
and I hope this will help you in your practice.
Thank you.