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FETAL SPINE WHEN TO RAISE ALARM
The mesenchymal cells from the sclerotome of the somites are present in three main areas
1. Around the notochord which is the vertebral body
2. Around the neural tube which forms the vertebral arches
3. In the body wall which becomes the ribs.
The mesenchymal cells migrate to the presumptive region of the vertebra. Thechondrification occurs around 6 weeks, which forms the centrum. The cartilageousmass transforms into a cartilaginous model over which the bone forms which is around 10 weeks of gestation.
The ossification of the vertebral body starts in the T12 – L1 level and progresses in cephalic and caudal directions. The vertebral arches ossification progresses from cranial to the caudal end.
The bony ossification (primary ossification) occurs in utero. There is one vertebral body and two vertebral arches (laminae). The secondary ossification centres - tip of the spinous process, two transverse processes, articular processes are cartilaginous inutero. Ossification of typical vertebrae begins during the embryonic period and usually ends by the twenty-fifth year
Ultrasound of the spine
On sonography, the ossification center of the neural arch was visible in the cervical region in all fetuses by 18–19 weeks. The ossification center of the neural arch was first seen in the thoracic region during the 18–19-week period and was consistently demonstrated in the 20–22-week period. In the lumbar and sacral regions ossification was not seen before 19 weeks and was consistently demonstrated in 22–24 weeks (Ref. 1). However with high resolution ultrasound the ossificationcentres are visible from cervical to lumbar level around 12-13 weeks of gestation.
“c” shaped curvature sacral tapering
The sacral tapering is observed in the parasagital scan in all fetuses beyond 22 weeks of gestation.
The three planes of imaging and the structures seen
1. Parasagital section vertebral body, one lamina and skin line (seen as three parallel lines)
2. Axial section vertebral body and the two laminae seen converging with skin line on top of it.
3. Coronal section - Anterior coronal -Vertebral body, Posterior coronal -Laminae (two in number)
Platyspondyly in thanatophoric dysplasia
Unossified laminae in hypophosphatasia
Abnormalities of the vertebral arch (Laminae)
· Open neural tube defect – Head signs present (abnormal posterior fossa)
· Closed neural tube defect – Normal posterior fossa. Diastematomyeliaotherwise termed as split cord malformation is suspected when there is widening of the laminae with a bony spicule or fibrous spur which splits the cord into two. This commonly occurs in the thoracolumbar region,rarely can affect cervicodorsal region. Associated hemivertebrae can be present.
5. The Ultrasound Review of Obstetrics & Gynecology, Volume 5, Issue 1 March 2005, pages 29 - 33.