Fetal skeleton





Objectives


On completion of this chapter, you should be able to:




  • Describe in detail the embryology of the fetal skeleton



  • Describe the variety of musculoskeletal anomalies that can occur in the fetus



  • Differentiate sonographically among the most common skeletal dysplasias



  • List limb abnormalities and the anomalies that are associated with specific defects









Embryology of the fetal skeleton


The majority of the musculoskeletal system forms from the primitive mesoderm arising from mesenchymal cells that are the embryonic connective tissue. These cells arise from different regions of the body. The vertebral column and ribs arise from the somites, and the limbs arise from the lateral plate mesoderm. The formation of the head is more complex in that the cranial bones that form the roof and base of the skull arise from mesenchymal cells of the primitive mesoderm, but the facial bones actually arise from mesenchymal cells arising from the neural crest, which is ectodermal in origin. The skeleton initially appears as cartilaginous structures that later undergo ossification.


Limb development begins the 26th or 27th day after conception with the appearance of upper limb buds. Lower extremity development begins 2 days later. Although the stages of development for the upper and lower extremities are the same, lower extremity development continues to lag behind that of the upper extremities. Initially the limbs have a paddle shape with a ridge of thickened ectoderm, known as the apical ectodermal ridge, at the apex of each bud. Digital rays begin to differentiate from the apical ectodermal ridge around day 41 through a process of cell death of the ridge between the digits. The fingers are distinctly evident by day 49, although they are still webbed, and by the eighth week of development the fingers are longer. The development of the feet and toes is essentially complete by the ninth week, although the soles of the feet are still turned inward at this time.


Anomalies of the skeletal system often result from genetic factors, though the cause may be unknown or be the result of environmental factors, including drug or mechanical effects.




Abnormalities of the skeleton


Skeletal dysplasia is the term used to describe abnormal growth and density of cartilage and bone, and the incidence is 3 in 10,000 births, with a significantly greater incidence in stillbirths. Dwarfism is the condition of a disproportionately short stature; it occurs secondary to a skeletal dysplasia. There are more than 450 types of skeletal anomalies, and not all of them are amenable to sonographic detection. The perinatal team may be able to isolate a skeletal dysplasia when abnormal skeletal structures are observed, such as bone shortening or hypomineralization.


Skeletal dysplasias are considered rare and many are incompatible with life. The lethal forms characteristically are extremely severe in their prenatal appearance, as with severe micromelia. Nonlethal skeletal dysplasias tend to manifest in a milder form. The sonographer should become familiar with the sonographic characteristics of the more common skeletal dysplasias that can be diagnosed in utero.


There are multiple anomalies of the musculoskeletal system that may be identified with ultrasound. Many of these osteochondrodysplasias have similar features, although often there are distinguishing features that can lead to a diagnosis. The primary focus should be on identifying those features suggestive of lethality. A list of short-limb skeletal dysplasias, ultrasound characteristics, and their distinguishing features are listed in Table 65-1 .



TABLE 65-1

Osteochondrodysplasia Findings




































Anomaly Sonographic Findings Distinguishing Characteristics
Thanatophoric dysplasia Severe micromelia
Macrocephaly
Cloverleaf skull
Narrow thorax
Cloverleaf skull
Achondrogenesis Severe micromelia
Macrocephaly
Poor ossification of spine, skull
Short thorax
Decreased ossification
Severity of limb shortening
Achondroplasia Rhizomelia
Macrocephaly
Trident hands
Rhizomelic shortening
Trident hands
Camptomelic dysplasia Hypoplastic fibulas
Long bone bowing
Micrognathia
Small thorax
Talipes
Fibular hypoplasia
Bowing affects lower extremities
Osteogenesis imperfecta (type II) Severe micromelia
Generalized hypomineralization
Narrow thorax
Multiple fractures
Normal head size
Hypomineralization of skull
Multiple fractures
Short-rib polydactyly syndrome Micromelia
Narrow thorax
Facial cleft
Polydactyly
Facial anomalies
Polydactyly
Hypophosphatasia Mild limb shortening
Narrow thorax
Limb fractures and bowing
Hypomineralization of skull
Fractures


Sonographic evaluation of skeletal dysplasias


The patient whose fetus is at risk for a skeletal dysplasia is commonly referred to a maternal-fetal center for genetic counseling and a targeted ultrasound. Although many skeletal dysplasias are inherited, sporadic occurrences and new mutations do occur, so it is important to screen for skeletal dysplasias as part of every obstetric ultrasound examination. Most prenatally diagnosed skeletal dysplasias occur in association with polyhydramnios or other fetal anomalies or when there is a risk for recurrence.


When a skeletal dysplasia is suspected, the protocol of the obstetric ultrasound examination should be adjusted to include the following criteria:



  • 1.

    Assess limb shortening. All long bones should be measured. A skeletal dysplasia is suspected when limb lengths fall more than 2 standard deviations below the mean ( Tables 65-2 and 65-3 ).



    TABLE 65-2

    Length of the Bones of the Leg: Normal Values































































































































































































































































































    Week No. Tibia Percentile Fibula Percentile
    5th 50th 95th 5th 50th 95th
    12 7 : 6
    13 10 : 9
    14 7 12 17 : 6 12 19
    15 9 15 20 —:— 9 15 21
    16 12 17 22 : 13 18 23
    17 15 20 25 : 13 21 28
    18 17 22 27 : 15 23 31
    19 20 25 30 : 19 26 33
    20 22 27 33 : 21 28 36
    21 25 30 35 : 24 31 37
    22 27 32 38 : 27 33 39
    23 30 35 40 : 28 35 42
    24 32 37 42 : 29 37 45
    25 34 40 45 —:— 34 40 45
    26 37 42 47 : 36 42 47
    27 39 44 49 : 37 44 50
    28 41 46 51 : 38 45 53
    29 43 48 53 : 41 47 54
    30 45 50 55 : 43 49 56
    31 47 52 57 : 42 51 59
    32 48 54 59 : 42 52 63
    33 50 55 60 : 46 54 62
    34 52 57 62 : 46 55 65
    35 53 58 64 —:— 51 57 62
    36 55 60 65 : 54 58 63
    37 56 61 67 : 54 59 65
    38 58 63 68 : 56 61 65
    39 59 64 69 : 56 62 67
    40 61 66 71 : 59 63 67
    mm mm mm mm mm mm

    From Jeanty P, Romero R, editors: Obstetrical ultrasound , New York, 1984, McGraw-Hill.


    TABLE 65-3

    Length of the Bones of the Arm: Normal Values






























































































































































































































































































































    Week No. Ulna Percentile Radius Percentile
    5th 50th 95th 5th 50th 95th
    12 : 7 : 7
    13 : 5 10 15 : 6 10 14
    14 : 8 13 18 : 8 13 17
    15 —:— 11 16 21 —:— 11 15 20
    16 : 13 18 23 : 13 18 22
    17 : 16 21 26 : 14 20 26
    18 : 19 24 29 : 15 22 29
    19 : 21 26 31 : 20 24 29
    20 : 24 29 34 : 22 27 32
    21 : 26 31 36 : 24 29 33
    22 : 28 33 38 : 27 31 34
    23 : 31 36 41 : 26 32 39
    24 : 33 38 43 : 26 34 42
    25 —:— 35 40 45 —:— 31 36 41
    26 : 37 42 47 : 32 37 43
    27 : 39 44 49 : 33 39 45
    28 : 41 46 51 : 33 40 48
    29 : 43 48 53 : 36 42 47
    30 : 44 49 54 : 36 43 49
    31 : 46 51 56 : 38 44 50
    32 : 48 53 58 : 37 45 53
    33 : 49 54 59 : 41 46 51
    34 : 51 56 61 : 40 47 53
    35 —:— 52 57 62 —:— 41 48 54
    36 : 53 58 63 : 39 48 57
    37 : 55 60 65 : 45 49 53
    38 : 56 61 66 : 45 49 54
    39 : 57 62 67 : 45 50 54
    40 : 58 63 68 : 46 50 55
    mm mm mm mm mm

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May 29, 2019 | Posted by in ULTRASONOGRAPHY | Comments Off on Fetal skeleton
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