Musculoskeletal Disorders in Neonates

Extremities

The upper and lower extremities develop from the limb buds. They become recognizable during the 4th week of gestation. These buds grow and differentiate rapidly in a proximal to distal sequence during the next 4 weeks. The cells differentiate into three segments: the dermatomes, which become skin; the myotomes, which become muscle; and the sclerotomes, which become cartilage and bone. By the 5th week, the hand plate forms, and mesenchymal condensations occur in the limbs. By the 6th week, the digits become evident, and chondrification of the mesenchymal condensations occurs. Notches appear between the digit rays during the 7th week. The failure of the rays to separate at this time results in syndactyly. Also during the 7th week, the upper and lower limbs rotate in opposite directions. The lower limbs rotate internally to bring the toes to the midline, whereas the upper limbs rotate 90 degrees externally to the position of the thumb on the lateral side of the limb.

Spine

The differentiation of the spinal column begins during the 4th week of the embryonic period. The somites first appear in the occipital region of the embryo; further development occurs simultaneously in a cranial-to-caudal direction. Dorsal and anterolateral migrations of the mesodermal tissue derived from the somites give rise to the connective tissue elements of the trunk and limbs. Anteromedial extensions of the somatic mesoderm migrate to surround the notochord, separating it from the neural tube and forming the primitive anlage of the vertebral bodies. The differentiation of the neural and vascular elements of the spinal column occurs simultaneously to somite development.

Definitive formation of the spinal column occurs from the 4th through the 6th week of gestation. The somatic mesodermal tissue surrounding the notochord differentiates into a less cellular and dense upper portion and a more cellular and dense lower portion. The somites cleave together, the lower portion of the superior somite joining with the upper portion of the inferior somite. The intervertebral disc develops at the site of the cleavage. The notochord, which is contained within the newly joined primitive vertebral bodies, degenerates, and those portions at the site of cleavage become the nucleus pulposus of the intervertebral disc. The neural arches and ribs develop from the dense portions of the somite, and the vertebral body develops from the less dense portions.

Chondrification begins in the primitive mesodermal vertebrae during the 6th week of pregnancy. It progresses rapidly to form cartilaginous models of the vertebral body by the end of the first trimester. Ossification of the cartilaginous models begins during the second trimester. The ossification of each side of the neural arch and of the body at each level proceeds separately. In the neonate, the ossified vertebral body and neural arches at each level are clearly visible radiographically, separated as they are by the nonossified synchondritic junctions. The ossification centers of the neural arches and body coalesce during the first 3 years of postnatal development.

The first and second cervical segments are embryologically and anatomically distinct from the remainder of the spinal column. The first cervical vertebra—the atlas—lacks the physical form characteristic of other vertebrae, having instead only a narrow anterior arch. This arch is not ossified at birth or during the neonatal period but is most often visible by 1 year of age.

The most striking feature of the second cervical vertebra—the axis—is the prominent odontoid process, derived from the caudal portion of the first cervical somite. The odontoid process joins with the remainder of the C2 body through synchondritic links with each neural arch and the vertebral body. The synchondrosis with the centrum lies below the level of the neural arches and may be radiographically confused with a fracture line; it closes by 3 years of age. The vertical synchondroses separating the centrum from the neural arches of C2 close by 7 years of age.

The radiographic evaluation of the neonatal spine requires experience. In the lateral view, the vertebral bodies are often notched at their waists and are trapezoidal rather than rectangular. In the anteroposterior view, the synchondritic links between the neural arches and vertebral bodies are not ossified and may give the false impression of a fracture. In the cervical region, the odontoid process may be confusing to those not familiar with the normal developmental anatomy of the region. Fortunately, fractures of the cervical spine are uncommon in the neonatal period and, when present, are usually associated with a suggestive history and other signs on physical examination.

Congenital Abnormalities

Teratogenic influences and genetic abnormalities occurring during the embryonic period can adversely affect the normal differentiation of the musculoskeletal system, resulting in malformations of the extremities or spine. Other organ systems differentiating at the same time are often concomitantly affected, such as the association of cardiac and genitourinary abnormalities with congenital spinal deformities (see Chapter 30 ).

Mendelian Disorders

Mendelian disorders are characterized by an abnormality in a single gene obeying the rules of Mendelian inheritance. Skeletal dysplasias such as achondroplasia and diastrophic dwarfism, Marfan syndrome, hemophilia, and rickets resistant to vitamin D are examples of Mendelian disorders. Skeletal dysplasias are often diagnosed in the neonate, whereas other entities do not become clinically recognizable until later in childhood.

Chromosomal Abnormalities

There are many varieties of chromosomal abnormalities. Aneuploidy is the presence of more or fewer chromosomes than normal and includes Down syndrome as well as Turner syndrome. Translocations are also changes that are detectable on ultrastructural analysis. Trisomies are common examples. Deletion and inversion can be more subtle and are discovered on specialized genetic testing. Although they are striking, the musculoskeletal manifestations of these conditions are frequently the least serious of the infant's problems in other organ systems.