Normal Examination

Normal limbs

      Fig 1, Fig 2, Fig 3, Fig 4, Fig 5, Fig 6, Fig 7, Fig 8

  1. Development: During the 8th week from the last menstrual period (LMP), a hyaline cartilage outline of the future appendicular skeleton begins to appear. The primary ossification centers of a long bone are located in the center of the shaft (diaphysis) and appear between weeks 7 and 12. Transabdominal and transvaginal sonography can clearly define the fetal skeleton even in the first trimester. The secondary ossification centers located in the epiphyses are at the distal ends of the long bone. During bone growth, the epiphyseal plate intervenes between the epiphysis and the end of the diaphysis (i.e. metaphysis). When the epiphyseal plate is replaced by bone, growth of the bone ceases. By 15-16 gestational weeks both the appendicular and axial skeleton can be consistently well imaged by sonography, although phalanges may be difficult to perceive in some instances. In the second trimester, the scapula, clavicle, humerus, radius, ulna metacarpals, femur, tibia, fibula, metatarsals and phalanges can be appreciated well sonographically in most cases.
  2. Systematic approach: All long bones should be visualized with a systematic and routine sonographic approach to minimize errors in detecting limb defects. For example, in a survey of the upper limbs, the scapula should first be located and then the probe is laterally moved to identify the humerus, which at first may be in the cross-section or longitudinal view of the long bones. After completion of the upper arm assessment, the transducer is moved to visualize the distal end of the humerus or the elbow joint and then the probe is meticulously adjusted to locate the proximal end of the radius and ulna. Similarly, after thorough examination of the forearm the wrist joints and hands will be demonstrated respectively. The lower survey must be approached in the same way from the iliac wing through the foot.
  3. Identification of long bones: The simple technique to identify the types of long bones is to obtain planes of sections that traverse the short axis of the limb. Such a plane through the forearm and calf will demonstrate two bones. In the lower leg, the more lateral bone is the fibula, and the medial bone is the tibia. The tibia and fibula and radius and ulna end at the same level distally, however, proximally the ulna is longer than the radius. This allows ready differentiation of these two bony sets and of the radius from the ulna in the upper extremity set.
  4. Femur length measurement: Femur length measurement is usually used as a means of predicting gestational age. The measurement is technically the easiest of the most common biometric measurements. The transducer need only be aligned to the long axis of the bone to obtain a proper plane of the section. Only the ossified portions of the diaphysis and metaphysis are measured. The cartilaginous ends of the femur are excluded.
  5. Identification of hands: With patience one can usually visualize all four fingers and the thumb. The hand is frequently clenched in a fist-like fashion, which can complicate the counting of fingers. The toes, although smaller than the fingers, can be seen relatively well. If difficulty arises, it is usually the functionally less important fourth and fifth toes that are not seen.
  6. If a skeletal dysplasia is suspected early in the second trimester, a follow-up study to assess the interval long bone growth should be performed. The growth rate of the long bones decreases before the absolute length falls below the cut-off point.
  7. Fetal parts ratios: The proportions between specific fetal body parts may be helpful in the diagnosis of a skeletal dysplasia. For example, foot length is not affected by most skeletal dysplasias whereas most other long bones are affected. In a normal fetus, the femur length/foot ratio (age-independent) is approximately equal to 1, and can effectively distinguish skeletal dysplasias from fetal growth restriction. Furthermore, the femur/head circumference and abdominal circumference/thoracic circumference ratios are also reliable for the detection of severe skeletal dysplasia.
  8. Degree of skeletal ossification: This should be routinely assessed, although it is usually evaluated by subjective impression. Examination of the acoustic shadow and the echogenicity of the bone itself are often helpful. However, the degree of skeletal ossification cannot be well judged. Only in the most extremely osteopenic bone can one appreciate diminished ossification on sonograms.

Fig 1:  Normal scapula  Coronal plane of the scapula: triangular shape

Fig 2:  Normal upper limb

Fig 3:  Normal radius and ulna   Note: proximal end not in the same level (arrow)

Fig 4:  Normal hand

Fig 5:  Normal hand in late first trimester

Fig 6:  Normal femur   Longitudinal scan of femur: standard plane for femur measurement

Fig 7:  Normal tibia and fibula  Note: proximal and distal end of the both bones are in the same level

Fig 8:  Normal toes

Video clips of normal examination

Normal upper limbs:  Scapula, humerus, radius and ulna; Note: ulna longer than radius at the proximal end, unlike tibia and fibula

Normal lower limbs:  Femur, tibia and fibula

Normal upper limb

Normal foot & toes

Normal hand

Pitfalls

  1. Fetal position: The fetal position is very important for a complete examination. All limbs are best visualized when floating freely in the amniotic fluid and imaging is difficult when they are tucked under the fetal body. Usually the limbs are readily imaged when the fetus is in the supine position whereas the posterior elements of the spine may be easily imaged in prone or decubitus positions.
  2. FL measurement error: A non-osseous tissue at the end of the femur can give equal brightness. Reflection is returned from tissues distal to the epiphyseal plate but in immediate contiguity with the distal femoral metaphysis.
  3. Short long bones: A fetal long bone length greater than 2 standard deviations below the mean for the gestational age does not necessarily indicate the presence of a skeletal dysplasia. Other possibilities for short fetal long bone include a normal physiologic variation in bone length, intrauterine growth restriction and chromosome abnormalities and other syndromes that may have a skeletal abnormality as part of their presentation.