The term clinodactyly derives from the two Greek words—kleinin, meaning “to bend,” and dactylos, which means “finger.” Clinodactyly is a descriptive term that refers to incurving or medial deviation of the finger at the distal interphalangeal joint (Figure 102-1). The fifth finger is most frequently affected. Usually, this is due to wedging of the middle phalanx so that the planes of the proximal and distal ends are not parallel but converge toward the radial side (Birkbeck, 1975). Clinodactyly is frequently accompanied by brachymesophalangy, which means that the middle phalanx of the fifth finger is short and has increased breadth. There have been several attempts to provide an objective definition of clinodactyly. In one approach, clinodactyly is defined as the relationship between the length of the fifth middle phalanx to the length of the fourth middle phalanx (Birkbeck, 1975). Other authors have used an angle of greater than 8 degrees between the long axis of the distal phalanx and the middle phalanx (Birkbeck, 1975). Yet other groups use a more stringent definition of a distal phalanx deviation of at least 15 degrees (Skvarilova and Smahel, 1984).

Clinodactyly may be isolated or part of a syndrome. It may be a sporadic developmental event or it may be familial (Poznanski et al., 1969). Approximately 60% of newborns with Down syndrome have bilateral fifth finger clinodactyly (Hall, 1970). The association between clinodactyly and Down syndrome has been known for more than 100 years (see Chapter 131). In 1896, Smith published the first X-ray illustrating fifth finger clinodactyly in a patient with Down syndrome (Smith, 1896).

Several large population studies have addressed the incidence of clinodactyly in healthy infants and children. In Czechoslovakia, Skvarilova and Smahel (1984) studied 911 healthy children from Prague, age 6 to 18 years. They defined clinodactyly as the presence of any distal phalanx axis deviation of greater than 15 degrees on clinical examination. Affected children underwent radiography and the hands of their immediate family members were examined. Clinodactyly of the fifth finger was seen in 10 individuals (1.1%). Of these, 6 individuals also had brachymesophalangy. In 9 of the 10 affected cases at least one parent was confirmed as being similarly affected. In the remaining case, the father was unavailable for examination. Affected children had considerable bilateral symmetry. The ratio of boys to girls affected was 3:2. An incidence of clinodactyly of approximately 1 % in the normal population was confirmed in a study of healthy white newborns, in which clinodactyly of the fifth finger was seen in 0.99% of cases (Marden et al., 1964). Poznanski et al. (1969) have noted significant racial and ethnic differences in fifth finger clinodactyly. For example, they cite an incidence of 3.4% in a Guatemalan population and an incidence of 5% in a population from Hong Kong. Clinodactylyis not an anomaly, per se, but rather a developmental delay or arrest. Mehes et al. (1973) reported that 0.5% of normal full-term Hungarian newborns had clinodactyly, but that this percentage increased to 0.9% in full-term small for gestational age infants. This study also noted an increased incidence of clinodactyly in preterm infants at less than 25 weeks of gestation, in which the incidence was 2.56%. Clinodactyly is seen in 12% of newborns with other major congenital anomalies (Marden et al., 1964).

Although ossification of the middle phalanx of the fifth finger occurs normally early during the second trimester of pregnancy, the major concern when fifth finger clinodactyly is detected on a prenatal sonogram is its potential association with trisomy 21 (Deren et al., 1998). In one study of ultrasound markers ofchromosomal disease, 3 of 21 cases of Down syndrome had postmortem or postnatal findings of fifth finger clinodactyly (Twining and Zuccollo, 1993).

In 1988, Benacerraf et al. demonstrated hypoplasia of the middle phalanx of the fifth digit in four of five fetuses with Down syndrome examined at 17 to 20 weeks of gestation. They described a radial curve to the fifth digit. One of the five affected fetuses had no visible ossification of the middle phalanx of the fifth digit (Figure 102-2). These authors suggested that fifth finger clinodactyly could be used in addition to other sonographic signs in screening for trisomy 21. This observation was followed by a prospective study examining the middle phalanx of the fourth and fifth digits in 1032 fetuses between 15 and 20 weeks of gestation prior to routine genetic amniocentesis (Benacerraf et al., 1990). These authors constructed a ratio of the middle phalanx of the fifth digit divided by the middle phalanx of the fourth digit and obtained a median value for normal fetuses of 0.85. In eight fetuses with trisomy 21, the median ratio was 0.59. These authors suggested a cutoff value of 0.70, which would identify 75% of the Down syndrome fetuses and 18% of normal fetuses at the same gestational age. This gave a positive predictive value of this finding of 3.2%. These authors noted that the ratio appeared to rise slightly between 15 and 16 weeks of gestation and at greater than 17.5 weeks of gestation in normal fetuses, implying that the ratios would be affected by normal developmental maturation of this bone. Ossification of the middle phalanx was completely absent in 65 (6.3%) of normal fetuses, although 50% of these fetuses were between 15 and 16 weeks of gestation. Thus, ossification of the middle phalanx of the fifth finger is a gradual process that is not normally complete at 15 weeks of gestation. This has also been confirmed by other investigators (Birkbeck, 1975). The conclusion of this report was that measurement of the middle phalanx of the fourth and fifth digits may be useful as an adjunct to sonographic screening for Down syndrome (Benacerraf et al., 1990).