Dermal ridge count To count the dermal ridges of a particular pattern, a line is drawn from the middle or center of the pattern to a triradius (a point at which three converging patterns meet) (Fig. 12.5). The number of ridges which are crossed by this line or touch the line is counted. The total dermal ridge count for the individual is the sum of the ridge count of
Figure 12.5 Measuring dermal ridge count.
the 10 digits. If there are two triradii, as in a whorl, the line is drawn to the most distal triradius. Arch patterns have no ridge counts.
The average dermal ridge count in males is 144; in females 127.
Triradii Triradii (or deltas) are the points at which three sets of converging ridge patterns meet. They are seen between the bases of the fingers, on the fingertips in loop and whorl patterns, and on the palm. Usually there is a triradius in the proximal part of the palm to the ulnar side of the center, just above the wrist flexion-crease: the proximal axial triradius (t). An angle can be established between the triradius at the base of the index finger (a), the proximal axial triradius (t), and the triradius at the base of the fifth finger (d). This is called the atd angle. A distal palmar triradius (t or t ) may also be present. When the at d angle is calculated, it is usually much larger than the atd angle. About 4 percent of Caucasians have a distal palmar triradius. Distal palmar triradii can be found in a number of syndromes, and their presence should alert the clinician to examine the dermatoglyphics more carefully.
Thenar, hypothenar, and hallucal patterns Patterns may be present in the thenar and hypothenar areas of the palm. Unusual patterns may be helpful in making a specific diagnosis. A lack of ridges in the hypothenar region of the palm can be seen in Cornelia de Lange syndrome. On the soles of the feet, in the hallucal area, a pattern (loop or whorl) is usually seen. If no pattern is present (i.e., there is an arch), the hallucal area is said to have an "open field." Open hallucal fields are very rarely found in normal individuals, but are present in about 50 percent of patients with Down syndrome.
Normal distribution of fingerprint patterns Evaluation of the fingertips generally reveals a variety of patterns on different fingertips (Fig. 12.6). The patterns tend to be familial. The most frequent patterns are whorls on the thumb and the fourth finger, and ulnar loops on fingers three and five. Among normal individuals, 0.9 percent have a predominance of arches on the fingertips, (i.e., 6 of 10 fingers show arches). However, this pattern is very frequently seen in patients with trisomy 18.
Three percent of normal individuals show 9-10 fingertips with a whorl pattern. Excessive whorl patterns derive from high fetal pads. They are seen frequently in patients with Turner syndrome or Noonan syndrome.
Radial loops on the fourth and fifth finger are unusual in normal individuals (1.5 percent) but are common in individuals with Down syndrome (12.4 percent).
Racial differences exist: Asians and Native Americans have an increased number of whorls.
Remarks Alterations in the dermal ridge patterns or in the patterns of creases are recognizable in various syndromes (Fig. 12.7, and Bibliography). They are rarely pathognomonic but may provide an additional clue to the
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