Simian lines have been noted on rudimentary palms of infants whose limb development was affected by thalidomide teratogen. The ridge patterns are of considerable clinical interest because they are affected by certain abnormalities of early development including genetic disorders. TFRC and AFRC are highly heritable, and the ridge patterns are genetically determined. Alternatively, the sum of all possible counts on all 10 fingers can be calculated yielding an absolute finger ridge count (AFRC), a measure of the total pattern size.
When the ridge count is used as a measure of a maximum pattern size on fingers, only the largest count from each finger is scored, and their sum is defined as the total finger ridge count (TFRC). A simple loop characterized by a single triradius, whorls have two triradii yielding two counts, while simple arches have no true triradii, resulting in a zero count. Traditionally, the ridge count is defined as the number of ridges that intersect or touch the line drawn from the easily recognized triradius (where three ridges meet) to the center of the pattern. This pattern has two triradii with the ridges forming various patterns inside. The shape of the pattern area may be either circular or elliptical. The whorls are the patterns so constructed that the characteristic ridge courses follow circuits around the core. A finger has an ulnar loop if its triradius is on the side of the thumb for that hand, and the loop opens toward the little finger. A finger possesses a radial loop if its triradius is on the side of the little finger for the hand in question, and the loop opens toward the thumb. The core is essentially a ridge that is surrounded by fields of ridges, which turn back on themselves at 180°. A triradius is a point at which three groups of ridges coming from three directions meet at angles of about 120°. The loop pattern has a triradius and a core. They may be subclassified as “plain” when the ridges rise slightly over the middle of the finger or “tented” when the ridges rise to a point. The arches are the simplest and least frequent pattern, which pass across the finger with slight bow distally.