A Missing Flexor Digitorum Brevis and its Evolutionary and Clinical Implications

Abstract

The flexor digitorum brevis belongs to the superficial layer of muscles of the foot and serves to flex the digits II–V at the proximal interphalangeal joint. Its tendons typically insert on the sides of the middle phalanx of each of the lateral four digits. During routine dissection for a human gross anatomy course, we discovered a bilateral variation in the structure of the flexor digitorum brevis in the feet of an older female cadaver. The variation consists of a bilaterally missing tendon to the digiti minimi (fifth digit). Further dissections were carried out on an additional 10 cadavers in the gross anatomy lab to ascertain the presence or absence of this variation, which is frequent in some populations. Preliminary results indicate that the variation is present in 40% of the Puerto Rican population.The flexor digitorum brevis develops embryologically from anterior condensations of abaxial muscle cell precursors starting at 6 weeks. This variation in particular is not expected to have serious functional or evolutionary consequences in humans, as flexion of the toes is not as important in human locomotor function as it would be in chimpanzees or other hominoids. A lack of functional consequences may result in greater variation than would otherwise be expected. Clinically, the tendon of the flexor digitorum brevis to digit V is often used for flexion of the proximal and intermediate phalanges. Our results suggest that because of the variation’s lack of functional adversities, the absence of this tendon might become increasingly prevalent in the population and that deeper muscles might become hypertrophied to supplement the action of the missing tendon.