The expanded plate exhibits five longitudinal mesodermal condensations called the digital rays. Later, a second constriction divides the rest of the limb into two segments.
It occurs due to failure of digital rays to separate from one another by a process of programmed cell death (apoptosis) during the seventh to eighth week of intrauterine life.
Hecht, "Experience with distraction lengthening of digital rays
in congenital anomalies," The Journal of Hand Surgery, vol.
: ABSTRACTObjectives: To report on six independent and isolated cases demonstrating thumb aplasia as an essentially limb-specific phenotype.Methods: The subjects were ascertained during 2011-2013 from six different geographic regions of Pakistan and underwent detailed clinical and phenotypic examination.Results: The affected arms of patients had complete absence of first digital rays medial inclinations of second and fifth fingers narrowing of palms missing carpals and shortening of zeugopod.
Congenital reduction of thumb is a relatively heterogeneous malformation of first digital ray (OMIM-188100)12 The condition ranges from hypoplastic thumb to a complete absence of first digit and its components.3 The variation in the clinical presentation of hypoplastic thumb has lead to the suggestion of their anatomical categorization into five entities which depict increasing severity of thumb deficiency.45 Complete reduction of first digit classified as type V is a rare malformation and was observed to be 0.25/10000 births.6 It occurs more often in males than in females and is bilateral in about two-third of the cases.235 Most of the isolated cases of absent thumb are sporadic.
Roentgenographic study revealed that first digital ray along with its components was completely omitted bilaterally (Fig.
As the tendon crosses the retinaculum it broadens out to become a flat sheet which splits longitudinally to send bundles of ligamentous fibers to digital rays