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In human and non-human primates, at the level of the proximal interphalangeal joint of the finger, the arrangement of tendinous bands in the extensor assembly as well as the shape of the trochlea of the proximal phalanx determine the rate of release of the third phalanx as seen in e.g., ordinary opening and closing of the hand. In view of the importance of skilled forelimb movements and hand- and finger joint coordination, in prehensile kinematics of human and other primates, we therefore analyse the various morphological characteristics of these structures to some detail, viz. in human, non-human primates, and in a primate ‘predecessor’. Our data corroborate most of the previously published conclusions. In the extensor assembly of the finger, intercrossing of the various fanning-out tendon fibres appears to be a prerequisite par excellence for well-defined tendon-bundle formation (also known as “tendon fasciculation and individualization”). Initially, “in opossum, at the dawn of mammalian eruption, the hand emerged with a [less] complex structure of the extensor assembly”. On the other hand, a predominant parallelism of tendon fibres in the extensor assembly results in more homogeneity. Especially small-sized mammalian species, with short gestation periods, appear to display such homogeneities. The fanning out of tendon fibres in the extensor assembly increases as contrahentes digitorum muscles of the hand become more and more incorporated within the interosseus muscles. As hands of mammals with rather long gestation periods lack mm. contrahentes digitorum in particular, it is challenging to consider some relation between tendon fasciculation and individualization in the extensor assemblies, and ontogenetic processes, in human and other primates.

Because in man, the knee joint and the Proximal Inter Phalangeal (P.I.P.-) joint of the finger show several similarities (Slattery, 1990), it is justified to analyze some of the human P.I.P.-joint´s functional morphology by making use of methodologies elsewhere applied on form and function of the human knee. Analyzing the shapes of the articular surfaces of the finger P.I.P.-joint is also needed in view of somber perspectives of some P.I.P. joint arthroplasties after a certain time (Sweets and Stern, 2011).