The human hand is well known for its unique dexterity which is largely facilitated by a highly mobile, long and powerful thumb that enables both tool manufacturing and use, a key component of human evolution. The bonobo (Pan paniscus), the closest extant relative to modern humans together with the chimpanzee (Pan troglodytes), also possesses good manipulative capabilities but with a lower level of dexterity compared with modern humans. Despite the close phylogenetic relationship between bonobos and humans, detailed quantitative data of the bonobo forelimb musculature remains largely lacking. To understand how morphology may influence dexterity, we investigated the functional anatomy of the bonobo hand using a unique sample of eight bonobo cadavers, along with one chimpanzee and one human (Homo sapiens) cadaver. We performed detailed dissections of unembalmed specimens to collect quantitative datasets of the extrinsic and intrinsic hand musculature, in addition to qualitative descriptions of the forelimb muscle configurations, allowing estimation of force-generating capacities for each functional group. Furthermore, we used medical imaging to quantify the articular surface of the trapeziometacarpal joint to estimate the intra-articular pressure. Our results show that the force-generating capacity for most functional groups of the extrinsic and intrinsic hand muscles in bonobos is largely similar to that of humans, with differences in relative importance of the extensors and rotators. The bonobo thumb musculature has a lower force-generating capacity than observed in the human specimen, but the estimated maximal intra-articular pressure is higher in bonobos. Most importantly, bonobos show a higher degree of functional coupling between the muscles of the thumb, index and lateral fingers than observed in humans. It is conceivable that differentiation and individualization of the hand muscles rather than relative muscle development explain the higher level of dexterity of humans compared with that of bonobos.
The thumb plays a crucial role in basic hand function. However, the kinematics of its entire articular chain have not yet been quantified. Such investigation is essential to improve our understanding of thumb function and to develop better strategies to treat thumb joint pathologies. The primary objective of this study is to quantify the in vivo kinematics of the trapeziometacarpal (TMC) and scaphotrapezial (ST) joints during flexion and adduction of the thumb. In addition, we want to evaluate the potential coupling between the TMC and ST joints during these tasks. The hand of 16 asymptomatic women without signs of thumb osteoarthritis were CT scanned in positions of maximal thumb extension, flexion, abduction, and adduction. The CT images were segmented and three-dimensional surface models of the radius, scaphoid, trapezium, and the first metacarpal were created for each thumb motion. The corresponding rotations angles, translations, and helical axes were calculated for each sequence. The analysis shows that flexion and adduction of the thumb result in a three-dimensional rotation and translation of the entire articular chain, including the trapezium and scaphoid. A wider range of motion is observed for the first metacarpal, which displays a clear axial rotation. The coupling of axial rotation of the first metacarpal with flexion and abduction during thumb flexion supports the existence of a screw-home mechanism in the TMC joint. In addition, our results point to a potential motion coupling between the TMC and ST joints and underline the complexity of thumb kinematics. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1556-1564, 2017.
The human thumb is specialized for manual tasks as it is no longer typically involved in locomotion. However, members of the genus Pan -the closest extant relatives of modern humans -also have a highly mobile thumb, which allows complex manual tasks such as tool-crafting and use. Here, we investigate the thumb kinematics of bonobos (Pan paniscus) in relation to the morphology of their trapeziometacarpal joint using unembalmed bonobo specimens and compare that with the human condition. We use computed tomography-based models of skeletal elements of the thumb during positions of maximum abduction/adduction and flexion/extension to determine the kinematics of the first metacarpal in bonobos and healthy human volunteers. In addition, the 3D geometry of the trapeziometacarpal joint is quantified and, together with an assessment of the ligaments surrounding the joint, is related to the obtained kinematics of the first metacarpal. Our results show a similar trapezial 3D morphology and similar kinematics of the first metacarpal in bonobos and humans, with a markedly higher extension of the first metacarpal in humans. This study provides an integrated analysis of thumb anatomy and kinematics in a unique sample of bonobo specimens.
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