Internal bone architecture in the capitate of extant hominids

Due to its central role in wrist function and distinctive morphology, the capitate has been an important element for drawing inferences about fossil hominin locomotor and manipulative capacities [1-3]. However, difficulties in studying the numerous, intricate joints of the wrist have led to disagreement about the functional significance of various aspects of its morphology [1-3]. As internal bone is known to adapt to load experienced during life, its structure may reflect the actual postures and behaviours of an individual [4,5]. To date, it is unclear whether the internal bone of the capitate might reflect differences in hand use among hominids. Furthermore, it is not currently understood how cortical and trabecular bone of carpals interact and adapt to cope with the high load and mechanical demands of suspensory or quadrupedal locomotion. To examine the relationship between internal bone architecture and locomotor behaviour, we test for group differences in bone volume to total volume (BV/TV), degree of anisotropy (DA), and cortical thickness (Ct.Th). Further, we analyse the relative distributions of cortical and trabecular bone to assess whether distribution is similar between the locomotor groups. Using a whole-bone method (http://www.dr-pahr.at/medtool/), BV/TV, DA, and Ct.Th were measured in four extant ape genera (n=69) representing bipedal (Homo), knuckle-walking (Pan, Gorilla) and suspensory (Pongo) locomotion. The capitate was partitioned for separate analysis as the proximal portion forms part of the midcarpal joint, while the distal portion forms part of the carpometacarpal joint [3]. BV/TV was measured twice, quantifying just the trabecular region and then the combined cortical and trabecular region. Mean group differences were examined with a KruskalWallis one-way ANOVA and Pairwise Wilcoxon Rank Sum test. Intraspecific ratios were calculated to facilitate comparison of the bone parameters between the proximal and distal regions of each genus. Parameters differed between the genera across all capitate regions (p=<0.001 for all tests), except distal DA (p= 0.593). Ct.Th was the most important parameter for differentiating locomotor groups as distribution across the bone was distinctly different between the human and non-human apes. Compared to the proximal, the distal cortex was thicker in all genera; a 12% increase was seen inHomo, 52% in Pan, 62% inGorilla and 79% in Pongo. This Ct.Th increase coincides with the attachment sites for several ligaments, and might indicate that tensional strain, which has a lower failure load than compressive strain, has an impact on cortical bone functional adaptation. These intergeneric differences further suggest that the hand of bipedal Homo may not experience a similar magnitude of strain. Trabecular BV/TV was higher in the proximal capitate for all genera suggesting it plays a bigger role in adapting to midcarpal joint load than cortical bone in this region. DA in the distal capitate was not significantly different among the genera. This may indicate that the limited mobility at the distal region results in a similar DA, irrespective of hand use. Homo and Pongo were differentiated from the knuckle-walking taxa by having higher DA in the proximal capitate, which was unexpected given our assumption of more variable wrist postures than that of knuckle-walking apes. Results of this study indicate that (1) capitate bone structure is informative for reconstructing hand use and (2) partitioning of ‘cortical’ and ‘trabecular’ regions in carpals (and likely tarsals) may obscure functionally relevant information in the carpus of hominids.