Phalangeal curvature in a chimpanzee raised like a human: Implications for inferring arboreality in fossil hominins.

Arboreal primates such as chimpanzees exhibit pronounced curvature in their hand and foot phalanges, which is assumed to develop throughout life in response to mechanical loads produced by grasping and hanging from branches. Intriguingly, ancient fossil hominins also exhibit substantial phalangeal curvature, which, too, has been interpreted as a direct result of habitual arboreality during life. Here, we describe the phalangeal curvature of a chimpanzee who was raised during the 1930s in New York City to live much like a human, including by having very few opportunities to engage in arboreal activities. We show that the degree of hand and foot phalangeal curvature in this individual is indistinguishable from that of wild chimpanzees and distinct from humans. Thus, rather than being a direct effect of mechanical loads produced by lifetime arboreal activities, phalangeal curvature appears to be shaped largely by genetic factors. An important implication of this finding is that phalangeal curvature among fossil hominins is evidently best interpreted as a primitive trait inherited from an arboreal ancestral species rather than proof of engagement in arboreal activities during life.

Human manual distal phalanges from the Middle Stone Age deposits of Klasies River Main Site, Western Cape Province, South Africa.

Two new distal manual phalanges from the Middle Stone Age deposits of Klasies River Main Site are described. One (SAM-AP 6387) likely derives from ray II or ray III, whereas the other (SAM-AP 6388) is from the thumb. Both derive from a late adolescent or fully adult individual. They were recovered by H. Deacon from the same stratigraphic unit (submember W or possibly submember R) of the Shell and Sand Member of Cave 1, which places them between 100 and 90 ka. Both are comparatively small elements, and the possibility that they came from the same hand cannot be discounted at this time. These bones add to the meager and all too fragmentary postcranial human fossil sample from the Late Pleistocene of South Africa. These two specimens provide some additional evidence pertaining to the morphological attributes of the distal phalanges of the Middle Stone Age inhabitants of South Africa. Together with the distal pollical phalanx from Die Kelders (SAM-AP 6402), they are relatively small in comparison with homologs from recent human samples as well as Late Pleistocene specimens from Eurasia. Given their small sizes, the distal pollical phalanges from Klasies and Die Kelders are not dissimilar to Holocene Khoesan homologs. As expected, the Klasies elements differ noticeably from Neandertal homologs, especially in the narrowness of their shafts and distal tuberosities.

Insertion sites in manual proximal phalanges of African apes and modern humans.

OBJECTIVES: The primary aim of this study was to describe the insertion sites of the ligaments holding the flexor digitorum profundus and superficialis muscles (flexor ridges) in proximal phalanges 2-5 of African apes and modern humans. To interpret differences in flexor ridge size based on general behavioral differences among taxa. MATERIALS AND METHODS: We analyzed 3D models of manual proximal phalanges 2-5 from 29 gorillas (Gorilla beringei and Gorilla gorilla), 30 chimpanzees (Pan troglodytes) and 36 recent modern humans. Flexor ridges (mm2) were compared within and across genera. RESULTS: Gorillas and chimpanzees had larger flexor ridges for phalanges 2-4 than humans and this difference subsists when controlling for body size. Each genus had a unique insertion size pattern across the digits, with the most heterogeneous pattern found in chimpanzees, followed by humans, and lastly gorillas. These patterns corresponded strongly to the differences in the size of the phalanges within each genus, except for phalanx 5 in humans, which had a larger flexor ridge than expected. DISCUSSION: When comparing these genera, the flexor ridges signal differences between taxa that use their hands for manipulation and locomotion (gorillas and chimpanzees) and taxa that use them exclusively for manipulation (humans). This functional signal was also apparent in the PP5 of humans, whose larger FR may be indicating the high recruitment of this digit during forceful precision grip characteristic of humans.

Karate Training Improves Skeletal Status Assessed by Quantitative Ultrasound in Girls and Premenopausal Women.

PURPOSE: The aim of the study was to assess the influence of regular karate training on the skeletal status evaluated by quantitative ultrasound (QUS) in females. METHODS: A group of 132 karate training girls and women at mean age 19.57 (standard deviation [SD] 7.64) yr (range 7.3-45.3 yr) and 322 age-, sex- and body size-matched controls were enrolled into the study. Mean training duration in the karate group was 7.52 (SD 5.05) yr and mean training frequency was 2.97 (SD 1.21) per wk. The QUS measurements were performed at hand proximal phalanges, using a DBM Sonic 1200 (IGEA, Carpi, Italy) sonographic device, which measures amplitude-dependent speed of sound (Ad-SoS [m/s]). RESULTS: The results of Ad-SoS obtained in karatekas were generally higher than in controls with significant difference for prepubertal girls (1966.2 [SD 46.2] vs 1942.7 [SD 38.4]; p < 0.05) and for adult women (2124.4 [SD 48.0] vs 2105.3 [SD 54.0]; p < 0.05). CONCLUSIONS: Regular karate training is a factor that is positively associated with results of the QUS measurements at hand phalanges in exercising females and its impact is most strongly pronounced in prepuberty and adulthood.

Evaluation of Articular Surface Similarity of Hemi-Hamate Grafts and Proximal Middle Phalanx Morphology: A 3D Geometric Morphometric Approach.

PURPOSE: Hemi-hamate arthroplasty has been described as a viable treatment option for unstable proximal interphalangeal joint fracture-dislocations. The procedure uses a dorsal distal hamate osteochondral graft to recreate the injured volar middle phalanx (MP) proximal base. The purpose of this study was to evaluate the similarity in shape of these articular surfaces using quantitative 3-dimensional methods. METHODS: Three-dimensional virtual renderings were created from laser scans of the articular surfaces of the dorsal distal hamate and the volar MP bases of the index, middle, ring, and little fingers from cadaveric hands of 25 individuals. Three-dimensional landmarks were obtained from the articular surfaces of each bone and subjected to established geometric morphometric analytical approaches to quantify shape. For each individual, bone shapes were evaluated for covariation using 2-block partial least-squares and principal component analyses. RESULTS: No statistically significant covariation was found between the dorsal distal hamate and volar MP bases of the middle, ring, or little digits. Whereas the volar MP bases demonstrated relative morphologic uniformity among the 4 digits both within and between individuals, the dorsal distal hamates exhibited notable variation in articular surface morphology. CONCLUSIONS: Despite the early to midterm clinical success of hemi-hamate arthroplasty, there is no statistically significant, uniform similarity in shape between the articular surfaces of the dorsal distal hamate and the volar MP base. In addition, there is wide variation in the articular morphology of the hamate among individuals. CLINICAL RELEVANCE: The lack of uniform similarity in shape between the dorsal distal hamate and the volar MP base may result in unpredictable outcomes in HHA. It is recommended that the variation in hamate morphology be considered while reconstructing the injured volar MP base in the procedure.

[The distribution of the main types of the papillary patterns on the distal phalanges of human fingers]. / Raspredelenie osnovnykh tipov papilliarnykh uzorov na distal'nykh falangakh pal'tsev ruk cheloveka.

The present study included the investigation of the four main types of the papillary patterns on the distal phalanges of all the ten fingers. As many as three million fingerprint cards have been used for the first time. The investigation yielded the exact quantitative characteristics of the frequency of occurrence of various papillary patterns. The importance of these findings for dactyloscopy and dermatoglyphics is discussed.

Hominin hand bone fossils from Sterkfontein Caves, South Africa (1998-2003 excavations).

We describe eleven hominin metacarpals and phalanges recovered from Jacovec Cavern and Member 4 of the Sterkfontein Formation between 1998 and 2003. Collectively, the fossils date in excess of 2.0 Ma, and are probably attributable to Australopithecus africanus and/or Australopithecus prometheus. When combined with results of previous studies on Australopithecus postcranial functional morphology, the new data presented here suggest that at least some late Pliocene and/or early Pleistocene hominins from Sterkfontein were arboreally adept. This finding accords with the reconstruction of the site's >2.0 Ma catchment area as well-vegetated and containing significant woody components. In addition, most of the new specimens described here evince morphologies that indicate the hands from which they derived lacked complete modern humanlike manual dexterity, which is integral to the manufacture and use of intentionally shaped stone tools. The absence of lithic artifacts from both stratigraphic units from which the fossils were excavated is consistent with this conclusion.

Oldest evidence for grooming claws in euprimates.

Euprimates are unusual among mammals in having fingers and toes with flat nails. While it seems clear that the ancestral stock from which euprimates evolved had claw-bearing digits, the available fossil record has not yet contributed a detailed understanding of the transition from claws to nails. This study helps clarify the evolutionary history of the second pedal digit with fossils representing the distal phalanx of digit two (dpII), and has broader implications for other digits. Among extant primates, the keratinized structure on the pedal dpII widely varies in form. Extant strepsirrhines and tarsiers have narrow, distally tapering, dorsally inclined nails (termed a 'grooming claws' for their use in autogrooming), while extant anthropoids have more typical nails that are wider and lack distal tapering or dorsal inclination. At least two fossil primate species thought to be stem members of the Strepsirrhini appear to have had grooming claws, yet reconstructions of the ancestral euprimate condition based on direct evidence from the fossil record are ambiguous due to inadequate fossil evidence for the earliest haplorhines. Seven recently discovered, isolated distal phalanges from four early Eocene localities in Wyoming (USA) closely resemble those of the pedal dpII in extant prosimians. On the basis of faunal associations, size, and morphology, these specimens are recognized as the grooming phalanges of five genera of haplorhine primates, including one of the oldest known euprimates (∼56 Ma), Teilhardina brandti. Both the phylogenetic distribution and antiquity of primate grooming phalanges now strongly suggest that ancestral euprimates had grooming claws, that these structures were modified from a primitive claw rather than a flat nail, and that the evolutionary loss of 'grooming claws' represents an apomorphy for crown anthropoids.

Correlation between Cervical Vertebrae Volume Parameter and the Skeletal Maturation Status.

AIM: Assessing skeletal maturity is a critical factor in orthopedic treatment and orthognathic surgery. Quantitative methods have been suggested to decide the skeletal age. This study aims to assess the correlation between the middle phalanx of the third finger (MP3) method and cervical vertebral volume parameters (CVVP) in skeletal age determination. MATERIALS AND METHODS: This cross-sectional study was conducted on 21 boys and 29 girls between the ages of 8 and 16 years, who were divided into five groups of skeletal maturation: 100 MP3 radiographs and cone beam computed tomography (CBCT) were obtained and analyzed. Middle phalanx of the third finger maturation stages were evaluated according to Perinetti method (stages F-FG-G-H-I). The CVVP was determined on CBCT using Mimics Medical software. Analysis of variance (ANOVA) and Bonferroni tests were utilized to investigate the differences in volumetric parameters between MP3 stages. Spearman correlation coefficient was employed to obtain the correlation between MP3 stages and the CVVP. RESULTS: The differences in CVVP concentrated between MP3-I stage and the previous stages, with the highest values those for the 4th vertebra. Spearman test revealed a significant highest correlation between MP3 stages and the CVVP in the 4th vertebra; the lowest was in the 2nd, which was higher in the girls group. CONCLUSION: Strong level of agreement between the two methods was significant in the 4th vertebra. CLINICAL SIGNIFICANCE: Measurement of volumes of the 4th CVVP could be used as simple quantitative analysis in clinical practice to evaluate the maturity in orthodontic patients.

Phalangeal morphology of Shanghuang fossil primates.

Here, we describe hundreds of isolated phalanges attributed to middle Eocene fossil primates from the Shanghuang fissure-fillings from southern Jiangsu Province, China. Extending knowledge based on previous descriptions of postcranial material from Shanghuang, this sample of primate finger and toe bones includes proximal phalanges, middle phalanges, and over three hundred nail-bearing distal phalanges. Most of the isolated proximal and middle phalanges fall within the range of small-bodied individuals, suggesting an allocation to the smaller haplorhine primates identified at Shanghuang, including eosimiids. In contrast to the proximal and middle phalanges from Shanghuang, there are a variety of shapes, sizes, and possible taxonomic allocations for the distal phalanges. Two distal phalangeal morphologies are numerically predominant at Shanghuang. The sample of larger bodied specimens is best allocated to the medium-sized adapiform Adapoides while the smaller ones are allocated to eosimiids on the basis of the commonality of dental and tarsal remains of these taxa at Shanghuang. The digit morphology of Adapoides is similar morphologically to that of notharctines and cercamoniines, while eosimiid digit morphology is unlike living anthropoids. Other primate distal phalangeal morphologies at Shanghuang include grooming "claws" as well as specimens attributable to tarsiids, tarsiiforms, the genus Macrotarsius, and a variety of adapiforms. One group of distal phalanges at Shanghuang is morphologically indistinguishable from those of living anthropoids. All of the phalanges suggest long fingers and toes for the fossil primates of Shanghaung, and their digit morphology implies arboreality with well-developed digital flexion and strong, grasping hands and feet.

Quantitative Ultrasound Measurement at the Hand Phalanges Does Not Reveal Skeletal Disturbances in Children With Bronchial Asthma: A Longitudinal Observation.

OBJECTIVES: To assess the skeletal status in asthmatic children treated with inhaled corticosteroids in a longitudinal observation by quantitative ultrasound (US) measurement at the hand phalanges. METHODS: Thirty-four children were studied prospectively. Quantitative US measurements were performed at baseline and after a mean ± SD of 2.35 ± 0.20 years. The obtained results were compared to age-, sex-, and body size-matched control participants selected from a database of previously examined healthy children. Individual changes in the amplitude-dependent speed of sound (Ad-SoS) during the follow-up period were also analyzed. RESULTS: The mean age of the asthmatic children was 10.6 ± 2.5 years. The mean Ad-SoS in the asthmatic children at baseline was 1940.5 ± 49.6 m/s, and the mean Z score was -0.26 ± 0.80. Corresponding values at the follow-up examination were 1976.2 ± 63.6 m/s and -0.18 ± 1.16. The results did not differ significantly in comparison to the healthy controls. The analysis of individual changes in Ad-SoS revealed that 18 participants had a significant increase in this parameter (ie, exceeding the least significant change threshold), and 16 did not have a significant change in their values. No one had a significant decrease in Ad-SoS. CONCLUSIONS: Quantitative US at the hand phalanges applied as a diagnostic tool revealed no essential differences in the pattern of skeletal development between asthmatic children treated with inhaled glucocorticosteroids and healthy controls.

Volar Anatomy of the Proximal Phalanx: Implications for Screw Length Selection for Fixation of Shaft Fractures.

PURPOSE: To investigate the anatomy of the volar surface of the proximal phalanx of the hand, specifically the longitudinal groove running along the volar phalangeal shaft. METHODS: We measured skeletonized proximal phalanges from 10 embalmed human cadaver hands at 5 equidistant points along the shaft. The difference between the maximum dorsal-palmar thickness of the shaft and thickness measured from the center of the volar groove to the most dorsal aspect of the phalanx indicated the depth of the groove at each point. These specimens underwent microtomography to characterize their osseous morphology further. Screws placed dorsal to palmar into the specimens and viewed fluoroscopically simulated the appearance of screw protrusion into the volar groove under intraoperative imaging. Similarly, screws placed into a fresh-frozen cadaveric hand illustrated possible screw impingement on soft tissue in vivo. RESULTS: The volar groove was most pronounced at the proximal and distal ends of the phalangeal shaft, becoming shallower along the midportion of the bone. The average difference between total bone thickness and thickness measured from the depth of the groove was significant at each of the 5 points of measurement along the phalangeal shaft for each of the 5 digits of the hand, including the thumb. Average groove depths ranged from 4% to 14% of total bone thickness, with a maximum individual measurement of 22%. Average depth of the groove at each of these positions ranged from 0.19 to 1.64 mm, reaching a maximum of 2.31 mm. CONCLUSIONS: We demonstrated that there is a longitudinal groove running the length of the phalangeal shaft. CLINICAL RELEVANCE: Viewed laterally, the cupped edges of the groove obscure its depth. Dorsally placed bicortical screws could protrude into the groove, remaining unnoticed on intraoperative imaging. The resulting impingement on the flexor tendon could lead to postsurgical stiffness or flexor tendon attritional rupture.

Diagnostic reliability of the third finger middle phalanx maturation (MPM) method in the identification of the mandibular growth peak.

Background/objectives: The use of the sole third finger middle phalanx for a maturational method has been proposed but not fully investigated. Herein, the diagnostic reliability of an improved five-stage third finger middle phalanx maturation (MPM) method in the identification of mandibular growth peak has been investigated. Subjects/methods: From the files of the Burlington Growth Study, 35 subjects (20 males, 15 females) with at least 7 annual lateral cephalograms taken from 9 to 16 years were included. Mandibular growth was defined as annual increments in condylion-gnathion (Co-Gn) distance. Subsequently, individual annual increments in Co-Gn were arranged according to annual age intervals, with the first and last intervals defined as 9-10 years and 14/15-16 years, respectively. A full diagnostic reliability analysis (including positive likelihood ratio) was performed to establish the diagnostic reliability of the MPM stage 2 (MPS2) in the identification of the imminent mandibular growth peak. Results: The MPS2 had a satisfactory accuracy in the identification of imminent mandibular growth peak with an overall positive likelihood ratio of 10.3. However, reliability showed noteworthy variability being greater and lower for younger and older age intervals, respectively. Limitations: Secular trend, limited sample size, and annual recording in conjunction with the use of a discrete staging system. At the 15 years recording, 28 of 35 cases were missing. Conclusions/implications: The MPS2 and MPS3 may be considered associated with the onset and maximum mandibular growth peak, respectively, in most of the subjects, indicating their use in planning treatment timing.

Grasping primate development: Ontogeny of intrinsic hand and foot proportions in capuchin monkeys (Cebus albifrons and Sapajus apella).

Young primates have relatively large hands and feet for their body size, perhaps enhancing grasping ability. We test the hypothesis that selection for improved grasping ability is responsible for these scaling trends by examining the ontogeny of intrinsic hand and foot proportions in capuchin monkeys (Cebus albifrons and Sapajus apella). If selection for improved grasping ability is responsible for the observed patterns of hand and foot growth in primates, we predicted that fingers and toes would be longer early in life and proportionally decline with age. We measured the lengths of manual and pedal metapodials and phalanges in a mixed-longitudinal radiographic sample. Bone lengths were (a) converted into phalangeal indices (summed non-distal phalangeal length/metapodial length) to test for age-related changes in intrinsic proportions and (b) fit to Gompertz models of growth to test for differences in the dynamics of phalangeal versus metapodial growth. Manual and pedal phalangeal indices nearly universally decreased with age in capuchin monkeys. Growth curve analyses revealed that metapodials generally grew at a faster rate, and for a longer duration, than corresponding phalanges. Our findings are consistent with the hypothesis that primates are under selection for increased grasping ability early in life. Relatively long digits may be functionally adaptive for growing capuchins, permitting a more secure grasp on both caregivers and arboreal supports, as well as facilitating early foraging. Additional studies of primates and other mammals, as well as tests of grasping performance, are required to fully evaluate the adaptive significance of primate hand and foot growth.

Digit ratio (2D:4D): Is it possible to use it for sex determination in the study of human skeletal remains?

OBJECTIVES: Sexual dimorphism in the relative length of the second-to-fourth digits (the digit ratio, or 2D:4D) in humans has been reported in many studies. The aim of our study was to ascertain possibility of using the 2D:4D ratio as an additional marker for sex determination in the study of human skeletal remains. METHODS: We have studied 2D:4D ratios obtained from measurements of finger phalanges and metacarpal bones in Russian (45 adult males and 26 adult females) and German (58 adult males and 29 adult females) skeletal series. RESULTS: The difference in 2D:4D ratio between the male and female subsamples in both skeletal series was not statistically significant. Analysis of variance revealed that the 2D:4D ratios in our sample varied more by ethnicity than by the sexual identity of the skeletal material. CONCLUSION: Our results suggest that the 2D:4D ratio cannot be used as an appropriate trait for the sex determination of human skeletal remains. Am. J. Hum. Biol. 28:591-593, 2016. © 2015 Wiley Periodicals, Inc.

Distal phalanges of Eosimias and Hoanghonius.

Seven primate distal phalanges have been identified from two middle Eocene fossil localities (Locality 1 and Nanbaotou) in the Yuanqu Basin, China, providing the first evidence of distal phalangeal morphology in Asian Eocene adapiform and eosimiid primates. The bones are best allocated to the basal anthropoid Eosimias centennicus and to hoanghoniine adapiforms. All distal phalangeal specimens display a morphology consistent with nail-bearing fingers and toes. The hallucal distal phalanx of the basal anthropoid Eosimias is more similar to that of primitive tarsiiforms than to crown group anthropoids. The adapiform distal phalanges from Locality 1 are allocated to Hoanghonius stehlini while those from Nanbaotou are tentatively assigned to an indeterminate hoanghoniine because dental remains of adapiforms have yet to be identified from this site. The distal phalangeal anatomy of hoanghoniines differs slightly from that documented for adapines and notharctines. One distal phalanx from Locality 1 shows a second pedal digit "grooming claw" morphology as noted for notharctines by Maiolino et al. (2012) and cercamoniines by Von Koenigswald et al. (2012).

Early Pleistocene human hand phalanx from the Sima del Elefante (TE) cave site in Sierra de Atapuerca (Spain).

In this study, a new Early Pleistocene proximal hand phalanx (ATE9-2) from the Sima del Elefante cave site (TE - Sierra de Atapuerca, Spain), ascribed to Homo sp., is presented and comparatively described in the context of the evolution of the genus Homo. The ATE9-2 specimen is especially important because of the paucity of hand bones in the human fossil record during the Early Pleistocene. The morphological and metrical analyses of the phalanx ATE9-2 indicate that there are no essential differences between it and comparator fossil specimens for the genus Homo after 1.3 Ma (millions of years ago). Similar to Sima de los Huesos and Neandertal specimens, ATE9-2 is a robust proximal hand phalanx, probably reflecting greater overall body robusticity in these populations or a higher gracility in modern humans. The age of level TE9 from Sima del Elefante and morphological and metrical studies of ATE9-2 suggest that the morphology of the proximal hand phalanges and, thus, the morphology of the hand could have remained stable over the last 1.2-1.3 Ma. Taking into account the evidence recently provided by a metacarpal from Kaitio (Kenya) from around 1.42 Ma, we argue that modern hand morphology is present in the genus Homo subsequent to Homo habilis.

La Ferrassie 8 Neandertal child reloaded: New remains and re-assessment of the original collection.

The first evidence of the partial infant Neandertal skeleton La Ferrassie 8 (LF8) was discovered in 1970, although most of the remains were found in 1973 as part of the 1968-1973 work at the site by H. Delporte. This individual and the other Neandertal children from La Ferrassie were published in the early 1980s by J.-L. Heim, and since then LF8 has been regarded as coming from a poorly documented excavation. The recent rediscovery of the box that contained the hominin bones given by Delporte to Heim in the Muséum national d'Histoire naturelle (MNHN) collection provided new fossils and helped to locate LF8 in the site: level M2 in square 1. Two visits to the Musée d'Archéologie nationale et Domaine national de Saint-Germain-en-Laye (MAN) yielded additional fossil remains from both the 1970 and 1973 excavations and resulted in the discovery of all of the notes from the excavation of H. Delporte between 1968 and 1973. Here the new fossil remains (47 after performing all possible refits), representing significant portions of the cranium, mandible, and vertebral column together with fragmentary hand and costal remains, are described. Unsurprisingly, the morphology of the bony labyrinth and of a complete stapes from the nearly complete left temporal show clear Neandertal affinities. Additionally, a complete reassessment of the original LF8 collection has resulted in the identification of several errors in the anatomical determination. Despite the significant increase in the anatomical representation of LF8, the skeletal remains are still limited to the head, thorax, pelvis, and four hand phalanges, with some very fragile elements relatively well preserved. Different hypotheses are proposed to explain this anatomical representation, which can be tested during future fieldwork.

First discovery of postcranial bones of Ouranopithecus macedoniensis (Primates, Hominoidea) from the late Miocene of Macedonia (Greece).

Two well preserved phalanges, one proximal and one intermediate, are identified and described. They constitute the first postcrania ever described of Ouranopithecus macedoniensis (Primates, Hominoidea) from the late Miocene locality of Ravin de la Pluie (RPl), Macedonia (Greece). They are isolated specimens, and the only ones known for their genus. The hypotheses that these specimens derive either from the hand or from the foot were tested. Comparisons with living apes of known positional behaviour were made to assess the functional signal in these specimens. The proximal phalanx, either manual or pedal, closely matches the proximal phalanges of terrestrial quadrupedal primates or the bipedal primate Homo. With respect to the intermediate phalanx we show that it closely matches phalanges of quadrupedal terrestrial primates. The terrestriality of Ouranopithecus, here reconstructed from phalangeal remains, is in agreement with our previous results of dental studies (thick check teeth enamel and microwear pattern of incisors and molars), which indicate that it was a hard object feeder living near the ground.

Micro-finite element (µFE) modeling of the siamang (Symphalangus syndactylus) third proximal phalanx: the functional role of curvature and the flexor sheath ridge.

Phalangeal curvature is a commonly used morphological feature for the interpretation of extant and fossil primate locomotor behaviour. Here, we build on a recent biomechanical study (Richmond, 2007) in two ways: first, we use a 3D micro-FE model, which models the real internal microstructure (i.e., cortical thickness and trabecular bone structure) and, second, we model four siamang third proximal phalanges. We test identical 2D homogenized FE models and two 3D micro-FE phalanx models that are mathematically straightened to isolate the biomechanical significance of curvature. We further investigate how varying the loading configuration (e.g., boundary constraints) and modeling (e.g., 2D versus 3D) affects the biomechanical behaviour of the phalanx. Finally, we examine how intraspecific variation in external and internal bony morphology affects the biomechanical behaviour of the phalanx. Simulation results demonstrate that the general pattern of strain and displacement is similar between the 3D micro-FE and 2D homogenized FE models but the absolute values differ substantially. The biomechanical behaviour of the 3D FE models more closely match the relative strain patterns from the validation experiment than the 2D homogenized FE models, indicating the 3D microstructure model is preferable. Varying the loading configuration can have dramatic effects on the biomechanical behaviour of the phalanx depending on individual morphology, but overall a cantilevered beam model is an equally valid, if not better, configuration for modeling the phalanx as other previously-proposed models. Variation in flexor ridge morphology has a substantial effect on phalanx strain; the taller the ridge, the less strain incurred by other regions of the palmar shaft. Finally, phalangeal curvature reduces overall strain experienced by the phalanx, but does not necessarily reduce bending or increase the compression-to-tension ratio. These results confirm the adaptive role of phalangeal curvature during flexed-finger grasping postures and demonstrate that modeling variation in cortical thickness and flexor ridge morphology improves the behaviour of the FE model, which has important implications for the functional interpretation of phalanx form.