Internal bone architecture in the triquetrum of extant hominids and Homo neanderthalensis

Bones on the radial side of the wrist have dominated research on the evolution and functional anatomy of the hominoid carpus. However, soft and hard tissue structures at the ulnar side of the wrist, such as reduced ulna-triquetrum articulation and the Triangular Fibrocartilage Complex (TFCC), are some of the most conspicuous synapomorphies among hominoids linked to high degrees of ulna-deviation and forearm supination [1,2]. However, there is also notable variation in the bony and soft tissue anatomy of the ulnar wrist among hominoids that affects how loads are transferred from the hand to the forearm but are not clearly understood, in part because the functional morphology of the triquetrum is understudied. Here we investigate for the first time the internal trabecular bone structure of the triquetrum and test the null hypothesis that great apes will exhibit similar distributions of relative trabecular bone volume to total volume (BV/TV) and degree of anisotropy (DA).

Using a whole-bone methodology (http://www.dr-pahr.at/medtool/) this study quantifies BV/TV and DA in the triquetrum in Pongo (n=12), Gorilla (n=10), Pan (n= 10) and a temporogeographically diverse sample of modern humans (n=21), in comparison to three Homo neanderthalensis individuals (El Sidron SD-1227, Tabun C1_1-154, and Amud 1). The distribution of BV/TV and DA values are visualised across the entire bone using Paraview (3.89.0). Low DA was identified as values equal to or less than 0.20. High BV/TV was identified as the top 20% of values for that individual’s range. In line with presumed force transfer at the ulna-side wrist in hominoids [2], it is predicted that low DA values will be present at the TFCC insertion site, with high BV/TV at the lunate and hamate articulations.

Low DA was observed at the TFCC insertion site in 41% of Pongo, 100% of Gorilla, 90% of Pan, 100% of Homo neanderthalensis, and 76% of Homo sapiens. High BV/TV was observed at the hamate articulation site in 75% of Pongo, 90% of Gorilla, 70% of Pan, 100% of Homo neanderthalensis and 61% of Homo sapiens. High BV/TV was observed at the lunate articulation site in 91% of Pongo, 20% of Gorilla, 0% of Pan, 33% of Homo neanderthalensis, and 0.04% of Homo sapiens.

Overall, these results suggest relatively consistent interspecific patterns in the trabecular structure of the triquetrum which contrasts the interspecific variation reported in some previous studies, including the capitate [3]. DA predictions were supported for all species except Pongo, where there was only partial support. Predictions for high BV/TV at the triquetrohamate joint were broadly supported while predictions for high BV/TV at the triquetrolunate had mixed support. This pattern suggests that force transfer between the lunate and triquetrum is lower than that between the hamate and triquetrum. Pongo is the exception to this pattern with 91% exhibiting high BV/TV at the triquetrolunate joint, which, when combined with the DA results, may reflect a less-developed TFCC [2] and a greater reliance on the (relatively large) lunate to support ulna-side force transfer compared with other great apes. Although the Neanderthal triquetrum morphology is primitive relative to H. sapiens [4], all three show a BV/TV and DA pattern most similar to that of modern humans. This suggests a similar force transfer pattern at the ulna-side wrist in Neanderthals despite this primitive morphology. There is considerable debate as to whether Neanderthals utilised the same grip patterns as modern H. sapiens and the ulna-side wrist may shed light on the importance of carpal morphology in predicting force transfer and grip types among hominins.