First evidence of a bipartite medial cuneiform in the hominin fossil record: a case report from the Early Pleistocene site of Dmanisi

Abstract

A medial cuneiform exhibiting complete bipartition was discovered at the Early Pleistocene site of Dmanisi, Georgia. The specimen is the oldest known instance of this anatomical variant in the hominin fossil record. Here we compare developmental variation of the medial cuneiform in fossil hominins, extant humans and great apes, and discuss potential implications of bipartition for hominin foot phylogeny and function. Complete bipartition is rare among modern humans (< 1%); incomplete bipartition was found in 2 of 200 examined great ape specimens and also appears in the form of a divided distal articular surface in the Stw573c Australopithecus africanus specimen. Although various developmental pathways lead to medial cuneiform bipartition, it appears that the bipartite bone does not deviate significantly from normal overall morphology. Together, these data indicate that bipartition represents a phyletically old developmental variant of the medial cuneiform, which does not, however, affect the species-specific morphology and function of this bone.

Bipartite Medial Cuneiform and Calcaneo-Navicular Coalition in a Single Individual

“…During this time the bipartite deformity may become apparent on imaging with a characteristic 'E-Sign' [4][5][6], which can be seen on both sagittal MRI and CT scans as demonstrated in Figure 2. The volume of a bipartite medial cuneiform is recognised as being slightly larger than that of a normal medial cuneiform with an increased dorso-plantar height of between 5-8mm [1,5,[11][12]. Variations in the articular surfaces include a split facet for articulation of the plantar and dorsal segments with the first metatarsal, as well as that for the navicular [11][12].…”

“…Cases of partial and complete bipartition have been described [1]. In partial bipartition a cleft or crease is present within the articular facet without separation into two distinct plantar and dorsal segments [11] as shown in Figure 1. Ossification of the cuneiforms begins in the first year of life, starting with the lateral cuneiform Bipartition of the medial cuneiform is a mal-segmentation defect of the midfoot in which the normal cuneiform is separated into dorsal and plantar segments [1][2], often referred to as os cuneiforme I dorsale and os cuneiforme I plantare [1,[3][4].…”

“…The volume of a bipartite medial cuneiform is recognised as being slightly larger than that of a normal medial cuneiform with an increased dorso-plantar height of between 5-8mm [1,5,[11][12]. Variations in the articular surfaces include a split facet for articulation of the plantar and dorsal segments with the first metatarsal, as well as that for the navicular [11][12]. The plantar segment receives the tendinous insertions of tibialis posterior and peroneus longus on its inferomedial and inferolateral surface respectively [2,6] Tibialis anterior attaches to the superomedial surface of the dorsal segment [2,6].…”

“…There have been relatively few studies large enough to estimate the incidence of bipartition however the three largest series quote frequencies between 0.27% and 0.31% [1,12]. A genetic predisposition for bipartite medial cuneiform has been highlighted with a high frequency of bilateral cases of both partial and complete bipartition [1,4,11]. Additionally, there is also a higher than anticipated frequency of bipartite medial cuneiform found within archaeological populations of low genetic diversity [7,9,[11][12].…”

“…A genetic predisposition for bipartite medial cuneiform has been highlighted with a high frequency of bilateral cases of both partial and complete bipartition [1,4,11]. Additionally, there is also a higher than anticipated frequency of bipartite medial cuneiform found within archaeological populations of low genetic diversity [7,9,[11][12]. Pathways responsible for the normal patterning and shape of the skeleton such as Hox and Sox transcription factors alongside local regulators of mesenchyme condensation have been cited as potential genetic factors [11].…”