Please cite this article in press as: Burnett, S.E., et al., Six new examples of the bipartite trapezoid bone: Morphology, significant population variation, and an examination of pre-existing criteria to identify bipartition of individual carpal bones. Ann. Anatomy (2014), http://dx.doi.org/10.1016/j.aanat.2014.11.002 ARTICLE IN PRESS G Model AANAT-50911; No. of Pages 8 Annals of Anatomy xxx (2014) xxx–xxx Contents lists available at ScienceDirect Annals of Anatomy j ourna l h omepage: www.elsevier.de/aanat Research article Six new examples of the bipartite trapezoid bone: Morphology, significant population variation, and an examination of pre-existing criteria to identify bipartition of individual carpal bones Scott E. Burnett a,* , Christopher M. Stojanowski b , Pasuk Mahakkanukrauh c a Comparative Cultures Collegium, Eckerd College, 4200 54th Avenue South, St Petersburg, FL 33711, USA b School of Human Evolution & Social Change, Arizona State University, Tempe, AZ 85287-2402, USA c Department of Anatomy, Faculty of Medicine, Chiang Mai University, 50120, Thailand a r t i c l e i n f o Article history: Received 13 May 2014 Received in revised form 7 November 2014 Accepted 20 November 2014 Available online xxx Keywords: Carpal bone bipartition Trapezoid Carpal malsegmentation Bipartite scaphoid a b s t r a c t Carpal bone bipartition is a developmental variant resulting in the division of a normally singular carpal into two distinct segments. Cases involving the scaphoid are best known, though many other carpals can be affected, including the trapezoid. Six new examples of bipartite trapezoids, identified in African and Asian anatomical and archeological samples, are reported here and compared with the eight pre- viously known. While the site of bipartition is consistent, the resulting segments exhibit variability in their articulations with neighboring carpals. Five of the six affected trapezoids were identified in African or African-derived samples, yielding a significantly higher frequency (0.323%) of bipartite trapezoid than seen in anatomical or archeological series of European origin. Bilateral bipartite trapezoids in archeolog- ical remains from the Mid Holocene site of Gobero (Niger) are potentially the oldest bipartite carpals yet identified in humans. Their discovery may indicate that trapezoid bipartition is a condition that has been present in African populations since prehistoric times, though more data are needed. Because bipartite carpals may be symptomatic and can occur as part of syndromes, the significant population variation in frequency identified here has potential utility in both anatomical and clinical contexts. However, a com- parison of the morphological appearance of bipartite trapezoids with the suggested criteria for bipartite scaphoid diagnosis indicates that these criteria are not equally applicable to other carpals. Fortunately, due to the rarity of fracture, identification of the bipartite trapezoid and separating it from pathological conditions is considerably easier than diagnosing a bipartite scaphoid. © 2014 Elsevier GmbH. All rights reserved. 1. Introduction Carpal bone bipartition is a developmental variant resulting from the formation of two mesenchymal or cartilaginous carpal centers instead of one, or from the non-union of two ossifica- tion centers within a single carpal bony template (Gruber, 1879; O’Rahilly, 1953). Bipartite carpal bones may be symptomatic (Richards et al., 1987; Kim et al., 2005; Et-tai et al., 2008), misdi- agnosed as an ununited fracture (Dwight, 1907), and occur as part of malformation syndromes (Tate et al., 2000; Saccomanni, 2009; Unruh and Shin, 2011) or endocrinopathies including cretinism (O’Rahilly, 1953). Most published cases of bipartite carpal bones involve the scaphoid, though the trapezium, trapezoid, triquetral, * Corresponding author. Tel.: +1 727 864 8932; fax: +1 727 864 7995. E-mail address: burnetse@eckerd.edu (S.E. Burnett). and lunate may each also be represented by two independent bony elements (e.g., Thilenius, 1894, in Windle, 1895; O’Rahilly, 1953; Loh et al., 2011; Barnes, 2012). Bipartition of the trape- zoid has been particularly infrequently reported, with seven cases published since the first in 1879 (Table 1), and only two cases described since 1887. As a result of the sparse and dated lit- erature on its occurrence, little is known about this anatomical variation. The purpose of this report is to: (1) describe five new cases, including the earliest known archeological case, occurring bilaterally in a mid Holocene burial from the Saharan site of Gob- ero (Niger), (2) describe the morphological characteristics of the bipartite trapezoid, (3) demonstrate statistically significant pop- ulation variation in the occurrence of this carpal variant, and (4) discuss issues with the differential diagnosis of carpal bipartition. These new findings are important for the proper interpretation and identification of bipartite carpals in both anatomical and clinical contexts. http://dx.doi.org/10.1016/j.aanat.2014.11.002 0940-9602/© 2014 Elsevier GmbH. All rights reserved.