The pisiform is one of the smallest bones you’re likely to come across when dealing with archaeological remains (though I once found one of the auditory ossicles when excavating a commingled burial site in Portugal – that was an exciting afternoon, lemme tell you); as such, it can occasionally be confused with the smaller, unnamed sesamoid bones of the hand or foot. If you have a particularly devious individual compiling your bone quizzes, it pays to know the difference. Luckily, a few simple tips can help you tell the distinguish the pisiform from impostor sesamoids.
First and foremost, the pisiform bears a striking resemblance to one of our great American megafauna: the bison. To wit:
The tag “superior” in the set of drawings above refers to anterior view in SAP – basically, it’s what you would see if you flipped your palm up in supination and looked down at your carpals. To orient you, from this perspective the pisiform will be sitting on the side of your wrist that is closest to Ray 5, your little finger.
The rugose, ‘superior’ surface of the pisiform visible in this view is suggestive of the hump and head of a bison. While it’s easiest to see the bovid resemblance in proximal or distal view, another distinctive and diagnostic feature of the carpal is its round and decidedly flat articular surface for the triquetral.
(i) The lunate: The lunate’s concave articular facet for the capitate is easily distinguised from the flattened, circular or oval facet for the triquetral on the pisiform. Also, the lunate will be much larger. (ii) Sesamoids: The various small sesamoid bones of the hands and feet are much less regular in form, and lack this distinctively flat articular surface
Siding Tips: Unfortunately, I’ve never found a reliable method for siding the pisiform, and I have yet to meet an osteologist who is passionately invested in discovering one. In his description of the pisiform, White notes that “The morphological variation of this bone makes siding accurate in only about 85-90% of all cases” (White et al., 2012: 205). He then provides a siding technique that doesn’t work on the bone photographed, which is telling. If Tim White doesn’t side something. I don’t side it either. JB out!
White, Tim D., Michael T. Black & Pieter Folkens. 2012. Human Osteology. 3rd Edition. Academic Press.