The pisiform bone ( or ), also spelled pisiforme (from the Latin ''pisifomis'', pea-shaped), is a small knobbly,

sesamoid bone In anatomy, a sesamoid bone () is a bone embedded within a tendon or a muscle. Its name is derived from the Arabic word for ' sesame seed', indicating the small size of most sesamoids. Often, these bones form in response to strain, or can be pres ...

that is found in the wrist. It forms the

ulna The ulna (''pl''. ulnae or ulnas) is a long bone found in the forearm that stretches from the elbow to the smallest finger, and when in anatomical position, is found on the medial side of the forearm. That is, the ulna is on the same side of t ...

r border of the

carpal tunnel In the human body, the carpal tunnel or carpal canal is the passageway on the palmar side of the wrist that connects the forearm to the hand. The tunnel is bounded by the bones of the wrist and flexor retinaculum from connective tissue. Normall ...

Structure

The pisiform is a sesamoid bone, with no covering membrane of

periosteum The periosteum is a membrane that covers the outer surface of all bones, except at the articular surfaces (i.e. the parts within a joint space) of long bones. Endosteum lines the inner surface of the medullary cavity of all long bones. Structure ...

. It is the last

carpal bone The carpal bones are the eight small bones that make up the wrist (or carpus) that connects the hand to the forearm. The term "carpus" is derived from the Latin carpus and the Greek καρπός (karpós), meaning "wrist". In human anatomy, t ...

ossify Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in t ...

. The pisiform bone is a small bone found in the proximal row of the wrist (

carpus In human anatomy, the wrist is variously defined as (1) the carpus or carpal bones, the complex of eight bones forming the proximal skeletal segment of the hand; "The wrist contains eight bones, roughly aligned in two rows, known as the carpal ...

). It is situated where the

joins the wrist, within the

tendon A tendon or sinew is a tough, high-tensile-strength band of dense fibrous connective tissue that connects muscle to bone. It is able to transmit the mechanical forces of muscle contraction to the skeletal system without sacrificing its ability ...

of the

flexor carpi ulnaris muscle The flexor carpi ulnaris (FCU) is a muscle of the forearm that flexes and adducts at the wrist joint. Structure Origin The flexor carpi ulnaris has two heads; a humeral head and ulnar head. The humeral head originates from the medial epicondyle o ...

. It only has one side that acts as a

joint A joint or articulation (or articular surface) is the connection made between bones, ossicles, or other hard structures in the body which link an animal's skeletal system into a functional whole.Saladin, Ken. Anatomy & Physiology. 7th ed. McGraw ...

, articulating with the

triquetral bone The triquetral bone (; also called triquetrum, pyramidal, three-faced, and formerly cuneiform bone) is located in the wrist on the medial side of the proximal row of the carpus between the lunate and pisiform bones. It is on the ulnar side of th ...

. It is on a plane anterior to the other carpal bones and is spheroidal in form. The pisiform bone has four surfaces: # The ''dorsal surface'' is smooth and oval, and articulates with the triquetral: this facet approaches the superior, but not the inferior border of the bone. # The ''palmar surface'' is rounded and rough, and gives attachment to the

transverse carpal ligament The flexor retinaculum (transverse carpal ligament, or anterior annular ligament) is a fibrous band on the palmar side of the hand near the wrist. It arches over the carpal bones of the hands, covering them and forming the carpal tunnel. Structur ...

, the

flexor carpi ulnaris The flexor carpi ulnaris (FCU) is a muscle of the forearm that flexes and adducts at the wrist joint. Structure Origin The flexor carpi ulnaris has two heads; a humeral head and ulnar head. The humeral head originates from the medial epicondyle o ...

and the abductor digiti quinti. # The ''lateral surface'' is rough, and concave. # The ''medial surface is rough and usually convex.

Etymology

The etymology derives from the Latin ''pīsum'' which means "pea" ultimately derived from the Greek "pison" (pea).

Function

The pisiform bone is most recognizable as an unassuming palmar projection forming the heel of your hand. The pisiform bone, along with the hamulus of the

hamate The hamate bone (from Latin hamatus, "hooked"), or unciform bone (from Latin ''uncus'', "hook"), Latin os hamatum and occasionally abbreviated as just hamatum, is a bone in the human wrist readily distinguishable by its wedge shape and a hook-l ...

, defines the medial boundary of the

because the pisiform body acts as one of the four attachments points of the flexor retinaculum. It also acts as an attachment site for tendons of the abductor digiti minimi and for the

- the tendon in which it develops. The pisiform is the only carpal bone with insertions and attachments for the abductor digiti minimi and the flexor carpi ulnaris. It is suggested that due to the pisiform's surprisingly large range of movement along its articulation surface with the triquetral bone (about 1 cm of movement is allowed), contraction of the flexor carpi ulnaris is necessary for the pisiform to remain stable enough for the abductor digiti minimi to function effectively. In clinical studies, the pisiform has been removed as treatment for

osteoarthritis Osteoarthritis (OA) is a type of degenerative joint disease that results from breakdown of joint cartilage and underlying bone which affects 1 in 7 adults in the United States. It is believed to be the fourth leading cause of disability in the w ...

in the pisotriquetral joint. While some studies came to the conclusion that the pisiform "contributes to the stability of the ulnar column of the wrist", others suggested that while excision slightly impairs the range of motion of the wrist (especially wrist extension), the forces generated within the wrist are not significantly impacted. Subjects in the latter study did report impaired function after excision when performing heavy lifting and weightbearing activities, but this is suggested to be subjective considering that they did not have to change occupation or their level of activity as a result of the excision.

Development

Compared with other non-human primates,

human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...

s have a short pisiform bone. This dramatic size difference is suggested to be the outcome of a lost growth plate in hominins some time between

Australopithecus afarensis ''Australopithecus afarensis'' is an extinct species of australopithecine which lived from about 3.9–2.9 million years ago (mya) in the Pliocene of East Africa. The first fossils were discovered in the 1930s, but major fossil finds would not ...

, who has been shown to have an elongated and ape-like pisiform, and

Homo neanderthalensis Neanderthals (, also ''Homo neanderthalensis'' and erroneously ''Homo sapiens neanderthalensis''), also written as Neandertals, are an extinct species or subspecies of archaic humans who lived in Eurasia until about 40,000 years ago. While the ...

, who is suggested to have a pisiform resembling the modern human condition. It is suggested that the first signs of human pisiform ossification, observed between the ages of 7 and 12, corresponds to the period of secondary pisiform ossification in apes. This can point to a couple different changes in development: either this growth plate loss in humans is also accompanied by a developmental shift in the timing of pisiform formation, or it is the primary center that fails to form in humans and as a result our pisiform is homologous to the

epiphysis The epiphysis () is the rounded end of a long bone, at its joint with adjacent bone(s). Between the epiphysis and diaphysis (the long midsection of the long bone) lies the metaphysis, including the epiphyseal plate (growth plate). At the join ...

of other mammalian pisiforms. Studies looking at the effect of

Hox gene Hox genes, a subset of homeobox genes, are a group of related genes that specify regions of the body plan of an embryo along the head-tail axis of animals. Hox proteins encode and specify the characteristics of 'position', ensuring that the cor ...

knockouts on the formation of the pisiform in mice have suggested that the modification of Hoxa11 or Hoxd11 genes, or the downstream targets they affect, could have acted as the mechanism for the reduction we see in the human pisiform condition.

Evolution

There are several hypotheses that seek to explain why we see pisiform reduction during the course of hominin evolution. Some suggest that the reduction of the pisiform allowed for ulnar deviation and that allowed for greater extension in the human wrist which increased our capacity for throwing. Scholars with this point of view would believe that these anatomical changes would improve the action of clubbing in our hominin ancestors. Others suggest that the pisiform's link with Hoxa11 and Hoxd11 could tie its developmental history to that of the

forearm The forearm is the region of the upper limb between the elbow and the wrist. The term forearm is used in anatomy to distinguish it from the arm, a word which is most often used to describe the entire appendage of the upper limb, but which in anat ...

, whose length is determined by Hox gene expression. Within the context of this hypothesis, because modern forearm proportions are not seen until

Homo erectus ''Homo erectus'' (; meaning "upright man") is an extinct species of archaic human from the Pleistocene, with its earliest occurrence about 2 million years ago. Several human species, such as '' H. heidelbergensis'' and '' H. antecessor' ...

at 1.5 million years ago, it is possible that pisiform reduction would have also occurred around this time. Alternatively, the same group suggests that the reduction could be a reflection of independent selection associated with the production and use of stone tools, but changes in pisiform

morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines * Morphology (archaeology), study of the shapes or forms of artifacts * Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies ...

have yet to be studied in relation to their effect on wrist function.

Other animals

All other

tetrapods Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids (reptiles, including dinosaurs and therefore birds) and synapsids ( pelycosaurs, extinct therapsi ...

have a pisiform, being the most common sesamoid.Liem (2005) functional anatomy of the vertebrates In

mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...

s and non-human

primate Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians (monkeys and apes, the latter including huma ...

s, the pisiform is an enlarged and elongated bone that articulates with the distal ulna. In some taxa, the pisiform even articulates with the hammate or radius. In these non-human taxa, the pisiform develops from two

ossification center An ossification center is a point where ossification of the cartilage begins. The first step in ossification is that the cartilage cells at this point enlarge and arrange themselves in rows.Gray and Spitzka (1910), page 44. The matrix in which t ...

s that are divided by a palmar

epiphyseal plate The epiphyseal plate (or epiphysial plate, physis, or growth plate) is a hyaline cartilage plate in the metaphysis at each end of a long bone. It is the part of a long bone where new bone growth takes place; that is, the whole bone is alive, wi ...

. Because in other mammals, the bone does not follow a typical sesamoid development pattern and can be seen articulating with more than one bone, the pisiform is not a true sesamoid bone.

Additional images

File:Pisiform bone (left hand) - animation01.gif, Position of pisiform bone (shown in red). Left hand. Animation. File:Pisiform bone (left hand) - animation02.gif, Pisiform bone of the left hand. Close up. Animation. File:Pisiform bone (left hand) - animation03.gif, Pisiform bone (red) forms ulnar border of the

. Left hand. Animation. Image:Pisiform bone.jpg, Pisiform bone.

References

External links

* *
Illustration at ntu.edu.tw
{{Authority control Skeletal system Wrist Sesamoid bones Bones of the hand Carpal bones