Carpal bones
The carpal bones are a set of eight small, irregularly shaped bones located in the wrist that connect the distal ends of the radius and ulna bones of the forearm to the bases of the five metacarpal bones of the hand.[1] These bones are organized into two transverse rows of four each, forming a flexible yet stable structure essential for wrist mobility. The proximal row, positioned closest to the forearm and arranged from lateral (thumb side) to medial (pinky side), includes the scaphoid, lunate, triquetrum, and pisiform bones. The distal row, adjacent to the hand, consists of the trapezium, trapezoid, capitate, and hamate bones. Each carpal bone has unique articular surfaces that interlock with neighboring bones and ligaments, creating a concave anterior arch roofed by the flexor retinaculum to form the carpal tunnel, through which flexor tendons and the median nerve travel.[2] Functionally, the carpal bones contribute to the radiocarpal and midcarpal joints, enabling a wide range of wrist movements including flexion, extension, abduction, adduction, and circumduction while providing structural support for load transmission during gripping and weight-bearing activities. They serve as attachment sites for numerous ligaments and tendons, enhancing joint stability and facilitating precise hand coordination. Blood supply to the carpals primarily arises from branches of the radial and ulnar arteries, with the scaphoid being particularly vulnerable to avascular necrosis due to its retrograde vascularization.[1][2] The carpal bones develop through endochondral ossification from mesenchymal precursors during fetal hand morphogenesis, which occurs primarily between 6 and 14 weeks of gestation, though full ossification centers appear postnatally in a sequential manner starting with the capitate and hamate around 1 to 3 months of age. Clinically, the carpals are prone to injury, with scaphoid fractures being the most common carpal injury due to falls on an outstretched hand, often requiring immobilization or surgical fixation to prevent complications like nonunion. Disorders such as carpal tunnel syndrome arise from compression within the tunnel, affecting median nerve function and leading to pain, numbness, and weakness in the hand.[3][4][5]Anatomy
Bones
The carpal bones comprise eight short bones that form the wrist, arranged in two transverse rows between the distal forearm and the metacarpal bases. The proximal row, situated closest to the forearm, consists of four bones from radial (lateral) to ulnar (medial): the scaphoid, lunate, triquetrum, and pisiform. The distal row, adjacent to the hand, similarly includes four bones in radial-to-ulnar order: the trapezium, trapezoid, capitate, and hamate. This arrangement allows for the complex mobility and stability of the wrist.[1][2] A widely used mnemonic for recalling the sequence of carpal bones from proximal radial to distal ulnar is "She Looks Too Pretty; Try To Catch Her," where each word's initial letter corresponds to scaphoid (She), lunate (Looks), triquetrum (Too), pisiform (Pretty), trapezium (Try), trapezoid (To), capitate (Catch), and hamate (Her). The following table summarizes the morphology, position, key features, and primary articulations of each carpal bone:| Bone | Row | Shape and Morphology | Key Features | Primary Articulations |
|---|---|---|---|---|
| Scaphoid | Proximal | Boat-shaped (navicular) with a peanut-like body, elongated waist, and concave distal surface; largest bone in the proximal row. | Prominent tubercle on the anterior (palmar) surface, forming part of the anatomical snuffbox; nutrient foramina typically on dorsal ridge. | Proximal: radius; Distal: trapezium, trapezoid, capitate; Interosseous: lunate. |
| Lunate | Proximal | Crescent- or moon-shaped (luna) with a concave proximal surface and convex distal; central position in proximal row. | Concavo-convex articular facets; variable morphology including type I (single facet) or type II (medial facet for hamate). | Proximal: radius; Distal: capitate; Interosseous: scaphoid, triquetrum. |
| Triquetrum | Proximal | Pyramidor irregular-shaped with a flattened ulnar surface and three articular facets. | Small facet on palmar surface for pisiform; vascular supply via dorsal and palmar branches. | Proximal: triangular fibrocartilage complex (ulnar side); Distal: hamate; Interosseous: lunate, pisiform. |
| Pisiform | Proximal | Small, nodular, and pea-like sesamoid bone; ovoid with irregular surfaces. | Embedded within the flexor carpi ulnaris tendon; unique secondary ossification center among carpals. | Interosseous: triquetrum (single facet). |
| Trapezium | Distal | Saddleor irregularly shaped with a ridged palmar surface and four articular facets. | Tubercle on anterior ridge for transverse carpal ligament attachment; supports thumb opposition. | Proximal: scaphoid; Distal: first metacarpal; Interosseous: trapezoid, second metacarpal. |
| Trapezoid | Distal | Wedge- or quadrangular-shaped; smallest carpal bone overall. | Concave proximal surface; minimal surface projections. | Proximal: scaphoid; Distal: second metacarpal; Interosseous: trapezium, capitate. |
| Capitate | Distal | Head-, body-, and neck-shaped with a rounded proximal head and elongated shaft; largest carpal bone overall. | Rounded head for scaphoid articulation; waist-like constriction at neck. | Proximal: scaphoid, lunate; Distal: third metacarpal; Interosseous: hamate, trapezoid. |
| Hamate | Distal | Wedge-shaped with a triangular base and prominent hook. | Hamulus (hook) process on palmar surface for flexor tendon and ligament attachment; three articular facets. | Proximal: triquetrum, lunate (via capitate); Distal: fourth and fifth metacarpals; Interosseous: capitate, pisiform (via coalition variant). |
Joints
The carpal bones articulate at several synovial joints that facilitate wrist mobility, primarily classified into the radiocarpal, midcarpal, and carpometacarpal joints. The radiocarpal joint, also known as the wrist joint, is an ellipsoid synovial joint formed between the distal radius and the proximal row of carpal bones, including the scaphoid, lunate, and triquetrum. The midcarpal joint represents a complex synovial articulation, consisting of multiple gliding interfaces between the proximal and distal rows of carpal bones. The carpometacarpal joints connect the distal row of carpal bones to the metacarpals; those for digits 2 through 5 are plane synovial joints, while the thumb's carpometacarpal joint is a saddle synovial joint.[15][16][17] Within the midcarpal complex, specific intercarpal joints include those of the proximal row, such as the scapholunate and lunotriquetral joints, which are synovial articulations between adjacent bones in that row. The distal row features synovial joints between the trapezium and trapezoid, the trapezoid and capitate, and the capitate and hamate. These intercarpal joints contribute to the overall segmented nature of the wrist's synovial architecture.[18][19] Each carpal joint is enclosed by a capsule comprising an outer fibrous layer, which provides structural integrity and attaches to the periosteum of the articulating bones, and an inner synovial layer that lines the joint cavity and secretes synovial fluid for lubrication. The capsules of the radiocarpal and midcarpal joints exhibit palmar and dorsal reinforcements, where the fibrous layer is thickened to enhance stability without direct tendon attachments.[18][20] Synovial sheaths and compartments associated with the carpal joints include the common flexor sheath, which extends through the carpal tunnel and envelops multiple flexor tendons, and separate extensor sheaths that also traverse this region to minimize friction during motion. These sheaths form distinct synovial compartments that communicate with certain joint cavities, such as the ulnar bursa linking to the radiocarpal joint in some individuals.[21][22]Ligaments
The ligaments of the carpus are classified into extrinsic and intrinsic types based on their connections. Extrinsic ligaments originate from the radius, ulna, or metacarpals and insert onto the carpal bones, providing broad support to the wrist joint. Intrinsic ligaments, in contrast, interconnect adjacent carpal bones within the carpus itself.[23][24] Additionally, carpal ligaments are categorized by location as volar (palmar) or dorsal, and by association with the proximal or distal carpal rows. They can also be distinguished as membranous (thin, intra-articular sheets) or capsular (thicker, reinforcing the joint capsules).[25][26]Extrinsic Ligaments
The palmar radiocarpal ligament arises from the anterior rim of the distal radius and attaches to the palmar surfaces of the lunate and capitate bones, forming part of the volar joint capsule. The radioscaphocapitate ligament, a distinct volar extrinsic band, originates from the styloid process of the radius and courses distally to insert on the scaphoid tuberosity and capitate, traversing the space of Poirier between the palmar radiocarpal and radioscaphocapitate ligaments. The dorsal radiocarpal ligament extends from the posterior distal radius to the dorsal aspects of the lunate and triquetrum.[26][27][28] The radial collateral ligament originates from the tip of the radial styloid process and inserts onto the radial aspect of the scaphoid, blending with the dorsal and volar radiocarpal ligaments. The ulnar collateral ligament arises from the ulnar styloid process and attaches to the ulnar side of the triquetrum and pisiform, contributing to the ulnocarpal complex. The triangular fibrocartilage complex (TFCC), located ulnarly, is a key component of the ulnocarpal complex and incorporates the ulnar collateral ligament along with dorsal and palmar radioulnar ligaments and an articular disc, attaching from the ulnar styloid and distal radioulnar joint to the triquetrum, lunate, and pisiform. The pisohamate ligament, a volar extrinsic structure, connects the palmar aspect of the pisiform bone to the hook of the hamate.[29][30][25][26][31]Intrinsic Ligaments
Intrinsic ligaments primarily link bones within the proximal and distal carpal rows. The scapholunate ligament interconnects the scaphoid and lunate, consisting of three parts: a thick dorsal band, a membranous central portion, and a volar band. The lunotriquetral ligament similarly joins the lunate and triquetrum, with comparable dorsal, membranous, and volar components, forming part of the proximal row's interosseous stabilizers.[25][32] The dorsal intercarpal ligament forms an arcuate structure connecting the dorsal surfaces of the scaphoid, lunate, triquetrum, and trapezium to the distal row bones, including the capitate and hamate. On the palmar side, the intrinsic ligaments include the palmar scaphotriquetral ligament and other capsular bands reinforcing the proximal-distal row interfaces.[26][31]Accessory bones
Accessory ossicles of the carpal region are supernumerary bones that develop independently from the standard eight carpal bones, arising either from separate secondary ossification centers that fail to fuse during development or from avulsion fractures that become rounded and corticated over time.[33] These ossicles are typically asymptomatic and discovered incidentally on imaging, but they hold clinical significance as they may mimic acute fractures, leading to unnecessary interventions, or cause pain through impingement, inflammation, or fracture in symptomatic cases.[34] Their overall prevalence in the wrist is approximately 1.6% to 9.7%, varying by population and imaging modality, with higher rates observed in radiographic surveys of trauma patients.[35][33] Among the more common accessory ossicles, the os centrale carpi is a small, ovoid bone located in the distal row of the carpus, adjacent to the scaphoid and capitate, often fusing with the scaphoid in early development but persisting separately in about 1% of individuals.[36] It may contribute to symptoms such as intermittent wrist pain, clicking, or crepitus due to abnormal mobility, and can be mistaken for a scaphoid fracture or lead to osteonecrosis.[36] The os styloideum, a variant near the radial styloid process, appears as a dorsal prominence at the base of the second or third metacarpal and has a prevalence of approximately 0.3% to 2% in radiographic studies of the general population, with cadaveric studies reporting up to 19%; notably higher prevalence (e.g., 81%) observed in athletes like NHL hockey players, likely due to selection bias.[37][38] Clinically, it is associated with carpal boss syndrome, presenting as a painful dorsal wrist mass exacerbated by repetitive motion.[33] Pisiform variants, such as the pisiforme secundarium, involve irregular or multipartite ossification of the pisiform bone, typically at its proximal pole, and occur during the late ossification phase between ages 8 and 12; these are often incidental but can cause ulnar-sided wrist discomfort if associated with tendon impingement.[39] The os triangulare, sometimes referred to as an accessory near the lunate due to its proximity, is positioned distally to the ulnar fovea between the ulnar styloid, lunate, and triquetrum, with a prevalence of about 2.4% in radiographic studies.[33][40] It is usually asymptomatic but may simulate an ulnar styloid avulsion fracture.[41] Rarer accessory ossicles include the os epilunatum, a small bone on the dorsal surface of the lunocapitate joint adjacent to the lunate, with an estimated prevalence of 0.3% to 0.5%.[42][43] This ossicle is exceptionally uncommon, absent in many large radiographic reviews, and can cause bilateral wrist pain if symptomatic, often requiring surgical excision for relief.[35] The os hamuli proprium represents a separate hook of the hamate, resulting from failure of the hamulus ossification center to fuse with the hamate body between ages 12 and 15, and is rare with prevalence below 1%.[44][45] It may be confused with a hamate hook fracture on imaging and, in isolated cases, contribute to carpal tunnel-like symptoms necessitating resection.[46] The os triquetrum accessorium (or secundarium), located near the dorsal aspect of the triquetrum, has a prevalence of approximately 1.3% and forms via segmentation of the triquetral ossification center.[43] Though generally asymptomatic, it can become symptomatic following trauma, presenting with pain due to fracture or ligamentous attachment stress, and must be differentiated from avulsion injuries using cortical margins on advanced imaging.[47] Overall, recognition of these ossicles prevents misdiagnosis, with CT or MRI aiding in confirmation of their benign, developmental nature.[33]Development
Embryonic formation
The carpal bones begin their embryonic development as mesenchymal condensations within the somatic layer of the lateral plate mesoderm, emerging during weeks 4 to 6 of gestation as the upper limb buds form. These precursors arise from the activation and proliferation of mesenchymal cells in the lateral mesoderm, which contribute to the foundational skeletal elements of the wrist. This initial phase establishes the basic framework for the eight carpal bones, aligning with the proximodistal outgrowth of the limb bud.[1][48] The proximal-distal identity of these carpal precursors is regulated by specific expression patterns of Hox genes, particularly Hoxa-11 and Hoxd-13, which are essential for patterning the appendicular skeleton. Hoxa-11 is predominantly expressed in the proximal regions, influencing the zeugopod (forearm) and adjacent carpal elements, while Hoxd-13 drives distal autopod (hand) development, including the carpal row formation. Disruptions in these genes can alter the segmental identity, leading to malformations in carpal differentiation.[49] Chondrogenesis of the carpal anlagen commences around week 6 and is largely complete by week 8, transforming the mesenchymal condensations into cartilaginous models. This process first manifests in the central carpal elements, such as the capitate and hamate, and progresses to form distinct pre-axial (radial, or thumb-side) structures like the scaphoid and trapezium, alongside post-axial (ulnar-side) elements including the triquetrum and pisiform. These cartilaginous precursors provide the template for subsequent skeletal maturation.[50][51] Congenital anomalies of the carpal bones, such as synostosis, originate from failures in the segmentation of these early cartilaginous anlagen during embryogenesis. For instance, scapholunate fusion results from incomplete separation of the interzones between adjacent precursors, often linked to genetic or developmental disruptions in the chondrogenic phase. These conditions highlight the precision required in embryonic segmentation for normal wrist formation.[52][53]Ossification
The ossification of the carpal bones occurs through primary endochondral ossification centers from the cartilaginous precursors for most bones.[54] Unlike long bones, carpal bones lack secondary ossification centers and epiphyseal plates, resulting in growth primarily through surface apposition rather than endochondral elongation or fusion events.[55] The sequence of ossification is predictable and begins shortly after birth, providing a reliable marker for skeletal maturity. The capitate ossifies first, typically between 1 and 3 months of age, followed closely by the hamate at 2 to 4 months. The triquetrum and lunate appear next, around 2 to 3 years and 2 to 4 years, respectively. The trapezium and trapezoid ossify between 4 and 5 years, the scaphoid at 4 to 6 years, and the pisiform last, between 8 and 12 years.[55] This timeline exhibits variability, with ossification generally occurring 6 to 12 months earlier in females than in males due to hormonal influences on skeletal development.[4] The pisiform is unique among the carpals as a sesamoid bone that ossifies intramembranously within the tendon of the flexor carpi ulnaris, rather than in a preformed cartilaginous model.[56] Its delayed ossification reflects its functional role in tendon mechanics, and it shows greater variability in appearance compared to other carpals.[57] In pediatric radiology, the radiographic appearance and sequence of carpal ossification centers on hand and wrist X-rays serve as a standard for assessing bone age and overall skeletal maturity, aiding in the diagnosis of growth disorders.[58] These images reveal the progressive mineralization, with the number and size of visible centers correlating to chronological age within typical ranges.[59]| Carpal Bone | Typical Ossification Age Range |
|---|---|
| Capitate | 1–3 months |
| Hamate | 2–4 months |
| Triquetrum | 2–3 years |
| Lunate | 2–4 years |
| Trapezium | 4–5 years |
| Trapezoid | 4–5 years |
| Scaphoid | 4–6 years |
| Pisiform | 8–12 years |