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The carpus, or wrist joint, is made up of eight irregularly-shaped bones that connect the forearm to the hand. These carpal bones are arranged in two rows: the proximal row, which includes the scaphoid, lunate, triquetrum, and pisiform bones, and the distal row, which includes the trapezium, trapezoid, capitate, and hamate bones. The carpal bones form joints, such as the antebrachiocarpal, middle carpal, and carpometacarpal joints, which allow for flexion, extension, lateral and medial bending, and axial rotation of the wrist. These bones also provide attachment points for various muscles, such as the flexor carpi ulnaris, and allow for the passage of neurovascular structures to the hand.
| Characteristics | Values |
|---|---|
| Number of carpal bones | 8 |
| Carpal bones | Scaphoid, Lunate, Triquetrum, Pisiform, Trapezium, Trapezoid, Capitate, Hamate |
| Sesamoid bone | Located in the tendon of the abductor pollicis longus muscle |
| Proximal row of carpal bones | Scaphoid, Lunate, Triquetrum, Pisiform |
| Distal row of carpal bones | Trapezium, Trapezoid, Capitate, Hamate |
| First metacarpal | Most mobile and can move on its own |
| Second to fifth metacarpals | Move alongside each other |
| Fourth and fifth metacarpals | Most mobile among the second to fifth metacarpals |
| Joint types | Ginglymi, Arthrodial |
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What You'll Learn
The role of the carpal bones
The carpal bones are the eight small bones that make up the wrist (carpus) that connects the hand to the forearm. The terms "carpus" and "carpal" are derived from the Latin "carpus" and the Greek "καρπός" (karpós), meaning "wrist". The carpal bones are divided into two rows: the proximal and distal rows. The proximal row of carpal bones consists of the scaphoid, lunate, triquetrum, and pisiform bones. The distal row includes the trapezium, trapezoid, capitate, and hamate bones. Each carpal bone has a unique shape, allowing them to articulate with several surrounding bones, muscles, and ligaments of the forearm and hand. This articulation provides flexibility and a range of movements to the soft tissues of the hand.
The main role of the carpal bones is to form a highly mobile condyloid joint, or wrist joint, by articulating with the radial and ulnar heads. This joint allows for flexion, extension, lateral and medial bending, and axial rotation of the wrist. The carpal bones also provide attachments for thenar and hypothenar muscles, contributing to the hand's ability to grasp objects. Additionally, the carpal bones form part of the rigid carpal tunnel, which allows the transmission of the median nerve and tendons of the anterior forearm muscles to the hand and fingers. The carpal tunnel is formed by the arrangement of the carpal bones in a transverse arch, providing support and protection to the finger flexor tendons and the median nerve.
The scaphoid is the largest bone in the proximal row, with a boat-like shape ("scaphos" in Greek means boat). It articulates proximally with the radius and distally with the trapezium, trapezoid, lunate, and capitate bones. The scaphoid is the most frequently fractured carpal bone, usually occurring when a person falls onto an outstretched hand. The lunate is the next bone in the proximal row, located between the scaphoid and triquetrum. It gets its name from its moon-like shape and articulates with the head of the radius and the articular disc of the distal radioulnar joint.
The triquetrum is a pyramid-shaped bone located on the medial aspect of the carpus. It articulates with the lunate, hamate, and pisiform bones. The pisiform is a small, pea-shaped bone found on the distal palmar surface of the triquetrum. It is a sesamoid bone, fully embedded in the tendon of the flexor carpi ulnaris muscle. The distal row of carpal bones articulates with the metacarpals at the carpometacarpal joints. The trapezium is the first and most lateral bone in this row, forming articulations with the scaphoid, trapezoid, and the first and second metacarpals. The trapezoid, with its wedge shape, articulates with the scaphoid, trapezium, capitate, and the second metacarpal bone.
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The carpus and its joints
The carpus, or wrist joint, is comprised of eight carpal bones, which are irregularly shaped and located in the wrist region. These eight bones are organised into two rows: the proximal row and the distal row. The proximal row of carpal bones includes the scaphoid, lunate, triquetrum, and pisiform bones. The distal row includes the trapezium, trapezoid, capitate, and hamate bones. Each carpal bone has a unique shape and is multifaceted, allowing them to articulate with the surrounding bones, muscles, and ligaments of the forearm and hand. This provides flexibility and a range of movements to the soft tissues of the hand.
The carpal bones form three types of joints: the antebrachiocarpal, middle carpal, and carpometacarpal joints. The antebrachiocarpal joint is located between the distal aspect of the radius and the proximal row of carpal bones, while the carpometacarpal joint is located between the distal carpal and metacarpal bones. The middle carpal joint lies between the proximal and distal rows of carpal bones. These joints are classified as ginglymi, or hinged joints, allowing for flexion, extension, lateral and medial bending, and axial rotation of the wrist.
The carpal bones also form arches, with each row forming an arch that is convex proximally and concave distally. On the palmar side, the carpus forms the carpal tunnel, which is covered by the flexor retinaculum. The carpal tunnel provides protection and support to the finger flexor tendons and the median nerve. The carpal bones articulate with the radial and ulnar heads, forming a highly mobile condyloid joint, or wrist joint.
The carpus can be surgically accessed through general or specific carpal exposures. General exposures provide broad access to large areas of the wrist, while specific exposures permit access to a single bone or joint. Surgical approaches to the carpus require meticulous care due to the proximity of important ligaments, tendons, nerves, and blood vessels in the hand and wrist.
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Muscles and ligaments of the carpus
The carpus, or wrist, is a highly mobile joint that allows the wrist to move and rotate vertically. It is made up of eight small, irregularly shaped bones, which are organised into two rows: the proximal row and the distal row. The proximal row of carpal bones includes the scaphoid, lunate, triquetrum, and pisiform bones. The distal row includes the trapezium, trapezoid, capitate, and hamate bones.
The carpus is also comprised of several muscles and ligaments. The thenar muscles control the thumb, and the hypothenar muscles line the outer edges of the palm on the outside of the pinky finger. The hamulus, an elongated bony structure that projects from the distal aspect of the hamate bone's palmar surface, serves as an attachment point for several muscles and ligaments of the hand and forearm, including the flexor retinaculum. The pisiform is a sesamoid bone, meaning it is fully embedded in a tendon, specifically in the tendon of the flexor carpi ulnaris muscle. The accessory carpal bone is the distal attachment site of the flexor carpi ulnaris and ulnaris lateralis muscles.
The carpal tunnel is a rounded space in the wrist that lets nine tendons, four ligaments, and one nerve pass through it to reach the rest of the hand. The four ligaments are the ulnocarpal, radiocarpal, volar carpal, and dorsal radiocarpal ligaments. The ulnocarpal and radiocarpal ligaments stabilise the wrist while it moves. The volar carpal ligaments support and stabilise the bottom (palmar side) of the wrist, and the dorsal radiocarpal ligaments support and stabilise the back side of the wrist. The palmar fascia is a thick, triangle-shaped ligament-like structure that runs under the skin of the palm, helping the hand keep its shape while moving and preventing skin from sliding when holding something.
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Injuries and conditions of the carpus
The carpus is comprised of 7 or 8 carpal bones that form the wrist and connect the hand to the forearm. The carpus allows for flexion, extension, lateral and medial bending, and axial rotation.
Injuries of the Carpus
Injuries to the carpus are extremely common in children, who frequently attend the emergency department (ED) due to hand injuries. Carpal fractures account for around 8-19% of all hand injuries worldwide, with fractures of the scaphoid being the most common, making up 70% of carpal fractures. The triquetrum and trapezium bones are the second and third most commonly fractured, accounting for 15% and 3% of carpal fractures, respectively. The remaining carpal bones (lunate, hamate, capitate, and pisiform) each account for approximately 1% of carpal fractures, while the trapezoid is the least commonly fractured bone, at 0.2%. Carpal fractures tend to occur in young males who are engaging in sports or manual work.
Carpal fractures can be challenging to diagnose due to the complex three-dimensional anatomy of the carpus, and standard radiographs may not always be sufficient for visualization. More comprehensive evaluations, such as cross-sectional imaging, may be necessary but are not always economically feasible. Therefore, a careful clinical examination is crucial, including a meticulous history of the mechanism of injury and a search for areas of swelling, bruising, or deformity. Systematic palpation for tenderness, crepitus, and instability should also be performed, as signs can be subtle.
Conditions of the Carpus
The most common abnormality of the carpus is traumatically induced osteoarthritis, which is often associated with chip or slab fractures. Osteoarthritis can occur in any age, breed, or performance category and can lead to lameness of varying severity. In some cases, inflammation unresponsive to treatment may indicate the need for arthroscopic evaluation.
In small animal practice, conditions affecting the structure and function of the carpus are common and can have debilitating consequences for thoracic limb function. These conditions can include congenital abnormalities, developmental abnormalities, traumatic carpal injuries, and arthrodesis.
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Surgical approaches to the carpus
When performing surgery on the carpus, meticulous care is required due to the intricate anatomy of the hand and wrist. The initial skin incision must consider the proximity of important ligaments, tendons, nerves, and blood vessels to avoid damage to essential components. The carpus, or wrist, consists of eight small bones arranged in two transverse rows: the proximal row and the distal row. The proximal row comprises the scaphoid, lunate, triquetrum, and pisiform bones, while the distal row includes the trapezium, trapezoid, capitate, and hamate bones.
The carpus forms a highly mobile condyloid joint, allowing for flexion, extension, lateral and medial bending, and axial rotation. It also provides attachments for thenar and hypothenar muscles and contributes to the rigid carpal tunnel, protecting the finger flexor tendons and the median nerve. The blood supply to the carpus is derived from regional vessels, including the radial, ulnar, anterior interosseous, and deep palmar arches.
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Frequently asked questions
The carpal bones are the eight small bones that make up the wrist (carpus) that connects the hand to the forearm.
The carpal bones provide attachments for the thenar and hypothenar muscles. The pisiform bone is located in the tendon of the flexor carpi ulnaris muscle. The hamulus of the hamate bone serves as the attachment point for a number of different muscles and ligaments of the hand and forearm, including the flexor retinaculum.
The carpus allows flexion and extension in the sagittal plane, lateral and medial bending (abduction and adduction, respectively, of the metacarpus and digits), and axial rotation (pronation and supination).
The joints of the carpus include the antebrachiocarpal, middle carpal, and carpometacarpal joints. The antebrachiocarpal joint is the most mobile, allowing almost all of the range of motion in flexion.
