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The human hand is made up of five metacarpal bones, which are the intermediate bones between the fingers and the wrist. These bones are susceptible to fractures, especially in contact sports, and are also the insertion points for various muscles. The muscles that insert into the metacarpals include the interossei muscles, which can be divided into dorsal and palmar groups, and the lumbricals, which are crucial for finger movement. Other muscles that attach to the metacarpals include the flexor carpi radialis, the extensor carpi radialis longus, and the adductor pollicis.
| Characteristics | Values |
|---|---|
| Metacarpal bones definition | Appendicular bones that form the intermediate part of the hand between the phalanges (fingers) and the carpal bones (wrist bones) |
| Number of metacarpal bones | Five |
| Metacarpal bones shape | Long |
| Metacarpal bones location | Between the carpal bones and the proximal phalanges of the hand |
| Metacarpal bones composition | Bones and muscles |
| Muscles that insert into metacarpals | Opponens digiti minimi, Extensor carpi radialis longus, Flexor carpi radialis, Ulnar head of the first dorsal interosseous, Second palmar interosseous, Second dorsal interosseous, Transverse head of adductor pollicis, Ulnar head of the second dorsal interosseous, Radial head of the third dorsal interosseous, Third palmar interosseous, Fourth dorsal interosseous |
| Metacarpal bones fractures | Bennett's fracture, Rolando's fracture, intra-articular comminuted fractures, neck fractures, shaft fractures, base fractures |
| Causes of metacarpal fractures | Punching, direct blow, fall, athletic activities, contact sports, work-related injuries |
| Treatment for metacarpal fractures | Non-operative techniques, operative techniques using closed or open reduction and internal fixation (ORIF), surgical fixation |
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What You'll Learn
Interossei muscles
The interossei muscles are intrinsic hand muscles found near the metacarpal bones. They help to control the fingers and are considered voluntary muscles. The interossei muscles are categorised into two groups: palar interossei and dorsal interossei, each comprising four muscles. The palar interossei facilitate finger adduction, while the dorsal interossei enable abduction. Both muscle groups contribute to metacarpophalangeal (MCP) joint flexion and interphalangeal joint extension.
The palar interossei are unipennate muscles originating from the metacarpals of the respective digit to which they are associated. Adduction occurs at the MCP joints. Additionally, the palar interossei contribute to flexion at the MCP joints and extension at the proximal interphalangeal (PIP) and distal interphalangeal (DIP) joints of their respective digits. The first palmar interosseous originates from the medial, palmar surface of the first metacarpal and inserts into the base of the proximal first phalanx and the extensor hood. This muscle is frequently considered rudimentary, with many anatomical sources classifying it as part of either the adductor pollicis or the flexor pollicis brevis. The second palmar interosseous originates from the medial surface of the base of the second metacarpal and inserts into the medial portion of the extensor hood and base of the proximal first phalanx. The third and fourth palmar interossei originate from the lateral aspects of the fourth and fifth metacarpals and insert into the lateral portions of their respective extensor hoods and the bases of their corresponding proximal phalanges.
The dorsal interossei are the most dorsally located intrinsic hand muscles. They are bipennate and are associated with the abduction of the second, third, and fourth fingers. Their tendons run dorsally to the deep transverse metacarpal ligament. The first dorsal interosseous muscle can be tested by placing the patient's palm flat on a table and asking the patient to abduct their index finger against the examiner's resistance. The muscle belly can be both seen and felt and is a reliable test for the ulnar nerve. The second dorsal interosseous muscle attaches to the lateral side of the metacarpal shaft, while the third dorsal interosseous attaches to the medial side. The fourth dorsal interosseous originates from the lateral aspect of the fourth metacarpal and the medial side of the fifth metacarpal, inserting into the lateral base of the fourth proximal phalanx and the extensor hood of the fourth digit.
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Extensor carpi radialis longus muscle
The extensor carpi radialis longus is a long muscle that extends between the humerus and the second metacarpal bone. It is one of the five main muscles that control movements at the wrist. It is also one of three primary wrist extensors. The muscle is located in the posterior compartment of the forearm, where it is partly overlapped by the brachioradialis muscle.
The extensor carpi radialis longus is responsible for wrist extension and abduction (radial deviation) of the hand, which are vital for effective hand gripping. It also helps to flex the elbow joint and is active during fist clenching. The muscle is most effective as a wrist extensor when the elbow is extended and when radial deviation is balanced by the primary ulnar deviator- extensor carpi ulnaris.
The muscle originates from the lateral supracondylar ridge of the humerus (distal third), the anterior aspect of the lateral intermuscular septum of the arm, and a small portion of fibres originate from the common extensor tendon attached to the lateral epicondyle of the humerus. The fibres unite into a muscle belly, which extends to the middle of the forearm. The muscle belly then transitions into a flat tendon that travels along the lateral surface of the radius, beneath the abductor pollicis longus and extensor pollicis brevis.
The tendon of the extensor carpi radialis longus inserts on the posterior aspect of the base of the second metacarpal bone (metacarpal of the index finger). Some tendon slips can also insert into the first and third metacarpal bones. Proximal to the wrist, the tendon passes behind the radial styloid process within a common synovial sheath, along with the tendon of the extensor carpi radialis brevis.
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Flexor carpi radialis muscle
The flexor carpi radialis (FCR) is a long, thin muscle in the forearm that acts to flex and abduct the hand and wrist. It is one of four muscles in the superficial layer of the anterior compartment of the forearm. This muscle originates from the medial epicondyle of the humerus as part of the common flexor tendon, which is shared by six long flexor muscles in the forearm. The flexor carpi radialis is relatively thin and becomes visible as the wrist comes into flexion.
The muscle belly of the flexor carpi radialis courses obliquely, crossing from the ulnar to the radial part of the forearm. In the lower third of the forearm, it gives off a long tendon that passes below the flexor retinaculum into the palmar surface of the hand. The tendon of the flexor carpi radialis is visible on the anterior surface of the forearm, just proximal to the wrist, when the wrist is flexed. It is the tendon seen most lateral, closest to the thumb.
The flexor carpi radialis inserts at the bases of the second and third metacarpal bones. The attaching tendinous fibres lie deep to the oblique head of the adductor pollicis muscle. The innervation of this muscle is provided by the median nerve (C6-C7), with axons from cervical nerve roots. The main blood supply to the flexor carpi radialis comes from branches of the ulnar and radial arteries.
The flexor carpi radialis has been suggested to act as a dynamic scaphoid stabilizer. The FCR tendon uses the scaphoid tuberosity as a pulley to reach its distal insertion onto the second metacarpal. It is hypothesized that the contraction of this muscle generates a dorsally directed vector that resists the scaphoid from rotating into flexion. The radial artery runs between the tendons of the brachioradialis and flexor carpi radialis, a common site for palpation to measure someone's radial pulse.
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Adductor pollicis muscle
The adductor pollicis muscle is an intrinsic muscle of the hand, which is located in the deepest muscular plane of the palm, within the adductor compartment. It is a unique muscle, with a 2-headed structure, and a triangular shape. The two heads are the oblique head and the transverse head. The former originates at the capitate, as well as the bases of the second and third metacarpals, while the latter originates from the volar aspect of the third metacarpal.
The adductor pollicis is the most powerful of the intrinsic muscles of the hand. Its main function is the adduction of the thumb, which is the movement of the thumb towards the index finger from an abducted position. This action is essential for functions that require pinching and gripping. The adductor pollicis also aids the later stages of opposition of the thumb. The strength of the adductor pollicis can be tested by pushing the thumb against the index finger while attempting to pull them apart.
The adductor pollicis is innervated by the deep branch of the ulnar nerve (C8-T1). The radial artery passes between the two heads, travelling from the back of the hand into the palm, where it forms the deep palmar arch. The adductor pollicis is also vascularized by the deep palmar arterial arch, which is formed by the anastomosis of the radial artery and the deep palmar branch of the ulnar artery.
The adductor pollicis evolved from the contrahens I muscle as man's ancestors' thumbs and big toes became opposable. It might also contain an element of the thumb's interosseous muscle.
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Fractures and their management
A metacarpal fracture is a type of broken bone. The metacarpals are the bones in the hand that connect the wrist to the finger and thumb bones. Metacarpal fractures are one of the most common types of broken bones, especially among athletes in contact sports such as boxing and football, and manual labourers.
Causes
Metacarpal shaft fractures are often caused by longitudinal compression, torsion, or direct impact. They can be categorised as transverse, oblique, spiral, or comminuted. Metacarpal base fractures are rare, as the motion of the joint is small. Fractures of the base of the fifth digit are more common, resulting from a longitudinally directed force.
Symptoms
Symptoms of a metacarpal fracture include knuckle asymmetry, finger misalignment, and severe discomfort. Movement of the wrist or longitudinal compression can exacerbate the pain.
Treatment
The treatment for a metacarpal fracture depends on the type of fracture, the degree of bone damage, and the stability of the fracture. Surgery may be required if there is a large degree of displacement, comminution, and rotational malalignment. During surgery, the bones are realigned and secured with metal plates, screws, or pins. If surgery is not required, a custom splint may be used to support the healing fracture, and hand exercises may be recommended to reduce stiffness and improve range of motion.
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Frequently asked questions
Metacarpals are the five long bones of the hand that lie between the carpal bones (wrist bones) and the proximal phalanges (fingers). They form the intermediate part of the hand, also known as the "palm bones".
The interossei muscles insert into the metacarpals. These muscles can be divided into two groups: dorsal and palmar. The dorsal interossei muscles are responsible for the abduction of the fingers, while the palmar interossei muscles are responsible for adduction. Other muscles that insert into the metacarpals include the opponens pollicis, abductor pollicis brevis, flexor pollicis brevis, and lumbricals.
Metacarpal fractures are common injuries, especially in contact sports and manual labour. The most common type of fracture is the metacarpal neck fracture, often referred to as a "boxer's fracture". This occurs when a person punches an object or another person, resulting in a break at the extra-articular neck. Metacarpal shaft fractures are also common and are often caused by longitudinal compression, torsion, or direct impact.
