The human hand shows incredible dexterity. It performs countless daily tasks. Each human hand contains 27 bones. This complex skeleton of the hand allows for its amazing function. This post focuses on the carpal bones and metacarpal bones. These are crucial hand bones.
We provide a detailed anatomical map of the anatomy of the bones of the hand. This map explains their structure, function, and common issues. This anatomy also explores the bones of the hand, the overall anatomy of the hand, and hand and wrist anatomy.
Key Takeaways
Your hand has 27 bones. The carpal bones form your wrist. The metacarpal bones form your palm. These bones work together for hand movement.
Eight carpal bones make up your wrist. They are in two rows. These bones help your wrist bend and move side to side.
Five metacarpal bones form your palm. They connect your wrist bones to your fingers. These bones give your hand its shape and help you grip things.
The carpal tunnel is a small space in your wrist. It protects nerves and tendons. Problems like carpal tunnel syndrome happen when a nerve gets squeezed there.
Common hand injuries include scaphoid fractures and Boxer’s fractures. A scaphoid fracture is a broken wrist bone. A Boxer’s fracture is a broken bone in your palm, often from punching.
Carpal Bones: The Wrist’s Foundation in Hand Anatomy
The carpal bones form the carpus, or wrist. These eight irregular bones create a strong yet flexible foundation for the hand. They arrange themselves into two rows: a proximal row and a distal row. These rows work together. They allow for complex movements of the hand and wrist.
Proximal Carpal Bones
The proximal carpal bones are closest to the forearm. They connect the forearm bones to the rest of the hand. There are four bones in this row.
The scaphoid bone is the largest carpal bone in this row. It has a boat shape. It sits below the anatomical snuffbox. The scaphoid connects with the radius bone of the forearm. It also connects with the trapezium, trapezoid, lunate, and capitate bones. A small bump, the scaphoid tubercle, is easy to feel on the palm side.
The lunate bone sits between the scaphoid and triquetrum bones. It looks like a moon. The lunate connects with the radius and the articular disc of the distal radioulnar joint. It also connects with the capitate bone.
The triquetrum bone has a pyramid shape. It is on the inner side of the carpus. It connects with the lunate bone and the hamate bone. It also has a surface for the pisiform bone.
The pisiform bone is small and pea-shaped. It sits on the palm side of the triquetrum bone. It is a sesamoid bone. This means it is embedded within the tendon of the flexor carpi ulnaris muscle. It is easy to feel because it is close to the surface.
Distal Carpal Bones
The distal carpal bones are closer to the fingers. They connect the proximal carpal bones to the metacarpal bones. There are also four bones in this row.
The trapezium bone is on the thumb side. It connects with the scaphoid and the first metacarpal bone. This connection allows the thumb to move freely.
The trapezoid bone is small and wedge-shaped. It sits between the trapezium and the capitate. It connects with the scaphoid and the second metacarpal bone.
The capitate bone is the largest carpal bone. It sits in the center of the wrist. It connects with the scaphoid, lunate, trapezoid, hamate, and the second, third, and fourth metacarpal bones.
The hamate bone has a hook-like projection. This projection is called the hook of the hamate. It connects with the lunate, triquetrum, capitate, and the fourth and fifth metacarpal bones.
Carpal Articulations
The carpal bones have many connections. They connect with each other. They also connect with the radius and ulna bones of the forearm. Distally, they connect with the metacarpal bones.
These connections allow for a wide range of wrist movements. These movements include bending, straightening, and side-to-side motion. The complex arrangement of these hand bones provides both stability and flexibility.
Carpal Tunnel Formation
The carpal tunnel is a narrow passageway in the wrist. It is on the palm side. The carpal bones and a strong band of tissue form this tunnel. This band of tissue is the flexor retinaculum.
The carpal tunnel has specific boundaries.
The medial (ulnar) side has the pisiform bone and the hook of the hamate.
The lateral (radial) side has the scaphoid tubercle and the ridge of the trapezium.
The carpal tunnel forms from two main parts. A deep carpal arch forms the base and sides. This arch is concave on the palm side. The scaphoid and trapezium tubercles form the lateral part of the arch. The hook of the hamate and the pisiform form the medial part. The flexor retinaculum is a thick connective tissue. It forms the roof of the carpal tunnel.
It stretches between the hook of hamate and pisiform (medially) and the scaphoid and trapezium (laterally). This structure protects important nerves and tendons that pass into the hand.
Common Carpal Injuries
Injuries to the carpal bones can be painful and limit hand function. One common issue is carpal tunnel syndrome. This condition happens when the median nerve, which passes through the carpal tunnel, becomes compressed. The prevalence of carpal tunnel syndrome is about 5% in the general population.
This includes 3.8% based on clinical examination and 4.9% based on nerve conduction testing. When both clinical and electrophysiological tests confirm it, the prevalence is 2.7%. Undetected carpal tunnel syndrome is more common in adult women (5.8%) than in men (0.6%).
Another frequent injury is a scaphoid fracture. The scaphoid bone is the most commonly fractured carpal bone. A fall onto an outstretched hand often causes a scaphoid fracture. This forces the wrist to bend backward. The scaphoid then hits the radius bone. Other causes include a direct blow to the wrist or heavy impact with the wrist in a neutral position. A scaphoid fracture can also happen from a force applied to a hyperextended wrist, such as in falls from high places or car accidents.
A non-displaced scaphoid fracture can take time to heal. Immobilization with a splint usually lasts three to five weeks. If a cast is needed, it might be for six to eight weeks. Generally, people need about three months to heal from a scaphoid fracture. Healing times range from 6 to 10 weeks. Fractures closer to the thumb heal faster because they have a better blood supply.
Metacarpal Bones: The Palm’s Framework of Hand Bones
The five metacarpal bones form the palm of the hand. They are often called “palm bones.” These bones create the intermediate part of the hand. They connect the carpal bones to the finger bones. Each hand has five metacarpal bones. They are numbered I to V, starting from the thumb side.
Metacarpal Structure
Each metacarpal bone has a distinct structure. It consists of a base, a shaft, and a head. A neck connects the shaft to the head.
Base (Carpal Extremity): This is the proximal end of the metacarpal. It is cuboidal and wider at the back. The base connects with the carpal bones.
Shaft (Body): This is the main part of the metacarpal. It has a prismoid shape. The shaft curves slightly. It is convex on the back side and concave on the palm side. It has medial, lateral, and dorsal surfaces.
Neck (Subcapital Segment): This is the narrow area. It connects the shaft to the head.
Head (Digital Extremity): This is the distal end of the metacarpal. It has an oblong, convex surface. The head connects with the proximal phalanx of each finger.
Metacarpal Articulations
The metacarpal bones form important joints. Proximally, their bases connect with the distal row of carpal bones. For example, the first metacarpal articulates with the trapezium. The second metacarpal connects with the trapezoid and capitate. The third metacarpal articulates with the capitate.
The fourth metacarpal connects with the capitate and hamate. The fifth metacarpal articulates with the hamate. Distally, the heads of the metacarpals connect with the proximal phalanges. These joints are called metacarpophalangeal (MCP) joints. They allow for bending and straightening of the fingers.
The metacarpal bones also play a crucial role in forming the hand’s arches. These arches give the hand its shape and function. The distal transverse arch, also known as the metacarpal arch, forms from the heads of the metacarpals. The second and third metacarpals provide stability within this arch. The fourth and fifth metacarpals offer relative mobility.
This combination allows for a balance of stability and movement. One can observe this balance when forming a fist. The metacarpals also influence the longitudinal arch’s behavior. The fourth and fifth metacarpals show movement when a person tightens their fist. These arches are essential for gripping and manipulating objects.
Metacarpal Fractures
Metacarpal fractures are common injuries to the hand. They can happen from direct trauma or twisting forces.
Boxer’s Fracture: This fracture typically affects the neck of the fifth metacarpal bone. It gets its name because it often occurs when someone punches a hard object with a clenched fist. A direct axial load with a clenched fist is the most common mechanism of injury. Getting hit on the back of the hand can also cause a Boxer’s fracture. This type of metacarpal fracture causes pain, swelling, and sometimes a visible deformity of the little finger knuckle.
Bennett’s Fracture: This is a fracture of the base of the first metacarpal bone. It involves the joint surface. This fracture is often unstable. It happens when a force pushes the thumb into the palm. This can occur during a fall or a punch. The fracture line extends into the carpometacarpal (CMC) joint of the thumb. This makes it a more complex injury.
Understanding the structure and connections of these hand bones helps explain their function. It also helps in treating injuries to the metacarpal bones.
Phalanges: Finger Bones of the Hand
The phalanges are the bones that make up the fingers and thumb. They complete the skeletal context of the hand. These bones allow for the fine motor skills and gripping ability of the hand. Each finger has three phalanges, while the thumb has two. These are the phalanges of the fingers.
Phalangeal Structure
Each phalanx has a distinct structure. It consists of a central part, called the body or shaft. Each phalanx also has two extremities: a proximal extremity and a distal extremity. Specifically, each phalanx is composed of a base, a shaft, and a head.
The distal phalanx, found at the fingertip, consists of a base, a shaft, and a tuft or ungual tuberosity. This tuft provides support for the fingernail. Each middle phalanx has a head, a body (shaft), and a base. The proximal phalanges, closest to the palm, also follow this base, shaft, and head pattern.
Phalangeal Articulations
The phalanges form several important joints within the hand. Proximally, the bases of the proximal phalanges connect with the heads of the metacarpal bones. These connections form the metacarpophalangeal (MCP) joints. The MCP joints are a type of condyloid joint.
They link the metacarpus (palm) to the fingers. There are five such joints, each linking a metacarpal bone to the corresponding proximal phalanx of a finger. The convex heads of the metacarpal bones articulate with the concave bases of the proximal phalanges. Hyaline cartilage covers the articular surfaces of both the metacarpal and phalangeal bones.
The phalanges also articulate with each other. The proximal phalanges connect with the middle phalanges at the proximal interphalangeal (PIP) joints. The middle phalanges then connect with the distal phalanges at the distal interphalangeal (DIP) joints.
The thumb, having only two phalanges, has one interphalangeal joint. These joints allow the fingers to bend and straighten. While the phalanges are vital for hand function, this discussion primarily focuses on the carpal and metacarpal bones.
