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The Hand Bones – Anatomy, Nerve Supply, Functions

The Hand Bones/Bones are often thought of as static structures which only offer structural support. However, they truly function as an organ. Like other organs, bones are valuable and have many functions. Besides providing shape to the human body, bones permit locomotion, motor capability, protect vital organs, facilitate breathing, play a role in homeostasis, and produce a variety of cells in the marrow critical for survival. Bones are continually undergoing structural and biological change, and remodeling of bone continues throughout life based on the demands placed upon them.

The skeletal system is capable of responding to increased stress such as during resistance training, by increasing osteogenesis, or new bone formation. In fact, resistance exercise has been shown to be a viable therapeutic option in osteosarcopenia,[rx] which is the loss of bone and muscle density due to aging. In addition to responding to external stimuli, they can also respond to internal stimuli for mobilization of their content. Bones can enlarge or become smaller, grow strong or weaker, and break when subjected to the application of excessive force. In the event of damage, they are one of the very few organs in the body that can regenerate without an obvious scar. There are typically around 270 bones in human infants, which fuse to become 206 to 213 bones in the human adult. The reason for the variability in the number of bones is because some humans may have a varying number of ribs, vertebrae, and digits. They vary in size, shape, and strength to respond to the demands of performing delicate or gross motor tasks. Bones of the middle ear have minimal strength but play a role in the transmission of sound waves to the auditory organs of the inner ear. Other bones, such as the femur, are exceptionally robust and can withstand enormous force before they fracture.

Each hand consists of 27 bones. The osseous anatomy of the human hand is integral to its impressive functionality. The purpose of this article is to provide a review of hand osteology for the education of current and future healthcare providers.

Structure of The Hand Bones

The carpus (proximal and distal rows), metacarpal bones, phalanges, and various sesamoid bones form the skeletal hand.[rx][rx] There are five metacarpals, fourteen phalanges, and four consistently present sesamoid bones in the hand.

The wrist connects to the hand at the carpometacarpal (CMC) joints.[rx] The first CMC forms from the articulation between trapezium and base of the first metacarpal. The base of the second metacarpal articulates with trapezium, trapezoid, and capitate. The base of the third metacarpal articulates with capitate. The base of the fourth metacarpal articulates with capitate and hamate. The fifth metacarpal’s base articulates with the hamate.[rx][rx]

The first metacarpal bone corresponds to the thumb, the second to the index finger, the third to the long finger, the fourth to the ring finger, and the fifth to the small finger. The thumb has two phalanges, named proximal and distal phalanges. The proximal phalanx base forms the metacarpophalangeal (MCP) joint with the first metacarpal head.[rx] The head of the proximal phalanx articulates with the base of the distal phalanx to form the interphalangeal (IP) joint. The index, long, ring and small fingers each have proximal, middle, and distal phalanges. The proximal phalanges form MCP joints with their respective metacarpal bones. The head of each proximal phalanx articulates with the base of each middle phalanx to form proximal interphalangeal (PIP) joints. The head of each middle phalanx articulates with the base of each distal phalanx to form distal interphalangeal (DIP) joints.[rx]

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Two sesamoid bones are present at the first MCP joint. These are consistently present and have significant functional implications for the motion of the thumb. Single sesamoids appear over the second and fifth MCP joints in 60.8% and 59.1% of the population, respectively.[rx]

Blood Supply of The Hand Bones

Blood supply to the hand comes from ulnar and radial arteries, which are branches of the brachial artery. Dorsal hand vascular supply comes from the dorsal carpal arch. This arch forms from anastomosis of dorsal carpal branches of radial and ulnar arteries. Metacarpal branches subsequently arise from the dorsal carpal arch. The first metacarpal branch courses along the radial border of the second metacarpal, supplying the first web space and second metacarpal. The second through fifth branches course along the ulnar borders of the second through fifth metacarpals and provide branches to the interosseous muscles as well as the periosteum.[rx]

Superficial palmar arterial arch is primarily formed by the ulnar artery, while the radial artery mainly supplies deep palmar arterial arch. Metacarpal branches generally arise from the deep arch, while digital arteries arise from the superficial arch. Metacarpal branches form anastomosis with digital arteries.[rx]

Physiologic variation of arterial flow in the hand is plentiful with a multitude of anastomoses and variations in flow dominance.[rx]

Nerves of The Hand Bones

Innervation to the hand is from the median, ulnar, and radial nerves.[rx][rx][rx] Each digit/ray has two palmar and two dorsal digital nerves. The cutaneous volar aspect of the first three and a half digits is innervated by the median nerve. The ulnar nerve entirely innervates the dorsal and volar ulnar half of the fourth and the fifth digit. The radial nerve innervates the remainder of the dorsal hand. Due to the complex functionality of the hand, it is vital that clinicians reliably report and understand proprioception and grasp strength. A multiple innervation distribution is provided to the MCPs, PIPs, and DIPs to facilitate this understanding.[rx]

Muscles of The Hand Bones

Three volar Interossei adduct the second, fourth, and fifth digits about an axis formed by the third digit. These muscles originate on the metacarpal of the digit being adducted and insert to their respective proximal phalangeal base and extensor hood. Adduction occurring along this third digit axis means that the volar interosseous muscle of the second digit is located on its ulnar border, while the interossei of fourth and fifth digit reside on their radial borders.[rx]

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Four dorsal interossei work to abduct the second, third, and fourth digits while assisting with flexion of the MCPs, extension of the PIPs, and extension of the DIPs. The first dorsal interosseous muscle originates from the first and second metacarpals, inserting to the radial base of proximal phalanx of the second digit as well as its extensor hood. The second dorsal interosseous muscle originates from second and third metacarpals, inserting to the radial base of proximal phalanx of the third digit as well as its extensor hood. The third dorsal interosseous muscle originates from third and fourth metacarpals, inserting to the ulnar base of proximal phalanx of the third digit as well as its extensor hood. The fourth dorsal interosseous muscle originates from fourth and fifth metacarpals, inserting to the ulnar base of proximal phalanx of the fourth digit as well as its extensor hood.[rx]

Abductor pollicis longus (APL) originates in the forearm, at the midshaft of the radial border of ulna and the ulnar border of radius, as well as the interosseous membrane. It inserts to the radial side of the base of the first metacarpal in addition to the trapezium and opponens pollicis fascia.[rx] This muscle acts to abduct the first digit.

Abductor pollicis brevis (APB) originates at trapezium and scaphoid and inserts to the lateral base of proximal phalanx of the first digit. This muscle acts in first digit abduction in addition to MCP flexion.[rx]

Extensor pollicis longus (EPL) originates at the radial border of ulna and interosseous membrane, distal to the origin of APL. EPL inserts to the base of the distal phalanx and acts to extend the interphalangeal joint.[20]

Extensor pollicis brevis (EPB) originates at the ulnar border of the radius and interosseous membrane at the level of EPL. It inserts to the base of proximal phalanx of the first digit and aids in first digit extension at the MCP.[20]

Opponens pollicis acts to pronate the first metacarpal. It originates at trapezium and inserts to the anterolateral aspect of the first metacarpal.[rx]

Flexor carpi radialis (FCR) originates at the medial humeral epicondyle and inserts to trapezium, second metacarpal, and third metacarpal. FCR acts to flex and radially deviate the wrist.[rx]

Flexor pollicis brevis (FPB) has a deep and superficial head. Deep head originates at second metacarpal, while superficial head originates at transverse carpal ligament and trapezium. These insert to the lateral base of proximal phalanx of the first digit and act to flex the MCP.[rx]

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Flexor pollicis longus (FPL) is the primary flexor of the first digit, originating from the volar interosseous membrane and radius. It inserts to the base of the distal phalanx.[rx]

Adductor pollicis has two heads, an oblique and transverse. The oblique head originates at the capitate, second, and third metacarpals. The transverse head originates at distal half of third metacarpal shaft. These both insert to the medial base of thumb proximal phalanx and act to adduct first digit.[rx]

Abductor digiti minimi (ADM) acts to abduct the fifth digit and has been found to originate at the pisiform, piso-hamate ligament and flexor carpi ulnaris. It inserts to the ulnar aspect of proximal phalangeal base and extensor apparatus of the fifth digit.[rx]

Flexor digiti minimi (FDM) originates from hamate, pisiform, and flexor retinaculum. Distally, it fuses with ADM to insert to the proximal phalangeal base of the fifth digit, leading to its action of MCP flexion.[rx]

Extensor carpi ulnaris originates at the lateral humeral epicondyle and inserts to the dorsal base of the fifth metacarpal, acting in wrist extension and ulnar deviation.[rx]

Flexor digitorum superficialis (FDS) has a broad origination from the medial epicondyle of humerus and coronoid process of the ulna to the volar aspect of the radial shaft. It then inserts to the middle phalangeal bases of second through fifth digits. This acts primarily in PIP flexion.[rx]

Flexor digitorum profundus (FDP) originates from the volar-medial aspect of the ulna and interosseous membrane and inserts at the distal phalangeal bases of second through fifth digits. This performs DIP flexion.[rx]

Extensor digitorum communis (central slip and terminal tendons) originates at the lateral epicondyle of humerus and inserts at the dorsal aspect of the distal phalangeal base of the second through fifth digits. There is significant anatomic variation in the accessory slips and insertions of this muscle, but the classic teaching is that there is an insertion to the middle phalanx and MCP in addition to this consistently present terminal insertion.[rx]

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