The Latin word "carpus" is derived from Greek καρπὁς meaning "wrist". The root "carp-" translates to "pluck", an action performed by the wrist. In human anatomy, the main role of the wrist is to facilitate effective positioning of the hand and powerful use of the extensors and flexors of the forearm, and the mobility of individual carpal bones increase the freedom of movements at the wrist.
There are 8 carpal bones, organized into two longitudinal rows, the proximal row contains Scaphoid,
Lunate, Triquetrum and Pisiform and the distal row has Trapezium, Trapezoid, Capitate and
Hamate.
SCAPHOID
It is a boat shaped bone and has a tubercle which is laterally, forwards and downwards.
The tubercle of the scaphoid gives attachment to flexor retinaculum and a few fibres of
abductor pollicis brevis.
Scaphoid articulates with Radius, Lunate, Capitate, Trapezium and Trapezoid.
LUNATE
It is half-moon shaped or crescentic bone.
As it has a semi lunar surface, it articulates with the scaphoid on its lateral side. A quadrilateral
surface for the triquetral on its medial side.
It is a pyramid shaped bone.
The oval facet for the pisiform lies on the distal part of the palmar surface.
The medial and dorsal surfaces are continuous and nonarticular.
It articulates with pisiform, lunate, hamate and articular disc of the inferior radioulnar joint. PISIFORM
It is a pea shaped bone.
The oval facet for the triquetral lies on the proximal part of the dorsal surface.
The lateral surface is grooved by the ulnar nerve.
It only articulates with Triquetral.
TRAPEZIUM
It is quadrangular in shape, has a crest and a groove anteriorly.
The palmar surface has a vertical groove for the tendon of the flexor carpi radialis.
The groove is limited literally by the crest of the trapezium.
The distal surface bears convexo-concave articular surface for the base of the 1st metacarpal bone.
It articulates with scaphoid, trapezium, capitate and 1st and 2nd metacarpal. TRAPEZOID
It looks like a shoe of a baby.
The distal articular surface is bigger than the proximal.
The palmar nonarticular surface is prolonged laterally.
It articulates with scaphoid, trapezium, 2nd metacarpal and capitate. CAPITATE
It is the largest bone in the carpal bones which has a rounded head.
The dorsomedial angle is the distal most projection from the body of the bone which bears a small facet for the 4th metacarpal bone.
It articulates with scaphoid, lunate, hamate, trapezoid and 2nd, 3rd & 4th metacarpals. HAMATE
It is a wedge shaped with a hook near its base.
The hook projects from the distal parts of the palmar surface and is directed laterally.
MUSCLES AND LIGAMENTS
The intricate movements of the hand are facilitated by a delicate balance of muscular forces and a robust ligamentous network within the wrist. Two primary muscle groups contribute: extrinsic muscles, originating in the forearm, stabilize the wrist by maintaining hand position on the radius during coordinated muscle contractions. Intrinsic muscles, originating within the hand, fine-tune movements by balancing flexor and extensor forces.
The flexor carpi ulnaris, an extrinsic muscle, significantly influences wrist movement by inserting onto the hamate, pisiform, and the base of the fifth metacarpal. Intrinsic muscles demonstrate diverse origins: thenar muscles arise from the scaphoid and trapezium, the adductor pollicis originates from the capitate and second/third metacarpals, and hypothenar muscles originate from the pisiform and hamate.
A complex system of ligaments provides stability. Radiocarpal ligaments connect the radius to various carpal bones (scaphoid, lunate), while ulnocarpal ligaments connect the ulna to the lunate and capitate. Intercarpal ligaments bind the carpal bones together (e.g., lunotriquetral, scapholunate), forming a strong, interconnected structure. These ligaments, along with carpometacarpal and intermetacarpal ligaments, ensure stability during a wide range of hand movements.
BLOOD SUPPLY, LYMPHATICS AND NERVOUS SUPPLY
The radial artery, ulnar artery, and their anastomoses provide the blood supply of the wrist. The radial artery predominantly supplies the thumb and the lateral side of the index finger while the ulnar artery supplies the rest of the digits and the medial side of the index finger. In particular, vascular supply takes place via the anastomotic network consisting of three dorsal and three palmar arches, which arise from both the radial and ulnar arteries, that overlie the carpal bones. The scaphoid, capitate, and a minority of lunates (20%) all have one intraosseous vessel supply. Of note, the scaphoid has a single blood supply from the radial artery that enters from the distal portion of the bone to supply the proximal portion, thus making its proximal pole most vulnerable to avascular necrosis. The trapezoid and hamate both have two areas of blood supply without intraosseous anastomoses. The trapezium, triquetrum, pisiform, and most lunates (80%) have two areas of blood supply and consistent intraosseous anastomoses. Therefore, the rest of the carpal bones, excluding the scaphoid, capitate, and the minority of lunates, have a lower risk of developing avascular necrosis following a fracture.
Innervation of the wrist joint comes from the:
- anterior interosseous branch of the median nerve
- posterior interosseous branch of the radial nerve
- the dorsal and the deep branches of the ulnar nerve
The lateral antebrachial cutaneous nerve, the posterior interosseous nerve, the dorsal branch and the perforating branches of the ulnar nerve, and the superficial branch of the radial nerve innervate the wrist joint from the dorsum. The palmar cutaneous branch of the median nerve, the anterior interosseous nerve, and the main trunk and deep branch of the ulnar nerve innervate the wrist joint from the palmar side.
EMBRYOLOGY
Upper limb development initiates with the activation of a group of mesenchymal cells in the lateral mesoderm towards the end of the fourth week, with the limb buds becoming visible around day 26 or day 27. Each limb bud comprises a mass of mesenchyme covered by ectoderm. This mesenchyme remains undifferentiated until it is ready to develop into bone, cartilage, and blood vessels later in development. Meanwhile, at the apex of each limb bud, the ectoderm thickens to form the apical ectodermal ridge, which stimulates the growth and development of the upper limb bud in the proximal-distal axis. Other signaling centers and primary morphogens such as the zone of polarizing activity, derived from an aggregate of mesenchymal cells in the limb bud, and the Want pathway, expressed from the dorsal epidermis of the limb bud, also contribute to the development of the upper limb buds by regulating growth along the anteroposterior axis and the dorsoventral axis, respectively.
At the end of the sixth week of development, digital rays form in the hand plate. By the seventh week, the carpal chondrification process begins. The capitate and the hamate carpal bones are the first chondrogenic centers to appear as immature cartilage early in the eighth week while the pisiform is the last to appear later in the eighth week. The hamulus, otherwise known as the “hook of the hamate,” also appears as an immature cartilaginous tissue towards the end of the eighth week and does not complete its development until the thirteenth week. Last, in the fourteenth week, a vascular bud penetrates the lunate cartilage mold, an early sign of the osteogenic process that will complete during the first year of life.
Complied & Written by Dr. Palak Shah
