The carpal bones (carpals, carpus) are the eight small bones that sit between the distal ends of the radius and ulna and the five metacarpals. They vary in size considerably, from the small pisiform and trapezoid to the larger capitate and scaphoid.
|Function:||Support structure wrist and articulate with radius and ulnar proximally, metacarpals distally|
|Search for Carpal bones in Gray's.|
The capitate can be felt by identifying the middle metacarpal and then sliding proximally until reaching a slight depression. This is the location of the capitate, which becomes more prominent on flexion of the wrist.
The bones are arranged in two carpal rows with four bones in each row. From radial to ulnar sides, the bones are:
The pisiform articulates only with the triquetral, lying anteriorly. The other bones of the proximal row form an arch which is convex proximally, allowing articulation at the wrist. The distal row bulges proximally into this arch to fit tightly when the carpus is extended.
The dorsal surface of the carpal bones is convex, whilst the palmar surface is concave with a central groove, the borders being accentuated by the anterior position of pisiform and the anteriorly projecting hook of hamate medially, and the anterior tubercles of scaphoid and trapezium medially.
The flexor retinaculum is an accessory ligament, formed from a thickening of the deep fascia on the palmar surface of the wrist, that bridges the carpal bones. It is attached medially to the to the pisiform and hook of the hamate. Laterally it attaches to the scaphoid and trapezium. It forms the roof of the carpal tunnel.
- A dorsal ligament connects scaphoid to lunate and lunate to triquetral
- A palmar ligament connects scaphoid to lunate and lunate to triquetral
- Interosseous ligaments connect scaphoid to lunate and lunate to triquetral
- A capsular ligament covers the pisotriquetral joint
- In addition the pisohamate and pisometacarpal ligaments, neither true ligaments, connect the pisiform to the hamate and little metacarpal respectively.
- A dorsal ligament connects trapezium to trapezoid, trapezoid to capitate, and capitate to hamate
- A palmar ligament connects trapezium to trapezoid, trapezoid to capitate, and capitate to hamate
- Interosseous ligaments connect trapezium to trapezoid, trapezoid to capitate, and capitate to hamate. The ligament connecting capitate to hamate is strong, one or both of the other two may be absent.
- The ulnar collateral carpal ligament attaches the tip of the ulna styloid to the triquetral and pisiform
- The radial collateral carpal ligament attaches the tip of the radial styloid to the scaphoid and occasionally trapezium
- A radial collateral ligament connects the scaphoid and trapezium at the midcarpal joint
- An ulnar collateral ligament connects the triquetral and hamate at the midcarpal joint
- The palmar radiocarpal ligament connects the distal radius to scaphoid, lunate, triquetral and capitate on the palmar surfaces
- The palmar ulnocarpal ligament connects the distal ulna and triangular articular disc to lunate and triquetral on the palmar surfaces
- The dorsal radiocarpal ligament connects the distal radius to the scaphoid, lunate, triquetral on the dorsal surfaces
- Abductor pollicis brevis arises from the palmar surfaces of scaphoid and trapezium
- Opponens pollicis arises from the palmar surface of trapezium
- Flexor pollicis brevis has two heads:
- A portion of the oblique head of adductor pollicis arises from the palmar surface of capitate
- Flexor digiti minimi arises from the hook of hamate
- Opponens digiti minimi arises from the hook of hamate
- Abductor digiti minimi arises from pisiform
- Flexor carpi ulnaris surrounds and/or inserts into pisiform, continuing onwards as the pisohamate ligament to finally insert into hamate
- The wrist is an articulation between the triangular articular disc attached to the ulnar styloid process, the distal radius and the proximal articular surfaces of the scaphoid, lunate and triquetral.
- The midcarpal joint between the two rows articulates the scaphoid, lunate and triquetral proximally with the trapezium, trapezoid, capitate and hamate distally
- The small pisotriquetral joint between pisiform and triquetral
- The other intercarpal joints have a very limited degree of movement due to the strong ligaments binding the rows together
- The carpometacarpal joint of the thumb is an articulation between the trapezium and the metacarpal bone of the thumb.
- The other carpometacarpal joints:
- Between the trapezium, trapezoid and the index metacarpal
- Between the capitate and the middle metacarpal
- Between the capitate, hamate and ring metacarpal
- Between the hamate and little metacarpal.
- The tendon of flexor carpi radialis lies adjacent to scaphoid and trapezium in the carpal tunnel
- The tendon of flexor pollicis longus lies adjacent to scaphoid, trapezoid and trapezium in the carpal tunnel
- The tendons of flexor digitorum profundus lie adjacent to lunate, triquetral, capitate and hamate in the carpal tunnel
- The radial artery lies adjacent to scaphoid in the anatomical snuffbox, continuing across the lateral surface of trapezium
- The ulnar artery and ulnar nerve lie anteriorly to the hook of hamate
Each carpal bone is ossified from a single centre, starting with the largest (capitate and hamate) in the first few months and continuing until the smallest (pisiform) which starts to ossify in the ninth to twelfth years.
An os centrale may occur between scaphoid, trapezoid and capitate; this usually fuses with the scaphoid during the second prenatal month.