Evolution has influenced the human body, something very evident in our upper limb. Mobility of the upper limb was crucial for our tree-climbing ancestors, and still is for us today. The wrist joint is crucial for the functioning of the upper limb, primarily its distal part, the hand. Our ability to move our hand in space is essential to our advanced and precise hand function. This article will detail the anatomy of the wrist joint as well as its function and numerous clinical correlates. The best ways of learning the anatomical structures will also be described.
In simple terms the wrist joint connects the forearm to the hand. More specifically, the carpal bones connect the forearm to the hand and are crucial for torque generation, which generates grip strength (note how the flexed wrist gives weak hand grip), extension stretches the wrist flexors and primes them to give maximum tension and strength. The wrist joint actually comprises a number of joints each working toward a specific purpose. Before describing these joints I will discuss the bony anatomy.
The radius articulates with the scaphoid laterally and the lunate medially. The scaphoid has a cashew nut shaped appearance with a narrowing in its centre, like a waist.
The lunate is connected to the scaphoid via the scapholunate ligament, and provides stability to the radiocarpal joint (See clinical points for ‘Terry Thomas sign’). It is said to be crescent shaped, hence its name.
The medial most bone of the proximal carpal row is the triquetrum that lies medial to the lunate on the proximal row of carpal bones. This is the carpal bone with the strongest connection to the ulna, although it does not directly articulate with it. The There is an interposed pad of triangular fibrocartilage complex extends from the ulna to insert into the lunate and triquetrum via the ulnolunate and ulnotriquetral ligaments respectively. The apex of the triangle points toward the radius. It has a deep layer (ligamentum subcruentum) that attaches to the fovea on the periphery of the ulnar head, and a superficial part that articulates with the triquetrum.
Its functions include bridging the gap between the ulnar and triquetrum bones and spreading the load of the ulnocarpal joint. The presence of a pad of cartilage rather than direct bony continuity is of great evolutionary benefit. Imagine our Stone Age ancestors using hammers to build homes and hunt prey. The ulnar deviation that occurs when you strike an object with a hammer is a mechanism for enhancing the velocity you can create. It gives the hammering upper limb a whip like momentum and ensures a high striking force.
The pea shaped pisiform (both start with the letter p) sits on top of it, and is embedded within the tendon of flexor carpi ulnaris as a sesamoid bone (a bone that lies within a tendon, the patella is another example).
The carpal bone immediately distal to the scaphoid is the trapezium (trapez-i-um, with the thumb). The trapezium articulates with the first metacarpal in a unique biconcave, biconvex saddle shaped joint that allows the thumb to move in numerous planes (abduction, adduction, flexion, extension and opposition). The ‘screw home torque rotation’ imparted by this saddle shaped joint (the dorsal ligamentous complex connecting the first metacarpal to the trapezium is also essential) allows for powerful key and pulp-to-pulp grip and thumb opposition with the fingers (a feature that differentiates us from other primates).
Medial to the trapezium is the small four-sided trapezoid (the two similarly named bones are adjacent), this articulates with the second metacarpal.
Imagine for a minute, the capital city of a country. Its name subtly implies that it lies in the centre of the country (although obviously this is not always the case). If you follow this analogy through, it makes sense that the capitate articulates with the third, or middle metacarpal.
The hamate is a unique bone with a pronounced hook or ‘hamulus’ on its volar surface, to which the tendon of flexor carpi ulnaris attaches. It also forms the medial wall of Guyon’s canal in which the ulnar nerve and artery pass into the hand. The pisiform and pisohamate ligament forms the ulnar or lateral wall, the roof is formed by the superficial palmar carpal ligament, the floor is the hypothenar muscles and deep flexor retinaculum, ending at the distal border of the aponeurotic arch of the hypothenar muscles. Distally the hamate articulates with the fourth and fifth metacarpals.
Distal Radioulnar joint- this joint connects the base of the radius to the head of the ulna. It is important to remember that supination and pronation occur at the proximal radioulnar joint.
Ulnocarpal joint/TFCC- if you observe the head of the ulna on an x-ray, it never directly articulates with the carpus. There is an interposed pad of cartilage that bridges the gap. This pad of cartilage is part of the triangular fibrocartilaginous complex (TFCC) and is an essential bridging and load spreading structure. The ulnar collateral ligament connects the ulnar styloid process to the pisiform and thus stabilizes the joint medially.
Radioscapholunate/radiocarpal joint- this joint connects the radius to the scaphoid and lunate. The scapholunate ligament connects the two carpals mentioned and the radial collateral ligament connects the radial styloid process to the base of the scaphoid.
Intercarpal ligaments- There are ligaments that connect the bones of the carpus, all are named after the bones they connect. The ligaments on the volar side are thicker and more developed than the dorsal side.
The pisohamate ligament forms the lateral wall of Guyon’s canal and the pisometacarpal ligament stabilizes the wrist joint and connects the pisiform to the fifth metacarpal, which is important for force transmission.
The flexor retinaculum or transverse carpal ligament attaches onto the pisiform and hook of the hamate medially and the trapezium and scaphoid laterally. Beneath we find the carpal tunnel, the floor of which is formed by the carpals. This tunnel houses the tendons of flexor digitorum superficialis and flexor digitorum profundus muscles, two flexors of the fingers as well as the median nerve. There is a great deal of variation among the intercarpal ligaments.
Mid carpal joint- This is the joint that lies between the proximal and distal carpal rows. The initial phase of wrist flexion occurs at this joint, with both proximal and distal rows flexing towards full wrist flexion. A similar pattern of movement emerges in extension. In both flexion and extension, the scaphoid acts as the bridging bone connecting the proximal and distal rows. The proximal row of carpal bones is not directly attached to any tendon and as a result, any movement of this row of bones only passively follows the movement of the distal row of carpal bones under muscle contraction.
Before we discuss the movements of the wrist, it is essential to describe the anatomy a bit further. The carpals are best thought of as a proximal and distal row (lunate, triquetrum, pisiform and scaphoid in the proximal row, and the trapezium, trapezoid, capitate and hamate in the distal row). The bones within each row tend to move in unison. The scaphoid is the connection between the proximal and distal row in flexion and extension movements.
Any muscle that crosses the joint can cause wrist flexion. These include flexor digitorum superficialis and flexor digitorum profundus. Other less powerful muscles such as the palmaris longus also have a role. The flexor carpi radialis and flexor carpi ulnaris cause flexion when they contract in unison. In flexion, the largest movement occurs at the capitolunate segment of the midcarpal joint.
Similarly, any muscle that crosses the wrist joint on its posterior side can cause extension. These include extensor carpi radialis longus and brevis, extensor carpi ulnaris, and extensor digitorum. The first two muscles insert into the base of the second and third metacarpal, and the third onto the base of the fifth metacarpal bone. It is easy to see how shortening of these muscles would cause extension. The tendons of all four muscles are held close to the wrist joint by the extensor retinaculum; an essential function for generating the strength required of these movements.
This movement is caused by the extensor carpi radialis longus and brevis as well as flexor carpi radialis. Crucially, they must contract together to cause abduction.
The flexor and extensor carpi ulnaris must also contract together to cause adduction.
Carpal arches- the blood supply derives from a palmar and dorsal carpal arch (much like the palmar arches of the hand). The palmar carpal arch is derived from the palmar carpal branch of the radial and ulnar arteries. These are joined by the anterior interosseus artery from above, and by the penetrating deep branches of the deep palmar arch from below.
The dorsal carpal arch is formed from the dorsal carpal branches of the radial and ulnar arteries, anastomosing with the anterior and posterior interosseus arteries. This anastomosis gives off three dorsal metacarpal arteries that run forwards.
Scaphoid- The bloody supply to the scaphoid shows interesting and clinically relevant anatomy. As the radial artery runs forward into the hand, it gives off a nutrient branch. The artery enters the bone at its distal pole and runs proximally to supply the proximal pole (see clinical points for clinical details). The rest of the carpals get their blood supply from nutrient vessels too.