Joints are the flexible parts of the body that allow for various degree of movements between two or more bones. In order to understand the anatomy of the wrist and hand joints, it is first important to identify all the bones in this part of the upper limb. The bones of the forearm include the radius and ulna which extend from the elbow joint to the wrist joint where it articulates with the wrist bones (carpal bone).
The wrist itself is known as the carpus and is made up of eight carpal bones. It also articulates with the hand bones known as the metacarpals of which there are five in the human hand. These metacarpals in turn articulate with the finger bones known as the phalanges (singular ~ phalanx). In the four fingers, excluding the thumb, there is three finger bones known as the proximal, middle and distal phalanges. The thumb has only two finger bones – the proximal and distal metacarpals.
The wrist is made up of 8 carpal bones. Four of these bones known as the pisiform, triquetrum, lunate and scaphoid form the proximal row of bones that lie next to the radius and ulna. The other four bones known as the hamate, capitate, trapezoid and trapezium form the distal row and lie next to the hand bones (metacarpals).
The wrist joint is known as the radiocarpal joint – radius (forearm long bone) and carpal bones (wrist bones). The ulnar is not involved in the wrist joint. Similarly, the pisiform bone from the proximal row of carpal bones is also not involved in this joint. Therefore the triquetrum, lunate and scaphoid bones articulate with the radius to form the wrist joint. On the surface, this joint lies at the level of the proximal wrist crease.
Capsule and Ligaments
The capsule of the radiocarpal joint has an outer fibrous layer and inner synovial membrane. The fibrous layer encapsulates the entire joint attaching to the ulnar, radius and proximal carpal bones. Internally the synovial membrane forms several folds that line the articular surfaces. Several ligaments strengthen this joint including the dorsal and palmar radiocarpal ligaments, as well as the ulnar collateral and radial collateral ligaments.
There are four set of joints within the carpal bones itself. Firstly, each carpal bone articulates with neighboring bones – joints between the proximal row of carpal bones and the distal row of carpal bones. Then the promximal and distal row articulate with each other at the midcarpal joint. Lastly, the pisiform and triquetrum form the pisotriquetral joint. The capsule of these joints is an extension of the carpometacarpal joints – between the wrist (carpal) and hand (metacarpal) bones. However, the capsule of the radiocarpal (wrist) and wrist and thumb bone (carpometacarpal thumb joint) are separate. The anterior, posterior, and interosseous ligaments secure the small carpal bones in place and ensure that it locks in to articulate with each other.
The hand bones are known as the metacarpals. These are single continuous bones that extend from the distal row of carpal bones (wrist) to the phalanges (fingers). There are five metacarpal bones that fan out in the hand. At the base, the metacarpal bones articulate with the wrist bones at the carpometacarpal (CMC) joints and with each other at the intermetacarpal (IM) joints.
The carpometacarpal joints, or CMC joints for short, are not as flexible as the other joints of the hand. The CMC joint of the thumb is highly mobile and the fourth and fifth CMC joints as well but to a lesser degree. However, there is almost no movement at the CMC joints of the second and third metacarpals.
Capsule and Ligaments
A common joint capsule surrounds the four CMC joints while the CMC joint of the thumb has its own capsule. The bones are secured in place and the cavity strengthened by palmar and dorsal carpometacarpal and intermetacarpal ligaments the interosseous intermetacarpal ligaments. The superficial and deep transverse ligaments ensure that the metacarpals do not separate to a large degree and ultimately limits the movement of the CMC and IM joints.
There are three intermetacarpal joints between the second and third, third and fourth, and fourth and fifth metacarpals. These joints are also surrounded by the same capsule as that of the medial four CMC joints. The thumb is extremely flexible in humans (opposable thumb) and this is facilitated by the very loose capsule between the base of the first metacarpal bone (thumb) and trapezium (carpal bone).
There are 14 phalanges (singular ~ phalanx) that make up the fingers – 3 phalanges for each finger except the thumb which has 2 phalanges. The proximal phalanges articulate with the metarcarpals (hand bones), the middle phalanges articulate with the proximal phalanges on one end and the distal phalanges at the other, while the distal phalanges articulate with the middle phalanges and its other end is free and makes up the fingertips. The thumb only has a proximal phalanx and distal phalanx.
Metacarpophalyngeal Joints and Interphalangeal Joints
The metacarpophalynheal joints are condyloid synovial joints between the hand bones (metacarpals) and the first row of finger bones (proximal phalanges). The interphalyngeal joints (IP joints) are hinge joints that lie between the phalanges (proximal and middle; middle and distal). Each MP and IP joint has its own joint capsule with an outer fibrous layer and inner synovial membrane. The joint capsule is strengthened by collateral ligaments on either side (medial and lateral).