The wrist complex is a complicated structure and is mainly made up of:
- the wrist joint (radiocarpal joint)
- the midcarpal joint
The midcarpal joint allows augmentation of the movements at the wrist joint when it has reached its limit. These movements include flexion, extension, abduction and adduction of the wrist (movements that occur at both the wrist and midcarpal joint take place at the same time).
The midcarpal joint is formed by the eight carpal bones that make up the carpus. From lateral to medial, the proximal row of carpal bones is made up of the:
From lateral to medial, the distal row is made up of the:
The midcarpal joint is the articulation between the proximal row of carpal bones and the distal row of carpal bones. At the lateral portion of the joint, the scaphoid articulates with the trapezium and trapezoid. Centrally, the scaphoid and lunate articulate with the capitate. The lunate also articulates with the hamate, while the latter also articulates with the triquetrum of the proximal row.
The midcarpal joint is a series of synovial gliding joints between the proximal and distal rows of carpal bones. Like any other synovial joint, the adjacent carpal bones in the midcarpal joint are lined with hyaline cartilage and the joint cavity is encompassed in a fibrous capsule lined with synovial membrane.
The joint or synovial cavity of the midcarpal joint is large and irregular in shape and it separates the proximal and distal rows of carpal bones. It also extends out to the distal surfaces of the distal row of carpal bones to the proximal surfaces of the proximal bones, but it does not blend with the wrist joint cavity due to the intercarpal or interosseous ligaments (short ligaments that connect adjacent carpal bones).
As the carpal bones of the midcarpal joint are not held together by their shapes (such as the ball and socket joint of the hip), the joint capsule of the midcarpal joint is reinforced by several ligaments (along with the extensor and flexor retinaculum and various tendons of muscles) to stabilize the joint. Along with the intercarpal (interosseous) ligaments, there are the palmar and dorsal intercarpal ligaments and radial and ulnar collateral ligaments.
- Intercarpal ligaments: They link some adjacent carpals.
- Palmar intercarpal ligament: Anteriorly, it extends from the proximal row of the carpal bones to mainly the head of the capitate (sometimes known as the radiate capitate ligament or palmar ligament).
- Dorsal intercarpal ligament: Posteriorly, it projects from the proximal to distal row of carpal bones.
- Radial collateral ligament: A strong ligament that is a continuation of the radial collateral carpal ligament from the wrist joint that extends from the scaphoid bone to the trapezium.
- Ulnar collateral ligament: Like the radial collateral ligament, the ulnar collateral ligament is an extension. It is a prolongation of the ulnar collateral carpal ligament and it connects the hamate bone to the triquetrum.
The wrist is a relatively stable complex, however abnormal stresses on the wrist can lead to fracture or dislocation of the wrist, including the midcarpal joint and/or bones. One cause consists of falling onto an outstretched hand when trying to break a fall, for example. This can lead to midcarpal joint instability, a term that can cover both abnormalities of the midcarpal joint and wrist joint due to the close nature of the two joints that make up the wrist.
Midcarpal instability is loosely termed as inadequate support to maintain a stable wrist due to a lack of support. This could be a result of tearing or excessive stretching of the ligaments, carpal bone fractures and/or dislocation of the carpal bone leading to carpal bone misalignment. Based on where the pathology has taken place to cause midcarpal instability, the majority of cases can be classified as either intrinsic or extrinsic.
Intrinsic midcarpal instability is characterized by the looseness of the ligaments, whereas extrinsic midcarpal instability is due to secondary bone abnormalities that are not carpal bones, i.e. radius and ulna. There have also been various descriptions of midcarpal instability in different clinical aspects based on the source of pathologic anatomy, for example, radial midcarpal instability, palmar midcarpal instability, capitolunate instability, etc.