Intercarpal joints

Articular surfaces

Intercarpal joints are all classified as synovial plane joints, meaning that the articular surfaces are functionally considered as nearly flat and lined with fibrocartilage. The joints are enclosed by the thin fibrous capsules whose internal surfaces are lined by the synovial membranes.

The joints of the proximal carpal row connect the relatively flat/planar adjacent surfaces of the scaphoid and lunate, and triquetral bones, forming the scapholunate and lunotriquetral joints. The pisiform bone, which lies within the tendon of the flexor carpi ulnaris muscle, articulates with the palmar surface of triquetrum bone, forming the pisiform joint. The pisiform bone is also connected to the hook of hamate bone and fifth metacarpal bones via pisohamate and pisometacarpal ligaments, respectively.

The joints of the distal carpal row connect the adjacent surfaces of the trapezium, trapezoid, capitate and hamate bones. These articulations form the trapeziotrapezoid, trapezoideocapitate and capitohamate joints which are substantially less mobile than those of the proximal carpal row. 
  
The midcarpal joint is the compound articulation between the distal surfaces of the proximal carpal bones and the proximal surfaces of the distal carpal bones. More specifically, it is a joint formed collectively by the:

• Scaphoid, lunate and triquetral bones proximally
• Trapezium, trapezoid, capitate and hamate bones distally

The midcarpal joint is divided into medial and lateral compartments. The medial compartment presents two articular regions. The first is that formed by the convex surface of the head of capitate bone which is received by the collective concave distal surfaces of the scaphoid and lunate bones. The second part, the triquetrohamate component, is more complex, presenting both distally convex and concave surfaces. The lateral compartment consists of the plane surfaces of trapezium and trapezoid bones, which articulate with the distally convex surface of the scaphoid bone. Thus, this is a planosellar compound joint, slightly convex distally.

Learn and practise the bones of the wrist and hand with this interactive quiz.

Ligaments and joint capsule

The intercarpal and midcarpal joints are enclosed by an irregular two layered, joint capsule. The outer layer of the capsule is composed of fibrous connective tissue which provides structural support to the joint, while the inner layer is composed of a synovial membrane responsible for the secretion of synovial fluid, keeping the joint lubricated. It spans the distal surfaces of the proximal carpus to the proximal surfaces of the distal carpus. It also interdigitates between the bones, showing proximal projections over the scapholunate and lunotriquetral joints. In the same manner, the capsule projects between the bones of the distal carpal row. The distal projection between the trapezium and trapezoid bones often communicates with the joint space of the corresponding carpometacarpal joint. The joint between the pisiform and triquetrum bones is usually isolated, having its own thin fibrous capsule lined by a synovial membrane.

The joints of the carpal bones are supported by an array of ligaments, namely the interosseous intercarpal ligaments, dorsal intercarpal ligaments, and palmar intercarpal ligaments. It’s worth noting that these ligaments are variably described in the literature, which has led to a degree of confusion in regards to their anatomy. The dorsal and palmar intercarpal ligaments are bound within the joint capsule of the carpal joints, with the palmar ligaments being much more numerous than those found on the dorsum. The interosseous intercarpal ligaments are conversely intracapsular. Additionally, the stability of the carpus is also supported by the flexor retinaculum.

Innervation

The intercarpal joints are innervated by the anterior interosseous and posterior interosseous nerves, which are the branches of the median and radial nerves, respectively. The deep and dorsal branches of the ulnar nerve also contribute to the innervation of these joints.

Blood supply

The blood supply to the intercarpal joints comes from the palmar and dorsal carpal arches, which are the anastomoses of the terminal branches of the ulnar and radial arteries.

Movements

The movements of the intercarpal and midcarpal joints follow the movements of the radiocarpal joints and are usually described together with them. Concerning that the ligamentous apparatus of the carpus strongly binds the bones together, there isn’t much movement at the proximal and distal intercarpal joints. The proximal intercarpal joints allow noticeable flexion and extension, while the distal joints move significantly less. Instead, these movements are important for adjusting the shape of the hand while the movements on the radiocarpal and midcarpal joints occur.

Recall that the radiocarpal joint is a biaxial joint which moves into two degrees of freedom; 

• Flexion (range of motion 35°) - extension (RoM 50°) 
• Abduction (RoM 8°) - adduction (RoM 15°). 

Circumduction is also possible, as a combination of the above movements. 

The slight movements on the midcarpal joint follow these occurring on the radiocarpal joint. They happen around transverse and sagittal axes that pass through the head of capitate bone. The movements are as follows;

• During flexion and extension of the wrist, the head of capitate rotates against the adjacent surfaces of the scaphoid and lunate, while the hamate rotates against the triquetrum. 
• In wrist adduction (ulnar deviation), the proximal end of the capitate rotates laterally. The hamate rotates in the same manner, approaching the lunate and separating it from triquetrum bone.
• In abduction (radial deviation), the proximal part of the hamate rotates towards the triquetrum, thus separating the hamate from the lunate.

Abduction and adduction are followed by the slight torsion movements between the carpal rows. In abduction, the distal row twists in the direction of supination and extension, while the proximal row rotates in the opposite direction (pronation and flexion). In adduction, the opposite occurs; the proximal row rotates in the direction of supination and extension, while the distal twists in the direction of pronation and flexion. 

The joint takes a close packed position when the hand is extended. The open (resting) position occurs when the hand is in a neutral position or slightly flexed. The capsular pattern hasn’t been described in the intercarpal joints, while for the midcarpal joint it’s equal limitation of flexion and extension. The proximal and distal intercarpal joints permit accessory movements of anteroposterior gliding of any two adjacent bones. This is also possible on the midcarpal joint, producing anteroposterior gliding between the proximal and distal rows of carpus.

Muscles acting on the intercarpal joints

Muscles that produce the movements on the intercarpal joints are the same that act on the radiocarpal (wrist) joint.

• Flexion is produced chiefly by the flexor carpi radialis, flexor carpi ulnaris and palmaris longus. It is assisted by the flexor digitorum superficialis, flexor digitorum profundus and flexor pollicis longus.
• Primary extensors are the extensors carpi radialis longus, brevis and ulnaris, while the extensors digitorum, digiti minimi, indicis, and pollicis longus muscles assist.
• The adductors are the flexor and extensor carpi ulnaris.
• The primary abductors are the flexor carpi radialis, extensor carpi radialis longus and extensor carpi radialis brevis. The accessory muscles for this movement are the abductor pollicis longus and extensor pollicis brevis.