Metacarpophalangeal (MCP) joints
The metacarpophalangeal joints (MCP) are a collection of condyloid joints that connect the metacarpus, or palm of the hand, to the fingers. There are five separate metacarpophalangeal joints that connect each metacarpal bone to the corresponding proximal phalanx of each finger.
Each metacarpophalangeal joint is formed by the convex heads of the metacarpal bones which are received by the concave bases of the proximal phalanges.
The primary movements of the metacarpophalangeal joints are flexion, extension, abduction, adduction, circumduction and limited rotation. These joints play an important role in the functionality of the hand providing stability and flexibility of the fingers which is facilitated by the ligaments, joint capsule, and adjacent musculotendinous structures.
This article will discuss the anatomy and function of the metacarpophalangeal joints.
|Type||Synovial, condyloid joint|
Heads of the distal aspect of the metacarpal bones
|Ligaments||Collateral ligament, palmar ligament, deep transverse metacarpal ligaments|
|Innervation||Posterior interosseous nerve, deep terminal branch of ulnar nerve, palmar branches of median nerve|
|Blood supply||Princeps pollicis artery, radialis indicis artery, common palmar digital arteries, dorsal metacarpal arteries, palmar metacarpal arteries|
|Movements||Flexion, extension, adduction, abduction, circumduction and limited rotation|
- Articular surfaces
- Ligaments and joint capsule
- Blood supply
- Muscles acting on the metacarpophalangeal joint
- Clinical relations
Each metacarpophalangeal joint is the connection between the palm and the fingers. The proximal articular facet is located on the rounded biconvex head of the metacarpal bones. This articular surface is wider anteriorly than posteriorly. The metacarpal heads articulate with the much smaller concave articular facets located on the bases of proximal phalanges. The facets of the phalanges are elongated anteriorly by the palmar ligament that functions as a hinge. Both metacarpal and phalangeal articular surfaces are lined with hyaline cartilage.
Ligaments and joint capsule
The joint is enveloped by a loose fibrous capsule, attached close to the margins of articular facets. On its medial and lateral sides, the joint capsule is thicker and strengthened by collateral metacarpophalangeal ligaments. A palmar metacarpophalangeal ligament mainly replaces the capsule on its anterior aspect, while the posterior capsule receives fibres from the tendons of the long extensors of the forearm (namely extensor pollicis longus, extensor indicis, extensor digitorum and extensor digiti minimi) .
The collateral ligaments are major stabilizers of the MCP joint and located on both radial and ulnar aspects of the joint. They consist of proper collateral and accessory collateral ligaments. Working together, these strong ligamentous bands limit the range of motion in both flexion-extension and adduction-abduction axes.
The proper collateral ligaments expand from the posterior tubercles on the dorsolateral aspect of the metacarpal head to the palmar aspect of the adjacent proximal phalanx, just distal to the base. The primary role of these ligaments is to limit flexion of the MCP joint.
The accessory collateral ligaments attach more proximal to the metacarpal head and run distally to attach onto the distal third of the palmar (also known as volar) plate. The accessory collateral ligaments are taut in extension, thus limiting this movement in the joint.
The palmar ligament (also referred to as the palmar, or volar, plate) is a dense fibrocartilaginous thickening located on the palmar aspect of the MCP joint capsule. They are loosely attached to the palmar aspect of the metacarpal neck, but firmly adhered to the palmar surface of the base of the adjacent proximal phalanx. The sides of the palmar ligament blend with the collateral ligament. In the thumb, the palmar ligament contains two sesamoid bones, which articulate with palmar facets on the thumb metacarpal head. The main function of this ligament is to prevent hyperextension of the MCP joint.
Deep transverse metacarpal ligaments
The deep transverse metacarpal ligaments are the narrow fibrous bands that run across the palmar aspect of the second to fifth metacarpophalangeal joints, connecting them together. They are situated anterior to interossei muscles and posterior to lumbricals. Their palmar surfaces are connected to the digital slips of the central palmar aponeurosis. These ligaments mainly contribute to the stability of the MCP joints during grip functions.
The metacarpophalangeal joints are innervated by the following nerves;
- Princeps pollicis artery
- Palmar and dorsal metacarpal arteries
- Radialis indicis artery
- Common palmar digital arteries
The movements in the metacarpophalangeal joints include flexion, extension, adduction, abduction, circumduction and limited rotation. Each joint has 2° of freedom.
Flexion and extension
In the metacarpophalangeal joint, flexion and extension are considered to be the primary movements. Active flexion has a range of motion of approximately 90° whereas extension ranges from 10° in the index finger to 30° in the little finger. Flexion in the third to the fifth MCP joint is accompanied by a slight lateral rotation, while the flexion that occurs in the second MCP joint is accompanied by a slight medial rotation.
The full range of motion for the thumb MCP joint is about 60° which is almost completely flexion. The movements in this axis are limited by ligamentous structures that surround the joint and antagonistic muscles.
Abduction and adduction
The full range of motion in this axis is around 25-30°. This fairly small range of motion belongs mostly to abduction. These movements are limited mainly by the shape and width of the metacarpal head, and by collateral ligaments. When the MCP joints are flexed, neither abduction nor adduction can occur, largely due to the arrangement of the collateral ligaments described above. Abduction and adduction in the MCP joints are always followed by the movements in the corresponding carpometacarpal joints.
Axial rotation is most important for the MCP joint of the thumb and it occurs during opposition. The rotation can be active or passive. The active axial rotation produced by simultaneous action of flexor pollicis brevis and abductor pollicis brevis muscles. Active rotation is always directed medially, while the passive movement can be directed in either direction.
The close packed position of the second to fifth metacarpophalangeal joints is full flexion, while the close packed position of the first metacarpophalangeal joint is maximum opposition. The open packed (resting) position for all joints is slight flexion. The joints’ capsular pattern is more limitation of flexion than extension.
Some accessory movements also occur in these joints. Mostly, this relates to accessory rotation in the thumb joint as well as anteroposterior and lateral translation of metacarpals or phalanges.
Muscles acting on the metacarpophalangeal joint
All the movements of the MCP joints are performed by the muscles of the forearm and hand.
- Flexion of the thumb is mainly produced by flexor pollicis brevis, aided by flexor pollicis longus muscle. Flexion of digits 2 to 5 are produced by flexor digitorum superficialis, flexor digitorum profundus, lumbricals and flexor digiti minimi brevis (fifth digit).
- Extension of the thumb (from the flexed position) is mainly produced by the extensor pollicis brevis, with some help from extensor pollicis longus muscle. Extension of digits 2 to 5 are produced by extensor digitorum, extensor indicis (second digit) and extensor digiti minimi (fifth digit).
- Adduction of the thumb is produced by the adductor pollicis, while the adduction of the other four digits is produced by palmar interossei muscles.
- Abduction of the thumb is produced by the abductor pollicis longus and abductor pollicis brevis; abduction of the fifth digit is produced by the abductor digiti minimi; abduction of fingers 2-5 is produced by the dorsal interossei muscles.
- Axial rotation is produced actively by the co-contraction of flexor pollicis brevis and abductor pollicis brevis.
Gamekeeper's thumb is a fairly common injury of the ulnar collateral ligament of the MCP of the thumb. The most common way to injure this ligament is by falling onto an outstretched arm with an abducted thumb. This injury is most commonly seen in skiers with a ski pole in the hand, which prevents the adduction of the thumb. It is not particularly easy to diagnose this condition since it is usually combined with damages of other hand structures. Most commonly, the combination of physical examination with some kind of imaging procedure is sufficient (x-ray or MRI). Mild injuries are usually treated with physical therapy while the complete ruptures and tears require a surgical approach.