Carpometacarpal (CMC) joints

Articular surfaces

The CMC joints are connections between the distal surfaces of the four distal carpal bones and the surfaces of the metacarpal bases of the medial four metacarpal bones.

The second CMC joint is formed by the apposition of the trapezium, trapezoid, capitate and second metacarpal bones. The distal portion of the trapezium directs a quadrilateral and flat bony protrusion (facet) medially which connects with a quadrilateral facet on the second metacarpal base. The triangular, distal surface of the trapezoid is convex in the transverse plane and concave coronally, ‘cupping’ a concave groove on the second metacarpal base. A deep ridge medial to this groove articulates with the concave, anterolateral corner of the capitate bone. These aspects make the second CMC joint quite distinct because it is the only one to have three carpal bones involved in forming the articulation.

The third CMC joint is between the capitate and third metacarpal bone. The capitate articulates with a convex facet on the metacarpal base via its triangular and mainly concave distal surface.

The capitate bone also takes part in forming the fourth CMC joint, together with the hamate and fourth metacarpal bones. In this case, the anteromedial corner of the capitate projects an articulating facet towards a large, oval, dorsal facet on the fourth metacarpal base. The partially convex and concave quadrangular proximal surface of the metacarpal base meets a small, anterolateral facet on the distal surface of the hamate bone.

The neighboring larger, anteromedial facet on the same distal surface of the hamate contributes to the formation of the fifth CMC joint, together with the fifth metacarpal bone. This metacarpal base meets the hamate via a transversely concave and coronally convex laterobasal surface. This concave-convex transition is not abrupt, but rather slopes downwards, almost flatly. This beveled edge on the articular surface of the hamate bone permits the fifth CMC joint a greater range of movement compared to the rest of the CMC joints.

From the above descriptions, the flat (non-planar) nature of the four CMC joints becomes evident. While some are indeed almost flat surfaces, others are concave, convex, or both. Therefore, they are structurally similar to ellipsoid or complex saddle synovial joints. However, the curvatures of the CMC joints are so small that they are usually disregarded, qualifying them functionally as plane synovial joints.

Learn more about the general features of various types of synovial joints using Kenhub’s articles, diagrams, videos and quizzes.

Joint capsule

The four CMC joints are surrounded and stabilized by a common fibrous capsule. This fibrous capsule is lined by a synovial membrane that secretes viscous synovial fluid, which acts as a lubricant. The synovial membrane is usually continuous with the lining of the intercarpal joints.

The joint cavity of the CMC joints extends proximally and distally, communicating with the midcarpal and intermetacarpal joint spaces, respectively. The articular surfaces of the CMC joints are lined by hyaline cartilage.

Ligaments

The CMC joints are stabilized by three sets of ligaments: dorsal carpometacarpal ligaments, palmar carpometacarpal ligaments and interosseous ligaments. These soft tissue structures are actually thickenings of the fibrous joint capsule surrounding the CMC joints.

The dorsal carpometacarpal ligaments, located on the dorsal aspect of the hand, are the strongest and offer the most reinforcement to the CMC joints. They comprise a total of seven ligamentous bands that extend obliquely between the dorsal surfaces of the distal row of carpal bones and the four medial metacarpal bases. Metacarpals 2, 3 and 4 each receive two bands; metacarpal 2 from the trapezium and trapezoid, metacarpal 3 from the trapezoid and capitate, and metacarpal 4 from the capitate and hamate. The fifth metacarpal is an exception because it receives only one ligamentous band, which is from the hamate bone. This band connects with the corresponding palmar carpometacarpal ligament to form an incomplete capsule.

The palmar carpometacarpal ligaments, located on the palmar aspect of the hand, are very similar to their dorsal counterparts. The only exception is the third metacarpal base, which receives three ligamentous bands; a lateral one from the trapezium/trapezoid, an intermediate one from the capitate and a medial one from the hamate.

The interosseous ligaments are the smallest stabilizers of the CMC joints. They are comprised of two thick, fibrous bands that extend between the inferior aspect of the distal margins of the capitate and hamate bones and the third and fourth metacarpal bases.

These bands can be either completely separated or united proximally. If separated, the lateral band connects the capitate with the third metacarpal base, while the medial band connects the hamate with the fourth metacarpal base. In addition, the medial interosseous ligament can sometimes divide the CMC joint space into medial and lateral compartments by creating a separate synovial cavity between the hamate and the fourth and fifth metacarpal bones.

Quiz yourself to reinforce what you have learned about the carpometacarpal joints and ligaments.

Innervation

The CMC joints are innervated from three sources, all of which initially originate from the brachial plexus:

• Anterior interosseous nerve of forearm, which is a branch of the median nerve (C8-T1)
• Posterior interosseous nerve of forearm, which stems from the radial nerve (C7-C8)
• Deep and dorsal branches of ulnar nerve (C7-C8)

Blood supply

Blood supply to the CMC joints comes from the palmar and dorsal carpal anastomotic arches. These are formed by the union of the palmar and dorsal carpal branches of radial artery.

Movements

All four CMC joints are structurally classified as synovial joints. However, they are separated into two groups based on their articular surfaces, functional classification, degrees of freedom and ranges of movement (RoM). The latter increases gradually as one moves medially across the four CMC joints.

The second and third CMC joints share some common features and comprise the first group. Being planar type synovial joints, they permit only nonaxial, translational movements. This uniplanar motion involves a gliding, or sliding motion in a linear direction between the articular surfaces of the respective bones forming the second and third CMC joints. This articular arrangement makes these two joints synarthrotic, permitting almost no movement under physiological conditions. This lack of mobility makes the joints very stable, providing a strong connection between the wrist and hand. While grasping objects with the palm, the two CMC joints act as a pillar and remain fixed, allowing the other fingers to close around them.

The fourth and fifth CMC joints comprise the second group and they are a lot more mobile compared to their predecessors. As structural saddle type synovial joints (biaxial), they permit movements in two degrees of freedom (average maximum active RoM is provided in brackets):

• CMC joint 4

1. Flexion (10°) - Extension (0°)
2. Adduction (0°) - Abduction (5°)

• CMC joint 5

1. Flexion (20°) - Extension (0°)
2. Adduction (0°) - Abduction (13°)

However, the adduction and abduction movements have only been observed in cadaveric specimens. The physiological implications and capabilities of these movements in living humans are still unknown and remain to be elucidated.

Therefore, functionally speaking, the fourth and fifth CMC joints are physiologically uniaxial, being capable of only flexion and extension. These two movements take place around a transverse axis passing through the respective metacarpal bases. During flexion at the fourth CMC joint, the articular surfaces of the capitate and hamate translate over the articular surface of the fourth metacarpal base. The movement displaces the metacarpal towards the palmar aspect of the palm in the sagittal plane, decreasing the angle between the moving bones by a maximum of 10°. Extension happens in reverse, aligning the joints in a straight axis in the sagittal plane of the hand.

Did you find hard to remember anatomical structures? Here at Kenhub, we offer you one of the greatest strategies to cement your knowledge, which involves creating your own flashcards!

The fifth CMC joint is the most moveable one of the group, capable of the greatest range of motion out of all of them. The doubled amount of permitted flexion (20o) compared to the fourth CMC joint is facilitated by the beveled articular surface of the hamate bone. This permits the fifth metacarpal base to slide over a greater distance compared to the rest of the CMC joints. The additional degree of flexion facilitates opposition of thelittle finger with the thumb and palm cupping when trying to grasp objects, especially round ones. Another unique feature of the fifth CMC joint is the ability to perform some external and internal rotation. However, similar to the movements of adduction and abduction, rotation at the fifth CMC joint has only been observed in cadavers. Physiologically, this movement is mainly indirect, occurring due to the action of opponens digiti minimi and facilitated by a small degree of elasticity of the surrounding ligaments. Rotation of the fifth CMC joint facilitates opposition and palmar grasp further, increasing the usability of the human hand even more.

The closed-packed position of the medial CMC joints is full flexion, while open-packed (resting) position is halfway between flexion and extension. The capsular pattern limits the movements of the two CMC joints equally in all directions. Additional accessory (arthrokinematic) movements of anteroposterior translation can take place between the metacarpal base and adjacent metacarpal bones if external pressure is applied to the joints.

If you want to visualize and test your knowledge about the movements described above, take a look at the following study unit and custom quiz.
 

Muscles acting on the carpometacarpal joints

The movements of the medial four CMC joints do not depend on the direct action of any muscles. However, the translational movements of the joints are affected by the flexor digitorum profundus and extensor digitorum muscles. As these antagonistic muscles act on the phalanges, they indirectly force the CMC joints to glide and to produce flexion and extension respectively, in the fourth and fifth CMC joints.

The only exception is the direct involvement of the opponens digiti minimi in the rotational movement of the fifth CMC joint. When the muscle acts as an agonist, it brings the fifth finger towards the palm of the hand, rotating it externally and hollowing the palm. These movements are opposed and smoothly controlled by the antagonists of opponens digiti minimi: adductor pollicis, opponens pollicis, flexor pollicis brevis and abductor pollicis brevis. The synergistic muscles stabilising the joint during movements are palmaris brevis, abductor digiti minimi and flexor digiti minimi.

The four medial CMC joints are related to several soft tissue structures. On the palmar aspect of the hand, the CMC joints are overlaid superficially by the tendons of several muscles; flexor digitorum superficialis, flexor digitorum profundus, flexor carpi radialis (laterally) and flexor carpi ulnaris (medially). The hypothenar muscles also cover the medial CMC joints superficially.

On the dorsal aspect of the hand, the CMC joints are situated deep to the tendons of the extensor forearm muscles. Moving from lateral to medial, these include extensor carpi radialis longus, extensor carpi radialis brevis, extensor pollicis longus, extensor indicis, extensor digitorum, extensor digiti minimi and extensor carpi ulnaris.