Structure and Function of the Hand (2024)

Because the entire thumb is rotated almost 90 degrees in relation to the fingers, the terminology used to describe thumb movement is different from that used for the fingers (Figure 7-7, E through I). Flexion is the movement of the palmar surface of the thumb in the frontal plane across and parallel with the palm. Extension returns the thumb back toward its anatomic position. Abduction is the forward movement of the thumb away from the palm in a sagittal plane. Adduction returns the thumb to the plane of the hand. Opposition is a special term that describes the movement of the thumb across the palm, making direct contact with the tips of any of the fingers. This special terminology, which is used to define the movement of the thumb, serves as the basis for the naming of the “pollicis” (thumb) muscles, for example, the opponens pollicis, the extensor pollicis longus, and the adductor pollicis.

Carpometacarpal Joints

Overview

The carpometacarpal (CMC) joints of the hand form the articulation between the distal row of carpal bones and the bases of the five metacarpal bones. These joints are positioned at the extreme proximal region of the hand (see Figures 7-3 and 7-4).

The basis for all movement within the hand starts at the CMC joints—at the most proximal region of each ray. Figure 7-8 shows a simplified illustration of relative mobility at the CMC joints. The joints of the second and third digits, shown in gray, are rigidly joined to the distal row of carpal bones, forming a stable central pillar throughout the hand. In contrast, the peripheral CMC joints (shown in green) form mobile radial and ulnar borders, which are capable of folding around the hand’s central pillar.

The first CMC joint (known as the thumb’s saddle joint) is the most mobile, especially during the movement of opposition. (The CMC joint of the thumb is extremely important and is described separately in a subsequent section.) The fourth and fifth CMC joints are the next most mobile CMC joints, allowing a cupping motion of the ulnar border of the hand. Increased mobility of the fourth and fifth CMC joints improves the effectiveness of the grasp and enhances functional interaction with the opposing thumb.

The CMC joints of the hand transform the palm into a gentle concavity, greatly improving dexterity. This feature is one of the most impressive functions of the human hand. Cylindrical objects, for example, can fit snugly into the palm, with the index and middle digits positioned to reinforce grasp (Figure 7-9). Without this ability, the dexterity of the hand is reduced to a primitive, hinge-like grasping motion.

Carpometacarpal Joint of the Thumb

The CMC joint of the thumb is located at the base of the first ray, between the metacarpal and the trapezium (see Figure 7-5). This joint is by far the most complex and likely the most important of the CMC joints, enabling extensive movements of the thumb. Its unique saddle shape allows the thumb to fully oppose, thereby easily contacting the tips of the other digits. Through this action, the thumb is able to encircle objects held within the palm.

The capsule that surrounds the CMC joint of the thumb is naturally loose to allow a large range of motion. The capsule, however, is strengthened by stronger ligaments and by forces produced by the over-riding musculature. Rupture of ligaments secondary to trauma, overuse, or arthritis often causes a dislocation of the joint, forming a characteristic hump at the base of the thumb.

Consider this…

Osteoarthritis at the Base of the Thumb

The large functional demand placed on the carpometacarpal (CMC) joint of the thumb often results in a painful condition called basilar joint osteoarthritis. The term basilar refers to the location of the CMC joint at the base of the entire thumb. This common condition receives more surgical attention than any other osteoarthritis-related condition of the upper limb. Arthritis may develop at this joint secondary to acute injury or, more likely, from the normal wear and tear associated with a physical occupation or hobby. It is interesting to note that persons who needlepoint or milk cows for many years frequently develop painful arthritis at the base of the thumb.

Persons who require medical attention for basilar joint arthritis typically present foremost with pain, but also with functional limitations, ligamentous laxity (looseness), and instability of the joint. Loss of pain-free function of the thumb markedly reduces the functional potential of the entire hand and thus of the entire upper extremity. Persons with advanced arthritis of the base of the thumb demonstrate severe pain (made worse by pinching actions), weakness, swelling, dislocation, and crepitation (abnormal popping or clicking sounds that occur with movement). This condition occurs with disproportionately greater frequency in female individuals, typically in their fifth and sixth decades.

The more common conservative therapeutic intervention for basilar joint arthritis includes splinting, careful use of non-strenuous exercise, physical modalities such as cold and heat, non-steroidal anti-inflammatory drugs, and corticosteroid injections. In addition, patients are taught ways to modify their activities of daily living to protect the base of the thumb from unnecessarily large forces.

Surgical intervention is typically used when conservative therapy is unable to retard the progression of pain or the instability.

Saddle Joint Structure

The CMC joint of the thumb is the classic saddle joint of the body (Figure 7-10). The characteristic feature of a saddle joint is that each articular surface is convex in one dimension and concave in the other—just like the saddle on a horse. This shape allows maximal mobility and stability.

Kinematics

Motions at the CMC joint occur primarily in 2 degrees of freedom (Figure 7-11). Abduction and adduction occur generally in the sagittal plane, and flexion and extension occur generally in the frontal plane. Opposition and reposition of the thumb are special movements that incorporate the two primary planes of motion. The kinematics of opposition and reposition is discussed after the two primary motions are considered.

Abduction and Adduction.

In the (neutral) position of adduction of the CMC joint, the thumb lies within the plane of the hand. Maximum abduction, in contrast, positions the thumb metacarpal about 45 degrees anterior to the plane of the palm. Full abduction opens the web space of the thumb, forming a wide concave curvature useful for grasping objects like a coffee cup.

Flexion and Extension.

Actively performing flexion and extension of the CMC joint of the thumb is associated with varying amounts of axial rotation (spinning) of the first metacarpal. During flexion, the metacarpal rotates slightly medially (i.e., toward the third digit); during extension, the metacarpal rotates slightly laterally (i.e., away from the third digit). The axial rotation is evident by watching the change in orientation of the nail of the thumb between full extension and full flexion.

From the anatomic position, the CMC joint can be extended an additional 10 to 15 degrees. From full extension, the thumb metacarpal flexes across the palm about 45 to 50 degrees.

Opposition.

The ability to precisely oppose the thumb to the tips of the other fingers is perhaps the ultimate expression of functional health of this digit and, arguably, of the entire hand. This complex motion is a composite of the other primary motions already described for the CMC joint.

For ease of discussion, Figure 7-12, A, shows the full arc of opposition divided into two phases. In phase 1, the thumb metacarpal abducts. In phase 2, the abducted metacarpal flexes and medially rotates across the palm toward the small finger. Figure 7-12, B, shows the detail of the kinematics of this complex movement. Muscle force, especially from the opponens pollicis, helps guide and rotate the metacarpal to the extreme medial side of the articular surface of the trapezium.

As can be seen by the change in orientation of the thumbnail, full opposition incorporates at least 45 to 60 degrees of medial rotation of the thumb. The small finger contributes indirectly to opposition through a cupping motion at the fifth CMC joint. This motion allows the tip of the thumb to more easily contact the tip of the little finger.

Metacarpophalangeal Joints

Fingers

General Features and Ligaments

The metacarpophalangeal (MCP) joints, or knuckles, of the fingers are relatively large articulations formed between the convex heads of the metacarpals and the shallow concave proximal surfaces of the proximal phalanges (Figure 7-13). Motion at the MCP joint occurs predominantly in two planes: (1) Flexion and extension in the sagittal plane, and (2) abduction and adduction in the frontal plane.

Supporting Structures

Figure 7-14 illustrates many of the supporting structures of MCP joints.

•Capsule: Connective tissue that surrounds and stabilizes the MCP joint

•Radial and ulnar collateral ligaments: Cross the MCP joints in an oblique palmar direction; limit abduction and adduction; become taut on flexion

•Fibrous digital sheaths: Form tunnels or pulleys for the extrinsic finger flexor tendons; contain synovial sheaths to help lubrication

•Palmar (or volar) plates: Thick fibrocartilage ligaments or “plates” that cross the palmar side of each MCP joint; these structures limit hyperextension of the MCP joints

•Deep transverse metacarpal ligaments: These three ligaments merge into a wide, flat structure that interconnects and loosely binds the second through fifth metacarpals

Kinematics

In addition to the motions of flexion and extension and abduction and adduction at the MCP joints, substantial accessory motions are possible. With the MCP joint relaxed and nearly extended, appreciate on your own hand the amount of passive mobility of the proximal phalanx relative to the head of the metacarpal. These accessory motions permit the fingers to better conform to the shapes of held objects, thereby increasing control of grasp (Figure 7-15).

Metacarpophalangeal Joints of the Fingers Permit Volitional Movements Primarily in 2 Planes

•Flexion and extension occur in the sagittal plane about a medial-lateral axis of rotation.

•Abduction and adduction occur in the frontal plane about an anterior-posterior axis of rotation.

Figure 7-16 shows the kinematics of flexion of the MCP joints, controlled by two finger flexor muscles: The flexor digitorum superficialis and the flexor digitorum profundus. Flexion stretches and therefore increases tension in both the dorsal part of the capsule and the collateral ligaments. In the healthy state, this passive tension helps guide the joint’s natural arthrokinematics. Increased tension in the dorsal capsule and collateral ligaments stabilizes the joint in flexion; this is useful during grasp. The kinematics of extension of the MCP joints occurs in reverse fashion compared with that described for flexion.

Because the proximal surface of the proximal phalanx is concave and the head of the metacarpal is convex, the arthrokinematics of flexion and extension occurs as a roll and slide in similar directions.

The overall range of flexion and extension at the MCP joints increases gradually from the second (index finger) to the fifth digit: The second finger flexes to about 90 degrees, and the fifth to about 110 to 115 degrees. The MCP joints can be passively extended beyond the neutral (0-degree) position for a considerable range of 30 to 45 degrees.

Figure 7-17 shows the kinematics of abduction of the MCP joint of the index finger, controlled by the first dorsal interosseus muscle. During abduction, the proximal phalanx rolls and slides in a radial direction: The radial collateral ligament becomes slack, and the ulnar collateral ligament is stretched. The kinematics of adduction of the MCP joints occurs in a reverse fashion. Abduction and adduction at the MCP joints occur to about 20 degrees on either side of the midline reference formed by the third metacarpal.

Consider this…

Position of Function: Placing Useful Tension in the Metacarpophalangeal Joints’ Collateral Ligaments

Flexion of the metacarpophalangeal joints places a stretch within the collateral ligaments. As with a stretched rubber band, increased tension in these ligaments restricts the freedom of passive motion at the joints. (This can be appreciated by noting how abduction and adduction of the fingers are much less in full flexion than in full extension.) Increased tension in the collateral ligaments can be useful because it lends natural stability to the base of the fingers, which is especially useful during flexion movements such as holding a hand of playing cards. Furthermore, clinicians often use increased tension in the collateral ligaments to prevent joint stiffness or deformity. This strategy is commonly used with a hand that must be held immobile in a cast (or splint) for an extended time after, for example, fracture of a metacarpal (Figure 7-18). Maintaining the metacarpophalangeal joints in flexion (with interphalangeal joints usually close to full extension) increases passive tension within the ligaments of the MCP joints just enough to reduce the likelihood of their undergoing permanent shortening and developing an “extension” contracture that gives a “claw-like” appearance to the hand.

Thumb

The MCP joint of the thumb consists of the articulation between the convex head of the first metacarpal and the concave proximal surface of the proximal phalanx of the thumb (Figure 7-19). The basic structure of the MCP joint of the thumb is similar to that of the fingers. Active and passive motions at the MCP joint of the thumb are significantly less than those at the MCP joints of the fingers. For all practical purposes, the MCP joint of the thumb allows only 1 degree of freedom: Flexion and extension within the frontal plane. Unlike the MCP joints of the fingers, extension of the thumb MCP joint is usually limited to just a few degrees. From full extension, the proximal phalanx of the thumb can actively flex about 60 degrees across the palm toward the middle digit (Figure 7-20). Active abduction and adduction of the thumb MCP joint is limited and therefore these are considered accessory motions.

Interphalangeal Joints

Fingers

The proximal and distal interphalangeal joints of the fingers are located distal to the MCP joints (see Figure 7-19). Each joint allows only 1 degree of freedom: Flexion and extension. From both a structural and a functional perspective, these joints are simpler than the MCP joints.

General Features and Ligaments

The proximal interphalangeal (PIP) joints are formed by the articulation between the heads of the proximal phalanges and the bases of the middle phalanges (Figure 7-21). The distal interphalangeal (DIP) joints are formed through the articulation between the heads of the middle phalanges and the bases of the distal phalanges. The articular surfaces of these joints appear as a tongue-in-groove articulation similar to that used in carpentry to join planks of wood. This articulation helps limit motion at the PIP and DIP joints to flexion and extension only.

Except for being smaller, the same ligaments that surround the MCP joints also surround the PIP and DIP joints. The capsule at each interphalangeal (IP) joint is strengthened by radial and ulnar collateral ligaments and a palmar plate. The collateral ligaments restrict any side-to-side movements, and the palmar (volar) plate limits hyperextension. In addition, the fibrous digital sheaths house the tendons of the extrinsic finger flexor muscles (see index and small fingers in Figure 7-14).

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