Structure and Function of the Hand



Structure and Function of the Hand




When functioning normally, the 19 bones and 19 joints of the hand produce amazingly diverse functions. The hand may be used in a primitive fashion such as a hook or a club or, more often, as a highly specialized instrument performing complex manipulations that require multiple levels of force and precision. Evidence of the hand’s enormous functional importance is evident by observing the disproportionately large area of the cortex devoted to the sensory and motor functions of the hand (Figure 7-1). A hand that is totally incapacitated by arthritis, pain, stroke, or nerve injury, for instance, can dramatically reduce the overall function of the entire upper limb. The function of the entire upper limb depends strongly on the function of the hand.



This chapter describes the basic anatomy of the bones, joints, and muscles of the hand—information essential to understanding impairments of the hand, as well as the treatments used to help restore its function following injury or disease.


The digits of the hand are designated numerically from one to five, or as the thumb and the index, middle, ring, and little (small) fingers (Figure 7-2). Each of the five digits contains one metacarpal and a group of phalanges. A ray describes one metacarpal bone and its associated phalanges.



The articulations between the proximal end of the metacarpals and the distal row of carpal bones form the carpometacarpal joints (see Figure 7-2). The articulations between the distal end of the metacarpals and the proximal phalanges form the metacarpophalangeal (MCP) joints. Each finger has two interphalangeal (PIP) joints: A proximal interphalangeal joint and a distal interphalangeal joint (DIP). The thumb has only two phalanges and therefore only one interphalangeal joint.





Osteology


Metacarpals


The metacarpals, like the digits, are designated numerically as one through five, beginning on the radial (lateral) side.


Each metacarpal has the following similar anatomic characteristics: Base, shaft, head, and neck. These characteristics are shown for the third ray in Figure 7-3. As is indicated in Figure 7-4, the first (thumb) metacarpal is the shortest and thickest, and the length of the remaining bones generally decreases in a radial-to-ulnar (medial) direction.





With the hand at rest in the anatomic position, the thumb’s metacarpal is oriented in a plane different from that of the other digits. The second through fifth metacarpals are aligned generally side by side, with their palmar surfaces facing anteriorly. The position of the thumb’s metacarpal, however, is rotated almost 90 degrees medially (i.e., internally), relative to the other digits (see Figure 7-4). This rotated position places the sensitive palmar surface of the thumb toward the midline of the hand. In addition, the thumb’s metacarpal is positioned well anterior, or palmar, to the other metacarpals. This can be verified by observing your own relaxed hand. The location of the first metacarpal allows the entire thumb to sweep freely across the palm toward the fingers. Virtually all motions of the hand require the thumb to interact with the fingers. Without a healthy and mobile thumb, the overall function of the hand is significantly reduced.


The medially rotated thumb requires unique terminology to describe its movement and position. In the anatomic position, the dorsal surface of the bones of the thumb (i.e., the surface where the thumbnail resides) faces laterally (Figure 7-5). Therefore, the palmar surface faces medially, the radial surface anteriorly, and the ulnar surface posteriorly. The terminology used to describe the surfaces of the carpal bones and all bones of the fingers is standard: The palmar surface faces anteriorly, the radial surface faces laterally, and so forth.





Arches of the Hand


Observe the natural arched curvature of the palmar surface of your relaxed hand. Control of this concavity allows the human hand to securely hold and manipulate objects of many and varied shapes and sizes. This palmar concavity is supported by three integrated arch systems: Two transverse and one longitudinal (Figure 7-6). The proximal transverse arch is formed by the distal row of carpal bones. This static, rigid arch forms the carpal tunnel, permitting passage of the median nerve and many flexor tendons coursing toward the digits. As with most arches in buildings and bridges, the arches of the hand are supported by a central keystone structure. The capitate bone is the keystone of the proximal transverse arch.



The distal transverse arch of the hand passes through the metacarpophalangeal joints. In contrast to the rigid proximal arch, the ulnar and radial sides of the distal arch are relatively mobile. To appreciate this mobility, imagine transforming your completely flat hand into a cup shape that surrounds a baseball. Transverse flexibility within the hand occurs as the peripheral metacarpals (first, fourth, and fifth) fold around the more stable central (second and third) metacarpals. The keystone of the distal transverse arch is formed by the metacarpophalangeal joints of these central metacarpals.


The longitudinal arch of the hand follows the general shape of the second and third rays. These relatively rigid articulations provide an important element of longitudinal stability to the hand.



Arthrology


Before progressing to the study of the joints, the terminology that describes the movement of the digits must be defined. The following descriptions assume that a particular movement starts from the anatomic position, with the elbow extended, the forearm fully supinated, and the wrist in a neutral position. Movement of the fingers is described in the standard fashion using the cardinal planes of the body: Flexion and extension occur in the sagittal plane, and abduction and adduction occur in the frontal plane (Figure 7-7, A through D). In most other regions of the body, abduction and adduction describe movement of a bony segment toward or away from the midline of the body; however, abduction and adduction of the fingers is described as motion toward (adduction) or away (abduction) from the middle finger.



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.



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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.




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.






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



Supporting Structures

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



Mechanical stability at the MCP joint is critical to the overall biomechanics of the hand. As discussed earlier, the MCP joints serve as keystones that support the mobile arches of the hand. In the healthy hand, stability at the MCP joints is achieved by an elaborate set of interconnecting connective tissues (Figure 7-14).



As is shown in Figure 7-14, the concave component of an MCP joint is extensive, formed by the articular surface of the proximal phalanx, the collateral ligaments, and the dorsal surface of the palmar plate. These tissues form a three-sided receptacle that is aptly suited to accept the large metacarpal head. This structure adds to the stability of the joint and increases the area of articular contact.



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).




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.




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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).

Dec 5, 2016 | Posted by in MANUAL THERAPIST | Comments Off on Structure and Function of the Hand
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