Which carpal bone is in the proximal row between the scaphoid and triquetrum?

The wrist Anatomy comprises the distal radius and ulna, eight carpal bones, and the bases of five metacarpals.

In addition, there are the ligaments that arise and insert on carpal bones and stabilize the wrist joint and connect it to metacarpal bones distally and forearm bones proximally.

See Also: Forearm Muscles Anatomy

Wrist Bones Anatomy

Osteology Wrist Anatomy consists of 8 carpal bones that’s arranged in two rows, the proximal row which contains from lateral to medial (scaphoid, lunate, triquetrum, and pisiform), while the distal row contains the following from lateral to medial (trapezium, trapezoid, capitate, and hamate).

Scaphoid:

The scaphoid is the largest of the proximal carpal bones, and its shape resembles that of a boat or canoe (thus the old term navicular). The scaphoid bone links the proximal and distal carpal rows and helps provide stability to the wrist joint.

The scaphoid is tethered to the proximal carpal row by a number of strong ligamentous attachments, and two-thirds of its surface area is articular.

• The proximal surface of the scaphoid is convex and articulates with the radius.
• The medial surfaceis concave and articulates with the capitate. The articulating surface for the lunate is flat.
• The distal surface consists of two convex facets for articulation with the trapezium and trapezoid (the scapho-trapezio-trapezoid joint).

The round tubercle on the inferolateral part of its anterior (palmar) surface serves as the attachment of the flexor retinaculum and the abductor pollicis brevis (APB).

The blood vessels to this bone enter the scaphoid at or distal to the wrist. This configuration predisposes a fracture on the proximal aspect to aseptic necrosis.

In addition, as the scaphoid plays a critical role in coordinating and stabilizing movements between the proximal and the distal rows of the carpals, damage to the intrinsic and extrinsic ligaments that support the scaphoid can result in persistent pain and dysfunction with loading activities.

See Also: Scaphoid Shift Test

Lunate:

The lunate articulates between the scaphoid and the triquetrum in the proximal carpal bone row. Its smooth convex proximal surface articulates with the radius and the TFCC at the lunate fossa. Its lateral surface contains a flat semilunar facet for the scaphoid. The medial surface articulates with the triquetrum. The distal surface is deeply concave and articulates with the edge of the hamate in adduction and
the medial aspect of the capitate.

Triquetrum:

The triquetrum is a pyramid-shaped bone. It articulates with the pisiform on its distal anterior (palmar) surface at the pisiform–triquetral joint. The almost square distal–medial surface of the triquetrum articulates with the concavo–convex surface of the hamate. The ulnar collateral ligament (UCL) attaches to the medial and posterior (dorsal) surfaces of the triquetrum. The proximal surface of the triquetrum articulates with the TFCC in full adduction. The lateral surface of the triquetrum articulates with the lunate.

See Also: Lunotriquetral Ballottement Test

Pisiform:

The pisiform, as its name implies, is shaped like a “P” with a posterior (dorsal) flat articular facet for the triquetrum. The pisiform, formed within the tendon of the flexor carpi ulnaris (FCU), is a sesamoid bone and serves as an attachment for the flexor retinaculum, abductor digiti minimi (ADM), UCL, pisohamate ligament, and pisometacarpal ligament. The pisiform also functions to increase the flexion moment of the FCU. As mentioned, the pisiform articulates with the anterior (palmar) surface of the triquetral and is thus separated from the other carpal bones, all of which articulate with their neighbors. The pisiform is closely related to the ulnar artery and nerve on its radial border, the nerve being the closer.

Trapezium:

The trapezium has a groove on its medial anterior (palmar) surface which contains the tendon of
the flexor carpi radialis (FCR). To its margins are attached two layers of the flexor retinaculum. The opponens pollicis (OP) is between the flexor pollicis brevis (FPB) distally and the APB proximally. The lateral surface serves as an attachment site for the radial collateral ligament and capsular ligament of
the Thumb CMC joint. The distal articulating surface of the trapezium is saddle shaped. Medially, its concave surface articulates with the trapezoid, whereas more distally its convex surface articulates with the second metacarpal base. Proximally, its concave surface articulates with the scaphoid.

Trapezoid:

The trapezoid is small and irregular. The distal surface articulates with the grooved second metacarpal base. The medial surface articulates via a concave facet with the distal part of the capitate. The lateral surface of the trapezoid articulates with the trapezium, and its proximal surface articulates with the scaphoid bone.

Capitate:

The capitate is the most central and the largest of the carpal bones. Its distal aspect articulates with the third metacarpal base. Its lateral border articulates with the medial side of the second metacarpal base. The convex proximal head of the capitate articulates with the lunate and scaphoid. The medial surface of the head articulates with the lunate, and the lateral aspect of the head articulates with the scaphoid. Medially, the capitate articulates with the hamate. With its central location, the capitate serves as the keystone of the proximal transverse arch. This arch is important to the prehensile activity of the hand.

Hamate:

The hamate is a cuneiform bone and contributes to the medial wall of the carpal tunnel. To the hook (hamulus) of the hamate is attached the flexor retinaculum. The hamate articulates with three carpal bones and two metacarpals. The medial surface articulates with the triquetrum and by association with the pisohamate ligament, the pisiform. The lateral surface articulates with the capitate. On its distal aspect the hamate articulates with the fourth and fifth metacarpal heads.

Distal Radioulnar Joint

The distal radioulnar articulation, formed by the ulnar head and the ulnar notch of the radius, allows 1 degree of freedom of movement: pronation and supination. The distal and proximal radioulnar joints work together to produce those motions. Restriction of motion at either of these joints limits pronation and supination of the entire forearm. At the distal radioulnar joint, pronation and supination are produced by the radius’ gliding around the ulna.

Radiocarpal joint

Ellipsoid shape involving distal radius and the scaphoid, lunate, and triquetrum. It’s located at the level of the crease of proximal wrist flexion.

Extrinsic ligaments bridge carpal bones to radius or metacarpals (radioscaphocapitate); while intrinsic attach carpal bones together (scapholunate).

Radiocarpal joint Ligaments:

Palmar/volar radiocarpal ligaments are the strongest supporting structures.

Space of Poirier: it’s a central weak area in floor of carpal tunnel; implicated in volar dislocation of lunate in perilunate dislocation.

Ligament of Testut (radioscapholunate ligament) functions as a neurovascular conduit.

See Also: Carpal Tunnel Syndrome

Triangular Fibrocartilaginous Complex (TFCC)

The ulna is buffered from the proximal row of carpals by the triangular fibrocartilaginous complex (TFCC). The TFCC is composed of:

1. the articular disc (or central fibrocartilage),
2. the dorsal and palmar distal radioulnar ligaments,
3. the ulnolunate and ulnotriquetral ligaments,
4. the subsheath surrounding the extensor carpi ulnaris tendon,
5. the meniscus homolog (meaning like a meniscus), which attaches the fibrocartilage to the triquetrum.

The TFCC dissipates stresses imposed on the forearm during loading by extending the radiocarpal articulation, stabilizes the distal radioulnar joint, and provides stability during pronation and supination.

Midcarpal Joints

The midcarpal joint lies between the two rows of carpal bones. It is referred to as a “compound” articulation because each row has both a concave and a convex segment. Wrist flexion, extension, and radial deviation are mainly midcarpal joint motions.

Approximately 50 percent of the total arc of wrist flexion and extension occur at the midcarpal level with more flexion (66 percent) occurring than extension (34 percent).

The proximal row of the carpals is convex laterally and concave medially. The scaphoid, lunate, trapezium trapezoid, and triquetrum present with a concave surface to the distal row of carpals. The scaphoid, capitate, and hamate present a convex surface to a reciprocally arranged distal row.

Transverse articulations between proximal and distal rows are reinforced by palmar and dorsal intercarpal ligaments and carpal collateral ligaments. The radial ligament is stronger.

See Also: Extensor compartments of the wrist

Proximal row

• Scaphoid, lunate, and triquetrum form gliding joints. They are connected by dorsal and palmar intercarpal ligaments.
• Dorsal intercarpal ligaments are stronger.
• Interosseous ligaments are narrow bundles connecting the scaphoid and lunate and the lunate and triquetrum.

Pisiform articulation:

• Pisotriquetral joint has a thin articular capsule.
• Ulnar collateral and palmar radiocarpal ligaments also connect the pisiform proximally.
• Pisohamate and pisometacarpal ligaments help extend the pull of the FCU.

Distal row

• Trapezium, trapezoid, capitate, and hamate gliding joints.
• Dorsal and palmar intercarpal ligaments connect trapezium with trapezoid, trapezoid with capitate, and capitate with hamate.
• Interosseous ligaments are much thicker in the distal row, connecting capitate and hamate (strongest), capitate and trapezoid, and trapezium and trapezoid (weakest).

Thumb CMC Joint

The thumb is the most important digit of the hand and greatly magnifies the complexity of human prehension. Functionally, the sellar (saddle-shaped) Thumb CMC Joint is the most important joint of the thumb and consists of the articulation between the base of the first metacarpal and the distal aspect of the trapezium.

The articular surfaces of the trapezium and the proximal end of the first metacarpal are reciprocally shaped.

Three other adjacent articulations are functionally related to this joint, which include:

1. the joints between the trapezium and the scaphoid,
2. the trapezium and the trapezoid,
3. the base of the first metacarpal and the radial side of the base of the second metacarpal.

Motions that can occur at Thumb CMC Joint include:

• flexion/extension,
• adduction/abduction,
• opposition, which includes varying amounts of flexion, internal rotation, and anterior (palmar) adduction.

Although the joint capsule of the Thumb CMC joint is large and relatively loose, motions at the joint are controlled and supported by muscle actions and by at least five ligaments:

1. anterior oblique,
2. ulnar collateral,
3. intermetacarpal,
4. posterior oblique,
5. radial collateral.

In general, most of the thumb ligaments are placed on tension with abduction, extension, and opposition.

Other CMC Joints

The distal borders of the distal carpal row bones articulate with the bases of the metacarpals, thereby forming the CMC joints. The CMC articulations of the fingers permit only gliding movements.

The CMC joints progress in mobility from the second to the fifth, with the second and third metacarpal joints being relatively immobile, and thus the primary stabilizing joints of the hand. The fourth and fifth CMC joints are more mobile to permit the hand to adapt to objects of different shapes during grasping.

Stability for the CMC joints is provided by the anterior (palmar) and posterior (dorsal) CMC and intermetacarpal ligaments.

While the trapezoid articulates with only one metacarpal, all of the other members of the distal carpal row combine one carpal bone with two or more metacarpals.

References

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8. Watson HK, Ashmead D 4th, Makhlouf MV. Examination of the scaphoid. J Hand Surg Am. 1988 Sep;13(5):657-60. doi: 10.1016/s0363-5023(88)80118-7. PMID: 3241033.
9. NAPIER JR. The prehensile movements of the human hand. J Bone Joint Surg Br. 1956 Nov;38-B(4):902-13. doi: 10.1302/0301-620X.38B4.902. PMID: 13376678.
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Which carpal bones are in the proximal row?

Which carpal bone is located in the proximal row on the lateral side?

Is the pisiform in the proximal row?

What is the carpal bone on the thumb side and proximal row that is commonly fractured on a fall to an outstretched hand?