Wrist

Introduction

The wrist contains eight small carpal bones, which as a group act as a flexible “spacer” between the forearm and hand.

Distal forearm

Distal forearm

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Distal end of the radius

A. anterior view of the distal radius showing an ulnar tilt of about 25 degrees.

B. a medial view of the distal radius showing a palmar tilt of about 10 degrees.

Carpal Bones

From a radial(lateral)to ulnar direction, the proximal row of carpal bones includes the scaphoid, lunate, triquetrum, and pisiform.

The distal row includes the trapezium, trapezoid, capitate, and hamate.

Carpal Bones

The proximal row of carpal bones is joined in a relatively loose fashion.

The distal row of carpal bones is bound tightly by strong ligaments, providing a rigid and stable base for articulation with the metacarpal bones.

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Carpal Bones

Scaphoid

•The scaphoid has two convex surfaces called poles.

•The proximal pole articulates with the scaphoid facet of the radius.

•The distal pole has a slightly rounded surface, which articulates with the trapezium and trapezoid.

Lunate

•The lunate is the most inherently unstable of the carpal bones, in part because of its shape, but primarily because of its lack of firm ligamentous attachments to the relatively rigid capitate bone.

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Carpal Bones

Triquetrum

•The triquetrum is the third most frequently fractured bone of the wrist, after the scaphoid and lunate.

Pisiform

•The pisiform, meaning “shaped like a pea”, articulates loosely with the palmar surface of the triquetrum.

Capitate

•The capitate is the largest of all carpal bones.

•The large head articulates with the deep concavity provided by the scaphoid and lunate.

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Carpal Bones

Trapezium

•The trapezium has an asymmetric shape.

•The proximal surface is slightly concave for articulation with the scaphoid.

•Particular importance is the distal saddle-shaped surface, which articulates with the base of the first metacarpal.

Hamate

•The hamate base, or distal surface, articulates with the bases of the fourth and fifth metacarpals.

•This articulation provides important functional mobility to the ulnar aspect of hand, most noticeably when the hand is cupped.

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What is Carpal Tunnel?

The tunnel serves as a passageway for the median nerve and the tendons of extrinsic flexor muscles of the digits.

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Arthrology

Joints of the Wrist

Radiocarpal joint

Midcarpal joint

•Medial compartment

•Lateral compartment

Intercarpal joints.

Intercarpal joints contribute to wrist motion through small gliding and rotary.

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Radiocarpal joint

The proximal components of the radiocarpal joint are the concave surfaces of the radius and an adjacent articular disc.

The distal components of the radiocarpal joint are the convex proximal surfaces of the scaphoid and the lunate.

The thick articular surface of the distal radius and the articular disc accept and disperse the forces that cross the wrist.

Approximately 20% of the total compression force that crosses the radiocarpal joint passes through the articular disc.

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Radiocarpal joint

The remaining 80% passes directly through the scaphoid and lunate to the radius.

The contact areas at the radiocarpal joint tend to be greatest when the wrist is partially extended and ulnarly deviated.

This is also the wrist position at which maximal grip strength is obtained.

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Midcarpal joint

The midcarpal joint is the articulation between the proximal and distal rows of carpal bones.

The midcarpal joint can be divided descriptively into medial and lateral joint compartments.

The lager medial compartment is formed by the convex head of the capitate and apex of the hamate, fitting into the concave recess formed by the distal surfaces of the scaphoid, lunate, and triquetrum.

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Midcarpal joint

The lateral compartment of the midcarpal joint is formed by the junction of the slightly convex distal pole of the scaphoid with the slightly concave oroximal surfaces of the trapezium and the trapezoid

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Wrist ligaments

Wrist ligaments are essential to maintaining the natural intercarpal alignment and for transferring forces within and across the carpus.

Extrinsic ligaments have their proximal attachments on the forearm but attach distally within the wrist.

Intrinsic ligaments have both their proximal and distal attachments within the wrist.

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Extrinsic ligaments

The dorsal radiocarpal ligament courses distally in an ulnarly direction, attaching primarily between the distal radius and the dorsal surfaces of the lunate and triquetrum.

The fibers that attach to the lunate provide an especially important restraint against anterior(volar)dislocation of this inherently.

Extrinsic ligaments

The palmar radiocarpal ligaments become maximally taut at full

wrist extension.

The primary global function of the TFCC is to securely bind the distal ends of the radius and ulna while simultaneously permitting the radius, with attached carpus, to freely rotate(pronate and supinate) around a fixed ulna.

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Intrinsic ligaments

Thin dorsal intercarpal ligament provides transverse stability to the wrist by interconnecting the trapezium, scaphoid, lunate, and triquetrum.

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Kinematics of wrist motion

The osteokinematics of the wrist are defined for 2 degrees of the freedom.

Flexion-extension and ulnar-radial deviation.

Osteokinematics

The wrist rotates in the sagittal plane about 130 to 160 degrees.

On average, the wrist flexes from 0 degrees to about 70 to 85 degrees and extends from 0 degrees to about 60 to 75 degrees.

Total flexion normally exceeds extension by about 10 to 15 degrees.

End-range extension can be limited by stiffness in the thick palmar radiocarpal ligaments.

Osteokinematics

The wrist rotates in the frontal plane approximately 50 to 60 degrees.

Radial and ulnar deviation is measured as the angle between the radius and the shaft of the third metacarpal.

Ulnar deviation of the wrist occurs from 0 degrees to about 35 to 40 degrees.

Radial deviation occurs from 0 degrees to about 15 to 20 degrees.

Because of the ulnar tilt of the distal radius, maximum ulnar deviation normally is double that of radial deviation.

Arthrokinematics

The axis of rotation for wrist movement is assumed to pass through the head of the capitate.

The axis runs in a medial-lateral direction for flexion and extension.

Anterior-posterior direction for radial and ulnar deviation.

Wrist extension and flexion

Radiocarpal joint is represented by the articulation between the radius and lunate.

Midcarpal joint is represented by the articulation between the lunate and capitate.

Dynamic interaction within the joints of the central column of the wrist

Ulnar and radial deviation of the wrist

Carpal instability

Two common types of carpal instability

1. rotational collapse of wrist: the “zig-zag” deformity

2. Translocation of the carpus

Rotational collapse of the wrist

The lunate is the most frequently dislocated carpal bone.

Scaphoid forms an important mechanical link between the lunate and the more stable, distal row of carpal bones.

Volar intercalated segment instability(VISI)

Dorsal intercalated segment instability(DISI)