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Normal
conditions of Bone in the Wrist
The bony structures about the
wrist are the distal portions of the radius and ulna, the proximal and
distal rows of carpal bones, and the metacarpals.
The distal aspects of the
radius and ulna articulate with the proximal row of carpal bones. On the
lateral surface of the radius is the radial styloid process, which extends
more distally than the remainder of the bone, and from which arises the
radial collateral ligament of the wrist joint. The articular surface of
the radius is divided into an ulnar and a radial portion by a faint central
ridge of bone. The ulnar portion articulates with the lunate and the radial
portion articulates with the scaphoid. The articular surface is continuous
medially with that of the triangular fibrocartilage. The medial surface
of the distal end of the radius contains the concave ulnar notch, which
articulates with the distal end of the ulna. The posterior surface of the
distal radius is convex and grooved or irregular in outline to allow passage
of tendons and tendon sheaths. A prominent ridge in the middle of this
surface is the dorsal tubercle of the radius. The anterior surface of the
distal portion of the radius allows attachment of the palmar radiocarpal
ligaments.
The distal end of the ulna
contains a small round head and a styloid process. The lateral aspect contains
an articular surface for contact with the ulnar notch of the radius. The
ulna also has a distal articular surface, which is intimate with the triangular
fibrocartilage. The ulnar styloid process, which extends distally from
the posteromedial aspect of the bone, gives rise to the ulnar collateral
ligament. Between the styloid process and inferior articular surface, the
ulna has an area for attachment of the triangular fibrocartilage and a
dorsal groove for the extensor carpi ulnaris tendon and sheath.
The proximal row of carpal
bones consists of the scaphoid, lunate, and triquetrum, as well as the
pisiform bone within the tendon of the flexor carpi ulnaris. The distal
row of carpal bones contains the trapezium, trapezoid, capitate, and hamate
bones. The dorsal surface of the carpus is convex from side to side, and
the palmar surface presents a deep concavity, termed the carpal groove
or canal. The medial border of this palmar carpal groove contains the pisiform
and hook of the hamate. The lateral border of the carpal groove contains
the tubercles of the scaphoid and the trapezium. A strong fibrous retinaculum
attaches to the palmar surface of the carpus, converting the groove into
a carpal tunnel, through which pass the median nerve and flexor tendons.
Evaluation of this tunnel in cases of median nerve entrapment is best accomplished
with CT and MR imaging.
The distal row of carpal
bones articulates with the bases of the metacarpals. The trapezium has
a saddle-shaped articular surface for the first metacarpal. The trapezoid
fits into a deep notch in the second metacarpal. The capitate articulates
mainly with the third metacarpal, but also with the second and fourth metacarpals.
The hamate articulates with the fourth and fifth metacarpals. The bases
of the metacarpals articulate not only with the distal row of carpal bones
but also with each other.
The alignment of the bones
of the wrist joints varies with wrist position. When the wrist is in neutral
position without dorsal or palmar flexion, the distal end of the radius
articulates with the scaphoid and approximately 50 per cent of the lunate.
The degree of radial shift of the carpus at the radiocarpal joint is determined
by comparing the distance between the central axis of the distal end of
the radius and the distal aspect of the radial styloid process in the two
wrists. Several methods exist for the measurement of ulnar translocation,
or ulnarward displacement, of the carpus at the radiocarpal joint. The
degree of radial deviation of the radiocarpal compartment can be measured
on a posteroanterior radiograph with the wrist in this neutral attitude.
A line is drawn through the longitudinal axis of the second metacarpal
at its radial cortex. A second line is constructed from the ulnar limit
of the distal end of the radius to the tip of the radial styloid process.
The second line intersects the first, creating an obtuse angle, which normally
averages 112 degrees (range, 92 to 127 degrees). In the neutral position,
the spaces between carpal bones are approximately equal in the normal wrist.
An abnormal widening of the scapholunate space is termed scapholunate dissociation.
On a posteroanterior radiograph of a normal wrist, a line drawn tangentially
from the distal tip of the radial styloid through the base of the ulnar
styloid process intersects a second line drawn along the midshaft of the
radius with an average angle of 83 degrees (72 to 95 degrees).
On a lateral radiograph
of a normal wrist in neutral position without palmar flexion or dorsiflexion,
a continuous line can be drawn through the longitudinal axes of the radius,
lunate, capitate, and third metacarpal. 6 A second line through the longitudinal
axis of the scaphoid intersects this first line, creating a scapholunate
angle of 30 to 60 degrees. A scapholunate angle of less than 30 degrees
or more than 60 degrees suggests carpal instability, which can be classified
as (1) dorsiflexion instability, or dorsal intercalated segment instability,
in which the lunate is dorsiflexed and displaced in a palmar direction
and the scaphoid is displaced vertically, or (2) palmar flexion instability,
or volar intercalated segment instability, in which the lunate is flexed
in a palmar direction. On the lateral view, a line drawn tangentially along
the distal articular surface of the radius intersects a second line through
the midshaft of the radius with an average angle of 86 degrees (79 to 94
degrees).
Radial and ulnar deviation
and flexion and extension of the wrist cause changes in the alignment of
the carpal bones. In radial deviation, palmar flexion of the proximal carpal
bones is noted as the distal end of the scaphoid rotates into the palm.
In ulnar deviation, the scaphoid is seen in full profile, the lunate and
triquetrum become more closely apposed, and the pisiform becomes intimate
with the tip of the ulnar styloid. Dorsiflexion of the wrist is particularly
prominent at the capitate-lunate space, and palmar flexion is pronounced
at the lunate-radial space. During wrist flexion, the pisiform tilts on
the triquetrum and moves in a volar direction for 2 to 3 mm. During wrist
extension, the pisiform slides distally and undergoes some rotation.
The complexity of wrist
motion has led to differing concepts of functional osseous anatomy. Some
regard the wrist as composed of carpal bones arranged in two rows (proximal
and distal) with the scaphoid bridging the two. Others describe a vertical
arrangement of the joint as consisting of three columns. A mobile lateral
column contains the scaphoid, trapezium, and trapezoid, in which osteoarthritis
is most frequent. The central column, containing the lunate and capitate,
is concerned with flexion and extension and is primarily implicated in
most varieties of carpal instability. The medial column is composed of
the triquetrum and hamate, and, on the axis of this column, the rotation
of the forearm is extended into the wrist. A third concept considers the
wrist as a dynamic ring, with a fixed distal half and a mobile proximal
half. Distortion or rupture of the mobile segment with respect to the rigid
part explains both instability and dislocation. |