Radioulnar Joints Anatomy
The radius and ulna articulate at
the proximal and distal radioulnar joints, which are synovial. At the proximal
joint, the head of the radius articulates with the radial notch of the ulna
(Fig. 3.88). The head and neck are encircled by the anular ligament, which
attaches to the anterior and posterior margins of the notch on the ulna (Fig.
3.92) and blends with
the capsule and radial collateral ligament of the elbow. Thus
the cavity of the proximal joint is continuous with that of the elbow.
The distal radioulnar joint occurs
between the head of the ulna and the ulnar notch of the radius (Fig.
3.93). An articular
disc (a triangular cartilage) attaches the ulnar styloid process to the distal
end of the radius and separates the cavity of the distal joint from that of the
wrist.
The anular ligament of the proximal
joint and the articular disc of the distal joint prevent separation of the
radius and ulna, yet allow the radius to rotate freely. In addition, the shafts
of the bones are joined by the interosseous membrane, whose fibres incline
downwards and medially from the interosseous border of the radius to that of
the ulna. The membrane may be regarded as a fibrous radioulnar joint.
The
movements of supination and pronation occur through approximately 180° at the
radioulnar joints. During pronation the radius rotates across the ulna and
twists the forearm and hand so that the palm faces posteriorly. Supination
returns the limb to the anatomical position. The axis of the movement passes
through the head of the radius and the styloid process of the ulna. Supination
is the more powerful movement and is produced by biceps and supinator, although
biceps is ineffective when the elbow is fully extended. Pronation is produced
by pronator teres and pronator quadratus. Also, when the elbow is flexed,
brachioradialis rotates the forearm and returns the limb to the midposition
from the extremes of supination or pronation. The head of the radius can be
felt rotating about 2 cm distal to the lateral epicondyle during these
movements.
In a fall onto the hand, the
interosseous membrane may transmit force from the radius to the ulna,
protecting the radial head from compression against the capitulum of the
humerus. However, since the membrane does not oppose distraction, a sudden tug
on the hand may dislocate the radial head downwards from within the anular
ligament of the proximal radioulnar joint.
