ELBOW LIGAMENTS
As discussed previously, the elbow is
not just a simple hinge joint but instead possesses two degrees of freedom or
motion: flexion-extension and pronation-supination. However, with the primary
range of motion occurring in the flexion-extension direction, there are
reciprocally convex and concave articular surfaces along the distal humerus,
radius, and ulna; a capsule, loose on the sides toward which movement takes
place; strong collateral ligaments; and a grouping of muscle masses at the
borders where they are not in the direction of movement.
The articular surfaces are the
spool-shaped trochlea and the rounded capitellum of the humerus proximally and
the trochlear notch of the ulna and the cupped upper surface of the humeral
head of the radius distally. The capitellum of the humerus is directed forward
and downward, with the articular surfaces most completely in contact when the
elbow is flexed to a 90-degree angle. Contact is weak between the humerus and
the radius, and both the stability of the joint and its limitation of motion to
flexion and extension are due to the ridged and grooved relationship of the
humerus and the ulna. The articular capsule is weak in front and behind
but strengthened at the sides by the ulnar and radial collateral ligaments.
In front, it is attached on the humerus from the medial to the lateral
epicondyles along the superior borders of the coronoid and radial fossae.
Distally, it is attached to the anterior border of the coronoid process of the
ulna and to the annular ligament of the radius; it is continuous on either side
with the collateral ligaments. The posterior portion of the capsule is
membranous. Its attachments are the margins of the olecranon and the edges of
the olecranon fossa, the lateral epicondyle, the annular ligament, and the
posterior border of the radial notch of the ulna.
The collateral ligaments are
strong, triangular thickenings of the articular capsule, attached by their
apices to the medial and lateral epicondyles of the humerus. Their broader
distal attachments are to the forearm bones and the annular ligament of the
radius. These ligaments place strict limitations on side-to-side displacements
of the joint.
The ulnar collateral ligament has
thickened borders, with the anterior band reaching the medial edge of
the coronoid process and the posterior band attaching to the
corresponding edge of the olecranon. The thinner intermediate portion ends
below, in transverse fibers stretched between the coronoid
process and the olecranon.
The radial collateral ligament, a
narrower, less distinct thickening, is stretched between the underside of the
lateral epicondyle above and the annular ligament and the margins of the radial
notch of the ulna below.
The synovial membrane of the
elbow joint lines the capsule and is reflected onto the borders of the radial
and coronoid fossae of the humerus in front and the olecranon fossa behind.
Below, it continues into the proximal radioulnar articulation.
Movements
The hinge action at the elbow joint is
not exactly in the line of the long axis of the humerus. In extension, the
forearm deviates from a straight line with the arm, forming the carrying angle
of the forearm, which is obliterated when the hand is pronated. As discussed
previously, the carrying angle has been reported to range from 11 to 14 degrees
of valgus in men and from 13 to 16 degrees of valgus in women. Because of a
slight spiral orientation of the ridge of the trochlear notch and of the groove
of the trochlea, flexion does not bring the forearm bones medial to the
humerus. The habitual ease with which the hand is carried to the mouth in elbow
flexion is due to the slight medial rotation of the humerus and the
semipronated position of the hand.
Proximal Radioulnar Articulation
The head of the radius rotates in a
ring formed by the radial notch of the ulna and the annular ligament of the radius.
The annular ligament of the radius is a strong, curved band attaching to
the anterior and posterior margins of the radial notch of the ulna. It serves
as a restraining ligament, which prevents withdrawal of the head of the radius
from its socket. The annular ligament receives the radial collateral ligament
and blends with the capsule of the elbow joint. Below, a lax band, called the quadrate
ligament, passes from the lower border of the radial notch of the ulna to
the adjacent medial surface of the neck of the radius.
The synovial membrane of this
joint is continuous with that of the elbow joint. A reflection of the membrane
below the annular ligament forms a loose sac around the neck of the radius,
which accommodates to the rotation of the head of the radius.
