STRUCTURE AND FUNCTION OF THE ELBOW AND FOREARM
The distal end of the humerus has two articular surfaces: the
trochlea, which articulates with the ulna, and the capitulum, which articulates
with the head of the radius. Flexion and extension occur between these two
joint surfaces. The radius also articulates with the radial notch on the ulna
and is called the proximal radioulnar joint. This joint participates in
pronation and supination along with the distal radioulnar joint. The capsule of
the elbow encloses the humeroulnar, humeroradial, and proximal radioulnar articulations.
The distal radioulnar joint is structurally separate from the elbow complex
even though its function is directly related to the proximal radioulnar joint.
JOINTS OF THE ELBOW AND FOREARM
There are four joints involved in elbow and forearm function: the
humeroulnar, humeroradial, proximal radioulnar, and distal radial ulnar joints.
Elbow Joint Characteristics and Arthrokinematics
The elbow is a compound joint with a lax joint capsule, supported
by two major ligaments—the medial (ulnar) and lateral (radial) collateral—which
provide medial and lateral stability, respectively.
Humeroulnar Articulation
Characteristics. The humeroulnar (HU) articulation is a
modified hinge joint. The medially placed hourglassshaped trochlea at the distal
end of the humerus is convex. It faces anteriorly and downward 45_ from the shaft of the humerus. The concave trochlear fossa, on
the proximal ulna, faces upward and anteriorly 45_ from the ulna. The primary
motion at this articulation is flexion and extension.
Arthrokinematics. During flexion/extension the concave fossa
slides in the same direction in which the ulna moves, so with elbow flexion the
fossa slides around the trochlea in an anterior and distal direction. With
elbow extension, the fossa slides in a posterior and proximal direction. There
is also slight medial and lateral sliding of the ulna, allowing for full elbow
range of motion (ROM); it results in a valgus angulation of the joint with
elbow extension and a varus angulation with elbow flexion. When the
bone moves in a medial/lateral direction, the trochlearridge
provides a convex surface, and the trochlear groove provides a concave surface—so
with varus the ulna slides in a lateral direction and with valgus the ulna
slides in a medial direction.
Humeroradial Articulation
Characteristics. The humeroradial (HR) articulation is a hinge-pivot
joint. The laterally placed, spherical capitulum at the distal end of the
humerus is convex. The concave bony partner, the head of the radius, is at the
proximal end of the radius. Flexion/extension and pronation/supination occur at
this articulation.
Arthrokinematics. As the elbow flexes and extends, the concave
radial head slides in the same direction as the bone motion so with elbow flexion
the concave head slides anteriorly and with elbow extension it slides
posteriorly. With pronation and supination of the forearm, the radial head
spins on the capitulum.
Ligaments of the Elbow
Medial (ulnar) collateral ligament.
The medial collateral ligament
complex consists of bundles of fibers that may be differentiated into anterior,
posterior, and transverse portions. Various portions of the ligament are taut
in different ROMs, providing medial support to the elbow against valgus stresses
and limiting end-range elbow extension. The ligament also keeps the joint
surfaces in approximation. Activities such as throwing and golfing impose
significant stresses to the medial collateral ligament complex.
Lateral (radial) collateral
ligament. The lateral collateral ligament
complex, a fan-shaped ligament on the lateral surface of the elbow, is composed
of the lateral collateral ligament, the lateral ulnar collateral ligament, and
the annular ligament. This complex provides stability to the lateral
aspect of the elbow against varus forces and prevents posterior translation of
the radial head.
MUSCLE FUNCTION AT THE ELBOW AND FOREARM
Primary Actions at the Elbow and Forearm
Elbow Flexion
Brachialis. The brachialis is a one-joint muscle that inserts
close to the axis of motion on the ulna, so it
is unaffected by the position of the forearm or the shoulder; it
participates in all flexion activities of
the elbow.
Biceps brachii. The biceps is a two-joint muscle that crosses
both the shoulder and elbow and inserts close to the axis of motion on the
radius, so it also acts as a supinator of the forearm. It functions most
effectively as a flexor of the elbow between 80_ and 100_ of flexion. For the optimal length–tension relationship, the shoulder
extends to lengthen the muscle when it contracts forcefully for elbow and
forearm function.
Brachioradialis. With its insertion a great distance from the
elbow on the distal radius, the brachioradialis mainly functions to provide
stability to the joint. However, it also participates as the speed of flexion
motion increases and a load is applied with the forearm from mid-supination to full
pronation.
Elbow Extension
Triceps brachii. The long head of the triceps brachii crosses
both the shoulder and elbow; the other two heads are uniaxial. The long head
functions most effectively as an elbow extensor if the shoulder simultaneously
flexes. This maintains an optimal length–tension relationship in the muscle.
Anconeus. The anconeus muscle stabilizes the elbow during
supination and pronation and assists in elbow extension.
Forearm Supination
Supinator. The proximal attachment of the supinator at the
annular and lateral collateral ligaments may function to stabilize the lateral
aspect of the elbow. Its effectiveness as a supinator is not affected by the
elbow position as is the biceps brachii.
Biceps brachii. The biceps muscle acts as a supinator if the elbow
simultaneously flexes or if resistance is given to supination when the elbow is
in extension.10
Brachioradialis. The brachioradialis contributes to pronation and
supination only as an accessory muscle when resistance is provided to the
motion.10 It cannot function alone as a rotator or
stabilizer of the forearm joints when other forearm muscles are paralyzed.
Forearm Pronation
Pronator teres. The pronator muscle pronates as well as stabilizes
the proximal radioulnar joint and helps approximate the humeroradial
articulation.
Pronator quadratus. The pronator quadratus is a one-joint muscle
and is active during all pronation activities.
0 Comments