Elbow and Forearm; structure and function

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.

Elbow and Forearm


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.

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