Structure and Function of the Elbow and Forearm Complex

Objectives

• Identify the primary bones and bony features relevant to the elbow and forearm complex.

• Describe the supporting structures of the elbow and forearm complex.

• Describe the structure and function of the four main joints within the elbow and forearm complex.

• Cite the normal range of motion for elbow flexion and extension and for forearm supination and pronation.

• Describe the planes of motion and axes of rotation for the joints of the elbow and forearm complex.

• Cite the proximal and distal attachments and innervation of the muscles of the elbow and forearm complex.

• Justify the primary actions of the muscles of the elbow and forearm complex.

• Cite innervation of the muscles of the elbow and forearm complex.

• Explain the primary muscular interactions involved in performing a pushing and pulling motion.

• Explain the primary muscular interactions involved in tightening a screw with a screwdriver.

Key Terms

actively efficient

actively insufficient

Colles’ fracture

cubitus varus

end feel

excessive cubitus valgus

valgus

varus

The ability to actively flex and extend the elbow is essential for many important functions such as those involved with feeding, grooming, reaching, throwing, and pushing. The elbow itself actually consists of two separate articulations: the humeroulnar and the humeroradial joint (Figure 5-1). The forearm complex allows the movements of pronation and supination—motions that rotate the palm upward (supination) or downward (pronation). Similar to the elbow, the forearm consists of two articulations: the proximal and distal radioulnar joint (Figure 5-1). The interaction among the four joints of the elbow and forearm enables the hand to be placed in a nearly infinite number of positions, greatly enhancing the functional potential of the entire upper extremity.

Figure 5-1 Articulations of the elbow and forearm complex. (From Neumann DA: Kinesiology of the musculoskeletal system: foundations for physical rehabilitation, ed 2, St Louis, 2010, Mosby, Figure 6-1.)

Joints of the Elbow and Forearm Complex

• Humeroulnar joint

• Humeroradial joint

• Proximal radioulnar joint

• Distal radioulnar joint

Osteology

The four bones that relate to the function of the elbow and forearm complex include the (1) scapula, (2) distal humerus, (3) ulna, and (4) radius.

Scapula

The scapula has three bony features that are important to the muscles of the elbow. The coracoid process serves as the proximal attachment for the short head of the biceps. The supraglenoid tubercle serves as the proximal attachment for the long head of the biceps. The infraglenoid tubercle marks the proximal attachment for the long head of the triceps. These bony landmarks were reviewed in the previous chapter (see Figure 4-4).

The trochlea is a spool-shaped structure located on the medial side of the distal humerus (Figures 5-2 and 5-3) that articulates with the ulna to form the humeroulnar joint. The coronoid fossa is a small pit located just superior to the trochlea that accepts the coronoid process of the ulna when the elbow is fully flexed. Just lateral to the trochlea is the ball-shaped capitulum, which articulates with the head of the radius to form the humeroradial joint.

Figure 5-2 The anterior aspect of the right humerus. Proximal attachments of muscles are shown in red. The *dotted line* represents the capsular attachments of the elbow. (From Neumann DA: Kinesiology of the musculoskeletal system: foundations for physical rehabilitation, ed 2, St Louis, 2010, Mosby, Figure 6-2.)

The medial epicondyle is the prominent projection of bone on the medial side of the distal humerus. This easily palpable prominence serves as the proximal attachment for most of the wrist flexor muscles, the pronator teres, and the medial collateral ligament of the elbow. The lateral epicondyle is less prominent; however, it is the proximal attachment for most of the wrist extensor muscles, the supinator muscle, and the lateral collateral ligament of the elbow. Immediately proximal to both epicondyles are the medial and lateral supracondylar ridges.

The olecranon fossa is the relatively deep, broad pit located on the posterior side of the distal humerus. With the elbow fully extended, a portion of the olecranon process projects into this fossa.

Consider this…

The “Funny Bone”

“Hitting your funny bone” technically means hitting your ulnar nerve. The ulnar nerve travels through a groove between the olecranon process and the medial epicondyle. When this area is bumped into a table edge, for example, the nerve is compressed between the table edge and its bony surroundings, sending tingling and numbness down the area of skin supplied by the nerve, specifically on the medial forearm and the fourth and fifth digits (ring finger and small finger).

Ulna

The ulna (Figures 5-4 and 5-5) has a thick proximal end with distinct processes. The olecranon process is the large, blunt, proximal tip of the ulna commonly referred to as the elbow bone. The rough posterior surface of the olecranon process is the distal attachment for the triceps muscles.

The trochlear notch is the large, jaw-like curvature of the proximal ulna that articulates with the trochlea (of the humerus), forming the humeroulnar joint (Figure 5-6). The inferior tip of the trochlear notch comes to a point, forming the coronoid process. The coronoid process strengthens the articulation of the humeroulnar joint by firmly grabbing the trochlea of the humerus. Slightly inferior and lateral to the trochlear notch is the radial notch, which articulates with the head of the radius to form the proximal radioulnar joint.

Located distally, the styloid process is a pointed projection of bone that arises from the ulnar head. Both of these structures can be palpated on the ulnar side of the dorsum of the wrist, with the forearm fully pronated.

Radius

In a fully supinated position, the radius lies parallel and lateral to the ulna (see Figures 5-4 and 5-5). The radial head is shaped like a wide disc on the proximal end of the radius. The superior surface of the radial head consists of a shallow, cup-shaped depression called the fovea that articulates with the capitulum of the humerus, forming the humeroradial joint.

The bicipital tuberosity, sometimes called the radial tuberosity, is an enlarged ridge of bone located on the anterior-medial aspect of the proximal radius. The bicipital tuberosity is so named because it is the primary distal attachment for the biceps brachii.

The distal end of the radius is wide and flat with two notable structures: the styloid process and the ulnar notch. The styloid process is the pointed (and easily palpable) projection of bone off the distal lateral radius. The ulnar notch is a small depression on the medial side of the distal radius that articulates with the ulnar head, forming the distal radioulnar joint.

Arthrology of the Elbow

General Features

As was mentioned in the previous section, the elbow joint is composed of two articulations: the humeroulnar joint and the humeroradial joint. The humeroulnar joint provides most of the structural stability to the elbow as a whole. This stability is provided primarily by the jaw-like trochlear notch of the ulna interlocking with the spool-shaped trochlea of the humerus (Figure 5-6). This hinge-like joint limits the motion of the elbow to flexion and extension.

The humeroradial joint is formed by the ball-shaped capitulum of the humerus articulating with the bowl-shaped fovea of the radius (Figure 5-7). This configuration permits continuous contact between the radial head and the capitulum during supination and pronation, as the radius spins about its own axis; and during flexion and extension, as the radial head rolls and slides over the rounded capitulum. Compared with the humeroulnar joint, the humeroradial joint provides only secondary stability to the elbow.

Figure 5-7 Anterior view of the right elbow disarticulated to expose the features of the humeroulnar and humeroradial joints. The synovial membrane lining the internal side of the capsule is shown in *blue*. (From Neumann DA: Kinesiology of the musculoskeletal system: foundations for physical rehabilitation, ed 2, St Louis, 2010, Mosby, Figure 6-11.)

With the forearm supinated and the elbow fully extended, it should be evident that the forearm projects laterally about 15 to 20 degrees relative to the humerus. This natural outward angulation of the forearm within the frontal plane is called normal cubitus valgus (Figure 5-8); valgus literally means to “bend outward.” The natural cubitus valgus orientation is also called the carrying angle because of its apparent function of keeping a carried object away from the body. Trauma to the elbow can alter the normal valgus angle, resulting in excessive cubitus valgus (Figure 5-8, B) or cubitus varus (Figure 5-8, C).

Figure 5-8 A, Normal cubitus valgus of the elbow. The radius and the ulna deviate 15 degrees from the longitudinal axis of the humerus. The red line represents the medial-lateral axis of rotation of the elbow. B, Excessive cubitus valgus. C, Cubitus varus. (From Neumann DA: Kinesiology of the musculoskeletal system: foundations for physical rehabilitation, ed 2, St Louis, 2010, Mosby, Figure 6-9.)

Clinical insight

Position of Comfort—A Double-Edged Sword

Patients with a painful and inflamed elbow often hold their arm in about 70 to 90 degrees of elbow flexion. This so-called position of comfort reduces intracapsular pressure and reduces pain in inflamed tissues. Although the flexed position improves comfort, extended periods of time in this flexed position significantly increase the chance of an elbow flexion contracture.

The primary function of the collateral ligaments is to limit excessive varus and valgus deformations of the elbow. The medial collateral ligament is most often injured during attempts to catch oneself from a fall (Figure 5-10). Because these ligaments also become taut at the extremes of flexion and extension, the extremes of these sagittal plane motions—if sufficiently forceful—can damage the collateral ligaments.

Figure 5-10 Attempts at catching oneself from a fall may induce a severe valgus-producing force that over-stretches or ruptures the medial collateral ligament. (From Neumann DA: Kinesiology of the musculoskeletal system: foundations for physical rehabilitation, ed 2, St Louis, 2010, Mosby, Figure 6-13.)

Clinical insight

Tommy John Surgery

Tommy John surgery refers to a surgical reconstruction of the medial (ulnar) collateral ligament of the elbow. This surgery is commonly performed on the elbow of throwing athletes, most often baseball pitchers who have over-stretched or torn this ligament. In fact, this surgery is named after the major league pitcher, Tommy John, who is considered the first person to have undergone this surgery, in 1974. Remarkably, after an 18-month rehabilitation program, Tommy John returned to pitching and won another 164 games before he retired at the age of 46.

Throwing athletes are highly susceptible to this injury because a vigorous over-head throwing motion places a large valgus stress on the elbow that, over multiple exposures, can cause laxity or tearing of the joint’s medial collateral ligament. Although surgical techniques differ, the goal is to replace the injured ligament with a stronger substitute tissue. Typically, the donor tissue is the patient’s palmaris longus tendon (autograft), which is woven within holes drilled into the medial epicondyle of the humerus and the proximal medial ulna. The holes are drilled carefully in locations that are similar to the ligament’s natural attachments. It is interesting to note that the palmaris longus tendon is often used because it is thin, relatively strong, and, in most persons, functionally insignificant. This surgery is often so successful that many baseball pitchers have reported that their pitch velocity increased after surgery (and full rehabilitation)—even compared with their “pre-injury” pitch velocity.

Supporting Structures of the Elbow Joint

The following structures are illustrated in Figure 5-9:

Figure 5-9 An anterior view of the right elbow showing the capsule and collateral ligaments. (From Neumann DA: Kinesiology of the musculoskeletal system: foundations for physical rehabilitation, ed 2, St Louis, 2010, Mosby, Figure 6-10.)

• Articular Capsule: A thin, expansive band of connective tissue that encloses three different articulations: the humeroulnar joint, the humeroradial joint, and the proximal radioulnar joint

• Medial Collateral Ligament: Contains fibers that attach proximally to the medial epicondyle and distally to the medial aspects of the coronoid and olecranon processes; provide stability primarily by resisting cubitus valgus–producing forces. This ligament is also called the ulnar collateral ligament.

• Lateral Collateral Ligament: Originates on the lateral epicondyle and ultimately attaches to the lateral aspect of the proximal forearm. These fibers provide stability to the elbow by resisting cubitus varus–producing forces. This ligament is also referred to as the radial collateral ligament.

Kinematics

From the anatomic position, elbow flexion and extension occur in the sagittal plane about a medial-lateral axis of rotation, which courses through both epicondyles. The range of motion at the elbow normally spans from 5 degrees beyond extension to 145 degrees of flexion (Figure 5-11). Most typical activities of daily living, however, use a more limited 100-degree arc of motion, between 30 and 130 degrees of flexion. Excessive extension is normally limited by the bony articulation between the olecranon and the olecranon fossa.

Figure 5-11 Normal range of motion at the elbow allows an arc of motion from 5 degrees of hyperextension to 145 degrees of flexion. The red area signifies the “functional arc” from 30 to 130 degrees of flexion. (Modified from Morrey BF, Bryan RS, Dobyns JH, et al: Total elbow arthroplasty: a five-year experience at the Mayo Clinic, J Bone Joint Surg Am 63[7]:1050–1063, 1981.)

The elbow can be flexed and extended while the forearm is free, as when performing a biceps curl, or fixed, as when performing a push-up. Although both open- and closed-chained functions are important, unless stated otherwise, this chapter describes open-chain motions. In either case, restricted mobility of the elbow can greatly decrease a person’s functional abilities.

Consider this…

Assessing a Joint’s End Feel

Clinicians must be able to describe the feel of a joint as it reaches its maximal range of motion. The term end feel has evolved for this purpose. Compare the end feel of full elbow extension versus full elbow flexion. Full extension results in an abrupt stop or bony end feel as the olecranon runs into the bony floor of the olecranon fossa. Full flexion, in contrast, results in a springy or soft end feel because of the soft tissue approximation of the forearm with the elbow flexor muscles and other soft tissues.

Clinicians with an awareness of the normal end feel of a joint can better determine the reason for the joint’s lack of motion (or excessive motion) and therefore can implement more effective treatments to address the underlying problem.

Arthrology of the Forearm

General Features

The forearm is composed of the proximal and distal radioulnar joints (see Figure 5-1). As the names imply, these joints are located at the proximal and distal ends of the forearm. Pronation and supination occur as a result of motion at each of these two joints. As is shown in Figure 5-12, A, in full supination, the radius and the ulna lie parallel to one another. However, in full pronation, the radius crosses over the ulna (Figure 5-12, B). As is emphasized in subsequent sections of this chapter, pronation and supination involve the radius rotating around a relatively fixed ulna. Although pronation and supination are typically used to describe motions or positions of the hand, these motions occur at the forearm. However, it is useful to observe this motion by noting the position of the hand relative to the humerus. The firm articulation between the distal radius and the carpal bones (at the wrist) requires that the hand follow the rotation of the radius; the ulna typically remains relatively stationary because of its firm attachment at the humeroulnar joint.

Figure 5-12 Anterior view of the right forearm. A, In full supination, the radius *(orange)* and the ulna are parallel. B, In full pronation, the radius is crossed over the ulna. The *dotted line* signifies the axis of rotation that extends from the radial head to the ulnar head. Note how the hand follows the radius. (From Neumann DA: Kinesiology of the musculoskeletal system: foundations for physical rehabilitation, ed 2, St Louis, 2010, Mosby, Figure 6-23.)

Supporting Structures of the Proximal and Distal Radioulnar Joints

• Annular Ligament: A thick circular band of connective tissue that wraps around the radial head and attaches to either side of the radial notch of the ulna (see Figures 5-9 and 5-13). This ring-like structure holds the radial head firmly against the ulna, allowing it to spin freely during supination and pronation.

Figure 5-13 The right proximal radioulnar joint as viewed from above. Note how the radius is held against the radial notch of the ulna by the annular ligament. (From Neumann DA: Kinesiology of the musculoskeletal system: foundations for physical rehabilitation, ed 2, St Louis, 2010, Mosby, Figure 6-24.)

• Distal Radioulnar Joint Capsule: Reinforced by palmar and dorsal capsular ligaments, this structure provides stability to the distal radioulnar joint.

• Interosseous Membrane (see Figure 5-5): Helps bind the radius to the ulna; serves as a site for muscular attachments, and as a mechanism to transmit forces proximally through the forearm

Kinematics

Supination occurs in many functional activities that require the palm to be turned up, such as feeding, washing the face, or holding a bowl of soup. Pronation, in contrast, is involved with activities such as grabbing an object from a table or pushing up from a chair, which require the palm to be turned down.

Supination and pronation occur as the radius rotates around an axis of rotation that travels from the radial head to the ulnar head (see Figure 5-12). The 0-degree or neutral position of the forearm is the thumb-up position (Figure 5-14). From this position, 85 degrees of supination and 75 degrees of pronation normally occur. People who lack full range of motion of these movements often compensate by internally or externally rotating the shoulder, so clinicians must be aware of this possible substitution when testing the range of motion of the forearm.