Anatomy

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Chapter 2

Anatomy

Franklin D. Shuler and Matthew R. Schmitz

Contents

section 1 Introduction

I OVERVIEW

A Osteology: The human skeleton has 206 bones: axial skeleton (80) and appendicular skeleton (126)

1. Ossification:

Intramembranous (direct laying down of bone without a cartilage model [skull]) or enchondral (with a cartilage precursor [most bones]).

Enchondral growth begins in the diaphyses of long bones at primary ossification centers, most of which are present at birth (Table 2-1).

Table 2-1

Summary of Ossification Patterns

2. Secondary ossification centers usually develop in the periphery of bones and are important for growth and the treatment of childhood fractures.

3. Heterotopic ossification is the formation of bone tissue in an atypical, extraskeletal location.

4. Anatomic landmarks of the skeleton and their related structures are listed in Table 2-2.

Table 2-2

Skeletal Grooves, Notches, and Points

B Arthrology: Joints are commonly classified into three types on the basis of their freedom of movement

1. Synarthroses: joining of two bony elements with no motion during maturity; skull sutures

2. Amphiarthroses: have hyaline cartilage and intervening discs with limited motion; symphysis pubis

3. Diarthroses: characterized by hyaline cartilage, synovial membranes, capsules, and ligaments

C Myology: classification based on the arrangement of muscle fibers

1. Parallel (e.g., rhomboids)

2. Fusiform (e.g., biceps brachii)

3. Oblique (with tendinous interdigitation): further classified as pennate, bipennate, multipennate

4. Triangular (e.g., pectoralis minor)

5. Spiral (e.g., latissimus dorsi)

D Nerves

1. Peripheral nerves

Originate from the ventral rami of spinal nerves and are distributed via several plexuses (cervical, brachial, lumbosacral)

The mnemonic “SAME” can be used to help understand the function of nerves: sensory = afferent; motor = efferent.

Efferent (motor) fibers carry impulses from the central nervous system to muscles.

Afferent (sensory) fibers carry information toward the central nervous system.

2. Autonomic nerves

Control visceral structures

Consist of the parasympathetic (craniosacral) and sympathetic (thoracolumbar) divisions

Preganglionic neurons of parasympathetic nerves:

Arise in the nuclei of cranial nerves III, VII, IX, and X and in the S2, S3, and S4 segments of the spinal cord

Synapse in peripheral ganglia

Preganglionic neurons in the sympathetic system

Located in the spinal cord (T1 to L3)

Synapse in chain ganglia adjacent to the spine and collateral ganglia along major abdominal blood vessels

E Vessels: arteries, veins, and lymphatic vessels

1. Of primary concern is avoiding major injury to these structures.

2. Courses and relationships are important and are highlighted in this chapter.

section 2 Upper Extremity

Table 2-3 summarizes upper extremity innervation. Table 2-4 summarizes standard surgical approaches to the upper extremity.

Table 2-3

Summary of Upper Extremity Innervation

Nerves	Muscles Innervated
Musculocutaneous (lateral cord)	Coracobrachialis, biceps, brachialis
Axillary (posterior cord)	Deltoid, teres minor
Radial (posterior cord)	Triceps, brachioradialis, extensor carpi radialis longus and brevis
Posterior interosseous	Supinator, extensor carpi ulnaris, extensor digitorum, extensor digiti minimi, abductor pollicis longus, extensor pollicis longus and brevis, extensor indicis proprius
Median (medial and lateral cord)	Pronator teres, flexor carpi radialis, palmaris longus, flexor digitorum superficialis, abductor pollicis brevis, supinator head of flexor pollicis brevis, opponens pollicis, first and second lumbrical muscles
Anterior interosseous	Flexor digitorum profundus (first and second), flexor pollicis longus, pronator quadratus
Ulnar (medial cord)	Flexor carpi ulnaris, flexor digitorum profundus (third and fourth), palmaris brevis, abductor digiti minimi, opponens digiti minimi, flexor digiti minimi, third and fourth lumbrical muscles, interossei, adductor pollicis, deep head of flexor pollicis brevis

Table 2-4

Standard Orthopaedic Surgical Approaches to the Upper Extremity

PIN, posterior interosseous nerve.

I SHOULDER

A Osteology

1. Scapula

Spans the second through seventh ribs and serves as an attachment for 17 muscles and four ligaments

Glenoid is retroverted approximately 5 degrees.

Scapular spine: separates supraspinatus from infraspinatus.

Coracoid: Attachments to the coracoid include the coracoacromial ligament, coracoclavicular ligaments (conoid and trapezoid [lateral]), conjoined tendon (coracobrachialis and short head of biceps), and pectoralis minor.

Acromion

Suprascapular notch has the superior transverse scapular ligament separating the suprascapular artery (superior) from the suprascapular nerve (inferior).

Spinoglenoid notch has both the artery and nerve inferior to the inferior transverse scapular ligament; long-term nerve compression at the spinoglenoid notch (i.e., ganglion; assume labral disease) results in infraspinatus atrophy.

2. Clavicle

It is the fulcrum for lateral movement of the arm.

It has a double curvature (sternal-ventral, acromial-dorsal) and serves as an attachment for the upper extremity.

The clavicle is the first bone in the body to ossify (at 5 weeks of gestation) and the last to fuse (medial epiphysis at 25 years of age; see Table 2-1). Fracture of the clavicle is the most common musculoskeletal birth injury.

B Arthrology: one major articulation (glenohumeral joint) and several minor articulations (sternoclavicular, acromioclavicular, scapulothoracic joints)

1. Glenohumeral joint (Figure 2-1): Spheroidal, ball and socket, with the greatest joint range of motion; motion is at the expense of stability with static and dynamic restraints.

Figure 2-1 Glenohumeral ligaments and rotator cuff muscles. The rotator interval is between the anterior border of the supraspinatus and the superior border of the subscapularis. This interval helps limit flexion and external rotation of the shoulder. Within the rotator interval is the superior glenohumeral ligament, the primary restraint both in inferior translation of the adducted shoulder and in external rotation of the adducted or slightly abducted arm. The middle glenohumeral ligament, absent in up to 30% of shoulders, is the primary stabilizer in anterior translation, with the arm slightly abducted (45 degrees). The inferior glenohumeral ligament complex is the primary stabilizer for anterior and inferior instability in abduction. The inferior glenohumeral ligament complex is composed of the anterior and posterior bands of the inferior glenohumeral ligament. (From Miller MD, et al: Orthopaedic surgical approaches, Philadelphia, 2008, Saunders, Figure SA-4.)

Static restraints include the articular anatomy, glenoid labrum, negative pressure, capsule, and ligaments.

Dynamic restraints include the rotator cuff and biceps tendon, and scapulothoracic motion is restrained.

Important glenohumeral stabilizers summarized in Table 2-5

Table 2-5

Glenohumeral Stabilizers

Structure	Function
Coracohumeral ligament	Primary restraint in inferior translation of the adducted arm and to external rotation
Glenoid labrum	Increases surface area, static stabilizer
Superior glenohumeral ligament	Primary restraint in external rotation of the adducted or slightly abducted arm Primary restraint in inferior translation of the adducted arm
Middle glenohumeral ligament (absent up to 30% of shoulders)	Primary stabilizer in anterior translation, with the arm abducted to 45 degrees
Inferior glenohumeral ligament complex	Primary stabilizer for anterior and inferior translation in abduction

2. Sternoclavicular joint:

This joint is double-gliding, with an articular disc.

Ligaments include the anterior and posterior sternoclavicular ligaments, an interclavicular ligament, and a costoclavicular ligament.

The sternoclavicular joint rotates 30 degrees with shoulder motion.

3. Acromioclavicular joint:

Plane/gliding joint with a fibrocartilaginous disc

Ligaments (Figure 2-2):

Figure 2-2 Ligaments about the shoulder. The acromioclavicular ligaments (superior, inferior, anterior, and posterior) prevent anteroposterior translation of the distal clavicle. The superior ligament is the most important and is reinforced by fibers from the trapezius and deltoid muscles. The coracoclavicular ligaments—conoid (posteromedial) and trapezoid (anterolateral)—prevent superior translation of the distal clavicle. The coracoacromial ligament should be preserved in massive rotator cuff defects because it provides superior restraint to the humeral head. Bleeding encountered during release of the coracoacromial ligament comes from the acromial branch of the thoracoacromial artery (second part of axillary artery; see Figure 2-6). (Adapted from Jenkins DB: Hollinshead’s functional anatomy of the limbs and back, ed 6, Philadelphia, 1991, Saunders, p 71.)

Acromioclavicular ligament: prevents anteroposterior displacement

Coracoclavicular ligaments: prevent superior displacement of the distal clavicle (trapezoid [anterolateral] and conoid [posteromedial and stronger])

When the arm is maximally elevated, about 5 to 8 degrees of rotation is possible at the acromioclavicular joint, although the clavicle rotates approximately 40 to 50 degrees.

4. Scapulothoracic joint:

Though not a true joint, this attachment allows scapular movement against the posterior rib cage.

Fixed primarily by the scapular muscular attachments.

Glenohumeral motion in comparison with scapulothoracic motion is in a 2:1 ratio.

5. Intrinsic ligaments of the scapula:

Superior transverse scapular ligament (which separates the suprascapular nerve [inferior] and vessels [superior] at the suprascapular notch; mnemonic: “Army over Navy” for artery over nerve)

Inferior transverse scapular ligament (spinoglenoid notch)

Coracoacromial ligament

Frequent cause of impingement.

The coracoacromial ligament is important for superoanterior restraint in rotator cuff deficiencies and should be preserved during débridement of painful massive rotator cuff tears that cannot be surgically repaired.

The acromial branch of the thoracoacromial artery runs on the medial aspect of the coracoacromial ligament.

C Muscles (Figure 2-3)

Figure 2-3 Origins and insertions of the muscles about the shoulder and upper arms (A, B) and shoulder girdle (C, D). A, Anterior view. B, Posterior view. C, Anterior view. D, Posterior view. (From Jenkins DB: Hollinshead’s functional anatomy of the limbs and back, ed 6, Philadelphia, 1991, Saunders, Figure 5-3.)

1. Muscles connecting the upper limb to the vertebral column: trapezius, latissimus, both rhomboid muscles, and levator scapulae

2. Muscles connecting the upper limb to the thoracic wall: both pectoralis muscles, subclavius, and serratus anterior

3. Muscles acting on the shoulder joint itself: deltoid, teres major, and the four rotator cuff muscles (supraspinatus, infraspinatus, teres minor, subscapularis)

The rotator cuff muscles depress and stabilize the humeral head against the glenoid; all attach to the greater tuberosity except the subscapularis, which has a lesser tuberosity insertion (shoulder internal rotator).

The shoulder internal rotators (pectoralis major, latissimus dorsi, teres major, and subscapularis) are stronger than the external rotators (teres minor and infraspinatus), which is why posterior shoulder dislocations are more common than anterior dislocations after electrical shock and seizures.

Table 2-6 presents the specific characteristics of these muscles, and Figure 2-4 and Table 2-7 describe the four layers of shoulder musculature.

Table 2-6

Muscles of the Shoulder

GT, greater tuberosity; ITG, intertubercular groove; SP, spinous process.

Table 2-7

Shoulder-Supporting Anatomic Layers

Layer	Structures
I	Deltoid; pectoralis major; trapezius
II	Clavipectoral fascia; conjoined tendon, short head of biceps, and coracobrachialis
III	Deep layer of subdeltoid bursa; rotator cuff muscles (supraspinatus, infraspinatus, teres minor, subscapularis [SITS])
IV	Glenohumeral joint capsule; coracohumeral ligament

Figure 2-4 Anterior aspect of the right shoulder, depicting the four layers (circled Roman numerals). In this illustration, the lateral retractor is placed deep to layer II, demonstrating the ease of dissection in the plane of the bursa around the lateral aspect of the proximal humerus. The subscapularis and supraspinatus (layer III) have been reflected, disclosing layer IV (capsule and coracohumeral ligament). The usual shape and position of the defect in the rotator interval (if present) are depicted and show variability. 3, Deltoid; 5, pectoralis major; 6, biceps; 9, coracoacromial ligament; 10, fasciae (layer II); 11, deep layer of subdeltoid bursa; 12, conjoined tendon; 14, tip of coracoid process; 17, biceps brachii long head; 18, deep layer of subdeltoid bursa and subscapularis; 19, coracohumeral ligament; 20, supraspinatus under deep bursa layer; 23, joint capsule; 24, hiatus in capsule. (From Cooper DE, et al: Supporting layers of glenohumeral joint, Clin Orthop 289:151, 1993.)

D Nerves

1. Anatomy of brachial plexus (Figure 2-5)

Figure 2-5 Brachial plexus. There are four preclavicular/supraclavicular branches: the long thoracic nerve (serratus anterior muscle), dorsal scapular nerve (rhomboid muscle), suprascapular nerve (supraspinatus and infraspinatus muscles), and nerve to the subclavius. (From Miller MD, et al: Orthopaedic surgical approaches, Philadelphia, 2008, Saunders, Figure SA-7.)

The brachial plexus is formed from the ventral primary rami of C5 to T1 and lies under the clavicle between the scalenus anterior and scalenus medius.

Dorsal rami of C5 to T1 innervate the dorsal neck musculature and skin.

Brachial plexus consists of roots, trunks, divisions, cords, and branches (mnemonic: “Ron Taylor drinks cold beer”).

Five roots (C5 to T1, although contributions from C4 and T2 can be small)

Three trunks (upper, middle, lower)

Six divisions (two from each trunk)

Three cords (named because of their anatomic relationship to the axillary artery: posterior, lateral, and medial); the termination of each cord is shown in Table 2-8

Table 2-8

Brachial Plexus Cord Terminations

Cord	Termination
Lateral	Musculocutaneous nerve* Lateral pectoral nerve
Posterior	Radial and axillary nerve* Upper and lower subscapular nerve Thoracodorsal nerve
Medial	Ulnar nerve* Medial pectoral nerve Medial brachial cutaneous nerve Medial antebrachial cutaneous nerve
Medial and lateral	Median nerve*

^*Major branches.

Multiple branches: four preclavicular branches (from roots and upper trunk):

Dorsal scapular nerve

Long thoracic nerve

Suprascapular nerve

Nerve to the subclavius

2. Muscle innervation: innervation of all rotator cuff muscles derived from C5 and C6 of the brachial plexus (Table 2-9; see also Table 2-3)

Table 2-9

Rotator Cuff Muscle Innervation: All C5 and C6 Muscle Innervation

3. Brachial plexus injury

Preganglionic brachial plexus lesions

Proximal to the dorsal root ganglion

Produce medial scapular winging (because of paralysis of the preclavicular long thoracic nerve with resultant serratus anterior dysfunction) and Horner’s syndrome (injury to brachial plexus at C8 to T1 involving the inferior/stellate ganglion)

Postganglionic brachial plexus injuries

Do not produce Horner’s syndrome, a winged scapula, diaphragmatic paralysis, or rhomboid paralysis

Obstetric brachial plexus palsy (Table 2-10)

Table 2-10

Obstetric Brachial Plexus Palsies

Injury to the spinal accessory nerve (cranial nerve XI)

Causes trapezius dysfunction and scapular-trapezius winging (lateral winging)

Results in shoulder depression with scapular translation laterally and the inferior angle rotated laterally because of the unopposed pull of the serratus anterior

Injury to the long thoracic nerve (C5 to C7)

Causes serratus anterior dysfunction and medial scapular winging

Results in superior elevation with scapular translation medially and the inferior angle rotated medially

E Vessels

1. Subclavian artery

The left subclavian artery arises directly from the aorta, and the right subclavian artery arises from the brachiocephalic trunk.

It then emerges between the scalenus anterior and medius muscles and becomes the axillary artery at the outer border of the first rib.

2. Axillary artery (Table 2-11, Figure 2-6)

Table 2-11

Axillary Artery Branches

Figure 2-6 Branches of the axillary artery. At the point where the subclavian artery passes beneath the clavicle, it becomes the axillary artery. The axillary artery is divided into three sections on the basis of their relationship to the pectoralis minor muscle. The first part of the axillary artery is medial to the pectoralis minor muscle and supplies the supreme thoracic artery. The second part is beneath the muscle and supplies the thoracoacromial artery and the lateral thoracic artery. The third part is lateral to the muscle and supplies the subscapular artery and the posterior and anterior humeral circumflex arteries. The third part of the axillary artery, at the origin of the anterior and posterior humeral circumflex arteries, is the part most vulnerable to traumatic vascular injury. (From Miller MD, et al: Orthopaedic surgical approaches, Philadelphia, 2008, Saunders, Figure SA-9.)

This artery is conceptualized as divided into three parts on the basis of its physical relationship to the pectoralis minor muscle (the first part is medial to it, the second is under it, and the third is lateral to it).

Each part of the artery has as many branches as the number of that portion (e.g., the second part has two branches: thoracoacromial and lateral thoracic).

The third part of the axillary artery, at the origin of the anterior and posterior humeral circumflex arteries, is the most vulnerable to traumatic vascular injury.

F Surgical approaches to the shoulder (Table 2-12; see Table 2-4):

Table 2-12

Surgical Approaches to the Shoulder

Approach	Interval	Structures at Risk
Anterior (Henry’s)	Deltoid (axillary nerve) and pectoralis major (medial and lateral pectoral nerve)	Axillary nerve limits inferior exposure; place arm in adduction and external rotation Musculocutaneous nerve: avoid vigorous retraction and medial dissection to the conjoined tendon/coracobrachialis
Lateral	Deltoid splitting (axillary nerve)	Avoid deltoid split >5 cm below acromion, to avoid damaging axillary nerve
Posterior	Infraspinatus (suprascapular nerve) and teres minor (axillary nerve)	Dissection inferior to the teres minor puts quadrangular space structures at risk: axillary nerve and posterior humeral circumflex artery Avoid excessive medial retraction on infraspinatus, which can injure suprascapular nerve

1. Anterior (Henry’s) approach (Figure 2-7)

Figure 2-7 Anterior (Henry’s) surgical approach to the shoulder. In this approach, the interval between the deltoid (axillary nerve) and the pectoralis major (medial and lateral pectoral nerves) is explored. To prevent injury to the musculocutaneous nerve, avoid excessive medial retraction (see medial retractor) on the coracobrachialis or avoid dissection medial to this muscle. Also avoid the axillary nerve, which is inferior to the shoulder capsule. Positioning the arm in adduction and external rotation helps displace the axillary nerve from the surgical field. (From Miller MD, et al: Orthopaedic surgical approaches, Philadelphia, 2008, Saunders, Figure SA-12.)

Interval: deltoid (axillary nerve) and the pectoralis major (medial and lateral pectoral nerves)

Dissection:

Dissect the cephalic vein, and retract it laterally with the deltoid, thereby exposing the underlying subscapularis.

Then divide the subscapularis (preserving the most inferior fibers in order to protect the axillary nerve); then the shoulder capsule is visualized.

A leash of three vessels (one artery and the superior and inferior venae comitantes) marks the lower border of the subscapularis.

Risks:

Musculocutaneous nerve (lateral cord brachial plexus)

Protect by avoiding vigorous retraction of the conjoined tendon and avoiding dissection medial to the coracobrachialis.

This nerve usually penetrates the biceps/coracobrachialis 5 to 8 cm below the coracoid, but it enters these muscles proximal to this 5-cm “safe zone” in almost 30% of shoulders.

Palsy of the musculocutaneous nerve would affect the coracobrachialis, biceps brachii, and brachialis muscles, and it would affect sensation in the lateral antebrachial cutaneous nerve (termination of the musculocutaneous nerve).

Axillary nerve

Just inferior to the shoulder capsule

Protected during procedures in this area with adduction and external rotation of the arm

2. Lateral approach

Interval: none; this is a deltoid-splitting approach

Dissection:

Either split the deltoid muscle or subperiosteally dissect it from the acromion.

The supraspinatus tendon is exposed, which allows for repairs of the rotator cuff.

Risks:

Deltoid should not be split more than 5 cm below the acromion, in order to avoid injury to the axillary nerve (posterior cord brachial plexus exiting from quadrangular space).

If dissection extends more than 5 cm inferiorly, denervation of the deltoid can occur in its anterior portion.

3. Posterior approach (Figure 2-8)

Figure 2-8 Posterior approach to the shoulder. A, Superficial exposure. B, Deep exposure. In this approach, the interval between the infraspinatus (suprascapular nerve) and the teres minor (axillary nerve) is explored. Do not dissect below the teres minor. Dissections below the teres minor place the structures in the quadrangular space at risk for injury: the posterior humeral circumflex artery and the axillary nerve. Also, to prevent suprascapular nerve palsy, avoid excessive retraction of the infraspinatus. (From Miller MD, et al: Orthopaedic surgical approaches, Philadelphia, 2008, Saunders, Figures SA-24 and SA-25.)

Interval: infraspinatus (suprascapular nerve) and teres minor (axillary nerve)

Dissection: Split the posterior deltoid, thereby exposing the interval between the infraspinatus and teres minor; the posterior capsule lies immediately beneath it.

Risks: Both the axillary nerve and the posterior circumflex humeral artery run in the quadrangular space below the teres minor, so it is important to stay above this muscle. Excessive medial retraction of the infraspinatus can injure the suprascapular nerve.

G Arthroscopy (discussed in Chapter 4, Sports Medicine)

1. Anterior-superior portal: musculocutaneous nerve at risk for injury

2. Anterior-inferior portal: must be above the subscapularis and lateral to the conjoined tendon

3. Posterior portal (inferior portals may put the axillary nerve at risk for injury)

II ARM

A Osteology: humerus

1. Humeral head:

Articulates with the smaller scapular glenoid cavity

Retroverted 30 degrees (in relation to the transepicondylar axis of the humerus)

Scapular glenoid: retroverted 5 degrees

2. Anatomic neck, directly below the humeral head, serves as an attachment for the shoulder capsule.

3. Surgical neck is lower and is more often involved in fractures.

4. Greater tuberosity is lateral to the humeral head.

Serves as the attachment for the supraspinatus, infraspinatus, and teres minor muscles (anterior to posterior, respectively)

5. Lesser tuberosity, located anteriorly, has only one muscular insertion: the last rotator cuff muscle, the subscapularis.

6. Bicipital groove (for the tendon of the long head of the biceps brachii) is a bony groove between the two tuberosities.

7. Humeral shaft has a posterior spiral groove (for the radial nerve) adjacent to the deltoid tuberosity and approximately 13 cm above the articular surface of the trochlea.

8. Distally, the humerus flares into medial and lateral epicondyles.

Forms half of the elbow joint

Medial spool-shaped trochlea (which articulates with the olecranon of the ulna)

Lateral globular capitellum (which opposes the radial head)

Normal articular alignment of the distal humerus has a 7-degree valgus tilt (carrying angle)

B Arthrology

1. Joints

Elbow is composed of a hinge joint (the humeroulnar articulation) and a pivot joint (the humeroradial articulation) (Table 2-13).

Table 2-13

Elbow Joint Articulations

Articulation	Components
Humeroulnar	Trochlea and trochlear notch
Humeroradial	Capitulum and radial head
Proximal radioulnar	Radial notch and radial head

Axis of rotation for the elbow is centered through the trochlea and capitellum and passes through a point anteroinferior on the medial epicondyle.

Elbow joint has capsuloligamentous tissues (Figure 2-9) that are a key source of testable material.

Figure 2-9 Elbow ligaments. A, Medial view. The most important portion of the medial collateral ligament is the anterior bundle or ulnar collateral ligament. B, Lateral view. The most important portion of the lateral collateral ligament complex is the lateral ulnar collateral ligament (LUCL) or radial collateral ligament. A deficiency of the LUCL results in posterolateral rotatory instability. (From Jenkins DB: Hollinshead’s functional anatomy of the limbs and back, ed 6, Philadelphia, 1991, Saunders, p 108.)

Capsule allows maximum distension at approximately 70 to 80 degrees of flexion, which is why patients with effusion hold their arms in this position, which is most comfortable.

Anterior capsule attaches at a point approximately 6 mm distal to the tip of the coronoid.

Coronoid tip is an intraarticular structure that is visualized during elbow arthroscopy.

2. Ligaments (Table 2-14)

Table 2-14

Elbow Ligaments

Ligament	Components	Comments
Medial collateral	Anterior bundle of MCL (ulnar collateral); posterior bundle; transverse bundle (Cooper ligament)	Anterior bundle (strongest of all elbow ligaments): anterior band taut from 60 degrees of flexion to full extension, posterior band taut from 60-120 degrees of flexion
Lateral collateral	LUCL; annular ligament; quadrate (annular ligament to radial neck) and oblique cord	Deficiency of LUCL results in posterolateral rotator instability

LUCL, lateral ulnar collateral ligament; MCL, medial collateral ligament.

The medial collateral ligament (MCL) (anterior, posterior, and transverse bundles) arises from the anteroinferior portion of the medial humeral epicondyle and provides stability in valgus stress.

The anterior bundle is the most important in helping resist valgus forces and attaches 18 mm distal to the coronoid tip.

Valgus stability with the arm in pronation suggests that the anterior bundle of the MCL is intact.

The lateral or radial collateral ligament (annular, radial, and ulnar parts) originates on the lateral humeral epicondyle near the axis of elbow rotation.

The lateral ulnar collateral ligament (LUCL) is an essential elbow stabilizer and runs from the lateral epicondyle to the ulna crista supinatoris (supinator crest).

A deficiency of the LUCL is manifested as posterolateral rotatory instability of the elbow (see Table 2-14).

C Muscles: four muscles of the arm controlling elbow motion (Table 2-15)

Table 2-15

Muscles of the Arm

1. Flexors (biceps, brachialis, and brachioradialis); the brachialis attaches to the coronoid at 11 mm distal to the tip

2. Extensors (triceps); also helps form borders for three important spaces (Figure 2-10 and Table 2-16)

Table 2-16

Shoulder Spaces and Intervals

Figure 2-10 Borders of key spaces and intervals, posterior view: quadrangular space (containing the axillary nerve and posterior humeral circumflex artery), triangular space (circumflex scapular vessels), and triangular interval (radial nerve and profunda brachii artery). (From Kaplan EB: Surgical approaches to the neck, cervical spine, and upper extremity, Philadelphia, 1966, Saunders, p 53.)

Triangular space: bordered by teres minor (superiorly), teres major (inferiorly), and long head of biceps brachii (laterally)

Contains the circumflex scapular vessels

Quadrangular space: bordered by the teres minor (superiorly) and teres major (inferiorly); medial border formed by the long head of the triceps, and lateral border formed by the humerus

Transmits the posterior humeral circumflex vessels and the axillary nerve

Triangular interval: immediately inferior to the quadrangular space and bordered by the teres major (superiorly), long head of the triceps (medially), and lateral head of the triceps or the humerus (laterally)

Through this interval, the profunda brachii artery and radial nerve can be seen

D Nerves

1. Anatomy

Four major nerves traverse the arm; two give off branches to arm musculature, and two innervate the distal musculature (Figure 2-11). Most of the cutaneous innervation of the arm arises directly from the brachial plexus.

Figure 2-11 Principal nerves (A) and arteries (B) of the upper extremity. FCR, flexor carpi radialis; FCU, flexor carpi ulnaris; FDP, flexor digitorum profundus; FDS, flexor digitorum superficialis; PT, pronator teres. (From Miller MD, et al: Orthopaedic surgical approaches, Philadelphia, 2008, Saunders, Figures SA-8 and EF-12.)

Musculocutaneous nerve (lateral cord):

Pierces the coracobrachialis 5 to 8 cm distal to the coracoid

Branches to supply the coracobrachialis, the biceps, and the brachialis

Gives off a branch to the elbow joint before it becomes the lateral antebrachial cutaneous nerve of the forearm, which is located deep to the cephalic vein

Radial nerve (posterior cord):

Spirals around the humerus (medial to lateral) in the spiral groove at a distance of approximately 13 cm from the trochlea

Emerges on the lateral side of the arm after piercing the lateral intermuscular septum approximately 7.5 cm above the trochlea between the brachialis and brachioradialis anterior to the lateral epicondyle (where it supplies the anconeus muscle)

Median nerve (medial and lateral cords):

Accompanies the brachial artery along the arm, crossing it during its course (lateral to medial)

Supplies some branches to the elbow joint but has no branches in the arm itself

Ulnar nerve (medial cord):

Passes medial to the brachial artery in the arm and then runs behind the medial epicondyle of the humerus, where it is superficial

Also has branches to the elbow but none to the arm

Cutaneous nerves:

Supraclavicular nerve (C3 and C4) supplies the upper shoulder.

Axillary nerve supplies the shoulder joint and the overlying skin.

Medial, lateral, and dorsal brachial cutaneous nerves supply the balance of the cutaneous innervation of the arm.

Lateral antebrachial cutaneous nerve is the termination of the musculocutaneous nerve (Figure 2-12 summarizes the dermatome patterns).

Figure 2-12 Dermatome patterns.

2. Compressive neuropathies (Table 2-17)

Table 2-17

Nerve Compression Syndromes of the Arm and Forearm

Syndrome	Nerve Involved	Sites of Compression
Pronator	Median	Supracondylar process of humerus and ligament of Struthers Lacertus fibrosis (bicipital aponeurosis) Pronator teres Arch of flexor digitorum superficialis
AIN	AIN of median	Deep head of pronator teres Flexor digitorum superficialis Aberrant vessels Accessory muscles (i.e., Gantzer’s muscles)
Cubital tunnel	Ulnar	Arcade of Struthers Medial intermuscular septum Medial epicondyle Cubital tunnel Proximal edge of flexor carpi ulnaris (Osborne fascia) Deep flexor pronator aponeurosis
PIN Radial tunnel	PIN of radial	Fibrous bands Recurrent leash of Henry Extensor carpi radialis brevis Arcade of Frohse (proximal edge of superficial head of supinator) Supinator distal margin
Superficial radial nerve	Superficial radial	Between the brachioradialis and extensor carpi radialis longus

AIN, anterior interosseous nerve; PIN, posterior interosseous nerve.

3. Muscle innervation (see Table 2-3)

E Vessels

1. Brachial artery

Originates at the lower border of the tendon of the teres major and continues to the elbow, where it bifurcates into the radial and ulnar arteries (see Figure 2-11)

Lies medial in the arm, curving laterally to enter the cubital fossa

Cubital fossa: formed by the distal humerus proximally, the brachioradialis laterally, and the pronator teres medially

2. Principal branches

Deep brachial (also known as the profunda, this artery accompanies the radial nerve posteriorly in the triangular interval)

Superior and inferior ulnar collateral arteries

The nutrient and muscular branches

The supratrochlear artery (the least flexible branch)

These collateral vessels can bind up the brachial artery with distal humerus fractures

F Surgical approaches to the humerus. (Table 2-18)

Table 2-18

Surgical Approaches to the Humerus

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Approach	Interval	Structures at Risk
Anterolateral—proximal	Proximal—deltoid (axillary nerve) and pectoralis major (medial and lateral pectoral nerve) Distal—brachialis (radial and musculocutaneous nerve)	Radial nerve; axillary nerve; anterior humeral circumflex artery
Posterior	Triceps (radial nerve); lateral and long heads	Radial nerve; deep brachial artery