Week 10 - Elbow Joint, Anterior and Posterior Compartments of Arm

What separates the two compartments of the arm?

The humerus and two sheets of fascia called the medial and lateral intermuscular septums.

Where is the anterior compartment of the arm located?

Anterior to the humerus and intermuscular septums.

Where is the posterior compartment located relative to the intermuscular septums and humerus?

Posterior to these structures.

What structures are contained in the “posterior compartment” list in the first section?

• Brachial artery and its main branches

• 3 muscles: biceps brachii, coracobrachialis, brachialis

• 3 nerves: median nerve, ulnar nerve, musculocutaneous nerve

What type of muscle is biceps brachii and what joints does it cross?

A two-headed fusiform muscle; crosses both the shoulder and elbow joints.

Where is the biceps brachii located?

Most anterior and superficial muscle of the anterior compartment.

What is the origin of the long head of biceps brachii?

Supraglenoid tubercle of the scapula.

What path does the tendon of the long head take?

Passes through the superior aspect of the shoulder joint capsule → runs down the front of the humerus in the bicipital groove.

What is the origin of the short head?

Coracoid process of the scapula.

Where is the short head located relative to the long head?

More medially.

Where do the two heads of biceps merge, and where does the tendon insert?

Fuse about halfway down the arm; tendon inserts into the tuberosity of the radius.

What is the bicipital aponeurosis?

A flat sheet of connective tissue from the medial side of the tendon that fans out to blend with deep fascia covering the forearm.

What is the most effective position for biceps brachii to flex the elbow?

When the elbow is flexed and the forearm is supinated.

What stabilizing roles does the long head play at the shoulder?

Resists superior translation of humeral head during deltoid contraction.

What stabilizing role does the short head play at the shoulder?

Resists inferior dislocation of the shoulder.

Where is coracobrachialis located?

Medial to the short head of biceps brachii.

What is its origin and insertion?

Origin: coracoid process
Insertion: middle third of medial aspect of humerus

Does coracobrachialis cross the elbow?

No — only acts on the shoulder.

What are coracobrachialis actions?

• Assists shoulder flexion

• Helps horizontal adduction

Where is brachialis located?

Deepest muscle of the anterior compartment, underneath biceps brachii.

What is the origin and insertion of brachialis?

Origin: anterior aspect of distal half of the humerus
Insertion: tuberosity of the ulna

What joint does brachialis act on?

Only the elbow joint.

What is the sole action of brachialis?

Flexes the elbow joint.

How does the musculocutaneous nerve enter the anterior compartment?

By piercing the coracobrachialis muscle.

What muscles does the musculocutaneous nerve supply?

• Biceps brachii

• Brachialis

• Coracobrachialis

What nerve does musculocutaneous nerve become at the elbow and where?

Becomes the lateral cutaneous nerve of the forearm, emerging lateral to the biceps brachii just above the elbow.

What is musculocutaneous nerve course through the compartment?

Pierces coracobrachialis → descends under biceps brachii and over brachialis → emerges lateral to biceps.

How does the median nerve enter the arm?

At inferior margin of teres major.

What is Median Nerve course relative to the brachial artery?

At inferior margin of teres major.

What is Median Nerve course relative to the brachial artery?

Proximally: lateral to brachial artery
Distally: crosses to the medial side of the brachial artery

Does the median nerve supply muscles in the anterior compartment?

No.

What is the course of the ulnar nerve through the arm?

Travels down medial side → halfway down passes between medial intermuscular septum and medial head of triceps → passes behind elbow posterior to medial epicondyle.

What does the ulnar nerve supply

• Muscles of the forearm (mostly in the anterior forearm):

  • Flexor carpi ulnaris

  • Medial half of flexor digitorum profundus (to 4th and 5th fingers)

• Intrinsic muscles of the hand (all except those supplied by the median nerve):

  • Hypothenar muscles

  • Interossei

  • Medial two lumbricals

  • Adductor pollicis

  • Deep head of flexor pollicis brevis

In the arm itself:

• It travels through the medial arm but does not supply any muscles there. Its main role is as a pass-through nerve until it reaches the forearm..

What is the brachial artery a continuation of?

Axillary artery.

Brachial Artery location proximally vs distally?

Proximal: medial to coracobrachialis
Distal: more anterior, medial to biceps tendon

What are the 3 Branches of the Brachial Artery

• Profunda brachii artery

• Superior ulnar collateral artery

• Inferior ulnar collateral artery

What is the profunda brachii artery?

Largest branch of the brachial artery.

What is the profunda brachii artery origin?

Just distal to inferior border of teres major.

What does the profunda brachii artery supply?

Posterior compartment of the arm.

Superior & Inferior Ulnar Collateral Arteries contribute to?

The vascular network around the elbow.

Which muscles are located in the anterior compartment of the arm?

The muscles in the anterior compartment of the arm are the biceps brachii, brachialis, and coracobrachialis.

Which nerves travel through the anterior compartment of the arm?

The anterior compartment contains the musculocutaneous nerve, the median nerve, and the ulnar nerve (although only the musculocutaneous supplies the muscles).

Which nerve supplies all muscles of the anterior compartment of the arm?

The musculocutaneous nerve supplies every muscle in the anterior compartment: biceps brachii, brachialis, and coracobrachialis.

Which vessels run through the anterior compartment of the arm?

The vessels in the anterior compartment are the brachial artery and its branches: the profunda brachii artery, the superior ulnar collateral artery, and the inferior ulnar collateral artery.

Which structures are found in the posterior compartment of the arm?

The posterior compartment contains the triceps brachii, anconeus, the radial nerve, and the profunda brachii artery.

What are the three heads of the triceps brachii muscle?

The triceps brachii consists of the long head, lateral head, and medial head.

Where do the three heads of the triceps converge?

All three heads converge about the middle of the arm to form a common tendon.

Where does the triceps tendon insert?

The common tendon inserts on the proximal posterior aspect of the olecranon process of the ulna and into the deep fascia of the forearm.

How visible is the medial head of the triceps brachii?

The medial head is mostly covered by the long and lateral heads and is only visible distally.

What is the origin of the long head of triceps brachii?

The long head originates from the infraglenoid tubercle of the scapula, and its tendon blends with the inferior part of the glenohumeral joint capsule.

Why is the long head’s capsular attachment clinically important?

Because it reinforces the inferior glenohumeral joint capsule and helps prevent inferior dislocation of the humeral head

What is the origin of the lateral head of the triceps?

The lateral head originates from the posterior humerus below the greater tubercle, extending down to just above the radial groove.

What is the origin of the medial head of the triceps?

The medial head originates from the posterior humerus below the radial groove, near the trochlea region.

Where is the medial head located relative to the other heads?

The medial head lies deep to both the long and lateral heads.

What are the two layers of the triceps tendon?

The triceps tendon has a superficial layer (externally visible) and a deep layer (embedded within the tendon).

What is the main action of the triceps brachii?

Its main action is powerful extension of the elbow joint.

What additional actions does the long head of the triceps perform?

The long head assists with shoulder extension from a flexed position and shoulder adduction.

What functional role does the triceps brachii play in daily activities?

It controls elbow extension during movements like pushing up from a chair or during push-up-type activities.

What is the specific stabilizing function of the long head of the triceps?

It reinforces the inferior glenohumeral joint capsule and resists inferior humeral head dislocation.

Where does the anconeus originate and insert?

Origin: Lateral epicondyle of humerus
Insertion: Olecranon and adjacent posterior ulna

Why is the classification of anconeus debated?

It is debated because some sources see it as a detached part of the triceps (posterior arm), while others categorize it with the posterior forearm due to its role in ulna abduction during pronation

What actions does the anconeus perform?

It assists elbow extension and slightly abducts the ulna during pronation, helping keep the hand centered during forearm rotation.

Which muscles does the radial nerve supply in the arm?

The radial nerve supplies all muscles of the posterior compartment:

• Triceps brachii (long, lateral, medial heads)

• Anconeus

How does the radial nerve enter the posterior compartment of the arm?

It enters through the triangular interval, formed by the inferior border of teres major, the medial border of the humerus, and the lateral border of the long head of triceps.

What branch does the radial nerve give off before entering the triangular interval?

It gives off a slender muscular branch to the medial head of the triceps.

What is the course of the radial nerve in the posterior arm?

It passes through the triangular interval → travels laterally and deep to the long & lateral heads → runs in the radial groove → gives branches to long & lateral heads and a branch to anconeus.

Where does the radial nerve go near the distal arm?

It pierces the lateral intermuscular septum and enters the anterior compartment between the brachialis and brachioradialis.

How does the profunda brachii artery enter the posterior compartment?

It enters with the radial nerve through the triangular interval.

What is the course of the profunda brachii artery?

It spirals around the radial groove with the radial nerve and terminates as the middle and radial collateral arteries.

Which muscles belong to the posterior compartment of the arm?

The triceps brachii and the anconeus.

What is the primary action of the muscles in the posterior compartment?

Their primary action is extension of the elbow joint.

Which nerve supplies all muscles in the posterior compartment?

The radial nerve.

Which artery supplies the posterior compartment?

The profunda brachii artery (deep artery of the arm).

What are the three articulations of the elbow joint complex?

• Humeroulnar joint

• Humeroradial joint

• Proximal radioulnar joint
  (All three share a common fibrous joint capsule.)

What are the two articulations that make up the elbow joint itself?

• Humeroulnar articulation

• Humeroradial articulation

What type of joint is the elbow joint classified as?

Synovial hinge joint.

What bones form the humeroulnar articulation?

The trochlea of the distal humerus fits into the trochlear notch of the ulna; both covered in hyaline cartilage.

Describe the shape of the trochlea.

Hourglass-shaped with the medial end larger → contributes to the carrying angle.

What bones form the humeroradial articulation?

The spherical capitulum of the humerus articulates with the slightly concave head of the radius; both covered in hyaline cartilage.

What movements occur at the elbow joint?

• Extension: 0°

• Flexion: 140°

What movement does the proximal radioulnar joint allow?

Pronation and supination.

What type of joint is the proximal radioulnar joint?

Synovial pivot joint.

What bones form the proximal radioulnar joint?

Head of radius + radial notch of ulna (a hollowed-out concavity that accommodates the circular radial head).

How much of the radius head contacts the radial notch at any time?

Only ¼ of the head of the radius is in contact at a time.

What provides major bony stability for the elbow joint?

High bony congruence of the articular surfaces.

What is the carrying angle, and why does it occur?

• Angle between forearm and humerus in anatomical position

• Caused by differing diameters of trochlear surfaces

• Larger medial trochlea causes forearm abduction

• More pronounced in women due to wider pelvis

What does the fibrous joint capsule of the elbow joint complex enclose?

• Humeroulnar joint

• Humeroradial joint

• Proximal radioulnar joint

Describe the tightness of the joint capsule.

• Anterior and posterior portions are loose to allow flexion/extension

• Thicker medially and laterally due to collateral ligaments

What are the proximal attachments of the anterior capsule?

• Medial epicondyle of humerus

• Around distal humerus margins

• Encloses radial fossa + coronoid fossa

What are the distal attachments of the anterior capsule?

• Coronoid process of ulna

• Annular ligament of radius

What are the proximal attachments of the posterior capsule?

• Medial epicondyle

• Over posterior capitulum

• Covers most of olecranon fossa

What are the distal attachments of the posterior capsule?

• Superior + posterior surface of olecranon

• Annular ligament

What lines the fibrous capsule?

Extensive synovial membrane.

What does the synovial membrane cover?

• Radial fossa

• Coronoid fossa

• Olecranon fossa

• Coronoid process

• Head of radius

What is the sacciform recess?

A pocket formed around the head of the radius that allows radius rotation during supination/pronation.

What are the three fat pads of the elbow joint?

Fat pads in:

• Radial fossa

• Coronoid fossa

• Olecranon fossa

They move out of the way during flexion/extension

What ligaments reinforce the elbow joint capsule?

• Ulnar (medial) collateral ligament

• Radial (lateral) collateral ligament

Describe the ulnar (medial) collateral ligament.

Fan-shaped ligament running from medial epicondyle → coronoid process and olecranon.

What are the three bands of the ulnar collateral ligament?

• Anterior band: strongest; medial epicondyle → olecranon

• Posterior band: medial epicondyle → medial olecranon

• Oblique band: connects anterior & posterior bands between coronoid process and olecranon

What is the function of the ulnar collateral ligament?

Resists medial separation due to abduction (valgus stress).