The Elbow Complex

💡 Core Concept: The "Elbow Complex" is a compound joint system consisting of three articulations inside a single joint capsule: the Humeroulnar, Humeroradial, and Proximal Radioulnar joints. It links the shoulder to the hand, adjusting the functional length of the limb.

1. Structure of the Elbow Joint

A. Articular Surfaces

• Humerus:
  • Trochlea: Spool-shaped, medial. Articulates with the Ulna.
  • Capitulum: Hemispherical, lateral. Articulates with the Radius.
• Ulna: Trochlear Notch (Concave). Has the Coronoid Process (anterior) and Olecranon Process (posterior).
• Radius: Radial Head (Concave fovea superiorly).

[Image of distal humerus anatomy]

B. The Carrying Angle (Cubitus Valgus)

Because the medial lip of the Trochlea extends further distally than the lateral lip, the forearm naturally deviates laterally in extension.

• Normal: 10-15° Valgus (Greater in females).
• Cubitus Varus (Gunstock Deformity): Forearm deviates medially (Angle decreases).
• Function: Clears the hips during walking/swinging arms.

C. Ligaments (Stability)

Ligament	Components	Function
Medial Collateral (MCL) / Ulnar	Anterior, Posterior, Transverse Bundles	Resists Valgus Stress (Primary stabilizer). Anterior bundle is strongest.
Lateral Collateral (LCL) / Radial	Radial Collateral, Lateral Ulnar Collateral	Resists Varus Stress.
Annular Ligament	Wraps around Radial Head	Holds the radial head against the ulna; allows spinning.

2. Function: Flexion and Extension

A. Osteokinematics

• Axis: Passes through the centers of the Trochlea and Capitulum (Oblique axis).
• ROM: 0° to 145° (Active). Functional arc is 30-130°.

B. Arthrokinematics (Humeroulnar Joint)

• Surface: Concave Ulna moves on Convex Trochlea.
• Rule: Roll and Slide occur in the SAME direction.
• Flexion: Anterior Roll, Anterior Slide.
• Extension: Posterior Roll, Posterior Slide.

3. Radioulnar Joints: Pronation & Supination

Allows the hand to rotate without moving the shoulder.

A. Proximal Radioulnar Joint (PRUJ)

• Structure: Convex Radial Head spins inside the Concave Radial Notch of the Ulna (+ Annular Ligament).
• Motion: Spinning.

B. Distal Radioulnar Joint (DRUJ)

• Structure: Concave Ulnar Notch of Radius moves on Convex Ulnar Head.
• Motion: Concave on Convex (Roll/Slide Same).
• TFCC: Triangular Fibrocartilage Complex stabilizes the DRUJ.

⚠️ Nursemaid's Elbow (Pulled Elbow): Common in children. A sudden traction force on the forearm pulls the Radial Head out from under the Annular Ligament (inferior subluxation).

4. Muscles of the Elbow

A. Flexors

Muscle	Characteristics	Best Position
Brachialis	"Workhorse" of the elbow. Only pure flexor.	Active in all positions (Sup/Pron).
Biceps Brachii	Flexor + Powerful Supinator.	Most effective in Supination. Least effective in Pronation.
Brachioradialis	Stabilizer / Compressor.	Neutral rotation (Beer drinking muscle).

B. Extensors

• Triceps Brachii: Long head is bi-articular (crosses shoulder). Medial head is the first to recruit ("workhorse").
• Anconeus: Small stabilizer, pulls capsule out of the way during extension.

C. Law of Parsimony

The nervous system activates the fewest muscles possible for a task. For light extension, only the Anconeus/Medial Triceps fire. The Long Head is recruited only for heavy loads.

🏆 Golden Points: Clinical Biomechanics

• Elbow Dislocation: Usually posterior. The coronoid process is a key bony block to posterior instability.
• Ulnar Nerve: Runs in the cubital tunnel (medial). Susceptible to compression/traction injuries, especially in Cubitus Valgus.
• FOOSH (Fall On Outstretched Hand): Transmits force: Radius → Interosseous Membrane → Ulna → Humerus. This sharing of load protects the humeroradial joint.

📝 20 High-Yield MCQs

Test your knowledge for Exams.

Q1. Which bundle of the Medial Collateral Ligament (MCL) is the primary restraint to Valgus stress?

A. Anterior Bundle B. Posterior Bundle C. Transverse Bundle D. Oblique Bundle

Rationale: The Anterior bundle is mechanically the strongest and tightest throughout the ROM, specifically resisting valgus force.

Q2. The "Carrying Angle" is formed primarily by:

A. The Olecranon Process B. The distal projection of the Medial Trochlea C. The Radial Head D. The Capitulum

Rationale: The trochlea is not horizontal; the medial lip extends further down, pushing the ulna laterally when extended (Valgus).

Q3. During Open Kinetic Chain flexion of the elbow, the Ulna:

A. Rolls Anteriorly and Slides Posteriorly B. Rolls Anteriorly and Slides Anteriorly C. Rolls Posteriorly and Slides Anteriorly D. Spins only

Rationale: Concave Ulna on Convex Trochlea = Roll and Slide in the SAME direction (Anteriorly for flexion).

Q4. Which ligament encircles the Radial Head and allows it to spin?

A. Quadrate Ligament B. Annular Ligament C. Oblique Cord D. Radial Collateral Ligament

Rationale: The Annular ligament forms a sling around the radial head, holding it against the ulna while allowing rotation (pronation/supination).

Q5. The "Workhorse" of elbow flexion, active irrespective of forearm position, is the:

A. Biceps Brachii B. Brachialis C. Brachioradialis D. Pronator Teres

Rationale: The Brachialis inserts on the Ulna, so its function is unaffected by pronation/supination of the Radius. It works in all positions.

Q6. Biceps Brachii is most effective as a flexor when the forearm is:

A. Supinated B. Pronated C. Neutral D. Extended

Rationale: Since the Biceps inserts on the Radial Tuberosity, pronation wraps the tendon under the radius, putting it at a mechanical disadvantage. Supination aligns it for maximum pull.

Q7. A "Nursemaid's Elbow" involves subluxation of the:

A. Ulna from the Humerus B. Radial Head from the Annular Ligament C. Distal Radioulnar Joint D. Olecranon from the Fossa

Rationale: Traction on a child's arm pulls the radial head inferiorly, slipping it out of the annular ligament sling.

Q8. Which structure acts as a "dynamic splint" and force transmitter between the radius and ulna?

A. Annular Ligament B. Interosseous Membrane C. Oblique Cord D. TFCC

Rationale: The Interosseous Membrane transfers force from the Radius (which takes load from the wrist) to the Ulna (which transfers load to the Humerus).

Q9. At the Distal Radioulnar Joint (DRUJ), the arthrokinematics during Pronation involve:

A. Radius rolling and sliding Anteriorly (Medially) B. Radius rolling Anteriorly and sliding Posteriorly C. Ulna rotating around Radius D. Pure Spin

Rationale: At the DRUJ, the Concave Radius moves on the Convex Ulna. Roll and Slide are in the same direction (Anterior/Medial for Pronation).

Q10. The closed-packed position of the Humeroulnar joint is:

A. Full Extension B. 90 degrees Flexion C. Full Flexion D. 70 degrees Flexion

Rationale: Full extension locks the Olecranon process into the Olecranon fossa, providing maximal bony stability.

Q11. Which head of the Triceps is considered the "Workhorse" (recruited first)?

A. Long Head B. Lateral Head C. Medial Head D. Anconeus only

Rationale: The Medial Head is deep and active in most extension movements. The Long Head is recruited for high resistance.

Q12. Functional range of motion for most daily activities at the elbow is:

A. 0-145 degrees B. 30-130 degrees C. 90-100 degrees D. 0-90 degrees

Rationale: This 100-degree arc (30-130) allows reaching the mouth and perineal care.

Q13. The "Gunstock Deformity" refers to:

A. Excessive Cubitus Valgus B. Cubitus Varus C. Hyperextension D. Flexion Contracture

Rationale: Malunion of a supracondylar fracture can cause the forearm to deviate medially (Varus), resembling a rifle stock.

Q14. Which muscle prevents the capsule from being pinched during elbow extension?

A. Anconeus B. Triceps Long Head C. Supinator D. Brachioradialis

Rationale: The Anconeus pulls the posterior capsule tight and out of the way of the olecranon process during extension.

Q15. To stretch the long head of the Triceps, you must position the shoulder in:

A. Extension B. Flexion C. Abduction D. Internal Rotation

Rationale: The long head crosses the shoulder joint posteriorly. Flexing the shoulder (arm overhead) and flexing the elbow lengthens it maximally.

Q16. The Radial Head articulates with which structure on the Humerus?

A. Trochlea B. Capitulum C. Olecranon Fossa D. Coronoid Fossa

Rationale: Cap on Head. The Capitulum ("Little Head") articulates with the Radial Head.

Q17. Which nerve passes through the Cubital Tunnel?

A. Median Nerve B. Radial Nerve C. Ulnar Nerve D. Musculocutaneous Nerve

Rationale: The Ulnar nerve runs posterior to the medial epicondyle in the cubital tunnel ("Funny Bone").

Q18. What is the primary function of the Triangular Fibrocartilage Complex (TFCC)?

A. Stabilize the Proximal Radioulnar Joint B. Stabilize the Distal Radioulnar Joint (DRUJ) C. Flex the wrist D. Extend the elbow

Rationale: The TFCC is the major stabilizer of the distal forearm, connecting the radius and ulna at the wrist.

Q19. If the Musculocutaneous nerve is severed, which elbow function is most severely weakened?

A. Flexion B. Extension C. Pronation D. Adduction

Rationale: The Musculocutaneous nerve innervates the Biceps Brachii and Brachialis, the two primary flexors.

Q20. Pronator Teres is also capable of assisting in:

A. Elbow Extension B. Elbow Flexion C. Wrist Extension D. Supination

Rationale: Because it crosses anterior to the elbow joint axis, the Pronator Teres acts as a weak elbow flexor, especially when resistance is high.