Elbow Biomechanics and Osteology

Elbow Biomechanics

• The elbow joint is composed of the ulnohumeral, radiocapitellar, and proximal radioulnar joints.
• It plays a crucial role in daily activities, acting as a lever arm for positioning the hand and as a fulcrum for forearm movement.
• Functional range of motion (ROM) for flexion/extension is 0-150 degrees, and 50 degrees for supination/pronation.
• Normal carrying angle ranges from 5-10 degrees in males and 10-15 degrees in females. This carrying angle decreases with flexion.
• In an extended elbow, 40% of weight is through the ulnohumeral joint, and 60% is through the radiohumeral joint.

Osteology & Arthrology

• The humerus shaft has a spiral groove posteriorly, containing the radial nerve, roughly 13 cm proximal to the trochlea's articular surface.
• The distal humerus flare includes the medial and lateral epicondyles, contributing to half of the elbow joint's structure.
• The trochlea is spool-shaped and medially located, while the capitellum is laterally positioned.
• The sublime tubercle on the ulna is a site where the anterior bundle of the medial ulnar collateral ligament attaches distally
• The distal humerus comprises medial and lateral columns.

Arthrology - Axis and Alignment

• The joint surface of the elbow is anteriorly tilted approximately 30 degrees relative to the humerus shaft.
• The elbow exhibits a 6-degree valgus rotation.
• The axis of rotation is centrally located at the trochlea and capitellum, passing through the anteroinferior medial epicondyle.

Joint Type

• The radiohumeral articulation is a pivot joint.
• The radial head is covered by cartilage for approximately 240 degrees. The lateral 120 degrees lacks cartilage.
• The ulnohumeral articulation is a hinge joint.
• The coronoid fossa on the distal humerus receives the coronoid tip during deeper flexion.
• The coronoid tip acts as a buttress, preventing posterior dislocations.
• The elbow capsule is maximally distended at 70-80 degrees of flexion.
• The distal anterior capsule attachment is 6 mm distal to the coronoid tip.
• The coronoid is within the joint cavity.

Muscles of the Elbow

• Flexors: Biceps (radial tuberosity attachment), Brachialis (11mm distal to coronoid attachment), Brachioradialis
• Extensors: Triceps

Ligaments & Stability of Elbow

• The ulnohumeral joint (coronoid) is a primary static stabilizer. Loss of 50% or more of coronoid height can lead to instability.
• The medial collateral ligament (MCL) is composed of anterior, posterior, and transverse bundles, offering resistance to valgus and distracting forces.
• The MCL's origin is the anteroinferior aspect of the medial epicondyle, and insertion is the sublime tubercle and medial coronoid process.
• The anterior bundle of the MCL is crucial for resisting valgus stresses, while the radial head is another significant stabilizer.
• The posterior bundle forms the cubital tunnel floor, and is the primary restraint during maximal flexion.

Lateral Collateral Ligament Complex (LCL)

• The LCL comprises the radial collateral ligament (RCL) and the lateral ulnar collateral ligament (LUCL).
• It's a primary restraint for varus and external stress during full elbow motion.
• Origin is the posterior lateral epicondyle.
• Insertion is the radial collateral ligament on the supinator crest of the proximal ulna.
• The accessory collateral ligament contributes to lateral elbow stability.
• The annular ligament stabilizes the proximal radioulnar joint.

Secondary Static Stabilizers

• The radiocapitellar joint functions as a constraint for valgus stress.
• The radial head contributes approximately 30% of valgus stability.
• The capsule's most crucial role is at 0-30 degrees of flexion and pronation.
• Dynamic stabilizers include muscles crossing the elbow joint, such as anconeus, brachialis, triceps, and biceps. These muscles provide compressive stability during movement.

Nerves of the Elbow

• Musculocutaneous nerve: originates from the lateral cord of the brachial plexus and exits laterally, distal to the biceps tendon. It innervates the biceps and brachialis.
• Radial nerve: originates from the posterior cord of the brachial plexus and runs between the brachialis and brachioradialis. This nerve is found superficially to the joint capsule, located at the radiocapitellar joint. Runs through the spiral groove.
• Median nerve: originates from the medial/lateral cord of the brachial plexus and runs with the brachial artery from lateral to medial. It is superficial to the brachialis muscle and provides innervation to elbow.
• Ulnar nerve: originates from the medial cord of the brachial plexus. It passes medial to the brachial artery, pierces the medial intermuscular, and enters the posterior compartment. It then passes posterior to the medial epicondyle, through the cubital tunnel. Provides innervation to the elbow.

Blood Supply of Elbow

• The brachial artery is centrally located in the upper arm, entering the cubital fossa laterally.
• It splits into the radial and ulnar arteries at the elbow level.
• Key branches include the superior and inferior ulnar collateral and nutrient/muscular branches, and the supratrochlear.

Kinematics

• Flexion/extension axis: center of trochlea.
• Pronation/supination axis: capitellum through radial head to distal ulna.
• Large joint reaction forces due to short lever arms contribute to elbow degenerative changes.

Free Body Diagram

• Free body diagrams demonstrate elbow inefficiencies, using sum M = 0, and equations like 5B = 15W (B=3W).
• Static loads are close to body weight.
• Dynamic loads exceed body weight.

Arthrodesis

• Optimal unilateral arthrodesis: 90° flexion, 0-7° valgus.
• Bilateral arthrodesis: one elbow 110° flexion (feeding), one elbow 65° flexion (perineal hygiene).