Elbow Biomechanics and Osteology

Elbow Biomechanics

• The elbow joint consists of the ulnohumeral, radiocapitellar, and proximal radioulnar joints.
• The elbow is crucial for daily living activities, acting as a lever arm for hand positioning and a fulcrum for forearm leverage.
• Functional range of motion (ROM) for flexion/extension is 0-150 degrees.
• Supination/pronation is 50 degrees.
• Normal valgus carrying angle is 5-10 degrees for males and 10-15 degrees for females, diminishing with flexion during axial loading.
• 40% of weight is transmitted through the ulnohumeral joint, and 60% through the radiohumeral joint in the extended elbow position.

Osteology & Arthrology

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

Arthrology

• The joint surface of the elbow is anteriorly tilted approximately 30 degrees relative to the humerus shaft.
• The elbow displays 6 degrees of valgus and 5 degrees of internal rotation.
• The axis of rotation is centred 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 lack cartilage, crucial for radial head fracture internal fixation.
• The ulnohumeral articulation is a hinge joint.
• The coronoid fossa on the distal humerus receives the coronoid process in deeper flexion.
• The coronoid tip acts as a buttress to prevent posterior dislocations.
• The elbow capsule is maximally distended at 70-80 degrees of flexion.
• The distal attachment of the anterior capsule is located 6 mm distal to the coronoid tip.
• The coronoid is an intra-articular structure.

Muscles of the Elbow

• Flexors: Biceps (distal attachment at the radial tuberosity), brachialis (11 mm distal to the coronoid tip).
• Extensors: Triceps.
• Other: Brachioradialis.

Ligaments & Stability of Elbow

• The ulnohumeral joint (coronoid) is a primary static stabilizer.
  • Loss of 50% or more of coronoid height may cause elbow instability.
• The medial collateral ligament (MCL) comprises anterior, posterior, and transverse bundles; it resists valgus and distractive stress.
• An important component of the MCL is the anterior bundle.
• The posterior bundle acts as the cubital tunnel floor, limiting flexion to an extent.
• Radial collateral ligament (RCL) and lateral ulnar collateral ligament (LUCL) make up the lateral collateral ligament complex (LCL), resisting varus and external stress during full elbow motion.
• The LCL's origin is the posterior lateral epicondyle and insertion is the supinator crest of the ulna.
• The annular ligament stabilizes the proximal radioulnar joint.

Secondary Static Stabilizers

• The radiocapitellar joint helps restrain valgus stress; the radial head provides approximately 30% of valgus stability, notably important in 0-30 degrees of flexion/pronation.
• The capsule contributes most to stability during full elbow extension.

Dynamic Stabilizers

• Muscles crossing the elbow joint, including anconeus, brachialis, triceps, and biceps, contribute compressive stability.

Nerve of the Elbow

• Musculocutaneous nerve: Originates from the lateral cord of the brachial plexus.
• Radial nerve: Originates from the posterior cord of the brachial plexus; runs between the brachialis and brachioradialis muscles, located superficially to the joint capsule at the level of the radiocapitellar joint.
• Median nerve: Originates from the medial/lateral cords of the brachial plexus.
• Ulnar nerve: Originates from the medial cord of the brachial plexus; traverses behind the medial epicondyle through the cubital tunnel.

Blood Supply of Elbow

• The brachial artery courses medially in the upper arm.
• It enters the cubital fossa laterally, containing the biceps tendon (lateral) and the brachial artery (medial).
• Continuing proximally, it branches into the radial and ulnar arteries.

Kinematics

• The axis of flexion/extension is at the trochlea centre.
• The pronation/supination axis is along the capitellum, radial head, and distal ulna.
• Joint reaction forces are considerable due to the elbow's lever arm design.
• This contributes to degenerative changes.

Free Body Diagram

• Free body diagrams demonstrate elbow inefficiencies.
• Static loads are close to body weight; dynamic loads exceed body weight.
• Sum of moments(M) = 0.

Arthrodesis

• Optimal unilateral arthrodesis position: 90° flexion, 0-7° valgus.
• Optimal bilateral arthrodesis position: One elbow at 110° flexion (feeding), the other at 65° flexion (perineal hygiene).