Elbow Biomechanics
The elbow joint functions as a fulcrum for the forearm and helps position the hand in space.
Anatomy
The elbow joint complex allows two types of motion: flexion-extension and pronation-supination. The humeroulnar and humeroradial articulations allow elbow flexion and extension and are classified as ginglymoid or hinged joints. The proximal radioulnar articulation allows forearm pronation and supination and is classified as a trochoid joint. The elbow joint complex, when considered in its entirety, is therefore a trochleoginglymoid joint. The trochlea and capitellum of the distal humerus are internally rotated 3° to 8° and in 94° to 98° of valgus with respect to the longitudinal axis of the humerus. The distal humerus is anteriorly angulated 30° along the long axis of the humerus. The articular surface of the ulna is oriented in approximately 4° to 7° of valgus angulation with respect to the longitudinal axis of its shaft.
The articular surface of the ulna is rotated 30° posteriorly with respect to its long axis. This matches the 30° anterior angulation of the distal humerus, which helps provide stability to the elbow joint in full extension. The radial neck is angulated 15° from the long axis in the anterior-posterior plane away from the bicipital tuberosity. Four-fifths of the radial head is covered by hyaline cartilage. The anterolateral one-fifth lacks articular cartilage and strong subchondral bone, explaining the increased propensity for fractures to occur in this region.
Kinematics
Elbow flexion and extension take place at the humeroulnar and humeroradial articulation. The normal range of flexion-extension is from 0° to 146° with a functional range of 30° to 130°. The normal range of forearm pronation-supination averages from 71° of pronation to 81° of supination. As the elbow is flexed, the maximum angle of supination increases, while the maximum angle of pronation decreases. Most activities are accomplished within the functional range of 50° pronation to 50° supination. Clinically, patients can tolerate flexion contractures of up to 30°, which is consistent.
Some movement occurs other than flexion/extension, from gliding to rolling at extreme range of motion. NOT A SIMPLE HINGE JOINT. Therefore, the ulnohumeral joint could be assumed to move as a uniaxial articulation except at the extremes of flexion-extension.
Carrying Angle
The carrying angle is a small degree of cubitus valgus. The angle is formed between the axis of a radially deviated forearm and the axis of the humerus.
The function of cubitus valgus is to allow the arms to swing while walking without them hitting the hips.
Normally it is 14° (female) and 11° (male) away from the body. The angle is less in children as compared to adults. In full flexion the angles become aligned
A decreased carrying angle can result in the forearm pointing towards the body, known as gunstock deformity or cubitus varus.[2]
Elbow Stability
Valgus forces at the elbow are resisted primarily by the anterior band of the medial collateral ligament (MCL). The anterior bundle of the MCL tightens in extension whereas the posterior bundle tightens in flexion. Radial head secondary stabilizer to valgus. Baseball pitchers are frequently at risk for MCL injury due to the repetitive valgus stress placed on their elbows by the nature of the throwing motion.
- In extension, resistance to valgus stress - shared equally by the MCL, capsule, and joint articulation.
- In flexion, the primary resistor to valgus stress is the MCL.
- In extension, the elbow articulation provides most of the resistance to varus stress followed by the anterior capsule.
- In flexion, the elbow articulation remains the primary restraint to varus stress followed by the anterior capsule and LCL, respectively, with the LCL contributing only 9%.
Musculature such as FCU and FDS provide some stability. Nevertheless MCL provides most valgus stability.
The LCL consists of- annular lig, radial collateral lig and lateral ulnar collateral lig.
| Position | Stabilising Element | Distraction | Varus | Valgus |
|---|---|---|---|---|
| Extension | MCL | 12 | - | 31 |
| LCL | 10 | 14 | - | |
| Capsule | 70 | 32 | 38 | |
| Articulation | - | 55 | 31 | |
| Flexion | MCL | 78 | - | 54 |
| LCL | 10 | 9 | - | |
| Capsule | 8 | 13 | 10 | |
| Articulation | - | 75 | 33 |
Posterolateral rotatory instability
Posterolateral rotatory instability of the elbow in which the ulna supinates on the humerus and the radial head dislocates in a posterolateral direction. Lateral ulnar collateral is the primary restraint to posterolateral rotatory instability of the elbow followed by the radial collateral ligament and capsule.
Longitudinal stability of the forearm is provided by both the interosseous membrane and the triangular fibrocartilage. The coronoid process also plays a role in longitudinal stability and has been shown to prevent posterior displacement of the ulna.
Movement
Elbow Flexors
The primary flexor of the elbow is the brachialis, which arises from the anterior aspect of the humerus and inserts on the anterior aspect of the proximal ulna. The biceps arises via a long head tendon from the supraglenoid tubercle and a short head tendon from the coracoid process of the scapula and inserts on the bicipital tuberosity of the radius. It is active in flexion when the forearm is supinated or in the neutral position. The brachioradialis, which originates from the lateral two thirds of the distal humerus and inserts on the distal aspect of the radius near the radial styloid, is active during rapid flexion movements of the elbow and when weight is lifted during a slow flexion movement. The brachialis, biceps, brachioradialis, and extensor carpi radialis are the major flexors of the elbow, the brachialis possessing the greatest work capacity.
Extensors
The primary extensor of the elbow, the triceps, is composed of three separate heads. The long head originates from the infraglenoid tubercle, and the medial and lateral heads originate from the posterior aspect of the humerus. The three heads coalesce to form one tendon that inserts onto the olecranon process of the ulna. The medial head is the primary extensor, and the lateral and long heads
act in reserve. The anconeus muscle, which arises from the posterolateral aspect of the distal humerus and inserts onto the posterolateral aspect of the proximal ulna, is also active in extension. This muscle is active in initiating and maintaining extension.
Supination
Muscles involved in supination of the forearm include the supinator, biceps, and the lateral epicondylar extensors of the wrist and fingers. The primary muscle involved in supination is the biceps brachii. The biceps generates four times more torque with the forearm in the pronated position than in the supinated position. The supinator arises from the lateral epicondyle of the humerus and the proximal lateral aspect of the ulna and inserts into the anterior aspect of the supinated proximal radius.
Pronation
Muscles involved in pronation include the pronator quadratus (PQ) and pronator teres (PT). PQ and PT are active throughout the whole rotation, being most efficient around the neutral position of the forearm. The pronator quadratus originates from the volar aspect of the distal ulna and inserts
onto the distal and lateral aspect of the supinated radius. The pronator teres is more proximally located, arising from the medial epicondyle of the humerus and inserting onto the lateral aspect of the midshaft of the supinated radius. The pronator quadratus is the primary pronator.
Supination strength was shown to be 20% to 30% greater than pronation strength! Anconeus is active in all positions and is considered to be a dynamic joint stabilizer. 43% of longitudinal forces are transmitted through the ulnotrochlear joint and 57% are transmitted through the radiocapitellar joint.
The force generated in the elbow has been shown to be up to three times body weight with certain activities.
Summary
The carrying angle of the elbow is defined as the angle between the anatomic axis of the ulna and humerus in the AP plane and in full elbow extension. It averages between 10° and 15° of valgus.
The primary stabilizer to valgus stress at the elbow is the anterior band of the medial collateral ligament with the radial head acting as a secondary stabilizer. The primary restraint to varus stress is the elbow articulation. The lateral ulnar collateral ligament is the main stabilizer to posterolateral rotatory instability of the elbow.
The primary flexor of the elbow is the brachialis whereas the primary extender is the triceps. The anconeus is active in initiating and maintaining flexion and is considered to act as a dynamic joint stabilizer. The main source of supination is the biceps brachii. The pronator quadratus is the primary
pronator of the forearm regardless of position of the forearm or degree of elbow flexion.
Force generated in the elbow has been shown to be up to three times body weight when performing activities of daily living.
References
Study notes from Nordin textbook
