Elbow and forearm
The elbow is a joint that connects between the arm and the forearm. It is a complex hinge-type joint, located 2-3 cm inferior to the epicondyles of the humerus. It involves three separate articulations that share a common synovial cavity. The humeroulnar articulation is between the trochlear notch of the ulna and the trochlea of the humerus. The humeroradial articulation is between the head of the radius and the capitulum of the humerus. The two are considered the traditional primary constituents of the elbow. The third articulation is that of the proximal radio-ulnar joint, it is a more pivot type of joint that connects between the head of the radius and the radial notch of the ulna. The three articulations form two different aspects of movement. They allow the flexion and extension of the elbow, as well as the supination and pronation of the forearm and wrist.
Joint capsule and ligaments
A fibrous layer of the joint capsule encloses the elbow. Its internal surface is lined by the synovial membrane. The joint capsule is weak both anteriorly and superiorly, but is reinforced on the sides by the collateral ligaments. The collateral ligaments of the elbow are lateral and medial thickenings that stabilize the elbow’s motion. These two ligaments are the lateral fan-like radial collateral ligament, and the medial triangular-shaped ulnar collateral ligament.
Bursae are membranous sacs filled with synovial fluid. They prevent degenerative damage to the joints by reducing the friction between the moving parts and allowing free movement. Only three olecranon (the bony prominence at the very tip of the elbow) bursae around the elbow have clinical significance, and they are the intratendinosus, subtendinosus, and the subcutaneous bursa.
The forearm is the second largest segment of the upper limb. It extends between the elbow and wrist joints, and is separated by layers of fascia, bones and ligaments, into anterior and posterior regions.
The forearm bone framework consists of two bones, the radius and ulna. The radius lies laterally. It is small at its proximal end where it articulates with the humerus, and large at its distal end where it forms the wrist joint with the carpal bones of the hand. The ulna is the exact opposite. It lies medially in the forearm, and has reversed proximal and distal end dimensions. This allows the distal end of the radius to swing over that of the ulna, permitting pronation and supination movement of the hand.
As mentioned before, the forearm is divided into anterior (flexor) and posterior (extensor) compartments. They are separated by the lateral intermuscular septum, interosseous membrane, and the attachment of the deep fascia along the posterior border of the ulna.
Muscles in the anterior compartment are flexors present in three layers. The superficial layer contains the flexor carpi ulnaris, the palmaris longus, the flexor carpi radialis, and the pronator teres. In the intermediate layer, there is the flexor digitorum superficialis. The flexor digitorum profundus, flexor pollicis longus and the pronator quadratus lie in the deep layer. All these anterior compartment muscles are responsible for the flexion of the wrist and digits, and the pronation of the hand. They are innervated by the median nerve, except for the flexor carpi ulnaris and the medial half of the flexor digitorum profundus, both of which are innervated by the ulnar nerve.
Muscles of the posterior compartment are present in two layers, superficial and deep. The superficial layer consists of the brachioradialis, extensor carpi radialis longus, extensor carpi radialis brevis, extensor digitorum, extensor digiti minimi, extensor carpi ulnaris, and the anconeus. The deep layer on includes the supinator, abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus, and the extensor indicis. Muscles of the posterior compartment are responsible for the extension of the wrist and digits, and the supination of the hand. They are all innervated by the radial nerve.