Maximus Gage
The ulna is one of the two long bones in the forearm, spanning from the elbow to the wrist on the medial side of the forearm. It is larger and longer than the other bone in the forearm, the radius, and acts as the stabilising bone. The ulna is the origin and/or insertion site for over a dozen muscles, including the triceps brachii, anconeus, brachialis, pronator teres, abductor pollicis longus, extensor pollicis longus, and extensor indicis. The ulna also articulates with the humerus at its most proximal point, forming the elbow in a hinge joint.
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What You'll Learn
Triceps brachii
The triceps brachii is a three-headed muscle in the arm, with long, medial, and lateral heads. It is the only muscle in the posterior compartment of the arm and is responsible for extension of the forearm at the elbow joint. The triceps brachii is innervated by the radial nerve, which gives off a separate branch for each head. The C6 root value of the radial nerve innervates the lateral head, the C7 root value innervates the long head, and the C8 root value supplies the medial head. However, recent studies have shown that in some individuals, the long head of the triceps brachii is innervated by the axillary nerve, or by both the radial nerve and axillary nerve. The triceps brachii can be trained in a variety of ways, including isolation movements such as lying triceps extensions and cable push-downs, and compound movements such as push-ups and bench presses.
The long head of the triceps brachii arises from the infraglenoid tubercle of the scapula, which is a rough area at the inferior margin of the glenoid fossa. Its attachment extends slightly above to the adjacent glenoid labrum and blends with the glenohumeral capsule of the shoulder joint, contributing to its stability. The medial head arises proximally in the humerus, just inferior to the groove of the radial nerve, and from the dorsal surface of the humerus. The lateral head originates from a narrow, linear ridge on the posterior surface of the humerus, just superior to the radial groove. A portion of the muscle fibres also arise from the lateral intermuscular septum. Its attachment ascends obliquely, beginning from the lateral border of the humerus behind the deltoid tuberosity.
The triceps brachii is an important surgical landmark, as it plays a role in creating anatomical spaces which are traversed by neurovascular structures. The superior portion of the long head of the triceps is covered by the posterior border of the deltoid muscle. On its course, the long head of the triceps descends through the wedge-shaped interval between the teres major and minor and the humerus, creating three distinct anatomical spaces. The upper triangular space is bounded by the teres minor and subscapularis superiorly, the teres major inferiorly, and the long head of the triceps laterally.
The triceps brachii attaches to the superior surface of the olecranon, a large projection of bone that forms part of the trochlear notch. The olecranon is located near the elbow, at the proximal ulna. The triceps brachii is supplied with oxygen and nutrients from the branches of the deep brachial artery. Ruptures of the triceps brachii muscle are rare and typically only occur in anabolic steroid users.
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Brachialis
The ulna is a long bone in the forearm that acts as the stabilizing bone, with the radius pivoting to produce movement. The ulna and the radius are attached by the interosseous membrane. The ulna has a large proximal end that articulates with the humerus and a small distal end. The distal ulna consists of a small rounded head and an ulnar styloid process. The lateral convex articular surface of the head of the ulna articulates with the ulnar notch of the distal radius to form the distal radioulnar joint. The inferior surface of the head of the ulna articulates with an articular disc called the triangular fibrocartilage (TFC), which separates the head from the carpal bones.
The olecranon is a prominent, proximal projection from the posterior aspect of the proximal ulna and serves as the point of insertion for the triceps brachii muscle. Its topmost portion bends anteriorly, fitting into the olecranon fossa of the distal humerus during extension, preventing hyperextension of the elbow joint. The coronoid process is a triangular eminence projecting forward from the upper and front part of the ulna. Its apex is pointed, slightly curved upward, and in flexion of the forearm, it is received into the coronoid fossa of the humerus.
The tuberosity of the ulna, an important landmark of the proximal ulna, is a roughening immediately distal to the coronoid process. It is where the brachialis muscle attaches. The brachialis muscle is the primary flexor of the elbow and is located in the anterior compartment of the arm along with the biceps brachii and coracobrachialis. It is a broad muscle, with its broadest part located in the middle rather than at the extremities. It is sometimes divided into two parts and may fuse with the fibres of the biceps brachii, coracobrachialis, or pronator teres muscles. The brachialis acts as the floor of the cubital fossa and is part of the radial tunnel. It is the only pure flexor of the elbow joint, producing the majority of force during elbow flexion. It is not affected by pronation or supination of the forearm and does not participate in pronation and supination due to its lack of attachment to the radius.
The brachialis muscle can be clinically assessed by palpating the contracting muscle fibres during flexion of the elbow joint against resistance while the forearm is in the semi-prone position. If the muscle cannot be palpated, testing of its function can be used to assess it. This can present as a weakness when flexing the arm against resistance or as an inability to fully extend the elbow joint due to painful stretching of the brachialis tendon. Brachialis tendonitis can be caused by overuse of the muscle during exercises such as pull-ups and elbow curls. It is also commonly seen in climbers due to the pronation of the hand and the extended starting position.
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Pronator teres
The pronator teres is a muscle located mainly in the forearm. It is a long, round muscle that is found in the superficial layer of the anterior compartment of the forearm. The pronator teres has two heads: the humeral head and the ulnar head. The humeral head is larger and more superficial, arising from the medial supracondylar ridge immediately superior to the medial epicondyle of the humerus, and from the common flexor tendon (which arises from the medial epicondyle). The ulnar head, or ulnar tuberosity, is deeper and joins the humeral head at an acute angle, forming one muscle belly. The ulnar head is sometimes more tendinous, sometimes more muscular, and occasionally absent. The pronator teres originates at the medial side of the coronoid process of the ulna.
The pronator teres is innervated by the median nerve and nerve roots C6 and C7. The median nerve typically runs between the two heads of the pronator teres, making it a possible site of nerve entrapment. The median nerve is thus at risk for compression at this site in cases of pronator teres overuse or even hypertrophy. Clinically, median nerve compression at this site is known as pronator teres syndrome, which can cause pain and numbness, and may also result in motor dysfunction.
The primary function of the pronator teres is to pronate the forearm, turning the palm downwards. It works synergistically with the pronator quadratus to pull the radius medially, causing the radial head to rotate around the proximal ulna. This action also assists in flexion of the elbow joint. If the elbow is fully flexed, the muscle fibres of the pronator teres are shortened and less able to produce force.
In C5 tetraplegia or radial nerve palsy patients, the pronator teres tendon can be rerouted to the extensor carpi radialis brevis tendon to restore wrist extension.
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Abductor pollicis longus
The ulna is a long bone in the forearm that lies medially and parallel to the radius, the second forearm bone. The ulna acts as the stabilising bone, with the radius pivoting to produce movement. The ulna has a large proximal end that articulates with the humerus and a small distal end. The distal ulna consists of a small rounded head and an ulnar styloid process. The lateral convex articular surface of the head of the ulna articulates with the ulnar notch of the distal radius to form the distal radioulnar joint. The inferior surface of the head of the ulna articulates with an articular disc called the triangular fibrocartilage (TFC), which separates the head from the carpal bones. The proximal ulna features the olecranon, coronoid process, trochlear notch, radial notch, and the tuberosity of the ulna. The triceps brachii muscle attaches to the superior surface of the olecranon. The coronoid process is a triangular eminence that projects forward from the upper and front part of the ulna. Its apex is pointed and slightly curved upward, and in flexion of the forearm, it is received into the coronoid fossa of the humerus. Its upper surface is smooth, concave, and forms the lower part of the semilunar notch. Its antero-inferior surface is concave and marked by a rough impression for the insertion of the brachialis. The radial notch is a narrow, oblong, articular depression on the lateral side of the coronoid process that receives the circumferential articular surface of the head of the radius. The body of the ulna at its upper part is prismatic in form, curved, and convex. Its central part is straight, and its lower part is rounded, smooth, and bent a little lateralward. The distal end of the ulna is much smaller in diameter than the proximal end. It terminates in a rounded head with a distal projection, the ulnar styloid process. The head articulates with the ulnar notch of the radius to form the distal radio-ulnar joint.
The abductor pollicis longus (APL) is a deep extensor of the forearm and is responsible for facilitating movement and stabilisation of the thumb. Its tendon is present in the first extensor compartment of the wrist. The APL is one of five deep extensors in the forearm, along with the supinator, extensor pollicis brevis, extensor pollicis longus, and extensor indicis. The three pollicis muscles attach to the bones of the thumb, producing various movements within its joints. The APL is innervated by the posterior interosseous nerve (C7, C8), which is a continuation of the deep branch of the radial nerve. The blood supply to the APL comes from the interosseous branches of the ulnar artery. The proximal part of the muscle is supplied by the lateral branch of the posterior interosseous artery, while the distal part is vascularised by a perforating branch of the anterior interosseous artery. The APL lies immediately below the supinator and is sometimes united with it. It arises from the lateral part of the dorsal surface of the body of the ulna, below the insertion of the anconeus, from the interosseous membrane, and from the middle third of the dorsal surface of the body of the radius. Passing obliquely downward and lateralward, it ends in a tendon, which runs through a groove on the lateral side of the lower end of the radius, accompanied by the tendon of the extensor pollicis brevis. The insertion is divided into a distal, superficial part and a proximal, deep part. The superficial part is inserted with one or more tendons into the radial side of the base of the first metacarpal bone, and the deep part is variably inserted into the trapezium, the joint capsule and its ligaments, and into the belly of the abductor pollicis brevis (APB) or opponens pollicis. The chief action of the APL is to abduct the thumb at the carpometacarpal joint, thereby moving the thumb anteriorly. It also assists in extending and rotating the thumb. By its continued action, it helps to abduct the wrist (radial deviation) and flex the hand. The APL insertion on the trapezium and the APB origin on the same bone is the only connection between the thumb's intrinsic and extrinsic muscles. Acting alone or with the abductor pollicis brevis, the APL pulls the thumb away from the palm.
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Extensor pollicis longus
The extensor pollicis longus is a skeletal muscle located on the dorsal (posterior) side of the forearm. It is one of the deep forearm extensors that control the movement of the thumb. The extensor pollicis longus arises from the dorsal surface of the ulna, originating from the middle third of the ulna's posterior surface, along its radial border. The muscle's origin extends onto the adjacent interosseous membrane, which connects the ulna to the radius, and is situated proximal to the origin of the extensor indicis muscle.
The muscle belly of the extensor pollicis longus runs obliquely in a radial direction towards the lateral aspect of the wrist joint. It passes through the third tendon compartment, lying in a narrow, oblique groove on the back of the lower end of the radius. The muscle then crosses the wrist close to the dorsal midline before turning towards the thumb, using Lister's tubercle on the distal end of the radius as a pulley.
The extensor pollicis longus tendon obliquely crosses the tendons of the extensores carpi radialis longus and brevis. At the level of the radiocarpal joint, the tendon sits medial to the extensor pollicis brevis tendon, forming a depression known as the anatomical snuffbox when the thumb is extended. The tendon of the extensor pollicis longus is supplied by branches from various arteries. Before the tendon enters its synovial sheath, it is supplied by the anterior interosseous artery or its muscular branches. The sheath itself is supplied by the posterior ramus of the same artery.
The main action of the extensor pollicis longus is the extension of the thumb at the metacarpophalangeal and interphalangeal joints. This extension occurs in synergy with the extensor pollicis brevis muscle. When the thumb reaches full extension or abduction, the extensor pollicis longus can also assist in adduction of the thumb. The tendon of the extensor pollicis longus forms an extensor expansion or dorsal aponeurosis as it is joined by the tendons of the abductor pollicis brevis and adductor pollicis. Together, this aponeurosis forms the extensor mechanism of the thumb.
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Frequently asked questions
The ulna serves as the attachment site for numerous muscles, including the triceps brachii, anconeus, brachialis, pronator teres, abductor pollicis longus, extensor pollicis longus, and extensor indicis.
The ulna is one of the two long bones in the forearm, spanning from the elbow to the wrist on the medial side of the forearm.
The ulna acts as the stabilizing bone of the forearm, with the radius pivoting to produce movement. It also articulates with the humerus at its most proximal point to form the elbow in a hinge joint.
Common pathologies associated with the ulna include fractures, such as Monteggia's fracture, Hume fracture, and Galeazzi's fracture, which involve fractures of the ulna and/or dislocation of the radial head.
