The proximal radioulnar joint has been mentioned in connection with the elbow joint. The essential structure here is the annular ligament which imprisons the head of the radius. The annular ligament is attached to the radial notch of the ulna and its fibers encircle the head and neck of the radius. The ligament has no attachment to the radius, which is free to rotate within it. Superiorly it blends with the capsule of the elbow joint. The proximal radioulnar joint and the elbow joint form one continuous synovial cavity. The synovial membrane is attached to the radius at the lower margin of the cylindrical articular surface; it is supported between the ulna and the radius by the quadrate ligament. This stretches between the neck of the radius, proximal to the tuberosity, and the upper part of the supinator fossa of the ulna just distal to the radial notch. Its fibers run crisscross, so that some are tense while others relax; its overall tension remains constant in all positions of supination and pronation.
The distal radioulnar joint is closed distally by a triangular fibrocartilage, which is attached by its base to the ulnar notch of the radius and by its apex to a small fossa at the base of the ulnar styloid. The capsule is loose and pouches upwards between the two bones behind the surface of the deep part of pronator quadratus, forming the sacciform recess. It is unusual for the fibrocartilage to be incomplete; in such rare cases, of course, the joint communicates with the wrist joint.
The interosseous membrane joins the interosseous borders of the two bones. Its fibers run from the radius down to the ulna at an oblique angle, and are supposed to have an effect in transmitting thrust from the wrist to the elbow via lower end of radius to upper end of ulna and so to the humerus. It tends to be relaxed when the weight is suspended from the hands; but in this case, of course, the weight is suspended by muscles, not by bones or ligaments. It is taut in pronation and lax in supination.
The oblique cord has been considered to represent a degenerated portion of either flexor pollicis longus or supinator. Its fibers run in opposite obliquity to those of the interosseous membrane. The cord slopes upwards from just below the radial tuberosity to the side of the ulnar tuberosity, distal to the quadrate ligament. The posterior interosseous vessels pass through the gap between the oblique cord and the upper end of the interosseous membrane.
In accordance with Hilton’s law both joints and the interosseous membrane are innervated from the median nerve and its anterior interosseous branch (the pronator muscles being the operative structures).
Examine the articulated bones of the upper limb. The basic movements of pronation and supination are produced by movements of radius (and hand) around an immobile ulna. The axis around which the radius rotates obviously passes through the centre of curvature of each radioulnar joint. The axis of pronation-supination is oblique along the supinated forearm — it joins the centre of the head of the radius and the base of the styloid process of the ulna, and when prolonged passes near the little finger. It lies in the line of the shaft of the humerus, hence the carrying angle of the elbow. For the ulna, as a result of the opposite curvatures of its upper and lower articular surfaces, must lie oblique to the axis of pronation-supination and therefore oblique to the shaft of the humerus, and this obliquity constitutes the carrying angle.
The axis of pronation-supination is fixed in relation to the ulna, but the axis of the forearm itself is not fixed in relation to the ulna. The axis of the forearm runs from the midpoint between the epicondyles of the humerus down to the midpoint between the styloid processes of the radius and ulna. The axis of the forearm is therefore constantly changing with the position of the bones throughout the range of pronation and supination. This can easily be seen by inspection of the dry bones and can be confirmed in the living forearm. In supination the axis of the forearm is parallel with the ulna; it is oblique to the shaft of the humerus (carrying angle). In pronation the axis of the forearm crosses the ulna obliquely and lies parallel with the axis of pronation- supination; it is in line with the shaft of the humerus though the ulna has not moved. The carrying angle exists so that the forearm may be in line with the humerus in the working position, which is that of almost full pronation. Few acts except carrying are performed in the fully supinated position.
A simple experiment is worth performing. Flex the elbow to a right angle and lay the forearm on a table. Supination and pronation in this position are seen to cause a wide movement of the thumb from right to left; note also that the anterior surface of the distal extremity of the ulna remains facing towards the ceiling in all positions of the hand, even when the latter lies palm downwards. During this experiment the ulna remains fixed and the hand moves around the axis of the little finger. Now repeat the movement with the flexed forearm free in mid-air. The axis of rotation of the forearm now no longer passes through the little finger, but has moved to the middle finger. The lower extremity of the ulna still faces forward as before; but it moves into slight adduction and abduction, during supination and pronation, in a manner beautifully coordinated. To the basic rotation of the radius around an immobile ulna is superadded a movement of the ulna itself; the actual motion performed by wrist and hand is the resultant of these two movements.
Ulnar movement is produced in two ways. In the usual working position with elbow flexed (e.g. turning a screwdriver, a doorknob) abduction and adduction occur between ulna and trochlea. Contraction of the anconeus causes slight abduction and pronation of the ulna, and these movements accompany pronation of the forearm. Ulnar abduction is opposed by the humeral head of pronator teres. This oblique pull adducts the radius (and hence the ulna).
In the rarer movements around a more laterally placed centre (lateral to the centre of the palm) ulnar movement at the trochlea is insufficient. Ulnar abduction and adduction with the elbow flexed are then produced by rotation of the shaft of the humerus, under the action of the short scapular muscles.
The forearm can be pronated and supinated around any finger tip placed on a fixed point. Ulnar movement is least when the little fingertip is fixed, greatest when index finger or thumb is fixed. The axis of rotation at the level of the wrist can thus be made to pass through any point between the styloid processes of radius and ulna; and each styloid process describes a semicircle around that point. Ulnar abduction and adduction are each accompanied by slight extension and then flexion at the elbow in order to bring this about. Thus during pronation the first half of the movement- is accompanied by abduction and slight extension of the ulna, and the second half by further abduction and slight flexion of the ulna. Likewise during supination the first half of the movement is accompanied by adduction and slight extension of the ulna, and the second half by further adduction and slight flexion of the ulna.
Summarizing, simple pronation and supination about an immobile ulna are rare and unnatural movements. Almost always they are accompanied by synergic movements at shoulder and elbow to produce simultaneous movement of the ulna.
The muscles producing pronation are principally pronator quadratus and pronator teres, with some small assistance from flexor carpi radialis and palmaris longus (and brachioradialis as far as the midprone position). Supination is carried out by supinator and biceps, with some assistance from extensor pollicis longus and extensor carpi radialis longus. Note that pronation is essentially a function of the median nerve, while supination depends on the musculocutaneous nerve and the radial nerve.