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(Figs. 305 and 306) is a fan-shaped structure of unequal thickness, but its margins, which are its strongest bands, are continuous with the adjoining parts of the anterior and posterior ligaments. By its proximal end it is attached to the anterior, distal, and posterior aspects of the medial epicondyle of the humerus. By its broad distal end it is attached to the medial margin of the semilunar notch, so that the anterior band is associated principally with the medial margin of the coronoid process, and the posterior band with the medial margin of the olecranon, while the intermediate weaker portion sends its fibres downwards to join a transverse band, sometimes very strong, which bridges the notch between the adjoining medial margins of the coronoid process and the olecranon.
Lig. Collaterale Radiale. The radial collateral ligament (O.T. external lateral) (Fig. 305) is a strong flattened band attached proximally to the distal and
posterior aspects of the lateral epicondyle of the humerus. It completes the continuity of the articular capsule on the lateral side, and blends distally with the lig. annulare radii, on the surface of which its fibres may be traced both to the anterior and posterior ends of the radial notch of the ulna. Both of the collateral ligaments are intimately associated with the muscles which take origin from the medial and lateral epicondyles of the humerus.
Synovial Pads of Fat (Fig. 307).-Internal to the fibrous stratum of the articular capsule, there are several pads of fat situated between it and the synovial stratum. Small pads are so placed as to lie immediately in front of the coronoid. and radial fossæ, but a larger one projects towards the olecranon fossa.
A stratum synoviale (Fig. 307) lines the entire fibrous stratum and clothes. the pads of fat referred to above, as well as those portions of bone enclosed within. the capsule which are not covered by articular cartilage. By its disposition the elbow and the proximal radio-ulnar joints possess a common joint cavity. It should be specially noted that the proximal part of the neck of the radius is surrounded by this synovial layer.
Movements at the Elbow-Joint.-The movements of the radius and ulna upon the humerus have already been referred to as those characterising a uniaxial joint constructed on the plan of a hinge. In this case the axis of the joint is obliquely transverse, so that in the extended position the humerus and ulna form an obtuse angle open towards the radius, whereas in the
flexed position the hand is carried medially in the direction of the mouth. Extreme flexion is checked by the soft parts in front of the arm and of the forearm coming into contact, and extreme extension by the restraining effect of the ligaments and muscles. In each case the movement is checked before either the coronoid process or the olecranon come into contact with the humerus. The anterior and posterior bands of the ulnar collateral ligament are important factors in these sults. Lateral movement of the ulna is not a characteristic movement, although it may occur to a slight extent, owing to a want of complete adaptation between the trochlear surface of the humerus and the semilunar notch of the ulna. This incongruence is noteworthy since the medial lip of the trochlea is prominent in front, and the lateral lip is prominent behind. Consequently, this latter part is associated with a surface on the lateral side of the olecranon which is only utilised in complete extension.
Olecranon pad of fat
The capitulum and the opposing surface upon the head of the radius are always in varying degrees of contact. The head of the radius participates in the movements of flexion and extension, and is most closely and completely in contact with the humerus during the position of semi-flexion and semi-pronation. In complete extension a very considerable part of the capitulum is uncovered by the radius.
THE RADIO-ULNAR JOINTS.
These articulations, which are two in number, are situated at the proximal and distal ends of the radius and ulna. They provide an adaptation whereby the radius rotates around a longitudinal axis in the movements of pronation and supination, and hence this form of uniaxial diarthrosis is termed lateral ginglymus.
Articulatio Radioulnaris Proximalis.-The proximal radio-ulnar joint forms a part of the articulatio cubiti or elbow-joint. The articular surfaces which enter into its formation are the radial notch of the ulna and the lateral aspect of the head of the radius. In each case the articular cartilage is continuous with an articular surface entering into the formation of the humero-radial and humeroulnar joints, consequently the joint cavity is continuous with the cavities of those joints, and therefore, in a sense, it lies within the cover of the articular capsule of the elbow-joint; but its special feature is the annular ligament of the radius.
Lig. Annulare Radii.-The annular ligament of the radius (O.T. orbicular ligament) (Figs. 305 and 308) has been mentioned above as the distal line of attachment of the radial collateral ligament and the ligaments on the front and back of the elbow-joint.
It is a strong, well-defined structure, attached by its extremities to the volar and dorsal margins of the radial notch of the ulna, and thus it forms nearly four-fifths of an osseo-tendinous circle or ring. The circle is somewhat wider at the proximal than at the distal margin of the annular ligament of the radius, which, by encircling the proximal part of the neck of the radius, tends to prevent
displacement of the head of that bone in a distal direction. The distal margin of this ligament is not directly attached to the radius.
The synovial stratum is continuous with that which lines the elbow-joint. It closes the joint cavity at the distal unattached margin of the annular ligament, where it is somewhat loosely arranged in its reflexion from the ligament to the neck of the radius. The epiphyseal line at the proximal end of the radius is intra-capsular. Articulatio Radioulnaris Distalis. The distal radio-ulnar joint is situated between the lateral aspect of the head of the ulna and the ulnar notch on the medial side of the distal end of the radius. In addition, it includes the distal surface of the head of the ulna, which articulates with the proximal surface of a triangular articular disc by means of which the joint is excluded from the radio-carpal articulation.
Discus Articularis.-The triangular articular disc (Figs. 309 and 311), besides presenting articular surfaces to two separate joints, is an important ligament concerned in binding together the distal ends of the radius and ulna. It is attached by its apex to the depression at the lateral side of the root of the styloid process of the ulna, and by its base to the sharp line of demarcation between the ulnar notch and the carpal articular surface of the radius.
Capsula Articularis.-The fibrous stratum is very imperfect, and consists of scattered fibres, termed the anterior and posterior radio-ulnar ligaments (Fig. 310). These ligaments pass transversely between adjoining non-articular surfaces on the radius and ulna, and are of sufficient length to permit of the movements of the radius in pronation and supination.
The synovial stratum completes the closure of the joint cavity. It forms a loose bulging projection (recessus sacciformis), passing proximally between the distal ends of the shafts of the radius and ulna, and it also clothes the proximal surface of the articular disc (Fig. 311). The cavity of this joint is quite distinct from that of the radio-carpal articulation, except when the articular disc presents a perforation.
Between the proximal and distal radio-ulnar articulations there are two accessory ligaments, viz., the chorda obliqua and the interosseous membrane, which connect together the shafts of the radius and ulna.
Surface for navicular bone
Groove for tendon
of extensor longus
Chorda Obliqua.-The oblique chord (Fig. 306) is a slender fibrous band of very varying strength which springs from the lateral part of the coronoid process of the ulna, and stretches obliquely distally and laterally to the radius where it is attached immediately distal to the tuberosity of the radius.
Membrana Interossea Antibrachii. The interosseous membrane of the fore
Head of ulna
arm (Fig. 306) is a strong fibrous membrane which stretches across the interval between the radius and ulna, and is firmly attached to the interosseous crest of each. Distally it extends to the distal limit of the space between the bones, whilst
proximally it only reaches a point about one inch distal to the tuberosity of the radius. A gap, called the hiatus interosseus, is thus left between its proximal margin and the chorda obliqua, and through this the dorsal interosseous vessels pass back
Surface for lunate bone
FIG. 308.-ANNULAR LIGAMENT OF THE RADIUS.
Styloid process of ulna
Apex of articular disc
FIG. 309.-CARPAL ARTICULAR SURFACE OF THE RADIUS,
wards between the bones to reach the dorsal aspect of the forearm. The fibres which compose the interosseous membrane run for the most part distally and medially from the radius to the ulna, although on its dorsal aspect several bands may be observed stretching in an opposite direction. The interosseous membrane augments the surface available for the origin of the muscles of the forearm; it braces the radius and ulna together; and when shocks are communicated from the hand to the radius, owing to the direction of its fibres, the interosseous membrane transmits them, to a large extent, to the ulna.
Movements of the Radius on the Ulna.-The axis around which the radius moves is a longitudinal one, having one end passing through the centre of the head of the radius and the other through the styloid process of the ulna and the line of the ring-finger. In this axis the head of the radius is so secured that it can only rotate upon the radial notch of the ulna within the annular ligament of the radius, and consequently the radial head remains upon the same plane as the ulna; but the distal end of the radius being merely restrained by the articular disc, is able to describe nearly a half-circle, of which the apex of this ligament is the centre. In this movement the radius carries the hand from a position in which the palm is directed forwards, and in which the radius and ulna lie parallel to each other (supination), to one in which the palm is directed backwards, and the radius lies diagonally across the front of the ulna (pronation).
The ulna is unable to rotate upon a long axis, but while the radius is travelling through the arc of a circle from lateral to medial side in front of the ulna, it will usually be seen that the ulna appears to move through the arc of a smaller circle in the reverse direction, viz., from medial to lateral side. If the humerus is prevented from moving at the shoulder-joint, a very large proportion, if not the entire amount, of this apparent movement of the ulna will disappear. At the same time some observers maintain that it really occurs at the elbow-joint, associated with lateral movement during slight degrees of flexion and extension at that joint.
The radio-carpal joint is a bi-axial diarthrosis, frequently called a condyloid joint.
The articular elements which enter into its formation are: on its proximal side, the carpal surface of the distal end of the radius, together with the distal surface of the discus articularis; on its distal side, the proximal articular surfaces of the navicular, lunate, and triquetral bones, and the interosseous ligaments between them. The articular surface of the radius is concave both in its antero-posterior and transverse diameters, in order to adapt itself to the opposing surfaces of the navicular and lunate, which are convex in the two axes named. In the ordinary straight position of the hand the articular disc is in contact with the lunate bone, and the proximal articular surface of the triquetral bone is in contact with the capsule of the joint. When, however, the hand is bent towards the ulna the triquetral bone is carried laterally as well as the lunate and navicular and the articular disc comes into contact with the triquetral. The articular surface of the radius is subdivided by an antero-posterior, slightly elevated ridge, into a lateral triangular facet which usually articulates with the navicular, and a medial quadrilateral facet for articulation with a portion of the lunate bone.
In the intervals between the navicular, lunate, and triquetral bones, the continuity of the distal articular surface is maintained by the presence of interosseous ligaments which are situated upon the same level as the articular cartilage.
Capsula Articularis. An articular capsule completely surrounds the joint. It is somewhat loosely arranged, and its fibrous stratum permits of subdivision into the following four portions:
Lig. Radiocarpeum Laterale. The lateral radio-carpal ligament (O.T. external lateral) (Fig. 310) is a well-defined band which is attached by one end to the tip of the styloid process of the radius, and by the other to a rough area at the base of the tuberosity of the navicular bone, i.e. lateral to its radial articular surface.
Lig. Ulnocarpeum Mediale.-The medial ulno-carpal ligament (O.T. internal lateral) (Fig. 310) is also a distinct rounded structure, having one end attached to the tip of the styloid process of the ulna, and the other to the rough non-articular border of the triquetral bone, some of its fibres being prolonged to the pisiform bone. Lig. Radiocarpeum Volare. The volar radio-carpal ligament (O.T. anterior ligament) (Fig. 310) is attached proximally to the volar margin of the distal end
of the radius, as well as slightly to the base of the styloid process of the ulna. Some transverse fibres may be seen, but the greater number pass obliquely distally and medially to the volar non-articular surfaces of the
navicular, lunate, and Medial ulno-carpal
triquetral bones, while
ing from it Hamulus of os hamatum
obliquely laterally. On its deeper aspect this ligament is closely adherent to the volar border of the articular disc of the distal radioulnar articulation.
Collateral radio-carpal ligament
Tubercle of navicular bone
angular bone Greater multangular bone
Lig. Radiocarpeum Dorsale.-The dorsal radio-carpal ligament (O.T. posterior ligament) extends from the dorsal margin of the distal end of the radius, in an oblique direction distally and medially, to the dorsal non-articular areas on the proximal row of the carpal bones. The slip to the latter assists in forming the fibrous sheath through which the tendon of the extensor carpi ulnaris muscle travels to its insertion. The principal bundle of fibres is connected with the triquetral bone.
The stratum synoviale (Fig. 311) is simple, and is confined to the articulation, except in those cases in which the articular disc is perforated, or in which one of the interosseous ligaments between the carpal bones of the first row is absent. The epiphyseal lines at the distal ends of radius and ulna are extra-capsular.
FIG. 310.-LIGAMENTS ON VOLAR ASPECT OF RADIO-CARPAL,
Movements at the Radio-carpal Joint. The radio-carpal joint affords an excellent example of a bi-axial articulation, in which a long transverse axis of movement is situated more or less at right angles to a short axis placed in the antero-posterior direction. The nature of the movements which are possible around these two axes is essentially the same in both cases, viz., flexion and extension. The movements around the longer transverse axis are anterior or volar flexion, extension, and its continuation into dorsi-flexion. Around the shorter antero-posterior axis we get movements which result from combined action by certain flexor and extensor muscles, whereby the radial or ulnar borders of the hand may be approximated towards the corresponding borders of the forearm. Lateral movement also may be possible to a slight extent. The range of movement in connexion with either of the principal axes is largely a matter of individual peculiarity, for, with the exception of the lateral ligaments, there is no serious obstacle to the cultivation of greater mobility at the radio-carpal joint.
Carpal Joints. The articulations subsisting between the individual carpal bones are all diarthroses, and although the total amount of movement throughout the series is considerable, yet the extent of movement which is possible between the two rows or between any two carpal bones is extremely limited. For this reason, as well as because of the nature of the movement, these articulations are called gliding joints (arthrodia).
It is advisable to consider, first, the articulations between individual bones of the proximal row; second, the articulations between the separate bones of the distal row; third, the articulation of the proximal and distal rows with each other; fourth, the pisiform articulation.