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LIGAMENTS OF THE SCAPULA.
These ligaments are not directly connected with any articulation.
Lig. Coracoacromiale.-The coraco-acromial ligament (Fig. 302) completes the arch between the coracoid process and the acromion, and thus provides a secondary socket for the greater protection and security of the shoulder-joint. It is a flat triangular structure stretched tightly between its attachments. By its base it is fixed to a varying amount of the postero-lateral border of the coracoid process, and by its narrower apical end to the tip of the acromion, immediately lateral to the acromio-clavicular joint. Its surfaces look upwards and downwards, and its free borders laterally and medially. It is thinnest in the centre, where it is sometimes perforated by a prolongation of the tendon of the pectoralis minor muscle.
Lig. Transversum Scapula Superius.-The superior transverse scapular ligament (O.T. suprascapular ligament) is a distinct but short flat band which bridges the scapular notch. It may be continuous with the conoid ligament, and it is frequently ossified. As a rule the foramen completed by this ligament transmits the suprascapular nerve, while the transverse scapular vessels pass superior to the ligament to reach the supraspinous fossa.
A small duplicate of this ligament may often be found bridging the foramen on its costal aspect, subjacent to which small branches of the transverse scapular artery return from the supraspinous to the subscapular fossa.
Lig. Transversum Scapulæ Inferius.-The inferior transverse scapular ligament (O.T. spino-glenoid ligament) consists of another set of bridging fibres which are situated on the posterior aspect of the neck of the scapula. By one end they are attached to the lateral border of the scapular spine, and by the other to the adjacent part of the posterior aspect of the head of the scapula. The suprascapular nerve and the transverse scapular vessels pass subjacent to this ligament.
The shoulder-joint is one of the largest as well as the most important of the joints of the upper limb. It is an example of the enarthrodial, i.e. ball-and-socket, variety of a diarthrosis, and, at the cost of a certain amount of security, it has obtained an extended range of movement.
The bones which enter into its formation are the glenoid cavity of the scapula and the head of the humerus.
The glenoid cavity is a shallow piriform articular surface, having its narrow end directed upwards and slightly forwards. The superior half of the anterior margin of the fossa is characterised by a shallow notch which accommodates the narrow part of the subscapularis muscle as it runs laterally to its insertion. At the apex of the cavity there is a flat area for the attachment of the long tendon of the biceps brachii muscle. The head of the humerus is hemispherical and articular, while, lateral to its articular margin, there is a slight constriction (the anatomical or true neck of the humerus), which is most strongly marked in relation to the greater and lesser tubercles of the humerus.
Under ordinary conditions the two articular surfaces are maintained in apposition by muscular action, aided by atmospheric pressure, and thus, when the muscles are removed, the bones fall asunder to the full extent of the restraining ligaments. Only a small part of the head of the humerus is in contact with the glenoid cavity at any particular moment, because the humeral head is much larger than the cavity, but, by reason of the shallow character of the cavity, all parts of the two articular surfaces may successively be brought into contact with each other.
In the position of rest, as the limb hangs parallel to the vertical axis of the trunk, the inferior aspect of the neck of the humerus is brought into close relation with the inferior part of the glenoid cavity.
Labrum Glenoidale. The labrum glenoidale (O.T. glenoid ligament) (Fig. 303) deepens the glenoid cavity, and thus extends the articular surface. It is situated within the fibrous stratum of the articular capsule, and to some slight extent increases the security of the articulation. It consists of a strong ring of dense fibrous
tissue attached to the margin of the glenoid cavity. Many of its fibres are short, and pass obliquely from the inner to the outer aspect of the ridge, so that its attached base is broader than its free edge, and therefore in cross section it appears somewhat triangular. The long tendon of the biceps, which arises from the apex of the glenoid cavity, becomes to a considerable extent incorporated with the labrum glenoidale.
Capsula Articularis.-The fibrous stratum (O.T. capsular ligament) (Fig. 302) of the articular capsule presents the general shape which is characteristic of the corresponding part in other ball-and-socket joints, viz., a hollow cylinder. By its proximal end the fibrous stratum is attached to the circumference of the glenoid cavity, external to the labrum glenoidale, and also, to a considerable extent, to the labrum glenoidale itself.
By its distal end it is attached to the neck of the humerus, and therefore beyond the articular area of the head. The fibrous stratum is strongest on its superior aspect, while inferiorly, where the neck of the bone is least defined, it
FIG. 302.-CAPSULE OF THE SHOULDER-JOINT AND CORACO-ACROMIAL LIGAMENT.
extends distally for a short distance upon the humeral shaft. Its fibres for the most part run longitudinally, but a certain number of them pursue a circular direction. The greater part of the epiphyseal line of the proximal end of the humerus is extra-capsular, but it is intra-capsular on the medial side of the bone.
A prolongation of the fibrous stratum, the transverse humeral ligament presenting both longitudinal and transverse fibres, bridges that part of the intertubercular groove which is situated between the tubercles of the humerus. At this point an interruption in the fibrous stratum, beneath the transverse humeral ligament, permits the long tendon of the biceps to escape from its interior. In addition to the opening just referred to, there is another very constant deficiency in the superior and anterior part of the fibrous stratum, where the narrowing tendon of the subscapularis muscle is brought into contact with a bursa formed by a protrusion of the synovial stratum. This defect in the fibrous stratum has its long axis in the direction of the longitudinal fibres. Occasionally there is a similar but smaller opening under cover of the tendon of the infraspinatus muscle. Through the two latter openings the joint cavity communicates with bursa situated between the capsule and the muscles referred to.
The tendons of the subscapularis, supraspinatus, and infraspinatus muscles fuse with, and so strengthen, the articular capsule as they approach their respective
On the superior aspect of the articulation the capsule is augmented by an
accessory structure, the ligamentum coracohumerale (Fig. 302). By its proximal end, which is situated immediately above the glenoid cavity, but subjacent to the coraco-acromial ligament, it is attached to the lateral border of the root of the coracoid process, while its distal end is attached to the humeral neck close to the greater tubercle. This ligament forms a flattened band, having its posterior and inferior border fused with the articular capsule, but its anterior and superior margin presents a free edge, slightly raised above the level of the capsule. This structure is believed to represent that portion of the pectoralis minor to which reference has already been made in connexion with the coraco-acromial ligament (p. 320).
The coraco-glenoid ligament is another accessory structure, which is not always present. It springs from the coracoid process along with the former ligament, and extends to the superior and posterior margin of the head of the scapula.
Gleno-humeral Ligaments (Fig. 303). If the articular capsule is opened from behind, and the head of the humerus removed, it will be seen that the longitudinal fibres of the anterior part of the fibrous stratum are specially developed in the form of thick flattened bands which extend from the anterior border of the glenoid cavity to the anterior aspect of the neck of the humerus. These gleno-humeral ligaments are three in number, and occupy the following positions: the
FIG. 303.-CAPSULAR LIGAMENT OF SHOULDER-JOINT CUT ACROSS AND HUMERUS REMOVED.
superior is placed above the aperture in the front of the capsule; the middle and inferior on the antero-inferior aspect of the capsule, and below the aperture mentioned.
The superior gleno-humeral ligament, which some believe to represent the ligamentum teres of the hip-joint, springs, along with the middle gleno-humeral band, from the superior part of the cavity. The inferior ligament is the strongest of the three, and springs from the inferior part of the anterior margin of the glenoid.
Intra-capsular Structures.-1. The labrum glenoidale, already described. 2. The long tendon of the biceps passes laterally from its attachment to the apex of the glenoid cavity and the adjoining part of the labrum glenoidale, above the head and neck of the humerus, to escape from the interior of the capsule by the opening between the tubercles of the humerus, subjacent to the transverse humeral ligament.
A synovial stratum (Fig. 304) lines the fibrous stratum of the capsule, and extends from the margin of the glenoid cavity to the humeral attachments of the fibrous stratum, where it is reflected towards the margin of the articular cartilage. It is therefore important to note that the inferior aspect of the humeral neck has the most extensive clothing of the synovial stratum. Further, the synovial stratum envelops the intra-capsular part of the tendon of the biceps, and although this tubular sheath is prolonged upon the tendon into the proximal part of the
intertubercular sulcus, yet the closed character of the synovial cavity is maintained. Thus, while the tendon is within the capsule, it is not within the synovial cavity. The synovial stratum is continuous with those bursa which communicate with the joint cavity through openings in the fibrous stratum of the capsule.
Long head of biceps
Bursa (a) Communicating with the Joint Cavity.-Practically there is only one bursa which is constant in its position, viz., the subscapular, between the capsule and the tendon of the subscapularis muscle. It varies considerably in its dimensions, but its lining membrane is always continuous with the synovial stratum of the capsule (Figs. 301 and 302), and therefore it may be regarded merely as a prolongation of the articular synovial stratum. Occasionally a similar but smaller bursa occurs between the capsule and the tendon of the infraspinatus muscle.
(b) Not communicating with the Joint Cavity.-The subdeltoid or sub-acromial bursa is situated between the muscles on the superior aspect of the shoulder-joint on the one hand and the deltoid muscle on the other. It is an extensive bursa, and is prolonged subjacent to the acromion and the coracoacromial ligament. It does not communicate with the
FIG. 304.-VERTICAL SECTION THROUGH THE SHOULDER-JOINT.
shoulder-joint, but it greatly facilitates the movements of the proximal end of the humerus against the inferior surface of the coraco-acromial arch.
Movements at the Shoulder-Joint.-A ball-and-socket joint permits of a great variety of movements, practically in all directions; but if these movements are analysed, it will be seen that they resolve themselves into movements around three primary axes at right angles to each other, or around axes which are the possible combinations of the primary ones.
Thus, around a transverse axis, the limb may move forwards (flexion) or backwards (extension). Around an antero-posterior axis it may move laterally, i.e. away from the median plane of the trunk (abduction), or medially, i.e. towards, and to some extent up to, the median plane (adduction)
Around a vertical axis, the humerus may rotate upon its axis in a medial or lateral direction to the extent of a quarter of a circle.
Since these axes all pass through the shoulder-joint, and since each may present varying degrees of obliquity, it follows that very elaborate combinations are possible until the movement of circumduction is evolved. In this movement the head of the humerus acts as the apex of a cone of movement with the distal end of the humerus, describing the base of the cone.
The range of the shoulder-joint movements is still further increased owing to the mobility of the scapula as a whole, and owing to its association with the movements of the clavicle already described.
The elbow-joint 1 provides an instance of a diarthrosis capable of performing the movements of flexion and extension around a single axis placed transversely, i.e. a typical ginglymus diarthrosis or hinge-joint.
The bones which enter into its formation are the humerus, ulna, and radius. The trochlea of the humerus articulates with the semilunar notch of the ulna (articulatio humeroulnaris); the capitulum of the humerus articulates with the shallow depression or cup on the proximal aspect of the head of the radius (articulatio humeroradialis). The articular cartilage clothing the trochlea of the humerus terminates in a sinuous or concave margin both anteriorly and posteriorly, so that it does not line either the coronoid or the olecranon fossa. Medially, it merely rounds. off the medial margin of the trochlea, but laterally it is continuous with the encrust
The articulatio cubiti or elbow-joint includes the humero-radial, humero-ulnar, and the proximal radio-ulnar joints; but, for convenience, the description given here is limited to the humero-radial and humero-ulnar joints.
ing cartilage covering the capitulum, to the margin of which the cartilage extends in all directions, and thus it presents a convex edge in relation to the radial fossa. The cartilage which lines the semilunar notch of the ulna presents a transverse interruption, considerably wider on its medial as compared with its lateral aspect. Thereby the coronoid and olecranon segments of the notch are separated from each other. The cartilage which clothes the coronoid segment is continuous with that which clothes the radial notch of the ulna. The shallow cup-shaped depression on the head of the radius is covered with cartilage which rounds off the margin, and is prolonged without interruption upon the vertical aspect of the head, extending to its most distal level on that part opposed to the radial notch of the ulna.
Capsula Articularis.Taken as a whole, the ligaments form a complete fibrous stratum of the articular capsule, which is not defective at any point, although it is not of equal thickness throughout, and certain bands of fibres stand out distinctly because of their greater strength.
The common epiphyseal line for the trochlea, capitulum, and the lateral epicondyle of the humerus, is partly intra-capsular and partly extra-capsular; that for the medial epicondyle is extra-capsular. The epiphyseal line of the olecranon is intra-capsular only anteriorly, and it may be altogether extra-capsular.
Lig. Anterius. - The anterior ligament (Fig. 305) consists of a layer whose fibres run in several directions-obliquely,
versely, and longitudinally -and of these the vertical fibres are of most importance. It is attached proximally to the proximal margins of the coronoid and radial fossa; distally, to the margins of the coronoid process and to the annular ligament of the proximal radio-ulnar joint, but some loosely arranged fibres reach as far as the neck of the radius. The marginal portions of this ligament, which are situated in front of the capitulum and the medial margin of the trochlea respectively, are much thinner and weaker than the central part. Fibres of origin of the brachialis muscle are attached to the front of this ligament.
Lig. Posterius. The posterior ligament is an extremely thin, almost redundant layer. Proximally it is attached, in relation to the margin of the olecranon fossa, at a varying distance from the trochlear articular surface, and distally to the summit and sides of the lip of the olecranon. Laterally some of its fibres pass from the posterior aspect of the capitulum to the posterior border of the radial notch of the ulna. This ligament derives material support from, and participates in the movements of, the triceps brachii muscle, since they are closely adherent to each other in the region of the olecranon.
Lig. Collaterale Ulnare. The ulnar collateral ligament (O.T. internal lateral)