- shoulder complex is a set of four mechanically interrelated articulations involving the sternum, clavicle, ribs, scapula, and humerus
- These joints provide extensive range of motion to the upper extremity, hereby increasing the ability to reach and manipulate objects
Palpation of the inferior angle provides a convenient method for following the movement of the scapula during arm motion
At rest the scapula is normally positioned against the posterior-lateral surface of the thorax, with the glenoid fossa facing about 30–40 degrees anterior to the frontal plane. This orientation of the scapula is referred to as the scapular plane. The scapula and humerus tend to naturally follow this plane when the arm is raised overhead.
these crests receive the distal attachments of the pectoralis major and teres major . Between these crests is the intertubercular (bicipital) groove, which houses the tendon of the long head of the biceps brachii.
The latissimus dorsi muscle attaches to the floor of the intertubercular groove, medial to the biceps tendon.
At rest the scapula is normally positioned against the posterior-lateral surface of the thorax, with the glenoid fossa facing about 30–40 degrees anterior to the frontal plane. This orientation of the scapula is referred to as the scapular plane. The scapula and humerus tend to naturally follow this plane when the arm is raised overhead.
The joints of the shoulder complex function as a series of kinematic links, all cooperating to maximize the range of motion available to the upper limb. A weakened, painful, or unstable link anywhere along the chain significantly decreases the effectiveness of the entire complex and arguably the entire upper limb
The glenohumeral joint is the articulation formed between the relatively large convex head of the humerus and the shallow concavity of the glenoid fossa.
Several anatomic features of the glenohumeral joint contribute to a design that favors mobility at the expense of stability. stability at the GH joint is achieved by a combination of passive and active mechanisms. Passive mechanisms include (1) restraint provided by capsule, ligaments, glenoid labrum, and tendons; and (2) mechanical support predicated on scapulothoracic posture and (3) negative intracapsular pressure. Active mechanisms rely on the forces produced by muscle, provided primarily by the embracing nature of the rotator cuff group
This joint operates in conjunction with the moving scapula to produce an extensive range of motion of the shoulder
The GH joint is surrounded by a fibrous capsule that isolates the joint cavity from most surrounding tissues. The capsule attaches along the rim of the glenoid fossa and extends to the anatomic neck of the humerus. To generate stabilizing tensions across the joint, the inherently loose capsular ligaments must be elongated or twisted to varying degrees; the resulting passive tension generates mechanical support for the GH joint and limits the extremes of rotation and translation
Rotator Cuff Muscles: four muscles (The subscapularis lies just anterior to the capsule, The supraspinatus, infraspinatus, and teres minor lie superior and posterior to the capsule) form a cuff that protects and actively stabilizes the GH joint, especially during dynamic activities. In addition to the belly of the rotator cuff muscles being located very close to the joint, the tendons of these muscles actually blend into the capsule. This unique anatomic arrangement helps explain why the mechanical stability of the GH joint is so dependent on the innervation, strength, and control of the rotator cuff muscles.
The rotator cuff fails to cover two regions of the capsule: inferiorly, and a region between the supraspinatus and subscapularis known as the rotator (cuff) interval. This region of the anterior-superior capsule is often thin and presents with openings or deficits of variable sizes but not indicate pathology.
The rotator interval is typically reinforced by the tendon of the long head of the biceps, the coracohumeral ligament, and by superior and (sometimes upper parts of the) middle GH ligaments. The rotator interval is a relatively common site for anterior dislocation of the GH joint and therefore the anatomic detail is a concern to the arthroscopic surgeon attempting to reinforce the region.
The passive mechanism described above to produce static stability at the GH joint is often adequate for activities such as standing with the relatively unweighted arm hanging freely at the side (A). A secondary, muscular-based mechanism may be needed to ensure additional stability when the upper limb encounters a significant distractive load, such as when holding a load by hand at waist level (B). The secondary, active source of static support is furnished primarily by the rotator cuff muscles. The overall force vector generated by the rotator cuff is oriented nearly horizontally, roughly parallel with the compression force generated by the passive mechanism. Isometric activation of the rotator cuff muscles effectively compresses the humeral head firmly against the shallow glenoid. Dynamic stability at the GH joint relies heavily on the interaction of these active and passive forces, particularly because of the lack of bony containment of the joint.
Elevation of the arm is performed by muscles that typically fall into three groups: (1) muscles that elevate (i.e., abduct or flex) the humerus at the GH joint; (2) scapular muscles that control the upward rotation of the scapulothoracic joint; and (3) rotator cuff muscles are regulators of dynamic joint stability (Forces produced primarily by the rotator cuff (and their attachments into the capsule) not only actively rotate the humeral head but also compress and centralize it against the glenoid fossa. Dynamic stability at the GH joint therefore requires healthy neuromuscular and musculoskeletal systems) and controllers of the arthrokinematics
The entire rotator cuff group is active during shoulder adduction and extension. Forces produced by these muscles assist with the action directly or stabilize the head of the humerus against the glenoid fossa
