The most common dislocation after a severe trauma is a glenohumeral dislocation. Overhead sports such as tennis, volleyball, and baseball are associated with glenohumeral instability. (2) These activities cause the joint to be in abduction and external rotation. Repetition of motion, collision, or falling on an outstretched arm can lead to instability and/or dislocation. The Glenohumeral joint is already prone to dislocation, because of it being a large head of the humerus going into a relatively small socket. Almost ninety five percent of dislocations in the glenohumeral joint are anterior.(2)
The shoulders dynamic joint components provide the shoulder with the stability. The muscles and tendons form a cuff like arrangement around the joint.(2) The glenohumeral joint relies on support from a group of four muscles know as the rotator cuff. These muscles allow the shoulder to function, while maintaining balance between mobility and stability.(3) The rotator cuff allows the humeral head to stay within 1-2 millimeters of the middle of the glenoid fossa.(3) These muscles compress and depress the humeral head to prevent it from rolling off the top of the fossa.(3) More Specifically, the Supraspinatus works closely with the deltoid for arm flexion and abduction. The Supraspinatus comes from the Supraspinatus fossa of the scapula, and attaches to the greater tuberosity of the humeral head .(3)The subscapularis, is an internal rotator, and is found in the subscapular fossa, and then inserts on the lesser tuberosity.(3) The infraspinatus is found in the infraspinatus fossa on the posterior surface of the scapula, it also attaches to the greater tuberosity of the humerus.(3) The teres minor is another muscle, which attaches at the greater tuberosity, it along with the infraspinatus perform external rotation.(3) The rhomboids retract and rotates the scapula to depress the glenoid cavity inferiorly and fixes the scapula to the thoracic wall.(6) The interior of the joint has attachment sites for the triceps, biceps, and deltoid.(10) The glenoid fossa (socket ) is made two times deeper by the labrum surrounding the edge.(2,10) The labrum is a fibro cartilaginous ring formed around the edges of the fossa. (3) The inferior glenohumeral ligament is the most important static stabilizer of the glenohumeral joint.(7) The inferior ligament is also aided in stability by the middle and superior glenohumeral ligaments.(7) The superior ligament provides resistance to inferior translation, while the middle ligament resists anterior translation.(7) The middle ligament is aided by an anterior band that resists anterior motion.(7) There is also a posterior band that is resistant to flexion, adduction, and internal rotation.(7) When dealing with a shoulder dislocation you should always be aware that there could be nerve or artery damage, seeing as how there are nerves as well as vessels that run between the humerus and scapula as well as around the structures.(6)
Common Mechanism of Injury
Falling onto an outstretched arm, or a collision is the most probable cause for shoulder dislocations, because the humeral head is forced out of the glenoid joint.(1,2,9,10) Although overuse, and any arm position where the shoulder is abducted and externally rotated can lead to a shoulder dislocation. An anterior shoulder dislocation is usually from external rotation, extension, and abduction, the action used preparing for a volleyball spike.(9) Posterior dislocations are usually caused from severe internal rotation and adduction, this occurs most during a seizure.(9) Inferior dislocations are rare, but may be caused by an axial force to a arm raised overhead.(9) A bankart lesion could be a possible cause for instability leading after a shoulder dislocation. A bankart lesion is often caused as the shoulder "pops" out of the joint, causing the labrum to tear.(4) If the injury occurs on the...
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