Over the past months its seems that many people are complaining of shoulder pain. When I evaluate these clients I find that they are suffering from dysfunction related to the rotator cuff muscles. This has prompted me to write about the rotator cuff in a way that you can understand. I will be breaking this blog up into two parts. Part one will be about the biomechanics of the rotator cuff and clinical pathology. Part two will be about exercises for working the rotator cuff area.
Biomechanic Function of The Rotator Cuff
The rotator cuff consists of tendinous insertions of the subscapularis, supraspinatus, infraspinatus and teres minor muscles. These muscles form a roof, that surrounds the head of the humerus anteriorly (front side), superiorly and posteriorly. Co-contraction of these muscles stabilizes the glenohumeral joint during regular activities. In particular, abduction in the plane of the scapula will be accomplished principally by action of the deltoid muscle, but this acts nearly vertically when the humerus is still close to the side of the body, and so tends to sublux the head of the humerus superioly. The rotator cuff muscles act more horizontally, and their tensions combine with that of the deltoid to direct the force of the superior concavity of the glenoid, which is normally in a stable state.
Conversely, rotator cuff deficiency leads to superior subluxation of the head of the humerus, leading to impingement against the coracoacromial ligament, accompanied by adbuction weakness and a loss of motion. Pain is referred to the superior lateral aspect of the humerus. Causes of supraspinatus degeneration is not fully understood. It is believed that the cause of degeneration is a result of shearing forces between the layers of the histological and mechanical properties of the different layers of the supraspinatus tendon. Also, the area is sensitive to compressive forces, such as superior impingement beneath the acromion. Another root cause could be excessive humeral rotation causing tension and slack to the edges of the supraspinatus tendon.
The rotator cuff’s primary function is to centralize the humeral head, limiting superior translation during abduction. The supraspinatus, infraspinatus, and teres minor tendons insert on the greater tuberosity, whereas the subscapularis tendon inserts on the lesser tuberosity. The subscapularis tendon lies on the anterior aspect of the anterior capsule of the glenohumeral joint, and its superior portion is intraarticular. The subscapularis bursa lies between the subscapularis tendon and the scapula. As the subscapularis muscle becomes attenuated from repeated episodes of dislocation, it may be the source of recurrent instability. The rotator cuff interval is located between the superior aspect of the subscapularis tendon and the inferior aspect of the suprspinatus tendon. This interval contains the coracohumeral ligament and the superior glenohumeral ligament. The rotator interval lesion has been attributed to a possible deficiency of the superior glenohumeral ligament. Surgical closure of the interval appears to eliminate excessive inferior translation.
Tears developed by acute trauma or chronic impingement tend to be the major dyfunction associated with the rotator cuff. Clients will express complaints associated with the anterior lateral aspect of the humeral head and will have a reduced range of motion. An impingement disorder has many variables attached to it; the rotator cuff muscles, the long head of the biceps, the subacromial bursa, the AC joint, the acromion and the humeral head can all be associated to acting upon the stress of the shoulder. The impingement occurs when the subacromial space is decreased to the point where tearing is evident compromising the centralizing effect of the cuff allowing the humerus to shift beyond regular range.
There are three stages to rotator cuff impingement disorder. Type I: Slight rotator cuff degeneration or irritation (tendonitis) no visible tearing. Type II: Moderate rotator cuff degeneration with partial tearing of the articular or bursal surfaces. Type III Complete tearing, degeneration and functional incapacity.
Many of the rotator tears are associated with improper shoulder biomechanics and over use syndrome. Insufficient contractile strength in the deltoids, trapezeus, pectoralis minor can also be a leading factor in the development of rotator dysfunction. Muscle imbalances and weakness will force the smaller inferior muscles of the shoulder to generate contractile forces beyond their capacity resulting in acute inflammatory factors and degeneration. Rotator cuff tendonitis can be attributed to chronic repetitive eccentric tensile overload of the rotator tendons. Most tears are attributed to degenerative lesions associated with increasing age. Athletes, from over use and high impact, tend to be more susceptible to tears as well.
Depending on the severity of the damage to the rotator cuff a person can successfully remedy the situation with proper exercise and biomechanical leveraging. I will discuss this further in the exercise for the rotator cuff part two. I hope that this has helped you gain a better understanding of the mechanisms of the rotator cuff.
Daryl Conant, M.Ed
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