Understanding Shoulder Impingement Etiology

Normal shoulder joint structures

To understand the etiology of subacromial impingement, it is important to understand the unique anatomical characteristics of the subacromial space. Within this space, a number of soft-tissue structures are situated between two rigid structures that move.
The superior border (the roof) of the space is the coracoacromial arch, which consists of the acromion, the coracoacromial ligament, and the coracoid process. The acromioclavicular joint is directly superior and posterior to the coracoacromial ligament.
The inferior border (the floor) consists of the greater tuberosity of the humerus and the superior aspect of the humeral head.
The height of the space between the acromion and the humeral head ranges from 1.0 to 1.5 centimeters as seen on radiographs³².

However, interposed between these two osseous structures are the rotator cuff tendons, the long head of the biceps tendon, the bursa, and the coracoacromial ligament. Therefore, the true height of this space is considerably less than that seen on radiographs. Normally, the bursa facilitates the motion of the rotator cuff beneath the arch. Any abnormality that disturbs the relationship of these subacromial structures may lead to impingement^13,104.

Intrinsic Factors

1.Muscle Weakness

Impingement occurs as a result of weakness of the muscles of the rotator cuff and that the pathological change in the supraspinatus tendon occurs primarily as a result of tension overload.

Although this phenomenon is most dramatic in patients who swim or who participate in racquet or throwing sports, it also can occur in carpenters, mechanics, plumbers, and other manual laborers who use overhead motions in their work. Proximal migration of the humeral head has been associated with muscle fatigue, injury, and degenerative changes in the tendons.

2.Overuse of the Shoulder

Subacromial impingement also may develop as a result of inflammation and thickening of the rotator cuff tendons or the subacromial bursa. The primary cause is soft-tissue inflammation resulting from overuse of the shoulder, which increases the area occupied by the soft tissues in the subacromial space and leads to friction and wear against the coracoacromial arch.Inflammation also can result from a variety of systemic diseases.The overuse syndrome, which is caused by repetitive microtrauma, is another source of intrinsic tendinitis, bursitis, and impingement.

3.Degenerative Tendinopathy:Intrinsic degenerative tendinopathy may play an important role in impingement syndrome

Extrinsic Factors

1.Acromial Morphology

It is a potential source of symptoms in the shoulder and it is noted that the acromial epiphysis could limit motion because of its slight downward displacement.t variations in the shape and slope of the anterior aspect of the acromion were responsible for subacromial impingement and associated tears of the rotator cuff. Furthermore, a spur that apparently had been caused by tensile forces on the coracoacromial ligament often was found to be protruding into the subacromial space. Eburnation with erosion of the acromion was noted in several specimens and was thought to be a later manifestation of impingement.

A substantially higher prevalence of full-thickness tears of the rotator cuff was noted in association with type-III (HOOKED)acromions.

2.Glenohumeral Instability

When managing young competitive athletes who have symptoms of impingement, it is imperative to rule out underlying glenohumeral instability as the primary source of the problem. Subtle glenohumeral subluxation may cause an adjustment in the mechanics of the overhead motion, which may lead to secondary impingement.

3.Degeneration of the Acromioclavicular Joint

Degeneration of the acromioclavicular joint may contribute to subacromial impingement. Osteophytes that protrude inferiorly from the undersurface of a degenerative acromioclavicular joint can contribute to impingement when the cuff passes beneath the joint⁸⁹

X-ray - AP Shoulder shows acromioclavicular spur

02 - Anteroinferior Acromial Spur of the Shoulder

Patients who had so-called painful arc syndrome could be divided into three categories.

I.Approximately two-thirds of the ninety-seven patients in the study had lesions of either the posterior (CATEGORY 1)or the anterior aspect(CATEGORY 2) of the rotator cuff. All of these patients had resolution of the symptoms after subacromial injection of a local anesthetic and a steroid or division of the coracoacromial ligament.

II. The remaining patients had pain in the superior aspect (CATEGORY 3) of the rotator cuff, which usually was associated with degeneration of the acromioclavicular joint.

Shoulder Outlet view showing the acromioclavicular joint spur

01- Subacromial Impingement by Type 3 Spur

Excision of the distal one centimeter of the clavicle provided consistent relief of pain.

4.Impingement by the Coracoacromial Ligament

The coracoacromial ligament as a source of impingement. The coracoacromial ligament was an offending structure in painful shoulders and was responsible for the so-called snapping shoulder, a condition in which the bursa thickens and snaps back and forth under the leading edge of the coracoacromial ligament on the undersurface of the acromion.

5.Os Acromiale

Os acromiale is an unfused distal acromial epiphysis and was first described, in 1863, by Gruber⁴⁴. Folliasson³³ classified the lesion into four distinct types on the basis of its anatomical location, with the mesoacromion being the most common type.