Thus the undersurface of anterior third of the acromion process, the coracoacromial ligament and the coracoid process form together the coracoacromial arch, which provides a hood-like protection for the shoulder joint.
The anatomical factors that may excessively narrow the subacromial space include variations in shape and inclination of the acromion or prominent osseous changes in the inferior aspect of coracoacromial arch.  The impingement may also occur as a result of thickening of the rotator cuff from acute or chronic inflammation.
The coracoid process is a key connection between the scapula and the clavicle and an important anchor in the coracoacromial arch
; it is part of the superior shoulder suspensory complex (SSSC) as proposed by Goss et al.
Subacromial impingement involves compression or mechanical irritation of the subacromial bursa, supraspinatus tendon, infraspinatus tendon, and/or long head of the biceps tendon between the coracoacromial arch
and the humeral head (Figure 2(a))  and is purported to occur at lower humeral elevation angles.
The coracoacromial arch
consists of the acromion, coracoacromial ligament, and coracoid process.
If the coracoacromial arch
is violated, these patients may also demonstrate superior escape of the humeral head.
The examiner then attempts to further internally rotate the shoulder, driving the greater tuberosity into the coracoacromial arch
Extrinsic impingement refers to pathological changes in the coracoacromial arch
This view shows the coracoacromial arch
, including a significant curve or spur on the anterior and inferior acromion.