Subacromial and subcoracoid impingement have been categorized as external impingement, one type of shoulder impingement syndrome (SIS). Differentiation between subacromial impingement and subcoracoid impingement are important in determining the treatment target. The thickness of the coracohumeral ligament (CHL) may affect coracohumeral distance (CHD), which has been suggested as a possible factor in developing subcoracoid impingement with subscapularis (SSC) lesion. Evidence that indicates the existence of abnormal SSC, CHL thickness and CHD in people with SIS is limited. The purposes of the study were (1) to evaluate the correlations between CHL thickness and CHD by using ultrasonography in people with SIS with and without SSC lesion, (2) to examine the difference in SSC/CHL thickness and CHD between people with SIS with and without SSC lesion.

Based on previous studies, a total sample size of 40 participants was calculated to provide 80% power with detection of a difference of more than 1.0 mm CHD between 2 groups. The inclusion criteria of the participants were age of 20-60 years old and positive unilateral shoulder results on at least 3 of 5 tests: 1) Neer's test, 2) Hawkins' test, 3) the Empty can test, 4) the pain or weakness with resisted ER test, and 5) tenderness in the tendon of the rotator cuff. Participants with a history of shoulder dislocation, fracture or surgery, history of direct contact injury to the neck or upper extremities within the past month, glenohumeral joint instability (positive apprehension test, sulcus sign), neurologic disorder (upper motor neuron diseases, cervical radiculopathy), passive ER Range of Motion (ROM) less than 30 degrees, or pain (visual analogue scale, VAS more than 5) during the experimental tasks were excluded. After the impingement tests were performed to ensure that the participants met our inclusion criteria, all participants were assessed with 3 special SSC tests, namely, the lift-off test, belly-press test (Napoleon sign) and bear-hug test, for group allocation. Patients with positive results on at least 2 of the 3 special tests were allocated to the SSC lesion (SSCL) group.

The characteristics of the participants were collected by one assessor, including age, gender, height, weight, dominant side, involved side, duration of symptom, pain (VAS), occupation ratio, the Flexilevel Scale of Shoulder Function (FLEX-SF) and internal rotator strength.

USG measurements We measured the following outcomes: (1) CHL thickness, (2) CHD and (3) AHD. Each outcome was measured in 3 trials and the mean of the 3 trials was used for data analyses. All of the measurements were measured with THI turned on except for that of CHD, due to the deeper anatomical structure.

For measurement of the CHL thickness, the position of the linear probe was on the lateral border of the coracoid process to obtain a longitudinal image of the CHL. Each participant was instructed to lie in supine position and relax while the examiner maintained the elbow of the participant at flexion of 90° and the shoulder under maximal ER without shoulder abduction or flexion (arm by side) (Figure 2). Maximal ER of the shoulder was achieved when the examiner could not further externally rotate the shoulder of the participant. The thickness of the CHL at a 2-mm distance from the coracoid process was measured.

CHD was measured with the probe positioned on the lateral border of the coracoid process to obtain images of the coracoid process and humeral head in 4 different shoulder rotation positions: (1) shoulder neutral rotation (CHD-NR), (2) external rotation (CHD-ER) and (3) shoulder internal rotation with maximal forward flexion and full adduction (CHD-IRFA, with the arm adducted across the chest reaching for the opposite shoulder) and (4) shoulder internal rotation (CHD-IR) (Figure 2). Participants were asked to sit with their arms by their sides and to perform the 4 different positions respectively. The measurements were repeated for 3 trials with repositioning of the arm to a neutral position for intervals of 10 seconds. The distance measured was that between the coracoid process and the lesser tuberosity of the humerus.

SSP tendon thickness was evaluated with the patient's palm placed over his/her iliac wing, or "back pocket", with the elbow flexed and directed medially. The transducer was placed over the anterior aspect of the shoulder, perpendicular to the supraspinatus tendon and just anterior of the anterior-lateral margin of the acromion. A transverse glide was then performed at the site to determine the exact position where the observer judged that the tendon thickness was at its maximum. The thickness of the SSP tendon was measured 2 cm away from the biceps long head tendon. For measuring SSC tendon thickness with a short axis of view, the probe was positioned horizontally on the bicipital grove. The participant's forearm was placed with the elbow flexed to 90° in slight internal rotation, with the palm facing upward and medially. Then the patient was asked to rotate the forearm externally, keeping the palm up and the elbow strictly close to the iliac crest.

Acrociohumeral distance (AHD) was measured under 0° and 60° of scapular plane shoulder elevation. The transducer was placed on the most anterior aspect of the acromion edge, with the long axis of the transducer placed in the plane of the scapula and parallel to the flat surface of the acromion. The participants sat in an upright position without back support and with their feet flat on the floor, holding their shoulders back and looking straight ahead, to achieve retracted shoulders and extension in the thoracic and cervical spine. AHD was measured at 0° and 60° of active shoulder elevation in the scapular plane. A 60° scapular elevation of AHD was then measured with a goniometer placed on the patient's arm to determine 60° of active shoulder elevation, with the thumb pointing up.

Inclusion criteria:

1. Age from 20 to 60 years old
2. Unilateral shoulder symptoms

2. At least three positive of five tests, including Neer's test, Hawkin's test, Empty can test, tenderness in tendon of rotator cuff and resisted external test.

Exclusion criteria:

1. Shoulder dislocation
2. Fracture
3. Surgery
4. History of direct contact injury to the neck or upper extremities within the past month
5. Glenohumeral joint instability (positive apprehension test, sulcus sign)
6. Neurologic disorder (upper motor neuron diseases, cervical radiculopathy)
7. Passive ER ROM less than 30 degrees
8. Pain (visual analogue scale, VAS more than 5)

Forty subjects whose age from 20 to 60 years old were recruited from National Taiwan University Hospital and through general local internet media. The inclusion criteria of subjects will be at least three positive of five tests, including Neer's test, Hawkin's test, Empty can test, tenderness in tendon of rotator cuff and resisted external test. In addition, participants with a history of shoulder dislocation, fracture or surgery, history of direct contact injury to the neck or upper extremities within the past month, glenohumeral joint instability (positive apprehension test, sulcus sign), neurologic disorder (upper motor neuron diseases, cervical radiculopathy), passive ER ROM less than 30 degrees and pain (visual analogue scale, VAS less than 5) during the experimental tasks will be excluded.