Covering the anterior, upper and posterior parts of shoulder joint, rotator cuff is mainly composed of supraspinatus, infraspinatus, teres minor and subscapularis. The major function of rotator cuff is to press humeral head into glenoid cavity during the abduction of ipsilateral upper arm, which helps to provide a pivot for the abduction and lifting activities of shoulder joint through dynamic stabilization of humeral head. Rotator cuff injury is one of the most common diseases of shoulder joint. The progression of this disease leads to the broken of balance and the upshift of humeral head, which may ultimately result in shoulder joint dysfunction and limitations of abduction and lifting activities. Some scholars believe that the distance between humeral head and acromion can be measured to assess the rotator cuff injury and to assist the selection of treatment options. Although MRI and shoulder arthroscopy have been widely used in clinical practice, conventional radiographic examination is still the preferred option. With retrospective analysis and research on the previous case, this study aims to determine the correlation between acromiohumeral distance (AHD) and rotator cuff injury, discuss the application value of conventional X-ray measurement in the differential diagnosis of rotator cuff injury, and provide guidance for clinical treatment options and prognosis.

(1) General information. From September 2014 to March 2016, 115 patients (55 males and 60 females) with full-thickness rotator cuff injury were treated with shoulder arthroscopy in Shanghai Tenth People's Hospital. There were 52 cases of left and 63 cases of right, and the age ranged from 28-72 years with an average of 59.3 years. All patients were conducted with anteroposterior X-ray and MRI examinations before and after operation. The preoperative MRI examination and intraoperative arthroscopic exploration were evaluated. According to the positions of rotator cuff tears, the experimental group was divided into 4 subgroups: anterior tear (involving subscapularis) , superior tear (involving supraspinatus and it's both sides) , posterior tear (involving infraspinatus and teres minor) and extensive tear (involving multiple tendons) . Based on the levels of tendon contracture, the experimental group was alternatively divided into 3 subgroups including Patte Grade I, II and Ⅲ. During the same period, 50 adults (24 males and 26 females) with negative results of shoulder X-ray and MRI examinations were randomly selected as the control group from outpatient clinic. There were 21 cases of left and 26 cases of right, and the mean age ranged from 26-63 years with an average of 53.8 years. (2) Inclusive criteria.① Age ≥18 years; ②No previous history of shoulder fracture or surgery;③No glenohumeral arthritis;③ No shoulder skeletal malformation. (3) Examination methods. The rotator cuff contracture was assessed by the preoperative MRI images. The AHD, the shortest distance from lateral boarder acromion to humeral head tip, was measured on the anteroposterior view of shoulder X-ray film. The analysis of radiographic image and the measurement of parameter were accomplished by 2 experienced clinicians, and the study was included when the results were consistent between the two. (4) Statistical analysis. SPSS 17.0 software was used for statistical analysis. The measurement data was presented as mean ± standard deviation (±s) , and t-test was used for comparison among groups. variance analysis and LSD-t test were used for AHD comparison among different subgroups. A P value < 0.05 was regarded as statistically significant.

The mean values of AHD for the experimental group and control group were 9.3 mm and 10.6 mm respectively. The difference between two groups was statistically significant (P <0.05) . According to the locations of rotator cuff tear, there were 12 cases of anterior tear, 57 cases of superior tear, 25 cases of posterior tear and 21 cases of extensive tear. Based on the degrees of tendon contracture, there were 38 cases of Patte Grade I, 45 cases of Patte Grade II and 32 cases of Patte Grade Ⅲ.The variance analysis showed that there was statistical difference among subgroups (F=25.080, P <0.05) ; the AHDs for posterior tear and extensive tear subgroups were remarkably lower than those for anterior tear and superior tear groups (P <0.05) ; there was no statistical difference between anterior tear group and superior tear group (P>0.05) ; the ADH for extensive tear group was lower than that for posterior tear group (P <0.05) . Through the comparisons of AHD between various subgroups and their corresponding normal control groups, no statistical difference was found between anterior tear group and its control group or between superior tear group and its control group (P>0.05) ; The AHD in the posterior group or extensive group was remarkably less than that in the control group (P <0.05) .

The reduction of AHD (upward movement of humeral head) is directly related to the position and size of rotator cuff tear. The upward movement of humeral head can be cause by both the extensive tear and the posterosuperior tear of rotator cuff, which is more obvious when there is rotator cuff amyotrophia and fatty degeneration. The measurement of AHD can provide effective information for clinicians to evaluate rotator cuff injury and to choose treatment scheme. Therefore, routine shoulder radiographic examination can be performed for the patient with shoulder pain, and the patient with abnormal AHD is required to be further examined by MRI.