A survey of upper extremity morbidity was conducted with support from public health officers. The study cohort was comprised of 1,149 uncompensated volunteers from the studied rural region. One of those recruited volunteers had an amputated shoulder; therefore, 2,297 shoulders were included in the study cohort. Of these volunteers, study subjects were enrolled according to the following inclusion and exclusion criteria. The inclusion criteria were the completion of a written consent and of a questionnaire, physical examinations, fasting blood tests, and simple radiographs (true anteroposterior, axillary lateral, and outlet views) and MRI evaluations of bilateral shoulders. The exclusion criteria were a lack of participation in shoulder MRI studies (n=17), a relevant history of trauma (n=26), previous shoulder surgery (n=13), glenohumeral joint osteoarthritis (n=12), calcific tendinitis (n=15), frozen shoulder (n=9), and/or use of medications that could affect serum lipid profiles (n=118). After exclusion, a total of 939 enrolled subjects, of whom 462 were male and 477 were female with a mean age of 59.2±8.4 years, were included in the study. Because several non-systemic variables are shoulder-related factors that would not affect both bilateral shoulders similarly, only one shoulder per subject was included in the analysis as the studied side to evaluate the strength of associations among variables. For subjects with either bilateral SBT or no SBT, one shoulder was randomly included (using random number generation by Excel). For each subject with unilateral SBT, only the involved shoulder was included as the studied side (Fig. 1).

MRIs were performed using a 1.5-T scanner (Siemens Medical Systems, Erlangen, Germany). Four sequences, each with a slice thickness of 3 mm, a field of view from 15.9 to 18.0 cm, and one excitation, were obtained as follows: (1) oblique sagittal T1-weighted spin echo, (2) oblique sagittal T2-weighted turbo-spin-echo (TSE) with fat saturation, (3) oblique coronal T2-weighted TSE with fat saturation, and (4) axial T2-weighted TSE with fat saturation. All MRIs were interpreted by one experienced musculoskeletal radiologist who was blind to the clinical findings (JBN). Full thickness RCTs were diagnosed based on a discontinuity or gap in the tendon or an increased signal intensity on T2-weighted images, extending from the articular to the bursal surfaces. Partial thickness RCTs were diagnosed based on partial high intensity in the rotator cuff tendon or on a slight increase in signal intensity in the cuff tendon, without a definite defect on either the intra-articular or the bursal side. Biceps tendon injuries were determined by MRI, then classified as partial or complete tear, or subluxation. Partial biceps tendon tear was identified by increased intra-tendinous T2-weighted signal intensity. A complete tear was identified by absence of the LHBT intra-articularly or within the bicipital groove. Subluxation was identified by displacement of the LHBT from the bicipital groove [15].

The studied variables were as follows. The demographic or general physical factors included age, sex, waist circumference, and dominant side involvement. The social factors included tobacco smoking, alcohol use, and manual labor and the comorbidities included diabetes, hypertension, metabolic syndrome, and dyslipidemia. Previous diagnoses of diabetes and hypertension were accepted. New diagnoses were made during the study using current standards for blood test and blood pressure findings as follows: diabetes, by serum levels of glycated hemoglobin (HbA1c) ≥6.5% or of fasting glucose ≥126 mg/dL [16] and hypertension, by blood pressure >140 mmHg in systolic or >90 mmHg in diastolic [17]. Clinical identification of metabolic syndrome involved meeting at least three of these five criteria: (1) fasting plasma glucose level ≥100 mg/dL or use of antidiabetic medication, (2) systolic blood pressure ≥130 mmHg or diastolic blood pressure ≥85 mmHg, or use of antihypertensive medication, (3) serum triglyceride (TG) level ≥150 mg/dL, (4) serum high-density lipoprotein (HDL) level <40 mg/dL for men or <50 mg/dL for women, and (5) waist circumference ≥90 cm for men or ≥85 cm for women [18,19]. The serological factors were cholesterol, TG, low-density lipoprotein (LDL), HDL, non-HDL (non-HDL), and TG/HDL ≥3.5. Dyslipidemia was determined, using these criteria: hypercholesterolemia (total cholesterol ≥200 mg/dL), hyper-LDLemia (LDL ≥100 mg/dL), hyper-TGmia (TG ≥150 mg/dL), hypo-HDLemia (HDL <40 mg/dL for men and <50 mg/dL for women), and hyper-non-HDLemia (non-HDL ≥130 mg/dL) [20].

Factors related to tear chronicity detected on MRI were Patte retraction degree [21], global fatty degeneration index [22], Goutallier grade of infraspinatus [23], tangent sign [24], and occupation ratio [25]. The radiographic factor was superior displacement of the humeral head [26]. The factors related to tendon involvement were posterosuperior cuff tear and biceps tendon injury. These factors and their prevalence are summarized in Table 1.

The prevalence and 95% confidence intervals (CIs) of SBTs were analyzed. Using univariate logistic regression analyses, the odds ratios and 95% CIs were calculated to identify any association between SBT and the studied variables. Then, multivariable logistic regression analyses, using only the significant variables identified in the univariate analyses, were performed. Multivariable logistic regression analysis was performed after assessment of multicollinearity using factors with both a variance inflation factor and a condition index <10, indicating no multicollinearity [27]. The goodness of fit for a multivariable logistic regression model was determined using the Hosmer-Lemeshow test. All statistical analyses were performed using the IBM SPSS ver. 24.0 (IBM Corp., Armonk, NY, USA). The significance of the logistic regression analyses and the Hosmer-Lemeshow test were set at p<0.05.