Patients with conservatively treated proximal humerus fracture who admitted to Ege University Physical Medicine and Rehabilitation outpatient clinic have been included in the study. All patients who met inclusion criteria were enrolled into the study at the first week of PHF. The orthopedic rehabilitation programme was applied to all patients three times a week for four weeks under the guidance of the same physiotherapist. The patients also received a complete set of premade exercise cards, which showed all exercises to ensure that the training program was learned correctly. The patients were recruited at the first week after proximal humerus fracture and then allocated into the groups. The patients were separated into two groups as interferential current or sham using a simple randomization method managed by an impartial observer. Flipping a coin was used for simple randomization (heads - sham, tails - treatment). The patient's group was reported to the physiotherapist who would apply interferential current in a closed envelope. Patients and the outcome assessor were blind to the treatment groups. The IFC therapy was applied using a combined electrotherapy device SONOPULS 692® (brand: Enraf-Nonius). The medium frequency of the IFC was 4000 Hz and 4100 Hz to produce the amplitude-modulated frequency at 100 Hz. Two rubber electrodes (8×6 cm) were fitted on the fractured shoulder. One of the electrodes was placed on the lateral part of the deltoid muscle; the other one was placed on the trapezius muscle near the shoulder. The current intensity was set to achieve a "strong but comfortable tingling" without visible muscle contraction. The sham therapy consisted of placing the same pads for the same time, but no electrical stimulation was applied to the probes. The primary outcome was shoulder global function which was measured by the Constant-Murley score (CMS). The questionnaire assesses four shoulder functions: 1) pain; 2) activities of daily living (sleeping, work, leisure); 3) range of motion; and 4) muscle strength. The total score ranges from 0 to 100, with a higher score indicating better shoulder function. Activity pain was measured with the visual analog scale (VAS). In addition, in order to evaluate shoulder function and disability, the Disability of the Arm, Shoulder and Hand questionnaire (DASH) was used. This self-administered questionnaire includes 30 questions evaluated on a 5-point Likert scale, most of which relate to the individual's capacity to realize a task. The patients were allowed to use paracetamol during the study and the paracetamol intake was recorded as gr/week.

Clinical assessments were evaluated at the end of the treatment, at 6-weeks and 18-weeks post-treatment. Only VAS resting pain was evaluated at the enrollment because the fractured side was immobile when the patients were included in the study. The physician who assessed the treatment outcomes, the patients and the physiotherapist who administer the exercises were unaware of the patient's group of treatment.

A clinically meaningful difference was considered to be a change of 15 points in total CMS. In order to detect this improvement, the number of patients needed was 17 per group according to a power of 0.80, a high size effect (d=0.88), a p value of 0.05, and a 15% dropout rate.

Statistical analysis was performed using SPSS version 20.0 (IBM, Armonk, NY, USA). An intention-to-treat analysis was employed for all analysis. The variables were investigated using visual (histogram, probability plots) and analytic methods (Kolmogorov-Smirnov, skewness and curtosis) to determine whether they were normally distributed or not. . Continuous data were described as median (inter-quartile range, IQR) or mean (standar deviation, SD) and categorical variables as percentages. Chi-square or Fisher's exact test was used to compare categorical variables and Mann-Whitney U test/Student's T-test was used to compare continuous variables. All outcome values were presented in mean and standard deviation. The repeated measures ANOVA test was used in comparison of the means of the repeated measures. The results of the repeated measures ANOVA were analyzed by Mauchly's sphericity test. If the parametric tests (factorial design for repeated measures analysis) did not provide the preconditions, the Greenhouse-Geisser correction was used for corrections to the degrees of freedom or Friedman's Test. The Bonferroni correction was used for multiple comparisons. A p value of less than 0.05 was considered to show a statistically significant result.