This trial was a prospectively registered, randomized, placebo-controlled trial conducted at the Department of Physical Therapy, Prince Sattam bin Abdulaziz University, Al Kharj, Saudi Arabia. Participants were recruited between May 1, 2020 and Jan 31, 2023. The uniqueness of this study is its methodology, which is more comprehensive and understandable. This study was designed and conducted according to the principles of the Declaration of Helsinki 1964 and its later amendments. The study has been approved by the Department Ethics Committee (DEC) with an ethical approval number RHPT/020/013. Written informed consent was obtained from all the study participants as per the ethical guidelines. The study was registered prospectively in a trial registry with reference number CTRI/2020/04/024834 on 24/04/2020.

An orthopedic surgeon with 20 years of clinical experience had diagnosed the frozen shoulder (International Classification of Diseases 10th revision [ICD-10] group M75.1-M75.8, M19.8) patients as per the frozen shoulder diagnosing criteria recommended by American Orthopedic Association (AOA) [1]. Participants that were between 18 and 60 years of age, had pain intensity of 3–8 on the visual analogue scale (VAS), and pain duration of 4–12 months (stiffening stage—pain starts to subside, progressive loss of glenohumeral motion in the capsular pattern) were chosen to participate in the study. Excluded from the study were any participants with prior steroid injection therapy, associated neck or arm pain, glenohumeral osteoarthritis, severe musculoskeletal, neural (Parkinson's disease), cardiovascular (heart disease and stroke) somatic and psychiatric problems, metabolic diseases, waiting for any surgery, having medications, alcohol or drug abuse, or involved in any weight training programs. In addition, participants with other soft tissue injuries, fractures at the upper limb, or those that were immobilized for a prolonged period were also excluded from the study.

All participants received a rehabilitation referral letter from the referring hospital to participate in the trial. They also received a study pamphlet detailing the procedure for participating in the study. After signing the written informed consent form, participants were randomized through a computer-generated randomization method and allocated into two groups: lidocaine injection with ESWT—Active group (n = 30, age 52.12 ± 5.2, male-14, female-16) and lidocaine injection with ESWT—Placebo group (n = 30, age 53.56 ± 5.5, male-13, female-17). Participants were allocated by a physiotherapy assistant through an on-site computer system in which allocation was concealed. The person enrolling the participants should ideally not be the same person generating the sequence to prevent manipulation. Each participant’s group allocation was only informed to the treating therapist immediately before the first intervention. The participants were not aware of which treatment they were receiving (blind participants); however, they were informed that they would receive one of the two interventions. Due to the nature of the interventions, it was not possible to blind the therapist who treated the patients. Both groups received the concerned intervention for a period of three sessions per week for 4 weeks. The primary and secondary outcome measures were collected by a blinded therapist at baseline after 4 weeks, 8 weeks, and at 6 months of follow-up and the methods were followed as per our previous study [22].

An orthopedic surgeon conducted a regular physical and orthopedic examination before the administration of the injection. All the participants were treated with a posterior approach intra-articular injection containing a mixture of 5 cc of 1% lidocaine (Xylocaine) and 2 cc (80 mg) methylprednisolone acetate (Depo-Medrol) injected at a 90° angle via a sterile 18-gauge spinal needle [23]. They were asked to rest and not engage in strenuous activities for 1 week following the injection, even if they experienced pain relief. Any adverse consequences such as allergic reactions, nausea, vomiting, and unusual tiredness and weakness were noted and treated by the treating doctor.

One week after receiving the injection therapy, all the participants were allowed to take physical therapy interventions by a licensed physical therapist with 15 years of clinical experience in treating shoulder conditions. Participants in both groups received physical therapy treatment for three sessions per week for 4 weeks and each session lasted for 30–40 min. To avoid intervention bias, a fixed physiotherapy protocol (Fig. 1 Permission from the patient has been received) was prepared.

Fixed physiotherapy protocol

Initially, the calibration of the ESWT (Zimmer, enPuls Version 2.0, Junkersstrabe, Germany) was done by an expert to obtain better and more consistent output from the device. In the active ESWT group, the participant was asked to sit with the shoulder passively abducted at 80°, the elbow flexed at 90°, and the forearm rested on a flat surface. The treatment started with 250 “warm up” pulses at 1.5 bar of air pressure, which accommodates the participant for the radial ESWT treatment. Once the patient was comfortable with the treatment, the air pressure was increased to 3.5 bar and 2000 pulses with an energy flux density (EFD) ¼ 0.16 mJ/mm2, and the impulses were applied with a 15-mm applicator at a frequency of 8 Hz of dose and were administered in two shoulder regions. The first 1000 impulses were applied in an anterior-to-posterior direction at the anterior shoulder joint, and the upper margin of the treatment zone was about one finger’s breadth lateral to the coracoid process (Fig. 2a). The remaining 1000 impulses of the total 2000 impulses per session were applied in a posterior-to-anterior direction on the posterior side of the shoulder joint located beneath the lateral border of the scapular spine (Fig. 2b) [20]. For the placebo group, the same set of treatments was provided but in providing ESWT, a special head that blocked the shockwaves from occurring was used but it was indistinguishable to the study participants.

a Application of ESWT in the anterior compartment of the shoulder. b Application of ESWT in posterior compartment of the shoulder

Progressive resistance exercises (PRE) (Fig. 1) were prescribed with a Thera band (THERABAND, Akron, OH, USA) for the shoulder muscles based on the assessment of individual muscles and personally tailored. Initially, the painful movements are trained with minimal resistance and then progress to the next level of resistance for the other joint movements. The therapist selected the exercise parameters (intensity, frequency, and duration) in every treatment session purely based on the individual capacities without exaggerating the symptoms. Throughout the treatment session, the participants were instructed to follow the correct form and posture to facilitate healing [24]. The patients were instructed to perform the home exercises daily during the 4 weeks of intervention and after 4 weeks of intervention [pendulum exercise, cane exercises (up, back, and out), wall slides (forward and side) three times 30 repetitions] and isolated stretching of shoulder muscles (shoulder stretch, towel stretch, chest stretch, inferior capsule stretch and sleeper stretch three times daily for 30 s). The treatment adherence at home was monitored by a treating therapist by checking the exercise logbook and frequent contact with the patient through the WhatsApp application.

Pain intensity was measured with a visual analogue scale (VAS) and the participant was asked to note the perceived pain intensity on the 10-cm point scale, where scores ranged from 'no pain' (0) to 'worst imaginable pain' (10). VAS is considered a moderate-to-good (r = 0.60–0.77) reliable tool for measuring pain intensity in frozen shoulder patients. [25]

Magnetic resonance imaging (MRI) has been established as a reliable and valid assessment tool for measuring the thickening of coracohumeral ligaments in frozen shoulder patients. It was performed with a 3.0-T MR unit (Siemens Medical Solutions, Germany) with a phased-array surface coil (Philips, Netherlands) centered over the glenohumeral joint and strapped in place. The arm position was standardized, with the thumb pointing upward in a neutral position. In the sagittal oblique plane, parallel to the glenohumeral joint (550/15, 3-mm section thickness, 0.3-mm intersection gap, 1,806,180-mm field of view, 5,126,512 matrix size) T1-weighted sections were taken. [26]

Range of motion Shoulder abduction and lateral rotation were measured with a universal goniometer. The goniometric passive ROM measurements for the shoulder appear to be highly reliable when taken by the same physical therapist, regardless of the size of the goniometer used [27].

Functional disability The Disabilities of the Arm, Shoulder and Hand questionnaire (Quick-DASH) was used to measure the upper limb physical disabilities and symptoms in frozen shoulder. It contains 11 items, which are measured in a five-point Likert scale and summarized into a total score from 0 ‘no disability’ to 100 ‘most severe disability. The Quick-DASH has adequate reliability, validity, and the ability to measure changes in disability among people with shoulder problems [28].

Kinesiophobia The Tampa Scale for Kinesiophobia-adjusted version (TSK-AV) was used to measure the status of fear of injury. The scale consists of 13 items, which are marked on a four-point Likert scale (strongly disagree, disagree, agree, strongly agree). Getting a maximum score indicates more fear of injury and less score such as 13 indicates less fear of injury. The scale shows a high level of test–retest reliability (ICC = 0.887) and moderate validity [29].

Depression The Hospital Anxiety and Depression Scale (HADS) was used to measure the depression status of frozen shoulder patients. It consists of seven items each for depression and anxiety subscales. Scoring for each item ranges from 0 to 3, with 3 denoting the highest anxiety or depression level. A total subscale score of > 8 points out of a possible 21 denotes considerable symptoms of anxiety or depression. It is an acceptable, reliable, and valid practical tool for measuring anxiety and depression [30].

Quality of life The EuroQol EQ-5D was used to measure the health-related quality of life, expressed as utility values ranging from 1 to 3, where 1 represents perfect health. The five domains of mobility, self-care, usual activities, pain or discomfort, and anxiety or depression were measured [31].

With a power of 0.8 and a significance level of 0.05, at least 30 participants were needed to be included in each treatment arm (60 participants in total) to detect a clinically important mean difference between groups of 2 points on the VAS scores at 6 months, when assuming a standard deviation of 0.8 points and considering a 10% drop to follow-up. For other outcomes, we considered a between-group difference of 10% of the outcome measure’s scale to be clinically worthwhile.

The data analysis was performed by a statistician who did not participate in the recruitment, evaluation, and treatment aspects of the study. The study homogeneity was analyzed through the Kolmogorov–Smirnov test. The data analysis was performed on an intention-to-treat analysis, in which the missing data were assumed to be included in the analysis. For the missing data, results obtained in the last available assessment of each participant were repeated. The time and group (4 × 2) linear mixed model (LMM) of all the outcome variables between active and placebo groups at baseline, 4 weeks, 8 weeks, and at 6 months of follow-up was performed and the analysis proceeded stepwise, with initial unadjusted covariates such as age, gender, duration of pain, stage characteristics, medication use, and occupation, etc. Multivariate logistic regression was used to test for heterogeneity between groups, and the maximum likelihood and least squares examined the standard error of each of the parameter estimators. The mean difference (MD) and 95% confidence interval (CI) were also calculated for each between-group comparison. The statistical analyses were processed using commercial statistical software (IBM SPSS Statistics for Windows, Version 26.0. Armonk, NY, IBM Corp) and a level of significance of p ≤ 0.05 was adopted for all tests.