Baseline data will include gender, age, height, weight, body-mass index, level of physical activity, unilateral vs. bilateral symptoms, duration of symptoms, work status, whether on sick leave or disability, highest level of education achieved, use of pain or anti-inflammatory medication, a plain film x-ray including skyline view, and MRI examination of the knee joint (previous examination within 6 months of baseline will be accepted, assuming no significant change in symptoms has occurred during this period). We will also record the Beighton score with regard to joint mobility [
45].
Blinded observers will assess all participants at baseline, three months and one year after inclusion. Pain and function, health related quality of life, activity level, strength, self-efficacy, anxiety and depression, number of pain regions, and kinesiophobia will be assessed by standardised, validated questionnaires at each time point. At six weeks, a mid-term assessment will be performed including pain and function (AKPS and VAS) and strength measurements. The treating physiotherapists will be blinded to all baseline data. Due to the nature of the study, blinding of the patients and physiotherapists to the intervention is not possible, but patients’ expectations about the effectiveness of each intervention will be assessed at baseline.
Secondary outcome measures
Include a range of standardised, generic self-report measures including VAS for pain, global score of change, step-down test, strength and endurance testing, quality of life, kinesiophobia, anxiety and depression, self-efficacy, number of pain regions, and activity level.
A 10-cm Visual Analog Scale (VAS) will be used to measure pain, where 0 corresponds to no pain and 10 corresponds to worst imaginable pain. The scale is validated for use with patellofemoral pain [
47]. We will record VAS for assessment of usual pain the past week (VAS-U) and worst pain during the past week (VAS-W). For these measurements a MCID of 2 has been reported [
47].
An 18-point Likert scale for measuring patients’ global assessment of change compared with baseline will be carried out at three and 12 months. The scale ranges from −9 (maximum deterioration) to +9 (maximum improvement) [
48].
According to Loudon et al. [
49] the step-down (from a 20 cm step) is the only functional test which discriminates between PFSS patients and normal controls. The step down as performed according to standardised instruction will be used to quantify changes in patients’ function. The measure is number of repetitions in 30 seconds.
Isometric strength will be measured for hip abduction, hip external rotation and knee extension. Strength will be measured with a force gauge (MuscleLab Force Gauge 300 kg) using stabilizing straps to ensure correct positioning of the patient, according to the current standard for studies of this type. Standardized positions and procedures are based on techniques proposed by Caldwell et al. [
50] and Thorborg et al. [
51]. Research personnel will be trained in the procedures by qualified researchers at University of Agder, Norway.
A novel test for hip abduction endurance has been developed for use in this study. A parallel article will be published quantifying normal values for this test, values for PFPS patients (men and women), its inter- and intra-rater reliability, validity compared to strength testing with force measurer, and its predictive value for clinical response to hip strengthening in PFPS patients. Testing is carried out in side-lying position with a 5 kg weight around the ankle of the upper leg, which is in 0-10° hip extension and abducted to 30° above the horizontal plane. Time successfully held in target area is measured in seconds. All participants in the study will be tested at baseline, six weeks, three and 12 months. Parallel with the RCT, normative data will be collected in a group of 75 healthy controls of both sexes from 16–40 years old.
Euro-Qol – 5 Dimensions (EQ-5D) is a questionnaire used to measure health related problems and quality of life [
52]. The questionnaire is widely used both internationally and in Norwegian [
53]. EQ-5D-5 L includes five questions about mobility, usual activities, self care, pain and discomfort, and anxiety and depression, each has five possible answers. The results are translated to a single summary index value through the use of a table [
54]. In addition the subject scores his/her overall health on a 0–100 scale (EQ-VAS).
The Tampa scale for kinesophobia (TSK) is a 13 –item questionnaire aimed at the assessment of fear of movement/re-injury. Each item is scored on a 4-point Likert scale with alternatives ranging from strongly disagree (0) to strongly agree (4). This gives a possible total score range from 0 to 52. The TSK is translated to Norwegian and a cross-cultural adaptation and validation study showed satisfactory validity and reliability in a Norwegian population with sciatica [
55].
The Hopkins Symptom Checklist (HSCL) is a symptom inventory which measures symptoms of anxiety, depression and somatisation, originally consisting of 58 items [
56]. Comparisons of different versions of the HSCL indicate that a shorter version (HSCL-10) perform almost as well as the full version, also in the Norwegian population [
57]. The scale for each question includes 4 categories of response ranging from 1 (not at all) to 4 (extremely). The average HSCL score is calculated by dividing the total score by ten (number of items). In accordance with recommendations by the Regional Ethical Committee, patients who score >2.00 on HSCL-10 (with suspected clinical depression or anxiety disorder that warrants treatment) will receive an offer of referral to psychologist or psychiatrist for follow-up.
Self-efficacy will be measured using the Knee Self-Efficacy Score (K-SES). This score is originally developed in both Swedish and English for measuring self-efficacy in patients with anterior cruciate injury [
58] and will be translated to Norwegian for the purpose of this study. K-SES is a self-administered instrument consisting of in total four sections in which patients score how certain they are about specific activities currently and in the future. The K-SES has not been validated for use in PFSS patients, but we consider that it is preferable to a more general self-efficacy score.
An adaptation of the Standardized Nordic Questionnaire [
59] will be used to register the number of painful areas. This modified questionnaire is used in a recent Norwegian study [
60] which showed a high prevalence of patients with more than one pain site in a Norwegian population, and that there was a strong correlation between increasing number of pain sites and decreasing functional ability.
Activity level will be monitored using a multi-sensor which includes an accelerometer (SenseWear Pro
2 Armband) using the techniques validated in a study by Berntsen et al. [
61]. Study participants will wear the monitor continuously for one week at baseline, three months and 12 months follow-up.
Compliance will be registered weekly by the treating physiotherapist, recording number of completed home sessions and sessions with the physiotherapist. We will also register use of pain medication including non-steroidal anti-inflammatory drugs, sick leave from work, and whether the patient has sought other treatment (type and number).