MATERIALS AND METHODS Participants The sample consisted of 105 people diagnosed with frequent (47.6%) and chronic (52.4%) TTH derived from two primary care centres, selected according to the criteria of IHS3,4, and suffering from headache for more than three months. Participants were aged between 18 and 65 years (mean ± SD: 38.9 ± 10.9 years), which included 23 men (21.9%) and 82 women (78.1%). Patients were excluded if they had suspected malignant disease, rheumatoid arthritis, pregnancy, vertigo, continuous intake of prescribed medication or those who received physical therapy in the past 3 months for headache or neck pain. Finally, 102 subjects completed the study (2 from the treatment group dropped out due to work problems and one from the control due to lack of improvement).
The software G*power14 was to calculate the required sample size, using data from previous studies to estimate the effect sizes to be expected. Our review of the extant literature showed that across studies, the mean N was 44.28 and in general large effect sizes were found (Cohen's d = .40).15-19 We accepted a 5% alpha risk (α = .05) and 10% beta risk (β = .1) as study parameters which showed that 51 subjects were required in each group to achieve a medium-to-large effect size (f = .35). We predicted a 5% dropout rate in the follow-up period.
Study design The study was a factorial, randomized, double-blinded, placebo-controlled clinical trial. It was conducted from January to November, 2014. After the initial clinical interview, the allocation of patients to control or treatment groups was randomized by an external assistant using a specific software (name) who monitored the sequence and documentation at all times. Both the external assistant and the therapist and examiner were blinded to the study objective.
Patients were randomly assigned into 2 groups (treatment and control). The treatment group received occiput-atlas-axis manipulation (OAA) and soft tissue treatment by massage. The control group received massage only. Thus both groups received the same duration of massage therapy and the exposure times were the same, however the difference between groups lay in the manipulative OAA technique which only the treatment group received. Four sessions were conducted (once per week) and the assessment was carried out in 3 stages: at baseline, end of treatment (at 4 weeks) and follow-up at 8 weeks following completion of treatment. All patients in both groups were assessed under the same conditions before and after the treatment by an examiner blinded to the group allocation.
The study was conducted at the University of Valencia (Spain) from January 2014 to November 2014. Prior to data collection, informed written consent was obtained from all patients, and all procedures were conducted according to the Declaration of Helsinki.
