ISRCTN14602568.

Study name	Orthotics for treatment of symptomatic flat feet in children ‐ the OSTRICH study
Methods	Interventional randomised controlled trial including a qualitative study within a trial (treatment)
Participants	N = 1055 Inclusion criteria: 1. Aged between 6 and 14 years, inclusive 2. Have one or both symptomatic pes planus* 3. The child, parent, or legal guardian is able to speak, write and understand English 4. The parent or legal guardian is able to give informed consent *Symptomatic pes planus is described as the manifestation of foot and lower limb symptoms, secondary to altered foot alignment (reduced medial longitudinal arch, everted rearfoot, and abducted forefoot). The diagnosis will be made pragmatically, by treating clinicians, in line with current practice. Eligibility for the birthday card SWAT All participants recruited into the host trial will be eligible to take part in this SWAT.
Interventions	Participants will be allocated to one of three groups: 1. A package consisting of an exercise programme and advice covering topics, such as typical arch development in children, coping strategies, and footwear advice 2. The exercise and advice package plus a pair of prefabricated, off‐the‐shelf orthoses (i.e. insoles that are mass produced to a generic shape, but can be adapted by a clinician) 3. The exercise and advice package plus a pair of custom‐made foot orthoses, where the shape of the insole is made for a specific person, based on a 3D impression or scan of the patient's foot The participant will be informed which group they have been allocated to at their first trial appointment. Participants will be asked to wear their orthoses every day in their shoes, and to do their exercises. Participants in group 1 and 2 will, on the whole, need one clinic appointment, but those in group 3, may need a total of 2 or 3 appointments. Blinding of participants to the treatment allocation is not possible. Measures will be collected at baseline, weeks 1 to 12, and at three, six, and 12 months after a participant is enrolled into the study. The measures are all self‐reported by either the participant or their parent or legal guardian. Birthday card study: In the birthday card SWAT, the researchers will evaluate whether sending a participant a birthday card increases the number of questionnaires they return to the study team. Participants will be allocated to one of three groups; birthday card, birthday card informed by nudge theory to encourage completion of questionnaires, or no birthday card.
Outcomes	Physical domain subscale of the Oxford Ankle Foot Questionnaire for Children (OxAFQ‐C); Time point(s): over the 12‐month follow‐up period
Starting date	18 May 2020
Contact information	David Torgerson, PhD; Sarah Cockayne, MSc; sarah.cockayne@york.ac.uk www.isrctn.com/ISRCTN14602568
Notes	University of York, UK