Study	Reason for exclusion
Akca 2013	Not an RCT ‐ no specific details on method of allocation concealment, participant allocation, blinding of outcome assessment (detection bias)
Al‐Zubeidi 2012a	Although this was an RCT on maxillary overdentures, more than 1 implant system was used on the same number of implants comparing different attachment systems
Alsabeeha 2011	Although this was an RCT, more than 1 implant system was used on the same number of implants (1 implant only) comparing different attachment systems
Burns 2011	Although this was an RCT using 1 implant system, different numbers of implants were used
Cehreli 2010b	Although this was termed as an RCT, and the same number of implants was used, 2 different implant systems were used each with different attachment systems Note: authors advised that this was a retrospective study as see excluded from Esposito 2014
de Souza 2015	Although this was an RCT, different implant systems were used, different numbers of implants were used with different attachment systems
ELsyad 2012	The RCT compares between 2 subtypes of bar attachment, and not between the main attachment system types (ball/stud, magnetic, telescopic or bar attachment systems) listed under Types of interventions
Gotfredsen 2000	The study did not use the same number of implants in all included participants. Study classified as an RCT, and even though termed a randomised prospective study, no details on method of random sequence generation, allocation concealment, participant allocation, or blinding of outcome assessment (detection bias) are included
Karabuda 2008	Not an RCT. The study also used different implant systems and different number of implants Note: see also excluded from Esposito 2014
Kleis 2010	The study compares between 2 subtypes of the ball attachment, and not between the main attachment system types (ball/stud, magnetic, telescopic or bar attachment systems) listed under Types of interventions. Study classified as not an RCT although termed as a randomised, clinical prospective study
Krennmair 2012	The study compares between 2 subtypes of the ball attachment, and not between the main attachment system types (ball/stud, magnetic, telescopic or bar attachment systems) listed under Types of interventions. Study classified as not a cross‐over RCT with consecutive participants admitted to the study
Krennmair 2006	Not an RCT ‐ participants were randomised according to their clinician treating preference
Krennmair 2008	The study compares between 2 subtypes of the bar attachment, and not between the main attachment system types (ball/stud, magnetic, telescopic or bar attachment systems) listed under Types of interventions
Krennmair 2012b	Not an RCT ‐ participants were randomly assigned according to clinician preference
Kronstrom 2010	Although this was an RCT and 1 implant system was used, different numbers of implants were used with only 1 attachment system
Mericske‐Stern 2009	Study classified as not a cross‐over RCT. The study compares between 2 subtypes of the bar attachment, and not between the main attachment system types (ball/stud, magnetic, telescopic or bar attachment systems) listed under Types of interventions
Osman 2014	Although this was an RCT ‐ it was on maxillary and mandibular overdentures with 1 implant system, using the same number of implants, there were titanium and zirconia implants used as well as only 1 attachment system used per jaw. Different attachment systems were not used in the maxilla and mandible for comparison
Payne 2000b	Study not an RCT ‐ confirmed by author correspondence
Slot 2013	Although this was an RCT on maxillary overdentures and 1 implant system was used, different numbers of implants were used with only 1 attachment system
Slot 2014	Although this was an RCT on maxillary overdentures and 1 implant system was used, different numbers of implants were used with only 1 attachment system
Walton 2009	Although this was an RCT and 1 implant system was used, different numbers of implants were used with only 1 attachment system
Watson 2002	Although this was an RCT more than 1 implant system was used on the same number of implants comparing different attachment systems
Wismeijer 1997	Although this was an RCT using 1 implant system, different numbers of implants were used

RCT = randomised controlled trial.