PICO question **Population** For human studies, patients prescribed an RBB for the replacement of a space (metal or all-ceramic retainer wing/ fixed-fixed or cantilever, anterior or posterior) will be included. For lab studies, studies that are undertaken on human extracted teeth will be included. Studies undertaken on animal or simulated teeth will be excluded. **Intervention and Comparison** Studies which compare preparation of abutment teeth (any preparation design including axial reduction, grooves, slots and boxes) to each other and to control group (no preparation of abutment tooth (adhesive only) will be included **Outcome** *Primary outcomes:* For human studies • Survival of the RBB within 5 years. Failure includes - any hard or soft tissue disease that requires changing the restoration (e.g. secondary caries, periodontal disease), fracture of the RBB (pontic or retainer), or any other issues that requires that the whole RBB would be constructed again • De-bonding of RBB requiring re-attachment (considering time-to-debonding, number of de-bonding episodes in five years but also whether a successful re-cementation led to further de-bond within five years) • Aesthetics of the RBB For in-vitro studies: • Inclusion of any outcome measures evaluated in the laboratory studies but discussed their indirectness towards our main clinical outcomes. • There is expected to be tests around bond strength tested using different loadings and forces e.g. dynamic loading, tensile shear strength under different temperature or chemical conditions to replicate the conditions in the mouth. *Secondary outcomes:* For human studies: marginal discoloration of the RBB or periodontal inflammation that does not lead to failure of the RBB. It is expected that there might be studies undertaken on humans that the outcome measures is tested in in-vitro We will consider and discuss other confounders but we will not conduct any analysis. In the data extraction, we will collect data around type of teeth (mandible or maxilla, posterior of anterior, type of preparation design, thickness of retainer wing). For the in-vitro studies, we will also collect data on the storage and experimental conditions to attempt to simulate some of the chemical or mechanical situation in the mouth.