Split-mouth design

A split-mouth design places one treatment on one side of a patient's mouth and a comparison treatment on the other, so each patient effectively serves as their own control rather than being compared against a separate group of different patients. Because the two treatments are being tested in the identical mouth (same saliva, same bite forces, same oral hygiene habits, same overall health) this design removes a huge amount of the patient-to-patient variation that can muddy a comparison between two separate groups of people.

This makes split-mouth trials particularly well suited to questions about materials and techniques where the biology of the individual patient plausibly affects the outcome as much as the treatment itself does: comparing two composite materials, two curing lights, or two crown techniques, for example, where a between-groups trial might need many more patients to detect the same real difference.

The tradeoff is that a split-mouth design only works for conditions and treatments that genuinely exist on both sides of a patient's mouth in a comparable way, which limits which clinical questions it can actually be used to answer.