To evaluate, using micro‐computed tomography, the preparation of long‐oval root canals using a single reciprocating system versus a multiple‐file rotary system.

Distal canals of thirty mandibular molars were selected and randomly assigned to one of two instrument groups (n = 15): Reciproc 40 (VDW, Munich, Germany) or BioRaCe system (FKG Dentaire, La Chaux‐de‐Fonds, Switzerland). The teeth were scanned before and after preparation of the canal by a SkyScan 1172 micro‐computed tomography scanner at 11‐μm resolution. Morphometric variations were measured by volume increases and by the remaining untreated canal surface area in the entire canal and as well as in each third of the canal. Data were compared using the Mann–Whitney test.

The Reciproc system left significantly more areas untouched (P < 0.001) in the cervical and middle thirds (18.14% and 21.82%) as compared to BioRaCe (8.14% and 11.35%). The Reciproc system had the greatest increase in volume of both the entire canal and the apical third (P < 0.5).

Neither technique was able to completely prepare the outline of long‐oval canals. The Reciproc system removed more tooth structure. The BioRaCe left fewer untouched dentine walls in the more coronal thirds of the canal, whilst Reciproc left fewer in the apical third.