Moving from analogue to digital workflows in dentistry: Understanding undermilling and overmilling as detrimental factors in fabricating CAD/CAM crowns

Due to the limitations caused by the specific dimensions of milling burs, milling machines may inaccurately fabricate restorations by either over-reducing or under-reducing restorative material, respectively known as "overmilling" and "undermilling". Overmilling occurs when the bur is unable to accommodate areas smaller than the size of the bur, especially at cusp tips and sharp line angles. This results in excess cement space and a weaker restoration. Undermilling occurs when small concavities are ignored by the larger milling bur, particularly at the margins. This results in inadequate seating of the restoration and open margins due to premature binding, which can lead to restorative failure. To prevent these processing errors, clinicians should ensure that all incisal edges and cusp tips are flattened and rounded, verify that there are no undercuts, and prepare teeth with 1mm thick, smooth finish lines. Furthermore, clinicians may use the preparation check and milling simulation step of computer-aided design/computer-aided manufacturing (CAD/CAM) software to confirm the preparation is adequate and the restorative design does not have potential areas that will lead to overmilling.