Interproximal accuracy of CAD/CAM machine cut, machine bent and manually bent retainers : An in vitro comparison

The purpose of the present study was to compare the interproximal accuracy (IA) of two different CAD/CAM (computer-aided design/computer-aided manufacturing) manufactured and conventionally bent retainers.

On 20 digitized debonding casts (10 upper, 10 lower), a canine-to-canine retainer was designed using two different CAD/CAM software: OnyxCeph^® (Image Instruments, Chemnitz, Germany) and FixR™ (YOAT Corp., Lynnwood, WA, USA). Each design was bent using the Bender II™ retainer bending machine (YOAT Corp.) and 3‑ or 6‑stranded twistflex wires (0.018-inch Dentaflex^®, Dentaurum, Ispringen, Germany), respectively. For each cast, one CAD/CAM machine cut retainer (Memotain^®, SCHEU DENTAL custom-made, Hilden, Germany) was fabricated and one retainer was bent manually by a dental technician using the 6‑stranded wire. All retainers were passively waxed on the dental casts and digitized using a laboratory scanner (Atos, Carl Zeiss IQS Deutschland, Oberkochen, Germany). For each of the five interproximal areas, IA was measured (1) as vertical distance between retainer and the reference retainer plane and (2) as horizontal distance between the interproximal reference point and the retainer in millimeters. For both directions, it was assumed that the smaller the distance, the higher the interproximal accuracy of the retainer. Statistical analysis was performed using Mann-Whitney and Kruskal-Wallis tests.

Considering the vertical deviation, significant differences of interproximal accuracy were seen between the jaws (upper 0.25 mm > lower 0.18 mm, p < 0.001), planning software (FixR 0.25 mm > OnyxCeph 0.21 mm, p = 0.03) and settings (upper jaw: CAD/CAM bent [FixR] 0.34-0.32 mm > manually bent 0.27 mm > CAD/CAM bent [Onyx] 0.23-0.18 mm > CAD/CAM cut 0.17 mm, p = 0.001; lower jaw: CAD/CAM bent [FixR 3‑stranded and Onyx] 0.22-0.21 mm > manually bent 0.18 mm > CAD/CAM bent [FixR 6‑stranded] 0.13 mm > CAD/CAM cut 0.10 mm, p = 0.004). For horizontal deviation, significant differences were observed between the jaws (upper 0.97 mm > lower 0.66 mm, p < 0.001), planning software (OnyxCeph 0.90 mm > FixR 0.72 mm, p < 0.001), settings (upper jaw: CAD/CAM bent [Onyx] = CAD/CAM cut 1.16-1.10 mm > CAD/CAM bent [FixR] 0.96-0.89 mm > manually bent 0.76 mm, p < 0.001; lower jaw: CAD/CAM bent [Onyx] 0.84-0.71 mm > CAD/CAM cut 0.66 mm = CAD/CAM bent [FixR] 0.66-0.63 mm > manually bent 0.51 mm, p < 0.001) and location of the interdental area in the upper jaw (13-12 = 22-23 > 22-21 > 11-21 > 12-11, p < 0.001). The wire quality had no significant impact in either of the dimensions.

Interproximal accuracy in the vertical plane was achieved best by CAD/CAM cut retainers, while horizontally, manually bent retainers were the most accurate. Overall, the accuracy of lower retainers seems clinically acceptable for their use in the mandible irrespective of the manufacturing method, whereas maxillary retainers presented a larger variability especially in the more distal interproximal areas.