Objectives: The present in-vitro-study examines the marginal and internal fit of zirconia crown frames with preparation angle a/2 6° and compares findings of the old and the new CAM-software. The difference between adapted and non-adapted crowns was analysed also. The non-adapted crown-frames should reflect the CAM-systems precision and indicate whether adaptation by the dental technician is necessary. Methods: The crown-frames were manufactured by the CAM-system Cerconâ (DeguDent, Hanau/Germany) on 40 different master dies with preparation angle a/2 6°. 20 crown frames were milled by the old CAM software, 20 by the latest software update. In each case 10 crowns were coincidentally selected and adapted by the dental technician to the optimum fit. The time needed for adaptation was noted. All 40 crown-frames were fixed on the master dies by glass ionomer cement. The fixed crown frames were cut parallel to the milling direction and examined by the scanning electron microscope (Stereoscan 250 Cambridge, Leica, Wetzlar/Germany). Results: The average marginal fit shows 50,7µm for adapted crown frames of the old software and 40,3µm for the updated software. A Significant difference between old and new software and adapted and non-adapted crown-frames exists (Mann-Whitney-U-test, p<0,05). The crowns manufactured by the old software show a broad dispersion in the range of the cement layer from 15µm to 560µm (adapted crown frames) and from 5µm to more than 1380µm for the non-adapted crown frames. The adapted crowns milled by new software get a thin equalised cement layer. Conclusions: To conclude, the new software compensates more exactly for the zirconia blanks` shrinkage of sinter. The marginal and internal fit is significantly improved. An adaptation by the dental technician is still necessary.

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