Clinical Pearls: A JOMI review by AO's YCC
As an occasional feature of AO CONNECT, articles from The International Journal of Oral and Maxillofacial Implants (JOMI) — the official journal of AO— are reviewed from the viewpoint of a member of the Academy’s Young Clinicians Committee. Today’s “JOMI Clinical Pearl” by Dr. José Antonio Garcia was the “Featured Article” from Volume 32, Number 4, 2017 as selected by AO’s Website Education Committee.
Accuracy of Digital vs Conventional Implant Impression Approach: A Three-Dimensional Comparative In Vitro Analysis
(Kinga Basaki, DMD, MSc, FRDC(C)1/Hasan Alkumru, DDS, MSc, PhD, FRCD(C)2/Grace De Souza, DDS, MSc, PhD3/Yoav Finer, DMD, MSc, PhD, FRCD(C)4)
Here we are going to see the exactness of definitive casts generated from a fully digital implant impression system and to compare results to casts fabricated from a conventional implant impression technique. The model, had bilateral edentulous areas. Several implant sizes were used to restore from premolar to molar regions, implants were placed angulated and convergent on the left side to copy a non-ideal clinical situation and allows the examination of the influence of the secondary variable of implant angulation.
Conventional and digital impression approaches had some inexact reproduction to the exact implant orientations of the model, the last casts from the digital system where less exact. For the purpose of the study, casts with an error less than 60 μm in implant positioning were acceptable. The most common 3D error of a conventional impression technique was in a clinically acceptable range, errors where reported between 20 to 89 μm using similar measurement techniques. At variance, the digital impression resulted an average 3D inter-implant error of 116 μm, which was in excess of the error limit of 60 μm and demonstrated an enormous error and variability than the conventional impression. This data was given by the clinical assessment, as there was a better fit of the verification jig stent on conventional definitive casts, which demonstrated passive fit for 18 out of the 20 casts, than for those fabricated from the digital approach, which only had 11 out of the 20 casts showing passive fit. The data thus supported the rejection of the primary hypothesis.
Examining conventional open-tray impression techniques shows that the accuracy of definitive casts can be characterized by different situations. Defects can happen due to the machining sensitivity of the transfer coping, the variation of the impression material due to polymerization shrinkage, and the setting dimensional change of dental stone. With a digital impression system, several factors may affect the final accuracy of the definitive cast. iTero scanner uses confocal laser scanning and a processing software that recognizes and rebuilds objects from a scanning point using a registration algorithm. Any error with this digitization process would be reflected as error in the resultant casts. Repetitive mistakes of fit of the commercially available scan abutments, which range from 11 to 39 μm based on machining tolerance, contribute to recorded errors. Further inaccuracy may have been introduced during the milling process of the polyurethane definitive casts or could be the result of potential dimensional changes of this material in response to the ambient environment, which have been previously evaluated as ranging from 0.2% to 1.3%. Any variability in the manual positioning of the implant analogs into the prefabricated guidance sockets in the final casts would also have contributed to the recorded inaccuracy within the digital impression approach.
Several recent studies comparing digital vs conventional implant impressions have found a greater error using the digital approach. Some studies have found equivalent accuracy between digital and analog impression techniques. The variation in recorded error may be attributed to the differences in study design, measurement methodology, and the inherent differences of various digital impression systems. The accuracy of three intraoral scanners for implant impressions in vitro reported a significant difference between scanners,40 with an average error for the iTero scanner of 65.8 ± 55 μm for inter-implant distance and 0.38 ± 0.25 degrees for angulation.
Implant placement angulation did not have a significant influence on definitive cast accuracy within either the digital or conventional impression approach. Increased implant angulation can cause an increased strain within impression material leading to its deformation. For a digital impression approach, where material strain is not a concern, several studies have indeed not found an effect of implant angulation on digital impression accuracy. The lack of influence on the error in conventional approach could be potentially explained by the moderate angulation, choice of the impression material, and implant connection type. A relative distortion was favored over an absolute distortion measurement, as this approach does not rely on external reference points having to be transferred to each definitive cast, which can introduce additional confounding variables.42 A limitation arising from this technique, however, was that measurements could not be provided for individual direction of error, horizontal or vertical.
Further evaluation of the reliability and accuracy of this digital impression approach must be undertaken in both a laboratory and a clinical setting. Clinicians, must carefully evaluate the inherent advantages and disadvantages of the technique and ultimately interpret the clinical significance of any error on an individual basis.
The digital impression approach produced less accurate definitive casts than the conventional approach. Both 3D analysis and clinical evaluation of definitive casts concluded that a significant portion of those from the digital approach could not be regarded as clinically acceptable. Implant angulation did not have an effect on overall cast accuracy.
About the Author
Dr. José Antonio Garcia graduated from Universidad Autonoma de Nuevo Leon in 2004 and from Instituto Dental de Implantologia in 2007.