Evaluation of Accuracy of Casts Obtained from Various Implant Impression Techniques: An Invitro Study

A study has been conducted to evaluate the accuracy of casts obtained from open tray and closed tray impression techniques. To conduct this study a master model was constructed by placing two endosseous root form implants in lower premolar regions on either side (35 and 45). A bar was fabricated and laser welded for strain free seating of it on the master model. This was considered as the control. Test samples were grouped into two (Closed tray and Open tray) with 10 samples in each group. Impressions were made using addition polymerizing silicone - Express STD, Putty (Class 1) and Light body (Class 3), regular set, hydrophilic impression material, 3 M ESPE, U.S.A and casts poured with type IV - dental stone, high strength, low expansion die stone - kalrock. Then the samples were evaluated for accuracy using two different methods with electrical resistance strain gage (SYSCON) and coordinate measuring machine (TESA Microhite 3D). On analyzing the strain values from strain gage, the values obtained with direct transfer open tray impression technique is close to the master model. There is less strain on the bar on open tray impression casts compared with the bar on closed tray impression casts. The resulting values from coordinate measuring machine showed that the mean value for x – axis distance (in mm) of open tray technique 27.05 mm is closer to master model value of 27.21 mm. Hence the open tray technique has the least amount of distortion in x axis direction among the two techniques. The mean value of y axis values (in mm) of abutment at 35 position for open tray impression casts - 9.100 mm is close to the master model value of 9.115 mm. The mean value of y axis values (in mm) of abutment at 45 position for open tray impression casts - 8.79 mm is close to the master model value of 8.965 mm. Hence the open tray technique has the least amount of distortion in y axis direction among the two techniques. The mean value of angularity (in radians) of abutments in 35 position of open tray technique casts is 0.08298 which is close to the master model value of 0.08472. Similarly the mean value of angularity values (in radians) of abutments in 45 position open tray technique casts is 0.07452 and is close to the master model value of 0.07520. Among the impression techniques used for obtaining the casts from master model there is significant difference in values obtained on all the three axis analysed. Hence the direct transfer impression technique gives the high accuracy of transfer of implant positions from master model to the sample casts. CONCLUSION: From the foregoing study for evaluating the accuracy of casts obtained from various implant impression techniques following conclusions have been drawn. The open tray impression technique for transfer of 3 dimensional implant position from master model to specimen casts using direct impression coping for open tray internal hex is more accurate than the closed tray impression technique using direct impression coping for closed tray internal hex. The open tray impression technique showed better accuracy than the closed tray technique on all the three parameters evaluated (x – axis, y – axis and angularity). This clinically implies that, more the number of components used for the impression procedure, the more the chance for inaccuracy (error) getting incorporated. Hence a direct transfer impression technique with less number of components possible ensures the high accuracy of transfer of implant positions from master cast to the laboratory cast which implies the accurate transfer of implant location from the patient to the laboratory cast