The Effect of Crown/Implant Ratio and Crown Height Space on Stress Distribution in Unsplinted Implant Supporting Restorations

Purpose: To measure the transfer of axial and nonaxial load in unsplinted fixed implant supported restoration with varying crown to implant (C/I) ratios and crown height space (CHS). Materials and Methods: A photoelastic block model was constructed. Three holes were drilled vertically in a straight line in the mid axis of the photoelastic model at predetermined locations to lengths of 12 mm. Three implants were inserted into the model. Two strain gauges were cemented onto the neck of each implant on the buccal and lingual aspects, which provided a simultaneous direct reading of strain. Four groups of cement retained restorations with C/I ratios of 1: 1; 1:1.5; 1:1.75, and 1: 2 were used. CHS were 10, 15, 17.5, and 20 mm, respectively. Fifteen static loadings were carried out simultaneously with 20 kg weights via a custom-built loading apparatus at 0 and 30 degrees to the vertical axis. Descriptive analysis consisted of mean and standard deviation of microstrain values for each group. Groups were compared by the use of the 1-way parametric analysis of variance. A P value of less than .05 was considered statistically significant. Results: Occlusal force application at 30 degrees showed a statistically significant increase in both buccal (1,588 +/- 150 vs 2,610.59 +/- 150) and palatal (64.92 +/- 7 vs 146.59 +/- 15) microstrain values as C/I ratio increased from 1:1 to 1:1.5 (P < .001). Force application at 30 degrees in cases with C/I ratio of 1:1.75 and 1: 2 resulted in fracture of the abutment screw followed by dislodgment of the crowns. Failures were noted at CHS > 15 mm. Conclusions: CHS is more significant than the C/I ratio in assessing biomechanical-related detrimental effects. Prosthetic failure occurred at CHS >= 15 mm. (C) 2011 American Association of Oral and Maxillofacial Surgeons J Oral Maxillofac Surg 69: 1934-1939, 2011