An Evaluation of the Stress Distribution in Screw Retained Implants of Different Crown Implant Ratios in Different Bone Densities Under Various Loads-A FEM Study.

INTRODUCTION: Studies on stress distribution around screw retained implants in different bone densities are limited. In clinical situations crowns of different heights are placed on the implants and the effect of varying crown implant ratio on the bone is not understood properly. AIM: To evaluate and compare the stress distribution in different screw retained implants for different crown-implant ratios in different bone densities under various occlusal loads using three dimensional finite element analyses. MATERIALS AND METHODS: In this invitro study the stress distribution was evaluated and compared between two different crown heights (7.5mm, 10mm) retained on implants by using different screw materials (commercially pure titanium, titanium alloy) in two different densities of bone D2, D3 under various load (100N, 200N) applications by using finite element analysis. RESULTS: For crown height of 7.5mm, in D2 bone density when vertical load of 200N was applied, the maximum stress concentration was 1780N/cm(2), for oblique load of 100N it was 2936N/cm(2) respectively and in D3 bone density when vertical load of 200N was applied, the maximum stress concentration was 1820N/cm(2), for oblique load of 100N it was 3477N/cm(2) respectively. When the crown height is increased to 10mm, the maximum stress concentration in D2 bone was 1875N/cm(2) for vertical load, 4015N/cm(2) for oblique load and in D3 bone the maximum stress concentration was 2123N/cm(2) for vertical load and 4236N/ cm(2) for oblique load. In case of titanium screws for crown height of 7.5 mm, when vertical load was applied, stress concentration was 1603 N/cm(2) where as for titanium alloy screw it was 1820N/cm(2). In case of 10mm crown height stress concentration was 1904N/cm(2) for titanium screw and 2123N/cm(2) for titanium alloy screw. In case of oblique loading for 7.5mm crown height stress concentration was 3155N/cm(2) for titanium screw 3477N/cm(2) for titanium alloy screw. For 10mm crown height stress concentration was 4236N/cm(2) for titanium screw, 4663N/cm(2) for titanium alloy screw. CONCLUSION: Stress concentration was less and stress distribution was better in D2 bone density than in D3 bone density. Stress concentration was less and stress distribution was better in commercially pure titanium screw than in titanium alloy screw. With the increase in the height of crown (i.e., from 7.5mm to 10mm) stress concentration and stress distribution also increased.

Retention of denture bases fabricated by three different processing techniques - An in vivo study.

AIM: Distortion due to Polymerization shrinkage compromises the retention. To evaluate the amount of retention of denture bases fabricated by conventional, anchorized, and injection molding polymerization techniques. MATERIALS AND METHODS: Ten completely edentulous patients were selected, impressions were made, and master cast obtained was duplicated to fabricate denture bases by three polymerization techniques. Loop was attached to the finished denture bases to estimate the force required to dislodge them by retention apparatus. Readings were subjected to nonparametric Friedman two-way analysis of variance followed by Bonferroni correction methods and Wilcoxon matched-pairs signed-ranks test. RESULTS: Denture bases fabricated by injection molding (3740 g), anchorized techniques (2913 g) recorded greater retention values than conventional technique (2468 g). Significant difference was seen between these techniques. CONCLUSIONS: Denture bases obtained by injection molding polymerization technique exhibited maximum retention, followed by anchorized technique, and least retention was seen in conventional molding technique.

To Evaluate the Bone Mineral Density in Mandible of Edentulous Patients using Computed Tomography: An In Vivo Study.

BACKGROUND: To evaluate the bone mineral density in the mandible of edentulous patients at prospective intraoral implant sites. Pre-operative evaluation of bone density is essential to assist the clinician with the treatment planning of implant supported prosthesis. MATERIALS AND METHODS: A study group of 12 edentulous subjects comprising of six male and six female between the age group of 45-55 years seeking implant supported prosthesis were selected. A radiographic stent using auto polymerizing resins incorporating the gutta-percha cones were prepared for the computed tomography scan. The bone mineral density values were recorded in various sites (trabecular and cortical) of the mandibular jaws in Hounsfield units. The data thus obtained were tabulated and statistically analyzed using Mann-Whitney U-test and Kruskal-Wallis test. RESULTS: The bone mineral density in the buccal cortical region of mandible increases from incisors to molars and in the trabecular region it is more in the incisors and canines compared to the premolar and molar regions whereas in the lingual cortical region of mandible may lie on nearly the same level over the entire lingual cortex. The bone mineral density is little higher in males than females. CONCLUSION: There is variation in the bone mineral density in the buccal cortex and trabecular bone, but no significant variation in the lingual cortex when compared between male and female subjects.