Hamdi H. Hamama, BDS, MDS
Assistant Lecturer, Conservative Dentistry Department, Faculty of Dentistry, Mansoura University, Mansoura, Egypt.
Nadia M. Zaghloul, BDS, MDS, PhD
Associate Professor, Conservative Dentistry Department, Faculty of Dentistry, Mansoura University, Mansoura, Egypt.
Ossama B. Abouelatta, BEng, MSc Engg, PhD
Associate Professor, Production Engineering and Mechanical Design Department, Faculty of Engineering, Mansoura University, Mansoura, Egypt.
Abeer E. El-Embaby, BDS, MDS, PhD
Lecturer, Conservative Dentistry Department, Faculty of Dentistry, Mansoura University, Mansoura, Egypt.
Correspondence to: Dr Hamdi H. Hamama
Conservative Dentistry Department, Faculty of Dentistry, Mansoura University, PO Box 35516, Mansoura, Egypt.
Email: [email protected]
VOLUME 1 • NUMBER 1 • FALL 2011 VOLUME 1 • NUMBER 1 • FALL 2011
49 This study evaluated the influence of the application of flowable composite resin
on cuspal deflection using a computerized modification of the strain gauge method. Forty sound extracted mandibular molars, which received a mesio-occlusodistal slot preparation, were divided into two groups of 20 molars each based on the type of restorative materials used. Each group was further divided into two subgroups of 10 molars each relative to the application of flowable composite resin at cavity internal line angles. Cuspal deflection was measured using a new computerized modification of the strain gauge method. The mean cuspal deflection values (µm/m) and standard deviations were calculated and subjected to normality and homogeneity of variances tests. If they passed the tests, they were subjected to parametric statistical analysis (independent sample t test). The results showed that groups containing flowable composite resin exhibited higher cuspal deflection values than groups without flowable composite resin. The application of flowable composite resin at the internal cavity line angles increased cuspal deflection, possibly due to the material’s high volumetric shrinkage levels, which exerted more stress at the tooth-restoration interface.
Further, the validity of the new computerized modification of the strain gauge method was proven by the agreement found between the output results and those of previous studies of cuspal deflection. (Am J Esthet Dent 2011;1:48–59.)
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THE AMERICAN JOURNAL OF ESTHETIC DENTISTRY THE AMERICAN JOURNAL OF ESTHETIC DENTISTRY
R
apid development of resin-based dental composites is one of the main characteristics of modern esthetic dentistry. Resin-based composite is now widely used as an alternative to amal-gam in stress-bearing areas.1 Consider-ing the polymeric nature of composite resin, it has an inherited volumetric po-lymerization shrinkage property that leads to contraction stresses at the restoration-cavity interface.2 This poly-merization shrinkage has been reported to be one of the factors directly respon-sible for marginal leakage at the com-posite restoration–cavity wall interface.3The interaction of the polymerization shrinkage stresses and the adhesive bond plays a large role in the long-term function of a composite resin restora-tion. At sites where these stresses are higher than the bond strength between the restoration and dental substrate, a microgap will form, increasing the prob-ability of postoperative sensitivity and recurrent caries.4,5 On the other hand, if the bond strength is higher than the polymerization contraction stresses, the stresses will transfer to the cusps, resulting in cuspal deflection.2,6–8
A class of low-viscosity composite resins, commonly called “flowable”
composite resins, has been commer-cially introduced for restorative dentist-ry. Flowability is regarded as a desirable handling property because it allows the material to be injected through small-gauge dispensers, thus simplifying the placement procedure and amplifying the range of applications suggested by the manufacturers.9,10 The effective-ness of flowable composite resin as an intermediate layer at the internal cavity
line angles is one of the most contro-versial topics in dentistry. Some stud-ies support its use due to its stretching capability (ie, its low Young modulus of elasticity), which provides sufficient elasticity to relieve polymerization con-traction stresses.5,10–14 In contrast, some studies suggest that the appli-cation of flowable composite resin in-creases contraction stresses due to the material’s high resin content.15–17
Cuspal deflection is a common bio-mechanical phenomenon observed in teeth restored with composite resin. It results from the interactions between the polymerization shrinkage stresses of the composite resin and the com-pliance of the cavity wall.18 There are many methods to measure cuspal de-flection, including noncontact methods (photography,19 microscopy,20,21 laser scanning,22 and three-dimensional microcomputed tomography23) and contact methods (strain gauge,6,24 interferometers,25 and linear vari-able differential transformers26–29).
Because these methods depend pri-marily on measuring the difference between precuring and postcuring val-ues, they have not provided detailed data regarding how cuspal deflection happens in relation to time. However, the present authors introduce a modi-fication of the strain gauge method, which was developed in cooperation between the engineering and dental teams of this study.
This study was designed to evaluate the effect of the application of flowable composite resin on cuspal deflection of mesio-occlusodistal (MOD) composite restorations. Cuspal deflection was
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measured using the new modification of the strain gauge method. The null hypothesis was that application of flow-able composite resin does not increase the cuspal deflection of MOD compos-ite resin restorations.