Group Standard Deviation Research Articles

Effects of different silanes and acid concentrations on bond strength of brackets to porcelain surfaces

The aim of this study was to determine the optimum silane-coupling agent and the optimum concentration of acid agent when bonding to porcelain surfaces. Eighty deglazed feldspathic porcelain discs with a diameter of 10 mm and a thickness of 2 mm mounted in acrylic resin blocks were randomly divided into four groups. In groups 1 and 2, the porcelain surfaces were etched with 9.6 per cent hydrofluoric (HF) acid and in groups 3 and 4 with 5 per cent HF acid. In groups 1 and 3, the Dynalock maxillary central incisor brackets were bonded with Pulpdent silane and Unite bonding adhesive and in groups 2 and 4 with Reliance silane and Unite. Shear forces were applied to the samples using an Instron universal test machine. The non-parametric Kruskal-Wallis test was used to determine significant differences in bond strengths between the four groups and Dunn's multiple comparison test to compare subgroups. The mean bond strengths and standard deviations of groups 1 to 4 were 5.51 +/- 1.19, 6.54 +/- 0.002, 4.55 +/- 1.93, and 6.39 +/- 0.45 MPa, respectively. Specimens bonded with Reliance showed a statistically significantly higher in vitro bond strength than those bonded with Pulpdent. The concentration of etching gels did not result in any statistically significant difference on the in vitro bond strength when evaluated separately.

Evidence-biased medicine: Intention-to-treat analysis less conservative?

Evidence-based epidemiology requires a rigorous but creative analysis of data. Intention to treat (ITT) analysis (i.e. taking into account the last data available for any subject included and for whom a point baseline evaluation is available) is commonly accepted as more conservative than the per-protocol (PP), restricted to the analysis of the data of subjects who completed the study. Commonly, the withingroups differences being smaller in ITT than in PP, their statistical comparison lead to a smaller risk of type I error (i.e. inappropriately concluding to a difference while there is not). It also allows for keeping the randomization scheme (i.e. the balanced distribution of confounding factors), and thus not leading to a differential distribution of confounding factors between groups if more subjects are withdrawn from the study in a given group. However, there is a particular condition in which ITT is less conservative, and thus leads to a higher risk of concluding to a difference in absence of real one. The table below displays the results of a hypothetical trial. The mean values and their standard deviations for group I and group II, before and after intervention are shown. The treatment allocated in group I induces a smaller effect in limiting the decrease of the outcome than the treatment received in group II. In patients finishing the trial, we assume a 20 (34)% reduction in group I and a 30 (27)% in group II. The losses to follow-up values are considered normally distributed between “before” and “after” values. The loss to follow-up in Group I is 50% while 10% in Group II. Figure 1 If we compare the changes between group I and II in PP and ITT using a classical unpaired Student t-test, we obtain p=0.06 (NS) and p=0.008 (S), respectively. The higher loss to follow-up rate in group I made the delta smaller in ITT (more subjects left the trial before the benefits of the whole effect of the treatment). In group II, that rate being much smaller, the delta did not vary much, since only 10% of the subjects had an intermediate value between “before” and “after”. The resulting global difference of the variations observed within groups was therefore higher in ITT and the p-value smaller. In a clinical perspective, such a situation can be encountered when the investigational treatment aims at preventing the decrease of the main outcome e.g. a symptomatic effect. If the onset of action of the experimental drug is longer than for the comparator, notwithstanding its superior efficacy at the trial end, the probability that more patients quit the “treatment” arm than the “control” one is high. Since the mean effect observed for loss to follow-up patients is below the one of per-protocol ones (they quit the trial very early), the mean difference in ITT will be superior than the one observed in per protocol. Once in a blue moon...

Comparison of Enhancement, Image Quality, Cost, and Adverse Reactions Using 2 Different Contrast Medium Concentrations for Routine Chest CT on 16-Slice MDCT

To evaluate the degree of enhancement and image quality of chest computed tomographic (CT) examinations on 16-slice multidetector CT using low-concentration [300 milligrams of iodine per milliliter (mg I/mL)] and high-concentration (370 mg I/mL) contrast media; to assess the impact on cost and adverse reactions of the use of high-iodine concentration contrast medium. A total of 100 patients scheduled for routine chest CT examinations were administered nonionic contrast medium of 2 strengths: low-iodine concentration contrast medium (300 mg I/mL) [group A: n = 50; male-female ratio, 28:22; mean age, 58.4 years] and high-iodine concentration contrast medium (370 mg I/mL) (group B: n = 50; male-female ratio, 18:32; mean age, 57.6 years) with a constant amount of iodine (400 mg) injected per kilogram of body weight. Contrast media were injected using a dual injector at 2.5 mL/s followed by a 30-mL saline at 2.5 mL/s. The degree of enhancement was quantified by measuring Hounsfield unit values in different arteries and veins and was also rated on a 5-point scale for qualitative assessment. We also evaluated perivenous contrast-related artifacts. The data were compared using Mann-Whitney U test for both qualitative and quantitative enhancement ratings. A value of less than 0.05 was considered statistically significant. The value was adjusted using Bonferroni correction for statistical significance when multiple comparisons were performed. The difference in cost and the incidence of adverse reactions in both groups were calculated. The mean enhancement values in group B were significantly greater (P < 0.05) than those in group A. The mean Hounsfield units and standard deviation in groups A and B were aorta = 153 +/- 4, 216 +/- 20; pulmonary artery = 147 +/- 10, 208 +/- 20; superior vena cava = 155 +/- 27, 299 +/- 72; and pulmonary vein = 134 +/- 10, 215 +/- 30, respectively. The mean enhancement on a 5-point scale was greater in group B (4.2) than in group A (3.3) (P < 0.01). No significant difference between groups in perivenous artifacts was seen. Up to 5.5% savings in cost resulted from the use of a higher concentration of iodine, with no increase in adverse reactions. Use of higher-concentration contrast media provides a higher degree of contrast enhancement and image quality for a routine chest CT on a 16-slice multidetector CT. It also contributes to considerable cost savings with no increased risk of adverse reactions compared with low-concentration contrast media.

A randomized clinical trial of combined topical-intracameral anesthesia in cataract surgery

Objective To evaluate the level of patient discomfort during phacoemulsification and implantation of a foldable intraocular lens (IOL) while under topical lidocaine, alone or in combination with intracameral lidocaine. Design A prospective, randomized, controlled trial. Participants Between January and July 1997, a total of 162 patients (162 eyes) scheduled for cataract surgery were randomly assigned to either the placebo group (topical anesthesia with intracameral balanced salt solution [group 1, n = 81]) or the interventional group (combined topical-intracameral anesthesia [group 2, n = 81]). Interventions All patients underwent temporal clear corneal phacoemulsification and implantation of a foldable silicone IOL. All patients received a minimum total of five doses (two drops per dose) of 2% topical lidocaine hydrochloride. Three doses were administered before surgery into the superior and inferior cul-de-sac at 10 minutes, 5 minutes, and immediately before the initial corneal incision. One dose was administered before the commencement of phacoemulsification and another dose before implantation of the IOL. In addition, all patients received an intracameral injection immediately after entrance into the anterior chamber. Patients in group 1 received a 0.5-ml injection of balanced salt solution, and those in group 2 received a 0.5-ml injection of 1% lidocaine. Main outcome measure A 10-point visual analogue scale was used immediately after the surgery to assess each patient’s overall severity of pain intraoperatively. Results Ninety percent of patients in group 1 and 95% in group 2 assigned a score of 0 or 1 to the level of intraoperative discomfort. The mean pain score +/−1 standard deviation for group 1 was 0.63 ± 0.68 and for group 2 was 0.37 ± 0.58. The difference between the pain scores for the two groups was statistically significant ( P = 0.0053). A small but significant proportion (15%) of patients in group 1 expressed distress when the ciliary body was stretched or the iris was manipulated with instruments. Conclusion Although topical lidocaine alone provides adequate anesthesia for phacoemulsification and implantation of a foldable IOL, combined topical and intracameral administration of lidocaine can further minimize intraoperative discomfort.

Computerized Cephalometrics

A method to analyse cephalometric radiographs with the help of a computer has been described. With the system a cephalometric radiograph can be analysed and the results compared to corresponding analyses of individuals with ideal occlusion. The results can also be visualized as schematic figures of the patients. The computer calculates means and standard deviations of groups of individuals and can visualize the results as mean figures. A program to train operators in pinpointing radiographic reference points has also been developed. A methodological study has shown that the errors of the computerized method are of little importance compared to the errors made in pinpointing radiographic reference points. A clinical test of the system has shown that inexperienced operators are able to use the system after about one hour's training. The method is about twenty minutes faster compared to an ordinary manual radiographic analysis. Some future aspects are also discussed.

Intrinsic cardiac output variability in the anesthetized normal and splenectomized dog.

In an effort to determine variations in the normal hemodynamics of the dog, serial cardiac output and blood pressure measurements were made and peripheral resistance values calculated in the same animal over a prolonged period of time. The hemodynamic effect of splenectomy was observed by comparing seven lightly anesthetized normal dogs with seven splenectomized animals. The standard deviation for serial cardiac output values in individual normal dogs was ±6–15%. The mean value for the standard deviation in the entire group was ± 10.5%. The standard deviation for the cardiac output values in splenectomized dogs varied from ±6.5 to 29.5%. The mean value for the standard deviation in this group was ± 15%. Although the normal dogs appeared to have a more stable cardiac output, statistical evaluation of the data showed no significant difference in cardiac output variation between normal and splenectomized dogs. The intrinsic variation in the cardiac output of the dog is apparently physiological in origin and must be considered in interpreting serial observations of cardiac function.

Digital recording and automatic computation of film tensile stress–strain data

A system is described for automatic collection of stress–strain data for polymeric films on magnetic tape for subsequent automatic data reduction and computation of a variety of stress–strain parameters. Two shaft analog-to-digital encoders attached to a conventional tensile tester feed digital information to an incremental pulse recorder which stores all stress and strain data in two channels on 0.625-in. magnetic tape. Specimen information and test constants are inserted by the operator through a keyboard located at the tensile tester. The tape is subsequently fed into a standard digital computer through a special incremental pulse magnetic tape reader. The data pulses are internally stored and converted by means of a special program into eight significant stress–strain parameters for polymeric film samples, plus averages and standard deviations for groups of replicate determinations. These parameters include: corrected length of specimen, initial maximum tensile modulus, offset yield stress, strain at break, strain at maximum stress, maximum tensile stress, tensile stress at break, and work to break. While the program described applies to polymer film testing, both it and the digital recording equipment accessories are applicable with suitable modification, to any analog experimental data. Significant savings in man-hours and increased information output result from use of this system.