Volume 102 Reviews and abstracts 193 Number 2

bond strengths of the new Transcend Series 2000 ce- ramic brackets when bonded with one of three ortho- dontic resins (Orthon Cement, Phase II with sealant, and Transbond), subsequent to enamel conditioning with one of two techniques: (1) crystal bonding (35% polyacrylic acid with sulfate ions) and (2) conventional acid etching. Ninety-six bovine lower permanent in- cisors were divided so that equal sized groups were subjected to tensile tests with an Instron testing machine and a custom jig after 15 minutes, 1 week, and 1 month of water storage. The tensile jig was intended to sim- ulate the debonding instrument recommended by the bracket manufacturer. Shear tests were also performed on the 3-month sample. SEM was used to evaluate samples of each tested group with respect to site of bond failure. The majority of the brackets tested in tension displayed an intrabracket failure pattern. Strength in tension of these brackets did not exceed 7.44 _ 0.53 MPa. The 3-month shear test showed sim- ilar bond strength results for both enamel conditioning techniques. However, the majority of the crystal bond- ing specimens failed a the enamel/adhesive interface, leaving a clean enamel surface, whereas the acid etch- ing specimens displayed and intrabracket base failure. Leakage tests were negative for all the samples at the 1-month and 3-month intervals. In conclusion, both enamel conditioning techniques displayed comparable testing results. One could question, on the basis of the present in vitro results, whether this new debonding procedure using a pure tensile force underneath the bracket wings, would be clinically safe. Crystal bond- ing could provide a safer interface for ceramic brackets, significant time saving, and less likelihood of enamel damage. The results of this investigation are by no means conclusive as to the safest and most efficient way of debonding these brackets. Further investigation is needed.

A comparison between estimated and true hinge axis as a reference for measuring condylar distractions and deflections from centric relation position Paul Korne, MCID Orthodontics, Tlze University of Western Ontario, 1990.

The purposes of the investigation were: (1) to com- pare the amount of centric slide for casts mounted with an estimated versus a true hinge axis (determined by locating the point of pure rotation of the condyle); (2) to determine the amount of error in hinge axes location on the recording of sagittal centric slides; and (3).to define the precision of the mandibular position indicator (MPI, a method of measuring condylar distractions and deflections from centric relation position). The centric slides recorded for 37 dental students were divided into

vertical and horizontal components with the MPI, and comparisons were made for each component between the two axes mountings.

The routine use of the true hinge axis to obtain MPI readings for analysis of diagnostic casts is not war- ranted. The mean standard error of MPI readings at- tributed to both hinge axes location was 0.4 mm for each component in the sagittal plane, half of which were due to errors of measurement. The mean standard error for the true hinge (0.49 mm) was not statistically significantly greater than that for estimated hinge (0.36 mm). The condylar distractions and deflections from centric relation to maximum intercuspation were pri- marily in an inferior direction with a smaller posterior component.

A comparative study of three ceramic bracket systems and subsequent enamel damage upon debonding C. Bruce Hill, MClD Orthodontics, The University of Western Ontario, 1990.

Three commercially available ceramic bracket sys- tems were compared for their tendency to cause enamel fracture lines and surface loss after debonding. The manufacturers' recommended methods of bonding and debonding were used for each system. Ninety-six ex- tracted premolar teeth were divided into three equal groups. 1. One group had silanated smooth-based sin- gle crystal aluminum brackets bonded with a chemically cured resin. 2. The second group had silanated, irreg- ularly based polycrystalline aluminum brackets bonded with a chemically cured resin. 3. The third group had nonsilanated, highly irregularly based polycrystalline aluminum brackets bonded with a light-cured resin.

Light and scanning electron microscopy revealed both cohesive and cohesive/adhesive debonds. In ad- dition, enamel damage was recorded in 34% of teeth from group 1, 13% of group 2, and 0% in group 3. Loss of surface enamel was observed in one tooth from group 1.

It was concluded that the bracket systems of groups 2 and 3 pose significantly less risk to the integrity of the enamel on debonding than did group 1.

Mandibular and maxillary facial growth in Class I females 14, 16, and 20 years of age Timothy Foley, MCID Orthodontics, The University of Western Ontario, 1990.

The objective of this study was to determine the magnitude and direction of postpubertal mandibular and maxillary facial growth in females. Lateral cephalo- metric radiographs were used to measure differences in