As the demand for orthodontic treatment has grown, the variety of appliances and techniques to correct dental malocclusions has increased. Since the development of adhesive resins, bands are no longer used on anterior teeth, and the metal brac­ kets that are now bonded to the tooth are becom­ ing smaller and less obtrusive. The development of the lingual appliance, with the absence of any labial attachments on the anteriors, was hailed as being the ultimate in orthodontic esthetics.1 This lingual system, however, was not without its disad­ vantages. The increase in chairside time, both to fit and adjust the appliance, patient discomfort, and the management of the anchorage requirements were amongst the most problematical. These prob­ lems, together with the finer control afforded by labially bonded brackets, have resulted in the demise of the lingual system by most practicing orthodontists.
The development of polycarbonate plastic brac­ kets was the first step towards an esthetic labial orthodontic system. These brackets were also not without their clinical problems. Firstly, they were very prone to staining and discoloration, especially in patients who smoke or drink a lot of tea and cof­ fee. Secondly, the polycarbonate resin was much less dimensionally stable than the materials used, and so the bracket slot dimensions could not be manufactured to the same precision. Thirdly, fric­ tion generated between the plastic bracket and metal archwire made it very difficult to slide teeth along the archwire.2 These plastic brackets have now been superceded by the new generation of ceramic and quartz brackets.3
Ceramic brackets are most commonly made out of tooth colored polycrystalline alumina, and many different shapes and prescriptions are available. Due to strength limitations of ceramics, only Edgewise and Straightwire brackets are available at the present time since the Begg bracket system does not lend itself to the design demands of the material. Ceramic brackets have been investigated to determine their mechanical properties, and it has been found that their strength correlates closely with surface defects often found on their sur­ faces.4,5 The failure of the material is due to crack growth propagation originating at these surface defects and so the provision of a defect-free surface is paramount for clinical longevity of the brackets.
Ideally, the bracket-archwire system should be frictionless to enable free movement of the tooth in all three planes. In reality, however, a large part of the anchorage is consumed by the frictional com­ ponent of the system. Previous work on friction in fixed appliances has quantified some components of the frictional force, but these studies have only investigated metal brackets in conjunction with stainless steel, Nitinol and titanium molybdenum alloy archwires.6-8 It was found that frictional bind­ ing was related to the surface topography of the slot, the mesiodistal width of the bracket, the archwire dimensions, the type of archwire, the applied force and the method of archwire ligation. A recent study9 was undertaken to investigate the bracket-archwire interaction when a tooth was retracted through a viscous material using sliding mechanics with the Edgewise technique. The con­ clusions were in general agreement with the previ­ ous studies in identifying the importance of surface irregularities in the bracket slot as well as the archwire selection, both in terms of dimension and composition.
This study is the first of a two-part investigation into the clinical impression that sliding mechanics with ceramic Straightwire prescription brackets produces slower tooth movement than when using similar metal brackets. The study examines the sur­ face characteristics of the ceramic bracket slots to identify defects or irregularities which could increase frictional binding of the system. The sec­ ond part will attempt to quantify the frictional resis­ tance of moving ceramic brackets along a continu­ ous archwire.

  Fifty Transcend ceramic orthodontic brackets were prepared for examination by a Hitachi S405A scan­ ning electron microscope. The brackets were coated with a minimal layer of conducting medium to improve the signal generated by the ceramic surface within the microscope. These were compared with fifty stainless steel brackets, twenty-five Andrews' Siamese Straightwire, and twenty-five standard Edgewise.* The metal brackets were attached to the specimen mounts using magnetic tape. The fitting surface of the brackets (that part which lies against the enamel tooth surface) and the slot (that part which engages the wire) were examined at known magnification and photographed.
The dimensions of the bracket slot were also measured using the electron microscope at mag­ nifications between X15 and X60, and these values were compared to the manufacturers stated values.

  The scanning electron micrographs (SEM) of the metal brackets are shown in Figs. 1a and 1b. Notice the surface debris that is present. The SEM of a typical ceramic bracket slot (Transcend) is shown in Figs. 2a, 2b, and 2c under increasing magnification. It is observed that many of the defects on closer examination actually protrude from the bracket surface. In contrast, the fitting sur­ face of the ceramic bracket, as shown in Figures 3a and 3b, has a relatively free defect field which at higher magnification still shows minimal defects.
The dimensions of the bracket slot, as measured by the electron microscope, are compared with the manufacturers' figures in Table 1. The slot sizes are  given in inches (International Standard) together with the standard deviation values for all the metal and ceramic brackets. The values measured accorded well with the manufacturers' stated dimensions, and there was no significant difference between the ceramic and metal bracket slot size tolerances.

  With the ever increasing demand for orthodontic treatment, especially from adults, the need for an esthetic and clinically efficient bracket remains paramount. Metal brackets are becoming more compact, and with the recent development of ceramic and quartz designs, the labially placed bracket with its improved mechanics over the lingual approach is here to stay. The Straightwire appliance,10 with its various prescriptions, has become the orthodontic system of choice for most Edgewise operators. The inclusion of the Straightwire system in ceramic bracket design together with their superior esthetics is largely responsible for the increased usage of these brac­ kets worlwide.
Inherent to the Straightwire system, whether it be with metal or ceramic brackets, is the integral use of sliding mechanics. This has been extensively inves­ tigated in both wet and dry conditions9,11,12 and there is no doubt that oral saliva is important in reducing the frictional component between the bracket and the archwire. It has also been shown that the inhomogeneity of the bracket slot plays an important role in the development of friction which can lead to the hindrance of free tooth movement. This has important implications in terms of calculat­ ing the anchorage requirements for certain orthodontic tooth movements.13
The slots of the metal brackets were consistently smooth, whereas those of the ceramic brackets showed equally consistent surface defects which seemed to project above the surface layer in many cases [Fig. 2c]. This was distinct from the fitting sur­ face of the ceramic brackets which showed a smooth surface. Even allowing for the fact that this surface has a minimal coating of a silane bonding agent, similar to that used when bonding to porce­ lain crowns and veneers to enhance the tooth-bracket bond strength, the absence of surface defects was extremely obvious.
It would, therefore, seem advisable for the man­ ufacturers to incorporate this smooth surface within the bracket slot in order to reduce the fric­ tional component of the archwire- bracket interac­ tion to a minimum. This would then ensure that the ceramic brackets were comparable to metal brac­ kets in terms of reduced frictional interference.
The dimensions of the bracket slots accorded well with the manufacturers stated figures and there was no significant difference between the tolerance values of metal compared to ceramic brackets.

  The authors would like to thank the staff of the Scan­ ning Electron Microscope Department of Guys Hospital, England for their help and assistance, and to the Riyadh-Al-Kharj Hospital Program for the study and travel sup­ port given to Dr. Toms in presenting this paper to the 56th Congress of the European Orthodontics Society on 16-18 May 1989 in Weirzburg, Germany.