Improving palatal attachments

Transcription

1 European Journal oforthodontics 10(1988) European Orthodontic Society Improving palatal attachments P. A. Cook and G. Watts University of Leeds, Leeds, England SUMMARY A major disadvantage of removable palatal arch systems is the tendency for the palatal sheath to fracture from the band. The present study tests three methods of attaching the sheaths to molar bands. The results show that flush soldering after spot welding significantly increases the strength of their attachment to the band, without reducing the strength of the sheath. Introduction A recent evaluation of the methods of attachment of palatal arches (Cook 1987), found that although the majority of orthodontists preferred to use a fixed or soldered attachment, the most popular of the removable systems was the MIA (Mobile Intra-oral Arch).* However, the system was felt to have a number of drawbacks, one of these being the tendency for the bracket sheath to fracture from the band during clinical use. In the questionnaire used in that study a number of respondents pointed out that this disadvantage may be overcome by flushing the bracket tags with solder following spot welding. The present study was instituted to test the flush soldered attachments against a similar series of brackets which had been spot welded only. Material and method Thirty molar bands, of identical size, were divided into three groups of ten and a MIA curved sheath was attached to the palatal surface of each band using one of the following methods: Group 1. These brackets were attached by spot welding using an orthodontic laboratory welder (Vulcan 3). This was used on setting number 3 and each of the four tags was given three spot welds. Group 2. These brackets were attached to the bands at an Orthodontic supply companyt using the welder that is normally used for pre-welding standard orthodontic brackets on to stock bands * Unitek MIA Appliance, Friehamer Strasse 4b, 8032 Grafeling, West Germany. t Orthomax, Bankfield House, Carbotlom Road, Bradford BD5 9BJ. (Unitek, Dual Range Power Supply, model number 1/059/04). Group 3. The brackets in this group were attached exactly as described for Group 1 but the bracket tags were also flushed with solder (Fig. 1), taking care not to anneal either the band or the bracket sheath. Each bracket/band combination was then tested using an Instron 1195 universal testing instrument. The band to be tested was firmly secured on a brass jig using a locking screw, and a small testing arm was inserted into the bracket sheath. The other end of this arm was held firmly in the clamp of the testing machine (Fig. 2). Each of the thirty brackets was tested using a cross head speed of 5 mm/min. and the force required to either fracture the bracket from the band, or distort the band or bracket to such a degree as to render it useless, was recorded on a print-out. Bands from each of the three groups were tested in turn to reduce any method error due to possible distortion of the testing arm or jig during repeated use. On removal from the testing jig each band and bracket was closely inspected to determine the main site of failure, these being recorded either as: bracket tag weld fracture, distortion of the bracket sheath or distortion of the band. Results The forces required to render the bracket/band combination valueless for further use, for each of the three groups, are shown in Fig. 3. In analysing the site of failure, it was found that while all the brackets fractured from the bands in Group 1, the failures in Groups 2 and 3 were

2 Figure 1 A bracket from Group 1, the bracket tags have been spot welded and then flushed with solder. Figure 2 The testing apparatus, showing the band secured in the brass jig, the testing arm inserted into the bracket sheath at one end and secured in the testing machine clamp at the other.

3 382 P. A. COOK AND G. WATTS 100i Table 2 Results of comparing the forces required to produce bracket failure, using the Student's f-test. (-test Significance 801? «0> 2 E a 40. Group I/Group (Laboratory weld/pre-weld) Group 2/Group (Pre-weld/Flush soldered) Group I/Group (Laboratory weld/flush soldered) * Significant at the (0.1%) level, with 18 degrees of freedom. 20" Orow 1 Oroup 2 OrOMBS Figure 3 Histogram showing the mean and standard deviation for each of the three groups. entirely due to distortion of the bracket sheath, with the exception of two bands which distorted badly in Group 3 (Table 1). When comparing the forces required to cause bracket failure, statistically significant differences were found between the three groups by applying Student's Mest, the results of which are shown in Table 2. Discussion The failure of the band/bracket interface is a common problem when using the removable Quadhelix systems. The results of the present study clearly show that this interface may be significantly improved in one of two ways: (1) The bracket sheaths could be welded using a similar orthodontic welder to that used by Table 1 Assessment of the main cause of failure. Group 1 (Laboratory welded) Group 2 (Pre-welded) Group 3 (Flush soldered) Weld site fracture 10 Bracket distortion 10 8 Band distortion 2 the orthodontic supply company. However, this may well be impractical due to the greater cost of these welders. Alternatively, the brackets could be supplied pre-welded to molar bands. Although this has been shown to produce a significantly stronger bond, this would require keeping a stock of such bands to cover a variety of tooth sizes. As these appliances are rarely used in great numbers, this also may not be practicable. (2) The strength of the band/bracket interface can be improved by simply flushing the bracket tags following their spot welding in the usual way. Although it is advised that care is taken not to anneal the band or bracket during this process, the present study has shown that such flush soldered brackets were no more liable to undergo distortion. On the contrary, the results show that the brackets which had been flush soldered required a greater degree of force to distort them (Fig. 4). However, in thisflushsoldered group, two of the bands did distort sufficiently to be the main cause of failure (Fig. 5). It may be that these bands were annealed during the soldering process. If this is the case then, in a clinical situation, a regular check must be made for band distortion when using attachments that have been flush soldered. Other than the attachment fracturing from the band, two other main drawbacks of using removable palatal arch systems were found by Cook ( these being: (1) Displacement of the appliance between visits. (2) Difficulty with insertion and removal of the appliance.

4 PALATAL ATTACHMENTS 383 Figure 4 A distorted bracket sheath, from Group 2. I» *. ; t J r.- Figure S A distorted band, from Group 3.

5 384 P. A. COOK AND G. WATTS A suggested method of securing the removable palatal arch using either an elastic module or a steel ligature has been described (Cook, 1987). However, some clinicians using this method have experienced loss of the elastic modules between visits and the stronger elastic separator rings appear to be of greater value in securing the palatal arch in place. With regard to the difficulty of insertion and removal of the appliance, orthodontic pliers are now available* specifically designed to unlock the appliance from the sheath and then the serrated grooves present in the beaks of the pliers allow the arch to be removed without slipping. However, with experience satisfactory manipulation of the appliance can be obtained by using standard utility pliers (Weingart style). * Orthomax, Bankficld House, Carbottom Road, Bradford BDS 9BJ. Acknowledgements The authors would like to thank Mr David Burdess of Orthomax, Bradford, for supplying the brackets used in this study. We are also indebted to Mr Ian Smith, Leeds Dental School, for his help with the Instron testing machine and to Mr David Hawkridge, for the illustrations. Address for correspondence Mr P A Cook Department of Child Dental Health Leeds Dental School Clarendon Way Leeds LS2 9LU England Reference Cook P A 1987 Methods of attachment of palatal arches: an evaluation. British Journal of Orthodontics 14: