Page 39 - REMOVABLE ORTHODONTIC APPLIANCES

Appliance retention 31
aspects of the crown just below the gingival ing the depth of the undercut. A vicious circle
margin. The model must be carefully trimmed to can then occur as further tightening, in an
reproduce the anatomical contour of the crown attempt to improve retention, produces further
so that when the appliance is constructed, the tooth movement, which makes the situation
clasps engage the undercuts snugly. Overzealous worse. It is even possible to create a crossbite,
trimming of the model will make the appliance which can only be corrected by the use of an
impossible to insert without extensive adjust- additional appliance. Such problems are liable
ment. Too little trimming may mean that reten- to occur where retention was initially poor due
tion is inadequate and there may be little to faulty appliance design and construction,
prospect of improving it subsequently. sometimes compounded by an awkward crown
In adults the opposite problem can occur. form. It can be avoided only by careful scrutiny
Undercuts at the gingival margin may be too and trimming of the model, thoughtful clasp
deep, particularly if there has been gingival placement and careful routine adjustment.
recession. Clasps are too stiff to engage deep
undercuts and, if they are made to do so, the
Lower molars
appliance will be impossible to insert. Adjust-
ment of the clasps to allow the appliance to be In some cases, lower molars have little avail-
fitted means that they do not contact the tooth able undercut on the buccal surface and conse-
surface and so the appliance will be loose, with quently retention of a lower appliance may not
little prospect of satisfactory modification. In be satisfactory. Adjustment of the clasp to try
such cases the clasps must only engage the to increase the retention may result in an appli-
required depth of undercut and should not ance that does not seat satisfactorily. One solu-
extend as far as the gingival margin. tion to the problem of lack of buccal undercut
on lower molars is to use an appliance with
clasping on the lingual side (see below).
Advantages of the Adams' clasp
The advantages of the Adams' clasp are as
follows: Variants of the Adanns' clasp
Adams described variations of this clasp
• Its bridge provides a site to which the patient
can apply pressure with the fingertips during (Figure 4.2) suited to most teeth, deciduous and
pennanent.
Soldered
clasps
that
auxiliary
removal of the appliance
• Auxiliary springs can be soldered to the utilize further undercuts are seldom used, as
they are awkward to adjust. An alternative is to
bridge of the clasp add an accessory spur made from 0.8 mm wire
• Hooks can be soldered to the clasp or bent with a single arrowhead closed to provide a
in during its construction to accept inter- smooth end (Figure 4.3).
maxillary traction
• Tubes can be soldered to the bridge of the
clasp to accommodate a facebow for extra-
oral traction.
Soldering must be carefully carried out, since the
use of excess heat can lead to softening of the
wire, which will reduce the efficiency of the clasp.
Figure 4.2 An Adams' clasp (0.6 mm wire) on a
Adjustment of the Adams' clasp canine.
Construction of the Adams' clasp is not simple
(see Appendix 1), but the clasp is effective and
can be readily adjusted at the chairside (see
Figure 10.7, p. 90). When the appliance is
seated a clasp should be nearly passive. Active
clasps can exert a palatal force on the teeth, so
that if the appliance does not seat fully they will
tend to be tipped under the baseplate, so reduc- Figure 4.3 A single arrowhead (0.8 mm wire).