Secure, full archwire engagement

Absolute security of archwire engagement—as provided by a molar tube—would inherently reduce treatment inefficiencies compared with conventional ligation. Although the potential for decay or loss of elastomeric ligatures is well known, only decay has been extensively investigated. One study did quantify the loss of elastomeric ligatures, finding 15 lost ligatures in 25 consecutively seen patients in a 12-month treatment period. Some early self-ligating brackets did not deliver the secure engagement of a molar tube, but today most self-ligating brackets have mechanisms to deliver this advantage and would by definition ensure full engagement of all archwires and eliminate the need to regain control of the teeth when full engagement is lost. In a recent study of canine retraction with 0.018-in stainless steel wires, self-ligating brackets were found to give better rotational control of the canine than conventionally ligated brackets. Furthermore, 80% of the canines rotated more than 10° with conventional brackets, as opposed to 33% with self-ligating brackets, over the 12-week period.

Faster ligation and archwire removal

The original motive when developing self-ligating brackets was to speed the process of archwire ligation. Several authors have shown self-ligating brackets to be better in this respect, with savings of up to 9 minutes per visit compared with wire ligation and approximately 2 minutes compared with elastomeric ligation. In these studies, unassisted archwire ligation with self-ligating brackets was compared with conventional ligation performed with a chair-side assistant to speed the process. It is probable that many of the more recent bracket types would show greater time savings for archwire changes because the mechanisms have become significantly easier to use. Interestingly, almost no recent studies with these newer brackets have emerged, probably because the speed and ease of use of the more modern self-ligating brackets is now taken for granted.

Faster ligation and archwire removal

The original motive when developing self-ligating brackets was to speed the process of archwire ligation. Several authors have shown self-ligating brackets to be better in this respect, with savings of up to 9 minutes per visit compared with wire ligation and approximately 2 minutes compared with elastomeric ligation. In these studies, unassisted archwire ligation with self-ligating brackets was compared with conventional ligation performed with a chair-side assistant to speed the process. It is probable that many of the more recent bracket types would show greater time savings for archwire changes because the mechanisms have become significantly easier to use. Interestingly, almost no recent studies with these newer brackets have emerged, probably because the speed and ease of use of the more modern self-ligating brackets is now taken for granted.

Lower resistance to sliding

When drawing wires through well-aligned brackets, friction is the only source of resistance to sliding, and friction has consistently been measured as much lower in self-ligating brackets than in conventionally ligated brackets. Studies typically report values of almost zero with self-ligating brackets and values from 43 to 98 g per bracket for various elastomeric-archwire combinations. The obvious next requirement was to investigate resistance to sliding with active archwires, where binding is an important contributor; this has been thoroughly investigated, particularly by Thorstenson and Kusy. These authors examined the effects of varying active tip (angulation) on the resulting resistance to sliding ( Table ). They calculated that for an 0.018 × 0.025-in stainless steel archwire, an activation of 6° was clinically the most relevant, since beyond that angle uprighting forces from the archwire would limit further tipping and cause the tooth to “walk” along the archwire in a series of binding and releasing movements around this angle. At this angulation of 6°, the difference in resistance to sliding is 60 g per bracket. Fig 10 in Thorstenson and Kusy’s paper, which illustrates these data, shows “clinically relevant illustrations showing that in terms of sliding with second order angulation, this bracket is superior to the conventional bracket.” Interpretation of this important point can vary widely, even when the same research is being quoted. For example, Brauchli et al correctly quoted data from Thorstenson and Kusy to the effect that for an 0.018 × 0.025-in wire and a self-ligating bracket “with angulations of 7° between the archwire and slot, more than 94% of the [resistance to sliding] is caused by binding.” However, this has been taken to imply that the contribution to resistance to sliding from the lowered friction of self-ligating brackets is therefore irrelevant when archwire activation causes binding. In fact, it is the low friction with self-ligating brackets that makes any binding component such a high percentage of the overall resistance to sliding. If the friction is zero—and it is usually close to zero with self-ligating brackets—the binding component will correspondingly constitute 100% of the resistance to sliding. Pliska et al also found that at high angles of tip, the reduction in friction with self-ligating brackets is drowned by the high binding, but at lower and clinically more relevant levels of activation, reductions in resistance to sliding averaging 40 g were found with self-ligating brackets. Studies of aligning wires placed in irregularly aligned brackets have also shown large reductions in resistance to sliding with self-ligating brackets in all 3 planes of space. Three of these articles specifically measured the other side of the equation: the net residual aligning force available to act on the displaced tooth after the losses from resistance to sliding. Typically in these studies, a bracket displaced by 4 mm from its neighbors has no residual aligning force with conventional ligation but 80 g of aligning force with self-ligating brackets. Peterson et al showed that artificial aging of elastics reduced this differential, but Matasa demonstrated the consequent trade-off for elastomerics with the loss of ligating force available for tooth control. Interestingly, these laboratory results were similar to an in-vivo study by Crawford et al, who commented that “the results of our in situ study concur with those of the best outcomes from the laboratory studies.” Well-designed in-vitro studies may indeed give a good insight into in-vivo conditions. In a systematic review of this question, the authors concluded that the case is proven for lower resistance to sliding with self-ligation with round wires, but not for the larger rectangular wires where more evidence is still required. An interesting facet of the study selection for this review was that 2 of the seminal articles by Thorstenson and Kusy referred to above were not mentioned.

Lower resistance to sliding in clinical use seems probable, as does the effectiveness of lighter applied forces, since less force for the desired tooth movement is “eaten up” as resistance to sliding. The question is whether this difference translates into any measurable improvements in the treatment process or the outcome.

More rapid treatment

Several consecutive case series studies found that treatment with self-ligating brackets was quicker, required fewer visits, and resulted in as good or better final alignment and occlusion than treatment with conventional appliances. However, other similar studies and all randomized controlled studies to date have found no difference in these parameters between self-ligating and conventional brackets in various parts of the treatment process. Two recent systematic reviews have understandably concluded that there is insufficient evidence to support the view that treatment with self-ligating brackets results in fewer visits or shorter treatment. It seems probable that self-ligating brackets, by themselves, do not confer a blanket advantage of reduction in duration of treatment and that randomized controlled trials with a general case mix when the bracket type is the only tested variable and all other treatment mechanics are identical will continue to find no difference in average treatment duration. It is possible that in certain patients and with archwires and reappointment intervals customized to match the biomechanical differences of lower resistance to sliding and better ligation control, self-ligation might reduce treatment duration. However, no randomized controlled trial has succeeded in supporting the view that fully preadjusted straight-wire brackets are superior to plain edgewise brackets, although their superior efficiency is universally assumed and adopted. Similarly, after several decades of use, only 1 recent study has succeeded in supporting the view that nickel-titanium aligning archwires are more effective than multistrand stainless steel—and then, only with Begg brackets, not with preadjusted edgewise brackets. Randomized controlled trials are vitally important, and I have been an author of and a participant in several, but in orthodontics, the authors of these studies have for various reasons struggled to find differences between treatment modalities.

Is treatment less painful with self-ligation?

The notion that treatment with self-ligating brackets is less painful has 2 possible explanations: the forces on the teeth are lower because lighter archwires can be used with equal effectiveness, and the teeth move more readily in response to the applied forces because of decreased resistance to sliding. However, although it is probable that lower applied forces will be more effective with self-ligation because less force will be lost through resistance to sliding, lower net forces are certainly not inevitable and, with self-ligating brackets, are correspondingly more sensitive to the choice of archwire. Three studies found that patients reported lower pain levels with various self-ligating brackets ; 2 others found no difference. The authors of a systematic review concluded that the balance of evidence from the 3 published randomized controlled trials on this topic just favor a reduction of pain during alignment with self-ligation. An interesting split-mouth study examined the question via a different measure of pain—the level of the neuropeptidase substance P in gingival crevicular fluid, which is a marker of inflammation and associated pain resulting from orthodontic forces. These authors found that treatment with self-ligating brackets significantly lowered the levels of this marker of pain and inflammation when compared with conventional ligation at 24 hours after archwire placement. At the least, there is reason to believe that lower resistance to sliding and the use of lighter but still effective archwires might be less painful for our patients.