Active versus Passive Self-Ligation? : Control versus Low-Friciton?

“In Matters of Self-Control, As We Shall See Again And Again, Speed Kills. But a Little Friction Really Can Save Lives.”

~ Daniel Akst

In my previous post, I talked about my personal thoughts and experiences with Passive Self-ligating brackets. There are many Cons and Pros to PSL, but how do they compare to ACTIVE self-ligating systems (ASL)?… and would a Hybrid appliance of both Active and Passive SL design give us the best of both worlds?

Let’s look at the main design difference between active and passive systems: the locking mechanism. In PSl, the locking mechanism is usually a sliding door which doesn’t encroach on the bracket slot dimensions in any way, even with a full-sized archwire in place, hence the passive monicker. The situation in ASL brackets is quite different: in most designs, the locking mechanism consists of predominantly an active “spring clip” that encroaches on the slot space from the labial/buccal aspect – particularly when larger-size archwires are in place – thus “actively” providing a seating force on the archwire into the base of the slot. The proposed theory behind this design is that it should enable better and complete torque expression of the bracket’s prescription towards the end of treatment. This active clip action , coupled with the usually shallower depth of the bracket slot in such systems; 0.0175″ to 0.020” compared to PSL’s usual 0.028” slot depth, is also proposed to provide more control on the archwire during initial alignment.

Several studies have attempted to determine how both these designs affect friction during orthodontic sliding mechanics.  It was consistently reported that when a small round archwire lies passively in a self ligating bracket’s slot, less friction is produced than conventionally-ligated systems. However, as the archwire increases in size; 0.018” and larger, differences in friction started to manifest between various types of SL brackets (Brauchli et al, 2012; Berger, 1990; Thorstenson and Kusy, 2001) . Overall, current thinking is that lower friction would be achieved by use of PSL brackets or when smaller archwires are coupled with active SL brackets. Once larger dimensions are reached, PSL brackets seem to have the advantage of producing much less friction than active Sl brackets (Pizzoni et al, 1998; Ward et al, 1997).

When studying the effect on torque expression between PSL and ASL, research comparing different PSL and ASL brackets has shown that better torque expression and control was demonstrated by ASL systems due to their active “forcing” of the archwire into the slot, thus their clinically applicable range of torque activation was greater than PSL systems (Badawi et al, 2008). In terms of speed of treatment, no measurable differences between these systems were demonstrated when comparing the times required for alignment of comparable maxillary anterior crowding (Pandis et al, 2010).

So, would combining both PSL and ASL brackets into one system provide us with “ the best of both worlds” in terms of low friction sliding mechanics as well as better torque expression and control?.. I can only talk about my own modest personal experience with Empower; American Orthodontics’ Active/Passive bracket system, which I had used in only a relatively small number of cases. Empower has a “hybrid” or “dual” system where active clip design is used on the anterior brackets for better torque control and expression while posterior brackets use a passive design for enhanced lower-friction sliding mechanics. Overall the experience was positive and indeed the system certainly seemed efficient at doing what it was supposed to do in a reasonable time frame and with good results in terms of anterior torque expression. My only problem was actually with the clip reliability and weak design which led to open brackets and lost archwire control with rotations on several occasions. I’m not particularly familiar with any notable research regarding these hybrid systems but will be following up on the topic closely to see if such an experience can be scientifically corroborated.

As a final note, the use of self-ligating systems in general is quite a mind-opening experience and a refreshing take on conventional orthodontic mechanics, that seem to be producing some excellent clinical results, once the initial learning curve is overcome. It is interesting to see where this idea is going to take us in the near future.