Tricks of the trade: Pushing for success in lower arch space closure

Orthodontic tooth movement is subject to Newton's Third Law of Motion. Exerting forces on teeth creates equal and opposite reactionary forces with the potential to cause unwanted tooth movement and impede the achievement of treatment aims. How these reactionary forces are managed forms much of the basis of the art and science of orthodontics.

Space closure can be approached through a variety of means, but the key features of the occlusion and any unwanted consequences of the chosen mechanics must be carefully considered.

This case highlights the left buccal occlusion of a Class II division 2 case transferred midway through treatment (Figure 1). There is no further requirement for the space that has been created by the loss of the lower left first premolar (LL4), so ideally the posterior teeth in the lower left quadrant need to be protracted. The combined root surface area of the lower left second premolar and first and second molars (LL5, LL6 and LL7) is far in excess of those teeth in the labial segment in the same quadrant.¹ This situation then creates an unfavourable anchorage balance and routine intra-arch space closing mechanics here would result in a shift in the lower centreline, and an increase in the overbite and overjet because there would be reciprocal distal movement of the LL3 into the extraction space, leading to a Class II buccal relationship at the end of treatment.²

Here we illustrate an alternative method for space closure. The distal anchorage unit, comprising the LL5, LL6 and LL7 is ‘broken up’. Pushcoil is placed between the LL5 and LL6, and the premolar is ‘pushed’ into the space (Figure 2). The same is then done with the LL6. To achieve this, the archwire is put through the LL6 tube, and pushcoil threaded on to it, before it is placed through the LL7 tube. A tight bend-back at the distal end of the archwire maintains the arch length, preventing the distal movement of any of the lower left posterior teeth. Interarch Class II elastics lend further support to the desired movement (Figure 3). At the end of treatment, the canine relationship has been maintained and full closure of the space is achieved (Figure 4).