Current trends in canal preparation

Current thinking on canal preparation emphasises the development of shape in a crown-down manner removing infected dentine as it is encountered. Further cleaning of the canal is provided by correct choice and use of root canal irrigants (in particular sodium hypochlorite

because of its antimicrobial and tissue-dissolving properties), with the combination being termed chemomechanical debridement.

Instrument manipulation

The two most commonly used motions are watchwinding and balanced force.

Watchwinding refers to the gentle side-to-side rotation of a file (30° each way). This motion is useful for all stages of canal preparation especially initial negotiation and finishing the apical third.

Balanced forces (in many ways a development from watchwinding) involves rotating the instrument 60° clockwise to set the flutes and then rotating it 120° anticlockwise

while maintaining apical pressure sufficient to resist coronal movement of the file. Balanced forces is an efficient cutting motion and has been shown to maintain a central canal position even round moderate curvatures while allowing a larger size to be prepared apically (compared with other hand instrumentation techniques).

The danger zone where care needs to be taken in order to avoid strip perforation. (a) Lateral view. (b) Crosssectional view.

Coronal interferences influence the forces a file will exert within a canal. This is of particular importance in curved canals where files may prepare more dentine along the furcal (danger zone), as opposed to the outer canal wall. It is important to be aware of this, to limit the

size of enlargement in curved canals and direct files away from the furcal wall to avoid a strip perforation.

Irrigation

Irrigating solutions are usually delivered using a syringe with a 27 or 28 gauge needle. Care should be taken to ensure that the needle does not bind in the canal and that irrigating solution does not pass into periapical tissues. The role of the irrigant is to remove debris and

provide lubrication for instruments. Specifically, an irrigant such as sodium hypochlorite will dissolve organic remnants and, most importantly, also has an antibacterial action. It may be used in a range of concentrations from 0.5 to 5.25%, 2.5% being popular. It is important

that the irrigant is changed frequently; ideally irrigation should be performed between each file, at least every two to three files being the minimum. If removal of the smear layer is desired, then an irrigation solution containing ethylenediamine tetraacetic acid (EDTA) should be used. There is no clinical consensus as to whether or not the smear layer should be removed. A most effective way of delivering irrigating solutions is through an ultrasonic handpiece. Ultrasonic

agitation (acoustic microstreaming) has been shown to be effective at removing debris from canals.

Canal preparation

Several methods of canal preparation exist. The method described here encompasses current thought:

canal exploration

pre-enlargement (when necessary)

straight-line radicular access

length determination

apical third preparation.