Poor appliance adjustment

The use of excessive force or trying to move too many teeth at the same time may result in unwanted movement of the anchor teeth. To avoid loss of anchorage, simultaneous multiple teeth movement should be avoided. If the appliance is poorly adjusted so that it doesn't fit very well, or the force levels applied to the teeth are too high, then undesired tooth movement may occur. High force levels produced by over activation are one of the key reasons for anchorage loss.

The optimal force for movement of a single rooted tooth is about 25-40 g for tipping and about 75 g for bodily movement. If the force is too low there will be very little movement, whereas too much force may result in loss of anchorage. Excess force does not increase the rate of tooth retraction as illustrated in Fig. 10.7 As the force levels rise the rate of tooth tipping also increases up to about 40 g. Beyond this very little extra tooth movement occurs. Thus increasing the force levels above about 40 g will not increase the rate of tooth tipping.

The force levels that wires from fixed or removable appliances exert on teeth usually depends on the following:

• The material the wire is made from

• The amount it is deflected

• The thickness of the wire

Steel wire will exert a force that is directly proportional to the amount the wire is deflected up to its elastic limit. Figure 11 demonstrates how decreasing the wire thickness and increasing the length (sometimes by adding loops) controls the force produced.

Modern alloys such as super elastic nickel titanium wires do not act in the same way as steel. These remarkable wires are capable of producing

Fig. 9a, b A reverse, or protraction headgear

Rate of canine retraction (mm per month)

Fig. 10 The graph shows how increase force levels do not necessarily increase the rate of tooth movement. The y axis shows the rate of movement in mm. The x axis is the amount of tipping force applied to the tooth. As the force level initially rises the rate of tooth movement also increases. Above about 40 g the rate slows down and very little additional tooth movement occurs. There will however be a greater risk of loss of anchorage with increased force levels gm

Fig. 10 The graph shows how increase force levels do not necessarily increase the rate of tooth movement. The y axis shows the rate of movement in mm. The x axis is the amount of tipping force applied to the tooth. As the force level initially rises the rate of tooth movement also increases. Above about 40 g the rate slows down and very little additional tooth movement occurs. There will however be a greater risk of loss of anchorage with increased force levels

Fig. 11 A 0.5 mm diameter wire can be deflected more than a 0.6 mm wire without increasing the force level. Thus a greater degree of activation is possible and the appliance will require less frequent adjustments. Similarly increasing the length of the wire, for example by incorporating loops allows a greater degree of wire deflection. The force characteristics may also be changed by altering the material the wire is made from

0.6 mm

0.5 mm

0.5 mm

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•—,

•—,

Fig. 12a-c Super elastic heat activated wires produce a light continuous force almost regardless of the amount of deflection. When cooled they become very flexible (12a) but return to their original shape as they warm in the mouth (12b,c)

a continuous level of force almost independent of the amount of deflection and have transformed the use of fixed appliances in recent years. Heat activated wire is now available that will increase its force level as the temperature changes. These wires exhibit a so-called shape memory effect. If the wire is cooled and tied into the teeth it deflects easily into position. As the wire warms in the mouth it gradually returns to its original shape moving the teeth with it (Figs12a-c).

For optimal tooth movement it is important that continuous gentle forces are applied to the teeth. Fixed appliances are ideal for doing this. When removable appliances are worn, the patient should wear them full-time except for cleaning and playing contact sports. Part-time wear produces intermittent forces on the teeth and is likely to reduce the rate of movement.

When a force is applied to a tooth, there is an initial period of movement as the periodontal

Fig. 13 Tooth movement requires light continuous forces. In this graph tooth movement in mm is shown on the y-axis and time in days on the x-axis. If a force is applied to a tooth the periodontal membrane is compressed and there is a small amount of initial movement. Movement then stops as bone cells are recruited and the socket starts to be remodeled. After about 14 days sufficient recruitment and remodeling has occurred to allow the tooth to move

Lag period

Lag period

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Days membrane is compressed (Fig. 13). No tooth movement occurs for a few days after this, as cells are recruited in order to remodel the socket as well as the periodontal membrane. This cell recruitment takes a few days and is known as the lag effect. Part-time wear of appliances will not allow efficient cell recruitment and the lag phase will not be passed which may result in poor tooth movement. This is another reason why fixed appliances, which cannot be left out of the mouth by patients, are much more effective than removable appliances at achieving a satisfactory treatment outcome.

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