Orthodontic Anchorage: Maximum, Moderate and Minimum Types

Anchorage in orthodontics is the resistance to unwanted tooth movement. Every force that moves a tooth produces an equal and opposite reaction on the teeth used as support, the anchor teeth. If that reaction is not planned, the molars drift forward, the extraction space is consumed, and the incisors do not end where the plan said they would. This guide follows the reasoning taught in our D.U. course materials: quantify the anchorage need from the space analysis, estimate the anchorage the patient naturally has, including the "anchorage value" tables, then choose the simplest reinforcement that will do the job, from transpalatal arches to headgear and temporary anchorage devices.

Anchorage at a glance

Reinforcement methods, by increasing strength in the course materials: conventional intraoral means, then extraoral, then combined, then skeletal anchorage.

Anchorage is a reaction

When an archwire is engaged, the sum of the forces and the sum of the moments are zero (Newton’s third law). Two units face each other:

• the anchor unit, for example the molar block, which should move as little as possible;
• the active unit, for example the canine, which you want to move.

After first premolar extractions, the elastic chain that retracts the canine also pulls the molar forward: every millimeter the molar gains is lost for crowding relief and incisor retraction.

De Nevrézé’s rule, taught in our courses, describes the ideal: the force should exceed the resistance of the active unit without reaching that of the anchor unit. If the force is too strong, both units move: that is anchorage loss. In practice these resistances are not known precisely, which leads to the course’s first rule: it is always easier to lose anchorage than to keep it. The need is defined before the mechanics start.

Step 1: quantify the anchorage need

The anchorage need comes from the space analysis: what has to be gained anteriorly is compared with the space the extractions will free.

With the Steiner box

The Steiner box (in French) gives the total space need in the lower arch: crowding, incisor repositioning, curve of Spee, Class II mechanics. It is compared with the extraction space, about 15 mm for two lower premolars:

Two anchorage-loss items enter the balance and should not be confused: Class II elastics, counted at about 2 mm per side, and space closure in edgewise mechanics without precautions, estimated by Steiner at about 2.5 mm per side.

With Tweed-Merrifield total space

The Tweed-Merrifield total space analysis (in French) divides the lower arch into zones. For anchorage, the sum of the anterior and middle deficits is compared with the extraction space: equal, the lower first molars are held (maximum anchorage); a residual deficit, distalize or hold them with a compromise on incisor position; an excess, the molars can come forward. The courses add zone guidance after first premolar extractions: an anterior deficit of 5 to 7 mm allows anchorage loss, one of 7 to 15 mm calls for maximum anchorage.

Example: 8 mm of anterior plus 4 mm of middle deficit makes 12 mm to gain, against about 15 mm freed by two lower premolars; the remaining 3 mm can close by molar mesialization, so maximum anchorage is not needed. Our free space analysis tool helps with the crowding figure that feeds this calculation.

Which premolars to extract

The courses link anchorage and extraction choice: first premolars when the discrepancy is anterior and anchorage must be maximum or moderate; second premolars when the discrepancy is in the middle zone and anchorage loss is accepted or wanted. Profile, tooth condition and occlusal class weigh in too; see our French article on extraction criteria.

Classifications of anchorage

The courses use several overlapping vocabularies:

• Maximum, moderate, minimum, defined by the fate of the extraction space (table above).
• Burstone’s groups: A, the molars must not come forward; B, molars and incisors close the space reciprocally; C, the molars come forward with no incisor retraction.
• Tweed’s degrees of anchorage preparation: moderate (ANB 0° to 4°, discrepancy under 10 mm), maximum (mild Class II, discrepancy 10 to 20 mm), critical (severe Class II, discrepancy over 20 mm). The course text leaves a gap between 4° and 4.5° of ANB; our knowledge base assigns those values to the second degree.

Keep three verbs apart: preserve anchorage (do not load the anchor teeth), reinforce it (add auxiliaries), prepare it (tip-backs on the lower posterior teeth, in the Tweed technique).

Step 2: estimate natural anchorage

Anchorage value and root surface tables

The resistance of a tooth depends first on its roots: the larger the root surface, the lower the pressure per unit area for the same force, and the higher the anchorage value (Freeman, as reported in our course materials). This is the origin of the anchorage value tables.

Values as given in our D.U. course materials. The course writes "cm²" for Freeman’s figures, but the values are in mm².

"Mandibular first molar anchorage value 475"

That number, a common exam question, matches the root surface area of the mandibular first molar, 475 mm², in the Freeman table of our course materials. It is a surface area, not a dimensionless coefficient. We could not verify the original publication, and other tables use other values, so treat it as a teaching figure rather than a measured constant. Two points make the table easier to use correctly:

• Surface is not everything. The upper first molar has more root surface than the lower one (533 against 475 mm²), yet a lower Jarabak coefficient (9 against 10), consistent with the denser mandibular bone.
• Never mix two scales in one calculation. Jarabak’s coefficients, the Freeman surfaces and the Nabbout-Faure volumes (also taught in the courses) do not rank the teeth the same way. Our knowledge base uses Jarabak’s scale by default.

A worked example from the course, on Jarabak’s scale: the block of first molar plus second premolar is worth 15 to 16, against 7 to 8 for the canine, so without precautions the canine moves only about twice as much as the block. Adding the second molar (23 to 25) pushes the ratio above 3.5. Hence the advice to include the second molars in the anchor unit.

These tables assume full bone support. In an adult with a reduced periodontium, posterior teeth that have lost bone may anchor no better than the incisors; the remaining support is assessed tooth by tooth on periapical radiographs (Canal).

Other factors

• Arch: for the same force in the same patient, the displacement is about three times greater in the maxilla, less dense than a mandible with thick cortices (Château). The upper arch more often needs reinforcement.
• Vertical pattern: in high-angle patients, muscular anchorage is weak, the molars drift forward easily and vertical control is difficult; in low-angle patients natural anchorage resists better. Our mandibular plane angle article gives the norms.
• Type of movement: tipping, extrusion, intrusion and rotation cost little anchorage; bodily movement and torque cost a lot. Retracting the incisors bodily is the most demanding phase for the molars.

Step 3: reinforce anchorage

The transpalatal arch

The TPA holds intermolar width, helps vertical control and is used to derotate and torque molars. Its sagittal effect is doubtful, and the literature agrees with the courses. A systematic review of 14 studies found only one that compared the TPA with no anchorage device, without a significant difference in molar anchorage loss; TADs controlled anchorage better than the TPA by about 2.09 mm, and the authors concluded that the TPA alone should not be relied on for maximum anchorage during anterior retraction (Diar-Bakirly et al., 2016, Angle Orthod). Before any activation, a TPA must be passive in all three planes, or it creates unwanted movements itself.

Extraoral anchorage: headgear

The vertical effect of headgear depends on the direction of pull. High-pull headgear has an intrusive component and is the option our knowledge base keeps for high-angle patients with an open bite. Cervical headgear extrudes the molars, opens the bite and rotates the mandible clockwise, which Boileau advises against in high-angle patients; it also aggravates posterior crowding. And without regular wear there is no anchorage. Headgear used only for anchorage is not the same dose as orthopedic headgear in growing patients, and the sources do not give the same values for each.

Skeletal anchorage: TADs

Mini screws take their support from bone, so they move teeth without loading an anchor unit or depending on compliance. Across 15 randomized trials (561 patients), surgical anchorage reinforced anchorage by about 1.68 mm more than conventional anchorage, with moderate-quality evidence and substantial heterogeneity; mini screws were described as particularly promising, and no difference in overall treatment time was found (Jambi et al., 2014, Cochrane Database Syst Rev). In a retrospective comparison of 51 maximum-anchorage cases, a TPA supported by two midpalatal mini screws allowed 0.85 mm of upper molar mesial movement against 3.63 mm with high-pull headgear, TPA and elastics, and the molars were intruded rather than extruded (Lee et al., 2013, AJODO).

"Absolute anchorage" is still a misleading phrase: a unilateral pull tips the arch and the occlusal plane, a vertical component flares the teeth, and screws fail. The course rule for adults is to avoid a mini screw when conventional mechanics do as well. Indications, sites, failure rates and age are detailed in our guide to TADs in orthodontics.

Anchorage loss: the common errors

Anchorage loss is the unwanted mesial movement of the anchor teeth. The errors listed in the courses:

1. Defining anchorage after the fact instead of quantifying it in the treatment plan.
2. Using heavy forces: a heavy force does not speed up movement. For canine retraction, beyond 300 to 500 g it is the anchorage that moves; the course gives 150 to 200 g for this movement.
3. Forgetting the reaction: a molar tip-back intrudes and proclines the incisors and extrudes the molar.
4. Using a nickel-titanium wire as anchorage support on a very displaced tooth: vertical anchorage loss, lateral open bite.
5. Closing spaces or running Class II elastics on undersized wires, or before the curve of Spee is leveled.
6. Applying extrusive mechanics to high-angle patients: Tweed anchorage preparation, Class II elastics, cervical headgear.
7. Counting on the TPA to hold the molars sagittally, or on anchorage tables in an adult with a reduced periodontium.

When anchorage loss is wanted, it is managed too: a mesial force alone tips the molar, while an active tip-back (10° in our knowledge base; the course text also gives 5°) makes it translate. The biology behind force levels is covered in our French article on tooth movement and orthodontic forces.

In practice

1. Complete the space analysis: crowding, incisor repositioning, curve of Spee, Class II items.
2. Compare the need with the extraction space: equal, maximum anchorage; smaller, anchorage loss possible; greater, maximum anchorage and revised incisor goals.
3. Choose the premolars accordingly: first premolars for maximum or moderate anchorage, second premolars when molar mesialization is accepted.
4. Estimate natural anchorage on one scale only: second molars present, arch, vertical pattern, periodontium, age, compliance.
5. Plan anchorage phase by phase, anteriorly and posteriorly, in all three planes.
6. Choose the simplest reinforcement that suffices; keep mini screws for cases where conventional means fall short.
7. For each activation, write down the expected reaction and how you will control it.

Frequently asked questions

What is anchorage in orthodontics?

It is the resistance to unwanted tooth movement. Because every orthodontic force has an equal and opposite reaction, the teeth used as support, the anchor teeth, tend to move too. Anchorage planning decides how much of that movement is acceptable and how to limit it.

What is an anchor tooth?

An anchor tooth is a tooth used as support to move other teeth, usually a molar, often tied to the second premolar and second molar to form an anchor unit. Its resistance depends on its root surface, the bone around it, the vertical pattern and the type of movement asked of the teeth being moved.

What does "anchorage value 475" mean for the lower first molar?

In the Freeman root surface table of our course materials, the mandibular first molar has a root surface area of 475 mm². Anchorage value tables rank teeth by root surface or by coefficients derived from it. Use one table consistently, and remember that the values assume full bone support.

Is a transpalatal arch enough to hold the molars?

Not sagittally. It controls width, rotation and vertical position well, but a systematic review found no proof that it prevents molar mesialization, and recommended against relying on it alone for maximum anchorage (Diar-Bakirly et al., 2016). It is combined with other means, such as second molars in the anchor unit, headgear, directional forces or mini screws.

With GS Ortho

In GS Ortho, the diagnostic studio takes each case through 12 panels, so the anchorage need is reasoned from the clinical examination, photographs, cephalometrics and the space analysis of the same patient before any device is chosen. AI places 29 landmarks on the cephalogram for you to check and correct, and the Steiner and Tweed analyses follow. If a case makes you hesitate, "Request an opinion" sends the question, with the panel attached, to a colleague or supervisor you choose; the answer stays in the record. The choice of anchorage stays yours. The GS Ortho interface is currently available in French.

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