A deep curve of Spee is often associated with severe anterior deep bite. By extruding posterior teeth, mainly premolars, and intruding anterior teeth, it is possible to flatten the arches and achieve an ideal overbite.¹

It is difficult to define the net contribution of molar and premolar extrusion versus canine and incisor intrusion to the overall curve of Spee flattening, as they act as a reciprocal source of anchorage. Whenever attempting to extrude the premolars, canines and incisors will serve as an anchorage unit, and they will pay the price of a most welcome intrusion side effect. On the contrary, every time clinicians would love to achieve intrusion of the anterior teeth, the premolars represent the primary source of anchorage, and they may extrude a beneficial side effect of anterior intrusion. Even if, by using clear aligners and an attentive planification of tooth movements, clinicians may be persuaded that they can achieve specific tooth movements (i.e., intrusion of the anteriors only/extrusion of the posteriors only), they should be aware that Newton’s third law of physics (action and reaction) plays an important role in distinguishing the real world from the virtual on-screen world of setup, where the laws of physics are often violated.

It is a common belief that deep bite correction and curve of Spee flattening is easier to achieve in growing patients, as extrusion of molars and premolars can be supported by vertical growth while grow is still happening.⁸

On the contrary, curve of Spee correction in adults may be much harder, as the orthodontist cannot hope in any influence or help from the vertical skeletal dimension. Furthermore, curve of Spee tends to deepen with aging,⁹ with supererupted lower incisors and canines that may also show lingual inclination (upper incisors can also show lingual inclination as a consequence). This becomes clinically evident in a two-step mandibular occlusal plane with a net step between the first premolars and canines. Excessive wearing of the incisal edges may also be evident in such circumstances. While planning deep bite correction in an adult, the orthodontist should also plan any eventual restorative treatment that is needed to reestablish the proper crown anatomy.

Align Technology has created a proprietary protocol for deep bite correction called Invisalign G5. This protocol involves incisor and canine intrusion through a combination of intrusion forces exerted by the aligners on the occlusal edge of the teeth and a pressure area on the lingual surface (Figs. 10.1 and 10.2). This combination of force systems exerts a final intrusive force that is supposed to be parallel to the tooth long axis. To achieve the desired intrusion on the anterior teeth, an adequate anchorage should be provided in the premolar and molar area. G5 retention attachments have been specifically designed for premolars, and they may serve as pure anchorage attachments or as active extrusion attachments in case of extrusion of the premolars. Both movement of anterior intrusion and posterior extrusion are automatically activated if the threshold of movement is more than 0.5 mm. Molar anchorage should be provided with conventional attachments (rectangular and horizontal) to counteract the occlusal movement of the aligner determined by the anterior intrusion design.

Clinicians working with other clear aligner systems than Invisalign, or those who feel the need for alternative approaches even when using Invisalign aligners, may create a similar protocol using standard attachments and a personalized staging of intrusion.

Gingival beveled attachments may be used as an alternative to G5 retention attachments on premolars to achieve retention and extrusion. When planning extrusion, it is useful to ask for a slower extrusion rate (e.g., 0.15 mm per stage instead of the classic 0.20 mm) to avoid lack of tracking within the aligner by respecting the physiologic tolerance of the periodontal ligament.

Some clinicians recommend a superiorly convex (reverse) curve of Spee as final objective of the alternative. While this is not the real clinical goal, the assumption behind this prescription is that the elasticity and resilience of the plastic material will very unlikely allow a full expression of the prescribed movement. By the way the lack of expression of certain movements can be compensated by this requested hypercorrection, that is the aligner equivalent of the reverse curve NiTi wires.¹⁰ The clinician who has the feeling that the hypercorrection is really happening may always stop the use of the aligners to avoid unwanted side effects.

Curve of Spee correction should always begin with lower incisor proclination to obtain a relative intrusion and start to recover the space required during the real intrusion movement. Since the expression of the lingual root torque information on lower incisors has not yet been investigated, it can be useful to prescribe extra lingual root torque. Again, it is important to remember that interproximal spacing may help the intrusion movements.

A paper by Liu and Hu¹¹ explained how force changes as a consequence of different intrusion strategies for deep bite correction with clear aligners. With the same activation (0.2 mm of intrusion) and rectangular attachments placed on the premolars and first molars, the canines experienced the largest intrusive force when intruded alone. When applying contemporary intrusion of canines and incisors, the canines received a larger intrusive force than incisors. The incisors received similar forces of intrusion if intruded alone or together with canines. First premolars experienced the largest extrusive forces when all anterior teeth were intruded. Extrusion forces were exerted also on canines and lateral incisors when differential staging for intrusion of canines and incisors was used. It is not surprising that the intrusive force exerted by clear aligners is higher when less elements are involved, and it is partially lost when multiple elements are intruded at the same time. The incisors show an overall scarce tendency to feel intrusion forces. This may lead to the clinical suggestion of a staggered approach, alternating canine and incisor intrusion to exert higher and more specific forces on canines and incisors.

Therefore, a clinical suggestion in prescribing anterior intrusion with any clear aligner system could consider the following:

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