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How should we level the curve of Spee with aligners? 

Spee's Curve: A Cruciate Dental Anatomy

Spee's curve is the natural curvature of the occlusal plane that follows the anatomical shape of the mandibular arch. A Spee curve If it is excessively enlarged, this can lead to various problems such as excessive wear of the front teeth, periodontal problems or joint disorders.

The latter aspect is frequently found in cases where the Spee curve The enlarged mandible produces an anterior contact during closure that forces the patient to retroposition the mandible, impacting the condyle into the joint.

Impact of Spee's Curve on Orthodontics

If we focus on the mechanics of orthodontic treatment, a Spee curve augmented can affect the correction of different problems. Although the Spee curve is categorised as a vertical plane problem, it will also affect the sagittal problem if not fully corrected. This is often seen in Class II cases with an increased overbite.

When we do not correct the overbite, that is, when we do not level the lower curve of Spee, anterior contacts will appear that will block the mesialisation of the lower arch and will not allow us to finish the case in Class I.

Spee curve

Case where inadequate levelling of the lower Spee curve has caused contacts between the upper and lower canine, blocking upper distalisation and Class II correction.

Treatment Mechanics and Correction Strategies

If we look back to when we used to work with brackets, when we were working with the Spee curve was not corrected, the contacts between the brackets of the lower incisors and the palatal faces of the upper incisors also blocked this sagittal correction. 

Overcorrection Considerations and Strategies

There is no appliance in orthodontics that has a predictability of 100%. For this reason, when we want to level the Spee curve we make overcorrections.

In traditional orthodontics, as the archwires are also unable to flatten the Spee curve In order to increase the intrusion force in the anterior region and to level the lower arch properly, for example, bends were made between premolars and canines, for example, to increase the intrusion force in the anterior region. When working with aligners, we will do something similar.

We will apply mechanics in the virtual planning that will allow us to counteract the inability of the aligner to achieve the movements at 100%. To do this, we will increase the intrusion of the anterior teeth and the extrusion of the posterior sectors, premolars and first molars.

This will give us a different final position in ClinCheck or Approver than what we actually want to achieve, but, just as the reverse curve arc is not our real final target, what appears in a programme will not be either.

Treatment Monitoring and Final Advice

Fortunately, we are confident that these overcorrections, to a greater or lesser degree, work. Studies comparing the efficacy of braces and Invisalign for intrusive or leveling the Spee curve have found that both tools achieve relatively similar results.

Knowing this, we can expect that, applying the same strategy with aligners as we did with brackets, we will obtain similar results.

Photo of the spee curve

Measurement of Spee's Curve.

And now we ask ourselves the question: how many millimetres of overcorrection do we do? There is no perfect answer. If we base it on the average predictability percentage of intrusion, we can overcorrect by an estimated 40 or 50 % more than the necessary intrusion, but we cannot think of it as a perfect overcorrection either.

The averages are made up of peaks and valleys, i.e. within that 40 % of predictability, there are patients where the predictability will have been, for example, 60 % and in other patients, 20%. 

As we do not know the real predictability of the planned intrusion for each of our patients (there are many factors that affect it), it is advisable to always do more intrusion than necessary.

Do not be afraid to exaggerate overcorrections, as you can stop at any time during the treatment. It is not necessary to reach the last aligner of the phase. 

If, for example, we have a first stage of 40 aligners, but we observe that in stage 30 we have 40 aligners. the Spee curve is already level, we can stop the treatment there, avoiding giving the patient more aligners that could over-intrude the lower incisors and canines. Just as in some patients we will have the reverse curve arch active for more months than in others, we will try to do something similar with the aligners.

We are the ones who control the treatment.

Final advice: In orthodontic treatment, unless it is an open bite, it is advisable to finish with the curve of Spee flat. As the years go by the Spee curve tends to increaseThis will help us to prevent future problems and to finish the cases in a more stable way.

Invitation to the Presential Course

Interested in learning more about how to level the spee curve and its application in orthodontics? Join our classroom course! "SAS meeting in Madrid on the 18th and 19th of October and deepen your knowledge with experts in the field! Book your place now on our landing page and be part of this unique hands-on training opportunity - don't miss the chance to take your orthodontic practice to the next level!

Goh S et al. The predictability of the mandibular curve of Spee leveling with the Invisalign appliance. Am J Orthod Dentofacial Orthop 2022;162:193-200. 
Rozzi M et al. Leveling the curve of Spee: Comparison between continuous archwire treatment and Invisalign system: A retrospective study. Am J Orthod Dentofacial Orthop 2022;162:645-55. 

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