Question:

I am trying to get a better understanding of the biomechanics occurring with my clinchecks. I have attached the clincheck for the batch the the patient just completed. The patient presents with much less buccal crown torque than planned across the Mx incisors and I have poor posterior occlusion.
My questions are:

1. I realize that I should have placed rectangular horizontal attachments on the 11/21. But in addition, how much overcorrection should we plan for Mx incisor torque? I continue to have poor expression of this on all my cases.

2. Is the reason for the poor occlusion in the posterior because buccal root torque was not expressed and so the crowns tipped due to pushing forces from the lingual? If so, should I change out the attachments on premolars and molars for thicker rectangles?

3. To correct this do I simply add Buccal root torque or should I also add some buccal crown tip of the lower molars.

Answer:

Biomechanics and Torque in Orthodontic Treatment

In this conversation, a discussion of biomechanics focuses on torque, specifically concerning a patient with significantly less buccal crown torque than planned across some maxillary incisors, resulting in poor posterior occlusion.

Maxillary Incisor Torque Correction

The challenge of torque expression using aligners is a central issue. Unlike brackets and wires, which have built-in torque, achieving torque with aligners can be more difficult. In cases with severe Class 2 Division 2 malocclusions, additional auxiliaries are often required to create more palatal root torque.

Contrary to the initial belief that attachments on the central incisors were necessary for torque expression, it is clarified that attachments are not essential. The problem lies in the need for over-treatment of torque. In the initial treatment plan, the torque was under-corrected. A suggestion was made to increase torque by approximately 30% in future treatment plans. However, the application of excessive torque could cause the trays to stop fitting properly. It is recommended to apply torque up to a comfortable limit while considering the risk of ill-fitting trays.

Wagon Wheel Effect

An interesting phenomenon called the "wagon wheel effect" occurs when excessive torque is applied. While the incisal edges may appear corrected, the roots of the teeth can move too far mesially, resulting in a need for distal root tip adjustment. This side effect must be addressed to prevent unfavorable tooth movement.

Torque and Intrusion

To improve torque expression, a critical tip is introduced: for every degree of torque applied (e.g., 26 degrees for centrals), 0.1 mm of intrusion should be incorporated into the treatment plan. This advanced concept helps ensure better engagement of the aligners, allowing the necessary forces to be applied more effectively.

Aligners must tightly grip the incisal edge while applying force, preventing the aligner from slipping off the tooth. The incorporation of intrusion during torque adjustments provides better force distribution and improves the chances of torque expression. Even though the intrusion might not fully express, it assists in maintaining torque control.

Attachments and Positioning

Attachments are essential to create sufficient forces for torque correction. The conversation highlights the importance of ensuring attachments are positioned correctly. Attachments should not be beveled toward the gingiva but should be beveled toward the occlusal surface for better torque generation. This positioning provides the necessary two pushing forces: one from the gingival side and the other from the palatal cusp. Proper attachment placement, along with the cleaning of flash, ensures effective force distribution to create the desired buccal root torque.

Posterior Occlusion and Buccal Root Torque

Regarding the poor occlusion in the posterior region, it is suggested that the buckle root torque in the molars and premolars was not adequately expressed. This is likely due to improper attachment design and placement. Rectangular attachments on these teeth, beveled towards the occlusal, would have helped generate the correct forces needed for buccal root torque.

It is also noted that cortical plate limitations could restrict tooth movement, and care must be taken not to exceed these anatomical boundaries. However, the conversation concludes that, in this specific case, more expansion could be achieved based on the patient's bone structure.

Buccal Root Torque vs. Buccal Crown Tip

To correct the poor occlusion, the practitioner should add more buccal root torque rather than buccal crown tip. Adding lingual crown tip (instead of buccal crown tip) and increasing the Curve of Wilson is often necessary when trying to establish proper occlusion. This adjustment ensures a better cusp-fossa relationship, particularly in situations where over-expansion of the lower teeth has resulted in heavy palatal cusp contacts.

The discussion emphasizes the complexity of biomechanics in orthodontics, particularly in cases where aligners are used. Compliance and detailed planning, including proper attachment placement, over-treatment of torque, and consideration of biological limits, are critical to achieving successful outcomes.