INTRODUCTION

A number of clear aligner systems are on the market and a popular one in use today, which will be the focus of this chapter, is the Invisalign system. Although the concept of producing tooth movement using a removable tooth positioning device instead of conventional fixed appliances has been around since at least 1945,¹ it was not until Align Technology computerized the process in the late 1990s that it became practical to use the method for anything beyond minor tooth movement.² In order to accurately produce complex tooth movement with removable appliances, it is necessary to manufacture a series of patient casts with the teeth reset, each progressively incorporating a small amount of tooth movement until the teeth have been placed into their ideal position.³ A series of tooth positioners, or aligners, are then fabricated from those casts.

PROCESS

As with any case requiring orthodontic treatment, the first step in the Invisalign process is to obtain diagnostic records. However, unlike treatment with conventional fixed appliances, impressions must be taken with poly-vinyl siloxane (PVS) due to its superior accuracy and stability.⁴ The patient’s impressions, PVS bite registration, photographs (both intraoral and extraoral) and radiographs are submitted to Align Technology in addition to the completed treatment form so that the fabrication process can begin. The PVS impressions and bite registration are scanned by computed tomography (CT) in order to create accurate three-dimensional digital models, registered in maximum intercuspation. The models are digitally processed by a technician using a software program that recognizes and removes artefacts. Technicians digitally separate the teeth and add gingiva around them.

A trained Invisalign computer technician then moves the teeth to their final positions in a series of stages according to the doctor’s prescription. This preliminary plan is submitted to the doctor for approval in the form of a computerised movie that is downloaded from Invisalign’s website and viewed via their ClinCheck software. Using the software, the doctor can communicate any additional instructions to the Align Technology technicians until they are satisfied with the progression of tooth movement and the final results.

Once the sequence and amount of tooth movement per stage are approved by the doctor, a series of casts are created using stereolithography, a three-dimensional printing process that makes a solid object from a computer image by using a computer-controlled laser to draw the shape of the object onto the surface of liquid plastic. The complete set of clear plastic aligners are made from these casts and sent directly to the doctor.⁵ Each aligner is prescribed to be worn for 2 weeks and is only removed for eating, drinking, brushing and flossing.

INDICATIONS

Invisalign was developed when surveys showed that there was a huge potential adult market with unmet needs. Typically, adult orthodontic patients are apprehensive about the lack of aesthetics, difficult oral hygiene practices, pain and discomfort associated with conventional fixed appliances.⁶ Some patients present with relapse from previous orthodontic treatment and do not want to undergo the same treatment again. The Invisalign appliance offers a removable, aesthetic alternative to patients with these concerns. Studies have demonstrated that patients treated with Invisalign have improved periodontal health⁷ and experience less pain⁸ than those treated with conventional fixed appliances. Composite attachments bonded to specific teeth may be required in order to accomplish certain tooth movements.

Invisalign has been used successfully to treat a variety of malocclusions, albeit some more successfully than others. In order to improve case selection for the expertise level of the practitioner, Align Technology has developed software that will evaluate the difficulty level of a case and will not allow a practitioner to treat beyond their capabilities without the proper experience and training. Specific clinical parameters are identified as being appropriate to be treated by practitioners with limited case experience (Table 34.1). Auxiliary appliances and advanced orthodontic techniques may be necessary if a case exceeds the beginner-level parameters listed.

Table 34.1 Case complexity based on tooth movement and malocclusion type