CAD/CAM-fabricated lingual retainers made of nitinol

Besides the benefits of lingual retainers and their prevention of undesirable tooth movements, the use of these appliances has certain disadvantages. Dentists and dental hygienists have observed that lingual retainers impair the patient’s ability to perform oral hygiene. Additional complications include impaired patient comfort, defective bonding sites and retainer fractures. These scenarios force the patient to return to the orthodontist to have the defective bonding sites repaired or a new lingual retainer fabricated.

The six-point Twistflex retainer continues to rank as the gold standard among lingual retainers (Fig. 2). It is a hand-bent, braided steel wire bonded to the four incisors and the canines. The fact that this technology has not changed for over 40 years raises the concern of whether this type of treatment is still in keeping with the times and whether these lingual retainers do justice to the precision claims prevailing in dentistry today.

Positioning accuracy
The bonding of lingual retainers in the maxillae demands special attention because it differs from the procedure in the mandible.

Limited space
Unlike in the mandible, the occlusal circumstances in the maxillae must be scrutinised. Given the often limited space, it is difficult to position lingual retainers, including their bonding sites (Fig. 12). It is imperative to avoid pre-contacts on the lingual retainer or the bonding sites. Various sources of error can arise when fabricating and inserting a hand-bent retainer in the maxillae, including no exactly defined vertical positioning of the appliance and a lack of transfer accuracy from the model to the clinical situation. Consequently, it is difficult to be certain during bonding whether the selected vertical position will result in pre-contacts.

Morphology of the maxillary incisors
The morphology of the maxillary incisors is markedly more complex than that of the mandibular incisors. Firstly, the lingual surfaces of the maxillary anterior teeth are more bulgy and bulky and, secondly, these teeth frequently exhibit individual peculiarities and sometimes extensive marginal ridges.

CAD/CAM lingual retainers can be positioned with high precision in the digital design stage. The 3-D modelling program enables accurate positioning and makes it possible to measure the distance between the lingual retainer and the opposing dentition and between the lingual retainer and the gingiva. A study published in the March issue of the Journal of Orofacial Orthopedics aimed to investigate the extent to which this computer-assisted positioning and fabrication method ensures that the position of the lingual retainer determined in the fabrication process is exactly transferred to the intra-oral situation of the patient.[1]

By means of innovative CAD/CAM technology, individualised lingual retainers (CA DIGITAL) were fabricated (Fig. 9). Following intra-oral insertion using the manufacturer’s recommended transfer system, impressions of the intra-oral situation were taken and scanned for digitisation. On this basis, the intra-oral retainer positions were compared with the preceding virtual set-ups by superimposition with the manufacturer’s datasets (Fig. 11). 3-D processing software (Geomagic Qualify 2012, Geomagic) was used to analyse the retainers, based on 80 interproximal sites, for deviations from their planned positions along the horizontal (x-), sagittal (y-) and vertical (z-) axes.

The data shows that the actual intra-oral retainer position after appliance insertion correlated clearly with the positions digitally planned by the manufacturer. Deviations from the 3-D planned bonding position were significantly less than 0.5 mm. The largest deviations were found in the z-axis (vertical plane), whereas positional changes in the x- and y-axes were minor (Fig. 8).

In conclusion, with regard to transfer to the patient, the 3-D positioning of CAD/CAM-fabricated lingual retainers appears to be highly precise and to facilitate easily planned insertion of permanent retainers even in anatomically demanding areas or where space is limited (Fig. 12).
 

Fabrication process
After digital scanning of the intra-oral situation, the fabrication process, which can be divided into four steps, follows.

Digital design
The appliance is designed using a special 3-D modelling program. Here, maximum precision is achieved (Fig. 5).

Cutting
The lingual retainer is cut from a nitinol blank using CAD. Consequently, there is no bending or plastic deformation. A kink in the wire is always associated with a change in the metal stent and creates a nominal fracture site. This should be absolutely avoided in the process.

Electropolishing
An electropolishing step follows to give the retainer a smooth finish and round off the edges (Fig. 14). This involves a method used in stenting technology. Consistent with the principle of a reverse galvanic cell, the outer coating of the appliance is removed gradually, leaving a smooth, microbe-resistant surface.

Transfer jig
The retainer can be positioned effortlessly on the physical model because MEMOTAIN (CA DIGITAL) only fits optimally in the one position defined by the program. By virtue of the key–lock principle, this position is easy to identify. Next, a transfer aid is made of silicone to facilitate exact insertion and to minimise potential sources of error.

Bonding recommendation
Both Schneider and Ruf and Scheibe and Ruf deal with the question of which factors affect the longevity of lingual retainers.[2, 3] One finding of the studies was that the operator factor appears to be critical. This technology and practitioner sensitivity also confirms our clinical experience. A meticulous working procedure and compliance with the bonding protocol appear to be important prerequisites for the long-term outcome of lingual retainers.

Several practical tips include the following:

• Particularly in the maxillae, it is important to review the available space prior to bonding the retainer. In this context, a screenshot of the situation in occlusion or careful inspection of the model in occlusion is helpful.
• Thoroughly clean the lingual surfaces with an ultrasonic jet or polishing brush or rubber cup.
• Sand-blast lingual surfaces. According to the literature, this step statistically significantly improves adhesion of the composite to the enamel.
• Optimal dryness: A sublingual parotid roll is mandatory and parotid pads vestibular to the maxillary second molars can be an additional help. Cotton-wool rolls can be inserted to help retract the lips.
• The patient should be instructed to breathe through his or her nose as much as possible. The high moisture content of respirated air is counterproductive when bonding.
• The bonding sites should be shaped to have as broad a surface area as possible in order to enlarge the bonding surface between the tooth and acrylic as much as possible.
• The etching gel should be applied with the same care as the later modelling of the acrylic. The etching gel and acrylic should be carefully applied with a fine probe and positioned in a targeted manner. The tip of an etching gel or flow cartridge is not suitable for this purpose. When modelling the acrylic, care should be taken to ensure that the margins of the bonding sites cannot be probed after hardening.
• The patient must be instructed on proper cleaning of the retainer. It is obligatory to give a demonstration of threading Super Floss (Oral-B) at the tip of the papilla from the vestibular to the lingual direction to clean the areas below the retainer. Moreover, the patient should be instructed to have his or her retainer checked regularly.

Advantages of CAD/CAM-fabricated lingual retainers
Figure 3 shows a CAD/CAM-fabricated lingual retainer made of nitinol (MEMOTAIN). This is the same patient in Figure 2 who was treated with a conventional hand-bent lingual retainer made of steel. Several differences are evident at first glance:

• The seating of the MEMOTAIN retainer on the teeth is highly precise.
• The bonding sites of the MEMOTAIN retainer were shaped much flatter and with less bulk owing to the precise accuracy of fit.
• The retainer shines. Electropolishing of the appliance makes its surface extremely smooth to prevent microbial colonisation.

Besides these obvious differences between a conventional Twistflex retainer and the CAD/CAM-fabricated MEMOTAIN retainer, advantages of the new technology include

• Accuracy of fit: The CAD/CAM retainer is digitally designed for computer-supported fabrication. The innovative CAD/CAM fabrication method from CA DIGITAL enables precision unattainable for hand-bent lingual retainers. This is particularly advantageous in the case of lingual retainers in the maxillae and atypical tooth shapes and achieves a flat, less bulky bond that is comfortable for the patient.
• Accurate positioning: The CAD/CAM retainer is digitally designed using 3-D modelling software. This production path enables maximum positioning accuracy of the CAD/CAM retainer in the individually optimised position. This is particularly advantageous when treating clinically challenging situations where space is limited in the maxillary anterior dentition.
• Easy insertion: Owing to its accuracy of fit, MEMOTAIN is seated perfectly in exactly one position. The supplied silicone transfer coping can easily be inserted into the patient’s mouth.

Nitinol
This material is distinguished by its high degree of biocompatibility along with its excellent corrosion resistance. For this reason, nitinol is typically used for surgical instruments, endoscopes or implants, such as stents.

The pseudo-elastic material minimises any restriction of the teeth’s physiological intrinsic mobility. Steel, the material conventionally used for lingual retainers, does not have these pseudo-elastic material properties. Its behaviour is more rigid than that of nitinol and provides stronger obstruction of tooth movement; this can have potentially negative effects on the periodontium in the long term. Dental practitioners know the importance of physiologically stimulating the natural deflection of the teeth in the periodontal gap to preserve alveolar bone. Indeed, the periodontium can atrophy if this important natural stimulation is lacking because the teeth are held too rigidly. The insertion of a lingual retainer permanently bonds together or blocks the teeth by means of force transmission.

With nitinol, the conflict of forces fixates the teeth in their position and prevents undesirable tooth movements, while permitting tooth movements that occur under physiological loading of the teeth and comply with the special pseudo-elastic material properties of nitinol. Furthermore, the pseudo-elasticity of nitinol makes it impossible to kink the CAD/CAM retainers.

Electropolishing
Electropolishing reduces the surface roughness of the material to a minimum. The surface of the CAD/CAM retainers deflects microbes to prevent plaque accumulation and optimises the patient’s ability to perform oral hygiene. Moreover, electropolishing enhances the corrosion resistance, thereby enabling the creation of a CAD/CAM retainer with the longest possible service life.

Maximum comfort for the patient
Owing to its maximum precision, the CAD/CAM retainer is seated tightly on the teeth. Therefore, the bonding sites can be shaped very flat, and this has a positive effect on the patient’s wearing comfort. The clinical use of MEMOTAIN over two years has yielded very promising results. The complication rate is low, and patients report an improvement in comfort and their ability to perform oral hygiene compared with hand-bent retainers.

Conclusion
Owing to its positive material properties, its superior precision and accuracy of fit, the heightened comfort for the patient, as well as its facilitated handling, the CAD/CAM-fabricated retainer is superior to the conventional Twistflex retainer. Particularly when space is critical and treatment by conventional methods would be impossible, the use of a CAD/CAM-fabricated retainer is still feasible. More information, including bonding recommendations, order forms, price lists, ordering instructions, shipping boxes and patient brochures, are available on request from CA DIGITAL at www.ca-digit.com or info@ca-digit.com.

Editorial note: A complete list of references is available from the publisher.