The null hypothesis had to be rejected, because the results of the present study showed that CAD-FR3s differed from Con-FR3s regarding Fmax(FR3) and stiffness. The CAD-FR33×3 had the highest mean Fmax(FR3) and stiffness values, both of which were almost five times higher than those for the Con-FR3. However, the mean Fmax(FR3) and stiffness of the CAD-FR35×2 were only slightly lower. As a result, and because its palatal connector has a flatter design that potentially provides greater wearing comfort, the CAD-FR35×2 might be particularly suitable for clinical use. In contrast, the mean Fmax(FR3) and stiffness values for the CAD-FR34×1 were significantly (p ≤ 0.007) lower, and two individual cases had the lowest Fmax(FR3) of all FR3s tested. One CAD-FR34×1 even broke during cyclic stress testing. This design therefore appears to be clinically unusable.
Moreover, the results of the pre-tests suggest that it might be possible to integrate slender CAD-Ls into the final CAD-FR3 design. Low stiffness values combined with a sufficient Fmax(L) were found for the CAD-L2 design. Thus, this design might enable slight pre-activation of the labial arch to be included in the digital construction process, as is recommended for the Con-L24.
This is the first study to describe a CAD-FR3. So far, CAD/CAM technology has mainly been used for aligners6, 7 and fixed orthodontic appliances10, 11; it has very rarely been described in relation to functional orthopaedic appliances. This might primarily be because it is difficult to fuse the metal elements and plastic base of functional appliances by means of CAD/CAM technology. Nevertheless, Al Mortadi et al. demonstrated the CAD/CAM production of an Andresen activator21. However, their study included no biomechanical stress tests. It therefore remains unclear whether the activator would be sufficiently stable for clinical use. The concept proposed by Al Mortadi et al. seems interesting, but its realisation remains complicated. For example, individual guiding jigs, which are required to construct the metal elements of the activator must be constructed and printed in advance. Moreover, in order to insert the metal elements, which must be bent and adapted to the guiding jigs, the printing of the activator base must be paused. Therefore, the method described by Al Mortadi et al. does not permit fully automated manufacture of the activator.
Another CAD/CAM concept, for the production of an active plate, was presented in a proof-of-concept study by van der Meer et al.22. In this study, metal parts were not produced manually, but by a bending robot. This fully digital planning and manufacturing approach could be very promising. Nevertheless, the difficulties posed by fusing metal and plastic elements again means that it remains challenging to implement this concept in everyday clinical practice. Although the wire elements are bent completely digitally, they must be bonded manually to the base plate by a technician after printing.
Both studies explore interesting concepts regarding how functional orthodontic appliances might be manufactured in the future by using CAD/CAM techniques. However, both concepts use a manufacturing process that seems rather time-consuming and complicated and therefore needs further simplification before it can be integrated into clinical routine. Nevertheless, both concepts could have potential for the CAD/CAM realisation of orthodontic appliances that necessarily require activatable metal elements.
All CAD-FR3s in this study were constructed without any metal elements. In contrast to most functional orthodontic appliances, however, a metal-free design might be feasible for the FR3. This is because all its metal components, except for the protrusion spring, only provide stability and must not be activated23. Instead, the focus is on an optimum fit, in order to achieve the best possible therapeutic effect in terms of inhibited mandibular translation or dorsally directed growth redirection and uninhibited maxillary post-development28. It is therefore important that the maxillary and mandibular fit of the appliance remain unchanged throughout treatment. Regarding its maxillary effect in particular, the FR3 is only efficient if its palatal connector can withstand buccal forces to create enough functional space and ensure sufficient periosteal traction. In this regard, the CAD-FR3 might be superior to the Con-FR3: 28 of 30 CAD-FR3s had higher transversal stiffness values than the Con-FR3, which might mean that the CAD-FR3 can achieve more constant periosteal traction and therefore a greater maxillary effect. Conversely, this higher transversal stiffness led to a higher probability of breakage in the CAD-FR3s, whereas forces exceeding Fmax(FR3) in the Con-FR3s led only to plastic deformation. Nonetheless, the Fmax(FR3) that caused the CAD-FR3s to break were three times higher than those that caused the Con-FR3s to deform. Intraoral breakage of the CAD-FR3 is therefore rather unlikely. Extraoral breakage, in contrast, might uncover when the CAD-FR3 is non-functional, whereas the plastic deformation of a Con-FR3 could initially go unnoticed. This seems particularly important, because both the maxillary and the mandibular part must fit precisely to achieve the optimum therapeutic effect. Conversely, higher transversal stiffness might complicate the insertion of the FR3. However, the aim of this in-vitro study was solely to determine whether further clinical testing of the CAD-FR3 would be justified. Consequently, the effect of increasing transversal stiffness on clinical applicability and therapeutic effect can only be answered by further clinical trials.
The CAD-FR3 does not contain a protrusion spring. In most cases, however, the protrusion spring is only required in the initial stage of FR3 therapy to transfer the frontal crossbite. Normal frontal bite relation should be achieved as quickly as possible, so that the blocking of the FR3 in the molar region can be reduced to 1.5 mm23. This should reduce lip closure obstruction and ensure the wearing comfort of the appliance. Therefore, the short-term use of other appliances such as active plates before CAD-FR3 therapy might be conceivable, because they apply force to the incisors in a faster and more targeted way.
The Con-FR3 is a very complicated appliance to produce. Consequently, its manufacture requires specially trained technicians. These factors have prevented the Con-FR3 from being used more widely. This study sought to determine whether CAD/CAM-manufactured FR3s might be a viable alternative to Con-FR3s. If so, this could provide dentists throughout the world (including those without access to a trained technician) with another means of producing FR3s, and thus facilitate the wider use of this appliance. Many dentists could benefit from a simple chairside production of the FR3. To make this a reality, however, the production process requires further simplification. But for the CAD-FR3 in particular this seems feasible. Only the design process of the concept presented requires simplification; the manufacturing process can already be performed completely automatically. In the future, it might be possible to automatically digitally project the FR3 onto previously digitised models, thus enabling fast and easy chairside production of the CAD-FR3. The possibility of digitally archiving and processing both the model and construction data might also offer further advantages. Model archiving would require no physical storage space. Moreover, the digital models could be used to analyse therapeutic effects. For example, digitally matching patient models of different therapy stages or projecting models into lateral cephalograms or three-dimensional facial photographs, as already used for diagnostics and surgical planning29, 30, might enable more precise planning of the FR3. Saving the construction data could provide further advantages. Adjustments could be quickly and easily implemented in the saved construction data in order to print a new design, an option that would not be possible for a Con-FR3. Furthermore, should the appliance be lost or damaged, the dentist could re-print the archived design and insert it after about 1.5 h’ printing time. As a result, the patient would be without their appliance for a shorter amount of time. This appears to be very important, because complication rates for inflexible removable appliances are up to 25%31, 32. Faster reintegration might also have a positive effect on therapy, because the time taken for a conventional repair negatively affects therapy time and therapeutic effect by 16% and 9%, respectively33, 34.
Several limitations should be considered when interpreting the results of this study. First, biomechanical cyclic load force and direction can vary from one patient to another. However, load force for cyclic stress testing was first determined by means of a spring balance when inserting Con-FR3s on several patients in everyday clinical practice, and then set particularly high at 15 N. Second, because an in-vitro setting can only partly represent the clinical situation, it is impossible to draw specific conclusions concerning the in-vivo implications of our results.
Nonetheless, this study shows for the first time that FR3s can be constructed in CAD/CAM technology and that their technical properties can be superior to those of Con-FR3s if certain design parameters are followed. These results justify further clinical investigations. Further studies are now required to determine whether CAD-FR3s are also comparable to Con-FR3s regarding patient acceptance and therapeutic effect and can therefore be used in everyday clinical practice.