Patient-specific anatomical articulator

Currently available dental articulators have limitations for reproducing human mandibular movements. The present article describes an innovative device that is a true patient-specific anatomical articulator that accurately simulates patient anatomy and eliminates all technique-sensitive mounting procedures, thus substantially diminishing potential errors in mounting and articulator settings. By using three-dimensional (3D) printing technology, patient cone beam computed tomography (CBCT) data are used to print 3D replicas of the condylar fossae, as well as the actual condyles, at the correct intercondylar distance.

The maxilla (maxillary teeth and edentulous ridge) is printed with the correct spatial relationship to the condylar complexes and the Frankfort horizontal plane (FHP). Those printed structures are then premounted onto a modified articulator frame to render it “anatomic.” This new custom anatomical articulator, which accurately mimics patient anatomical movements rather than relying on average values, represents the first truly fully adjustable articulator that is more precise than can be generated by a pantographic tracing. It saves money, time, and effort by eliminating earbow transfers and mounting errors in complex prosthodontic treatment.

Simulating the human jaw movements to accurately fabricate complex prosthodontic restorations is challenging. More than 2 centuries ago, Gariot invented the first mechanical articulator in 1805, which became to be known as the plane hinge or plane line articulator, and, in 1840, Evans presented a modified version to simulate mandibular excursive movements.¹ In 1858, William Bonwill invented his articulator, which was essentially driven by the laws of mechanics as he envisioned them being related to mandibular movements. The term anatomical articulator was first mentioned by Bonwill in 1899 while attempting to defend his work and silence his critics, proving that his articulator reproduced the relationship of the glenoid fossa contours and the guidance of teeth.² Interestingly, he acknowledged that the shapes of the fossae are never the same angle on either side. He questioned the existence of any art or mechanical rules that would regulate the beginning and ending jaw movements.² The Bonwill articulator was certainly far from being anatomical.

Since that time, myriads of articulator devices have been produced and marketed to serve the dental profession. These instruments have been constantly modified over the years based on the increasing knowledge of the dynamics of human mandibular movements and the stomatognathic system as influenced by the elevator and depressor muscles, as well as the anatomy of the temporomandibular joints (TMJs) and associated structures such as the articular discs, joint ligaments, and bony articular eminences.^3,4 Multiple classification systems for these devices have been published, ranging from simple to fully adjustable types. These were based on criteria such as descriptive purposes, designs, concepts, and capability and on the types of intraoral records that the device accepts to allow the setting of its controls.5, 6, 7, 8, 9, 10, 11, 12, 13 Currently, 4 articulator classes are acknowledged by the American College of Prosthodontists (ACP): Class I articulators are simple holding instruments that may allow vertical movement; class II articulators allow horizontal and vertical movements but do not orient the motion to the TMJs; class III articulators (semi-adjustable) allow for cast orientation relative to the joints and are able to simulate condylar pathways by using mechanical averages for the movements; class IV articulators (fully adjustable) are additionally capable of accepting 3D dynamic registration within obvious limits and hence come close to simulating mandibular movements as far as is “mechanically” possible.¹⁴

For most oral rehabilitations, accurate mounting of patient casts onto a semi-adjustable articulator is considered a standard practice that entails several procedures. An earbow transfer with an arbitrary hinge axis location using a third point of reference is necessary to correctly capture the relationship of the maxillary teeth to the transverse hinge axis (THA), as well as to the Frankfort horizontal plane (FHP). This enables mounting of the maxillary cast onto the upper member of the articulator in the same spatial relationship as the patient.15, 16, 17, 18 The upper member of the articulator represents the FHP when set parallel to the floor/bench top (with incisal pin set at 0). The earbow transfer device orients the maxillary cast in space relative to the THA and the FHP. A centric relation (CR) record is then required to mount the mandibular cast to the already mounted maxillary cast. Furthermore, protrusive and lateral records are required to program the articulator to replicate function, expressed in protrusive and lateral jaw movements.

This entire mounting process is technique sensitive, must be accomplished as accurately as possible, and is still challenging. Teteruck and Lundeen¹⁹ compared mounting maxillary casts by using the traditional facebow and the classic 13-mm tragus-canthus point (arbitrary hinge axis) to the kinematic (true) hinge axis. It was found that 33% of the arbitrary axis points fell within a 6-mm radius of the kinematic axis. However, with the use of an earbow, this percentage increased to 56.4% and to 75.5% accuracy with a simple anterior modification of the ear plug.¹⁹ Others have investigated the resultant mismatch between the arbitrary and kinematic axes on the accuracy of the mounting process. They all agreed that an error within a ±5-mm radius between the 2 axes yielded clinically acceptable results that necessitated minor occlusal adjustments.^15,16,20, 21, 22, 23, 24, 25, 26, 27 Nevertheless, it has been reported that the earbow transfer is not statistically repeatable²⁸ and that since different articulator systems use different anterior third points of reference for their earbow transfer, this may result in variation of the superior inferior position of the mounted casts on the articulators, rendering the reliability of these anterior reference points questionable.²⁹

For that reason, the complex analog pantograph was praised as a device that would accurately simulate the patient’s border movements and transfer them onto a fully adjustable articulator via its 6 tracings.30, 31, 32 Later, the electronic pantograph was similarly reported to record the condylar determinants with acceptable range. The investigators highlighted the influence of correctly recording mandibular movements on the resultant occlusal morphology of posterior teeth, expressed in cusp angles and groove direction as a direct effect of variation in condylar determinants.33, 34, 35, 36, 37 One particular determinant of condylar movement, the immediate mandibular lateral translation (IMLT) of the condyles, has been the subject of considerable debate and confusion in the prosthodontic literature.38, 39, 40 However, a recent systematic review of the literature reported a lack of evidence for the clinical significance or implications of this movement.⁴¹

In general, when articulators are used to fabricate dental prostheses, average articulator values are usually used by practitioners and dental laboratory technicians. It is thus logical to expect that the resultant inevitable prosthetic errors need to be corrected or adjusted intraorally, consuming precious chairside time.

The purpose of this article was to introduce to the profession a true patient-specific anatomical articulator (patent pending) for accurately replicating in 3D the patient anatomy in its correct relationship and orientation. With this innovative device, all the aforementioned procedures for accurately mounting the patient casts are eliminated, thus substantially diminishing potential errors not only in mounting and articulator settings but, most importantly, also in dental prosthetic fabrication.