DENTAL INSTRUMENTATION

The History of Articulators: A Critical History of Articulators Based on Geometric Theories of Mandibular Movement: Part IEdgar N. Starcke, DDS

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INCE AS early as the 1860s, dental scientists and inventors investigated the nature of mandibular movement for the purpose of reproducing these movements in an articulator. Simple hinge articulators became commonplace, but by the turn of the 20th century, the natural variability of the condylar paths, both between individuals and from side to side in the same individual, had begun to be recognized and appreciated as important determinants of mandibular movement. Undoubtedly, the investigators interpretations of what they observed varied greatly. This is demonstrable in the features of their articulators. From the inspired to the near-genius and from the ridiculous to the sublime, these articulators simply reected what was perceived to be the anatomic and kinesthetic characteristics of mandibular movement. Despite differences in investigators perception and application of mandibular movement, the complexity of articulators began to evolve as a result of the important work of such scientists as W.E. Walker, Alfred Gysi, and George Snow. By 1910, most inventors had become more systematic in their attempts to reproduce the individual natural movements of the mandible.1

intense competition in the marketplace and dentists demands for simplicity, generated a trend toward average value instruments.1 The most noteworthy example is the Gysi Simplex articulator,2 which, incidentally, caused quite a reaction from Gysis critics when introduced in 1912 (Fig 1).3*

The Geometric (or Nonanatomic) School of Articulator DesignBy about 1900, a second major school of articulator design, the geometric, or nonanatomic, school, was emerging. This approach embodied principles contrary to the condylar school and proved to be both trend-setting and a source of controversy. The geometric school denied the existence of condylar axes and disregarded the condylar paths as inuences on occlusion, instead contending that the articulation of the teeth guides the mandible during mastication. The condylar paths need only be in accord with the plane of occlusion. Critics of the geometric school believed that this view was invalid for 2 primary reasons: (1) It did not take into

The Condylar (or Anatomic) School of Articulator DesignIn a broad sense, the school of articulator design that emphasizes condylar guidance and rotation centers can be called the condylar, or anatomic, school. During the early 20th century, articulators with adjustable condylar guides were becoming more popular; or at least so it seemed on the surface. However, undercurrents brought about byCorrespondence to: Edgar N. Starcke, DDS, Clinical Professor, Department of Prosthodontics, The University of Texas Health Science Center at Houston Dental Branch, 6516 M.D. Anderson Boulevard, P.O. Box 20068, Houston, TX 77225. E-mail: estarcke@mail.db.uth.tmc.edu Copyright 2002 by The American College of Prosthodontists 1059-941X/02/1102-0012$35.00/0 doi:10.1053/jpro.2002.124356

*Gysi was tireless in his resolve to promote his Simplex articulator, of course, with a little help from his friends. A booklet titled The Happy Average Way was published for practitioners of general dentistry in about 1912. It was endorsed by George Wood Clapp, the editor of Dental Digest, and promoted Gysis average complete denture technique, which included his Simplex articulator. By 1918, several theories of occlusion existed along with articulators designed to promote them. According to James E. House, since the principles of these theories varied so widely, it was decided that in the best interest of the profession, a study club would be created, limited to 50 men dedicated to testing their ideas on each other in a workshop setting. Their goal was to narrow the eld of articulator design to one acceptable articulator for the improvement of prosthodontics. This was one of the primary reasons that, in August 1919, the National Society of Denture Prosthetics was organized.4

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Journal of Prosthodontics, Vol 11, No 2 ( June), 2002: pp 134-146

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Figure 1. The rst and facing pages of The Happy Average Way. Probably published by the Dental Digest in about 1912, this booklet was intended to enable the general practitioner to provide efcient denture service without the need for scientic equipment. The Gysi Adaptable articulator (left) would be the ideal instrument, but the Simplex would sufce in 80% of the cases. The booklet advertised the services of the I.J. Dresch Laboratories of Toledo, OH, and the illustrations were provided by the Dental Digest, G.W. Clapp, editor. It was not copyrighted.

consideration individual variations (i.e., there was the notion that one size ts all), and (2) no provision was made for the BalkwillBennett movement. Articulators designed to reect geometric theories feature some type of mechanism that allows the mandible to move around a single central radial axis generally located above and/or posterior to the occlusal plane. Traditionally, these devices have been called arbitrary and single rotation center articulators. These terms are not adequately descriptive, however, because they are simply too vague and ambiguous. For example, to stretch a point, the simple hinge articulators might also be considered single rotation center articulators,4 and they certainly can be considered arbitrary. (Incidentally, it appears that over the years, the popularity of simple hinge devices has never waned.) The inventors most frequently associated with the geometric school of mandibular movement and articulator design are George S. Monson (for his spherical theory) and Rupert E. Hall (for his conical theory). It was earlier investigators, however, who laid the basic foundations on which the principles of the various geometric theories were built. William G.A. Bonwill and Francis H. Balkwill, who were contemporaries although oceans apart, were perhaps the earliest investigators to apply geometric principles to articulation, mandibular

movement, and the design of articulators. In 1864, Bonwill introduced his equilateral triangle theory, establishing the size of the mandible as 10 cm from condyle to condyle and from each condyle to the incisor point. Bonwill believed that articulation of the teeth guides the mandible during function, but that the centers of the condyles are also the centers of lateral rotation for the mandibles opening and closing movements.5 Balkwill presented his observations on mandibular movement in 1866. When describing the opening motion, he theorized thatthe articulating posterior outline of the condyle of the lower jaw appears formed of parts of two circles, the inner and larger forming part of an independent smaller circle. The condyle articulates with the glenoid cavity so as to allow a single hinge-like motion and a forward and backward motion. While there is only a slight lateral motion, both sides move on the radii of the same circle. The combined motion of both circles will give the [rotating] side nearly a simple lateral action, while the [orbiting] side will move forward and downward.6

In 1890, anatomist Ferdinand Graf von Spee of Kiel, Germany (Fig 2) called attention to the relationship between the curved arrangements of the occlusal planes of natural teeth and the corresponding curves of the condylar paths.7 As reported by Gysi, von Spee described the forward movement of the mandible (as viewed in the sagittal plane) in this manner:

136along the upper part of the skull are lying on the same cylindrical surface. The location of the axis of that cylinders curvature is at the level of the horizontal mid-orbital plane. The steeper the path of the condyles, the more pronounced the tooth curve would be, because both have the same radius.8

This was later to be known as the curve of Spee.

The Spherical Theory: Should the Credit Go to Christensen or Monson?There was never a raging controversy over who originated the spherical theory. On the contrary, most authors have traditionally awarded that distinction to George Monson. However, there were some early discussions on this issue, and even into the late 1940s, there was some question as to who actually originated the spherical theory of mandibular movement.9 Rupert Halls historical review of the work of various investigators on mandibular movement led him to believe that Carl Christensen had developed

Figure 2. Ferdinand Graf von Spee (18551937). (Reprinted by permission of ADA Publishing, a division of ADA Business Enterprises, Inc. Copyright 1980, American Dental Association.) The total visible contact of the molar masticatory surfaces lies on the same arc of a circle. The posterior continuation of this arc touches the most anterior point of the condyle. Accordingly, the points of the mandible that glide in contact

Figure 3. Sagittal view of the mandible. The concentric arcs demonstrate the nature of the protrusive movement of the mandible. The short black line represents the joint path. Christensen believed that the path of the condyle never differs much from a straight line. (Reprinted from Christensen.11)

Figure 4. A lateral view of the skull with a schematic drawing of dentures in centric occlusion and in protrusion. This illustrates the intraoral method for recording the condylar inclination, or Christensens phenomenon. Christensens Rational articulator is based on this principle. (Reprinted from Christensen.11)

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Figure 5. Christensens Rational articulator with plaster casts and wax occlusion rims mounted in the centric position. The plaster blocks, mounted for the simulated functional generated path procedure, would look similar to this. (Reprinted from Christensen.11)

the spherical theory.10 Christensens claim to fame, of course, was his practical technique for registering positional relations of the mandible. He was the rst to describe an intraoral method for recording a static protrusive record to determine the condylar inclination, and he produced an adjustable condylar guide articulator, the Rational articulator, to promote this technique.11,12 From his description of the technique came what Ulf Posselt coined Christensens phenomenon, or the posterior separation of the occlusion rims that occurs when the mandible moves from a centric to a protrusive position.13 In the late 1890s, Christensen discovered what was, until then, the largely unknown work of von Spee on the displacement path of the jaw.7 He believed that Spee should be credited with pointing out the importantand simpletruth that the path of the condyle during the bite movement must be in conformity with the bite-path.12 Christensen developed his method of recording the condylar inclinations for his Rational articulator as an extension of Spees principle, that is, harmonizing the articulation of the teeth with the movements of the condyles.14 Christensen was well aware that in Spees view, the nature of the temporomandibular joint during movement was of more a mechanical than an anatomic character and that his observations may not hold true in all cases. He pointed out

Figure 6. (A) Christensens Rational articulator with the condylar guides set at a high inclination. The maxillary and mandibular plaster blocks have been manually ground in and the surfaces have obtained spherical shapes. (Reprinted from Christensen.12) (B) Vulcanite rubber stints with wax occlusion rims on casts of badly worn natural teeth. The spherical contours of the rims were formed as a result of the subject moving his mandible freely and as far as capable while maintaining contact of the rims with moderate pressure. (Reprinted from Christensen.12)

that Spee himself admitted that there seemed to be a discrepancy between his hypothesis and the accepted conception of anatomic conditions. But Christensen proposed that during movement of the mandible in individuals with natural teeth, while the teeth remain in sliding contact, the condyles can only move downward and forward 4 to 5 mm, with a maximum distance of 12 mm. Therefore, he believed that the small distance and direction that the condyles traveled while the teeth remained in contact was of utmost importance for dentures to function properly. Christensen believed that, as von Spee indicated, if the articulation-path and the joint-path were similar, then whether the articulation-path is straight or curved, the joint-path must be parallel to it (Fig 3).11 In this gure, both paths are shown to conform to concentric arcs with a common center. Christensen considered the condylar path curves to have innite radii and, for all practical purposes for setting denture teeth, to be a straight line. His articulator was based on this principle (Fig 4).11

138 a laboratory demonstration, used his Rational articulator to manually simulate functionally generated path occluding surfaces on maxillary and mandibular rims. To simulate occlusion rims, Christensen mounted plaster blocks in his articulator (Fig 5). He then set the condylar guides at an especially high oblique position. Maintaining rm hand pressure on both bows of the articulator and using the guiding mechanism of the instrument, he functionally articulated the blocks to grind them in to balancing surfaces in all directions of the moving bite. The worn surfaces now showed perfect contact through all movements and obtained the shape of spherical surfaces, the mandibular surface concave upward and the maxillary surface convex downward (Fig 6A).12 Christensen claimed to conrm this indirect proof by another experiment that he carried out with a living subject, a man whose natural teeth were severely abraded. The subjects plane of occlusion was slightly curved but was not smooth. Christensen constructed vulcanite rubber stints to cover the teeth, and over the stints he placed wax occlusion rims of a few millimeters thickness. After lubricating the rims with soap, the subject was asked to move his mandible in all possible directions, holding the rims together with moderate pressure. Although not as dramatic, the outcome was the samethe occlusal surfaces of the wax rims obtained a spherical shape (Fig 6B and C).12

Christensens Spherical TheoryCarl Christensen pursued Spees ideas further but adopted different concepts of the nature of mandibular movement. By the early 1900s, he had studied the work of other investigators and had made his own observations on mandibular movement and occlusal wear patterns of natural teeth. Christensens new spherical hypothesis was based on the conclusions that he had reached regarding the factors that determine the nature of the occlusal plane and the relationship between the occlusal plane, tooth articulation, and condylar paths. Preferring to use the root word bite rather than the terms articulation and occlusion, Christensen claimed that the only way to prove that his theories were correct was to observe the bite-movement (articulation) phenomenon itself. But, he explained, it must be remembered that the minute details of [these movements]. . .in the living individual. . .are still a closed book to us, and. . .are hardly suitable as the real basis for [debate].12 Christensen held that it is the ideal jaw-path during bite movement of the edentulous mouth (related to the construction of complete dentures) that should be determined, not the accidental, more or less normal bite-path of the mouth with natural teeth.12 Christensen did not fully understand the nature of the lateral movements of the mandible, but he concluded that the mandible must make lateral movements similar to the forward movements and that only a spherical surface arrangement of the occlusal plane would allow continuous tooth contact during all excursions of the mandible. These spherical surfaces differ for each individual, ranging from an almost-plane surface with an innite radius to a highly curved surface with a radius of 4 to 5 inches. Christensen offered 2 of his several practical experiments to conrm that the principles of his spherical theory were correct. The rst experiment,

A Frank-ly Discouraging WordIn 1908, Bernard Frank of Amsterdam, took aim at von Spee and Christensen, harshly criticizing their work on mandibular movement and admonishing any inventors who had claimed that their so-called anatomic articulators could imitate the joint mechanism.15 Frank conducted experiments that he believed produced conclusive evidence that Spees ndings were inaccurate. He said that von Spee had stated emphatically that the sagittal occlusion curve of man has a radius of 6 to 7 cm, and claimed that his own experiments showed that this was the case in only 27% of the measurements.15 Frank also contended that Christensen did not prove the validity of his Rational articulator. Using cross-sections of dentulous mandibular casts, Frank demonstrated that there were vast differences among individuals in the curvatures of the occlusal planes. Moreover, by cutting cross-sections of each cast at the positions of the premolars and

In his 1905 paper, Christensen chose to avoid the use of the terms articulation and occlusion, but instead, chose the word bite as a general term meaning all the forms of contact in which both rows of teeth may meet. He went into detail dening his bite-related terms and his arguments for preferring their use; but his basic reason was simply because experience has taught me that neither articulation nor occlusion [are understood by] the great majority of dentists when a thorough explanation of the subject is attempted.11

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Figure 7. Cross-sections of mandibular dentulous casts of different individuals demonstrating how Frank calculated the differences between the lateral occlusal plane curvature variations (as viewed in the frontal plane.) Lines were drawn touching the highest points of the respective pairs of teeth. Points a and b identify the midpoint of the occlusal surfaces. The lines intersect at point c. Frank identied points a, b,and c as the inter-occlusal surface angle. At points a and b, perpendicular lines were drawn that intersected at point d, representing the common center of rotation of each pair of teeth. Frank noted that each tooth had a circle of occlusal contact, 1 with radius r and 1 with radius r . None of the radii constructed for the occlusal circles of each tooth pair ever appeared to be equal. (Reprinted from Turner.14)

molars, he showed that the radius of each of the 5 pairs of teeth would be different (Fig 7). Using Christensens Rational articulator, Frank repeated his simulated functionally generated path experiment using blocks made of a pumicestone mixture (Fig 8). The curved occlusal surfaces generated on Franks blocks were remarkably more complicated than the spherical surface reported by Christensen. Furthermore, Frank suggested that it was evident that the directions of the natural masticating surfaces differ so greatly from those obtained by repeating the experiment of Christensen that this ex-

periment entirely fails to prove the correctness of the [Christensen] articulator.15 Bernard Franks rhetoric was that of a man with a mission: to let the world know that it is utterly impossible to solve the problems of articulation by means of articulators. In the milieu of this early20th century dentist, it is doubtful that he found many colleagues to argue with that statement. Indeed, there are those today who would wholeheartedly agree with him. Clearly, Frank expressed some legitimate concerns. He understood the concepts of the facebow,

Figure 8. Cross-sections of the mandibular casts of occlusal rims that Frank generated by repeating Christensens simulated functionally generated path experiment. Frank made 5 transverse sections at the proper positions of the posterior teeth. He noted 10 different sloping surfaces, 5 for each side, and pointed out numerous discrepancies between Christensens ndings and his. (Reprinted with permission.15)

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Figure 11. Dr. Monson making measurements on a human mandible to demonstrate that from the 4-inch common center, the divider touches the incisal edges and the condyles. (Reprinted from Washburn.17)

Figure 9. George S. Monson, DDS (1869 1933). (Reprinted by permission of ADA Publishing, a division of ADA Business Enterprises, Inc.16 Copyright 1933, American Dental Association.)

the third point of reference, and the variability of the intercondylar distance. Christensen had not addressed these issues in his work. On the other hand, Franks choice of analysis to challenge Chris-

tensens theories could be described as comparing apples and oranges. Even though the spherical theory implies multidirectional movement, Christensen primarily studied the movement of the mandible in the anteroposterior direction (as observed in the sagittal plane) after the work of Spee, whereas Franks observations were in the frontal plane. Christensen also made it quite clear that the

Figure 10. George Monson demonstrated his spherical theory for the rst time on this Bonwill articulator. The casts were mounted in the articulator according to Bonwills equilateral triangle and with the spherical occlusion guide. (Reprinted from Washburn.17)

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Figure 12. L.A. Weinbergs schematic illustration of the 3-dimensional relationships of the components of Monsons theory. Lines projected from the apices (A, B, and C) of Bonwills triangle intersect at point D, forming a spherical pyramid. Monsons 8-inch diameter sphere touches the apices of the triangle, and point D is the center of rotation or radius of the sphere. Weinberg pointed out that a relationship between Bonwills triangle and Balkwills angle. Monsons theory requires a condylar inclination of close to 35 degrees and a Balkwill angle of 15.5 degrees. These angles do not correspond to those average angles found by Gysi (30-degree condylar inclination) and by Balkwill (26-degree Balkwill angle) (Reprinted with permission.22)

ideal occlusal curve would be considered only for the edentulous mouth in the context of constructing complete dentures.12 So what did Frank conclude from his own experiments with curves of occlusion and from his observations of the known articulators of his day? What he said was this: An anatomical articulator is good for nothing. Life cannot be imitated. It would seem then, that we must give up forever any idea of being able to construct a mechanical joint articulator that will enable us to construct a physiologically articulating denture for each individual case.15 Clearly, he was ahead of his time.

Monsons Spherical Theory and ArticulatorConducting experiments on mandibular movement during the same period as Carl Christensen was George Monson, of St. Paul, MN (Fig 9).16 H.B. Washburn (also of St. Paul, MN), writing on the history of occlusal concepts, reported that Monson had conceived the spherical theory. Washburn also considered

it signicant that Christensen and Monson, so close in ideas, knew nothing of each others work.17 Washburn reported that in 1898, speaking to a group at Mankato, MN, Monson presented for the rst time a method for setting denture teeth, using Bonwills equilateral triangle conforming to the surface of a sphere. Monson had been a student and close friend of Bonwill for many years, but the time came when he could no longer strictly follow all of Bonwills teachings. Nevertheless, this rst demonstration of his spherical theory was performed with a Bonwill articulator, and the casts were mounted according to Bonwills instructions. However, the teeth were set to conform to a wire spherical occlusal guide constructed by Monson (Fig 10).17 Through further studies, Monson concluded that prenatally, mandibles ideally tend to develop as equilateral triangles and, if the various interfering factors can be controlled during development, that the teeth also would conform to a sphere.18 To verify this hypothesis, Monson conducted experiments with both a human mandible and with casts of the mandibular dentition of highly developed individuals. By highly

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Figure 13. A frontal view of the mandible illustrating the relationship of the 8-inchdiameter sphere with the transverse plane of occlusion that Monson claimed must be the same as the anteroposterior plane for balanced occlusion to be achieved. The radial lines of force of 4-inch length converge forming the radial point at the apex from which the radius of occlusion of each tooth is determined. (Reprinted from Monson.20)

developed, he meant a person with an ideal mandible and dentition that had not been disturbed at some point by disease, trauma, or developmental anomaly. Monson afxed a metal rod to the center of the

occlusal surface of each posterior tooth, projecting the rod upward and parallel to the long axis of the tooth. These rods represented the radial lines of force of the teeth. When all of the rods were in place, Monson

Figure 14. A posterior view of the mandible, illustrating the application of the radial lines to the condyles. The center of the condyles are shown conforming to the surface of the sphere, giving the same radial dimensions from the centers of the condyles to the apex as from the occlusal surfaces of the teeth. This also illustrates Monsons concept that this radial center is the center for the entire muscular action because the angles of the mandible conform to lines centering at apex A. (Reprinted from Monson.20)

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Figure 15. Monsons Mandibulo-Maxillary instrument. Point A is the radial center of the instrument from which the occlusal surfaces of the teeth are determined. Point B is the position of the condylar hinge mechanism for the instrument. The teeth are arranged to conform to the 8-inch sphere at C. Slot D controls anteroposterior movement. The slot is concentric with the outer surface of the sphere. Jackscrews E are used to adjust the position of the lower cast to the center if required. This instrument was manufactured by M.F. Patterson Supply Co., St Paul, MN. (Reprinted from Campbell.21)

found that they intersected at a common point or center. On the human mandible (Fig 11), he discovered that when measuring from this common center, a dividing caliper not only touched the incisal edges of the anterior teeth and the buccal and lingual cusps of the posterior teeth, but also bisected both of the condyles. This, then, was the origin of Monsons spherical theory. It was based on the concept that the mandibular teeth move over the occlusal surfaces of the

maxillary teeth, as over the external surface of a segment of an 8-inch sphere, and that the radius (or common center) of the sphere is located in the region of the crista galli. Because of the way in which the mandible develops, Monson further believed that it would be logical to adapt Bonwills 4-inch equilateral triangle to the surface of the 8-inch sphere, because geometrically, such a spherical-based triangle would also be a segment of the 8-inch sphere, and the apex of a pyramid erected on

Figure 16. A sagittal view demonstrating the relationships of the 8-inch diameter sphere to Monsons articulator and to the anteroposterior plane of occlusion. (Reprinted from Monson.20)

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Figure 17. A schematic drawing illustrating the theoretical mechanics of transferring wax rims from the patient to the instrument. (Reprinted from Washburn.17)

the triangular base would be coincident with the common center of rotation, that is, the 4-inch radius of the sphere (Figs 12, 13, and 14). Initially, the spherical theory involved the articulation of natural teeth in the highly developed individual and the conviction that these principles apply to the edentulous mandible with highly developed ridges.18 Monson soon realized, however (and was quick to point out), that most patients encountered are not highly developed, because at some point in life an unbalanced condition replaced an earlier balance as a result of some disturbing inuence. In these individuals, the radius of the sphere may be greater or smaller than 4 inches and may not always be in the same location. Thus Monson provided a mechanism in his instrument and in the method for mounting casts whereby the relationship of the patients occlusal plane and condyles to the patients center is the same on the articulator as in the patient.17 In 1923, Monson was issued a patent for his articulator.19 The Mandibulo-Maxillary Instrument, as Monson named it, was based on

his spherical theory (Figs 15 and 16). The instrument had 2 rotational axes, spherical and condylar. The condylar axis feature was, of course, one of convenience but was also designed for a facebow transfer method used for the unbalanced [oral] conditions encountered in most patients. Both Washburn17 and R.G. Keyworth23 described their methods for using Monsons articulator in complete denture construction; both versions included a similar facebow transfer technique (Figs 17 and 18). In summarizing the principles of Monsons instrument, Washburn stated that it incorporated Monsons spherical principle and combined the Bonwill triangle with Walker and Gysis condyle movements. In addition, the instrument included Gysis idea that the forward and lateral movements must be combined and that the plane of occlusion conforms to the curve of Spee.17

Returning to the Original QuestionSo, who should receive credit for the spherical theory, Carl Christensen or George Monson? The answer may never be denitely known, because the exact date when and by whom the spherical idea was conceived may be too close to call. Is this answer important? Probably not. Because they were working independently at about the same time, either one of these men could have actually been the rst. In any event, it is George Monson who should and probably will be remembered for promulgating the spherical theory and for his conviction that its principles were sound.

James House states that Monson had applied for the articulator patent in 1918 and had presented and defended his spherical principles and his Mandibulo-Maxillary Instrument surprisingly well before his peers at the annual session of the National Society of Denture Prosthetists about 2 years later. Monson was very much in the center of the spirited dental controversy [over the various theories of mandibular movement and articulator design] because his idea of a single rotation center was an easy target.4

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Figure 18. (A) After a centric relation record is made, the rims are fastened together and transferred to the instrument with a facebow. (B) After the occlusion rims are related to the condylar axis with the facebow, the lower cast is adjusted by placing one end of the open calipers in the radial center of the articulator and touching the free end of the calipers to the incisor point on the lower wax rim. (C) A caliper is used to project the spherical curve to the occlusal surface of the mandibular wax rim as a guide for setting the teeth. (Reprinted by permission of ADA Publishing, a division of ADA Business Enterprises, Inc.23 Copyright 1929, American Dental Association.)

146 Carl Christensen was a practical clinician who devised a useful intraoral procedure to record the individual condylar paths for the purpose of setting the adjustable condyle controls of his articulator. Christensen was curious about the nature of mandibular movement and, through his experiments, recognized the spherical curvature of the occlusal plane and its relationship with the curvature of the condylar paths. However, Christensen believed that because of the innite radius of the sphere, for all practical purposes, the condyle paths would be a straight line. He did not promote his spherical theory, but he will always be associated with his method for making a protrusive intraoral record and for Christensens phenomenon. George Monson, on the other hand, believed that his spherical principles produced the ideal occlusion in the highest-developed type of individual and accordingly, the best-balanced articial dentures must conform to a spherical base.20 Monsons articulator and technique based on his spherical theory attracted a number of devoted followers. Even today, many of his principles persist as a part of the dental landscape. More on the history of articulators based on geometric theories of occlusion will appear in the next issue of The Journal of Prosthodontics.

5. 6.

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8. 9. 10.

11. 12. 13. 14.

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16. 17. 18.

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United States from 18401970. Masters thesis, Indiana School of Dentistry, Indianapolis, IN, 1970, pp 119-127 Bonwill WGA: Articulation and articulators. Trans Am Dent Assoc 1864; July 26:76-79 Balkwill FH: The best form and arrangement of articial teeth for mastication. Trans Odont Soc Great Britain 1866; 5:133-158 von Spee FG: Die Verschiebrangsbahn des unterkiefers am schadell. Arch Anat Physiol 1890;16:285-294. English translation, Niedenbach MA, Holtz M, Hitchcock HP: The gliding path of the mandible along the skull. J Am Dent Assoc 1980;100:670-675 Gysi A: The problem of articulation (Part II). Dent Cosmos 1910;52:148-169 Lufkin AW (ed): A History of Dentistry (ed 2). Philadelphia. PA, Lea and Febiger, 1948, p 292 Hall RE: An analysis of the work and ideas of investigators and authors of relations and movements of the mandible. J Amer Dent Assoc 1929;16:1642-1693 Christensen C: A rational articulator. Ashs Q Circular 1901;18:409-420 Christensen C: The problem of the bite. Dent Cosmos 1905;47:1184-1195 Posselt U (ed): Physiology of Occlusion and Rehabilitation. Philadelphia, PA, Davis, 1962, pp 42-43 Turner CR (ed): The American Textbook of Prosthetic Dentistry (ed 3). Philadelphia, PA, Lea Brothers, 1907, p 414 Frank B: An investigation on articulation and experiments with C. Christensens articulator. Brit Dent J 1908;29:289295 Cruttenden LM: Obituary of George S. Monson. J Am Dent Assoc 1933;20:1285-1287 Washburn HB: History and evolution of the study of occlusion. Dent Cosmos 1925;67:331-342 Washburn HB: The application of the Monson spherical principle to full dentures. J Am Dent Assoc 1927;14:648-654 Monson GB: Dental Articulator. US Patent No. 1,457,385. June 5, 1923 Monson GB: Occlusion as applied to crown and bridge-work. J Nat Dent Assoc 1920;7:339-413 Campbell DD (ed): Full Denture Prosthesis. St. Louis, MO, Mosby, 1924, p 355 Weinberg LA: An evaluation of basic articulators and their concepts. Part II: Arbitrary, positional, semiadjustable articulators. J Prosthet Dent 1963;13:645-663 Keyworth RG: Monson technic for full denture construction. J Am Dent Assoc 1929; 16:130-162