he history of articulators

8/12/2019 he History of Articulators

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DENTAL INSTRUMENTATION

The History of Articulators: A CriticalHistory of Articulators Based on Geometric

Theories of Mandibular Movement: Part I Edgar N. Starcke, DDS

SINCE AS early as the 1860s, dental scientistsand inventors investigated the nature of man-dibular movement for the purpose of reproducingthese movements in an articulator . Simple hingearticulators became commonplace, but by the turnof the 20 th century, the natural variability of thecondylar paths, both between individuals andfrom side to side in the same individual, had be-gun to be recognized and appreciated as impor-tant determinants of mandibular movement. Un-doubtedly, the investigators interpretations of what they observed varied greatly. This is demon-strable in the features of their articulators. Fromthe inspired to the near-genius and from the ridic-ulous to the sublime, these articulators simply reected what was perceived to be the anatomicand kinesthetic characteristics of mandibularmovement. Despite differences in investigatorsperception and application of mandibular move-ment, thecomplexity of articulators began to evolveas 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 systematicin their attempts to reproduce the individual natu-ral movements of the mandible. 1

The Condylar (or Anatomic) Schoolof Articulator Design

In a broad sense, the school of articulator designthat emphasizes condylar guidance and rotationcenters can be called the condylar, or anatomic,school. During the early 20 th century, articulators with adjustable condylar guides were becomingmore popular; or at least so it seemed on thesurface. However, undercurrents brought about by

intense competition in the marketplace and den-tists demands for simplicity, generated a trendtoward average value instruments. 1 The mostnoteworthy example is the Gysi Simplex articula-tor, 2 which, incidentally, caused quite a reactionfrom Gysis critics when introduced in 1912(Fig 1). 3*

The Geometric (or Nonanatomic)School of Articulator Design

By about 1900, a second major school of articulatordesign, the geometric, or nonanatomic, school, was emerging. This approach embodied principlescontrary to the condylar school and proved to beboth trend-setting and a source of controversy.

The geometric school denied the existence of con-dylar axes and disregarded the condylar paths asinuences on occlusion, instead contending that thearticulation of the teeth guides the mandible dur-ing mastication. The condylar paths need only be inaccord with the plane of occlusion. Critics of thegeometric school believed that this view was invalidfor 2 primary reasons: (1) It did not take into

Correspondence to: Edgar N. Starcke, DDS, Clinical Professor, De- partment 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: [email protected]

Copyright 2002 by The American College of Prosthodontists1059-941X/02/1102-0012$35.00/0 doi:10.1053/jpro.2002.124356

*Gysi was tireless in his resolve to promote his Sim-plex articulator, of course, with a little help from hisfriends. A booklet titled The Happy Average Way waspublished for practitioners of general dentistry in about1912. It was endorsed by George Wood Clapp, the editorof Dental Digest, and promoted Gysis average completedenture technique, which included his Simplex articula-tor.

By 1918, several theories of occlusion existed along with articulatorsdesigned to promote them.According to James E. House, since the principles of these theories varied so widely, it was decided that in the best interestof the profession, a study club would be created, limitedto 50 men dedicated to testing their ideas on each otherin a workshop setting. Their goal was to narrow the eldof articulator design to one acceptable articulator for theimprovement of prosthodontics. This was one of theprimary reasons that, in August 1919, the National Soci-ety of Denture Prosthetics was organized. 4

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consideration individual variations (i.e., there wasthe notion that one size ts all), and (2) noprovision was made for the BalkwillBennett move-ment.

Articulators designed to reect geometric theo-ries feature some type of mechanism that allowsthe mandible to move around a single centralradial axis generally located above and/or posteriorto the occlusal plane. Traditionally, these devices

have been called arbitrary and single rotationcenter articulators. These terms are not ade-quately descriptive, however, because they are sim-ply too vague and ambiguous. For example, tostretch a point, the simple hinge articulators mightalso be considered single rotation center articu-lators, 4 and they certainly can be considered arbi-trary. (Incidentally, it appears that over the years,the popularity of simple hinge devices has never waned.)

The inventors most frequently associated withthe geometric school of mandibular movement andarticulator design are George S. Monson (for his

spherical theory) and Rupert E. Hall (for hisconical theory). It was earlier investigators, how-ever, who laid the basic foundations on which theprinciples of the various geometric theories werebuilt.

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 the-ory, establishing the size of the mandible as 10 cmfrom condyle to condyle and from each condyle tothe incisor point. Bonwill believed that articulationof the teeth guides the mandible during function,but that the centers of the condyles are also thecenters of lateral rotation for the mandibles open-ing and closing movements. 5

Balkwill presented his observations on mandib-ular movement in 1866. When describing the open-ing motion, he theorized that

the 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 therelationship between the curved arrangements of the occlusal planes of natural teeth and the corre-sponding curves of the condylar paths. 7 As re-ported by Gysi, von Spee described the forwardmovement of the mandible (as viewed in the sagit-tal plane) in this manner:

Figure 1. The rst and fac-ing pages of The Happy Aver- age Way. Probably publishedby the Dental Digest in about1912, this booklet was in-tended to enable the generalpractitioner to provide ef-cient denture service with-out the need for scienticequipment. The Gysi Adapt-able articulator (left) wouldbe the ideal instrument, butthe Simplex would sufce in80% of the cases. The bookletadvertised the services of theI.J. Dresch Laboratories of Toledo, OH, and the illustra-tions were provided by the Dental Digest, G.W. Clapp, ed-itor. It was not copyrighted.

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The total visible contact of the molar masticatory surfaceslies 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

along the upper part of the skull are lying on the same cylindrical surface. The location of the axis of that cylin- ders 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 theCredit Go to Christensen or

Monson?There was never a raging controversy over whooriginated the spherical theory. On the contrary,most authors have traditionally awarded that dis-tinction to George Monson. However, there weresome early discussions on this issue, and even into

the late 1940s, there was some question as to whoactually originated the spherical theory of mandib-ular movement. 9

Rupert Halls historical review of the work of various investigators on mandibular movement ledhim to believe that Carl Christensen had developed

Figure 2. Ferdinand Graf von Spee (18551937). (Re-printed by permission of ADA Publishing, a division of ADA Business Enterprises, Inc. Copyright 1980, Amer-ican Dental Association.)

Figure 3. Sagittal view of the mandible. The concentricarcs demonstrate the nature of the protrusive movementof the mandible. The short black line represents thejoint path. Christensen believed that the path of thecondyle never differs much from a straight line. (Re-printed from Christensen. 11 )

Figure 4. A lateral view of the skull with a schematicdrawing of dentures in centric occlusion and in protru-sion. This illustrates the intraoral method for recordingthe condylar inclination, or Christensens phenomenon.Christensens Rational articulator is based on this prin-ciple. (Reprinted from Christensen. 11 )

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the spherical theory. 10 Christensens claim to fame,of course, was his practical technique for register-ing positional relations of the mandible. He was therst to describe an intraoral method for recording astatic protrusive record to determine the condylarinclination, and he produced an adjustable condylarguide articulator, the Rational articulator, to pro-

mote this technique.11,12

From his description of thetechnique came what Ulf Posselt coined Chris-tensens phenomenon, or the posterior separationof the occlusion rims that occurs when the mandi-ble moves from a centric to a protrusive position. 13In the late 1890s, Christensen discovered what was,until then, the largely unknown work of von Speeon the displacement path of the jaw. 7 He believedthat Spee should be credited with pointing out theimportantand simpletruth that the path of thecondyle during the bite movement must be inconformity with the bite-path. 12 Christensen developed his method of recording the condylar incli-

nations for his Rational articulator as an exten-sion of Spees principle, that is, harmonizing thearticulation of the teeth with the movements of thecondyles. 14

Christensen was well aware that in Spees view, the nature of the temporomandibular jointduring movement was of more a mechanical thanan anatomic character and that his observationsmay not hold true in all cases. He pointed out

that Spee himself admitted that there seemed to

be a discrepancy between his hypothesis and theaccepted conception of anatomic conditions. ButChristensen proposed that during movement of the mandible in individuals with natural teeth, while the teeth remain in sliding contact, thecondyles can only move downward and forward 4to 5 mm, with a maximum distance of 12 mm.Therefore, he believed that the small distanceand direction that the condyles traveled while theteeth remained in contact was of utmost impor-tance for dentures to function properly. Chris-tensen believed that, as von Spee indicated, if thearticulation-path and the joint-path were

similar, then whether the articulation-path isstraight or curved, the joint-path must be par-allel to it (Fig 3). 11 In this gure, both paths areshown to conform to concentric arcs with a com-mon center. Christensen considered the condy-lar path curves to have innite radii and, forall practical purposes for setting denture teeth,to be a straight line. His articulator was based onthis principle (Fig 4). 11

Figure 5. Christensens Rational articulator with plastercasts and wax occlusion rims mounted in the centricposition. The plaster blocks, mounted for the simulatedfunctional generated path procedure, would look similarto this. (Reprinted from Christensen. 11 )

Figure 6. ( A) Christensens Rational articulator withthe condylar guides set at a high inclination. The maxil-lary and mandibular plaster blocks have been manually ground in and the surfaces have obtained sphericalshapes. (Reprinted from Christensen. 12 ) ( B) Vulcaniterubber 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 man-dible freely and as far as capable while maintainingcontact of the rims with moderate pressure. (Reprintedfrom Christensen. 12 )

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Christensens Spherical TheoryCarl Christensen pursued Spees ideas further butadopted different concepts of the nature of man-dibular movement. By the early 1900s, he had stud-

ied the work of other investigators and had madehis own observations on mandibular movement andocclusal wear patterns of natural teeth.

Christensens new spherical hypothesis was basedon the conclusions that he had reached regarding thefactors that determine the nature of the occlusalplane andtherelationship between theocclusal plane,tooth articulation, and condylar paths. Preferring touse the root word bite rather than the terms artic-ulation and occlusion, Christensen claimed thatthe 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 move-ments]. . .in the living individual. . .are still a closedbook to us, and. . .are hardly suitable as the real basisfor [debate]. 12 Christensen held that it is the idealjaw-path during bite movement of the edentulousmouth (related to the construction of complete den-tures) that should be determined, not the accidental,more or less normal bite-path of the mouth withnatural teeth. 12

Christensen did not fully understand the natureof the lateral movements of the mandible, but heconcluded that the mandible must make lateralmovements similar to the forward movements andthat only a spherical surface arrangement of theocclusal plane would allow continuous tooth contactduring all excursions of the mandible. These spher-ical surfaces differ for each individual, ranging froman almost-plane surface with an innite radius to ahighly curved surface with a radius of 4 to 5 inches.

Christensen offered 2 of his several practicalexperiments to conrm that the principles of hisspherical theory were correct. The rst experiment,

a laboratory demonstration, used his Rationalarticulator to manually simulate functionally gen-erated path occluding surfaces on maxillary andmandibular rims. To simulate occlusion rims,Christensen mounted plaster blocks in his articula-tor (Fig 5). He then set the condylar guides at anespecially high oblique position. Maintaining rmhand pressure on both bows of the articulator andusing the guiding mechanism of the instrument, hefunctionally articulated the blocks to grind themin to balancing surfaces in all directions of themoving bite. The worn surfaces now showed per-fect contact through all movements and obtainedthe shape of spherical surfaces, the mandibularsurface concave upward and the maxillary surfaceconvex downward (Fig 6 A).12

Christensen claimed to conrm this indirectproof by another experiment that he carried out with a living subject, a man whose natural teeth were severely abraded. The subjects plane of occlu-sion was slightly curved but was not smooth. Chris-tensen constructed vulcanite rubber stints to coverthe teeth, and over the stints he placed wax occlu-sion rims of a few millimeters thickness. Afterlubricating the rims with soap, the subject wasasked to move his mandible in all possible direc-tions, holding the rims together with moderatepressure. Although not as dramatic, the outcome was the samethe occlusal surfaces of the waxrims obtained a spherical shape (Fig 6 B and C).12

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 admonishingany inventors who had claimed that their so-calledanatomic articulators could imitate the jointmechanism. 15 Frank conducted experiments thathe believed produced conclusive evidence thatSpees ndings were inaccurate. He said that vonSpee had stated emphatically that the sagittal oc-clusion 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 notprove the validity of his Rational articulator. Us-ing cross-sections of dentulous mandibular casts,Frank demonstrated that there were vast differ-ences among individuals in the curvatures of theocclusal planes. Moreover, by cutting cross-sectionsof each cast at the positions of the premolars and

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

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molars, he showed that the radius of each of the 5pairs of teeth would be different (Fig 7).

Using Christensens Rational articulator, Frankrepeated his simulated functionally generated pathexperiment using blocks made of a pumicestonemixture (Fig 8). The curved occlusal surfaces gener-ated on Franks blocks were remarkably more compli-cated than the spherical surface reported by Chris-tensen. Furthermore, Frank suggested that it wasevident that the directions of the natural masticatingsurfaces differ so greatly from those obtained by re-peating 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 witha 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 early-20th century dentist, it is doubtful that he foundmany colleagues to argue with that statement. In-deed, there are those today who would wholeheart-edly agree with him.

Clearly, Frank expressed some legitimate con-cerns. He understood the concepts of the facebow,

Figure 7. Cross-sections of mandibular dentulous casts of different individuals demonstrating how Frank calculated thedifferencesbetween the lateralocclusal plane curvature variations (as viewed in thefrontalplane.) Lines weredrawn touchingthe highest points of the respective pairs of teeth. Points a and b identify the midpoint of the occlusal surfaces. The linesintersect at point c. Frank identied points a, b,and cas theinter-occlusal surfaceangle.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 thateach tooth hada circleof occlusal contact, 1 with radius rand 1 withradius r . None of the radiiconstructedfor theocclusalcircles of each tooth pair ever appeared to be equal. (Reprinted from Turner. 14 )

Figure 8. Cross-sections of the mandibular casts of oc-clusal rims that Frank gener-ated by repeating Christen-sens simulated functionally generated path experiment.Frank made 5 transverse sec-tions at the proper posi-tions of the posterior teeth.He noted 10 different slop-ing surfaces, 5 for each side,and pointed out numerousdiscrepancies between Chris-tensens ndings and his. (Re-printed with permission. 15 )

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the third point of reference, and the variability of the intercondylar distance. Christensen had notaddressed these issues in his work. On the otherhand, Franks choice of analysis to challenge Chris-

tensens theories could be described as comparingapples and oranges. Even though the sphericaltheory implies multidirectional movement, Chris-tensen primarily studied the movement of the man-dible in the anteroposterior direction (as observedin the sagittal plane) after the work of Spee, whereas Franks observations were in the frontalplane. Christensen also made it quite clear that the

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

Figure 10. George Monsondemonstrated his sphericaltheory for the rst time onthis Bonwill articulator.The casts were mounted inthe articulator accordingto Bonwills equilateral tri-angle and with the spheri-cal occlusion guide. (Re-printed from Washburn. 17 )

Figure 11. Dr. Monson making measurements on ahuman mandible to demonstrate that from the 4-inchcommon center, the divider touches the incisal edges andthe condyles. (Reprinted from Washburn. 17 )

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ideal occlusal curve would be considered only for

the edentulous mouth in the context of construct-ing complete dentures. 12So what did Frank conclude from his own exper-

iments with curves of occlusion and from his observations of the known articulators of his day? Whathe said was this: An anatomical articulator is goodfor nothing. Life cannot be imitated. It would seemthen, that we must give up forever any idea of beingable to construct a mechanical joint articulator that will enable us to construct a physiologically articu-lating denture for each individual case. 15 Clearly,he was ahead of his time.

Monson s Spherical Theoryand Articulator

Conducting experiments on mandibular movementduring the same period as Carl Christensen wasGeorgeMonson, of St. Paul,MN (Fig 9). 16 H.B. Wash-burn (also of St. Paul, MN), writing on the history of occlusal concepts, reported that Monson had con-ceived thespherical 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 agroup at Mankato, MN, Monson presented for therst time a method for setting denture teeth, usingBonwills equilateral triangle conforming to thesurface of a sphere. Monson had been a student andclose friend of Bonwill for many years, but the timecame when he could no longer strictly follow all of Bonwills teachings. Nevertheless, this rst demon-stration of his spherical theory was performed witha Bonwill articulator, and the casts were mountedaccording to Bonwills instructions. However, theteeth were set to conform to a wire spherical

occlusal guide constructed by Monson (Fig 10). 17Through further studies, Monson concluded that

prenatally, mandibles ideally tend to develop as equi-lateral triangles and, if the various interfering factorscan be controlled during development, that the teethalso would conform to a sphere. 18 To verify this hy-pothesis, Monson conducted experiments with both ahuman mandible and with casts of the mandibulardentition of highly developed individuals. By highly

Figure 12. L.A. Weinbergs schematic illustrationof the 3-dimensional relationships of the components of Monsons theory.Linesprojectedfrom theapices(A,B,andC) ofBonwills triangleintersect at point D, forminga spherical pyramid. Monsons8-inch diameter sphere touches the apices of the triangle, and point D is the center of rotation or radius of the sphere.Weinberg pointedout thata relationshipbetweenBonwills triangle and Balkwillsangle. Monsons theory requiresa condylarinclination of close to 35 degrees anda Balkwill angleof 15.5 degrees.Theseangles do not correspond to thoseaverage anglesfound by Gysi (30-degree condylar inclination) and by Balkwill (26-degree Balkwill angle) (Reprinted with permission. 22 )

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

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

Figure 13. A frontal view of the mandible illustrating therelationship of the 8-inch-diameter sphere with thetransverse plane of occlusionthat Monson claimed mustbe the same as the antero-posterior plane for balancedocclusion to be achieved. Theradial lines of force of 4-inchlength converge forming theradial point at the apex from which the radius of occlusionof each tooth is determined.(Reprinted from Monson. 20 )

Figure 14. A posterior viewof the mandible, illustratingthe application of the radiallines to the condyles. Thecenter of the condyles areshown conforming to the sur-face of the sphere, giving thesame radial dimensions fromthe centers of the condyles tothe apex as from the occlusalsurfaces of the teeth. Thisalso illustrates Monsonsconcept that this radialcenter is the center for theentire muscular action be-cause the angles of themandible conform to linescentering at apex A. (Re-printed from Monson. 20 )

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found that they intersected at a common point orcenter. On the human mandible (Fig 11), he discov-ered 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 thecondyles.

This, then, was the origin of Monsons sphericaltheory. It was based on the concept that the man-dibular teeth move over the occlusal surfaces of the

maxillary teeth, as over the external surface of asegment of an 8-inch sphere, and that the radius (orcommon center) of the sphere is located in theregion of the crista galli. Because of the way in which the mandible develops, Monson further be-lieved that it would be logical to adapt Bonwills4-inch equilateral triangle to the surface of the8-inch sphere, because geometrically, such a spher-ical-based triangle would also be a segment of the8-inch sphere, and the apex of a pyramid erected on

Figure 15. Monsons Man-dibulo-Maxillary instrument.Point A is the radial center of the instrument from whichthe occlusal surfaces of theteeth are determined. PointB is the position of the con-dylar hinge mechanism forthe instrument. The teethare arranged to conform tothe 8-inch sphere at C. SlotD controls anteroposteriormovement. The slot is con-centric with the outer surfaceof the sphere. Jackscrews Eare used to adjust the posi-tion of the lower cast to thecenter if required. This in-strument was manufacturedby M.F. Patterson Supply Co., St Paul, MN. (Reprinted

from Campbell. 21 )

Figure 16. A sagittal viewdemonstrating the relation-ships of the 8-inch diametersphere to Monsons articula-tor and to the anteroposte-rior plane of occlusion. (Re-printed from Monson. 20 )

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the triangular base would be coincident with thecommon center of rotation, that is, the 4-inch ra-dius of the sphere (Figs 12, 13, and 14). Initially, thespherical theory involved the articulation of naturalteeth in the highly developed individual and theconviction that these principles apply to the eden-tulous mandible with highly developed ridges. 18Monson soon realized, however (and was quick topoint out), that most patients encountered arenot highly developed, because at some point inlife an unbalanced condition replaced an earlierbalance 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 notalways be in the same location. Thus Monsonprovided a mechanism in his instrument and inthe method for mounting casts whereby the re-lationship of the patients occlusal plane andcondyles to the patients center is the same onthe articulator as in the patient. 17

In 1923, Monson was issued a patent for hisarticulator. 19 The Mandibulo-Maxillary In-strument, as Monson named it, was based on

his spherical theory (Figs 15 and 16). Theinstrument had 2 rotational axes, spherical andcondylar. The condylar axis feature was, of course, one of convenience but was also de-signed for a facebow transfer method used forthe unbalanced [oral] conditions encoun-tered in most patients. Both Washburn 17and R.G. Keyworth 23 described their methodsfor using Monsons articulator in completedenture construction; both versions includeda similar facebow transfer technique (Figs 17and 18).

In summarizing the principles of Monsons in-strument, Washburn stated that it incorporatedMonsons spherical principle and combined theBonwill triangle with Walker and Gysis condylemovements. In addition, the instrument includedGysis idea that the forward and lateral movementsmust be combined and that the plane of occlusionconforms to the curve of Spee. 17

Returning to the Original QuestionSo, who should receive credit for the spherical the-ory, Carl Christensen or George Monson? The an-swer 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 thisanswer important? Probably not. Because they were working independently at about the same time,either one of these men could have actually beenthe rst. In any event, it is George Monson whoshould and probably will be remembered for pro-mulgating the spherical theory and for his convic-tion that its principles were sound.

James House states that Monson had applied for thearticulator patent in 1918 and had presented and defendedhis spherical principles and his Mandibulo-Maxillary In-strument surprisingly well before his peers at the annualsession of the National Society of Denture Prosthetistsabout 2 years later.Monsonwasverymuch inthe centerof the spiriteddental controversy [over thevarious theoriesof mandibular movement and articulator design] because hisidea of a single rotation center was an easy target. 4

Figure 17. A schematicdrawing illustrating the theo-retical mechanics of transfer-ring waxrims from thepatientto the instrument. (Reprintedfrom Washburn. 17 )

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Figure 18. ( A) After a centric rela-tion record is made, the rims arefastened together and transferredto the instrument with a facebow.( B) After the occlusion rims are re-lated to the condylar axis with thefacebow, the lower cast is adjustedby placing one end of the open cal-ipers in the radial center of the ar-ticulator and touching the free endof the calipers to the incisor pointon the lower wax rim. ( C) A caliperis used to project the sphericalcurve to the occlusal surface of themandibular 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, AmericanDental Association.)

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Carl Christensen was a practical clinician who devised a useful intraoral procedure torecord the individual condylar paths for thepurpose of setting the adjustable condyle con-trols of his articulator. Christensen was curiousabout the nature of mandibular movement and,through his experiments, recognized the spher-ical curvature of the occlusal plane and itsrelationship with the curvature of the condylarpaths. However, Christensen believed that be-cause of the innite radius of the sphere, for allpractical purposes, the condyle paths would bea straight line. He did not promote his spheri-cal theory, but he will always be associated withhis method for making a protrusive intraoralrecord and for Christensens phenomenon.

George Monson, on the other hand, believedthat his spherical principles produced the idealocclusion in the highest-developed type of individual and accordingly, the best-balanced articialdentures must conform to a spherical base. 20 Mon-sons articulator and technique based on his spher-ical theory attracted a number of devoted followers.Even today, many of his principles persist as a partof the dental landscape.

More on the history of articulators based ongeometric theories of occlusion will appear in thenext issue of The Journal of Prosthodontics.

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and articulate teeth for full denture construction. In Sharry JJ (ed): Symposium on complete denture prosthesis, DentClin North Am 1964; :629-663

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3. Starcke EN: The history of articulators: The appearance andearly use of the incisal-pin and guide. J Prosthodont 2001;10:52-60

4. House JE: The design and use of dental articulators in the

United States from 18401970. Masters thesis, IndianaSchool of Dentistry, Indianapolis, IN, 1970, pp 119-127

5. Bonwill WGA: Articulationand articulators. Trans Am Dent Assoc 1864; July 26:76-79

6. Balkwill FH: The best form and arrangement of articialteeth for mastication. Trans Odont Soc Great Britain 1866;5:133-158

7. von Spee FG: Die Verschiebrangsbahn des unterkiefers amschadell. Arch Anat Physiol 1890;16:285-294. English trans-lation, Niedenbach MA, Holtz M, Hitchcock HP: Theglidingpath of the mandible along the skull. J Am Dent Assoc1980;100:670-675

8. Gysi A: The problem of articulation (Part II). Dent Cosmos1910;52:148-169

9. Lufkin AW (ed): A History of Dentistry (ed 2). Philadelphia.PA, Lea and Febiger, 1948, p 292

10. Hall RE: An analysis of the work and ideas of investigatorsand authors of relations and movements of the mandible. J Amer Dent Assoc 1929;16:1642-1693

11. Christensen C: A rational articulator. Ashs Q Circular1901;18:409-420

12. Christensen C: The problem of the bite. Dent Cosmos1905;47:1184-1195

13. Posselt U (ed): Physiology of Occlusion and Rehabilitation.Philadelphia, PA, Davis, 1962, pp 42-43

14. Turner CR (ed): The American Textbook of ProstheticDentistry (ed 3). Philadelphia, PA, Lea Brothers, 1907,p 414

15. Frank B: An investigation on articulation and experiments with C. Christensens articulator. Brit Dent J 1908;29:289-295

16. Cruttenden LM: Obituary of George S. Monson. J Am Dent Assoc 1933;20:1285-1287

17. Washburn HB: History and evolution of the study of occlu-sion. Dent Cosmos 1925;67:331-342

18. Washburn HB: The application of the Monson spherical

principle to full dentures. J Am Dent Assoc 1927;14:648-65419. Monson GB: Dental Articulator. US Patent No. 1,457,385. June 5, 1923

20. Monson GB: Occlusion as applied to crown and bridge-work. J Nat Dent Assoc 1920;7:339-413

21. Campbell DD (ed): Full Denture Prosthesis. St. Louis, MO,Mosby, 1924, p 355

22. Weinberg LA: An evaluation of basic articulators and theirconcepts. Part II: Arbitrary, positional, semiadjustable ar-ticulators. J Prosthet Dent 1963;13:645-663

23. Keyworth RG: Monson technic for full denture construction. J Am Dent Assoc 1929; 16:130-162

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