DENTAL INSTRUMENTATION

The History of Articulators: A CriticalHistory of Articulators Based on GeometricTheories of Mandibular Movement: Part IEdgar 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 20th 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 ofwhat 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 simplyreflected what was perceived to be the anatomicand kinesthetic characteristics of mandibularmovement. Despite differences in investigators’perception and application of mandibular move-ment, the complexity of articulators began to evolveas a result of the important work of such scientistsas W.E. Walker, Alfred Gysi, and George Snow. By1910, 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 20th century, articulatorswith 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 Gysi’s 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 asinfluences 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 Centerat Houston Dental Branch, 6516 M.D. Anderson Boulevard, P.O. Box20068, Houston, TX 77225. E-mail: estarcke@mail.db.uth.tmc.edu

Copyright © 2002 by The American College of Prosthodontists1059-941X/02/1102-0012$35.00/0doi: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 Gysi’s “average” completedenture technique, which included his Simplex articula-tor.

†By 1918, several theories of occlusion existed alongwith articulators designed to promote them. According toJames E. House, since the principles of these theoriesvaried 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 fieldof 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

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

consideration individual variations (i.e., there wasthe notion that “one size fits all”), and (2) noprovision was made for the Balkwill–Bennett move-ment.

Articulators designed to reflect 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 deviceshave 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 neverwaned.)

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 his“conical” 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 applygeometric 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 mandible’s 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 lowerjaw appears formed of parts of two circles, the inner andlarger forming part of an independent smaller circle. Thecondyle articulates with the glenoid cavity so as to allow asingle hinge-like motion and a forward and backwardmotion. While there is only a slight lateral motion, bothsides move on the radii of the same circle. The combinedmotion of both circles will give the [rotating] side nearly asimple lateral action, while the [orbiting] side will moveforward and downward.6

In 1890, anatomist Ferdinand Graf von Spee ofKiel, Germany (Fig 2) called attention to “therelationship between the curved arrangements ofthe 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 first 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 “effi-cient” denture service with-out the need for “scientificequipment.” The Gysi Adapt-able articulator (left) wouldbe the ideal instrument, butthe Simplex would suffice in80% of the cases. The bookletadvertised the services of theI.J. Dresch Laboratories ofToledo, OH, and the illustra-tions were provided by theDental 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 continuationof 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 samecylindrical surface. The location of the axis of that cylin-der’s curvature is at the level of the horizontal mid-orbitalplane. The steeper the path of the condyles, the morepronounced the tooth curve would be, because both have thesame 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 intothe late 1940s, there was some question as to whoactually originated the spherical theory of mandib-ular movement.9

Rupert Hall’s historical review of the work ofvarious investigators on mandibular movement ledhim to believe that Carl Christensen had developed

Figure 2. Ferdinand Graf von Spee (1855–1937). (Re-printed by permission of ADA Publishing, a division ofADA 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 the“joint 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 Christensen’s phenomenon.Christensen’s “Rational articulator” is based on this prin-ciple. (Reprinted from Christensen.11)

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the spherical theory.10 Christensen’s claim to fame,of course, was his practical technique for register-ing positional relations of the mandible. He was thefirst 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-tensen’s phenomenon,” or the posterior separationof the occlusion rims that occurs when the mandi-ble 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 believedthat Spee should be “credited with pointing out theimportant—and simple—truth that the path of thecondyle during the ‘bite movement’ must be inconformity with the ‘bite-path’.”12 Christensen de-veloped his method of recording the condylar incli-nations for his “Rational” articulator as an exten-sion of Spee’s principle, that is, harmonizing thearticulation of the teeth with the movements of thecondyles.14

Christensen was well aware that in Spee’sview, 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 tobe a discrepancy between his hypothesis and theaccepted conception of anatomic conditions. ButChristensen proposed that during movement ofthe 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 the“articulation-path” and the “joint-path” weresimilar, then whether the “articulation-path” isstraight or curved, the “joint-path” must be par-allel to it (Fig 3).11 In this figure, both paths areshown to conform to concentric arcs with a com-mon center. Christensen considered the “condy-lar path curves” to have “infinite” 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. Christensen’s 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) Christensen’s Rational articulator withthe condylar guides set at a high inclination. The maxil-lary and mandibular plaster blocks have been manuallyground in and the surfaces have obtained sphericalshapes. (Reprinted from Christensen.12) (B) Vulcaniterubber stints with wax occlusion rims on casts of badlyworn natural teeth. The spherical contours of the rimswere 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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Christensen’s Spherical TheoryCarl Christensen pursued Spee’s 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.

Christensen’s new spherical hypothesis was basedon the conclusions that he had reached regarding thefactors that determine the nature of the occlusalplane and the relationship between the occlusal 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 correctwas to observe the “bite-movement” (articulation)phenomenon itself.‡ But, he explained, it “must beremembered 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 ideal“jaw-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 infinite radius to ahighly curved surface with a radius of 4 to 5 inches.

Christensen offered 2 of his several “practical”experiments to confirm that the principles of hisspherical theory were correct. The first experiment,

a laboratory demonstration, used his “Rational”articulator 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 an“especially high oblique position.” Maintaining firmhand 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 6A).12

Christensen claimed to confirm this “indirectproof” by another experiment that he carried outwith a living subject, a man whose natural teethwere severely abraded. The subject’s 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 outcomewas the same—the occlusal surfaces of the waxrims obtained a spherical shape (Fig 6B and C).12

A Frank-ly Discouraging WordIn 1908, Bernard Frank of Amsterdam, took aim atvon Spee and Christensen, harshly criticizing theirwork on mandibular movement and admonishingany inventors who had claimed that their so-called“anatomic” articulators could imitate the jointmechanism.15 Frank conducted experiments thathe believed produced conclusive evidence thatSpee’s findings 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, andclaimed that his own experiments showed that thiswas 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 defining 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 Christensen’s “Rational” articulator, Frankrepeated his simulated “functionally generated path”experiment using blocks made of a pumice–stonemixture (Fig 8). The curved occlusal surfaces gener-ated on Frank’s blocks were remarkably more compli-cated than the spherical surface reported by Chris-tensen. Furthermore, Frank suggested that it was“evident 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 Frank’s rhetoric was that of a man witha mission: to let the world know that it is “utterlyimpossible to solve the problems of articulation bymeans 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 thedifferences between the lateral occlusal plane curvature variations (as viewed in the frontal plane.) Lines were drawn 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 identified points a, b,and c as the “inter-occlusal surface angle.” At points a and b, perpendicular lineswere drawn that intersected at point d, representing the common center of rotation of each pair of teeth. Frank noted thateach tooth had a “circle of occlusal contact,” 1 with radius r and 1 with radius r�. None of the radii constructed for the “occlusalcircles” of each tooth pair ever appeared to be equal. (Reprinted from Turner.14)

Figure 8. Cross-sections ofthe mandibular casts of oc-clusal rims that Frank gener-ated by repeating Christen-sen’s simulated “functionallygenerated 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-tensen’s findings and his. (Re-printed with permission.15)

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

tensen’s 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 Frank’s 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 ofADA Business Enterprises, Inc.16 Copyright © 1933,American Dental Association.)

Figure 10. George Monsondemonstrated his “spherical”theory for the first time onthis Bonwill articulator.The casts were mounted inthe articulator accordingto Bonwill’s 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 forthe edentulous mouth in the context of construct-ing complete dentures.12

So what did Frank conclude from his own exper-iments with curves of occlusion and from his obser-vations 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 thatwill 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 wasGeorge Monson, of St. Paul, MN (Fig 9).16 H.B. Wash-burn (also of St. Paul, MN), writing on the history ofocclusal concepts, reported that Monson had con-ceived the spherical theory. Washburn also considered

it significant that Christensen and Monson, so close inideas, knew nothing of each other’s work.17

Washburn reported that in 1898, speaking to agroup at Mankato, MN, Monson presented for thefirst time a method for setting denture teeth, usingBonwill’s 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 ofBonwill’s teachings. Nevertheless, this first demon-stration of his spherical theory was performed witha Bonwill articulator, and the casts were mountedaccording to Bonwill’s instructions. However, theteeth were set to conform to a wire “spherical”occlusal guide constructed by Monson (Fig 10).17

Through further studies, Monson concluded thatprenatally, 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. Weinberg’s schematic illustration of the 3-dimensional relationships of the components of Monson’s theory.Lines projected from the apices (A, B, and C) of Bonwill’s triangle intersect at point D, forming a spherical pyramid. Monson’s8-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 Bonwill’s triangle and Balkwill’s angle. Monson’s theory requires a condylarinclination of close to 35 degrees and a Balkwill angle of 15.5 degrees. These angles do not correspond to those average 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 affixed 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 ofthe 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 fromwhich 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 Monson’sconcept 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 ofthe anterior teeth and the buccal and lingual cusps ofthe posterior teeth, but also bisected both of thecondyles.

This, then, was the origin of Monson’s 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 inwhich the mandible develops, Monson further be-lieved that it would be logical to adapt Bonwill’s4-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. Monson’s Man-dibulo-Maxillary instrument.Point A is the radial center ofthe 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 SupplyCo., St Paul, MN. (Reprintedfrom Campbell.21)

Figure 16. A sagittal viewdemonstrating the relation-ships of the 8-inch diametersphere to Monson’s 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.18

Monson 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 influence.In these individuals, the radius of the sphere maybe 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 patient’s occlusal plane andcondyles to the patient’s 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, ofcourse, one of convenience but was also de-signed for a facebow transfer method used forthe “unbalanced [oral] conditions” encoun-tered in most patients. Both Washburn17

and R.G. Keyworth23 described their methodsfor using Monson’s articulator in completedenture construction; both versions includeda similar facebow transfer technique (Figs 17and 18).

In summarizing the principles of Monson’s in-strument, Washburn stated that it incorporatedMonson’s spherical principle and combined theBonwill triangle with Walker and Gysi’s condylemovements. In addition, the instrument includedGysi’s 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 definitely known, because theexact date when and by whom the spherical ideawas conceived may be “too close to call.” Is thisanswer important? Probably not. Because they wereworking independently at about the same time,either one of these men could have actually beenthe first. 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. Monson “was very much in the center ofthe ‘spirited dental controversy’ [over the various theories ofmandibular 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 wax rims from the patientto 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 forsetting the teeth. (Reprinted bypermission of ADA Publishing, a di-vision of ADA Business Enterprises,Inc.23 Copyright © 1929, AmericanDental Association.)

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Carl Christensen was a practical clinicianwho 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 infinite 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 “Christensen’s phenomenon.”

George Monson, on the other hand, believedthat his spherical principles produced the idealocclusion in the “highest-developed” type of indi-vidual and accordingly, the “best-balanced artificialdentures” must conform to a spherical base.20 Mon-son’s 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.

References1. Paraskis CS: Criteria for selecting an articulator to occlude

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

2. Gysi A: Simplifying the correct articulation of artificialteeth. Dent Dig 1913;19:1-8

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 1840–1970. Masters thesis, IndianaSchool of Dentistry, Indianapolis, IN, 1970, pp 119-127

5. Bonwill WGA: Articulation and articulators. Trans Am DentAssoc 1864; July 26:76-79

6. Balkwill FH: The best form and arrangement of artificialteeth 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: The glidingpath of the mandible along the skull. J Am Dent Assoc1980;100:670-675

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