Verifying the Reliability ofInterchanging Casts With

Semiadjustable Articulators

Robert D. tow jn , ULI3, ms

Robert A. Sanchez, DDS"

Robert P. Chappell, DOS-

Alan G. Claros, PhD'"'

Witliam I. Hoyden, DDS, MPH

School ol DentistryUniversity of Missouri, Kansas City

Interchangeability of a semiadjustable articulator could be ofsignificant convenience and economit benefit to therestorative dentist. The manufacturer of the Whip-Mix Model2240 art:iculator claims such interchangeability. This studyevaluated the accuracy of the cross-mounting capability ofeight such articulators, then compared the measured resultswith those obtained from eight semiadjustable instrumentsfor which the manufacturer makes no such promise ofinterchangeability. It was determined that the Model 2240articulator did possess accuracy when a measuring devicewas transferred between the eight materials tested. IntjProsthodonI 1991,•4:258-264.

A semiadjustable articulator capable of acceptinga facebow record, various condylar settings,

and a custom incisai guide table is an importantadjunct for restorative dentistry. A semiadjustableinstrument is adequate for tbe majority of applica-tions. Although Wachtel and Curtis'- demonstratedlimitations of semiadjustable articulators when com-pared with a fully adjustable instrument, they alsodemonstrated that a semiadjustable instrument withprovisions for replicating immediate mandibularlaterotrusion produced acceptable accuracy in tbehorizontal plane, as well as in the frontal plane ifthe immediate side shift was 0.75 mm or less. Sim-ilarly, Lundeen and Shryock' demonstrated that 0.75mm was the average laterotrusive movement in 163asymptomatic subjects.

Most recent dental school graduates and manydentists who have been in practice for several yearshave been taugbt to use semiadjustable instru-ments. Gross and Cazif surveyed the departments

'Assistant Professor. Director AEGD."Assistant Professor, Department of Fixed Prosthodontics.

'"Professor Emeritus. Department of Oral Biology.""Associate Professor. Behavioral Science.

""•Associate Professor. Quality Assurance.

Reprint requests: Dr Robert D. Cowan, School of Dentistry.University of Missouri. Kamas City. 650 E 25th Street. KansasCity. Missouri 64108.

of prosthodontics and occlusion of 59 North Amer-ican dental schools and reported that 92% of the53 responding schools used a semiadjustable artic-ulator for training students. As part of tbeir survey,tbey asked if tbe respondents tbought that themajority of their former students who were now inpractice used hinge or plane line articulators in pref-erence to semiadjustable instruments. Sixty-ninepercent responded "yes." Possible reasons thatsemiadjustable articulators are not routinely used inpractice might include insufficient experience usingthem while in school or lack of dentist appreciationfor more sophisticated equipment. However,another reason may be the necessity of sending thearticulator to the dental laboratory.

In the past, mounted casts could not be accu-rately transferred from one semiadjustable articu-lator to another, even when the instruments werefrom the same manufacturer and had the samemodel number. Some manufacturers bave nowintroduced semiadjustable instruments tbat areclaimed to have the capability of accurately trans-ferring mounted casts from one articulator toanother. One such instrument is the Model 2240"Precision Mount" articulator (Whip-Mix Corp,Louisville, Kentucky).

This study was conducted to evaluate the reli-ability of tbe cross-mounting capability ofthe Model2240 articulator and to compare these findings withthose obtained using a semiadjustable articulator

Ttie International journal of Prosthodontics 258 Volume 4. Number .1. 19

Inlerchínging Cdsls Willi Semiadiuílabie Articulatois

(Model 8500) from the same manufacturer, but forwhich no claim of interchangeability has beenmade.

Materials and Methods

The condylar guidance elements of eight Whip-Mix Model 2240 semiadjustable articulators wereidentically preset, and the superior and inferiormembers were made parallel to one another. FightWhip-Mix Model 8500 articulators were similarlypreset. All articulators were purchased from openstock and were not provided by the manufacturer.

It was necessary to devise a measuring devicespecifically for this project. Three styli were madeby milling flat machine screws to a fine point. Thesestyli were attached to a flat aluminum plate usinglock washers and nuts, making them verticallyadjustahle. The styli were then positioned to sim-ulate point contact at the maxillary right and left first

molar mesiolingual cusp tips and at the contact areabetween the maxillary central incisors (Fig 1). Thestyli-bearing aluminum plate was then attached toa Whip-Mix metal mounting plate using three addi-tional flat metal screws, lock washers, and nuts (Fig2), The mounting plate was drilled with countersunkholes to allow flush mounting against the supenormember of the articulators. A sheet of graph paperwith 1-mm squares was attached to the table por-tion of a Whip-Mix remounting jig.

One articulator of each model was arbitrarilyselected as a master to be compared to the otherseven in its group. The test apparatus and remount-ing jig were placed on the master articulator, andeach stylus was adjusted to contact the graph paperequally. The incisai guide pin was then removed(Fig 3), and each stylus was readjusted until all threeexhibited equal, nontearing tug resistance to shim-stock (Artus Occlusal Registration Strips, The ArtusCorp, Englewood, New Jersey) (Fig 4). The styli were

Fig 1 Aiuminum plate wilh styli positioned at maxillary rightand left mesial linguai cusp tips and incisor midhne contactpoint.

Fig 2 Styli-bearing aluminum plate attached to Whip-Mixmounting piate.

Fig 3 Test apparatus mounted to upper articuiator memberand grapti paper-covered remounting jig mounted to lowermember with incisai guide pin removed.

Fig 4 Each styius adjusted to allow tug removal ot shimstockwithout tearing.

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Interctianging Casis With Semiadjuitable Articulator

then locked into position by the lock washers andnuts.

To examine the horizontal reliability of the Model2240 articulators, ink from a black felt-tip markerwas applied to the point of the styli on the masterarticulator. The condylar elements were checked toensure contact with the posterior wall of the con-dylar guides and the wall of the plates controllinglaterotrusion. The superior frame of the articulatorwas then rotated to allow contact of the styli with

the graph paper, arcing it around the condylar ele-ments, as the instrument is designed to be usedclinically. This produced three disti/.t t black dots onthe paper. The styli-bearing apfwratus and theremounting jig were sequentially mounted on theother seven Model 2240 articulators, using a differ-ent coior ink for each instrument. This marking pro-cess allowed determination of any positionaldiscrepancy of styli among the articulators (Figs 5ato 5c), When tnitial contact was made with the

M

Figs Sa to 5c Model 2240 stylus markings tor styius no, 3¡top), no, 8-9 (center), and no. 14 fttottom) usinq maqnificationX 40 ' a a

Figs 6a to 6c t̂ flodel 8500 stylus markings for stylus no 3(top), no. 8-9 (center), and no, 14 (bottom) using maqnificationX 40.

loumal of ProsIhodontic 260 Volume 4, Number 3, 1991

IntercliariBing Casis Willi Semi ad justa ble Articula i on

graph paper by one or more styli, the condylar ele-ments were checked to ensure that contact with theposterior wall had been maintained.

To examine reliability in the vertical dimension,each stylus was individually tested with shimstockfor contact. If contact with the graph paper providedresistance to tugging, as did the master, it wasrecorded as "tug," If the shimstock tore under thestylus, the result was recorded as "tear," If therewas no contact to the shimstock (0.013-mm thick-ness), the vertical space was measured using anautomotive feeler gauge with the thinnest leaf being0,038 mm. If no resisfance was felt to withdrawalof the 0,038-mm leaf, the next-largest leaf (0,051mm) was tested until resistance was felt. The thick-ness of that leaf was then recorded as fhe verticalspace. After each articulator measurement, theapparatus was remounted on the master articulatorto ensure that all styli were still in "tug" contact withshimstock.

This same horizontal and vertical assessment pro-tocol was followed for each of the eight 8500-seriesarticulators. Examples of fhe markings made by eachstyitjs are shown in Figs 6a to 6c, One investigatormade all measurements. A second investigator thenrepeated the vertical space assessment with shim-stock for "tear," "tug," or "no contact," He alsoindependently measured the graph paper markingsusing a universal measuring microscope (UnitronInstruments, Woodbury, New York),

The marked graph paper was removed from theremounting ¡ig for each series of articulators andmeasured for horizontal stylus mark differencestjsing the Unitron microscope. To allow for stylus

superimposition, the measurements were madefrom the outer edge of the master color mark (hiack)to the corresponding outer edge of the other marks.If one stylus of an articulator recording did not con-tact the graph paper, an approximate distance dis-crepancy was de te rmined , since actualmeasurements were made in the horizontal and sag-ittal planes only. This approximation of the distancediscrepancy from the master was calculated as thehypotenuse of a right triangle (see appendix). Thevertical distance was that measured by the feelergauge. The horizontal distance recorded was cal-culated as the averaged distances of fhe two con-tacting styli from their black master stylus mark. Ifonly one stylus contacted the paper, the distancebetween the mark left by that stylus and the markleff by the master was used as the horizontal devia-tion distance for all three styli, A multivariate analysisof variance (MANOVA) using the three styli posi-tions as the dependent variables was then used toassess measurement differences between fhe twogroups of articulators. Measurements for the masterarticulator (0,00) for each model were not includedin the analysis.

Results

The discrepancy distances in millimeters from fhemaster articulator styli are shown in Tahle 1, For theModel 2240 articulators, the horizontal deviationsranged from 0,00 to 0,08 mm with a mean of 0,032mm (SD = 0,026). There were no vertical devia-tions. Both investigators found equal tug, without

Table 1 Dimensional Discrepancies (in millimeters)

Articulatormodel

2200 Series

S500 Series

Articuiatornumber

Master1234567

Waster1234567

3

0.000.060,000,080.080.050.000.00

0.000,720,94t0,38-0,58t0,76-0,35t0.65

Stylus no.

8-9

0.000.040.020,030,040,000,020.05

0.000.790.94t0,38-0,63'0,740,33t0.70-

14

0,000,030,040.050.000.040.000,05

0,000.76t0.940.37t0.58'0.78t0.34-0.64t

•No contact ih vertical plane.'[Tear ot shimstock caused by excess contact.

lumber B, 1991 261 The International lournal of Prosthodontics

intercSanging Castí Wilh Semiadiustable Articulaforí

1.0 - -

g 0.9 - -lU

s 0.8 - -

i 0.7 - -

z 0.6 - -

i °-5 ::5 0.4 - -

s 0.3 - -

g 0.2 - -O

0.1 - -0 - #3 #8-9

STYLUS

#14

Fig 7 Distribution of the total distance deviation of each stylus of each test artioulator from the master, X axis (0| representsmaster arlicuiator for both groups. Model 2240 articulators Shown as clear circles; Model 8500 articulators shown as solidcircies.

tearing, to shimitock for all styli of the eight Model2240 articulator?. For the Model 8500 articulators,the horizontal deviation from the master rangedfrom 0,33 to 0,94 mm with a mean of 0,633 mm(SD = 0,206], The vertical recordings varied fromexcessive stylus contact, causing shimstock tearing,to a 0,279-mm gap.

The MANOVA showed a highly significant dif-ference between the two groups of articulators,F{3,10) = 14,93, P < ,0005, Figure 7 presents, ingraph form, the total deviation distribution for eachof the three styli for the 14 articulators, as measuredfrom the masters' baseline. The graph shows thenonoverlapping distributions of discrepanciesbetween the two models.

Discussion

At the beginning of the study, a cylindrical jig wasmade for each articulator model to standardize thedistance between ihe superior and inferior mem-bers prior to inserting the incisai guide pins. Follow-ing adjustment of the three styli to give equalshimstock tug on the master articulator, the incisai

guide pin was removed to determine if the weightof the superior member altered the styli contactagainst the remounting ¡ig. Although there was stillcontact of all three styli, it was no longer equal. Inthe clinical situation, the incisai guide pin is arbi-trarily extended to compensate for the thickness ofthe material used in making the centric relationrecord prior to mounting the mandibular cast. Afterthe mandibular cast is firmly mounted to the inferiormember, the incisai guide pin is usually raised toensure that the casts are in positive centric relationcontact. The pin is then brought into passive contactwith the incisai guide table. It was decided to testthe articulators with the incisai guide pin removedto more closely simúlale the clinical situation. Thetest apparatus was returned to the master articuiatorafter each test articulator was evaluated to ensurethat no abrasion of the styli tips occurred. Sinceequal shimstock tug was found by both investigatorsthroughout the multiple testing of the Model 2240articulators, a meastjring sensitivity of less than 2ÜAim appears to be present using this method.

To determine the diameter of each stylus tip, theaverage of the diameters made by all "tug-to-shim-stock" marks ior a given stylus was calculated. The

The International loumal of Prosthodontii 262 • 4 , N'.'

Interchanging Casis With Semiadjuslable AriiculaiorE

three stylus tips measured 0.3, 0.25, and 0.25 mm.For the 8500-5eries articulators, styli markings fromall eight instruments could be recorded and readilydistinguisbed with tbe unaided eye on the samesheet of graph paper. The horizontal distancebetween tbe master articulator mark and each con-tacting test stylus mark could easily be measured.However, for the 2240-series instruments, no morethan two articulators in addition to the master couldbe recorded on one sheet of graph paper. The near-total superimposition of markings of the styli fromone articulator onto the other made the measuringof a horizontal discrepancy difticult. With more thanthree markings, the colors fused into one slightlylarger mark and could not be differentiated.Although there appeared to be slight horizontalplane discrepancies for some of the 2240-seriesinstruments, none was greater than 80 ^m, withalmost total superimposition of the marks for allrecordings. The slight discrepancies observedactually may bave occurred from excessive inkingofthe styius, resulting in a wick-like spreading effecton the graph paper. Even when measured horizon-tal discrepancies existed, no studies have reportedthe amount of horizontal discrepancy tbat would beclinically unacceptable. Since tbe diameter of tbemark made by each stylus tip was approximately250 um, tbe maximum discrepancy measured (80iim) appears inconsequential.

To simulate tbe clinical use of articulators in thevertical dimension, shimstock was employed in atear/tug technique as is used to adjust occiusal con-tacts. In this plane, all styli for the eigbt Model 2240

articulators gave equal "tug" resistance to shim-stock. Since the most discriminating vertical mea-surement of the reliability of cross-mountinginterchangeability was the 13-mm shimstock, italone was used by the second investigator. If a "tug"but no "tear" resistance was obtained from eachstylus of a test articulator, then vertical contact equalto the master was validated. If the shimstock eithertore or did not resist dislodgment when drawn fromunder a stylus, the vertical position was not the sameas the master and the articulator did not allow cross-mounting interchangeability.

Additional research is indicated, since other man-ufacturers have introduced semiadjustable articu-lators that are described as being interchangeable.Such rescarcb could permit comparison among theInstruments of various manufacturers.

Summary and Conclusions

1. A technique for evaluating the accuracy oftransfer between multiple articulators wasdescribed.

2. Tbe claims of interchangeability of the Whip-Mix Model 2240 semiadjustable articulatorappear valid. Horizontal deviations rangedfrom 0.00 to 0.08 mm, and no vertical dis-crepancies were detected.

3. Interchanging mountings between Whip-MixModel 8500, which is not designated as allow-ing interchanging, resulted in horizontal devia-tions ranging from 0.33 to 0.94 mm. Largevertical deviations were also recorded.

Appendix

Schematic of method for calculating discrepancydistance for non-graph paper contacting stylus(styli). Point A is the black mark made by the stylusof the master articulator. Point B is the color mark{s)made by the contacting stylus (styli) of the test artic-ulator. V is the vertical distance from the noncon-tacting stylus to the graph paper measured with thefeeler gauge. H is the average horizontal distancebetween tbe measured distances when two test stylicontacted the paper. If only one test stylus con-tacted the paper, this distance was used.

\

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ûngiriB Casis Will i Semiadjuslabie Arricuiaiors

References

!. Wachtei HC, Curtis DA: Limitalions of semijdjustable arlic-ulators, Part I: Straight line artitulators without netting forimmediate side shift. ) Prosthet Dent l987;5S:43e-442,

2. Curtii DA, Wachtel HC: Limitations of semiadjuslable artic-

ulators. Part li: Straight line articulators with provision forimmediate side shift. / Prosthet Dent 19ö7;SH.569-573.

3, Lundeen HC, Shryock EF: An evaluation oí mandibular bor-der movements: Their character and significance. / ProsthetDent 1970,40^442-452.

4. Cross MD, Cazit E: Articulators uswl in North Ameritandental schools. ( Dent Educ 1985:49:710-711,

Literature Abstract-

A Comparison of Panoramic Radiography WithComputed Tomography in the Planning of ImplantSurgery

This stuQy compared the accuracy of panoramic radiography and computed tomography |CT) in theevaluation of depth of the mandible at recipient implant sites. The distance between the edentuiousbony crest and the superior border of the inferior aiveoiar canal in a group of 10 patients weremeasured. Correction for distortion was calculated for the panoramic radiographs using a metai bailtechnique and for the computed tomography using a computer scaie. The precise distance betweenthe bony crest and inferior aiveoiar canai was determined from postoperative radiographs using theknown length of the implahts. Although CT proved statisticaily superior, both were sufficiently accuratefor routine ciihical purposes. Computed tomography does have an advantage over panoramicradiography ih that it demonstrates the width and shape of the ¡aw at the proposed implant site. It issuggested that panoramic radiographs are sufficiently accurate for patients in wfiom impiants ofsuitable iength can be piaced safely, but for those patients in whom the measured verticai iengths areinadequate and the option of placing impiants laterai to the inferior alveolar canai is considered, CTscanning should serve the surgical team as a comprehensive examination to acfiieve the optimumresuits.

Tal H, Moses 0. Deníomax/'to'ac i?acfio/1991:2Û(1);40-42. References; 11. Reprints: Dr Haim Tal, Department ofPer I odontology. The Maurice and Gabriela Goldschleger School of Dental Medicine, Tel Aviv University, Tel Aviv,Israel.—flicnard R. Seals, Jr. DDS. MEd. MS. The University of Tetas Healtti Science Canter at San Antonio. SanAntcr)io, Tanas

The International lourral of Prosthodonlics 264