ch 10

Chapter 10

ProximalHalf-Crowns

An especially perplexing problem forthe dentist is that of the tilted mandibu-lar molar abutment. In most preparationdesigns the path of insertion for thepreparation more or less parallels thelong axis of the tooth, while being per-pendicular to the plane of occlusion.This permits adequate resistance to oc-clusal loading while facilitating the seat-ing of the restoration.

When the prospective abutment toothhas tipped toward the edentulousspace, it is no longer possible for thepath of insertion of the abutment prep-aration to be both parallel to the longaxis of the tooth and perpendicular tothe plane of occlusion.

Several solutions have been offeredfor this problem. Whenever possible,the tooth is uprighted orthodontically topermit a favorable path of insertion withoptimum preparation retention and toeliminate uncleanable periodontal de-fects on the mesial aspect of the root.Brown reported a decrease of 3.1 mmin pocket depth by uprighting mesiallyinclined molars.1

If orthodontic treatment is not feasiblefor any reason, other solutions may beused. A telescopic crown retainer onthe bridge may be fitted over a castcoping on the tooth if the clinical crownof the tooth has suffered moderate tosevere destruction.2,3 This approach,utilizing two crowns, telescope, andcoping on the abutment, requires thedestruction of considerable tooth struc-

ture and should not be used to restoreminimally damaged teeth.

Another, less destructive retainerdesign for. the tilted abutment is theproximal half-crown,4-7 which is athree-quarter crown variant.8-10 In con-cept it is a three-quarter crown whichhas been rotated 90 degress so that aproximal surface, rather than the facial,is left unveneered.11 It can be em-ployed if the tooth has been damagedonly slightly. Two criteria must be met,however:

1. The distal surface must be caries- ,free.

2. There should be minimal interproxi-mal caries throughout the rest of themouth.

There is some risk in leaving the inter-proximal surface of a bridge abutmentunrestored. The risk can be minimizedby using this retainer design only inthose mouths where there is little historyof interproximal caries and thereforeless likelihood of future occurrence.

The proximal half-crown has' alsobeen described for use as a retainer onmandibular premolars, especially whenthe tooth is somewhat malpositioned. Insuch applications it minimizes thedisplay of metal.4

The sequence for preparing a tippedmandibular second molar for a proximalhalf-crown is shown in Figs. 10-1through 10-39. Clinical examples areshown in Figs. 10-40 through 10-48.

189

Proximal Half-Crowns

A

B

~

v

Fig. 10-1 In this typical situation, a mandibular molar bridge abutment is tipped mesially, creatingan exaggerated curve of Spee (A). It would not be possible to prepare this molar for a full crownwith a path of insertion paralleling that of the premolar abutment preparation. In mouths where condi-tions are favorable, a proximal half-crown can be utilized as a retainer on the molar, allowing the dis-tal surface of the tooth to remain untouched (B).

190

Proximal Half-Crowns

Fig. 10-2 Planar occlusal reduction: round-endtapered diamond and no. 171 bur.

Fig. 10-3 Begin the occlusal reduction by plac-ing depth-orientation grooves on the occlusalsurface with the round-end tapered diamond.

Fig. 10-4 On the distal aspect of the occlusalsurface the occlusal reduction, and therefore thedepth-orientation grooves, will be the normaldepth.

Fig. 10-5 The grooves and the reduction thatwill follow may not be as deep in the mesial por-tion of the occlusal surface, since this segmentof the tooth has dropped below the occlusalplane.

191

Proximal Half-Crowns

\ ,~~..~

~ f

1.. L'~..

(I

..

i

\

...

192

Fig. 10-6 The normal occlusal reduction will berequired if the tooth opposing the edentulousspace has supererupted into the space. Correc-tion of the occlusal plane to prevent occlusaldisharmony often will require placement of a res-toration with occlusal coverage on that opposingtooth.

....

Fig. 10-7 Using the round-end tapered dia-mond, remove the tooth structure remainingbetween the depth-orientation grooves to repro-duce the geometric 'planes of the occlusal sur-face.

Fig. 10-8 Functiooal cusp bevel: round-end ta-pered diamond and no. 171 bur.

:J

Fig. 10-9 Begin the functional cusp bevel byplacing depth-orientation grooves with theround-end tapered diamond. These grooves, as'Well as the bevel which will follow, often will beshorter and shallower on the mesial cusp thanon the distal. There will be less need for thebevel where the tooth has tipped below the oc-clusal plane, providing the opposing tooth haseither not supererupted or has been restoredback to the proper occlusal plane.

Proximal Half-Crowns

Fig. 10-10 Complete the functional cusp bevelwith the round-end tapered diamond, removingthe tooth structure between the depth-orientationgrooves.

Fig. 10-11mond.

Mesial axial reduction: torpedo dia-

Fig.10-12 Begin the axial reduction by makingthe path of insertion of the mesial surface paral-lel with the long axis of the premolar abutment.At this point the diamond instrument will makecontact with only a small area of the mesial sur-face just apical to the marginal ridge. Do not at-tempt to produce a mesial gingival finish line atthis point, or an undercut will be produced.

Fig. 10-13 Continue cutting the mesial surfacewith the torpedo diamond oriented with the even-tual path of insertion of the preparation. Enoughtooth structure will be removed so that the end ofthe diamond will eventually make contact withthe tooth and produce a chamfer finish line inthe gingival area of the mesial surface.

.,

."\

",.

~

~

uaIII

193

Proximal Half-Crowns

194

Fig. 10-14 Facial and lingual axial reductiontorpedo diamond.

Fig. 10-15 Use the same diamond to produceaxial reduction with a chamfer finish line on the

/ facial surface. End the extension 1.0 mm ormore mesial to the distofacial embrasure.Overextension distally will leave the verticalfinish line in a position where it will be difficult tocapture in the impression, hard to finish, and im-possible for the patient to keep clean.

Fig. 10-16 Since mesially tipped molars also. frequently exhibit some lingual inclination as

well, take care to keep the diamond as uprightas possible to avoid excessive facial wall inclina-tion and resultant loss of retention.

Fig. 10-17 Often there will be a sharp anglewhere the facial and mesial reductions meetRound this angle from facial to mesial, and makesure that the chamfer does not have a "scallop"or a rise occlusally at the angle.

.Proximal Half-Crowns

Fig. 10-18 Repeat the process on the lingualsurface, creating a definite chamfer finish line,and keeping the surface as upright as possible.Again, do not extend the vertical distal finish linetoo far into the distolingual embrasure.

Fig.10-19 Axial finishing: torpedo bur.

Fig. 10-20 Go over all three axial surfaces witha torpedo-shaped carbide finishing bur to pro-duce a precise, well-defined chamfer finish line.

~Fig. 10-21 Smooth the planes and angles ofthe occlusal surface with a no. 171 bur at thistime.

195

Proximal Half-Crowns

..,

t-~~

196

....

Fig. 10-22 Nowfinishthe functionalcusp bevelwith the same carbide bur. This finishing step isdelayed to this time because of the large quanti-ties of tooth structure removed during theuprighting of the mesial surface. Going over thefunctional cusp bevel now enables the operatorto better blend it with the other occlusal and ax-ial features of the preparation.. ...

Fjg. 10-23 Occlusal isthmus and countersink:no. 171 bur.

Fig. 10-24 The addition of an occlusal isthmusincreases bulk and rigidity in the casting as wellas providing much needed retention. Thisfeature is usually "automatic" inasmuch as mostprospective abutments will have either an olerestoration or caries in the central groove.

II

Fig. 10-25 A countersink is added to the distfossa with the no. 171 bur. This feature not onsupplements retention and resistance, but it aliprovides greater bulk to the casting in the criticarea near the distal occlusal margin.

Proximal Half-Crowns

Fig.10-26 Facial and lingual grooves: no. 171bur.

Fig.10-27 Place a groove on the facial surfacewithin 1.0 mm of the vertical distal extension ofthe preparation. The groove should parallel themesial surface of the tooth and the long axis ofthe other abutment tooth. The groove must alsobe upright faciolingually and should not lean tothe lingual.

Fig. 10-28 Repeat the process on the lingualsurface, parallel.ing that groove with the one onthe faciall surface. Be careful not to place it toofar distaUy.The groove may be started with a mo.170 bur and jinished with,the no. 171 to preventits becoming 100large.

Fig.10-29 Distal occlusal offset: no. 171 bur.

t

~

J~

I .~~J. - . if '

I~' y/I

" J

197

Proximal Half-Crowns

~l,

r

~-

..,

/

.A

198

J Fig. 10-30 Create a V-shaped offset 0.5 to 1.0mm from the distal occlusal finish line, connect-ing the lingual groove to the countersink to thefacial groove. It will produce a rigid staple withthe grooves to reinforce the distal marginal areaof the casting.

Fig. 10-31 Flares and occlusal bevel: flamediamond and flame bur.

Fig. 10-32 Place a flare distal to the facialgroove with the flame diamond. The flare will bea flat plane, wider at the occlusal end than at thegingival.

Fig. 10-33 Repeat the process on the lingualsurface, creating a flare distal to the lingualgroove, tying it in with the gingival chamfer.

Proximal Half-Crowns

Fig. 10-34 Cut a bevel along the distal mar-ginal ridge with the flame diamond, taking care

. not to extend into the distal occlusal embrasure,where the compromised location of the finish linewould jeopardize the success of the restoration.

Fig. 10-35 Round over the angles between thedistal occlusal bevel and the facial and lingualflares. Sharp angles in these areas will causesevere problems in the restoration margin.

Fig. 10-36 Go over both of the flares with theflame carbide bur to produce the sharpest finishline possible.

Fig. 10-37 Redo the occlusal bevel with theflame carbide bur.

//'

J

L"

--1

199

Proximal Half-Crowns

Fig. 10-38 The completed preparation for aproximal half-crown on a tilted mandibularsecond molar.

Fig. 10-39 The features of a proximal half-crown preparation on a mandibular molar, andthe function served by each.

CountersinkRetention and resistance.Structural durability

Bevel

Marginal integrity

Occlusal offset

Structural integrity

GrooveRetention and resistance

Structural durability

200

Planar occlusal reduction

Structural durability

IsthmusRetention and resis

Structural durability

Axial reductionRetention and resistance

:...;--Structural durability

.J Periodontalpreservation

:l~ ChamferMarginal integrityStructural durability

Proximal Half-Crowns

Fig. 10-40 A proximal half-crown was used onthis tipped mandibular second molar as a re-tainer for a fixed bridge. This bridge was aremake of one that had been in the mouth forfourteen years. The original proximal half-crownon this tooth was still intact and had to be sec-tioned to be removed. The retainer on "the otherabutment failed. An occlusal view is seen in themouth (left), and the cast is seen from a mesiofa-cial angle (right).

Fig. 10-41bridge.

A facial view of the completed

Fig.10-42 An occlusolingual view of the bridgeshows the conservative extensions of the oc-clusal and distolingual margins.

Fig. 10-43 A proximal half-crown was used onthis mandibular second molar to compensate forits mesial inclination. A full veneer crown wasutilized as a retainer on the second premolar.

~

"'"

201

Proximal Half-Crowns

I

t

,'Ii r

1'1

'3i

202

Fig. 10-44 In this mesiofacial view of the cast,the two grooves on each of the facial and lingualsurfaces are clearly evident.

Fig. 10-45 This is the completed all-metalbridge fabricated for the preparations seen inFigs. 10-43 and 10-44.

Fig. 10-46 A proximal half-crown was selectedas a retainer for this tipped mandibular premolar.The preparation is seen here from the facial side(left) and the occlusal side (right).

Fig. 10-47 A stone cast of the half crowpreparation on the first premolar.

Proximal Half-Crowns

Fig. 10-48 Mesiofacial (left) and occlusal(right) views of the cemented retainer on thepremolar reveal less metal display than onemight expect. It is less than that of a standardthree-quarter crown on this tooth.

IIiI:I

....

203

Proximal Half-Crowns

References

1. Brown, I. W. The effect of orthodontic therapy oncertain types of periodontal defects. I. Clinicalfindings. J. Periodontol. 44:742,1973.

2. Eisenbrand, G. F. A method for constructing a fixedbridge with an extremely tipped abutment. Dent.Digest 68:514,1962.

3. Shillingburg, H. T., and Fisher, D. W. Bridge re-tainers for tilted abutments. N. M. Dent. J. 22:16,1972.

4. Smith, D.E. Abutment preparations. J. Am. Dent.Assoc. 18:2063, 1931.

5. Potter, H. R., and Smith, D. E. Practical bridgework.III. Non-vital teeth in bridgework. Pac. Dent. Gaz.40:51.9,1932.

204

6. Klaffenbach, A. O. An analytic study of modernabutments. J. Am. Dent. Assoc. 23:2275, 1936.

7. Ingraham, R., Bassett, R. W., and Koser, J. R. AnAtlas of Cast Gold Procedures. 2nd ed. BuenaPark, CA: Unitro College Press, 1969, 164.

8. Smith, D. E. Fixed bridge restorations with the tiltedmandibular second or third molar as an abutment.J. South. Calif. Dent. Assoc. 6:131,1939.

9. Schwartz, J. R. The basic or structural characterof abutment preparations. Dent. Items Interest56:897, 1934.

10. Willey, R. E. The preparation of abutments forveneer retainers. J. Am. Dent. Assoc. 53:141, 1956.

11. Shillingburg, H. T., and Fisher, D. W. The partialveneer restoration. Aust. Dent. J. 17:411, 1972.