Retention of dowels subjected to tensile and torsional forces

Ebtmtion forces

of dowels subjected to teruile and tomhaf

Dale R. Ruemping, D.D.S., M.S;D.,* Richard J. Schnell, D.D.S., M.S.*** Indiana

University

School of Dentistry,

Melvin Indianapolis,

R. Lund, D.M.D.,

M.S.,**

and

Ind.

M

ost of the designsused in restoring endodontitally treated teeth call for dowels and cores, and the criteria that have evolved are primarily based on general biologic and engineering principles. Unfortunately, the specific mechanical and physical properties required for an ideal dowel and core have not yet been defined. Until controlled clinical -mvestigations provide these guidelines, many materials and designsmust be evaluated in the laboratory. The forces that dowels must withstand are difficult to determine, and standards of evaluation are mainly empirical. Most investigators attribute failures either to the lack of strength of the dowel itself or to retentive problems. Retention studies performed in the laboratory have dealt with shape, diameter, and length, primarily under tensile loads, which partially duplicate the forces found in the mouth. Although a torsional test is not an exact duplication, it closely simulates one of the forces actually encountered clinically. This study evaluated retention of four commercial dowels under torsional and tensile forces. The purpose was to establishcriteria for selecting a dowel based on retentiveness.

LITERATURE

REVIEW

Black’ in 1869 advocated filling the root canal with gold foil and anchoring in it a threaded gold bolt which retained a denture tooth. Later dowel and crown techniques used a single unit.* Still later dowels were cast by either direct or indirect techniques. Other procedures employed a self-curing resin in the direct technique.3-7Precision became the key word in dowel construction. Attempts were *Major, USAF (DC). **Professor and Chairman, Department of Operative Dentistry. ***Associate Professor, Department of Dental Materials.

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Fig. I. Preparationof the roots. made to match a reamer, either hand-held or engine-held, to the dowel. The dowels could either be plastic cylinders or prefabricated stainlesssteel or platinized gold dowels. These techniques matched the dowel to a channel of predetermined size instead of matching it to the natural taper and contour of the existing tooth.* Kurers stated that a minimal thickness of dental cement is necessaryfor precise fit and proper retention. He said that having machined reamers and dowels minimizes the error. Criteria for successare dependent upon length, diameter, shape, and surface configuration of the dowel. The two main causesof dowel failure are (1) structural failure of the dowel itself and (2) loss of retention. Tests by Colley, Hampson, and Lehman”’ demonstrated the increased retention of parallel dowelsover tapered onesand roughened dowels over smooth ones. The choice of dental cement for a luting agent was of little consequence, as shown by Caputo and Hanson.”

METHODS

AND MATERIALS

All test specimens were prepared from sound human maxillary central incisors. The single canal and an adequate bulk of dentin atlowed standard dowel placement. The crown was severed at the cervix perpendicular to the long axis of the tooth.

159

RUEMPING,

Fig. 2. The four dowels shown with their matched reamers. l+om left to right: Tapering smooth-sided, parallel serrated-sided, parallel smooth-sided, and parallel threaded screw-in.

Fig. 3. Cage machine for length makes links reduce

apparatus used with the Riehle testing measuring tensile force. The extreme chain axial loading errors negligible. The multiple torsional stress.

The specimens were stored in distilled water, except during preparation, cementation, and testing. The roots were imbedded in self-curing acrylic resin, with the axis of the roots being parallel to the tensile force to be applied (Fig. 1). The channels were prepared with reamers provided in the commercial systems 160

Fig. 4. Specially constructed measured in ounce-inches.

LUND,

AND

torque apparatus.

SCHNELL

Torque

is

(Fig. 2). Both cast and prefabricated dowels were cemented with zinc phosphate cement under finger pressure. All specimens were stored for 2 weeks in tap water before testing. The tensile tests were conducted with a Riehle testing machine. A special cage apparatus was constructed to facilitate the testing (Fig. 3). The rate of load application was 0.02 inch/minute. The torsional tests were conducted with a specially built machine (Fig. 4). The torque or moment of force was measured in ounce-inches (applied load X lever arm length). The dowels were seated to a depth of either 5 or 8 mm. Four designs of dowels were tested (Fig. 2). These were: (1) Endowel,* size No. 140, tapering, smooth-sided dowels cast from a plastic form; (2) Parapostt parallel, serrated-sided dowels prefabricated in stainless steel; (3) Parapostt parallel, smooth-sided dowels cast from a plastic form; and (4) Kurer Crown Saver$ parallel, threaded dowels of prefabricated stainless steel. The diameters of all four dowels were within 0.003 inch of one another. All plastic dowels were invested and cast in type III gold in the manner prescribed by the manufacturer. In the torsional test the Kurer threaded dowels were screwed to place in a clockwise direction, with retention being measured in both clockwise and counterclockwise directions. This accounted for a fifth group in the torsional test. A total of 148 specimens were tested under both *Star Dental Mfg. Co., Conshohocken, Pa. tWhaledent International, New York, N. Y. SUnion Broach Co., Long Island City, N. Y.

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1979

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RETENTION

OF DOWELS

m

tapering, snwoth pxalklsmaxth

cl

pnallel.

serratd

parakl,

threaded

J

Fig. 5. Tensile loads (mean c SD) required dowel from channel (in pounds).

to separate

tensile and torsional forces. An analysis of variance and Newman-Keuls sequential range test were performed on the data.

RESULTS Table I shows the results for the different dowels after tensile force was applied. These results are also represented by the graph in Fig. 5. The analysis of variance indicated a significant difference among the four types of dowels. The Newman-Keuls test revealed that the parallel-threaded dowel was significantly more retentive (p < .Ol) than the other dowels with either smooth or serrated sides at both 5 and 8 mm embedment depths. Table II shows the results of the torsional tests. These results are represented in Fig. 6. Once again, the analysis of variance and Newman-Keuls revealed a difference between the dowels. The threaded and serrated dowels had values significantly higher (p < .Ol) than the smooth-sided dowels at 5 mm embedment depth. All dowels were significantly different from each other at 8 mm embedment depth THE JOURNAL

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DENTISTRY

Fig. 6. Torsional loads (mean t SD) required retention (in ounce-inches).

Table I. Tensile separate

load (pounds) from channel

dowel

required

for loss of

to

Dowel type Embedmerit depth

Tapering smoothsld%d

Par&%1 smmth-

28.7

38.8

Fadlel f3efrated-

ParaHel threaded SX%W-ill

5 mm (n = 8)

Mean SD 8 mm (n = 8) Mean SD

4.8 34.9 3.4

7.5 44.3 6.7

44.8 15.8 50.9 14.2

197.5 20.0 256.8 11.5

QJ < .05). On the average, the 8 mm dowels were only 1.20 times as retentive under tension and 1.28 times as retentive under torque as the 5 mm dowels. With regard to the screw-in dowels, it was determined by a t-test that there were no significant differences between a clockwise or a counterclockwise torque. 161

RUEMPING,

Table II. Torsional

load (ounce-inches)

required

LUND,

AND

SCHNELL

for loss of retention Dowel type

Emb+-thM 5 mm (n = 8) Mean SD 8 mm (a = 8) Mean SD

Tape*g smooth-sided I

ParaIM smooth-sided

Parallel serrated-sided

Parallel threaded scniw-in (clockwise)

6.20 0.98

10.60 2.00

14.30 5.90

16.90 1.80

15.80 4.20

7.60 0.35

12.00 0.80

19.60 4.30

22.80 2.10

21.00 2.00

DISCUSSlON The resuits indicate that surface configuration of the dowel is a more important variable than length. The depths of embedment yielded little overall difference in retention, with the 8 mm dowels only 1.23 times as retentive as the 5 mm dowels. The differences encountered with surface aherations agreed with those found by Standlee, Caputo, and Hanson” and Colley, Hampson, and Lehman.‘” These results suggesta marked increase in retention with roughened or serrated sides. The threaded screw-in dowels provided the greatest retention. The values needed for loss of retention under torque were much smaller than those under tension. This indicates that for increased resistance to dislodgement, the concentric-type dowels used in this study should incorporate an antirotational feature, such as auxilliary pins, grooves, or an irregular root face. The direction of torsional force is of little consequence,even with the screw-in type of dowel. An important clinical feature to consider is that loss of retention under torsional forces often results in an almost imperceptible movement as compared to the dramatic withdrawal of a dowel under tension. This movement can be a beginning of failure of the dowel. The results of this study indicate that retention under tension and torque is best provided by the screw type of dowel.

SUMMARY

AND CONCLUSIONS

This investigation measured the maximal tensile and torsional forces sustained by four different designs of dowels. 1. Under tensile force, the threaded screw-in dowels were significantly more retentive than the unthreaded dowels. 2. Under torque, both the threaded screw-in and serrated dowels were significantly‘ more retentive than the smooth-sided dowels. 162

Parallel threaded screw-in (counterclockwise)

3. Rotational direction of the applied torque was of no significance with the screw-in post, 4. Depth of embedment at 5 and 8 mm exerted lessinfluence upon retention than surface configuration. The 8 mm dowels were, on the average, 1.23 times as retentive as the 5 mm dowels. 5. Because of the relatively low values of forces causing dislodgement under torque, an antirotational lock should be incorporated into the dowels. REFERENCES I. 2.

3.

4. 5.

6. 7. 8. 9. 10.

11.

Black, G. V.: A method of grafting artiftcial crowns on roots of teeth MO Dent J 1:233, 1869. Baumhammers, A.: A simplified technique for one-unit cast dowel crown. Dent Dig f&468, 1962. Rosenberg, P. A.: GoId posts common problems in preparation and technique for fabrication. NY State Dent J 37:601, 1971. McPherson, J. L.: A simplified root dowel technique. J South Cahf State Dent Assoc 39:115, 1971. Jacoby, W. E., Jr.: Practical technique for the fabrication of a direct pattern for a post-core restoration. J PROSTH~F DENT 35~357, 1976. Asawa, G. N.: Cast dowel core fabrication on a pre-existing crown. Dent Surv 48:36, 1972. Stern, N.: A direct pattern technique for post and cores. J PROSTHET DENT 28:279, 1972. Perel, M. C.: Clinical criteria for post and core. .J PROSTHET DENT 28:405, 1972. Kurer, P. F.: The Kurer system for post crown preparation. J dntario Dent Assoc 45:57, 1968. Colley, I. T., Hampson, E. L., and Lehman, M. L.: Retention of post crown-an assessment of relative efficiency of posts of different shapes and sizes. Br Dent J 124:63, 1968. Caputo, A. A., and Hanson, E. C.: Cementing mediums and retentive characteristics of dowels. J PROSTHET DENT 32:55 1, 1974.

12.

Standlee, J. P., Caputo, A. A., and Hanson, tic dowels-effects of retentive parameters. Abst No. 912:B296, 1976.

Reprint

requests

E. C.: EndodonMIDR Dental

to:

DR. DALE R. RUEMPINC WILLFORD LACKLAND

HALL

AFB,

MEDICAL

CENTER

TEXAS 78236

FEBRUARY

1979

VOUlME41

NUMBER

z