sECTIONEDITORS

ments

for overdentures

with

Douglas B. Evans, DDS, MS,a and Raymond G. Koeppen, Wilford Hall U.S. Air Force Medical Center, San Antonio, Tex.

DDS,

nonparallel MSb

Bar attachments are easily fabricated and provide increased retention and support for overdentures. Nonparallel root abutments can preclude the routine use of bar attachments unless modifications are made in the design. This article reviews several existing techniques to successfully fabricate bar attachments in overdenture patients with nonparallel abutments. In addition, three alternative methods are presented for placing bar attachments on abutments with divergent roots. (J PROSTHET DENT 1992;68:6-11.)

The views expressed in this article are those of the authors and do not necessarily reflect the views of the United States Air Force or the Department of Defense. aMajor, U.S. Air Force, DC; Chief, Fixed Prosthodontics, Department of General Dentistry. bLieutenant Colonel, U.S. Air Force, DC; Chief, Removable Prosthodontics, Department of General Dentistry. 161Il36281

Fig. 1. Components of Schubiger attachment system. A, Cap nut for locking assembly in place. B, Receptacle sleeve for soldering or direct casting to bar. C, Attachment soldering base for soldering to cast dowel-copings. Assembled attachment height is 2.8 mm.

6

R

etaining teeth for overdentures has long been accepted as a means of maintaining alveolar bone,‘, 2 preserving proprioception and sensory function2 and increasing support of occlusal loads. 2, 3 Bar attachments often are incorporated because they provide a splinting mechanism between the overdenture abutment teeth and increase the stability and retention of the prosthesis2, 4, 5 Bar-attached overdentures have been shown to increase load tolerance 120 % in anterior regions and 30% in posterior regions due to their stabilizing effect.6 A common bar attachment assembly for overdentures is the one-piece cast bar, connecting two dowel-copings that are permanently luted to parallel, endodontically treated mandibular canines. Unfortunately, these abutments frequently are divergent. When parallelism is close, the root

Fig. 2. Tube and screw system. Tube is cast within el-coping.

JULY1992

VOLUME@

dow-

NUMBER1

BAR

ATTACHMENTS

FOR

OVERDENTURES

canals may be enlarged to allow the dowel-copings to draw. However, this procedure drastically reduces dowel retention and may produce abutments with thin, weakened canal walls.7 Alternative techniques and/or retentive devices are therefore required for the placement of bar attachments to teeth with nonparallel roots. This article reviews techniques for the placement of bar attachments in overdenture patients with divergent roots and presents three alternative designs. LITERATURE

REVIEW

One of the early methods of constructing a bar attachment for nonparallel abutment teeth was described by Marquard@; it used the Schubiger screw system (Cendres and Metaux SA, Bienne, Switzerland). The attachment complex consists of a threaded stud on a base that is soldered to the dowel-coping. A metal cylindrical sleeve slides over the threaded screw projection and is retained by a screw locknut (Fig. 1). The bar is then soldered or cast directly to the metal sleeve.2, 5,8 The threaded screw base must be made parallel to the other abutment with a surveyor before soldering. The Schubiger serew system may be used at both ends of the bar or at only one end with a conventional dowel-coping attached to the other end. In situations in which only one Schubiger screw is used, the stronger abutment should house the screw system. If the weaker abutment later requires extraction, the Schubiger system allows the bar assembly to be unscrewed and substituted with Gerber individual stud units (Cendres and Metaux SA).2, 5,8,g In 1981, Matsuo and ShimegiiO described a system in which cemented posts could be internally tapped to receive screws that retain a cast bar. With this technique, a dowel is cast to the internally threaded tube and then cemented within the endodontically treated root. The bar is then soldered to a metal collar and screwed into place. Newer screw systems bave become commercially available (Attachments International, Inc., San Mateo, Calif.), with the major components consisting of a tube, collar, screw, and waxing sleeve (Fig. 2). The cast-to tubes are available in either iridium-platinum or high-heat metal ceramic alloy. The collars are available as prefabricated, cast-to, metal ceramic alloy rings or as full-length plastic patterns incorporated with the waxing extension sleeves. The plastic waxing extension sleeves are cast to the bar and enable the assembly to be secured to the dowel-tubes with gold or titanium screws. Screw diameters range from 0.8 to 1.6 mm, with the tube providing an additional 0.6 mm. Total heights range from 6.0 to 10.3 mm. Tube and screw systems require abutments with sufficient root length and diameter to prevent tooth fracture. In 1985, Preiske15 described a dowel with a threaded screw hole tapped in the center. After the dowels are cemented into the roots, a bar is attached by screws threaded through diaphragms into the dowels.

THE

JOURNAL

OF

PROSTHETIC

DENTISTRY

Fig.

3. Hruska screw block and retaining screw.

The Hruska screw block (Cendresand Metaux SA) for bar attachments with divergent roots was described by Preiske15and Matsuo and Shimegi’O(Fig. 3). This assembly consistsof a preciousmetal block with a threaded screw hole that is soldered to a dowel-coping. The cast bar suprastructure is retained by a screw threaded into the block unit. However, the volume of the block metal may compromiseremaining abutment tooth structure and interocclusal space. Mensorg,l1 reported that the Kurer coping retaining screw (Teledyne-Getz, Elk Grove Village, Ill.) accurately related a bar system to divergent abutments. With this technique a bar is castor solderedto an open-facedcoping. The bar and copingcomplex is securedto the abutment by first tapping the canal, then threading and cementing the fixation screwthrough the coping into the root (Fig. 4). The manufacturer recommendsthe coping retaining screw be adjusted (1) for length before cementation, and (2) for uniformity between the flat screwhead and the coping after cementation. The need for clinical adjustment of the retaining screw may jeopardize bar assembly precision. The addition of a countersunk ledgewithin the coping to recessthe screw head would minimize postcementation grinding. Becausethe Kurer coping retaining screwsare

7

EVANS

Fig. 4. Root canals are tapped. Kurer threaded screwed and cemented into canals.

posts are passed through

ALTERNATIVE

AND

ROEPPEN

the copings, then

METHODS

In addition to the techniques and devices previously described, three additional methods are offered for placing bar attachments on nonparallel abutments. They include (1) individual dowels with surveyed cores to which copings and a cast bar are permanently luted, (2) a bar and dowelcoping unit cemented into one abutment with an interlocking, permanently cemented dowel placed through the bar into the second abutment, and (3) internally tapped and threaded cast dowel and cores to which a bar-coping is attached with gold~qcrews.

Parallel Fig. 5. Individual dowels with parallel cores. Parallelism of cores and grooves is verified with surveyor before permanent cementation.

threaded directly into dentin, they provide greater retention than cast dowels.12 However, this increased retention must be weighed against the possibility of detrimental tooth fracture.13 In 1989, Render and JenningsI described a bar-clip attachment for divergent roots using a two-piece system. The first portion combines a dowel-coping and a bar that are cast as one unit. The bar has a 2 mm attachment component at its distal end. A cast dowel-coping with a 2 mm receptacle is made for the second abutment tooth. The combined dowel-coping and bar unit is fitted to the dowelcoping with the receptacle. Each is then permanently luted within its respective abutment root. This technique does not provide a true splinting effect between the abutments. Farish et a1.15 reported the use of a Ney (J. M. Ney Co., Bloomfield, Conn.) dovetail, semiprecision attachment to orient a bar suprastructure to nonparallel screw implants. One end of the bar is rigidly attached to the implants with a screw assembly; the other end of the bar fits passively within the semiprecision attachment. Neither bar torque nor occlusogingival movement was addressed, and detrimental stresses may be transferred to the abutments. 8

core technique

In this technique dowels with parallel cores are placed in the individual abutments. First, each divergent root is prepared according to standard considerations for length, width, and conservation of tooth structure. Radiographs are made to ensure that a minimum of 3 mm,16 but preferably 5 mrn,17, l8 of gutta-percha remains as an apical seal. Occlusal counterbevels and antirotational keyways are recommended for additional support and stability of the dowel-cores.17 A nonaqueous elastomeric impression is made of the prepared teeth for the indirect fabrication of the castings. A surveyor is used to parallel the two waxed cores. Grooves or boxes may be placed in the cores to increase resistance and retention of the copings. The individual cast dowel-cores are permanently luted within the roots (Fig. 5). A second impression is made to fabricate a one-piece bar-coping assembly that is subsequently cemented over the parallel cores (Fig. 6). Coping margins may be placed either on abutment tooth structure or on the metal of the cores. The decision on margin placement will depend on remaining tooth structure, degree of divergence of the residual roots, and height of the core. This technique is indicated for patients with relatively long roots, good interarch distance, and sufficient anterior labiolingual dimension to accommodate the additional bulk of the prosthesis. The completed bar attachment assembly offers easy access and smooth, rounded contours to facilitate oral hygiene maintenance and promote tissue health (Fig. 7). JULY

1992

VOLUME

68

NUMBER

1

BAR

ATTACHMENTS

FOR

OVERDENTURES

Fig. 6. Bar attachment is fitted to parallel cores. Groove placement provides increased resistance and retention. Fig. 7. One-piece bar-coping assembly permanently luted to parallel cores. Round bar is cast from 1X-gauge plastic sprue. Fig. 8. First component of two-piece, interlocking cast bar and dowel-coping complex. Coping sleeve allows access to canal, provides vertical stop, and offers antirotational design. Fig. 9. Two-piece, interlocking bar attachment assembly ready for trial fitting. Cast handle will be removed after cementation.

Interlocking

bar assembly

Bar attachments for nonparallel roots also can be made with a two-piece, cast bar and dowel-coping assembly. One piece consists of a coping sleeve, a bar, and dowel-coping unit that is permanently luted into one abutment. The second component is a cast dowel that fits through to interlock the custom sleeve and is permanently cemented into the other abutment. The nonparallel abutments are prepared as described previously with concern for proper canal dimensions, occlusal counterbevels, and antirotational elements. A conventional dowel and coping are waxed on one abutment of a cast derived from an elastomeric impression. A 12-gauge, plastic bar pattern is attached to the coping. On the opposite end of the bar, a coping sleeve is waxed to fit the other abutment. The sleeve permits access to the canal. A keyway is made to supply resistance to rotation. A bevel is placed on the perimeter of the sleeve to ensure the marginal seal between the coping sleeve and subsequent dowel. This bevel also provides a vertical stop. An alternative vertical stop can be achieved by placing a countersunk ledge within the sleeve. The bar THE

JOURNAL

OF PROSTHETIC

DENTISTRY

complex is then cast in type IV gold and fitted in the laboratory (Fig. 8). A dowel pattern is waxed through the coping sleeve into the canal. The margins are waxed to the bevels of the coping sleeve. The sprue may be retained to provide a handle during fitting and then removed after cementation (Fig. 9). Finishing of the gold interface between the dowel and coping sleeve will produce a smooth, plaque-resistant surface (Fig. 10). The primary indication for this design is limited interarch distance. The reduced height of the suprastructure lever arm is especially beneficial for short-rooted abutments.

Threaded

core technique

The third design uses cast dowel and cores that are internally tapped and threaded. A bar-coping unit is later attached to the individual dowel and cores with gold screws (Fig. 11). Initially, the abutments are prepared for cast dowel and cores. Sufficient space within the canal, particularly in the mesial-distal dimension, is needed to accommodate both the screw and the surrounding gold of the dowel. A master cast is obtained, and individual dowel and 9

EVANS

AND

KOEPPEN

Fig. 10. Intraoral view of permanently luted, two-piece, interlocking bar attachment and dowel-coping assembly. Fig. 11. Threaded dowel and cores with screw-retained bar and coping sleeves. Branemark gold retention clips are positioned on bar to verify fit and path of insertion before resin processing. Assembly will be inserted with immediate overdenture. Fig. 12. Components of screw-retained bar attachment to threaded dowel-cores. Fig. 13. Intraoral view of screw-retained bar attachment 2 weeks after delivery of immediate overdenture.

cores are made. The cast dowel and cores are threaded for screws. Implant abutment screws are readily available and provide excellent precision and strength for fastening. Gold screws (Branemark System, Nobelpharma, Gothenburg, Sweden) of 1.4 mm thread diameter and 1.4 x 0.30 pitch were used in this instance. Tapping instruments are available through dental supply retailers (Attachments International, Inc.) and most jewelry suppliers. A round carbide bur is used to place a dimple on each dowel and core for the site of thread location. A hole is drilled to the length of the screw with a twist drill (TMS Thread Mate Systems, Whaledent, Inc., New York, N.Y.) that is at least 0.2 mm less in diameter than the screw. The proper tap size and holder are selected to tap the dowel either manually or with a milling instrument. A small amount of tapping oil is placed into the hole before the tap is turned two rotations clockwise and one rotation counterclockwise. This sequence is repeated until the thread is cut to the depth of the screw. If the tap breaks inside the dowel, pickle and ultrasonically clean them to remove debris from the threads. The tap then can be easily reversed from the dowel. After 10

the tapping process is completed, a 12-gauge plastic bar with coping sleeves is waxed to the dowel-copings and around the screws. The sleeves are not threaded but are countersunk to receive the screw heads. The bar attachment components are seen in Fig. 12. The individual cast dowel and cores are cemented within their respective abutments. The bar assembly is positioned over the dowelcores and secured with the screws. This technique is indicated for patients with ample interocclusal space and periodontally healthy abutments with good bony support. Despite the additional height requirement for this assembly, the resulting distance between the bar and tissue may reduce cervical plaque accumulation and mucosal proliferation (Fig. 13). One major advantage of this technique is that the complex can be disassembled for access to the abutments. SUMMARY This article has reviewed several traditional techniques for the fabrication of overdenture retention bars for patients with nonparallel abutments. Three alternative methJULY

1992

VOLUME

68

NUMBER

1

BAR

ATTACHMENTS

FOR

OVERDENTURES

ods have been presented. Two of these methods require no additional materials or equipment other than those routinely found in the dental laboratory. The third method requires only a twist drill, screw tap, and implant screws. These alternative techniques should be considered when planning for bar attachments to nonparallel abutments because of their simple design, readily available components, and relatively inexpensive approach.

7. Trabert KC, Caputo AA, Abou-Rass M. Tooth fracture-A comparison of endodontic and restorative treatments. J Endodont 1978;4:341-45. 8. Marquardt GL. Dolder bar joint mandibular overdenture: A technique for nonparallel abutment teeth. J PROSTHET DENT 1976;36:101-11. 9. Mensor MC Jr. Attachments for the overdenture. In: Brewer AA, Morrow RM, eds. Overdentures. 2nd ed. St Louis: CV Mosby Co, 1980:20851. 10. Matsuo E, Shimegi S. Screw-retained dental prostheses. Chicago: Quintessence Publishing Co, 1981:16-44, 89-97. 11. Mensor MC Jr. Attachments for overdentures. In: Rudd KD, Morrow RM, Rhoads JE, eds. Dental laboratory procedures: Removable partial dentures. Vol 3. 2nd ed. St Louis: CV Mosby Co, 1986:577-616. 12. Ruemping DR, Lund MR, Schnell RJ. Retention of dowels subjected to tensile and torsional forces. J PROSTHET DENT 1979;41:159-62. 13. Standlee JP, Caputo AA, Collard EW, Pollack MH. Analysis of stress distribution by endodontic posts. Oral Surg 1972;33:952-60. 14. Render PJ, Jennings DE. Simplified bar-clip attachment for an overdenture patient with divergent roots. J PROSTHET DENT 1989;61:127-8. 15. Farish SM, Pansch JL, Sherwood RL. Prosthetic management of unparallel titanium plasma-sprayed screw implants. J PROSTHET DENT 1989;61:129-32. 16. Dykema RW, Goodacre CJ, Phillips RW. Johnston’s modern practice in fixed prosthodontics. 4th ed. Philadelphia: WB Saunders Co, 1986: 364-78. 17. Shillingburg HT, Hobo S, Whitsett LD. Fundamentals of fixed prosthodontics. 2nd ed. Chicago: Quintessence Publishing Co, 1981: 143-59. 18. Rosensteil SF, Land MF, Fujimoto J. Contemporary fixed prosthodontics. St Louis: CV Mosby Co, 1988:198-217.

Our special thanks to Dr. Jeanne Hansen-Bayless for her clinical contributions and to Mr. Kenneth Kemple for his laboratory support. REFERENCES 1. Crum RJ, Rooney GE. Alveolar bone loss in overdentures: A 5 year study. J PROSTHET DENT 1978,40:610-13. 2. DeFranco RL. Overdentures. In: Winkler S, ed. Essentials of complete denture prosthodontics. 2nd ed. Littleton, Massachusetts: PSG Publishing Co, 1988:384-402. 3. Crum RJ. Rationale for the retention of teeth for overdentures. In: Brewer AA, Morrow RM, eds. Overdentures. 2nd ed. St Louis: CV Mosby Co, 1980:3-11. 4. Renner RP. Complete dentures: A guide for patient treatment. New York: Masson Publishing USA, 1981:257-71. 5. Preiskel HW. Precision attachments in prosthodontics: Overdentures and telescopic prostheses. Vol2. Chicago: Quintessence Publishing Co, 1985:243-306,360-51. 6. Caputo AA, Standlee JP. Biomechanics in clinical dentistry. Chicago: Quintessence Publishing Co, 1987:178-80.

he design Change-xi

of a two-part Shi,

DDSa

The Fourth Military China

Ji-hua

Chen,

Reprint requests to: DR. DOUGLAS B. EVANS 11350 WOODRIDGE FOREST SAN ANTONIO, TX 78249

acid-etched PhD,b

and

Medical University, Stomatological

He-ying

resin-bonded

fixed

YuanC

Hospital, Xian, The People’s Republic of

A new framework design is presented for resin-bonded, acid-etched, fixed partial dentures. This innovative design divided the restoration into two parts, each with a separate path of insertion. The preparation of abutment teeth required minimal reduction and the pontic could not be displaced during function. Thirty-five patients with missing posterior teeth have been treated with this imaginative approach and after approximately 2 years, there have been no debonding failures. (J PROSTHET

DENT1992;68:11-15.)

abrication of the first acid-etched, solid resinbonded fixed partial denture (FPD) in 1980 at the University of Maryland introduced a new era in conservative tooth rep1acement.l Several investigators have reported results

aProfessor, Prosthodontic Department. “Doctor, Prosthodontic Department. “Technician-in-charge, Prosthodontic Department. 10/l/35254 THE

JOURNAL

OF PROSTHETIC

DENTISTRY

on bonding between the restoration and abutment teeth. These include the appropriate treatment of the castings and tooth enamel, improvement in the design of the FPD framework, and tooth preparation. To improve retention of the FPD to the enamel of the abutment, authors have suggested the preparation of grooves, boxes, three-quarter or complete crowns, inlays, onlays, and pins in the abutment, including expansion of the wings.ss6 Clinically, the framework design must observe the basic 11