Prosthodontics

Corrosion of a nonprecious metal post: a case reportKhue Quan Luu"' / Richard T. Walker* *

This paper describes a case in which an endodontically treated molar, which wasrestored with a post-retained cast restoration, was eventually lost following corrosionof the nonpreciotis metal post and fracture of the distal root.(Quintessence Int 1992:23:389-392.)

Introdnction

Posts made of nonprecious metals are commonly usedfor the retention of artifieial crowns and large dentalrestorations following root canal therapy.'"^ Such postsare prefabricated and convenient to use, because theyrequire minimal preparation. They provide an eco-nomical alternative to custom-fitted posts made ofprecious metals. It has been claimed, however, thatthese posts can corrode over time, leading to fractureof the roots into whieh they have been inserted. ^^

Corrosion in connection with root fractures of post-retained restorations has been studied by several au-thors.' Dreyer-Jorgensen^ described tbe corrosion ofstainless steel pins as a causal factor of root fractures.Such fraetures, which sooner or later result inperiodontal infection, are either longitudinal oroblique, never transverse.'"'^ These findings havebeen confirmed by Rud et al" wbo used radiographsto traee the radiopaque corrosion products in andaround the fractured root. Angmar-Mansson et al'"made a comprehensive analysis of tbe metals involvedand their occurrence on posts and fractured surfaces.They found that when materials with different elec-

troebemical characteristics are used together, such asin nonprecious alloys, eorrosion often occurs, leadingto complications.'"

Most prefabricated posts available today are stain-less steei or consist mainly of chromium, nickel, andother metals,'^ In such systems, when metals of differ-ent potentials are in contact with one another, sur-rounded by an eleetrolyte sueh as sodium chloride, anelectric circuit can form, often leading to a corrosiveattack of the less-noble metal with a lower poten-jjgl x.iii.i.i Ultimately, galvanic corrosion occurs. Silnesset al' have further proposed that the corrosion prod-ucts often migrate into the adjacent dentinai tubules,obliterating them and gradually building up intratubu-lar pressures. These pressures may exceed the strengthof the root itself, resulting in fraetures, which in turnallow corrosion products to continue to migrate intothe periodontal space, leading to periodontal compli-cations.

The prognosis of these teeth is usually poor. Hemi-section and root amputation are considered when cir-cumstances are favorable, but most frequently extrac-tion is tbe treatment of ehoiee. This report describes acase in which corrosion of a post-retained cast restora-tion following root canal therapy occurred in conjunc-tion with the fracture of a root.

" Liitturer, Deparlmcn! ol Conservative Dentistry, University ofHong Kong, Faculty of Dentistry, Prince Phiiip Dental Hospi-tal, 34 Hospital Road, Hong Kong.

'* Reader and Former Head, Department of Conservative Den-tistry, Universily of Hong Kong.

Address all correspondence to Dr R. T. Walker, 1 Lisier House,11/12 Wimpole Street, London WIM 7AB, England.

Case report

The patient, a 37-year-old white man, was firstexamined on August 9, 1982. A periapical radiographwas taken of the mandibular left second molar, whichhad been endodontically treated and restored with apost inserted into the distal canal (Fig 1). The post

Quintessence Internationai Voiume 23, Number 6/1992 389

Prosthodontics

Fig 1 Preoperative radiograph of the mandibular ieftmoiar.

Fig 2 Radiograph taken 3 years later shows evidence ofcorrosion of post, as well as a iongitudinal fracture of thedistai aspect of the distal root.

Fig 3 Sixteen months later, the fractured fragment is com-pletely separated from the root aiong the whole length ofthe fragment.

provided radicular retention for an amalgam core anda cast gold restoration. The restoration had been inplace for more than 5 years. The patient had no med-ical history of dental significance and no immediatedental complaints.

Close examination of the periapical radiographtaken in 1982 revealed that the lamina dura of thetooth was intact on the interradicular surface of bothroots and on the mesial surface of the mesial root, butonly on the coronal half of the distal surface of thedistal root. No explanation eould be given for thisappearance at that time.

On February 20, 1984, another bitewing radiographwas taken of the tooth as part of routine examination.A loss of lamina dura, from the bifurcation down

to the midpoint of the mesial surface and at the mid-point of the distal surface of the distal root, was noted.Because the apical half of the distal root was notrecorded in this radiograph, it was assumed that theloss of lamina dura extended apically from the mid-point onward. Associated with this loss of lamina durawas a mild degree of rarefying osteitis adjacent to thedistal root. The post itself showed evidenee of dissolu-tion, and the luting material adjaeent to the post wasno longer homogeneous in appearance.

On August 8, 1985, the patient complained of aspontaneous pain restricted to the left side of his face.The pain had started 10 to 14 days previously and hadintensified in the last 5 days prior to the visit. No his-tory of trauma was ehcited, and no significant findingswere noted on visual examination. Radiographieevaluation, however, revealed evidence of corrosion ofthe post in the distal root extending to the middlethird of the post. Furthermore, a longitudinal fraetureof the distal aspect of the distal root, extending fromthe alveolar crest to two thirds the length of the root,was noted (Fig 2). The patient was not prepared tohave the tooth extracted at that time.

The patient was not seen again until December 31,1986, when he reported that the pain had subsided toa dull sensation. Bitewing and periapical radiographswere taken of the tooth to monitor the corrosion. Noadditional change was noted in the post itself or inthe interradieular region. The fractured fragment,however, was completely separated from the rootalong the whole length of the fragment (Fig 3).

On February 10, 1987, the patient reported that thepain had become persistent; it was described as aspontaneous and continuous pain that radiated to the

390 Quintessence Internationai Volume 23, Number 6/1992

Prosthodontics

Fig 5 The fractured fragment is compieteiy separatedfrom the root, which in turn is covered with iarge depositsof biack material.

Fig 4 (ieft) The extracted tooth reveals a severely discoloreddistal root.

left side cf bis bead, as v>'ell as the mandibuiar region.On clinical evaluation, an U-mm distal pocket waslocated in relation to tbe distal root of tbe molar.

Because of the periodontal involvement and pres-ence of the fracture, tbe prognosis of the tooth wasdeemed to be poor, and the tooth was extracted onAugust 10, 1987. The distal root of the molar wasseverely discolored (Fig 4). A longitudinal fracture ofthe distal root was noted; the fragment was completelyseparated from the root. Large deposits of blackmaterial were adjacent to and covering the fracturesite (Fig 5). The post was identified as a Wiptamnickel-chromium post (Fried Krupp).

Discussion

The present case report is an example of a root frac-ture and accompanying corrosion of a nonpreciouspost. The post was identified as a nickel-chromiumalloy. Such an alloy system will readily corrode in thepresence of an electrolyte.'"' The oral cavity serves asan ideal source of electrolytes. The question thatneeds to be addressed is, "How do the electrolytes gainaccess to tbe post to initiate the corrosion process?"There are three possible explanations. It may be pos-tulated that the electrolyte gains access to tbe post viathe eementum and dentin covering the root surface.Linden '̂' showed that the root cementum and dentin of

extracted teeth are permeable to both water andsahne. The second explanation would be that micro-leakage around the coronal restoration acts as a pos-sible conductor of electrolytes to the post. The niostplausible explanation is that tbe fracture in the distalroot was produced during post preparation or cemen-tation. Once initiated, the fracture provided a naturalentrance for tbe ingress of tissue fluids, leading to tbedissolution of the post, the buildup of corrosion prod-ucts, and eventual separation of the fractured portionof tbe distal root.

Silness et al' bave proposed a mechanism of corro-sion that may be responsible for tbe fracture of teethrestored with stainless steel posts. In the initial phaseof corrosion, tbe products, which consist mainly oftin, are formed on the suriace of the post. Thisaccounts for the appearance of the corroded post(see Fig 4). These products migrate to the availabledentinai tubules in the post-canal walls, as weU asadjacent root restoration, leading to a pressure build-up as well as staining of dentinai structures. Wbenthe tubules have been completely filled, and can nolonger absorb tbe pressure exerted by the growing cor-rosion products, fracture of the root lnay result. Tbesefractures have always been found to be longitudinal oroblique, wbicb coincides witb tbe patterti of growtband distribution of tbe corrosion products. Tbe pro-cess continues after the fracture has occurred, deposit-

Quintessence International Volume 23, Number 6/1992 39T

Prosthodontics

ing a second layer of corrosion products and some-

times displacing the fractured fragment, as in the case

described. Ultimately, the corrosion products migrate

into the periodontal space and cause pain and discom-

fort to the patient.

Conclusion

When materials with different electrochemical charac-

teristics are used as pins or posts for the retention of

cast restorations, corrosion may result in complica-

tions.

References

1. Standlec JP. CaputoAA. Collard EW, et al: Analysis of stressdistribution by endodontic posts. Oral Surg Oral Med OralPaihol 1972;33:952-960.

2. Hudis SE, Goldstein GR: Restoration of endodonticallytreated teetb: a review of the literature. .' Prosiliei Denl1986;55:33-38.

3. Silness J, et al: Distribution of corrosion products in teethrestored with metal crowns retained by stainless stt'cl posts.Scand Dent J I979;37:317-321.

4. Lewis R, Smith BG: A dinical survey of failed post retainedcrowns. Br Dem J 1988; 165:95-97.

5. Kurer PF: A clinical survey ot failed post retained crowns. BrDeniJ l9SS;lfi5:202.

6. Hunter A, Flood A: Tbe restoration of endodontically treatedteeth. I. Treatment planning and restorative principles. AmiD 7 19S833481490

7. Espevik S: Corrosion of base metal alloys in vitro. Scand Dem

8. Sarkar NK, Greener EH; In vitro eorrosion resistance of newdental alloys. Biomarer Mea Dev An Org 1973:1:121-129.

9. Dreycr-Jorgensen K: Korrosionssproengnin av rpddes. Tand-laegebtadet 1955;59:92y.

10. Angmar-Mansson B, Qmnell KA, Rud J: Root fractures dueto eorrosion. 1. Metallurgical aspects. Udunloi Rev1969;20:245-265.

11. Ruti J. Qmnell KA: Root fractures due to corrosion. Diag-nostic aspects. Scand J Dem Res 1970;78:397-403.

12. Meyer JM, Nally JN: Corrosion resistance of commercial Ni-Cr dental casting alloy. / Denl Res 1975:34(special issue):L1I3 (abstr No. 452).

1.1. Phillips RW: Skinners Scienee of Denial MaleriaL. Philadel-phia. WB Saunders Co, 19H2, pp 121-129.

Linden LA: Microscopic observations of fluid flow throughcementum and dentiti. An in vitro study on human tiictb.Odiintol Rev t968;19:367. D

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