Abstract Objectives: The aim of this study was to evalu-ate the frequency of technical failure rates and the main-tenance costs of clasp- or double crown-retained remov-able partial dentures (RPDs) (parallel-sided and conicaldouble crowns). Material and methods: According tothree different retainer systems used between January1992 to December 1998, three subgroups were randomlyassigned from a group of 181 patients consisting offorty cases each. The average observation time was4.2±1.7 years, with a minimum of 1.0 years and a maxi-mum of 6.9 years. Technical complications such as frac-tures of the artificial teeth, loss of cementation, loss offacings as well as fractures of the metal framework andthe soldering were recorded due to the medical reports.Results. Twenty percent of all clasp-retained denturesshowed technical complications during the observationperiod, whereas 50% of conical crown-retained (CC)and 32.5% of the parallel-sided retained dentures (PS)required repair. Most often reported was loss of cementa-tion for double crowns (n=13, 32.5% PS; n=8, 20% CC)and fractures of the clasps (n=5). Although clasp-retained dentures had a markedly lower frequency offailures (n=8) than double crown systems (n=31), thecalculated repair costs per event during the observationtime were more than twice as high for clasp-retaineddentures (172.5 Euro) than for double crown systems(8–78 Euro). The hypothesis that the maintenance costsof the more sophisticated double crown system are highermust be rejected.

Keywords Technical failure · Removable partial denture ·Maintenance costs · Double crown · Clasp retainer

Introduction

An unfavorable topography and a heterogeneous prognosisare often combined in dentitions with few remaining teethand are indications for the use of removable partial den-tures (RPD) [1, 2, 3, 22]. The concept of treatment forthese patients includes not only functional aspects but alsohas to fulfil the patients’ aesthetic expectations at anaffordable price regarding the durability of the denture.Thus, the type and frequency of technical complicationsfor different kinds of RPD are important factors that haveto be considered when a new prosthetic restoration isplanned. Concerning technical aspects, RPDs with doublecrown retainers [5, 21] are more sophisticated than den-tures with clasp retainers. Double crown systems havesome advantages: the “retainer” is invisible even in the an-terior area, where a clasp looks unfavorable during smil-ing. The double crown comprises the retainer completely,protects the tooth against caries, and stabilizes the retainertogether with the prosthesis against horizontal and verticalforces. The fixation of the prosthesis by friction of parallel-sided or conical surfaces withstand the chewing forcesmore effectively than a clasp-retention. Double crown-retained prostheses have a reputation of being successful inthe long term [12, 24]. Publications emphasize the favor-able periodontal situation and survival time of the retain-ers, even after observation periods of up to 15 years [12,19, 27, 28]. Despite the mentioned advantages, the clasp-retained prosthesis is preferred in many countries due tothe considerably lower costs and simple construction. Themore sophisticated “double crown” retainer system hashigher technical failure rates [9], and consequently themaintenance costs of the double crown-retained prosthesesis considered to be higher. The question arises whether thismore sophisticated system could be justifiable in view ofthe cost-cutting efforts of health care organizations.

The aim of this study was to compare the frequencyand costs of the technical failure rates of clasp- and doublecrown-retained RPDs during the observation period. Thehypothesis is stated that the maintenance of both systemsdid not differ in the long term.

E. Hofmann · M. Behr (✉ ) · G. HandelDental Clinic, Department of Prosthetic Dentistry,University of Regensburg, Franz-Josef-Strauss Allee 11,93055 Regensburg, Germanye-mail: Michael.Behr@klinik.uni-regensburg.deTel.: +49-941/944-6058, Fax: +49-941/944-6171

Clin Oral Invest (2002) 6:104–108DOI 10.1007/s00784-002-0160-9

O R I G I N A L A RT I C L E

E. Hofmann · M. Behr · G. Handel

Frequency and costs of technical failures of clasp- anddouble crown-retained removable partial dentures

Received: 14 August 2001 / Accepted: 22 February 2002 / Published online: 19 April 2002© Springer-Verlag 2002

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Material and methods

Patients receiving RPDs between January 1992 andDecember 1998 from the Dental School at the Universityof Regensburg, Germany, were investigated. In this study,patients were only included if their partial dentures hadtwo to five retainer teeth and three to ten replaced lostteeth. Furthermore, they had to belong to types B2–4,C1, or C2 of the Eichner Classification [8]. All patientswere treated with a removable partial denture with ametal base. The dentures were either clasp-retained[26] or provided with double crown attachments, whichincluded RPDs retained with parallel-sided crowns [5] orconical crowns [21]. According to the three differentretainer systems used, from the entire group of 181 patients,three subgroups were randomly assigned consisting offorty cases each. The randomization process was per-formed using the SPSS hazard generator. The averageage of the 120 randomized patients (58 female, 62 male)was 59.6±11.8 years, with a minimum of 31.1 and amaximum of 86.5 years. The patients participated in amaintenance program and attended regular follow-upcheckups at intervals of 12 months. Before the prosthetictreatment began, all patients were pretreated with specialregard to periodontal care and caries control.

Prosthetic treatment had been carried out by profes-sional dentists of the department of prosthetic dentistry.In the case of parallel-sided or conical crowns, a subgin-gival or isogingival chamfer finish line and an axial taperof the abutment teeth of about 3° was prepared, accord-ing to the recommendations of Shillingburg, Hobo, andWhitsett (1981) [25]. Temporary crowns (Pro TempGarant, 3 M Espe, Seefeld, Germany) were inserted usingzinc-oxide-eugenol cement (Temp Bond, Kerr, Romulus,Minn., USA). In the case of removable partial dentureswith clasp retainers, a cavity (2×2×1.5 mm) was preparedfor the clasp rest [26]. Silicone or polyether materialswere used to take impressions after preparation. Highlyprecious alloys were used for the double crowns andsoldered to the cobalt-chromium alloy framework of thedenture base [1, 2, 5, 6, 7]. In the case of conical crowns,a taper of 5° was chosen for the outer surface of theinner crowns. The inner crowns of the double crownsystems were fixed with zinc-oxide-phosphate cement(Havard, Berlin, Germany) on the abutment teeth.

The denture base acrylic resin consisted of PalapressVario (Heraeus-Kulzer, Wehrheim, Germany). The venee-ring material for the outer crowns was a composite resin(Dentacolor, Heraeus-Kulzer), bonded to the metalframework using the Rocatec tribochemical conditioning

system (3 M Espe). Acrylic artificial teeth (Vitapan,Vita, Bad Säckingen, Germany) were used to replace thelost teeth.

The average observation period for removable partialdentures with parallel-sided crowns was 4.6±1.8 years(range 1.2 to 6.8), and 5.3±1.3 years (range 3.0 to 6.8)for RPDs with conical crowns, respectively. For RPDswith clasp retainers, the mean observation time wascalculated to be 3.7±1.6 years (range 1.0 to 6.8). Remov-able partial dentures with double crowns had an averagenumber of 5.7 artificial teeth, while those with claspretainers had an average number of 5.1 artificial teeth;and the average numbers of retainer teeth per RPD were3.6 with double crowns and 4.1 for RPDs with claspretainers.

Statistical analysis (Spearman’s correlation) was per-formed using SPSS 10.0 software (SPSS, Chicago, Ill.,USA). The cumulative hazard rate was calculated usingKaplan-Meier-estimations [18]. The hazard function isan event rate that describes how likely an event is, giventhat the restoration has survived to that time [23]. Thehazard function is not a probability but an event rate perunit of time, so it need not be less than 1.

Dental technicians [30]. Only the first failure thatoccurred was noted, and repeated failures with the samepatient were not taken into account (Table 1). The repaircosts were calculated according to the German scaleof fees for dentists [20] and dental technicians [30](Table 2). Less than 2.5% of repeated events were noted.On this basis, the repair costs per event were calculatedfor double crown systems and the clasp-retained RPDs.If the possible costs ranged between values, the highestamount was taken.

Results

Fifty percent (n=20) of all partial dentures with conicalcrowns and 67.5% (n=27) of the parallel-sided crownsdid not exhibit any type of complications, while 80%

Table 1 Frequency of technicalfailures observed during theobservation time. Repeatedevents with the same patientare not noted

Type of failure Loss of Failure of Fracture Loss of Fracture Total ofcementation facing of the metal artificial of clasp complications

framework tooth

Telescopic crowns 13 (32.5%) – – – – 13 (100%)Conical crowns 8 (20%) 4 (10%) 3 (7.5%) 3 (7.5%) – 18 (100%)Clasp retainers – – – 3 (27%) 5 (63%) 8 (100%)

Table 2 Average costs for the repair of the different complicationsin Euro (June 2001)

Type of complication Average total cost

Loss of cementation 8 EuroFailure of facing 39–140 EuroFracture of the metal framework 50–150 EuroLoss of artificial tooth 30–110 EuroFractured clasp 75–210 Euro

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(n=32) of the RPDs with clasp retainers had not beenrepaired after their insertion.

The loss of cementation was the complication mostfrequently reported (n=21) for RPDs with double-crownretainers, and 32.5% (n=13) of the parallel-sided doublecrowns and 20% (n=8) of the conical crowns had to berecemented once.

Failures of facings turned up in 10% (n=4) of thedentures with double crown retainers, whereas the eventsturned out to be only distributed to conical crowns.

Fractures of the metal framework were prevalent withdouble crown treatment. The incidence of a fracture ofthe metal framework was about 7.5% (n=3) for the RPDswith conical crown retainers. This complication didnot occur in the group of RPDs with clasp retainers(Table 1).

Regarding the loss of artificial teeth 7.5% (n=3),dentures with clasp retainers needed repair due to fracturedartificial teeth, and 7.5% (n=3) of the RPDs with conicalcrowns retained partial dentures.

Fractures of the clasp retainers occurred in 63% (n=5)of the dentures. The rate of all technical complicationswas influenced neither by the type of opposing teeth northe Eichner classification of the remaining teeth. Therewas no statistically significant difference in the frequencyof failures for the different retainer systems concerningthe position of the denture in the upper or lower jaw.Furthermore, the rate of technical failures observed did

not depend on the number of artificial teeth or retainerteeth.

The period of time during which technical failureswere observed did not statistically significantly differ forthe different retainer systems (Fig. 1). For RPDs withdouble crowns, complications occurred after an averageof 1.7±0.26 years (range 0.01 to 4.4), while RPDs withclasp retainers experienced complications after an averageof 1.4±0.29 years (range 0.3 to 3.0). The hazard ratedemonstrated that the event rate per unit of time increasedexponentially after 4 years for all systems.

The costs to repair the technical failures are given inTable 2. The calculated repair costs per failure eventwere less than 80 Euro for double crown systems andabout 172 Euro for the clasp-retained RPDs (Table 3). Ifthe mode of failure was divided into failures which neededthe help of a dental technician such as fractures or loss offacings or artificial teeth and complications which couldbe directly (and easily) managed in the chair, the differ-ences between the systems decreased. Then, the moresophisticated system had no higher frequency of failuresand the maintenance costs were lower.

Discussion

Despite the popularity and the proposed indications forthe use of RPDs with different retainer systems, until

Fig. 1 Cumulative hazard rateof technical failures withparallel-sided conical doublecrowns and clasp retainers. Thehazard function is an event ratein time that describes how likelyan event case is, given that ithas survived to that time. Thehazard function is not a proba-bility but a event rate per unitof time, so it need not be lessthan 1

Table 3 Calculation of the costs per event (complication) during the observation time. The calculation was performed according to Table 1and Table 2. The highest cost level from Table 2 was assumed

Type of failure Loss of Failure of Fracture Loss of Fracture Frequency of all events Cost per eventcementation facing of the metal artificial of clasp and total costs during observation

framework tooth during observation timeFrequency Frequency Frequency Frequency Frequency Frequency Average repaircosts costs costs costs costs total costs costs per event

Telescopic crowns 13x=104 Euro – – – – 13x=104 Euro 8 EuroConical crowns 8x=64 Euro 4x=560 Euro 3x=450 Euro 3x 330 Euro – 18x=1404 Euro 78 EuroRetainers – – – 3x=330 Euro 5x=1050 Euro 8x=1380 Euro 172.5 Euro

recently many reports have been published about theclinical success rates [9, 10, 11, 13, 14, 22, 27, 28] butfew about the technical failure rate and maintenancecosts of these dentures. Therefore, the aim of this retro-spective study was to compare the technical failure rateand maintenance cost per event of RPDs with two differ-ent types of retainers: double crowns and clasp retainers.

Fractures of the metal framework of RPDs were exclu-sively reported for dentures which had been fixed withdouble crowns. Such complications are mostly caused byfaults during the casting process, e.g., wrong positioningof the sprue formers or casting temperature. Consequently,porosities or shrinkage cavities can be seen at the fracturesurfaces of the damaged metal frameworks [6].

Functional overload is another cause for the fractureof the denture bases. Ericson et al. (1990) [10] reported17 incidents of repaired RPDs due to fractures of theresin base in patients wearing conical crown-retaineddentures for 48 to 67 months, while Wagner [28] foundeight (16.7%) during an observation period of 10 years.Possible reasons for this kind of technical complicationare fractures due to fatigue as a result of insufficient fitof the dentures [29], damage of dentures by patients withinadequate skill, and deformation of the denture base asa result of functional strain [19].

The loss of artificial teeth was reported during theentire period of observation. This fact indicates that theloss of artificial teeth was caused not only by faultsduring the manufacturing process of the dentures butalso by factors concerning the construction principles ofRPDs. Artificial teeth for dentures are characterized byhigh conversion rates of the resins’ double bonds. Thus,due to the low number of double bonds on the surface ofthe artificial teeth, the polymerization process of theresin base cannot reinforce the bond between the artificialteeth and the denture base. A considerable amount of theforce that keeps the artificial teeth in position is amechanical one caused by undercuts in the teeth filledwith the resin of the base. Deformation of the denturebase resin under functional load and during insertion [11,13] cannot be transferred to the artificial teeth. Due totheir weak bond to the denture base resin, a gap forms atthe interface of the two materials that consequently leadsto the loss of artificial teeth under stress.

The fracture of the clasps was most often observed in63% (n=5) of all RPDs with clasp retainers. Complica-tions at an early stage are more likely to be caused byfaults during the manufacturing process, e.g., surfacecracks or porosities caused by problems during the cast-ing process (casting defects) [7]. The most probablecause for fractures of clasp retainers that occur after alonger period of oral service is the fatigue of the claspretainers due to either functional stress or removal andinsertion or cleaning of the denture. Cobalt chromium, asother metals or alloys, undergoes fatigue and deformationas a result of functional stress [6]. Clasp retainers aredesigned to withstand forces caused by correct insertionand removal of the dentures. However, patients frequentlyinsert their dentures by simply putting them into their

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mouth and clenching their teeth. Thus ignoring the pathof insertion intended by their dentist, the correct care forthe dentures by the patients appears to be another impor-tant risk factor for fractures of the clasp retainers [1, 2].

With technical failure rates of 32.5% respective 50%,RPDs with double crowns turned out to be less reliablethan RPDs with clasp retainers, at a technical failure rateof 20%. Common to all systems is that the event rate pertime increased markedly after 4 years. As a clinicalconsequence, recall efforts should be intensified to avoidfailures in the following years.

The most frequently reported failure of the doublecrown dentures, i.e., the loss of cementation, can easilybe treated by the dentist himself if the crowns are notdamaged. The recementation of retainer crowns needs noparticipation of a dental technician. On the other hand,the fracture of clasp retainers always requires an expen-sive repair of the denture in a dental laboratory. The fail-ure of facings, which occurred in 7.5% of the failures ofdouble-crown (conical) retained dentures equaled thepercentage of lost artificial teeth for the dentures withclasp retainers at 7.5%, and the costs of repairing thesefailures are at the same level. Repairing a fracture of themetal framework is much more expensive than thereplacement of lost artificial teeth. Whereas dentures withclasp retainers showed no fractures of the metal frame-work, the costs for repairing fractures of clasps were acostly event with this type of retainer (Table 3) [20, 30].

Calculation of the maintenance cost of removable par-tial dentures is very difficult. In this study, only the firstfailure was taken in account. Multiple failures of the samecase were neglected. However, the rate of multiple failureevents was noted to be less than 2.5% in this population.The costs were calculated according to the German scaleof fees for dentists [20] and dental technicians [30], whichcan differ from case to case and from place to place, andfurthermore the maintenance cost depends on the skills ofthe dentist and dental technicians as well as on the behaviorof the patient to take great pains over his denture.

Regarding these facts, no recommendation for a spe-cific type of retainer system can be given concerningeconomic considerations in the long term. Althoughclasp-retained RPDs had lower failure rates, the calculatedmaintenance cost per event during the observation timewas higher than the costs for the more sophisticateddouble crown system.

Conclusions

Clasp-retained RPDs had less frequent technical failuresthan double crown systems, but the fracture of a clasp wasa cost intensive event.

The most frequent complication of double crownsystems shown – loss of cementation – was easily reparableand did not limit the survival time of the denture. Theother failures occurred independently of the chosenretainer system and did affect the metal framework andthe artificial teeth. The hypothesis that the maintenancecosts of both systems do not differ must be rejected.

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