Unilateral complex partial denture performance evaluation: 5 years follow up clinical study

Summary Introduction Removable partial denture (RPD) is common treatment option for unilateral partially edentulous patients not indicated for implant therapy. Unilateral complex partial denture (UCPD) could be an alternative approach to RPD treatment, but there is lack of evidence about UCPD treatment outcomes during the long-term clinical performance. The aim of this this study was to use periodontal, prosthodontic and participant satisfaction measures to evaluate the long-term clinical performance of UCPD. Material and methods This 5 year follow-up clinical study evaluated pocket probing depth (PPD) and vertical clinical attachment loss (CAL-V) of direct abutment (DA), indirect abutment (IA) and control teeth (CT). Also complications and failures of UCPD were analysed using questionnaire of participant satisfaction with UCPD (stability, comfort and manipulation). Results Evaluation of data showed that CAL-V and PPD significantly increased over time for DA, IA and CT (p<0.0001), but the tooth function (DA, IA and CT) did not significantly influence changes in PPD and CAL-V. The fracture of one abutment tooth and increase of the number of artificial teeth deformations (p=0.039) were observed after 5 years. Participant satisfaction with denture after 1 year and 5 years vs. 7 days was significantly improved. Conclusion Despite limitations of this clinical study and assuming regular oral maintenance with proper indication, UCPD might be considered as good treatment option for Kennedy II rehabilitation in patients not indicated for implant therapy or who cannot tolerate extensive RPD design.


INTRODUCTION
Oral rehabilitation of unilaterally shortened dental arch includes removable, fixed and implant borne restorations [1,2,3]. Removable partial denture (RPD) is still common treatment option for unilateral partially edentulous patients who can't be candidates for an implant treatment due to various reasons including lack of bone support with anatomical limitations such as position of maxillary sinus, inferior alveolar nerve and health condition. Evidence has shown satisfactory clinical results with clasp-retained RPDs concerning survival and extraction rate of abutment teeth [4,5], but aesthetic outcomes for clasp-retained RPDs in comparison to precision attachment retained RPDs were low [6]. To enable proper assessment of the oral needs and treatment demands of dental patients with adequate prediction of therapeutic interventions outcomes, it is important to recognize patients' self-perception of quality of life in terms of oral-health. Namely, the presence of denture plate as a part of RPD might cause the so-called foreign body sensation and nausea whereas clasps often do not meet patients aesthetic and comfort needs [7,8]. To overcome those problems, unilateral complex partial denture (UCPD) without denture palate retained with latch type of attachment could be an optional treatment for unilaterally partially edentulous patients (Kennedy class II). However, the use of UCPD in dental practice might be a controversial subject, having in mind possibility of overloading abutment teeth and lack of major connector with contributing factor in denture retention. Hence, to date there have been few clinical reports concerning UCPD, while the actual impact of UCPD on oral health is not clarified yet.
The aim of this study was to use periodontal, prosthodontic and participant satisfaction measures to evaluate the long-term clinical performance of UCPD with a snap in latch attachment.

Participants
This study design and data collection methods were approved by the Ethics Committee of the School of Dental Medicine, University of Belgrade, Serbia (No: 36/26).
Fourteen participants provided informed consent forms in accordance with the World Medical Association Declaration of Helsinki, and participated in the study (10 men and 4 women; age range: 57 to 68 years). The predefined inclusion criteria were: participants were maxillary unilaterally partially edentulous (Kennedy class II) with missing second premolar and first and second molars; not indicated for implant treatment due to lack of bone and position of maxillary sinus; refused denture with palatal plate and therefore indicated for UCPD; and available for periodic checkups every 6 months up to 5 years. Since 3 patients didn't maintain oral hygiene properly and didn't come to the clinic for regular check-ups, they were excluded from the study. Also, one patient refused to use his denture for the reason of discomfort caused by the pin of latch attachment. The final number of participants enrolled in the study was 10.

Interventions
The design of UCPD considered precision attachment retained restoration (SD snap in latch attachment, Servo-Dental GmbH & Co. KG, Hagen, Germany) consisting of free-end denture saddle attached to splinted 2 abutment teeth, canine and first premolar covered with milled crowns due to the absence of major connector. The primary part of attachment made of fully combustible plastic was integrated in a milled crown on the abutment tooth and casted together with crowns. The secondary component of attachment made of titanium was adhered into unilateral saddle and created the latch type connection of the whole restoration.
Preprosthetic procedure included the following periodontal treatment: motivation for oral hygiene; instruction in oral hygiene procedures with adequate oral hygiene manual; scaling, root planning and polishing.
The procedures for making UCPD included teeth preparation, taking impression for crowns using addition polymerization silicone material (Elite HD+, Zhermack, Italy) and try-in phase of metal framework that involved primary part of attachment and try-in ceramic. The impression for unilateral metal framework of saddle was made with the crowns placed on teeth. After casting metal framework for free-end saddle, it was evaluated intraorally. The following phases included the placement of secondary component of attachment and adhering into metal framework. Glazed crowns and finished UCPD with maximally extended free-end saddle were attached together and placed in mouth. The crowns were cemented according to standard procedures for cementing milled crowns with attachments. The participants were instructed how to use and maintain denture and additionally educated and motivated to perform appropriate oral hygiene protocols.
The patients were asked to visit the clinic every 6 months for regular check-ups. During the appointments they were re-motivated to maintain oral hygiene of the teeth and appliance.

Outcomes
After denture insertion, during the appointments after 7 days, 1 year and 5 years, trained and calibrated external examiners (in most cases in the presence of treatment coordinator) measured periodontal indices, recorded prosthetic parameters and completed the questionnaire of participant satisfaction with UCPD during the study.

Periodontal outcomes
Teeth were classified according to their relation to the attachment and free-end saddle and divided into the three groups: direct abutment (first premolar), indirect abutment (canine) and control teeth (same tooth on the opposing side of the jaw). Periodontal conditions of direct abutment (DA), indirect abutment (IA) and control teeth (CT) were evaluated using pocket probing depth (PPD) and vertical clinical attachment loss (CAL-V). The measurements of PPD and CAL-V were conducted with graduated periodontal probe at 6 sites per tooth (buccal, distal-buccal, mesial-buccal, lingual, mesial-lingual and distal-lingual). CAL-V was measured from the crown margin.

Prosthetic Outcomes
At every recall, abutment teeth and UCPD were evaluated for complications according to slightly modified standard criteria for complications and failures of RPD presented by Saito et al. [9].
1. Teeth Fracture and/or missing teeth: yes or no 2. Attachment Fracture and /or deformation: yes or no 3. Denture base Fracture and /or deformation: yes or no 4. Artificial teeth Fracture and /or deformation of acrylic veneering: yes or no

Participants' satisfaction
The participants were asked to complete a questionnaire regarding the stability, comfort and difficulties in denture manipulation based on verbal rating scale (VRS), ranging from 1 to 5 (1= completely unsatisfied; 5= completely satisfied).

Statistical methods
All statistical analyses were performed using Statistical Package for Social Science (SPSS software package, version 18.0; SPSS Inc., Chicago, IL, USA). Mean, median, SD and range were used for descriptive data. Category variables were compared using Cochran test. Non-parametric data were analysed using Friedman and Wilcoxon test. The mixed between-within subject ANOVA was used to show the interaction of time and type of abutments.
Inter-group comparisons were analysed using one-way analysis of variance (ANOVA). P value less than 0.05 was considered statistically significant.

RESULTS
The influence of three abutments (direct abutment, indirect abutment or control) on CAL-V changes during the observation period (7 days, 1 year, 5 years) is shown in Figure 1. The results showed that CAL-V significantly increased over time for all observed teeth (p<0.0001), but interaction between groups for each parameter did not show significant difference. Inter-group comparison did not show statistically significant differences between abutment teeth for each time separately. Figure 2 shows PPD changes during the evaluation period (7 days, 1 year, 5 years) for each observed group of teeth (direct abutment, indirect abutment or control). PPD significantly increased in all observed groups over time (p<0.0001), but the results did not show statistically significant interactions between tooth function and time.
The results also showed that the tooth function (direct abutment, indirect abutment or control) did not significantly influence the changes in PPD for each time separately.
The incidence of prosthetic complications and denture failures are shown in Table 1. The results showed the fracture of one abutment tooth and deformation of one retainer after 5 years of wearing UCPD, but compared to data after 7 days and 1 year it was without statistical significance. On the contrary, the number of artificial teeth deformation was significantly increased over time (p=0,039). During the evaluation period, denture base deformation was not identified.  Table 2 shows participant satisfaction with denture stability, comfort and manipulation. The results showed that all three parameters were significantly improved after 1 year and after 5 years period of denture wearing compared to the data after 7 days of denture insertion. It is evident that maximum score was given for denture stability for all evaluation periods, while patients' satisfaction after 7 days was assigned number 3 on the scale from 1 to 5. During subsequent periods of evaluation comfort and manipulation parameters achieved higher scores.

DISCUSSION
Despite various clinical options offered by dental implants, therapy with RPD is still the most common treatment option in partially edentulous patients without posterior teeth [10]. Therefore, it is very important to assess treatment outcomes with RPD considering the preservation of supporting tissues and teeth and patients' comfort and aesthetic demands. Based on our evidence, there have been only few clinical studies investigating UCPD [11,12], which is considered to be a restoration with high functional and aesthetic values.
The results of our study showed that UCPD wearers reported high scores for all 3 measured prosthetic satisfaction parameters: stability, comfort and manipulation during the appointments after 7 days, 1 year and 5 years. Taking into account the lack of literature related to UCPD and if UCPD is compared to CFPD as a restoration also characterized by attached pontics, the result of this study is in compliance with the evidence estimating that lack of palatal base in CFPD gives advantages to RPD regard- ing patient comfort and satisfaction [12]. Interestingly, the results of the present study also showed that patients' satisfaction with UCPD significantly increased over time. One may speculate that the period of 7 days was not long enough for complete patient adaptation and that was the reason for significantly lower scores for stability, comfort and manipulation after 7 days in comparison to the scores after 1 and 5 years. As the time passed the patients got used to the appliance with evident increase of the comfort and manipulation.
The overload of abutment teeth for restorations without palatal plate and attached pontics is a controversial question. The results of the present study showed fracture of one abutment tooth after 5 years of UCPD function. This is in accordance with the findings of Schmitt et al. who performed 5-year follow up study comparing unilateral and bilateral dentures retained by miniaturized attachment system [13]. According to their findings, in contrast to unilaterally retained removable dentures where splinted abutment teeth had high survival rate, fracture of abutment teeth was the most common complication and cause for failure of bilateral partial dentures (with non-splinted abutment teeth). The findings of our study are also in accordance with the finite element method (FEM) analyses of UCPD [14]. Namely, comparing the UCPD and RPD through FME stress analysis, calculations showed that applied forces are within physiological limits on splinted direct and indirect abutments of UCPD (canine and first premolar), which behaviour was similar to the behaviour of direct abutment of conventional RPD.
The results regarding the technical complications showed reparable deformation of the sprue in one latch attachment after 5 years of wearing UCPD, in contrast to the results of Schmitt et al. [13] who reported high rate of irreversible mechanical wear of male part of used attachment after 4 years of clinical function with the need for restoration replacement. According to these findings, latch attachment with sprue might be more suitable for unilateral free end saddle treatment in comparison to miniaturized attachment system. Also, type of attachment offers denture stability in unilateral cases and mouth safety, since removal is not possible unless the aforementioned button is pressed and denture is deliberately displaced. A fracture of acrylic veneering present in our patients was also common complication similar to findings of Schmitt et al. [13] Due to the fact that primary causes of denture abutment teeth failures are periodontal disease and caries [15,16], periodontal status of abutment teeth involved in UCPD restoration was evaluated. The results of UCPD clinical prognosis through evaluation of periodontal status of direct and indirect abutment teeth and control teeth in the opposite side of the same jaw showed that PPD and CAL-V significantly increased for all observed groups over time. However, PPD and CAL-V values of direct abutment and indirect abutment teeth were not significantly different from control teeth that were not involved in UCPD. Therefore, we might estimate that teeth splinting increases load potential despite the presence of palatal base and latch attachment providing elastic connection between fixed and maximally extended free-end saddle. As mentioned by Jorge et al. [17] such favorable results could be attributed to well planned prosthetic treatment and properly designed removable partial dentures.
It's interesting to mention that follow-up clinical studies of RPD showed controversial findings regarding the influence of tooth function (direct abutment, indirect abutment or control) on periodontal attachment loss. Contrary to findings showing that tooth function significantly influences gingival recession and probing depth [18,19], Drake and Beck [20] did not find difference in PPD between patients wearing RPDs and not wearing RPDs. In accordance with this, Schmitt et al. [13] even identified improvement in periodontal parameters for abutments of unilateral denture compared to the baseline values suggesting that restoration is reliable treatment modality for periodontal health.
For better prediction of clinical performance of unilateral complex partial denture it is necessary however to enrol more participants for longer evaluation time, because the main limitation of this study was the small sample size and therefore it might be considered as preliminary study. Although the sample size was small, its distribution was in accordance with previous studies [21,22].