The increase in mean life expectancy (1) and the consecutive proportion of the older population have been related to increased need for prosthodontic treatment (2, 3). Removable Partial Dentures (RPDs) are still widely used for prosthetic restoration because they are generally associated with lower costs with predictability and functionality (4-6).
The knowledge of the type of partial edentulism, the distribution of the remaining teeth in arches treated with RPDs and the abutment teeth used, is a valuable basis for debates about treatment plans, RPDs design and the proper use of RPDs framework components thus promoting higher quality prosthetic dental care services (7-9).
It is well known that there are multiple classification systems of RPDs (10-15) with the most widely accepted one being the Kennedy Classification with modifications by Applegate (16). Furthermore, the majority of RPDs comprise a cast metal framework which includes a major connector, minor connectors, one or more bases, rests and retentive elements; the retentive elements have been defined as direct and indirect retainers (17, 18). The direct retainers (clasps and attachments), attached to the main or primary abutment teeth, provide support, retention and stability. The indirect retainers (occlusal, cingulum or incisal rests) attached to the secondary abutment teeth ensure additional support and stability (17-19). The most used direct retainers are clasps. In literature (17-19), a number of different clasp systems with various properties have been described. It is of major importance that RPDs retentive elements have to be chosen in such a way that appearance is not disturbed and the abutment teeth are not excessively loaded. In addition, the design must ensure minimal tooth coverage to avoid plaque accumulation (20). Thus, the circumferential clasp may be applied in tooth supported RPDs, at adequate bone supported teeth, considering aesthetic parameters. The ring clasp may be used in posterior teeth presenting lingual inclination and should be designed carefully to minimize the entrapment of food and debris (17, 18, 20). The back-action clasp, modification of ring clasp, may be designed in canines and premolar teeth with sufficient bone support (17). The embrasure clasp may be applied in posterior teeth to provide good retention and stability but it is prone to debris impaction (17). The bar clasps are considered to transfer less stress to the abutment teeth compared to the occlusally approaching clasps and are mainly designed to address the needs of Kennedy Class I and II situations (17, 18, 20). Rest, Proximal plate, I-bar (RPI) clasp is preferred to be applied to poorly bone supported teeth because of inducing low leverage forces during function. In addition to that, RPI clasps prevent food debris trapping (17, 18, 20).
Concerning attachments, it is well documented that they should be used when aesthetics is of primary importance considering the need for abutment preparation, splinting, the possible harmful effect of periodontal tissue, higher cost and the difficulty when a repair is needed (17, 18).
Several surveys have been carried out in many countries concerning different aspects on RPDs design and construction (7-9, 21-30). The analysis of their results has been proved to be helpful for the training of both dentists and dental technicians.
The purpose of this survey was to identify the retentive elements of RPDs and relate the designs to the type of edentulism and the abutment teeth used as noted in commercial dental laboratories in Athens, Greece.
Materials and Methods
After simple random sampling, three commercial laboratories were chosen out of a total of 10 certified dental laboratories with more than 15 employees. All selected laboratories cooperated with dentists practicing in Athens, Greece.
The material of this study included 628 master casts (276 maxillary and 352 mandibular) and the corresponding cast metal frameworks for RPDs construction collected in a period of 6 months. The number of the casts collected was estimated on the basis: a) of relevant studies (7, 9, 22, 24, 25) and b) that maximum sampling error should be less than ± 3.9. Each cast, with the metal framework in place, was photographed by the same author, using a digital camera (Canon Digital Ixus 750, Canon UK, Surrey, UK) ensuring that the number of existing teeth, the Kennedy Classes and the retentive elements of the cast metal frameworks could be identified. A special form was used to record the physical findings, when the photographs of each cast were examined with a PC monitor. The three authors recognized and classified the features of interest. When they were in disagreement, the finding was the one that two of the authors agreed on.
Classification of partial edentulism
Kennedy Classes and Αpplegate’s rules were used to classify partial edentulism (16).
The cases of fixed prosthetic restorations were recorded as existing teeth. Tooth numbering was in accordance with the Universal Numbering System.
1. Circumferential clasp (occlusal rest, buccal retentive arm and lingual reciprocal arm).
2. Ring clasp (occlusal rest and a ring-like arm surrounding the abutment tooth).
3. Embrasure clasp (two circumferential clasps opposing each other, joined into a common body).
4. Back-action clasp (modification of the ring clasp).
5. Bar clasps or Roach clasps (combination of a circumferential clasp - an occlusal rest, a lingual reciprocal arm - and a buccal retentive arm - T, Y or I shaped - originating from the cast metal framework mesh and approaching the retentive undercut in a cervical direction).
6. RPI clasp system (mesial occlusal rest, distal proximal plate, I-shaped retentive arm).
Circumferential, Ring, Embrasure and Back-action clasps were considered to be occlusally approaching clasps, while Roach and RPI clasps were considered to be gingivally approaching clasps.
The use of indirect retainers depends on any given case. It is of utmost importance to apply an indirect retainer in Kennedy Class I, II and IV situation with expanded edentulous areas (18).
a) Rests (occlusal, cingulum, incisal).
b) Continuous bar or Cingulum bar.
Prevalence tables and the x2 test were used for the statistical analysis of the collected data. The software program used was the SPSS (SPSS14, SPSS Inc. Chicago 2005) and statistical significance level was determined at α=.05.
The rate of absent maxillary right third molars was 96.7% and 96.0% for maxillary left third molars. Regarding the rates of the other maxillary teeth the results are seen in Figure 1 and the rates of missing mandibular teeth are seen in Figure 2.
Direct retainers and Kennedy Class
The direct retainers used in 577 of 628 RPDs, were clasps (91.9%) while the attachments accounted for the remaining 51 (8.1%) (p<0.001). Almost one half n=747 (48.9%) of the total clasps n=1527 were Roach Τ type clasps, whereas 65.6% (n=455) of them concerned the Kennedy Class I finding that is statistical significant among Kennedy classes (p<0.01). Circumferential clasps were less frequently used n=294 (19.3%), and 8.8% of them (n=61) found in Kennedy Class I finding that is statistical significant among Kennedy classes (p<0.01) (Table 1).
The embrasure clasps represented 8% (n=122) of total clasps been more common in Kennedy Classes ΙΙΙ and IV RPD designs as compared to Class Ι (p<0.01). Roach I clasps represented 7.7% (n=118) of all clasps; and there were no statistically significant differences among classes. Ring clasps encountered 6.5% (n=100) of total clasps (2.7% in Class Ι and 10.8% in Classes ΙΙΙ/ΙV) (p<0.01). Back action clasps were less frequently used n=45 (2.9%); the same was true for Roach Υ n=31 (2.0%), RPI n=12 (0.8%) and other clasps n=58 (3.8%) (Table 1).
Type of clasps and abutment teeth
Descriptively, the majority of circumferential clasps, as seen in Figure 3, were designed for maxillary first molars 31% (n=49), second molars 44% (n=71); and mandibular first molars 23% (n=30), second molars 38% (n=50); the teeth most frequently used for embrasure clasps were maxillary and mandibular first molars 46% (n=33) and 39% (n=20). Back action clasps were mainly applied on first maxillary premolars 50% (n=8) and mandibular canines 45% (n=13). Ring clasps were chosen for second maxillary 63% (n=34) and mandibular molars 54% (n=25) (Figure 3).
All types of Roach clasps were mainly placed on maxillary canines and mandibular first premolars (Figure 4).
In the twenty one (41.2%) cases of attachments, retention was achieved in combination with clasps, while in the remaining thirty (58.8%) cases retention was exclusively achieved through attachments (Table 1).
Indirect retainers and Kennedy Class
Indirect retainers were found in 389 (61.9%) of the 628 casts. In regard to Kennedy Classes, the prevalence was 60.6% (n=211) for class I, 66.0% (n=136) for class ΙΙ and 56.8% (n=42) for classes ΙΙΙ/IV (tooth supported RPDs) (p=0.28). For distal extension RPDs (Kennedy Class I and II), the rate of indirect retainers was 62.6% (Table 1).
The types of indirect retainers used in this report were occlusal rests n=316 (81.2%) and cingulum bars n=73 (19.3%). In class Ι, cingulum bars were found in n=56 (26.5%) of the 389 frameworks, as compared to 10.3% (n=14) in class ΙΙ and 7.1% (n=3) in remaining classes, finding that is statistical significant among Kennedy classes (p<0.01), while occlusal rests were observed in 73.5% of cases in class Ι, 89.7% in class ΙΙ and 92.9% in other classes (p<0.01) among Kennedy classes (Table 1).
In 37 of the 48 distal extension RPDs designs with attachments, indirect retention was not noted (p<0.001).
Epidemiological and clinical studies have used different methods to record remaining teeth, classes of partial edentulism and designs of metal frameworks used: namely, photographic records (7, 21), visual cast examination (29, 30), clinical examination (9, 32) and completing questionnaire forms (22, 33, 34). The reason for using photographic records in this study was because it allowed all authors to analyse data outside the laboratory at any convenient times.
In this study the mean number of remaining maxillary and mandibular teeth was 6.8 and 7.3, respectively; this is similar to what was reported in research studies performed in Poland (23) (5.8 and 7), Sweden (24) (6.5 and 6.6) and West Germany (8) (6.29 and 6.57), but contradicts findings by Öwall et al., in North America (7) (8.7 and 8.1) and Scotland (25) (9.9 and 8.4). It could be said that, according to these studies, the average number of remaining teeth did not significantly differ in developed countries.
It is noteworthy that the last remaining teeth recorded in this study were anterior maxillary and mandibular teeth, a finding that was similar to the results of the above mentioned studies (7, 8, 24, 25).
A clasp should be chosen on the basis of its characteristic features, indications and limitations of use (17). Previous research papers (26) indicate that there was a lack of data concerning the correct selection of clasps when designing RPDs.
According to the present study, gingivally approaching clasps were applied in 59.4% of clasp situations, mainly on maxillary canines and mandibular premolars. These results contradicted those of Öwall et al. (25) who reported that such clasps were noted in 15.4% of their cases.
The most frequently used clasps in distal extension removable partial denture designs were the gingivally approaching (79% and 47.6% in Kennedy Classes Ι & ΙΙ, respectively); this was probably because such clasps create less of an aesthetic problem and their function apply lower forces on abutment teeth (17-19). Our results are in agreement with those of Niarchou et al. (30) who reported that the Roach type clasp was the most frequently used (69.2%). On the contrary, the retentive arm of these clasps - which typically originates from components located in the edentulous area and projects horizontally across the soft tissues (18) - predisposes to plaque accumulation, fact considered to be a major disadvantage (18, 20). Basker et al. (27), in a study performed in the United Kingdom, found that the percentage of gingivally approaching clasps used in distal extension RPDs was 20% for the maxilla and 32% for the mandible.
Despite the indisputable advantages of RPI clasps, the low rate found in this study (1.6%) agrees with that of Basker et al. (27) (1.7%). In contrast, our result differs from the figure 29% reported by Curtis et al. (28) of the total number of clasps, and the 75% of AL-Dairy (22) which involved only class I and II situations, mainly for the mandible.
Occlusally approaching clasps are supposed to be reducing the risk of injury caused to surrounding tissues (22). Our results indicated that occlusally approaching clasps were chosen less frequently for Kennedy Class I (16.6%), as compared to other classes, a finding that is statistically significant. This contradicts the results of Basker et al. (27), who reported rates of 58% for the maxilla and 68% for the mandible, of Curtis et al. (28), who reported that occlusally approaching clasps were those most frequently used, even with distal extension partial dentures, and those of AL-Dairy (22), who reported that embrasure clasps were the most frequently used type for both jaws.
The fact that the circumferential clasps were found in 19.3% of clasp cases contradicts the findings of Curtis et al. (28), who reported rates of 62.7%.
The prevalence of ring clasps recorded in our study was similar in both jaws (6.5%), and this is in disagreement with what AL-Dairy (22) reported; according to his findings, this type of clasp was more frequently used in the mandible rather than the maxilla, due to teeth inclination.
Back action clasps were observed in 2.9% of all clasp cases and most frequently in Kennedy Classes Ι&ΙΙ, findings similar to those reported by Curtis et al. (28) (0.75%). The higher rates of Roach clasp use, as compared to other clasp types, was probably due to patients’ desire for a better aesthetic result and the greater familiarization of dentists and dental technicians with the design and construction of such clasps.
Attachments were employed in a mere of 8% of RPD cases, while a similar study performed in Western Germany by Öwall et al. (8) reported a rate of 15.3%.
What may be concluded from the above is that there are various approaches to be implemented when selecting the type of clasps to be used in RPDs. Although several types of retainers are described in the literature, providing clear indications for their application in relation to the class of partial edentulism, in daily practice only few of them are used, and not always those indicated for a particular case. We consider that further emphasis should be given to aspects of RPD design.
The results of this study revealed that:
1. The last remaining teeth found were the lower canines.
2. Roach type clasps were used in numerous cases.
3. There were very low rates of using RPI clasps in dental practice, despite their indisputable advantages.
4. Attachments were not extensively used.