Clinical Assessment of Flexible and Non-Metal Clasp Dentures: A Systematic Review

Abstract

Background/Objectives: The present study aimed to evaluate the oral health and patient satisfaction of flexible and non-metal clasp dentures (NMCD) compared to removable partial dentures (RPD) using a systematic review. Methods: The PICOS framework of this review was as follows: Do rehabilitations involving flexible dentures or NMCD have a similar success rate to those using RPD? Thus, the PICOS approach involves the following topics: (P) Population/Problem: partial edentulous adult patients; (I) Intervention: patients rehabilitated with flexible dentures or NMCD; (C) Comparison: patients rehabilitated with standard RPD; (O) Outcome: clinical parameters such as oral health, masticatory function, and patient satisfaction; and (S) Study Type: clinical trials and observational studies (cohort, case–control, and cross-sectional). No language restrictions were applied to the studies. The search strategy consisted of the following keywords in different databases: ((flexible) OR (nonmetal) OR (non-metal) OR (thermoplastic)) AND (denture). Only clinical trials and observational studies (cohort, case–control, and cross-sectional studies) from the last 15 years were included, and no language restrictions were applied. Studies that did not describe the denture material were excluded. Results: Of the 2197 potentially relevant records, 14 studies were included in the present review. Two studies reported retrospective results, while twelve reported a prospective evaluation. Considering the thermoplastic materials, five studies evaluated polyester, five polyamides, three polyacetals, and only one study evaluated polyetheretherketone (PEEK). Flexible dentures and NMCD demonstrated similar periodontal status and bone levels on abutment teeth to RPD after up to 12 months. Flexible dentures exhibited a higher degree of redness of the mucosa after 12 months. One study showed a lower maximum bite force for flexible dentures compared to RPD. No study has performed a clinical evaluation of mastication and chewing ability. Conclusions: Despite increased short-term patient satisfaction for flexible dentures and NMCD, there is weak evidence to support a similar clinical performance of flexible dentures and NMCD to RPD.

1. Introduction

The use of removable partial dentures (RPDs) can be an alternative for restoring missing teeth and supporting tissues when dental implants cannot be used due to anatomical, medical, or economic issues. Although RPDs have been used over the years to restore esthetics and masticatory function with minimal intervention and reduced clinical time [1,2,3], patients may hesitate when considering the presence of a metal clasp visible at the buccal face of the anterior teeth [4,5,6]. The use of attachments or telescopic dentures can be an aesthetic improvement for RPD designs for patients complaining about the exposure of metal clasps when smiling. However, using such retainers on vital abutment teeth is practically impossible and requires more complex clinical and laboratory work procedures.

In the 1950s, thermoplastic materials, including polyamide, polyester, and polycarbonate polymers, were developed [7]. The elastic properties and low likelihood of breakage of these polymers enabled the use of the primary connector and retainers made with thermoplastic resin [7,8], and they rarely include occlusal rests [2], a prosthetic design known as flexible dentures. Although it provides better comfort for the patient, the lack of rigidity in the primary connector and the absence of occlusal rests to support the vertical load of the RPD lead to uneven force distribution. This can cause trauma to the supporting tissues [9], resulting in extensive alveolar bone resorption, increased mobility of abutment teeth, and unstable occlusal contacts [4]. Therefore, because of the lack of prosthetic principles, flexible dentures cannot be recommended as a permanent prosthesis and should be limited to temporary use [4,10].

Thus, combining a rigid framework with retentive elements made of thermoplastic resin has been proposed to restore the biomechanical principles of RPDs for flexible dentures [11]. This construction is known as a non-metal clasp denture (NMCD) [4], which can be considered a more aesthetic RPD because the clasps are made of the same material as the denture base, and the framework reinforcement prevents deformations caused by occlusal forces [6]. Including a metal framework to better control prosthesis displacement is a common indication, and it is essential for long-term success [6]. Additionally, acetal and polyetheretherketone (PEEK) have been proposed as frameworks for manufacturing NMCD. However, limited clinical evidence supports their use [12,13,14].

Although these thermoplastic materials are more accepted by patients, there are some limitations because they cannot be relined in the office and they become rougher and stained over time [15]. Moreover, resin clasps covering the gingival margin promote caries and periodontal problems on the abutment teeth. Due to limited information in the literature, especially for long-term clinical studies, their use may be poorly supported, and there is no clear estimate of the success rate of rehabilitation using NMCD. Therefore, the purpose of this study was to evaluate the oral health properties of, and patient satisfaction with, flexible dentures and NMCD options compared to RPDs through a systematic review.

2. Materials and Methods

This systematic review was conducted following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA 2020) guidelines [16] (see Supplementary Table S1). The review protocol was prospectively registered on the Open Science Framework (OSF) platform under the title “Clinical assessment of flexible and non-metal clasp dentures: a systematic review” (https://osf.io/registries/osf/registrations, accessed on 9 April 2025).

2.1. Eligibility Criteria

The Population, Intervention, Comparator, and Outcomes (PICO) framework of this review was as follows: Do rehabilitation methods involving flexible dentures or NMCD have a similar success rate to those using RPD? Therefore, the PICOS approach covers the following topics: (P) Population/Problem: partial edentulous adult patients; (I) Intervention: patients rehabilitated with a flexible dentures or NMCD; (C) Comparison: patients rehabilitated with standard RPD; (O) Outcome: clinical parameters such as oral health, masticatory function, and patient satisfaction; and (S) Study Type: clinical trials and observational studies (cohort, case–control, and cross-sectional). No language restrictions were applied to the studies.

2.2. Information Sources

In January 2024, two researchers (C.R.T.R. and L.Z.) searched the following databases: MEDLINE/PubMed, EMBASE, Web of Science, Scopus, LILACS, BBO/Virtual Health Library, and OpenGrey (www.opengrey.eu, accessed on 3 January 2024). An additional search was conducted through the reference lists of the eligible studies. The keywords used in all databases for the search strategy were as follows: ((flexible) OR (nonmetal) OR (non-metal) OR (thermoplastic)) AND (denture). Synonyms for denture, such as removable prosthesis or prosthodontic appliance, were also checked and yielded no further results. The papers were imported into Mendeley Desktop Software (version 1.19.4) to remove duplicates. Two reviewers (P.M.S. and W.J.B.C.) independently examined the titles and abstracts of the records and discussed discrepancies until reaching consensus. If needed, a third reviewer (L.Z.) was consulted for the final decision. No articles required translation from another language using software programs. Studies from the past 15 years, including clinical trials and observational studies (cohort, case–control, and cross-sectional), were selected for full-text review. Next, the same reviewers independently screened full-text articles for inclusion. Studies that did not report the denture material were excluded. In the case of disagreement, a consensus was reached through discussion, and if necessary, the third reviewer was consulted. No articles required translation from another language using software programs.

2.3. Data Collection Process

A standardized form was created in Microsoft Excel for data extraction, which was undertaken independently by two reviewers (P.M.S. and W.J.B.C.). The extracted data were compared, and any discrepancies were resolved through discussion. Data extraction and synthesis included authors, study design, number and age of participants, Kennedy’s classification, type of denture, and follow-up period.

The outcomes included patient satisfaction, periodontal health, and masticatory function. A quality assessment of the included articles was conducted using the Risk Of Bias In Non-randomized Studies of Interventions (ROBINS-I) tool [17].

3. Results

It was found that 2197 potentially relevant records were identified through database searching, of which 1312 were duplicates. After screening titles and abstracts, 738 articles were excluded, leaving 147 articles for full-text review. We could not locate one full-text article, so we contacted the authors but received no response. An additional 137 articles were eliminated based on the inclusion and exclusion criteria. Eight articles were retrieved from a manual search of the references list. Ultimately, 14 studies evaluating flexible dentures or NMCDs in clinical settings were included.

Figure 1 presents the flowchart of the review and the reasons for excluding studies.

Table 1. Study characteristics.

Study (Authors and Year)	Type of Study	Patients (n)	Age (Mean/Range)	Kennedy’s Classification	Comparison Groups
Yoon et al., 2024 [18]	Crossover CT 1	15	29–81	Class I, II, and III	RPD 2 × NMCD 3
Fueki et al., 2022 [19]	Crossover CT	24	67.3 ± 7.6	Class I, II, and III	RPD × NMCD
Fueki et al., 2021 [20]	Crossover CT	24	67.3 ± 7.6	Class III	RPD × NMCD
Nakai et al., 2024 [21]	Crossover CT	24	59.0	Class II	RPD × NMCD
Kumar et al., 2021 [22]	Parallel CT	22	42.5 ± 7.5	Class I	RPD × Flexible
Saad et al., 2021 [12]	Parallel CT	14	NS 4	Class I	RPD × Flexible
Vozza et al., 2021 [23]	Parallel CT	120	>65	Class I	RPD × Flexible
Sadek & Elawady, 2019 [24]	Parallel CT	42	45–60	Class II mod. 1	RPD × Flexible
Manzon et al., 2019 [25]	Retrospective	120	73	Class I/II × III/IV	RPD × Flexible
Fueki et al., 2019 [26]	Crossover CT	24	67.3 ± 7.6	Class I, II, and III	RPD × NMCD
Aljabri et al., 2017 [27]	Retrospective	60	51.2	NS	RPD × Flexible
Fueki et al., 2017 [28]	Crossover CT	24	67.3 ± 7.6	Class I, II, and III	RPD × NMCD
Hundal & Madan, 2015 [29]	Parallel CT	30	25–45	Class II	RPD × Flexible
Mohamed et al., 2011 [30]	Parallel CT	20	NS	Class III mod. I	RPD × NMCD

¹ CT: clinical trial; ² RPD: removable partial denture with metal framework; ³ NMCD: non-metal clasp denture; ⁴ NS: not specified.

and

Table 2. Materials used in the selected studies.

Authors and Year	RPD Group	NMCD Group		Flexible Group
	Framework/Clasp	Framework	Clasp
Yoon et al., 2024 [18]	CoCr 1	PEEK 2	PEEK	×
Fueki et al., 2022 [19]	CoCr	CoCr	Polyester	×
Fueki et al., 2021 [20]	CoCr	CoCr	Polyester	×
Nakai et al., 2024 [21]	CoCr	CoCr	Polyester
Kumar et al., 2021 [22]	CoCr	×	×	Polyacetal
Saad et al., 2021 [12]	CoCr	×	×	Polyacetal
Vozza et al., 2021 [23]	CoCr	×	×	Polyamide
Sadek & Elawady, 2019 [24]	CoCr	×	×	Polyamide
Manzon et al., 2019 [25]	CoCr	×	×	Polyamide
Fueki et al., 2019 [26]	CoCr	CoCr	Polyester	×
Aljabri et al., 2017 [27]	CoCr	×	×	Polyamide
Fueki et al., 2017 [28]	CoCr	CoCr	Polyester	×
Hundal & Madan, 2015 [29]	CoCr	×	×	Polyamide
Mohamed et al., 2011 [30]	CoCr	CoCr	Polyacetal	×

¹ CoCr: cobalt–chromium alloy; ² PEEK: polyetheretherketone.

summarize the main characteristics of the included studies. Among the prospective studies, seven feature a crossover design and six a parallel design. Two studies are retrospective evaluations. Regarding thermoplastic materials, five studies examined polyester, five examined polyamide, three examined acetal, and only one evaluated PEEK.

Two studies with NMCD and four studies with flexible dentures conducted clinical assessments of the periodontal status of abutment teeth (

Table 3. Clinical assessment of the periodontal health of abutment teeth supporting removable prosthetic devices. ×: not evaluated.

Authors and Year	Follow-Up (Months)	Groups	Redness of the Mucosa Surrounding Abutment Teeth	Plaque Index	Gingival Index	Probing Pocket Depth	Tooth Mobility	Abutment Bone Loss	Abutment Teeth Health
NMCD
Fueki et al., 2022 [19]	3	RPD	×	22.4 ± 28.5	8.3 ± 13.1	1.7 ± 0.4	6.3 ± 22.4	×	×
		NMCD	×	24.0 ± 24.4	9.9 ± 14.3	1.8 ± 0.3	4.2 ± 14.1	×	×
Mohamed et al., 2011 [30]	12	RPD	×	×	×	×	×	0.6 ± 0.2	×
		NMCD	×	×	×	×	×	0.4 ± 0.1	×
Flexible Dentures
Saad et al., 2021 [12]	12	RPD	×	×	2.5 ± 0.5	4.1± 0.7	×	0.9 ± 0.1	×
		Flexible	×	×	1.2 ± 0.7	2.7 ±0.4	×	0.7 ± 0.1	×
Manzon et al., 2019 [25]	12	RPD Class I/II	35%	35%	×	×	×	×	95%
		Flexible Class I/II	65%	65%	×	×	×	×	85%
	12	RPD Class III/IV	40%	40%	×	×	×	×	100%
		Flexible Class III/IV	60%	60%	×	×	×	×	95%
Sadek & Elawady, 2019 [24]	24	RPD	×	×	×	×	×	×	71.4%
		Flexible	×	×	×	×	×	×	100%
Hundal & Madan, 2015 [29]	6	RPD	0%	×	×	×	×	×	×
		Flexible	0%	×	×	×	×	×	×

). NMCD showed similar periodontal status and bone levels on abutment teeth compared to RPD after 3 and 12 months. Although flexible dentures exhibited more mucosal redness, they resulted in higher survival rates of the abutment teeth.

Considering masticatory function, the studies present various clinical parameters, which makes the data non-comparable. Bite force was examined in only one study, while chewing ability and discomfort during mastication were assessed in another using questionnaire methodology (

Table 4. Clinical assessment of masticatory function.

Authors and Year	Evaluation Tool	Kennedy’s Classification	RPD Prosthesis	NMCD/Flexible Prosthesis
Maximum Bite Force
Vozza et al., 2021 [23]	Digital dynamometer	Class I in one arch	22 kg	15 kg
		Class I in both arches	18 kg	11 kg
Chewing Ability Satisfaction
Yoon et al., 2024 [18]	VAS 1	Classes I, II, and III	80.8 ± 2.6	82.2 ± 3.2
Manzon et al., 2019 [25]	Questionnaire	Classes I and II	4 (20%)	6 (30%)
		Classes III and IV	2 (10%)	2 (10%)
Discomfort/Pain During Mastication
Manzon et al., 2019 [25]	Questionnaire	Classes I and II	5 (25%)	11 (55%)
		Classes III and IV	3 (15%)	6 (30%)

¹ VAS: visual analog scale.

).

Table 5. Patient satisfaction with RPD, NMCD, and flexible dentures based on follow-up time. OHIP-49: version with 49 questions [0 (more satisfied) to 196 score].

Authors and Year	Kennedy’s Classification	Follow-Up	Methodology/Parameter	Satisfaction
				RPD	NMCD	Flexible
Yoon et al., 2024 [18]	Classes I, II, and III	1 week	VAS (mean satisfaction score)	71.5 ± 4.65	73.8 ± 3.9	×
Fueki et al., 2021 [20]	Class III	3 months	OHIP 1-49 [median (IQR 2)]	28 (31)	10.5 (11)	×
Nakai et al., 2024 [21]	Class II	2 weeks	OHIP-49 (mean ± s.d.3)	60.6 ± 28.9	42.7 ± 21.2	×
Fueki et al., 2019 [26]	Classes I, II, and III	3 months	VAS (mean satisfaction score)	81.0 ± 17.4	87.3 ± 15.5	×
Fueki et al., 2017 [28]	Classes I, II, and III	3 months	OHIP-49 (mean ± s.d.)	25.7 ± 20.7	16.3 ± 17.2	×
Hundal & Madan, 2015 [29]	Class II	6 months	Questionnaire score [0 to 3 (more satisfied)]	1.5 ± 0.5	×	2.07 ± 0.6
Kumar et al., 2021 [22]	Class I	12 months	OHIP-14 (mean ± s.d.)	16.9 ± 3.3	×	13.63 ± 2.4
Saad et al., 2021 [12]	Class I	12 months	Questionnaire score [0 to 3 (more satisfied)]	1.42 ± 0.53	×	2.71 ± 0.48
Manzon et al., 2019 [25]	Classes I and II	12 months	Questionnaire (prevalence of satisfied answers)	40%	×	95%
	Classes III and IV	12 months	Questionnaire (prevalence of satisfied answers)	35%	×	100%

¹ OHIP: oral health impact profile; ² IQR: interquartile range; ³ s.d.: standard deviation.

shows the data on patient satisfaction via Oral Health-Related Quality of Life (OHRQoL). Nine studies assessed patient satisfaction using different methods; the most common was the oral health impact profile questionnaire, used in four studies. One study conducted a phone interview to evaluate patient satisfaction after 12 months [27], but data for RPD and flexible dentures are not available in this study; therefore, they are not included in Table 5 [27].

Figure 2 shows the quality assessment of the included studies. Two studies received an overall grade indicating medium risk of bias, while the other twenty were rated as low risk. Six studies raise some concerns due to the inherent challenges in evaluating different prostheses, since neither the patient nor the operator can be easily blinded to the interventions. Additionally, the data are from studies with short follow-up periods.

4. Discussion

Although flexible RPDs have been available to the dental profession for nearly 65 years and have received significant attention in dental advertisements over the past two decades, there is a lack of evidence-based information in the relevant literature regarding their clinical performance, follow-up, or incidence data [3,4,31]. The present study, through a systematic review, showed that the clinical evidence supporting the use of NMCD and flexible dentures instead of RPD is weak. Information about abutment teeth health, residual bone ridge preservation, and oral function maintenance is limited in the literature.

Conventional RPD is considered the most predictable treatment in terms of patient satisfaction among prosthetic options [32]. In this type of prosthesis, a rigid primary connector in the RPD is crucial to evenly distribute the occlusal load across both sides of the jaw. Previous studies in Class-I and -II patients support this, showing that flexible dentures generate significantly lower bite force [23] and reduced chewing ability [25] compared to RPD. Therefore, NMCD designs should offer biomechanical advantages over flexible dentures, especially when torsional forces are present. However, no direct comparison has been made between NMCD and flexible dentures design. Unlike the prosthetic principles, the NMCD design for Class-II patients in Nakai et al.’s study was unilateral, and no primary connector was used [21].

The thermoplastic materials included in this study were PEEK, polyamide, polyester, and polyacetal. Polyester exhibits less flexibility than polyamide. However, the former can be repaired or relined with self-curing resins in clinical settings [31]. Since movement of the flexible denture is expected, the material with the lowest flexibility near the prosthetic equator can be placed in the resin clasp, preserving the marginal gingiva in case of any intrusion movement. Although removable prostheses made from materials with a lower modulus of elasticity than PMMA can deform easily, resulting in a larger load transmitted onto the mucosa [6,33], the studies in this review only investigated bone loss around the abutment teeth with polyacetal and polyamide clasps and did not evaluate the residual alveolar ridge [12,30]. Because polyester was always combined with a metal framework in NMCD [19,20,21,26,28], this material cannot be directly compared to polyamide in the same application [23,24,25,27,29]. Polyacetal used in flexible designs was considered only in relation to patient satisfaction, with no evaluation of oral health [12,22]. Only one study examined a one-piece PEEK partial denture as an NMCD design; however, only satisfaction was reported after a short follow-up of one week [18]. PEEK has a higher modulus of elasticity than PMMA and appears not to cause overload or resorption of the residual ridge [14].

Although patients reported higher short-term satisfaction with NMCD and flexible dentures, thermoplastic materials are vulnerable to water absorption in the oral environment, leading to deformation and loss of flexibility. Consequently, after 12 months, polyamide clasps show a loss of retention in 55% of the prostheses in Class-I and -II patients and in 45% of cases for Classes III and IV [25]. Additionally, the redness of the mucosa beneath the denture indicates a tissue reaction to the degraded thermoplastic materials [25].

When one clasp fails to provide retention, other clasps on the prosthesis may experience early fatigue. This is a critical point because a previous survey reported that after one year of using flexible dentures, dentists encounter discoloration of the base, fracture of clasps, and debonding of teeth. After two years, half of the flexible dentures are replaced due to problems with the abutments [34]. In contrast to this survey data, two studies reported higher survival rates of the abutment teeth when flexible dentures were used rather than RPD [24,25].

The retention and stability of a partial prosthesis type may influence mastication in comminuting tests [35]. However, in the present review, no study conducted a clinical evaluation of mastication and chewing ability. One study showed that maximum bite force decreases when no metal framework is used, especially with flexible dentures in both arches [23]. Chewing ability was assessed using VAS and a self-perceived satisfaction questionnaire [18,25]. Yoon and colleagues investigated PEEK structures and found similar chewing ability compared to standard RPDs [18]. On the other hand, Manzon and colleagues reported similar values for patients with Classes III and IV, while Class-I and -II patients preferred polyamide dentures [25].

Polyacetal shows better retentive force than polyamide [35], which explains why the type of partial prosthesis also affects mastication in the comminuting test [36]. However, there is no data in the literature about the retentive force of other materials like polyester and PEEK, nor their behavior when a rigid reciprocal arm is used, such as RPD clasps. Akinyamoju and colleagues evaluated patient satisfaction with removable prostheses in 2019 [37] and 2017 [38]. Although their results favor flexible dentures, these studies were excluded from the present review because they compare flexible dentures to PMMA-only dentures. This review did not discuss the effects of clasp designs and minor connectors on biomechanics and clinical performance. The studies included patients with Class-I, -II, -III, and -IV anatomy, and each class may receive different types of clasps in RPD groups and varying thickness and undercut positions in flexible and NMCD groups. Therefore, it is not possible to determine the role played by clasps in the results. Future research should focus on evaluating the biomechanics of flexible dentures and NMCD and their clinical parameters, as the current favorable data for these materials is based on weak evidence that lacks support from professional reports [34].

5. Conclusions

The studies may be biased in determining whether NMCD or flexible dentures perform comparably to RPD. Although there is higher patient satisfaction with flexible and NMCD prostheses, current clinical data do not support the use of any thermoplastic material for long-term treatments.