Examination of inter-rater and intra-rater reliability during retentive force measurement of different clasps using the developed small-sized retentive force measurement device

In a treatment aiming for the functional recovery of a partially edentulous patient’s masticatory system, intraoral installment of prosthetic devices functioning as artificial organs are often used. Removable partial denture (RPD) are widely used to replace missing teeth and lost alveolar tissue, thereby restoring aesthetic and function [1]. RPD should be esthetic with minimum periodontal tissue problem and sufficiently retentive to avoid begin displaced function. From the design of RPD considering occlusal pressure distribution, the design process of RPD consists of rests, major connectors, minor connectors, denture base, and retainers. The load capacity of the clasp depends on various factors such as type, the position of a clasp, tooth position, clasp length, and pullout location. The abutment tooth contour determines the location of the retention portion of the clasp at the retention areas [2]. The clasp of RPD is generally made of 0.25 mm, 0.50 mm, and 0.75 mm undercuts for near and far zones [3]. As a general rule, the amount of retention required to dislodge the RPD from the supporting structure should always be the minimum necessary to resist reasonable dislodging forces. Excessive force from the clasps may cause many problems [4]. Circumferential clasps are the most frequently used direct retainers, and long term success of RPD depends on properties of clasps [5]. The retentive force is the force that works when the RPD is removed, and an appropriate force must be applied to the abutment tooth. Unfortunately, an inadequate denture design or retainer often generates an excessive retentive force to cause the movement of abutment tooth, thereby resulting in the mobility of abutment tooth. Additionally, this increases the abutment tooth’s load, and eventually, the abutment tooth is lost. Inadequate use of RPD must be avoided so as not to miss a natural tooth, an integral part of a human body.

Regarding the measurement of the retentive force of the clasp for RPD, there are also several studies on clasps using the finite element method [6], on measuring the retentive force using a universal testing machine with a crosshead speed [7], on weighing the retentive force of removal and insertion cycling of clasp [8], on cyclic fatigue properties of alloy cast clasps [9], on an evaluation of retentive ability and deformation [10], on the comparison of buccal and lingual retention [11], and on clasps compared with the metal materials used [12]. These are all studies using large-scale experimental equipment, and until now, there was no equipment in the dental field that could easily measure the retentive force of a clasp of RPD.

In the present study, it is significant to present a retentive force measurement device that can be easily used in the laboratory to provide an RPD with adequate retentive force.

The purpose of the present study is to develop a small-sized retentive force measurement device that can easily measure the retentive force of a clasp used for an RPD, and it is to examine the inter-rater and intra-rater reliability.

In principle, the effect of support and bracing functions when wearing a partial denture. The impact of retention does not act during wearing a partial denture because the retentive force acts on the retentive arm that has entered the undercut when detaching the partial denture. When wearing a partial denture, the clasp closely conforms to the tooth surface and should not exert any force on the tooth. No burden on the abutment tooth is observed and prevents detachment of the denture. The retentive force of a clasp works only when the denture is removed. To date, the retentive force of a clasp has been reported in many laboratory studies [6‐12, 18‐26]. Ahmad et al. [18] mentioned that the mean retentive force for a framework engaging an undercut of 0.25 mm with Akers clasp was 4.77 N. Meenakshi et al. [10] showed the clasp required 7.24 N for 0.25 mm undercut and 8.37 N for 0.50 mm undercut. Frank and Nicholls [19] concluded that 300 to 750 g (2.94 to 7.35 N) represented an acceptable amount of retention for a bilateral distal extension RPD. Arda and Arikan [20] tested the retentive force of cast Akers clasp was 615.2 ± 14.5 g with the undercut depth at 0.25 mm, 858.2 ± 58.2 g with the undercut depth at 0.5 mm. Bridgeman et al. [21] showed retentive forces from 5 N to 10 N would be necessary in one clasp. Wie et al. [22] showed the initial retentive force of the clasps was higher than 5 N, the final retentive force of the 0.50 mm undercut clasps was approximately 5 N. de Torres et al. [23] showed mean retentive force of RPD clasp was 8.09 ± 3.05 N with Co-Cr circumferential clasps. Tse et al. [24] found that retentive forces of Co-Cr clasps at undercut depths of 0.25 mm, 0.50 mm, and 0.75 mm were 2.34 ± 0.23 N, 4.65 ± 0.35 N, and 7.56 ± 0.50 N, respectively. It has been reported that the retentive force varies depending on the difference in modulus of elasticity of metal used for clasp [25]. The mechanical properties of 12% Au-Pd alloy and Co-Cr alloy are Vickers hardness of 280 and 365, and tensile strength of 804 Mpa and 938 Mpa, respectively [26]. Although there are differences in the metals used, these numerical data are similar to the measurements of the retentive force of the clasp of the present study using the small-sized retentive force measurement device.

Intraclass correlation coefficients are used in reliability studies [16, 17, 27‐29]. In the present study, intraclass correlation coefficients were used to examine whether there was inter-rater reliability and intra-rater reliability when measuring the retentive force of 10 types of cast clasps. The standard of intraclass correlation coefficients is 0.0 to 0.20 for slight, 0.21 to 0.40 for fair 0.41 to 0.60 for moderate, 0.61 to 0.80 for substantial, 0.81 to 1.00 for almost perfect [17]. ICC (1,1) indicates the intra-rater reliability when one evaluator makes multiple evaluations. ICC (1,3) indicates the reliability of the average value when one evaluator makes multiple evaluations. As a result of the present study, ICC (1,1) and ICC (1,3) of 3 assessors showed 0.9 or more. Accordingly, it was confirmed that the reproducibility within the assessor of this measurement device was high. ICC (3,1) indicates the inter-rater reliability when multiple evaluators evaluate once. ICC (3,3) indicates the reliability of the average evaluation value when multiple evaluators evaluate once. As a result of the present study, ICC (3,1) and ICC (3,3) of 3 assessors were both 0.9 or more, and it was found that the reliability between assessors was high. Thus, it was confirmed that the retentive force measurement by the developed small-sized retentive force measurement device has high reliability within and between the assessors.

By using the developed small-sized retentive force measurement device, it is possible to observe the retentive force of the retainers manufactured on the technician side. It will be possible to easily measure the retentive force of RPD in vitro.

The present study successfully developed a small-sized retentive force measurement device that enables a secure measurement of the retentive force of various clasps applied to RPDs. From examining the intraclass correlation coefficients, it was confirmed that the developed small-sized retentive force measurement device has reproducibility within and between the assessors.