Evaluation of Retention Force of Polyetheretherketone (PEEK) and Cast Cobalt-Chromium Circumferential Clasps: A comparative study

Aims : This in-vitro study aimed to compare the retention force of Polyetheretherketone (PEEK) and Cobalt-Chromium (Co-Cr) clasps by mechanical cycling test simulating ten years of use. Materials and Methods : 72 clasps samples were fabricated on a standardized e-max upper right first premolar, it is divided into 24 Co-Cr clasps, and 48 PEEK clasps which were subdivided into 24 clasps (2.4mm width), and 24 clasps (3.00mm width).Each group was further subdivided (n=8 clasps) into amounts of abutment undercut 0.25mm, 0.50, and 0.75mm undercuts. Mechanical cycling of each clasp was performed 15000 times on its specific abutment crown. The retention force of each clasp in newton was measured every 1500 cycles by applying a tensile force using a universal tensile machine. R esults : The retention force of Co-Cr clasps was higher significantly than the PEEK clasps retention force for the three undercuts. Deeper undercuts showed a significantly higher retention force for both materials. All subgroups (except 0.75mm Co-Cr group) exhibit an increase in retention force after the first 1500 cycles followed by the gradual decrease till the end of 15000 cycles. Conclusions : The retention force of PEEK clasps with both widths along 15000 cycles of insertion/removal was lower than Co-Cr clasps but still sufficient for clinical use at


INTRODUCTION
Beauty, attractiveness, and function must be restored by removable partial dentures (RPDs) to guarantee psychological agreement of the patient (1) .
RPDs are most commonly made from Co-Cr alloy and eventually, the clasp (2) Esthetic problem and the probability of patient's metal sensitivity are the main drawbacks of the metal alloys (3) . Newly, thermoplastic resins have been used instead of metal alloys to resolve these drawbacks (4) .
Thermoplastic PEEK is biomedical, with a modulus of elasticity similar to dentin, and non-allergic proposed to be used in dentistry (5,6) . Little information is found about PEEK clasps performance, its retention force at a longterm use.

A. Abutment Fabrication
The first upper right anatomical

D. Retention Measurements
A total of 15000 cycles were executed, matching the simulated insertion and removal of RPD over ten years, supposing that the patient would execute four complete cycles per day (8,9) . The retention forces (in newton) of clasp were estimated by a universal testing machine which performs a tensile force at a constant speed of 5mm/min till the clasp separated from the abutment teeth and the mean of the peak forces of ten tests was reported as the retentive force of each specimen (10,11) .

RESULTS
The average retentive force registered in newton's unit (N), standard deviation from zero to tenth cycles for nine groups of both clasps' materials are analyzed using the SPSS program (version19).

DISCUSSION
The data obtained from this in vitro study showed that Co-Cr clasps had significantly higher retention force than PEEK clasps. Therefore, the null hypothesis that there would be the difference in retentive force between cast Co-Cr clasps and PEEK clasps has been accepted.
The factors affecting clasp retention are tooth undercut and flexibility of clasp material but also affected by other factors such as clasp design, surface topography, angle of gingival convergence (12) , and friction coefficient between the abutment teeth and the clasp (13) .
Thermoplastic resins clasps have to be thicker and wider than a metallic clasp to produce efficient retention, due to their high proportional limit and low elastic modulus (14) .Other authors claimed that the resin clasps arm must be shorter with bigger cross-sectional diameter and engage deeper undercuts to get sufficient retention (15) . The results of the present project following these findings, the 3.00mm wide retentive arm of PEEK clasp that engages 0.75mm undercut provides the greatest retentive force. This is due to low elastic modulus of PEEK material (4.0GPa) which renders PEEK more flexible with low rigidity in comparison to Co-Cr alloy (240GPa) which is rigid with low flexibility (16,17) .  Retention of 2.94-7.35N is substantial in clI RPD to protect against removal during chewing (18) . Other authors claimed that a retentive force of 5N is suitable for functioning RPD (19) . The results of the present study showed that  (20) .
Co-Cr clasp with 0.75mm undercut attitude was not harmonious with the aforementioned groups; this was because of the big amount of strain undergone as it was evicted from the undercut. The high modulus of elasticity of Co-Cr predetermines its use in an undercut smaller than 0.50mm (21) .
The subsequent decrease in retention force Co-Cr clasps along with the rest cycles because of the successive attrition of the inner surface of the clasps and outer surface of the abutment crowns which produce plastic clasp distortion leading to progressive reduction in frictional resistance (22,23) .
The initial rise in retention for PEEK clasps in the first cycle attributed to enhance the fitness of the clasps to the abutment crowns which subsequently cause more friction, and then the retention reduced progressively during the remaining cycles due to fatigue deformation which reduces the friction (24) .
Authors' similar studies on Co-Cr clasps mentioned that a severe decrease in retentive force had been inspected (22,25) , other study remembered a raise in retention force along five years of simulated use (8) . between initial and final retention forces at the end of 15000cycles (9) .
An in vitro study compared the retentive force of circumferential clasps made of Co-Cr and PEEK on lithium disilicate crowns revealed initial raise then progressive reduction in retentive force along 15000cycles, but the abutment undercut was not remembered (24) .

CONCLUSIONS
The PEEK clasps can be used in an

Limitation of the study:
The limitation of this study was that it was performed in a dry environment -without saliva-and that may influence the retentive force, so the retentive force may be diverse in a realistic -oral cavityenvironment.