Estimation of Fatigue Deformation for Polyetheretherketone (PEEK) and Cast Cobalt-Chromium Circumferential Clasps: A comparative study

*Correspondence: Hussein T. Abed E-mail: newera502@gmail.com Abstract Aims: This in-vitro study aimed to compare the fatigue deformation 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 two halves (2.4, and 3.00) mm width respectively. Each group was further subdivided (n=8 clasps) according to the amounts of abutment undercut (0.25, 0.50, and 0.75) mm respectively. Each clasp was digitally pictured by a microscope before cycling, and then mechanically cycled by a specifically designed insertion-removal apparatus for 15000 times on its specific abutment crown, and at the end of 15000 cycles, each clasp was digitally re-pictured, and the distance between the clasp tips in millimeter of the pre and post cycling pictures were measured with specific analyzing computer program. Results: The fatigue deformation of Co-Cr clasps was higher significantly than the PEEK clasps for the three undercuts. Deeper undercuts showed a significantly higher fatigue deformation for Co-Cr clasps but not significant for PEEK clasps. Conclusion: The fatigue deformation of PEEK clasps with both widths after 15000 cycles of insertion/removal was lower than Co-Cr clasps fatigue deformation.

, but Co-Cr is the most popular (2) . The popularity of Co-Cr alloys has been attributed to their low density (weight), high modulus of elasticity (stiffness), low material cost, greater stiffness, and lighter weight combined with suitable mechanical properties and resistance to tarnish (3,4) . Many studies had been investigating cast Co-Cr clasps performance (5,6) . These metal clasps were subjected to the deformation due to fatigue that has been happened due to repeated insertion and removal from the abutment teeth while thermoplastic resin clasps showed no or less deformation as had been reported (7,8) . PEEK is a synthetic, tooth colored polymeric material with a modulus of elasticity similar to dentin, and non-allergic proposed to be used in dentistry (9) . Little information are found about PEEK clasps and their fatigue deformation at long-term use.

MATERIALS AND METHODS
Natural beige PEEK (peekMED, Dental Direkt, Germany) and a conventional dental Co-Cr alloy (Co 63%; Cr 29%; Mo 6.5%; Magnum H60, MESA, Italy) were estimated in this study.  (11) .The resulted in three stone models of abutment were forwarded to the dental laboratory who completed the processing of these models to produce an IPS e-max press abutment (Ivoclar Vivadent, Germany), in accordance with the manufacturer's instructions.

A. Abutment Fabrication
Nakashima J. et al., 2016 claimed that the IPS e-max had a similar abrasion to enamel (12) .

B. Clasp Design and Fabrication
The

D. Fatigue Deformation Measurements
A digital microscope (Koolertron, China) at a magnification power of 120 times was used to take a photo for each clasp individually which is retained on an a-silicone impression material placed in a round small dish of 5cm diameter and 1cm thickness. Mechanical cycling for each clasps 15000 times, matching the simulated insertion and removal of RPD over ten years, supposing that the patient would execute four complete cycles per day (11,14) . The clasps were pictured again under the same position. The digital photos (Figure 2) of the clasps were tested by Image J program to investigate the distance in millimeters between the tips of the retentive and reciprocal arms of each clasp (7,8) .

RESULTS
The mean fatigue deformation measured in millimetre (mm), standard deviation from zero to tenth cycles for nine groups of both clasps' materials is analyzed using the SPSS program (version19).

DISCUSSION
The results of the present study showed that the fatigue deformation was higher in Co-Cr clasps than PEEK clasps.
On the other side, the fatigue deflection for PEEK clasps (both widths) showed no significant difference. So, the null hypothesis that there would be a difference in fatigue deformation between PEEK clasps and cast Co-Cr clasps was accepted.
Fatigue deformation of the clasp affected by the material from which it is constructed, design of the clasp, loading orientation (15), amount of deflection (undercut measurement) and the number of deflections which represent repeated insertion, and removal of the clasp (2,16) . *Duncan's Multiple Range Tests: Means with different letters are statically significant at p≤ 0.01.

Figure (3): Mean ± Standard deviation and Duncan's multiple range tests for Clasps
Deformation for Nine Groups The cast metal clasp is anticipated to be used for a long period with 0.25mm undercut, otherwise plastic distortion of the clasp or periodontal ligament problem of the abutment tooth will occur with deeper undercuts because the metal will work closer to its yield stress (17) and surpass its proportional limit (5,6) . In the present study the Co-Cr clasps at 0.50 and 0.75mm undercuts were significantly deformed more than 0.25mm undercut ( and insertion neglecting the probability of distortion (7) .
Other authors approved that there was a significant fatigue deformation for Co-Cr, and acetal resin clasps in 0.50mm undercut (8,20) , while Meenakashi A. et al., 2016 stated that Co-Cr clasps distorted more than acetal resin but without significance (21) .

CONCLUSSION
The PEEK clasps were not deformed even in deep undercuts (0.50, 0.75mm), and this encourages its use in case of unfavorable undercut instead of Co-Cr clasps. Co-Cr clasps deformed highly, so it will need to be adjusted frequently to increase adaptation and retention which make it subjected to fracture.

Limitation of the study:
The limitation of the present study is that it was accomplished in a mere rigid system, and fatigue deformation was inspected as a function for the mechanical factor only (clasp), while the influence of soft tissue, periodontal ligament, movement during functioning (mastication), physiological, physical, and muscular factors was not studied.