Biomechanics in Removable Partial Dentures: A Literature Review of FEA-Based Studies

Table 2

Studies reporting the influence of different retainer designs on stress and displacement of conventional RPD.

Authors	Type of prosthesis	The examined independent variable	Materials used in the denture	Reported dependent variable		Outcome
				Stress	Displacement deflection
Richert et al. 2021 [60]	FES RPD	Optimizing the length of I-bar clasp	Co-Cr	√	—	(i) I-bar clasp design could demonstrate optimal mechanical properties as long as the length of horizontal and vertical arms did not exceed 6 mm length
Tribst et al. 2020 [55]	BS RPD	18 3D designs of Aker clasps, with different materials, within 0.25, 0.5, and 0.75 mm undercuts	Six materials: (i) Polyamide; (ii) Polyoxymethylene; (iii) PEEK; (iv) Gold alloy; (v) Titanium (Ti–6Al–7Nb) Co-Cr	√	√ and force of removal	(i) The stress was concentrated at the shoulder of the circumferential clasp in all models; (ii) The highest stress was reported in Co-Cr with 0.75 mm, while the lowest stress was reported in polyamide, regardless of depths of undercuts; (iii) Polyamide showed the lowest forces of removal, followed by polyoxymethylene, while Co-Cr showed the highest removal force followed by titanium
Peng et al. 2020 [63]	—	72 3D models of PEEK clasps with different thickness/width ratios	(i) PEEK; (ii) Co-Cr	√	√ and force of removal	(i) The maximum stress concentration was located at the base of the clasp; (ii) The stress concentration increased when the thickness of the material increased; (iii) PEEK clasp showed higher flexibility when compared with Co-Cr clasp; (iv) PEEK clasp with a ratio of width/thickness at the tip 2.70/1.69, 1.50/1.13, or 1.75/1.53 was considered an optimal clasp to 0.5 mm undercut
Yamazaki et al. 2019 [64]	Mandibular FES RPD	Resin clasp with 6 areas of blocked-out undercut with 0.50 & 0.75 mm on the buccal surface of the main abutment	Co-Cr base denture with two thermoplastic resin clasps; (i) Polyester polyamide	√	√ and force of removal	(i) The stress was concentrated at the shoulder of the clasps but on the inner surface; (ii) No significant differences were reported between the two types of resin; (iii) The retention of thermoplastic clasps depends on the position and depth of undercut rather than the material itself
Reddy et al. 2016 [57]	Mandibular BS RPD	2 Aker clasps, with two different materials, in 0.25 mm undercut	(i) Co-Cr; (ii) Acetal resin	√	√ and force of removal	(i) The highest stress was reported in the Co-Cr clasp compared with the acetal resin; (ii) The force of removal of acetal resin was significantly lesser than that of the Co-Cr
Nakamura et al. 2014 [58]	Mandibular FES RPD	(i) 1 Aker and 1 reverse Aker; (ii) 1 embrasure clasp; (iii) 1 I-bar clasp	Co-Cr	√	√	(i) RPI clasp shows lower stress concentration in the buccal and apical region and areas of the cortical bone supporting the abutment tooth when compared with Aker and embrasure clasps; (ii) Embrasure clasp expressed slightly lesser vertical displacement compared with RPI and Aker clasps, while RPI showed significantly lesser distal displacement followed by embrasure and Aker clasps
Oyar et al. 2012 [44]	—	9 3D models of the I-bar clasp of three different materials and three modified tips	(i) Co-Cr; (ii) Titanium (Ti–6Al–7Nb); (iii) Gold alloy	√	—	(i) The maximum stress concentration was located at the horizontal curvature of the clasp and was reported in the Co-Cr specimen, while the gold alloy specimen showed the minimum stress; (ii) There is a direct relationship between lengths of the horizontal arm and development of stresses in the arms of the clasp
Wang et al. 2011 [67]	Mandibular class II	Rigid and nonrigid precision attachment (ERA attachment)	Ni-Cr	√	—	(i) Both attachments showed similar stress distribution in the alveolar bone and PL, but with more concentration in the case of rigid attachment; (ii) Compared with the rigid attachment, the nonrigid attachment resulted in higher stress in the mesial and distal end of the residual ridge when subjected to axial loads; however, the opposite was true regarding buccolingual and mesiodistal loads
Aoda et al. 2010 [59]	Mandibular FES RPD	(i) Reverse Aker; (ii) Embrasure; (iii) Back action	Co-Cr	√	√	(i) Reverse Aker clasp put more stress in abutment teeth compared with embrasure and back action clasps; (ii) Reverse Aker provided higher stability and lesser deflection to the denture compared with embrasure and back action clasps
Sandu et al. 2010 [62]	Maxillary FES RPD	Evaluation of round and half-round clasps with 9 diameters (from 0.5 to 1.3 mm) for each	Stainless steel	√	√	(i) The stress was concentrated in the inner surface of both half-round and round wires, in the part of the arm located above the height of contour of abutment teeth; (ii) Regarding the displacement, the clasp arm with half-round shape (with a diameter of 1 mm) showed a similar displacement to the clasp arm with round shape (with a diameter between 0.6 and 0.7 mm)
Judy 2009 [56]	FES RPD	Optimizing the width & length of the circumferential clasp arm	Co-Cr	√	—	The circumferential clasp with half-round shape and formula and showed the least stress concentration
Sato et al. 2001 [61]	FES RPD	Evaluation of the I-bar clasp with 6 widths & lengths	Co-Cr	√	√	(i) I-bar clasp with thin and wide arm, taper 0.020-0.023, and radius of curvature of 2.75–3.00 exhibited less stress compared with the thicker or shorter ones

FES: free-end saddle; BS: bounded saddle; Co-Cr: cobalt-chromium; Ni-Cr: nickel chromium; PEEK: polyetheretherketone; W2: the width of the clasp at the tip; W1: the width of the clasp at the base; T: thickness; L: length; PL: periodontal ligaments.