Effect of Distal Implant Inclination on Dislodging Forces of Different Locator Attachments Used for Mandibular Overdentures: An In Vitro Study

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1 on Dislodging Forces of Different Locator Attachments Used for Mandibular Overdentures: An In Vitro Study Moustafa Abdou ELsyad, BDS, MSc, PhD Tarek Mohy Ashmawy, BDS, MSc, PhD 2, 1 Radwa Mohsen Emera, BDS, MSc, PhD, 1 & 1 Department of Removable Prosthodontics, University of Mansoura, Faculty of Dentistry, Eldakahlia, Egypt 2 Department of Maxillofacial Prosthodontics, University of Cairo, National Cancer Institute, Cairo, Egypt Keywords Attachment; implant inclinations; Locator; overdenture; retention. Correspondence Moustafa Abdou ELsyad, Department of Removable Prosthodontics, Faculty of Dentistry, Mansoura University, Eldakahlia, P.O. Box 35516, #68 ElGomhoria Street, ElMansoura, Egypt. M_syad@mans.edu.eg. The authors deny any conflicts of interest related to this article. Moustafa Abdou ELsyad ORCID Accepted July 16, 2017 doi: /jopr Abstract Purpose: To evaluate the effect of distal implant inclination on axial and nonaxial retentive forces of different Locator attachments used to retain mandibular overdentures. Materials and methods: Four duplicate mandibular edentulous acrylic models received 2 implants in the canine areas with 0, 5, 10, and 20 distal inclinations. Experimental overdentures were connected to the implants with Locator extra light retention (Le), Locator light retention (Ll), and Locator medium retention (Lm). For the 20 model, extended range Locator extra-light retention (Lee) and Locator medium retention (Lem) were used. Axial and nonaxial (anterior, posterior, lateral) retentive forces were measured initially and after 540 cycles of denture insertion and removal. Three-way mixed ANOVA was used to analyze axial and nonaxial retentive forces Results: After wear simulation, 20 angulation showed the highest axial retention for Le and Ll while 5 showed the highest retention for Lm; 0 and 5 showed the highest anterior and posterior retention for Lm; 20 showed the highest lateral retention for Le and Ll. For all implant inclinations, Lm showed the highest axial and nonaxial retention, and Le and Lee showed the lowest retention for 10 and 20 inclined implants, respectively. Conclusions: Axial and nonaxial retention of Locator attachments for implantretained overdentures are significantly affected by the degree of distal implant inclination and the type of nylon inserts. Lm is recommended to retain overdentures when implants have 5 or 10 distal inclination, and Le and Ll are recommended with 20 inclination to maintain high axial and nonaxial retention after wear. For most edentulous patients, an overdenture on two implants is the minimum standard of prosthetic care, taking into account performance, stability, retention, patient satisfaction, cost, and clinical time. 1 Implant-retained overdentures employ different splinted or nonsplinted attachments. Splinted attachments include bars with different designs, 2 while nonsplinted attachments include spherical/ball-types, magnets, telescopic crowns, and Locator attachments 3 Locators are resilient stud attachments. 4 These attachments have become popular due to excellent (dual) retention, selfalignment, and ease of repair/replacement. 5 Moreover, Locators have a low profile and can be used with inadequate interridge space to reduce denture base fracture. 6 In addition, they can compensate for implant angulation. 7,8 The manufacturers recommend the use of conventional Locator inserts (with double frictional flanges) where interimplant angles are in the range of 0 to 20. Extended range inserts (with external frictional flange only) are recommended for use where interimplant angles exceed 20. 7,8 However, Locator inserts are not resistant to wear 9 and show reduced retention clinically compared to ball attachments 5 Although implants for overdentures should be placed parallel to each other, 10,11 implant inclination toward the ideal path of denture insertion may occur due to bone quality, anatomical structures, and clinical practice. 12 Walton et al 13 noted that less experienced surgeons may place implants that diverge from each other in the frontal plane (with a distal implant inclination) and have greater facial or lingual inclination. They also found a significant relationship between surgical experience and favorable angulation. Denture retention is defined as the resistance of the denture to vertical forces in a direction opposite to denture insertion. 14 e666

2 Figure 2 Controlling the degree of distal implant inclination using a semicircular protractor. inclinations and (2): no difference in retentive forces between types of Locator attachments. Figure 1 Implant inclinations: (A) 0 ; (B) 5 ; (C) 10 ; and(d)20. Stability is the resistance to horizontal and rotational forces that prevent lateral or anterior-posterior shunting of the denture base. 15 Implant angulation significantly affects peri-implant strain, retention, and prosthetic maintenance 8,13,19,22 of Locator attachments. The effect of different degrees of distal 8,20,23 and mesial 19,24 inclinations of two implants on the retention of Locator attachments have been investigated in several studies; however, these studies did not evaluate and compare the retention of all types of patrix nylon inserts (the conventional and extended range inserts) when used for angulated implants. Also, the effect of increased implant angulations (more than 10 ) on the retentive properties of conventional and extended range Locator patrix inserts was not evaluated. Moreover, the evaluation of the different degrees of implant angulations on the nonaxial retention (stability) of implantretained overdentures with Locator attachment was not investigated, as the aforementioned studies were usually conducted on acrylic resin blocks that did not simulate the mandibular edentulous ridge. The majority of studies have evaluated retentive properties of overdenture attachments by only measuring maximum force in the axial direction. 25,26 However, retentive properties of overdenture attachments depend on the type of dislodgement. 27 In function, overdentures are subjected to 3D displacements, 28,29 and the direction of forces can be a combination of vertical, oblique, rotational, and horizontal. 24 The aim of this in vitro study was to evaluate the effect of distal implant inclination on axial and nonaxial retentive forces of different types of Locator attachments used to retain mandibular overdentures. The authors state two null hypostheses: (1) no difference in retentive forces between degrees of implant Materials and methods Experimental models This in vitro study was conducted on four duplicate mandibular edentulous acrylic resin models without undercuts. For each model, two mm implants (TioLogic; Dentaurum, Ispringen, Germany) were inserted in the canine regions with 0 (parallel), 5 (slight), 10 (moderate), and 20 (severe) distal inclinations using consecutive drills mounted on a parallometer milling device (BF 2; Bredent, GmbH&Co, KG, Senden, Germany) (Fig 1). Implants were retained on the acrylic models with autopolymerized acrylic resin to simulate osseointegration. With a semicircular protractor (Fig 2), each implant inclination was established by pivoting the table of the milling device mesiodistally to make the long axis of each drill correspond to the degree of the proposed implant inclination. 16,18 For each model, an approximately 1.5-mm-thick layer of autopolymerized resilient silicone soft lining material (Softliner; Promedica, GmbH, Neumünster, Germany) was used to simulate resilient edentulous ridge mucosa. 16,18 The Locator abutments (TioLogic; medium, 3 mm gingival height) were screwed to the implants and tightened to 35 Ncm torque. Experimental mandibular overdentures For each model, a cobalt-chromium-reinforced experimental mandibular overdenture was constructed and used throughout the study. Each experimental overdenture consisted of an acrylic record block without teeth. 4,27 The occlusal plane of the record block was adjusted to the level between the upper and middle third of the retromolar pad. Four metal hooks were attached at the canine and second molar areas of the overdenture bilaterally. Locator matrices (TioLogic) were picked up to the overdentures using autopolymerized acrylic resin. The following nylon inserts were fitted to the Locator matrices (Fig 3): (A) Conventional inserts: Locator extra-light retention (Le, blue, 680 g), Locator light retention (Ll, pink, g), and Locator medium retention (Lm, transparent g). These inserts were used for 0, 5, 10, and 20 implant inclinations. e667

3 2. Posterior dislodgement: left and right molar chains were connected to the hooks, and the anterior chains were disconnected (Fig 5B). 3. Lateral dislodgement: right canine and molar chains were connected to the hooks, and the left chains were disconnected (Fig 5C). Figure 3 Types of nylon inserts: (A) conventional Locator extra-light retention (Le); (B) conventional Locator light retention (Ll); (C) conventional Locator medium retention (Lm); (D) extended range Locator medium retention (Lem); and (E) extended range Locator extra-light retention (Lee). The two-point force needed to dislodge the attachment (in N) was recorded as nonaxial retention. Five measurements were performed for axial and nonaxial dislodgements with each type of nylon insert, and the mean was recorded as initial retention. To simulate repeated insertions and removals of the overdenture over a 6-month period (assuming three daily insertions and removals for hygiene purposes), each overdenture was pulled out 540 times. 23 Five additional measurements were performed, and the mean was recorded as retention after wear simulation. 4,30 (B) Extended-range inserts: Locator extra-light retention (Lee, red, 680 g) and Locator medium retention (Lem, green, g). These inserts were used for 20 implant inclination only. For each implant inclination (model), five specimens for each type of insert were used based on the recommendations of other studies 16-18,22 for sufficient power in the results. No power analysis was performed. Measurement of retentive forces Axial (vertical) retention Four metal chains (15-cm long) were connected to hooks of the overdenture. A 5 5 cm metal plate with four tapped holes was attached to the end of the chains by adjustable screws (Fig 4). Another chain was screwed into the center of the metal plate to connect the plate to the head of a universal testing machine (Lloyd LRX; Lloyd Instruments Ltd., Fareham, UK). 4,30 The four chains were adjusted by tightening the screws connected to the plate before each measurement to reduce slackness of the chains. 31 An acrylic resin bar projection was constructed at the base of each model to fix the model to the base of the universal testing machine. 4,30 The testing machine was calibrated and balanced using a computer algorithm to account for the weight of the prosthesis and chains. 32 The universal testing machine was used to apply a four-point vertical tensile load on the metal plate until the attachments separated from the abutments. The testing machine was set at a constant crosshead speed of 50 mm/min to approximate the speed of denture movement away from the ridge during mastication. 33,34 The maximum load needed to dislodge the overdenture from the mandibular test model (axial retention) was recorded in Newtons (N). Nonaxial retention Three types of oblique dislodgement were used to represent nonaxial retention: 4,27,30 1. Anterior dislodgement: left and right canine chains were connected to the hooks, and the posterior chains were disconnected (Fig 5A). Statistical analysis For axial (vertical) and nonaxial (anterior, posterior, lateral) dislodging, three-way mixed ANOVA was applied to analyze the results of the retentive forces. The between-group independent variables were inclination (0, 5, 10, 20 ) andtype of insert (Le, Ll, Lm, Lee, Lem). The repeated-measures independent variable was time of retention measurement (initial retention and retention after wear simulation). Full term interaction (inclination*type of inserts*time) and correction for multiple comparisons (using Bonferroni test) were used. p Value was significant if <0.05. Results Axial (vertical) retention The main effects of retentive forces (N) during vertical dislodging are presented in Table 1. There was a significant effect of implant inclination (F[3,28] = , p < 0.001), type of insert (F[4,28] = , p < 0.001) and time (F[3,28] = , p < 0.001) on retentive forces; 5 recorded the highest retention, and 0 recorded the lowest. Lm recorded the highest retention, and Le recorded the lowest. Initial retention was significantly higher than retention after wear. Retentive forces (N) of different implant inclinations and different nylon inserts during vertical dislodging are presented in Table 2. The interaction of retention was significant for inclination*type of inserts*time (F [6,28] = , p < 0.001). For Le, Ll, and Lm, the highest initial retention was recorded with 10, and the lowest retention was recorded with 5 (Le and Ll) and 20 (Lm). For Le and Ll, the highest retention after wear was recorded with 20, and the lowest retention was recorded with 10. For Lm, the highest retention after wear was recorded with 5, and the lowest retention was recorded with 0 o.forall implant inclinations, the highest retention (initial/after wear) was recorded with Lm, and the lowest retention was noted with Le (0, 5, 10 ) and Lee (20 ). With exception of 20 inclination of Lm, initial retention was significantly higher than retention after wear. e668

4 Table 1 Comparison (main effects) of retentive forces (N) during vertical dislodging between levels of independent variables in the mixed ANOVA F value p Value Bonferroni test Degree of inclination 0 o (X ± SD) ± <0.001 A 5 o (X ± SD) ± B 10 o (X ± SD) ± C 20 o (X ± SD) ± 8.39 D Type of nylon inserts Le (X ± SD) ± <0.001 A Ll (X ± SD) ± B Lm (X ± SD) ± C Lee (X ± SD) ± 6.63 D Lem (X ± SD) ± 9.95 B Time of retention measurements Initial (X ± SD) ± <0.001 A After wear (X ± SD) ± B Figure 4 Measurement of axial retention. X = mean, SD = standard deviation, * = p is significant at 5% level of significance. Different letters indicate a significant difference between levels of independent variables Nonaxial retention The main effects of retentive forces (N) during anterior, posterior, and lateral dislodging are presented in Tables 3 to 5. For anterior dislodging, there was a significant effect of implant inclination (F[3,28] = , p < 0.001), type of insert (F[4,28] = , p < 0.001), and time (F[3,28] = , p < 0.001) on retentive forces; 0 recorded the highest retention, and 20 recorded the lowest. For posterior dislodging, there was a significant effect of inclination (F[3,28] = , p < 0.001), type of insert (F[4,28] = , p < 0.001), and time (F[3,28] = , p < 0.001) on retentive forces; 5 recorded the highest retention, and 20 recorded the lowest. For lateral dislodging, there was a significant effect of inclination (F[3,28] = 50.34, p < 0.001), type of insert (F[4,28] = , p < 0.001), and time (F[3,28] = , p < 0.001) on retentive forces; 5 recorded the highest retention, and 0 recorded the lowest. For anterior, posterior, and lateral dislodging, Lm recorded the highest retention, and Lee recorded the lowest retention. Initial retention was significantly higher than retention after wear. Retentive forces (N) of different implant inclinations and different nylon inserts during anterior, posterior, and lateral dislodging are presented in Tables 6 to 8 respectively. The interaction of retention was significant for inclination*type of inserts*time during anterior (F[6,28] = , p < 0.001), posterior (F[6,28] = , p < 0.001), and lateral (F[6,28] = 71.26, p < 0.001) dislodging. During anterior dislodging, the highest initial retention was recorded with 10, and the lowest retention was recorded with 20 for Le. For Ll and Lm, the highest initial retention was recorded with 0, and the lowest retention was recorded with 20. The highest retention after wear was recorded with 0,and the lowest retention was recorded with 20 for Le and Ll.For Lm, the highest retention after wear was noted with 20, and the lowest retention was noted with 0. Figure 5 Measurement of nonaxial retention during anterior (A), posterior (B), and lateral (C) dislodging. e669

5 Table 2 Retentive forces (N) of different implant inclinations and different nylon inserts during vertical dislodging Le Ll Lm Lee Lem X ± SD X ± SD X ± SD X ± SD X ± SD 0 o Initial retention ± ± ± 0.72 Retention after wear 3.52 ± ± ± o Initial retention ± ± ± 1.20 Retention after wear 9.13 ± ± ± o Initial retention ± ± ± 1.39 Retention after wear 2.97 ± ± ± o Initial retention ± ± ± ± ± 1.08 Retention after wear ± ± ± ± ± 1.03 Table 3 Comparison (main effects) of retentive forces (N) during anterior dislodging between levels of independent variables in the mixed ANOVA F value p Value Bonferroni test Degree of inclination 0 o (X ± SD) ± <0.001 A 5 o (X ± SD) ± B 10 o (X ± SD) ± 9.84 C 20 o (X ± SD) ± D Type of nylon inserts Le (X ± SD) ± <0.001 A Ll (X ± SD) ± B Lm (X ± SD) ± C Lee (X ± SD) ± 7.00 A Lem (X ± SD) ± D Time of retention measurements Initial (X ± SD) ± <0.001 A After wear (X ± SD) ± B Different letters indicate a significant difference between levels of independent variables. During posterior dislodging, the highest initial retention was recorded with 5, and the lowest retention was recorded with 20 for Le, Ll, and Lm. The highest retention after wear was recorded with 5, and the lowest retention was recorded with 20 for Le, Ll, and Lm. During lateral dislodging, the highest initial retention was recorded with 5, and the lowest retention was recorded with 20 for Le. For Lm, the highest initial retention was recorded with 20, and the lowest retention was recorded with 0 o.the highest retention after wear was noted with 20,and the lowest retention was noted for 0 for Le, Ll, and Lm. For all implant inclinations, the highest retention (initial/after wear) was recorded with Lm, and the lowest retention was noted with Le during anterior and posterior dislodging. During lateral dislodging, the highest retention (initial/after wear) was recorded with Ll (0 and 5 ) andlm(10 and 20 ), and the lowest retention was noted with Le (10 ) and Lee (20 ). With the exception of 20 inclination of Lm during anterior and posterior dislodging, initial retention was significantly higher than retention after wear. Table 4 Comparison (main effects) of retentive forces (N) during posterior dislodging between levels of independent variables in the mixed ANOVA F value p Value Bonferroni test Degree of inclination 0 o (X ± SD) ± <0.001 A 5 o (X ± SD) ± B 10 o (X ± SD) ± 3.94 C 20 o (X ± SD) ± 5.99 D Type of nylon inserts Le (X ± SD) ± <0.001 A Ll (X ± SD) ± B Lm (X ± SD) ± C Lee (X ± SD) 8.28 ± 1.33 D Lem (X ± SD) ± 4.27 E Time of retention measurements Initial (X ± SD) ± <0.001 A After wear (X ± SD) ± 9.11 B Different letters indicate a significant difference between levels of independent variables. Discussion The simulation of resilient mucosa upon which the overdenture rests is important when testing the axial and nonaxial dislodging of overdentures. The overdenture contact with the mucosa may alter the way of attachment disconnection, particularly during nonaxial dislodging, as the denture base periphery may fulcrum on the soft liner. Therefore, mucosal simulation was performed in this study. Effect of implant inclination In this study, the axial and nonaxial initial retentive forces of different Locator inserts ranged from to 70.1 N for all implant inclinations. The minimum initial retentive forces were recorded for Le (12.02 N) and Lee (13.7 N) at 20 inclination during lateral dislodging. These values are still greater than the minimum retention values reported in the literature for mandibular ovedentures (8 N). 33,35 The highest initial retention e670

6 Table 5 Comparison (main effects) of retentive forces (N) during lateral dislodging between levels of independent variables in the mixed ANOVA F value p Value Bonferroni test Degree of inclination 0 o (X ± SD) ± <0.001 A 5 o (X ± SD) ± 8.15 B 10 o (X ± SD) ± 3.95 C 20 o (X ± SD) ± 8.30 C Type of nylon inserts Le (X ± SD) ± <0.001 A Ll (X ± SD) ± 7.44 B Lm (X ± SD) ± 6.71 C Lee (X ± SD) 9.42 ± 3.76 D Lem (X ± SD) ± 1.55 E Time of retention measurements Initial (X ± SD) ± <0.001 A After wear (X ± SD) ± 6.13 B Different letters indicate a significant difference between levels of independent variables. was recorded with 10 for all inserts. A similar finding was also observed in another study 8 in which the authors found that 10 distal implant inclination displayed significantly more initial retention of Le inserts than 0 inclination. They attributed the increased retention to the divergence between the implants with respect to the path of travel of Locator patrices. Consequently, opposing enlarged undercuts are created, from which the patrices had to be removed in unison. They also found that Locators angled at 20 did not display significantly greater retention than those angled at 10, which is the same result obtained in this study. For Le and Ll, the highest axial retention after wear was recorded with 20. Similarly, Stephens et al 8 concluded that retention of Le was not impaired by interimplant divergence up to 20 after wear simulation. Also Yang et al 21 concluded that single Le maintained the retentive force until 30 inclination without a significant difference from the initial retentive values. This could be attributed to the effective distribution of the internal and external undercuts of Locator abutments when implants are inclined at 20, thus Le patrices wear evenly. 8 These findings suggest that Le and Ll inserts may be used in the clinical situation up to the 20 angulation limit suggested by the manufacturer, without negatively affecting attachment longevity or retention. For Lm, the highest axial retention after wear simulation was recorded with 5. Therefore, a slight implant angulation for Lm may be advantageous compared to parallel implant placement. In agreement with this finding, several authors 19,22,36 showed that angulations of 5 and 10 have no significant effect on the quality of retention, demonstrating that a small offset may be advantageous. Rabbani et al 24 found that 5 mesial implant inclination showed higher retentive values than the parallel one for all types of nylon inserts after Table 6 Retentive forces (N) of different implant inclinations and different nylon inserts during anterior dislodging Le Ll Lm Lee Lem X ± SD X ± SD X ± SD X ± SD X ± SD 0 o Initial retention ± ± ± 0.73 Retention after wear ± ± ± o Initial retention ± ± ± 1.15 Retention after wear 9.49 ± ± ± o Initial retention ± ± ± 1.05 Retention after wear ± ± ± o Initial retention ± ± ± ± ± 0.73 Retention after wear 9.01 ± ± ± ± ± 1.00 Table 7 Retentive forces (N) of different implant inclinations and different nylon inserts during posterior dislodging Le Ll Lm Lee Lem X ± SD X ± SD X ± SD X ± SD X ± SD 0 o Initial retention ± ± ± 1.00 Retention after wear ± ± ± o Initial retention ± ± ± 1.04 Retention after wear ± ± ± o Initial retention ± ± ± 0.68 Retention after wear ± ± ± o Initial retention ± ± ± ± ± 1.03 Retention after wear ± ± ± ± ± 0.51 e671

7 Table 8 Retentive forces (N) of different implant inclinations and different nylon inserts during lateral dislodging Le Ll Lm Lee Lem X ± SD X ± SD X ± SD X ± SD X ± SD 0 o Initial retention ± ± ± 1.02 Retention after wear 3.08 ± ± ± o Initial retention ± ± ± 0.87 Retention after wear 9.09 ± ± ± o Initial retention ± ± ± 1.05 Retention after wear ± ± ± o Initial retention ± ± ± ± ± 1.14 Retention after wear ± ± ± ± ± , 12, and 18 months of simulated clinical use. Also, Al-Ghafli et al 19 found that Locator attachments on implants angulated at 5 exhibited the least retention loss. A possible explanation for this phenomenon is that the 5 angulation probably provides an ideal friction between the patrix and matrix components, without acceleration of nylon wear. This is important clinically, as it is difficult to position implants exactly parallel to each other. With the exception of Lm during lateral dislodging, parallel (0 ) and slightly inclined (5 ) implants recorded the highest initial nonaxial retention during anterior, posterior, and lateral dislodging, while severly inclined implants (20 ) recorded the lowest retention. These findings may be attributed to the nylon inserts resiliency, which allow them to disengage more slowly from the abutments when the implants are slightly inclined. In contrast, inserts disengage more rapidly with excessive implant inclination. This may cause rapid loss of retention, except when Lm was used, as it had a reduced resiliency, which enabled it to disengage slowly. Anterior dislodgement simulates movement that may occur when the patient tries to remove the denture by applying pressure opposite to the attachments anteriorly. During anterior dislodging, 0 showed the highest retention after wear, and 20 showed the lowest retention for Le and Ll inserts. Similarly, Ortegon et al 36 found a significant reduction in retention after wear for 20 implant angulations. The decreased retention of Le and Ll during anterior dislodging when implant angulation increases may be due to deterioration and wear of the central projection of these inserts more than the outer retentive ring. 37 This wear increases as the angle of implant inclination increases. 8,19 Posterior dislodgement is one of the most anticipated movements that occurred clinically when the distal extension base of the mandibular overdenture lifts off the tissues during function. 33,38 Therefore, it is desirable to minimize or eliminate it. During posterior dislodging, 5 showed the highest retention after wear, and 20 o showed the lowest retention for all inserts. Again, a slight distal implant inclinaton may be advantageous, as it increases denture stability during posterior dislodgement of the denture. The decreased retention with 20 inclined implants may be attributed to the wear of the nylon components. Since most dislodging forces that occur during mastication are in the posterior direction and not perpendicular to the attachments, all types of inserts can be used sucessfully when the implants are slightly angulated. Lateral dislodging simulates the action that occurs when the patient tries to remove the overdenture from one side. During lateral dislodging, 20 showed the highest retention after wear, and 0 showed the lowest retention. This may be attributed to the direction of implant inclination. The lateral dislodging of the denture makes the denture rest on the implant of the contralateral side and rotate mesially in the coronal plane. The distally inclined implant on the side of dislodging counteracts the mesial rotation of the denture and increases the resistance to lateral dislodgement. On the other hand, when implants are inserted parallel to each other, the implant on the side of dislodging does not resist this mesial rotation, causing the denture to rapidly disengage from abutments. Effect of type of nylon inserts For all implant inclinations, Lm showed the highest retention (intial/after wear). Le (0, 5, 10 ) and Lee (20 ) showed the minimal axial and nonaxial retention. These findings are in line with the retention values provided by the manufacturer (6.6 N for Le, N for Ll, N for Lm); however, these values are reported for single Locator inserts. The increased retention with Lm concurred with the finding of Rabbani et al, 24 who observed high initial retention with Lm and great percentage of retention loss with Le on 0/10 o mesial inclined implants after 720 cycles (6 months of simulated use). Clinically, the best option with distal implant inclination in terms of cost effectiveness may be the Lm. The increased retention of Lm may be due to the frictional contact mode of retention of the Locator attachments, which arises from a dimensional misfit between the slightly oversized nylon patrix insert and the smaller diameter of the inner ring of the matrix abutment. 5,20 In contrast to this finding, Evtimovska et al 20 concluded that retention of the Lem and Lm are reduced after multiple pulls when they are used for 20 distally inclined implants due to wear of nylon inserts; however, in this study, Lem recorded higher retention compared to Le and Lee inserts after wear simulation. The decreased retention of Lee with 20 inclined implants may be because these inserts were originally designed by the manufacturer for severely inclined implants with external frictional flange only. The absence of internal frictional flange decreases frictional surface area and retention. In line with this explanation, 18 found a decreased strain around distally inclined implants when Lee were used. e672

8 Although Lm may increase retention values, it may cause destructive forces to the inclined implants when Locator attachments are used Geckili et al 39 concluded that higher retention force of an implant-retained overdenture provides better quality of life, but does not affect patient satisfaction. Therefore, clinicians should avoid the use of strong retentive elements (Lm) when implants present a distal inclination, and use light retentive elements (Le and Lee) instead to minimize destructive forces on the implants when Locator attachments are used. Another option is to use bar attachments to overcome the problem of divergence between implants. Effect of time of measurments Except Lm with 20 inclined implants, initial retention was significantly higher than retention after wear for all implant inclinations, type of inserts, and dislodging forces. This finding was not surprising, and in accordance with previous in vitro investigations. 8,24,27,36 The retention loss can be described by wear and degradation of the nylon components during loading; however, it seems that degradation of the inserts does not necessarily produce a proportional deterioration in retention, as it may increase surface roughness and produce a corresponding increase in retention through micromechanical friction. 37 This may explain the increased retention of Lm on 20 angled implants after wear simulation. In general, it could be concluded that different types of Locator attachments lose retention rapidly after 6 months of simulated denture use with parallel or angled implants. Therefore, replacement of these attachments may be needed after 6 months of clinical use; however, to ensure this finding, further clinical trials are needed to verify the retention behavior of these attachments in a clinical setting. Although in vitro studies differ from clinical studies, they allow standardization of test conditions. The limitations of this study included lack of simulation of in vivo conditions regarding the presence of saliva, amount of occlusal force, and power of masticatory muscles, which may affect the wear pattern and retentive values of Locator inserts. Moreover, restricting nonaxial dislodging forces to anterior, posterior, and lateral directions is somewhat simplistic and not reflective of the complex in vivo dislodging forces to which the denture base is subjected. Furthermore, the experimental set-up did not allow control of seating of the denture, which may limit the generalizability of the results. Conclusions Within the limitations of this in vitro study design, the following conclusions could be drawn: 1. Axial and nonaxial retention of Locator attachments for two-implant-retained overdentures are significantly affected by degree of distal implant inclination, type of nylon insert, and time of retention measurement. 2. For Lm, parallel and slightly inclined implants (5 ) showed the highest axial, anterior, and posterior retention. For Le and Ll, severely inclined implants (20 ) showed the highest axial and lateral retention 3. With parallel, slight (5 ) and moderate (10 ) distal implant inclination, Lm is recommended to retain overdentures to implants. With severely inclined implants (20 ) Le and Ll are recommended to maintain high axial and nonaxial retention after wear. References 1. Thomason JM, Feine J, Exley C, et al: Mandibular two implant-supported overdentures as the first choice standard of care for edentulous patients the York Consensus Statement. 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