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Occlusal Interferences and Cantilever Joint Stress in Implant-supported Prostheses Occluding With Complete Dentures. | Dentures | Strength Of Materials

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  JOMI on CD-ROM, 1990 Jan (70-77 ): Occlusal Interferences and Cantilever Joint Stre…Copyrights © 1997 Quinte… Occlusal Interferences and Cantilever Joint Stress in Implant-Supported Prostheses Occluding with Complete Dentures Hanne Falk, DDS/ Lars Laurell, DDS, LDS, Odont Dr/ Dan Lundgren, DDS, LDS, Odont Dr Naturally occurring closing and chewing forces were measured in dentitions with mandibular fixture-supported bilateral distal two-unit cantilever fixed prostheses occluding with complete dentures, and the vertical bending stress in the cantilever joint of the preferred chewing side was assessed. Occlusal forces were measured using eight miniature strain gauge transducers bilaterally and symmetrically mounted in complete dentures. Occlusal forces over the entire prosthesis were registered in basic (habitual) occlusion and with a 100- µµµµ m-high premature occlusal contact on the second cantilever unit. Closing and chewing forces increased distally. The premature occlusal contact did not significantly influence the total forces over the entire dentition but increased the local forces over the second cantilever unit. The group mean maximal vertical bending stress for closing forces reached the fatigue limit for unhardened type III dental gold alloy in basic occlusion. Although the interfered occlusion resulted in a significant redistribution of forces and significantly altered the stress contributions from each cantilever unit, maximal vertical bending stress in the cantilever joint was not significantly influenced. During chewing, the fatigue limit was not reached in basic or in interfered occlusion. (I NT J O RAL M AXILLOFAC I MPLAN TS 1990;5:70-77.) Key words: complete dentures, implants, occlusal interference, vertical bending stress O cclusal forces in dentitions with cross-arch fixed prostheses including posterior multiple-unit cantilevers have been found to decrease in the distal direction along the cantilever segment if the prosthesis is occluding with natural teeth. 1,2  In contrast, when the prosthesis occludes with a complete denture, local closing and chewing forces increase in the distal direction along the cantilever, while the total forces are of the same magnitude. 3-5  This difference in force distribution pattern has been suggested to be a consequence of the behavior of the complete denture. 4,5  From a material strength point of view, the distally increasing force distribution pattern is unfavorable. However, this pattern could be reversed by infraoccluding the distal cantilever until as little as 100 µ m. 5 By introducing a diminutive premature occlusal contact on the distal cantilever unit in dentitions with cross-arch fixed two-unit cantilever prostheses occluding with  JOMI on CD-ROM, 1990 Jan (70-77 ): Occlusal Interferences and Cantilever Joint Stre…Copyrights © 1997 Quinte… natural teeth, the distally decreasing force distribution pattern is altered to one characterized by distally increasing forces, resulting in increased stress on the prosthesis. 2,6 The aim of this study was to determine whether a small, experimentally introduced, premature occlusal contact on the distal cantilever unit in dentitions with fixture-supported prostheses occluding with complete dentures would further amplify the unfavorable distribution of axially directed occlusal forces along the cantilever segment and, as a consequence, increase the vertical bending stress in the cantilever joint. Material and Method Eight edentulous subjects, six men and two women, participated in this study. Their mean age was 56 years (range 43 to 65). They were selected from among edentulous patients referred to the Department of Prosthodontics at The Institute for Postgraduate Dental Education, Jönköping, Sweden, for mandibular implant-supported fixed prostheses. Each subject was treated according to the principles of Brånemark et al. 7  Five or six titanium fixtures were installed in the anterior region of the mandible, between the mental foramina. The fixture-supported prostheses included bilateral posterior two-unit cantilevers, the extension of which amounted to approximately 16 mm measured from the distal fixture (Fig 1).The metal frameworks were cast in type III dental gold alloy. The shape of the cantilever joints was roughly elliptic, and the dimensions of the joints as well as the whole metal framework of the construction were approximately 5 mm high and 4 mm wide. The yield strength for hardened type III gold alloy is approximately 440 N/mm 2 , and for unhardened it is approximately 310 N/mm 2 . Forces. The method of measurement of axially directed occlusal forces was recently described in detail. 4,8,9  Briefly, in duplicates of the maxillary dentures, eight strain gauge transducers (FFA TU 631, The Aeronautical Research Institute of Sweden, Bromma, Sweden) were mounted bilaterally, evenly distributed over the denture to occlude with mandibular fixture-supported prostheses at the first (F 1 ) and third (F 3 ) fixture areas and at the first (C 1 ) and second (C 2 ) cantilever units (Fig 1). Occlusal contacts with the cantilever segments were arranged to occur approximately 8 and 16 mm distal to the most posterior retainer. To direct all axial occlusal forces to the transducers, the occluding cusps and incisal edges were supraoccluded approximately 1 mm using self-retaining acrylic resins (Prisma-Fil, LD Caulk, Milford, Delaware). 9  For force recording, the transducers were connected to a Mingograf jet recorder (Elema Schönander, Solna, Sweden) via a 9-V battery-powered bridge amplifier. The occlusal forces were then measured according to a standardized test program 9  in two occlusal arrangements:1. In basic occlusion with simultaneous occlusal contacts on all transducers, as indicated by synchronous deflection in all channels when the test subjects  JOMI on CD-ROM, 1990 Jan (70-77 ): Occlusal Interferences and Cantilever Joint Stre…Copyrights © 1997 Quinte… performed light closings in habitual occlusion2. In interfered occlusion with a 100- µ m-high premature occlusal contact on the second cantilever unit (C 2 ) of the preferred chewing sideThe occlusal interference arrangement was accomplished in the following manner. 6  Beginning with the basic occlusion arrangement, a 100- µ m thickness of aluminium foil was attached to the occlusal surface of all transducers except the one occluding with the distal cantilever unit on the preferred chewing side. Self-retaining acrylic resin (Prisma-Fil) was then applied to the tip of the cusp to occlude with the center of the corresponding transducer. The subject was asked to close gently during light curing. The foils were then removed, leaving the distal cantilever unit with a 100- µ m-high premature occlusal contact. For each individual and occlusal arrangement, the following variables were assessed 4 : 1. Local and total maximal closing forces in habitual occlusion. The local maximal closing force is the local force obtained at each registration point (transducer) when the subject is asked to close the jaws with maximal strength in habitual occlusion. The local values from the largest total maximal closing force were used. The total maximal closing force is the sum of all local maximal closing forces at a given moment. The largest value obtained from five consecutive maximal closings was used.2.  Mean local and total chewing forces calculated from one randomly selected chewing sequence. The mean local chewing force for each region (transducer) represents the average of the peak values of all local chewing strokes of one randomly chosen chewing sequence. The mean total chewing force for each individual was calculated from the sum of the local chewing forces at each registration point (transducer) at a given moment. These values were added and divided by the number of chewing strokes. Vertical Bending Stress. The maximal vertical bending stress ( ! ) in the elliptically shaped cantilever joint of the preferred chewing side created during closing and chewing in basic and interfered occlusion was calculated for each individual using the formula !  = M/Wb, where M denotes the vertical bending moment (the registered force ˚ distance from the point of load application on the cantilever segment to the cantilever joint [8 and 16 mm]) and Wb denotes the bending resistance. The Wb for frames with an elliptical cross-section was calculated with the formula Wb = w ˚ h 2  ˚  /32, where w is the width and h is the height of the  joint. 10  The maximal vertical bending stress during chewing was calculated from forces of the chewing stroke giving the largest bending moment at the cantilever  joint. To avoid fatigue of the material, frequently occurring stress has been recommended not to exceed 60% of the yield strength, 10  corresponding to 180 N/mm 2  for unhardened and 260 N/mm 2  for hardened dental gold alloy of the type used in this study.  JOMI on CD-ROM, 1990 Jan (70-77 ): Occlusal Interferences and Cantilever Joint Stre…Copyrights © 1997 Quinte… Statistical Analysis. Data are presented as group mean values and standard deviations. Student's t test for paired data was used to test whether force and stress values in interfered occlusion differed significantly from those in basic occlusion. Probabilities of less than 5% (P < 0.05) were considered statistically significant. Results Forces. The occlusal force patterns during maximal closing and chewing in basic and interfered occlusion are shown in Figs 2 and 3. Group mean local and total closing and chewing force values are given in Table 1. In basic occlusion, local closing as well as local chewing forces increased distally. The mean total chewing force (162 ± 78 N) amounted to approximately 50% of the total maximal closing force (335 ± 87 N). The premature occlusal contact on C 2  of the preferred chewing side had no influence on the total maximal closing forces as compared to basic occlusion. The group mean local closing force at the interfered C 2  was significantly increased (P < 0.05), whereas the local closing force at C 1  of the same side was significantly reduced (P < 0.05) (Table 1, Fig 2). With C 2  in premature occlusion, the group mean total chewing force was reduced by an average of 25% to 122 ± 53 N compared to basic occlusion. However, this reduction was not statistically significant (Table 1), although the individual total chewing force was reduced in six subjects, unchanged in one subject, and increased in one subject with small chewing forces. The mean local chewing force at C 2  in premature occlusion was not altered as compared to the basic occlusion arrangement (Table 1 ). However, the premature occlusal contact at C 2  gave an average relative (38%) increase of the local chewing force over that unit (from 29% to 40%) in relation to the group mean total chewing force. Vertical Bending Stress. The maximal vertical bending stress in the cantilever  joint created by closing and chewing forces in basic and interfered occlusion showed great interindividual variations (Table 2). In basic occlusion, the fatigue limit of 180 N/mm 2  for unhardened type III dental gold alloy was exceeded in two subjects (subjects 2 and 5) during closing and chewing. During each chewing procedure, all individuals exhibited frequent chewing strokes in which the individual maximal vertical bending stress was approached. The individual frequency of such chewing strokes varied between 20% and 55%. The introduction of the premature occlusal contact at C 2  resulted in a slight, although nonsignificant, increase of the vertical bending stress during closing but had no effect on the chewing vertical bending stress (Table 2). The group mean maximal vertical bending stress in the cantilever  joint of the preferred chewing side and the contribution from each cantilever unit are presented in Table 3 and Fig 4. In basic occlusion, on average more than 75% of the vertical bending stress was derived from forces on C 2 . In interfered occlusion, the corresponding value was more than 90%. Discussion
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