| Backgrounds and ObjectivesImplant-supported fixed restorations are widely used in edentulous cases due to their long-term success rates,greatly improving the retention and stability of traditional dentures.According to the international dental implant clinical guidelines,the use of 6 implants for complete fixed mandibular prostheses is a reliable and standard treatment procedure.However,unreasonable framework design can bring considerable biological and mechanical complications in a complete implant-supported prosthesis,resulting in disorders in the masticatory system,such as temporomandibular joint disease,painful musculature and so on.Controversies exist regarding the use of segmented or splinted complete restorations in the edentulous mandible.On the one hand,the use of finite element analysis suggests that a 1-piece framework is more beneficial to stress distribution over implants and the surrounding bones compared to a 3-piece framework.Moreover,many retrospective studies have reported that cumulative survival rates of implant over ten years have reached more than 94%.On the other hand,mandibular flexure has been proven due to masticatory muscles during opening and functional movements.An unfavorable flexure in the posterior region of the mandible may result in greater stress and strain on implants and surrounding bones.Excessive stress concentration might facilitate marginal bone resorption under occlusal overloadThere is no present consensus regarding which prosthetic design,a 1-piece or 3-piece framework,is superior.The objective of this study was to evaluate how the framework design of implant-supported mandibular fixed complete prostheses influenced the masticatory system,based on clinical,occlusal and biomechanical analyses.Materials and Methods1.Collection and classification of clinical samplesThis clinical trial recruited participants from a group of patients with edentulous mandibles who underwent fixed implant supported restorations with 6 vertical implants between January 2012 and December 2019.Written informed consent was obtained from all participants.Sixty-five patients were assigned to 1 of 2 groups varying based on framework design.Thirty-six patients received 1-piece frameworks,while 29 patients received 3-piece frameworks.The framework designs of implant-supported mandibular fixed complete prostheses were evaluated based on clinical,occlusal and biomechanical outcomes.2.Clinical analysisClinical study included implant survival rates,mechanical complications and various biological indices.Oral health-related quality of life was recorded to evaluate patient satisfaction.3.Occlusal analysis(1)The patients then underwent occlusal examination at 12 months after prosthesis delivery.With the use of digital occlusion analysis system,four relative force-time graphs,including intercuspation,protrusion,and left and right lateral positions,were generated to record centric and eccentric occlusal characteristics,and occlusal parameters were measured.(2)Simultaneously,sEMG signals from the masseter and anterior temporalis muscles were measured via electromyography.sEMG signals and bilateral asymmetry indices of the masticatory muscles were recorded and calculated,respectively.4.Biomechanical analysisTwo finite element models,one simulating a 1-piece fixed complete restoration and the other simulating a 3-piece restoration with 2 posterior segments and an anterior segment,were created.Four framework materials and different trabecular bone were simulated.Occlusal loading and mandibular flexure from muscular forces were simulated.The stresses on implants and surrounding bones were analyzed and compared both quantitatively and qualitatively.Results1.Collection and analysis of clinical samplesCompleted fixed implant-supported prostheses,including ceramic and metal-ceramic frameworks,were delivered 4 to 12 months after implant surgery.Following prostheses removal due to peri-implantitis,three patients stopped participating in further occlusal examinations.Therefore,62 participants underwent follow-up examinations at 12 months.There were no statistically significant differences in age or sex between the 2 groups.2.Clinical analysisIn the 1-piece prostheses group,three-year and 5-year cumulative survival rates are 99.53%.In the 3-piece prostheses group,three-year and 5-year cumulative survival rates are 98.85%.The modified plaque index and modified sulcus bleeding index of 1-piece prostheses group are 0.77±0.13 and 0.86±0.1 1,respectively,whereas those in the 3-piece prostheses group are 0.67±0.10 and 0.75±0.13,respectively.There was a tendency towards increased inflammation around the implants in the 1-piece prostheses,as indicated by significant intergroup differences in modified plaque index and modified sulcus bleeding index.The annual average repair frequency was not significantly different between the 2 groups during follow-up.3.Occlusal analysis(1)In occlusal examination,intergroup differences in disclusion time of protrusion,left and right excursion were statistically significant.The mean disclusion time of protrusion in the two groups ranged between 0.61±0.18s and 0.71±0.14s,disclusion time of right excursion ranged between 0.62±0.26s and 0.81±0.34s,and disclusion time of left excursion ranged between 0.62±0.17s and 0.75±0.22s,respectively.(2)The significantly lower resting sEMG activity of 1-piece prosthesis group in both the right and left anterior temporalis muscles was observed when compared to the 3-piece prosthesis group.The resting sEMG of the right and left temporalis muscles in the 1-piece prosthesis group were 1.79±0.47μV and 1.81±0.41μV,respectively,at 12 months,whereas those in the 1-piece prosthesis group were 2.56±0.60μV and 2.23±0.60μV,respectively.Intergroup comparisons of masseter sEMG during resting and clenching were not significantly different.The asymmetry indices of the bilateral anterior temporalis and masseter muscles during clenching were not significantly different4.Biomechanical analysisSimilarly,in both designs,mandibular flexure from masticatory muscles,which was mainly distributed in the mandibular ramus,had little influence on the implants and surrounding bones.In both designs,the stresses were always concentrated on the buccal and distal cortical bone and the neck of the implants.The highest stress values were recorded in the posterior region in the 3-piece design.As framework materials were compared,pure titanium and zirconia frameworks exhibited the lowest stress values on the cortical bone and implants.As density of trabecular bone were compared,high density trabecular bone exhibited the lowest stress values on the cortical bone.Conclusion1.From the clinical viewpoint,a trend was observed towards higher inflammatory reactions around implants in the 1-piece framework group,as assessed by modified plaque index and modified sulcus bleeding index.2.From the occlusal viewpoint,the 1-piece framework group exhibited a more balanced physiological function for the occlusal outcomes of the masticatory system than the 3-piece restoration group.3.From the biomechanical viewpoint,the 1-piece framework group exhibited a pronounced reduction in stress levels for the implants and surrounding bones compared to the 3-piece framework group when the mandibular models were exposed to mandibular flexure and occlusal forces in the finite element analysis. |
