Prosthodontics design and dental implants Assignment Help
Prosthodontics design and its potential effect on bone level around dental implants
Introduction
Implant dentistry has long been described as a prosthetic discipline. This means the clinician should first envisage the final result before embarking on the surgical procedure. Of course the decision is influenced by a number of factors, including, the patient wishes, aesthetics, function and occlusion.
Although the importance of the prosthetic component on the implant is well established, the literature is divided when it comes to how much of an influence the prosthetic component may have on the bone levels around an implant. Furthermore, few have investigated the effects that differing prosthetic designs may have on the bone in a clinical scenario.
Mechanical load causes adaptation and remodeling of bone via a process of resorption and deposition. When similar amounts of bone are being resorbed and deposited, equilibrium is present that is characteristic for physiological loading of bone. Frost in 2004 explains that the strain is proportional to the stress and the mechanical properties of the bone. Where the bone is softer, such as the posterior maxilla, the same stress may cause more macrostrain than in the anterior mandible.
In the case of overload, equilibrium between bone resorption and deposition is being disturbed, thereby causing fatigue-related micro-fractures at, and around, the bone–implant interface. These fractures are being repaired by bone resorption and a subsequent ingrowth of connective tissue and epithelium instead of new bone. Isidor 1997 compared the effects of plaque retention versus occlusal overload on implants. In an animal model of monkeys none of the implants in the plaque retention group were lost over an 18 month period. On the other hand, 5 out of 8 with occlusal overload were lost, and the rest showed increased rates of bone loss. Piattelli in 1998 corroborated these findings.
However, in 2004 Heitz- Mayfield did not support these results. After evaluating supra occluded implant prosthesis in dogs over 8 months, there were no statistically significant changes for any of the parameters from baseline to 8 months in the loaded and unloaded implants. Histologic evaluation showed a mean mineralized bone-to-implant contact of 73% in the control implants and 74% in the test implants, with no statistically significant difference between test and control implants. Furthermore, in a short study of only 4 weeks, Miyata in 2000 showed no bone loss in monkeys when the occlusal overload was just 100 microns. However, in this study, when 180 microns of occlusal discrepancy is introduced, almost 50% bone loss on the buccal surface is recorded.
Human studies are just as equivocal, while some studies have found no difference in biological and technical complications related to bruxism, others have found significantly more complications.
In the case of biomechanical complications, one or more components of an implant system may fail. For example, fracture of an implant itself, loosening or fracture of connecting screws or abutment screws, loosening or excessive wear of mesostructured components in overdentures, and excessive wear or fracture of suprastructural porcelain or acrylic teeth.
When a cantilever is involved, the forces created are amplified by leverage action and moments of forces are created. Therefore case selection, design of the prosthesis, and the occlusal scheme become even more crucial.
Although the biology of bone is not one of a static structure, finite analysis commonly used by engineers, gives us some idea of force distribution. Stegaroiu 1998 showed the magnitude and complexity of force vectors created by cantilever prosthesis based on three-dimensional finite element analysis. Assessment of stress development under axial, buccolingual , or mesiodistal loads(under 1:1:1 ratio), for 3 unit prosthetic designs including cantilever supported by two implants, conventional fixed partial denture on two implants, and three connected crowns supported by three implants. Overall bone stress was highest in cantilever design, compared with other two designs. This was especially so in the axial load where in vivo may have substantial ratio of magnitude.
With the cantilever design, the high stress around the distal half of the distal implant, which was found under axial loads, resulted from both the rotation in the vertical plane and the load applied to that implant. The rotation acted so as to extract the mesial implant, but the axial load applied to that implant canceled this action. Thus, almost no stress was found around the mesial implant. Under Buccal-lingual loads, the deformations that occurred as a combined effect of the rotations in the transversal and horizontal planes yielded increased stress around the distal implant and lower stress around the mesial implant.
Rodriguez 1993 investigated strain generated within an implant-supported prosthesis and on a simulated bone surface during functional cantilever loading, using symmetrical mandibular fixed-implant framework supported by six fixtures matrix of polymethyl methacrylate resin. Microstrain distal to the terminal abutment increased significantly with increasing cantilever length. Overall, microstrain increased 306% when the cantilever length was increased from 7 mm to 20 mm. Furthermore, Strains observed at the simulated bone surface increased dramatically as the distance to the adjacent active abutment increased or as the anterior-posterior span of abutments decreased. Distal abutment microstrain also increased significantly at the bone site as cantilever length increased. The authour concluded that an optimum biomechanical environment should exist when cantilever spans exceeding 7 mm are planned, regardless of the number of supporting abutments. Maximizing the number and anterior-posterior spread of supporting fixtures can decrease strain transmitted to the crestal bone, while minimizing the distance between the distal abutment and its adjacent abutment.
Consistent with these findings in vivo, Rangert et al 1995 analysed 39 patients with implant fractures treated by three different clinicians. The majority of the fractures occurred in the posterior region (90%), one to two implant supported prosthesis had combination of cantilever load and bruxism / heavy occlusal forces (77%). Even so, review of the literature reveals a low occurrence of implant fracture.
Studies on loaded implants in humans found little statistical or clinical difference in the biological outcome between cantilevers in full mouth prosthesis with distal cantilevers, as well as FPD, in medium term follow up of 4-10 years.
Halg 2008 studied 54 partially dentate patients of which twenty-seven FDPs were with cantilever and 27 without cantilever, located in posterior maxilla or mandible. The primary outcome was change in peri-implant marginal bone during the observation period of 3 to 12.7 years. After a mean observation period of 5.3 years, there was a trend for increase in mean peri-implant bone loss for the FDPs with cantilevers in the order of 0.23mm compared with 0.09mm for FDPs without cantilever. However, not surprisingly, this was not statistically significant as the power analysis was for a difference of 1mm, been decided on as figure of clinical significance. Wennstrom et al based on their post hoc power analysis assumed they would have needed at least 102 patients to show any statistical significance.
Systemic reviews have concurred with above findings, however cantilevers had increase trend towards biological complications and definite increased risk of technical complications.
Zurdo 2009 performed a systemic review on this subject with at least a 5 year follow up. The 5-year survival rate of cantilever FPDPs in weighted mean was 91.9%, with implant fracture as the main cause for failures, and without cantilever extensions weighted mean of 95.8% . Technical complications rate of suprastructure of weighted mean 20.3% for cantilever FPDPs compared with 9.7% for non-cantilever FPDPs. The most common complications were minor porcelain fractures and bridge-screw loosening. For cantilever FPDPs, the 5-year event-free survival rate had weighted mean 71.7% and between 85.9% for non-cantilever FPDPs. No statistically significant differences were reported with regard to peri-implant bone-level change between the two prosthetic groups, either at the prosthesis or at the implant level.
