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Prosthodontic design and its potential effect on bone level around dental implants
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 remodelling 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 microstrain 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 mineralised 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 mesostructural 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.
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