The increasing use of implants in dentistry makes it necessary to reduce the cost and time of implant treatment. Babbush et al.  reported a reasonable survival rate for immediately loaded implants, and this treatment is clinically in great demand [2–4]. Immediate loading means that the occlusal load is applied to an implant within 48 h of implant placement . Nevertheless, the effectiveness of an immediately loaded implant is less predictable than that of the delay-loaded implant . The main concern is the occurrence of fibrous encapsulation instead of osseointegration around implants .
An important requirement of the immediately loaded implant is to minimize micromovement at the bone–implant interface (BII) during occlusion with the opposing dentition . The implant surface texture plays an important role in BII mobility, with roughening of the surface being beneficial to increasing the area of the BII  and the resistance to shear forces  due to the increased surface friction. Many techniques have been used to produce various types of microroughness structures on the implant surface, such as sandblasting, plasma spraying, and porous beading . Although the usefulness of a rough surface texture in implants for immediate loading procedures has been suggested , a deeper understanding is required for the effects of surface texture of immediately loaded implants on micromo- tion at the BII and the stress distribution in bone.
Implant macrodesign has been regarded as essential to the success of an immediately loaded implant [6,10]. The use of screw- type implants enhances more contact area in BII and improved implant stability . Other designs such as the stepped implant and the tapered body of threaded implant have also been proposed that mimic the root anatomy and enhance the bony support in spongy bone, thereby creating a favorable load distribution [11,12]. In addition, the size and shape of the thread might affect the stress pattern in the surrounding bone [13,14]. Nonetheless, the advan- tages ofminimizing the mobility at the BII and the surrounding bone stress in these designs in immediate loading applications remain to be proven, and hence require further investigation.
The present study compared the biomechanical effects of immediately loaded implants with various designs of implant shapes and designs of surface textures with different roughnesses in the edentulous mandible. Prior to osseointegration, the interface condi- tion between the immediately loaded implant and bone is in contact only and the interfacial micromotion between immediately loaded implant and bone can occur and influences the quality of osseointe- gration . Therefore, finite element (FE) modeling of nonlinear contact was used to simulate the contact interfaces between the immediately loaded implant and bone, and the interfacial sliding at BII was evaluated. Moreover, immediately loaded implants were subjected to in-vitro experiments to validate the accuracy of the nonlinear FE model that incorporated human mandible geometry.
This study revealed the biomechanical mechanisms (including bone stress and sliding at the BII) of immediately loaded mandibular implants with various implant designs and surface roughnesses. The FE models could be limited by the over- simplified loading conditions and the inhomogeneous material properties of human bone. Nevertheless, within the limitations of this study, the following conclusions can be drawn: 1. Both experimental and validated FE analyses confirm that the immediately loaded implant can induce disproportionate bone stresses during lateral loading, which might result in a high risk of surrounding bone loss due to overloading resorption. 2. Adding threading to an implant can significantly decrease the bone stress and sliding at the BII relative to nonthreaded implants (in cylindrical and stepped implants). 3. The stress in trabecular bone and sliding at the BII are lower for the rectangular threaded implant than for the v-thread implant. Using shorter threads in cortical bone decreases sliding at the BII but not bone stresses. 4. The stress reduction in cortical bone and sliding at the BII does not differ significantly between threaded implants with tapered and straight bodies, and stresses in trabecular bone are higher in the tapered body of threaded implant. 5. It is not purely advantageous for the implant surface texture to have a high roughness (such as produced by plasma spraying or a beaded porous surface), since this decreases sliding at the BII it increases the crestal bone stress around the implant.
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