# Biomechanical Rationale for a Novel Implant Design Reducing Stress on Buccal Bone
## 1 Introduction  
Achieving adequate primary stability still is one of the major goals in dental implant surgery and is governed by several parameters including bone quality [1], surgical protocol and implant design [2; 3; 4]. Host factors such as diabetes and osteoporosis are difficult to control for dentists but have been shown to also affect osseeintegration [5; 6]. Consequently, implant manufacturers advocate drilling protocols [7] with varying levels of undersizing depending on the bone quality present in a specific implant site [8]. As a general trend, undersizing an osteotomy and the use of tapered implant macrodesigns compressing cortical layers of alveolar bone [9] increases primary stability [3; 10; 11; 12]. Using both clinical and animal research designs, several authors have meanwhile reconsidered the concept of high initial implant stability being a desirable goal [13]. A retrospective clinical study showed that excessive insertion torque in dense bone can cause negative marginal bone responses [7] thereby confirming data from a randomized clinical trial recording more peri-implant bone remodeling and buccal soft tissue recession in implants inserted with high insertion torque [14]. While still concluding that high insertion torque would not have a negative effect, [PERSON] and coworkers found a three-year survival rate of only 98.6% and a considerable bone loss of up to 2.5 mm for implants placed with high insertion torque [15]. On the other end of the spectrum, a clinical study proved that lower insertion torques may yield favorable implant survival rates with optimal marginal bone levels [16]. In this context, it might be anticipated that less stress is inducedin maxillary sites as compared to mandibular sites due to their lower percentage of stiff cortical bone. While this question certainly cannot be addressed in a clinical study, a recent meta-analysis indeed showed greater risk for implant loss in the maxilla [17]. A clear trend depicting the negative effects of high insertion torques on alveolar bone can be derived from animal studies. It was pointed out that underpreparation of the implant socket compromised osseointegration of immediately loaded bone-level tapered implants [18] and reduces crestal bone-to-implant contact at early healing stages [11]. In addition, it has been shown that the speed of new bone formation was relatively slow while the speed of marginal bone loss was relatively fast when the final drill was excessively smaller than the implant to be placed [19]. Two animal studies showed a correlation between microcracks induced during implant placement and bone resorption [20, 21]. Knowledge on how immediate vs. conventional implant loading might affect bone in this context is sparse [22]. However, similar amounts of osseointegration have been described irrespective of the implant insertion torque applied [23]. Using various analyzing techniques up to molecular levels, it has recently been proven that placing implants with high insertion torque leads to a zone of dead and dying osteocytes, resulting in microfractures of bone increasing bone resorption but decreasing new bone formation [24, 25]. Insertion torque may be seen as a common denominator for assessing implant stability [13, 26] and has been correlated with resonance frequency analysis [26] and damping capacity assessments [27]. While a certain level of peri-implant marginal bone loss during healing is widely accepted [28, 29], it may be argued that this phenomenon results from overstressing of alveolar bone during implant installation. In response to that and as a potential alternative to common tapered bone-level implants (Figure 1a), a novel implant macrodesign characterized by an increase in the core diameter of the implant in the middle portion was fabricated (Figure 1b). The change in core diameter coincided with a continuous change in thread geometry starting with sharp threads in the apical region, dull, condensing threads in the middle portion and sharp threads in the cervical portion.