Introduction and Background
By the 1990's, Branemark's concept made such an impact in clinical dentistry that mainstream clinicians and public acceptance rose significantly to the extent that it adumbrated all other protocols and systems, specifcally one-piece (1P) implants. Improper clinical documentation, inclination towards reports of failed cases, and lack of university based implantology programs accelerated the isolation of the alternative approaches. Failure reports are not scientifically sufficient and conclusive enough to substantiate or refute any techniques or implant systems without proper investigation, hence there are discrepant reports of 1P and 2P success rate [1, 2, 3, 4, 5]. However, growth in reception and subsequent demand of dental implant phenomena have put forth considerations for shorter healing periods and better esthetics in last the decade or two [6, 7]. Therefore, 1P system is returning to the spotlight only after many studies [8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23] found 1P and immediate loading implants superior in terms of implant-bone interface (IBI), surgical protocol, and elimination of potential structural drawbacks of 2P implant in two-stage protocol. Regardless of the piece-count, primary stability and long-term success is the driving force behind the dramatic evolution of the implant design. The focus has been to introduce and marry a foreign device into the body as minimally invasive as possible, yet yielding perpetual stability. To achieve such tour de force, multiple components are scrutinized within surgical and biomechanical perspective .
Whether 1-stage, 2-stage, or immediate loading, a prosthetic abutment and an anchoring implant body, as the standard pieces, are accompanied by an array of expensive armamentariums and complicated protocols in all 2-stage-2P implants. In this approach, the implant is inserted into the bone surgically (first stage) followed by a healing screw. After months of prescribed hard tissue healing, a healing abutment is attached (second stage) for soft tissue healing and development of permucosal seal while the patient awaits another appointment for the final restoration [25, 26]. At the second stage, a minute yet redundant uncover surgery may be required to expose implant-abutment interface (IAI), although submerge healing is not a prerequisite of osseointegration .
In the case of 1-stage-2P, the hard and soft tissues are healed at the same time eliminating the second stage surgery and appointment. Finally, immediate loading requires attachment of the prosthetic abutment and final restoration, at the day of implant insertion, leaving healing abutment and screw dismissed.25 As alluded above, the implant is subjected to rotational load in multiple occasions during the course of the treatment. Considering the very low shear strength of the bone, consecutive tightening and un-tightening of the threaded components of 2P implants may potentially increase the risk of loss of IBI and already achieved primary stability .
Conversely, both major elements of the 1P implant are manufactured as one unit and practitioners require less than half of the armamentarium. 1P provides unparalleled surgical advantages in terms of surgical simplicity and level of invasion. It is accomplished in only 1-stage surgical procedure; and, often it is inserted immediately after extraction or ﬂapless with minimal osteotomy, substantially decreasing surgical trauma and post-operative edema, to provide for an uneventful and accelerated healing phase. Furthermore, it is associated with less bone grafting, sinuslift, and nerve transpositioning . Regardless of the patients' personality types, prolonged treatment period and unnecessary bloodshed are at inconvenience and depict undesirable impressions for already apprehensive ones.
The biological width in natural dentition is comprised of a connective tissue attachment and a junctional epithelium, respectively 1.07mm and 0.97mm on average, by which probing depth is determined . Similar to the tooth, biologic tissue encapsulates the implant by generating a band of soft tissue to provide for the integrity of the periodontium and protection from external factors such as mechanical and biological agents . Bone resorption occurs when the epithelium forges a defensive distance as an attempt to isolate the external factors by proceeding beyond them apically . It has been suggested that the location and presence of the microgaps, IAI and abutment-crown interfaces (ACI), are directly related to crestal peri-implant bone loss and stage two uncover surgery of the 2P implants [32, 33, 34, 35, 36, 37]. These studies found that exposure of the implant to the oral medium during the uncover surgery allows introduction of bacteria to the barely established biological width eliciting an inﬂammatory response and subsequent bone resorption at the crestal region, where the IAI approximates. In 1P implants, the IAI is excluded, the ACI location falls coronal to the biological width, and, thus, the risk of microgapinduced bone resorption is significantly reduced [35, 38] In another study comparing the 1P and 2P implants, the effects of micromovement and size of microgaps at the crestal bone were analyzed and concluded that the more components utilized in an implant system, the higher is the rate of crestal bone loss regardless of the size of the microgap .
Heat generation and dissipation are regarded as major concerns during implant surgery as well as abutment and prosthetic crown preparations [40, 41]. Osteotomy preparation is an inevitable direct assault to the labile bone. The resultant surgical trauma can be classified into mechanical and thermal injury from which the bone must recover by utilizing renewed blood supply in order to produce osseointegration at IBI . The amount of prepared bone, depth of osteotomy, and heat generated during drilling is detrimental to the implant success, especially at the crestal regions due to the presence of denser bone and insufficient blood supply [43, 44, 45]. Moreover, during prosthetic preparations, frictional heat easily conducts through the metal implant rapidly and jeopardizes osseointegration. Research has defined a thermal threshold of 47oC for 1 minute to avoid subsequent heat-induced cortical bone necrosis and impaired healing. The practitioners are strongly advised to utilize precautionary methods such as frequent coolant irrigation and short working intervals . When compared to 2P implant, 1P requires more abutment preparation by which excessive heat is generated fostering apprehensions amongst clinicians. However, Omer et al reported proper water irrigation as beneficial, serving to enhance the cooling capacity of 1P implant significantly and to prevent thermal induced injuries to adjacent hard and soft tissue . In another study on 2P implant, abutment preparation recorded a maximum temperature change of 2oC and 4.7oC, diamond and tungsten bur respectively, using standard turbine and water irrigation system .
Flow Chart 1: Over-all Pilot Study Design
"Stress treatment theorem", according to Misch, "is the key to implant treatment plans." Unlike the natural dentition, implant lacks the viscoelastic shock absorbing periodontal ligaments while fixated in the bone rigidly; and therefore, the surrounding bone and implant system are at high risks of fatigue and fractures under parafunctional forces. The width, length, and crestal cross-sectional shape of a transosseous structure, implant or tooth, become pertinent in diffusing such offense. The greater the width, the lesser transmitted stress to the bone, the length determines the location, and the cross-sectional shape resists and directs lateral and occlusal loads at the crest . However, in addition to the bone thickness and height limitations as the greatest obstacles to reckon with when choosing the right implant, no implant cross-sectional design comes close to mimic that of a natural tooth. Therefore, from a biomechanical perspective, one of the detrimental aspects in achieving adequate implant-bone approximation, stress distribution, and osseointegration of an implant is its design. Over-all geometry, prosthetic platform and abutment shape, macroand microgeometries, and material composition define what is referred to as the implant "design" [49, 50, 51, 52, 53].
Improper transition and dissipation of multiaxial functional load and bending moments is menacing to implantology success and marginal bone level preservation. The implant geometry and its boneimplant contact percentage (BIC%) greatly inﬂuences load distribution . Exact Reproduction of the manner by which natural tooth distributes stress and load to the adjacent bone is improbable by dental implants. However, geometrical similarity between a natural tooth and a tapered implant leads one to speculate that they may abide to similar principles when distributing forces . It has been suggested that tapered (conical) design has proven significantly superior to its counterparts, parallelwalled design, in achieving and maintaining primary stability even in D4 bone zones without any soft or hard tissue complications [55, 56, 57, 58, 59, 60, 61, 62]. Although parallel-walled implant scores 20-30% higher in providing surface area for osseointegration and lowering stress in cortical bone in a few studies [62, 63, 64]. Tapered implant compensates for deficiencies by obtaining much higher values in maximum insertion torque, maximum removal torque, and resonance frequency analysis [57, 60, 61].
Flow Chart 2: Over all Main Study Design
The coincidental release of Branemark's work at the time of technological revolution triggered a movement that lead to a multidimensional expansion in implantology in terms of materials and techniques. The material of choice has been titanium since 1940's, when Bothe et al observed the very first "bone fusing" . Titanium ubiquity is directly related to its chemical and mechanical properties. Anti-corrosive in biological ﬂuid, high strength-to-weight ratio, and machinability are unrivaled qualities that lend titanium "the gold standard" title . Although commercially pure Titanium (cpTi) has proven its clinical success, few alloys have been developed to compensate for its deficiencies. For instance, titanium-aluminum-vanadium alloy (Ti-6Al-4V) has shown to increase cpTi tensile strength at the cost of lower corrosion resistance. As any metal is bound to corrode, Ti- 6Al-4V corrosion toxicity was found to produce adverse local and immunological reactions. Yet, the most common commercial dental implants are manufactured from Ti- 6Al-4V [67, 68]. However, the binary titanium zirconium (TiZr) alloy poses as an integral and improved alternative in that it offers better strength without compromising biocompatibility and osseointegration [69, 70].
In conjunction with material, surface topography or roughness is pivotal complement to osseointegration. Generally, the main idea behind texturing the implant is to maximize surface area and BIC, thus, it is indicated in regions with poor bone quality . So far, three levels have been defined: macro, micro, and nano . Macro-level indeed produces favorable results in respect to primary stability; however, it is associated with ionic leakage and peri-implantitis. Nano-level has been advocated in the past few years as it encourages protein absorption and guides osteoblast adhesion to the titanium surface . Achieving nano-level roughness with current technology deems diffcult and expensive. More over, only a few studies have been conducted and many parameters are still unknown in respect to biological quantifcation and mechanism of action . On the other hand, Micro level yields the maximum bone-implant fixation as well as higher resistance to shear via configuration such as semi-spherical indentations of 1.5um in depth and 4 um in diameter [74, 75, 76].
Figure 1: Straight and Tapering Pre-Prototype Implants
By far, the most common dental implants are the root-form type due to their predictability and relative small size. According to Misch's terminology, the root-form implants are classified based on design into cylinder (press-ft), screw (threaded), or combination design . These models govern the transmission and conversion of occlusal load to the bone and different types of forces: compressive, shear, and tensile. Therefore, strategic engineering designs become more important than ever to counter and prevent the destructive shear and tensile forces . While the press-ft type benefits from macro- and microgeometries (e.g., surface topography, semi-spherical indentations) to obtain microscopic bond to the bone, screw type affixation is by means of microscopic elements of threads on the body of an implant . The best-known macro- and microgeometry designs and textures for osseointegration play major role in maintaining structural integrity of bone and implant as well as enhancing the rate and quality of bone-implant fixation.
Figure 2: Fully Seated PPI in SM 2
It is pertinent to mention that simply designing a perfect implant does not diminish the need to examine the cause(s) of implant failure, although science has yet to designate an exact reason for rejection . However, consensus is when establishment and/or maintenance of osseointegration is jeopardized or impaired, at early stages of bone healing, implant mobility is rendered as the epitome of unsuccessful implant surgery [78, 79, 80].
To date, geometrical studies on implant design have not investigated alternative shapes other than circular or oval in cross section of bone engagement. The aim of this in-vitro study was to develop a 1P, threadless, tapered, and hexagonal (in cross-section) implant design for immediate loading. It utilizes the concepts of fulcrum-lever force dissipation and wedging circumferentially and apically for primary stability while enhancing the clinical and functional aspects. It was assumed that the results of the present study would make it possible to explore alternative possibilities other than common implant devices, delivery methods, and protocols in implantology.
Methodology, Results, and Materials
In this study, the term "hybrid" refers to a combination of impact and drill methods. Also, "regular and irregular hex" refer to the cross-sectional shape of the implant prototypes. Regular hex is a symmetrical hexagon geometrically; while in the later, only two parallel planes of hexagon are equally and slightly longer than the other four planes in cross-section. Two human mandible specimens (Skulls Unlimited International Inc., Oklahoma City, OK, USA) were chosen to perform AllOn-6 substructures. The condition of each specimen was as following: one fully edentulous (FEM) and one with extracted sites (EM). Impact, drill, and hybrid delivery methods were used to insert 30 prototype dental implants in D1 and D2 bone zones. Initial anatomical landmarks were examined via pre-operative CBCT. Surgical kit drills were utilized to prepare osteotomies with respect to anatomical structures such as mental and inferior alveolar nerves. All the prototype implants were fabricated from Titanium Grade IV according to specifc chemical compositions (Table 1) and mechanical properties (Table 2). A total of 30 prototype dental implants were divided into 2 groups. In group 1, 15 Regular Hex Polygon Prototypes (RHPP) (Figure 3) and the FEM specimen were dedicated to the drill and hybrid methods. It was decided to utilize two delivery methods for placement of RHPPs for further analysis. In group 2, 15 Irregular Hex Polygon Prototypes (IHPP) (Figure 4) and the EM specimen were dedicated to the impact method only. Although all implants were 16mm in total length with a 10mm osseous engagement zone, RHPPs greatest width marked at 4.39mm and IHPPs greatest width were 4.20mm. Figure 5 and 6 demonstrate dimensional specifications for both prototypes.
Table 1: Chemical Composition of Titanium Grade IV
Table 2: Mechanical Properties of Titanium Grade IV
Figure 3: Fully Seated PPIs in HM
Figure 4: Bone Particles with in PPI Reservoirs
Figure 5: Mandible Specimens: IHPP Group (Left) and RHPP Group
Figure 6: Anatomical Landmarks: IHPP Group Pre-Op Analysis
Figure 7: Anatomical Landmarks: RHPP Group Pre-Op Analysis
Group 1: RHPP-Hybrid method
8mm vertical osteotomy preparations were made for placement of seven RHPPs. A precision drill was used to penetrate crestal bone for guiding holes. Then, a 2.4mm diameter tapered-tip pilot drill was used to prepare 8mm vertical initial osteotomies. This step was followed by osseous expansion with a 3.8x8mm and 4.25x8mm conical drills to a final depth of 8mm. RHPPs were initially driven to 8mm length of osseous engagement zone using a surgical handpiece at 45 N/cm (Figure 7). Then, they were impacted into an additional 2mm with a dental mallet.
Group 1: RHPP-Drill method
10mm vertical osteotomy preparations were made for placement of eight RHPPs. A precision drill was used to penetrate crestal bone for guiding holes. Then, a 2.4mm diameter tapered-tip pilot drill was used to prepare 10mm vertical initial osteotomies. This step was followed by osseous expansion with 3.8x10mm and 4.25x10mm conical drills to a final depth of 10mm. RHPPs were driven to 8mm length of osseous engagement zone using a surgical handpiece at 45 N/cm. Then, they were tightened further with a ratchet to a final depth of 10mm.
Group 2: IHPP-Impact method
A703FG bur and a surgical stent were used to penetrate crestal bone vertically for 5mm pilot holes. Then, the prototypes were tapped to the final length of 10mm using an industrialgrade electric hammer guided by a carrier.
All placements were studied with post-operative CBCT studies and digital photographs. Additionally, periotest was conducted to measure initial instability and resistance to micromovement of all the implants before extraction and evaluating the amount of autogenous bone graft collected during each delivery method.
Materials-Topography and Design
All prototypes were a 16mm long, 1P, and hexagonal structure in cross section of the Bone Engagement Zone (BEZ). A gradual taper mimics the shape of a root of the natural dentition from the Crestal Zone (CZ) to the apical end portion of the implant; such that, the implant device is thicker and wider at the BEZ than at the apical end portion (Figure 3 and 4).
By design, this prototype is divided into multiple zones each serving a/or multiple purpose(s). The 6mm Restorative Zone (RZ) is comprised of 3mm Attachment Zone (AZ) and 3mm Prosthetic Margin Zone (PMZ). Whereas, 3o taper is evident from the top of the RZ to the platform switch in IHPPs (Figure 6), RHPPs experience such taper only at the PMZ and its AZ remains parallel.
Finite Element Analysis
The Bone Engagement (BEZ) and Restorative Zone (RZ) of the RHPP and IHPP were subjected to a series of comparative behavior of fatigue failure. Four types of spherical titanium alloy coping (Ti-6Al-4V Gr. 5) was attached to each RZ to ensure the uniform transfer of applied load while stimulating prosthetic crowns of various lengths. Additionally, load was applied at 30o to the long axis of the device (Figure 5). Geometrically, the RZ is designed to accommodate for multi-unit restorations and delivery systems to engage an external hex at the most coronal 3mm of the prototype. The PMZ in collaboration with the initial 2mm of the Implant Body Zone (IBZ) provide the 5mm Machines Surface Zone (MSZ), which aids to prevent plaque accumulation. The 2mm Crestal Zone (CZ) represents the transition zone from the PMZ to IBZ at the crest of the ridge. A total of 36 semi-spherical indentations, which acted as autogenous bone graft reservoirs in this study, are engraved onto the 6mm Harvesting Zone (HZ) linearly, 6 on each plane and 1 mm deep at the most concave point. All the planes and beveled corners converge harmoniously into a chisel-like apical end portion to from the 2mm Fulcrum Zone (FZ). The 10mm BEZ is referred to the IBZ and FZ collectively
Figure 8: RHPP Implant
angled to better approximate standard masticatory force vector in the worst-case scenario. Four IHPPs and one RHPP implants were embedded onto separate bases. The objective was to evaluate the differences in the FEA and the maximum load that the best configuration test could tolerate during a fatigue test in multiple scenarios. Table 3, 4 illustrates the testing conditions for each implant in details. The list of implants that were tested as follows:
Table 3: Testing Conditions of Each Implant Configuration Test
Table 4: Composition of the Implant Components
Figure 9: IHPP Implant
Figure 10: RHPP Implant Dimensional Specifications
Figure 11: IHPP Implant Dimensional Specifications
Figure 12: RHPP Implant Insertion Assembly
Figure 13: Anatomical Landmarks: IHPP Group Post-Op Analysis
1) IHPP Implant Configuration 1 Test with support
2) IHPP Implant Configuration 2 Test with support
3) IHPP Implant Configuration 3 Test with support
4) IHPP Implant Configuration 4 Test with support
5) RHPP Implant Configuration 5 Test with support
The Mathematical calculation model of the implants was obtained by means of tetrahedral solid elements. The structural analysis was conducted in a linear elastic feld. Therefore, a linear increase in maximum stress in bone was expected. The interface between the titanium alloy coping and the RZ was considered continuous.
Load and Constrains Application Method
The structural analysis is conducted by applying a unit load at an intensity of 1N in accordance with the standard requirements. The linear elastic analysis allows applying the principle of superposition for determining the limit load of static resistance and fatigue resistance. In case of implants that do not have pre-angled components, the UNI EN ISO 14801:2017 standard requires that the load applied has a straight line of action, forming an angle of 30o with the axis of the implant. The fxture must be embedded onto a base so that the nominal bone level is distant 3 mm from the connecting section to stimulate bone resorption. The test scheme indicated by the standard is reported in Figure 14. The Figure 15 to 19 show the main dimensions for application of the loads and constraints on all configurations.
Figure 14: Anatomical Landmarks: RHPP Group Post-Op Analysis
Figure 15: Mandible Specimens: IHPP group (Left) and RHPP group
Figure 16: Extraction
Figure 17: Prototype Zones
Figure 18: IHPP Implant Configuration 1 test with Support-Mesh
of the Mathematical Model with Tetrahedral Elements
Figure 19: IHPP Implant Configuration 2 test with Support-Mesh of
the Mathematical Model with Tetrahedral Elements
Method adopted to test the static resistance
Comparing the Von Mises equivalent stress with the yield strength reported in Table 5 tests the static resistance.
Table 5: Mechanical Properties of the Material Used
Result of the static resistance structural tests
A series of illustrations showing the stress condition of the IHPPs and RHPP implants (Figures 20- 24). In alphabetical order, the conditions represent:
Figure 20: IHPP Implant Configuration 3 test with Support-Mesh of
the Mathematical Model with Tetrahedral Elements
Figure 21: IHPP Implant Configuration 4 test with support-Mesh of
the Mathematical Model with Tetrahedral Elements
Figure 22: RHPP Implant Configuration 5 test with support-Mesh of
the Mathematical Model with Tetrahedral Elements
Figure 23: Test scheme indicated by the UNI EN ISO 14801:2017
Figure 24: IHPP Implant Configuration 1 test with support-Main
dimensions for the Structural Analysis (Dimensions in millimeters)
a) Von Mises equivalent stresses on the external threading of the BEZ and RZ.
b) Equivalent Von Mises stresses on the implant.
c) Detail of Von Mises equivalent stresses on the implant.
d) Equivalent Von Mises stresses on the implant.
e) Detail of Von Mises equivalent stresses on the implant.
Result of the fatigue resistance structural tests
The fatigue load limit was calculated by applying the procedure described earlier. The results that emerged are shown in the table 6. It illustrates the fatigue load limit for each implant configuration, setting a limit value of nodes that does not exceed 0.5% and 5% of the total.
Figure 25: IHPP Implant Configuration 2 test with support-Main
Dimensions for the Structural Analysis (Dimensions in Millimeters)
Figure 26: IHPP Implant Configuration 3 test with support-Main
Dimensions for the Structural Analysis (Dimensions in millimeters)
Figure 27: IHPP Implant Configuration 4 test with support-Main
Dimensions for the Structural Analysis (Dimensions in Millimeters)
The Restorative Zone of (RZ) of the RHPP and IHPP were subjected to periotest to assess the mobility of the implant after insertion and subsequently the quality of primary stability. A Periotest M handpiece, type 3218, Medizintechnik Gulden Germany, was used, which measures the damping characteristics of the periodontium and, indirectly, tooth/implant mobility. During a measuring cycle of this device, an electrical motor activated a percussing rod to tap an implant surface approximately 16 times over a period of 4 seconds. It output measurements in the form of a Periotest Value (PTV) ranging from -8.0 to +50.0, which correlate to Miller's Mobility Index (Table 6, 7). The smaller the PTV, the higher is the stability of the implant. Table 8 illustrates the PTV and its interpretations in correlation to implant mobility clinically, according to the manufacturer guideline values.
Table 6: Fatigue Load Limits for each Implant with 0.5% and 5% Limit Value of Nodes
Table 7: PTVs Compared with Miller Mobility Index
Table 8: Guideline Values in Correlation to PTV
The device was held buccally in a horizontal position, +/- 25o, and directed perpendicularly to the long access of the implants at a distance between 0.6 and 2.5 millimeters to the center of the RZ end tip. The PTVs were taken three times for each implant and the average value was recorded. Additionally, the positions of placements were divided into Anterior and Posterior position in respect to D1 and D2 bone zone, for which the total average value was calculated individually for further analysis.
Figure 28: RHPP Implant Configuration 5 test with support-Main
Dimensions for the Structural Analysis (Dimensions in Millimeters)
Figure 29: Static resistance of IHPP Implant Configuration 1 test with
Perio Test Results
The recorded mean PTVs of all 30 implants are presented in table 9.
Table 9: The Mean PTVs of all the Implants
The present in-vitro study analyzed thirty titanium, solid, 1P, threadless, and cross-sectionally hexagonal implant design during and after insertion. During delivery, the biomechanics and protocol facilitation aspects of the device, which utilized the concepts of fulcrum-lever force dissipation and wedging circumferentially and apically, were assessed, while the quality of primary stability and fixation was evaluated after. Engineered and intended for immediate loading, the implants were delivered into two human mandibles D1 bone zones by means of conventional rotational, impact, and hybrid methods.
Figure 30: Static resistance of IHPP Implant Configuration 2 test with support
The concept of osseointegration has come a long way since Branemark introduced in the 1970's. Ever since, geometric designs and surface characteristics have been continuously modified and incorporated into varieties of implant designs for the sole purpose of achieving primary stability. How bone and implant interact essentially defines the long-term success in implant surgery. Profound osseointegration depends on the quality of initial rigid fixation at the time of implant placement, primary stability, and bone apposition onto implant surface during and after the healing process, secondary stability . On the other hand, biomechanical factors implemented in implant designs and prude treatment planning demand close considerations to minimize the potential risks of failure. Having the mentioned criteria in mind, recently we have designed a unique 1P implant system to provide for enhanced primary stability, conservative osteotomy, and controlled ridge expansion.
Figure 31: Static Resistance of IHPP Implant Configuration 3 test with Support
Inspired by an industrial chisel, our 1P implant can be easily differentiated by its shape and design, when compared to other common implant systems. The polygon design, hexagonal in cross-section, delivers six slightly rounded line angles and ﬂat planes with six semi-spherical indentations on each. These macrogeometries are present from the Crestal Zone (CZ) and are continuous along the tapering Bone Engagement Zone (BEZ) to finally merge together at the Fulcrum Zone (FZ) creating a miniature chisel. The objective behind such design was to produce a facilitating effect during and an antirotational effect after implant insertion within the osteotomy via harvesting the power of fulcrum-lever and wedging concept.
Figure 32: Static Resistance of IHPP Implant Configuration 4 test with Support
The FZ, a V-shaped tip coupled with a pair of auxiliary ﬂat planes on either ends, stands as the primary wedge, anchorage, and fulcrum. Together with the tapering BEZ, the lever and the secondary wedge, they act harmoniously effective to decrease amount of load necessary to insert the implant into osteotomy.82 During implant placement, this design allows operators to press-ft and/or rotate and guide the implant into freshly extracted socket or prepared osteotomy with enhanced control. Taking both the threadless and tapered design into account, a 1P design provides ease of implementation, in terms of fewer protocol steps and accessibility, and prevents inadvertent bone loss that the threaded implants may cause during insertion into the hard to reach D3 and D4 . In comparison to parallel-walled implants, previous studies have confrmed that tapered designs offer enhanced primary stability, particularly in bone types of soft quality, due to higher compressive force on the cortical bone regions [55, 56, 83, 84]. These fndings were confrmed in this clinical study as none of the prototypes exhibited micromovement that may held accountable for poor stability.
Figure 33: Static Resistance of RHPP Implant Configuration 5 test with support
The implant is designed to serve as an exceptional anti-rotational lock. Twelve ﬂat planes of the BEZ and FZ with different areas and angulations appose bone surface and direct bone remodeling and growth creating an accurate negative impression of the implant, as compression loads, by which rotational load is resisted during parafunctional activities . The crestal bone had conformed precisely to the CZ circumference without any evident gaps clinically. The corners were embedded into cortical bone rendering rotational movement impossible. Close approximation of bone-implant at the CZ seals the entrance of fbrous tissue and pathogens at early stage of healing and enhances primary stability [82, 85, 86]. Two studies [87, 88] have reported an angled geometry at the crest module of a design reduces the risks of bone resorption via imposing compressive component to adjacent bone. Additionally, it is hypothesized that such tapering design prevents surgical complications such as dislodgement into facial cavities.
Whereas other common implant systems may have only a few spherical indentations, the Bone Harvesting Zone (BHZ) holds a total of thirty-six along the BEZ that transfer occlusal load into the bone and resist strain and stress . It was discovered in our study that these indentations also served as reservoirs of autogenous bone harvested during insertion. Aside from their intended purpose, the BEZ and FZ macrogeometries shaved the osteotomy wall and produced autogenous bone while the implant was being seated rotationally. Significant amount of bone particles were harvested and collected during all three delivery methods by which the need for bone graft could potentially be decrease. In conjunction with precise bone-implant contact, bone-harvesting capability of this design promotes profound primary stability during bone healing and aid subsequent long-term fixation.
In the 1P implant systems, the prosthetic abutment and implant body are milled as one single unit. A prosthetic crown may be retained on the abutment portion either by a screw or cement . The screw-retained crowns (SRC) are most common due to retrievability and inconsequential retention and resistance-form [89, 90]. Although SRC have been the most popular choice, studies have reported that an inclination is on the rise towards cement-retained crowns (CRC) [91, 92]. In natural tooth restorations, the type of luting agent mandates the quality of resistance to dislodgment under compressive and shear forces . Rosenthal et al  proved high compressive strength cements are the best candidates to counter such forces thereby applicable for implant-supported crowns. The hex abutment design observes the same retention and resistance-form as natural tooth preparation. In despite of proportionally smaller surface areas, the parallel walls and angled-hexed designs can accommodate dimensionally larger crowns than natural tooth preparations [93, 94]. In another study, Kwan et al  demonstrated that CRCs of common hex abutment designs well-resisted displacement under off-axial load, which mimics physiologic load.
The other possible cause of affnity towards the traditional CRC is the possible structural drawbacks of the screw system. A SRC adds an additional screw to the 2P systems screw collection, abutment and healing screw, tallying to three. Although recent screw designs and concepts have reduced the rate of screw failure in the 2P designs dramatically , the resultant complications call its value into question. On the other hand, the screw-less CRC 1P implant conveniences both operators and patients as the hex abutment provides for the prosthetic crown attachment and eliminates possible uncover surgery.
Load distribution is greatly inﬂuenced by multiple aspects of the implant, bone, and force vectors. Finite Element Analysis (FEA) was designed in the late 1970's to predict functionality and feasibility of different implant designs in laboratory settings and predict their behavior and effects in real-life clinical cases. Therefore, some margin of error accompanies such theoretical analysis . In this in-vitro study, the results of FEA were divided into the BEZ, the fxture, and the RZ, the abutment. Four types of titanium alloy attachments were used that slightly differed in vertical height and the amount of the RZ they covered. Efforts were made to stimulate realistic clinical scenarios as closely as possible by designing multiple implant configuration tests.
In static resistance structural test, the implant configurations 1, 2, 4, and 5 tests demonstrated even load distribution vertically and laterally through out the RZ and the initial 1/3 of the BEZ, so that the load was completely dissipated and neutralized before reaching the embedded 2/3 of the BEZ (Figures 15, 16, 18, 19). In the mentioned configuration tests, the Von Mises equivalent stress never exceeded beyond 2.083e. The epicenters were at spherical head-RZ, RZ-BEZ, and BEZ-Base interfaces. IHPP implant configuration tests experienced identical load distribution patterns with almost similar stress resistance magnitudes. It was assumed that the minimal difference was due to the difference in the design of titanium coping attachment, which provided stability and support in resisting load. Therefore, the same principle might be duplicable clinically. The more embodiment of the RZ by prosthesis, the more occlusal load resisted and less transferred to the crestal bone. However, in IHPP implant configuration 3, where the BEZ was fully embedded, major stress of 4.583e was experienced at the BEZ-RZ interface diagonally opposite of load application area. A possible explanation could be a combination of the design of this interface, which is near butt joint, and higher bending movement aided by the least stable coping. In this particular scenario, the base and the BEZ-RZ interface were equi-level while the load source and the platform switch were the furthest apart when compared to other configuration tests. The long coping that covered the minimum amount of the RZ collaboratively provided longer moment of arm and consequently more bending movement. Additionally, load application parallel or perpendicular to the FZ tip did not make significant difference in terms of fatigue load limit, load dissipation pattern, and stress resistance tolerance in IHPP implant configurations 1 and 4.
In fatigue load and structural static resistance tests, RHPP implant configuration 5 test with a short coping demonstrated the best result (Figure 19 and Table 6). It tolerated 374-N with 0.5% and 767-N with 5% maximum nodes outside of the test. Load distribution pattern was much more spread uniformly, while the epicenters located at RZ-BEZ and BEZ-Base interfaces. Although the overall magnitude of stress equaled the other configuration tests, the area under the most stress was significantly the smallest in the RHPP configuration 5 test. The greatest constrain related to BEZ-Base interface at the level of first spherical indentation of the BEZ.
The methods for assessing implant mobility or stability are either subjective or objective. An objective evaluation becomes highly important when examiner dependent nature of the subjective method may be convoluted by bias . Periotest is a quantitative, noninvasive, and reproducible method that has been reported reliable to evaluate primary stability of implant upon delivery [96, 97, 98, 99]. In this study, periotest device measured impressive PTVs consistently from -7.5 to -2.9 as the most to least stable implant (Table 8). The placements were divided into anterior and posterior in respect to their mandibular position. The average PTV of anterior position group was determined greater than their posterior counterpart. The highest values on average belonged to Group 1 Hybrid method, RHPP implants. Additionally, our evaluation during extraction determined that the polygon implant provided sturdy primary stability within D1 and D2 bone when industrial plier had to be used often.
A major limitation of this study, however, was the small sample size of implants examined. Prospective clinical research is needed to determine the exact micromovement and heat generation statistical Figures as well as the long-term effects of load distribution around implants and abutments in respect to marginal bone preservation.
Within limitations of this study, we demonstrated that: 1) A polygon-shaped (in-cross section) and tapered design enhances primary stability and provides for antirotational lock. 2) Fulcrum-Levered force dissipation concept facilitates and simplifes common protocols in implantology. 3) The conical design prevents dislodgement into oral cavities. 4) Over all design provides clinicians with enhanced control and maneuverability during insertion.
The authors have no financial interest in any of the companies mentioned in this article and received no compensation for writing this article. For this work, no funding was received. Dr. Kianor Shahmohammadi: the inventor of the implant designs and related parts Dr. Masih Rezaei: research and article writing, data collection and data analysis, research concept and design. Ilia Deylami: research assistant and head of photography. Mallori Shahmohammadi: patent Assignee.
For this ex-vivo study, neither approval from an institutional review board nor an ethics review committee was necessary.