Ⅰ. Introduction
1. Definition of root analog implant and fabrication process
2. Finite element analysis
3. Mechanical test in vitro
4. Histologic evaluation
5. Histomorphometry analysis
6. Destructive mechanical test for osseointegration measurement
7. Micro computed tomography (CT)
8. Periapical radiography
9. Implant stability measurement
10. Clinical parameters
11. Perspective and future directions
Ⅰ. Introduction
When root analog implants are placed, following tooth extraction, according to the shape of the tooth, the discrepancy between the implant and the tooth, trauma, and surgical complications can be reduced.1 In addition, an esthetic implant prosthesis can be achieved by placing an appropriate implant in the planned location.2 For these reasons, efforts to manufacture and place root analog implants in the extraction socket have been in progress since the late 1960s, including the Re Implant® System, BioImplant, CAD/CAM fabricated titanium RAI, Replicate TM System, and 3D printed RAI.3 Nevertheless, there is a lack of literature on which factors should be evaluated and analyzed when utilizing a root analog implant.
To use root analog implants in clinical practice, it is necessary to determine whether the functional load distribution of the implant is appropriate through finite element analysis. Concentrated stress on an implant site may lead to implant fracture4,5 or surrounding bone loss.6,7 Usually, it is difficult to identify concentrated stress in a short period of time because fatigue fractures or peri-implant bone loss caused by overloads occur after long-term accumulation, that is, several years or more. Therefore, conducting finite element analysis (FEA) might be useful to evaluate whether it is appropriate to use root analog implants in clinical situations.
If an appropriate design is selected through finite element analysis, it is necessary to evaluate the mechanical strength of the fabricated root analog implant. Unlike conventional thread-types, root analog implants tailored to the patient’s extraction socket can be used. The validity and rationale of dynamic loading tests in these implants should be investigated to measure the mechanical strength, which has been applied to conventional thread-type implants. Recently, 3D printing technology has been applied to manufacture customized implants, followed by additional surface treatment.8 Since 3D-printed implants are manufactured by melting metal powder, the mechanical strength may differ from that of the powder itself after additive manufacturing. Therefore, it is necessary to identify whether the root analog implant has sufficient mechanical strength for clinical application with torsion, compressive fatigue, and dynamic loading tests.9
Although the biocompatibility of titanium or titanium alloys that form oxide layers in air is well known,10 the biological response with regard to the macrodesign and surface treatment of root analog implants could be confirmed through animal experiments.11,12 The experimental animal and organ should be carefully selected with regard to the characteristics of the animal, bone tissue ratio, and purpose of the study. The biological response of the implant should be evaluated using appropriate methods.
For the success of implants, osseointegration, which is regarded as a direct bone-to-implant contact, is guaranteed.13,14 No fibrous tissue is interposed between the implant surface and living bone in microscopic view.15,16 It is necessary to confirm whether the above-mentioned osseointegration is well-established, and if there are any specific inflammatory results in the histological observation.
It is also necessary to evaluate implant stability through objective quantitative monitoring to determine the appropriate time to apply functional load. In the conventional Branemark protocol, a non-loading healing period of 3–6 months is recommended to obtain adequate biological stability before functional load is applied.17,18 There is a case series study applying a non-loading healing period of 3–6 months, similar to that of conventional thread-type implants.2 However, there are insufficient data suggesting an appropriate non-loading period through randomized controlled clinical trials. As the root analog implant is a patient-specific medical device, the optimal non-loading healing period must be tailored to each patient to reflect the patient-specific healing period in clinical situations. This will enable patient-friendly treatment and application of an optimal non-loading healing period to each patient, compared with the conventional thread-type implant protocol by quantifying the implant stability at various time points before implant prosthesis delivery.
In this narrative review, we tried to find the optimal evaluation methods for root analog implants. For this, we thoroughly checked each of the various methods applied to conventional thread-type implants.
1. Definition of root analog implant and fabrication process
Conventional thread-type implants that are widely used in clinical practice can be divided into the following types: bone-level internal type, bone-level external type, and tissue-level internal type. Root analog implants differ from the three types of conventional thread-type implants mentioned above in terms of structure and/or shape. To date, there have been no reports of root analog implants manufactured with internal threads, as seen in conventional thread-type implants. Most root analog implants are applied in such a way that there are no components, such as screws that connect the suprastructure and implant fixture. The root analog implant structure can be divided into the following components: implant fixture (root area and abutment) and implant prosthesis. Both components are attached with dental cement.
Previously, root analog implants were manufactured by taking the impression of the extraction socket or extracted tooth. Virtual root analog implant design has been performed using software since the development of digital technology. Considering the manufacturing method, the milling method was initially applied to fabricate root analog implants. Recent studies have demonstrated that these implants can be fabricated using 3D printing technology. The materials used are titanium, zirconia, and titanium alloys to improve the mechanical properties.
2. Finite element analysis
Before investigating the biocompatibility of dental implants in histologic and histomorphometry analyses (Table 1), the behavior of structures or tissues can be evaluated under specific stimuli using finite element analysis, which can be applied to periodontal ligaments, traumas, fractures, post- and core restorations, obturators, or dental implants to analyze biomechanical changes in tissues.19 In particular, finite element analysis has been applied to investigate the optimal implant design, superstructure, and materials by evaluating the stress distribution within the bone during mastication, which could not be measured in vivo.20,21
Table 1.
Summary of histomorphometry results obtained using conventional thread-type implants
| Species | Bibliography | Implantation area | Time to retrieve | BIC* |
| Mouse |
Armand Keuroghlian et al. (2015)78 | Femur | 4 weeks | 73.70% |
| 8 weeks | 86.95% | |||
| Rat |
Dong et al. (2018)79 | Maxilla | 14 weeks | 46.36% |
| 28 weeks | 72.99% | |||
| Rabbit |
Anavi Lev et al. (2020)80 | Calvaria | 8 weeks | 41.23% |
| Rabbit |
Martins et al. (2018)81 | Tibia, cortical | 8 weeks | 48.8–56.3% |
| Dog |
Si et al. (2015)82 | Maxilla | 8 weeks | 20.33% |
| 24 weeks | 40.05% | |||
| Dog |
Lee et al. (2019)44 | Mandible | 2 weeks | 67.10% |
| 4 weeks | 81.42% | |||
| 12 weeks | 89.85% | |||
| Pig |
Cardoso et al. (2017)83 | Parietal bone | 4 weeks | 28.8% |
| 12 weeks | 42.8% | |||
| minipig |
Fabbro et al. (2017)84 | Tibia, cortical | 12 weeks | 76.05–83.53% |
| Minipig |
Bissinger et al. (2018)85 | Maxilla | 4 weeks | 34.23% |
| 8 weeks | 36.68% | |||
| Minipig |
Ruehe et Al. (2011)86 | Mandible | 8 weeks | 54.19% |
| Sheep |
Abdel-Haq et al. (2011)87 | Mandible | 3 weeks | 71.6–2.27% |
| 6 weeks | 76.8–80.64% | |||
| Sheep |
Haas et al. (2003)88 | Maxilla | 12 weeks | 20.0–30.4% |
| 16 weeks | 25.1–32.0% | |||
| 26 weeks | 20.7–35.5% | |||
| Sheep |
Stocchero et al. (2018)89 | Mandible | 5 weeks | 51.0–62.9% |
| 10 weeks | 58.2–61.3% | |||
| Goat |
Vercaigne et al. (1998)90 | Maxilla | 12 weeks | 8.8–29.5% |
| Goat |
Xu et al. (2015)91 | Mandible | 6 weeks | 41.3–55.2% |
| 12 weeks | 48.3–63.6% | |||
| 24 weeks | 67.8–76.1% | |||
| Goat |
van Oirschot et al. (2016)92 | Iliac bone | 4 weeks | 40.7–66.7% |
| Monkey |
Romanos et al. (2003)93 | Mandible |
12 weeks (no loading) | 50.20% |
|
12 weeks (immediate loading) | 64.25% | |||
|
24 weeks (delayed loading) | 67.93% | |||
| Human |
Kohal et al. ( 2016)94 | Maxilla & mandible | 38–75 months | 76.5% |
* The BIC values not presented in the text but presented in a graph, were measured indirectly by measuring the bar graph length. When the values were presented with both pathological and healthy states, those for the healthy state were selected. Results obtained from the use of specific growth factors are excluded.
Although various finite element analyses related to dental implants have been conducted over the past decades, precision and accuracy limitations still occur. Therefore, finite element analysis results should be used as basic data for preclinical experiments, and then the analysis should be validated through additional biological experiments, if possible.22,23
Studies related to finite element analysis have been performed more recently (Table 2). The first study in the table focused on the shape of the root analog implant abutment.24 The oval-shaped abutment resulted in lower stress and better durability than the circular abutment. Another study was related to the design of the root area of the root analog implant.25 Finite element analysis was performed on the following root shape designs: non-modified, wedge added at the root surface, lattice added at the root surface, and apex-anchor added at the root apex. The apex anchor added at the root apex design resulted in lowest stress concentration and microdisplacement among the designs. However, since finite element analysis related to the optimal design of a multi-root analog implant has not yet been established, more in-depth research is required.
Table 2.
Summary of finite element analysis of root analog implants
| Study |
Study design |
Material model |
Bone model |
Bone-to-implant interface |
Loading conditions | Results | |
|
Cortical layer |
Trabecular layer | ||||||
|
Moin et al. (2016)24 | 1. Standard 2. Prism 3. Fins 4. Plug 5. Bulbs |
Ti6Al4 V - E=110 GPa - ν=0.35 | - 1 mm - E=12.6 GPa - ν=0.3 - G=5.7 GPa |
- 2 mm - E=1.1 GPa - ν=0.3 - G=0.07 GPa |
-Frictional surface (nonosseointegrated situation) |
1. Oblique buccoapical force with a magnitude of 300 N on 135° to the long axis of the implant 2. Vertical force in the apical direction to the long axis of the implant with a magnitude of 150 N |
With the addition of targeted press-fit geometry including fins or bulbs, RAI had a positive effect on stress distribution and provided a better primary stability |
|
Lee et al. (2020)25 |
1. No modification 2. Wedge added at root surface 3. Lattice added at root surface 4. Apexanchor added at root apex |
Ti6Al4 V -E=113.8 GPa - ν=0.342 Esthetic ceramic -E=69 GPa - ν=0.3 |
- L=1 mm - E=13.7 GPa - ν=0.3 - G: no information |
-L: no information - E=1.37 GPa - ν=0.3 - G: no information |
-Frictional surface (nonosseointegrated situation) |
A static load of 100 N was distributed to the palatal surface of the esthetic ceramic crown at a 45° angle to the tooth extraction socket long axis |
The apex-anchor added at root apex design showed superiority in reducing the stress concentration on the supporting cortical bone and the microdisplacement of RAI |
3. Mechanical test in vitro
Implants placed in the alveolar bone can withstand axial forces; however, they are susceptible to lateral forces that create bending moments.26,27 Because the implant restoration of the missing tooth is continuously subjected to a dynamic load, various biomechanical complications may occur.28,29,30 Since root analog implants are different from conventional thread-type implants in terms of manufacturing method, geometry, and shape, further research is required to reduce stress concentrations.
Mechanical evaluations of dental implants include torsion, screw loosening, compressive fatigue, and dynamic loading tests. The torsion test of the implant body and connecting joints of the dental implant can be performed according to the international standard in ISO/TS 13498:2011. When the fixture and abutment are composed of one piece, this test may be difficult to apply as it is performed when the fixture and abutment are connected using internal screws. The screw loosening test using cyclic torsional loading is conducted when the superstructure is fixed to the implant fixture using screws, according to ISO/TR 18130:2016. However, this test cannot be performed on root analog implants that have no internal screws. The compressive fatigue test evaluates the durability of dental implants, and the dynamic loading test evaluates the fatigue strength of transmucosal-type single post dental implants in combination with their premanufactured prosthetic components with different designs or sizes according to ISO 14801:2016. This international standard does not apply to dental implants with lengths less than 8 mm. Since the length of most tooth roots is more than 8 mm,31,32 this test can be applied to root analog implants.
The mechanical complications of conventional thread-type implants include screw loosening or fracture,33,34 implant abutment fracture,35 or implant fixture fracture.36 When a root analog implant fixture is manufactured in one piece with the abutment, most complications are expected to be limited to fracture of the fixture or abutment. Conventional thread-type implant fixture fracture is a relatively rare complication.37 If the mechanical strength of the root analog implant is similar to that of the conventional thread-type implant, root analog implant fracture is not commonly expected to occur.24,25 However, when root analog implants are manufactured using additive manufacturing, it is necessary to secure adequate mechanical strength comparable to that of conventional thread-type implants made by milling processes. This is because the residual stress of the 3D printed implant after the titanium powder is melted can affect the mechanical strength. One study performed mechanical tests in vitro; however, no dynamic loading test was performed (Table 3). Further studies including compressive fatigue and dynamic loading tests should be performed to determine the mechanical stability of the implants.
Table 3.
Preclinical study of root analog implant
| Study |
Study design | Material |
Fabrication method |
Surface treatment |
Surface test |
Mechanical test |
Histologic test |
Crestal bone loss |
Implant stability test | Results |
|
Hodosh et al. (1969)3 | In vivo |
1. Polymetha crylate 2. 20% inorganic bone and 80% polymethacrylate |
Auto-polymerization and heat-processing using tooth as a template |
Airborne particle abrasion with corundum | - | - |
Collagen fiber insertion into the implanta) (Fibrointegration?) | - | - |
Periodontal ligament was regenerated around the implants with minimal inflammatory reaction. |
|
Lundgren et al. (1992)95 | In vivo | Titanium |
Machine coping with extracted tooth | - | - | - |
Bone-to-implant contact 2 months: 30.5±12.2% 12 months: 64.8±12.2% 36 months: 68.1±8.5% | - | - |
Successful osseointegration was shown in eighty eight percent of root analog implants. |
|
Kohal et al. (1997)96 | In vivo | Titanium | CAD/CAM |
Sandblasted with Al2O3 | - | - |
Bone-to-implant contact 6 months: 41.2±20.6% | - | - |
Custom made root analog titanium implants placed in the fresh extraction sockets showed osseointegration. |
|
Traini et al. (2008)97 | In vitro |
Ti6Al4 V alloy powder |
Selective laser sintering |
1. Untreated 2. Inorganic acid treated 3. Organic acid treated | - |
Young modulus of elasticity 1. Compact Ti: 104±7.7 GPa 2. Porous Ti: 77±3.5 GPa | - | - | - |
The mechanical tests indicated that compact Ti was similar to machined Ti while porous Ti present on the implant surface was similar to that of bone. |
|
Moin et al. (2013)98 |
Ex vivo (cadaver) |
Ti6Al4V alloy powder |
Selective laser melting | - | - | - | - | - | - |
With the use of CBCT data and selective laser melting 3D printing technology, 3D printed titanium RAI could be fabricated. |
|
Kontogiorgos et al. (2017)99 | In vivo |
Titanium (root analog implant) and zirconia (abutment) |
Not mentioned explicitly (Possibly CAD/CAM) |
Not mentioned explicitly | - |
Pullout test 366.7±182.8 N |
MAR 1. 21–35 days - closed osteons: 1.76±0.51 μm/day - open osteons: 2.00± 0.64 μm/day 2. 7–21 days - closed osteon: 2.55±1.10 μm/day - open osteons: 2.47± 1.25 μm/day BIC Maxilla: 68.3±13.6% Mandible: 70.77±16.31% | 0.35±0.63 mm | - |
Root shaped implants might be a viable option for single tooth replacement, exhibiting successful osseointegration. |
|
Moin et al. (2017)100 |
Ex vivo (cadaver) |
Commercial ceramic powder (ZrO2) with a liquid solution of polyacrylate |
CAD/DLP 3D printing | - | - | - | - | - | - |
With the use of CBCT data and CAD/DLP 3D printing technology, 3D printed zirconia RAI could be fabricated. |
|
Matys et al. (2017)101 |
Ex vivo (Pig mandible) | Zirconia |
Laser scanning of extracted tooth and milling of zirconium dioxide block | Macroretentions | - | - | - | - | O (DCA) |
CBCT data facilitated the making of RAI, leading to good primary stability. |
|
Osman et al. (2017)102 | In vitro | Zirconia |
DLP 3D printing | - |
Flexure strength, characteristic strength and Weibull moduli values, and surface topography, phase analysis | - | - | - | - |
Custom designed zirconia dental implants showed sufficient accuracy and flexural strength. |
|
Li et al. (2020)75 | In vivo |
Titanium grade 2 powder |
Direct metal laser melting |
Blasting –3D -3DS | - | - |
BIC: -3D:59.43±16.98% -3DS:44.28±15.99% BAFO: -3D:56.98±12.48% -3DS:45.58±10.77% | - | O (DCA) |
3D-printed implants (3D and 3DS) showed comparable implant stability as well as BIC and BAFO values with T implant up to 3 months after implant placement. |
|
Lee et al. (2021)10 |
In vitro and in vivo |
Titanium grade 2 powder |
Direct metal laser melting |
-3D-None -3D-SLA -3D-TIPS |
Surface roughness measurement and surface topography |
mBIC, OIC, tBIC, mBAFO, OAFO, and tBAFO in the inner and outer areas | - | - |
The surface treatment with TIPS in 3D-printed implants promoted osseointegration at early healing period. |
a) It is expressed as “collagenous fibers of the periodontal ligament insert into the implant”. Considering that the fiber direction was parallel to the implant in the transmucosal part of the implant, it should be understood that the fibers in this study were encapsulated fibrointegrated results rather than actual PDL. The understanding of histological observation was different from the present may be because the concept of histological osseointegration was not fully established at this time. CAD, computer-aided design; CAM, computer-aided manufacturing; DLP, digital light processing; DCA, damping capacity analysis; MAR, mineral apposition rate; BIC, bone-to- implant contact; BAFO, bone area fraction occupancy; 3D, 3D printed implant without spike; 3DS, 3D printed implant with spike; 3D-None, 3D printed implant with no surface treatment; 3D-SLA, 3D printed implant with sandblasted with a large grit and acid-etched; 3D-TIPS, 3D printed implant with target-ion-induced plasma sputtering; mBIC, mineralized bone-to-implant contact; OIC, osteoid-to-implant contact; tBIC, total bone-to-implant contact; mBAFO, mineralized bone area fraction occupancy; OAFO, osteoid area fraction Table 3. occupancy; tBAFO, total bone area fraction occupancy
4. Histologic evaluation
Biocompatible materials inserted into a living body can be categorized into the following groups: bioinert, bioactive, and bioresorbable. Among these, dental implants are bioinert. Dental implants with bioinert properties should have no inflammatory cells in the peri-implant soft and hard tissue, no fibrous capsule around the implant placed in the bone, and direct contact between the implant and bone interface. It has been reported that multinucleated giant cells (MNGCs) are observed at the implant interface during osseointegration.38,39 These are known as the M2-phenotype MNGCs which are involved in tissue healing and not the M1-phenotype MNGCs which cause foreign body reactions.40
Some particles from root analog implants may fall off the bone tissue around the implant during the implantation process, which may cause biological side effects. In particular, when the root analog implant is manufactured using 3D printing technology, the possibility of powder falling off cannot be excluded. Therefore, it is necessary to confirm the presence of these particles using histological specimens.
5. Histomorphometry analysis
The biocompatibility of root analog implants can be examined to some extent by qualitatively analyzing the histological observations mentioned above. However, a quantitative analysis is required to determine the degree of osseointegration following peri-implant bone healing. The measurement of direct bone-to-implant contact (BIC) has been used for quantitative analyses in various studies.41,42,43,44
A recent study demonstrated that functional loading is properly conducted if the interfacial boneimplant area is 20% or more.45 However, the threshold value for BIC analyzed using 2D histology has not yet been defined.46 Another issue related to the BIC is that it is difficult to present a specific range of BIC values as a reference because the values vary depending on the experimental species and the healing period after implant placement. Some excerpts from previous studies of conventional threadtype implants are summarized in Table 1. These are worthy of reference for research on root analog implants.
Another limitation of this method is that it does not reflect the bone-to-implant contact of the entire implant as it is analyzed using a part of the entire implant after cutting the block. In addition, errors might occur due to preparation artifacts during the histologic preparation process.47
6. Destructive mechanical test for osseointegration measurement
To measure the degree of osseointegration of the implant, destructive methods, including reverse torque, push-in, and pull-out tests that apply mechanical force, can be used. The critical torque threshold at which osseointegration is destroyed by applying reverse torque can be measured, indirectly providing information on the degree of osseointegration of the implants.
As the healing period increases, the reverse torque value and BIC in the histomorphometry analysis shows a positive correlation.48,49 Therefore, the reverse torque test is considered a relatively reliable method for measuring the degree of osseointegration. However, the threshold reverse torque test may also be affected by the peri-implant bone density. In other words, the test may cause tearing of type 4 bone with low bone density when a reverse torque is applied. Therefore, the reverse torque test may fail to quantify the degree of osseointegration. Crucially, there is a technical limitation in performing the reverse torque test in root analog implants because they are placed using the malleting technique rather than the rotational installation technique, which is applied to a conventional thread-type implant.
The push-in/pull-out test measures the shear strength by applying a pushing or pulling force to a load parallel to the implant-bone interface to investigate the bone-healing capabilities at the interface.50 Since peak pull out/push-in values are directly related to bone-implant interfacial stiffness,51 their measurements can be used to measure implant osseointegration. Numerous studies have used this method to measure the osseointegration of implants in orthopedics52 and dentistry.53
Interfacial failure due to these tests depends entirely on the shear stress without considering tensile or compressive stress.54 Therefore, it is possible to apply the methods to root analog implants that have no thread shape. Because this test is technique sensitive, it is important to hold the specimen firmly in the vise, ensuring that the force is applied in a consistent direction. Compared to the pull-out test, the pushin test can simulate in vivo occlusal loads, such as mastication, bruxism, clenching, and bruxism.53 However, there is a limit to confirming the effect of lateral force on implants, which are vulnerable to the forces.55,56
7. Micro computed tomography (CT)
As mentioned above, bone-to-implant contact (BIC) measurements are performed using twodimensional (2D) histological analysis. However, inter-examiner and intra-examiner errors can produce unreliable results. In addition, these are not the overall measurement values throughout the implant-tobone interface but the results of 2D histological analysis.
It has been demonstrated that measuring 3D BIC using synchrotron-radiated micro-CT shows reliable results with high resolution.57 However, this technique has limitations including being time-consuming and inaccessible owing to its high operation cost and large instrument size. Fortunately, recent advances in micro-CT and analytical tools have addressed these issues. In a recent study, the accuracy of 3D BIC measurement was obtained by reducing metal artifacts around implants observed in micro-CT images.58,59
8. Periapical radiography
Radiological analysis using periapical X-rays is a noninvasive method that can be used to monitor peri-implant marginal bone changes following implant placement. It is a recommended method to observe and measure peri-implant bone loss.60,61,62 Marginal bone loss around the implant within 1.5 mm for the first year following functional loading and bone loss within 0.2 mm per year thereafter is the standard for implant success.63 Additionally, changes in bone loss around implants within 1.0 mm must be interpreted with caution.64 Although it is possible to measure the amount of bone change at the mesial and distal sites of the implant, detecting peri-implant bone changes in the buccal or lingual (palatal) area is difficult.65 Radiographic analyses have been performed in various clinical studies (Table 4). However, long-term data with the radiological bone level must be analyzed along with various clinical parameters to examine peri-implant health for the evaluation of implant success.
Table 4.
Clinical study of root analog implant
| Study |
Study design | Material |
Fabrication method |
Design & surface treatment |
Implant stability test |
Healing period before definitive prosthesis |
Periapical X-ray |
Follow-up period | Results |
|
Strub et al. (1997)103 |
Case report | Titanium |
Extraction socket impression and milling |
Sandblasted and acid etched |
Manual evaluation with two dental mirrors | 6 months | yes | 6 months |
The Re Implant® system allowed immediate placement. |
|
Pirker and Kocher (2008)104 |
Case report | Zirconia |
Laser scanning of extracted tooth and milling of zirconium dioxide block |
Macroretentions with sandblast |
Palpation and percussion | 4 months | yes | 26 months |
Esthetic and functional result was achieved with no remarkable bone loss or soft tissue recession using root analog zirconia implant restoration. |
|
Pirker and Kocher (2009)105 | Prospective study | Zirconia |
Laser scanning of extracted tooth and milling of zirconium dioxide block |
- Group A: Sandblast - Group B: Macroretentions with sandblast |
Palpation and percussion | 3–5 months | yes | 6–34 months |
All implants in Group A were lost within 2 months, while 92% of implants in Group B survived. Macroretentions indicated primary stability and improved osseointegration, resulting in high implant survival. |
|
Pirker et al. (2011)106 |
Case report | Zirconia |
Laser scanning of extracted tooth and milling of zirconium dioxide block |
Macroretentions with sandblast |
Palpation and percussion | 4 months | yes | 30 months |
Anatomical root analog zirconia implant with macroretention for replacing two-rooted tooth achieved primary stability and osseointegration for 2.5 years. |
|
Mangano et al. (2012)107 |
Case report |
Ti6Al4 V powder |
CAD and direct laser metal sintering |
Acid etching with 50% oxalic acid and 50% maleic acid |
Palpation and percussion |
3 months (provisional restoration at surgery) | yes | 12 months |
One year after implant placement, the RAI functional without any sign of infection. |
|
Mangano et al. (2012)108 |
Prospective (n=62, 201 implants) |
Ti6Al4 V powder |
CAD and direct laser metal sintering |
Acid etching with 50% oxalic acid and 50% maleic acid | - |
2–3 months in the mandible 3–4 months in the maxilla | yes | 12 months |
One year follow-up prospective clinical study gave evidence of very high survival (99.5%) and success (97.5%). |
|
Figliuzzi et al. (2012)109 | Case report |
Ti–6Al–4 V alloy powder |
CAD and direct laser metal sintering |
Acid etching with 50% oxalic acid and 50% maleic acid |
Palpation and percussion |
3 months (provisional restoration at surgery) | yes | 12 months |
At the 1-year follow-up examination, the RAI showed stable and esthetic result. |
|
Mangano et al. (2013)110 |
Prospective case series |
Ti–6Al–4 V alloy powder |
CAD and direct laser metal sintering |
Acid etching with 50% oxalic acid and 50% maleic acid |
Palpation and percussion |
3 months (provisional restoration at surgery) | yes | 12 months |
No RAI were failed at the 1-year follow-up. No sign of infection was detected in any implant, with good peri-implant conditions. |
|
Patankar et al. (2016)111 |
Case report | Zirconia |
Laser scanning of the extracted tooth and milling of zirconium dioxide block |
Macroretentions with sandblast |
Palpation and percussion | 4 months | yes | 18 months |
Root analog zirconia implant restoration for replacing a right mandibular first premolar was performed successfully and maintained for 18 months. |
|
Tunchel et al. (2017)112 |
Prospective study (n=88, 110 implants) |
Ti6Al4 V powder |
CAD and direct laser metal sintering |
Thread shape acid-etched in a mixture of 50% oxalic acid and 50% maleic acid | - |
2–3 months in the mandible 3–4 months in the maxilla | yes | 36 months |
After 3 years of loading, six implants failed with an overall implant survival rate of 94.5%. |
|
Böse et al. (2020) 2 |
Retrospective study | Zirconia | Milling |
Macro and microretentions | - |
6.6±2.5 months | yes |
17.5±6.4 months after surgery (10.8±7.0 months after loading) |
Total survival rate of RAI was 94.4% with a mean follow-up period of 18.9±2.4 months after implant placement. Immediate placement of RAIs did not reduce peri-implant bone loss and might not be recommended for daily routine. Esthetic scores of RAIS showed satisfying results. |
9. Implant stability measurement
Raghavendra et al.66 proposed a quantitative measurement of overall implant stability followed by implant placement because primary stability decreased over time, while secondary stability increased. Quantitative measurements are necessary when considering the fundamental goal of a personalized treatment strategy for root analog implants. It is necessary to examine the implant stability measurement methods currently used in conventional thread-type implants and their applicability to root analog implants.
The percussion test is one of the simplest methods for evaluating the degree of osseointegration.67,68 This test is based on the vibration-acoustic science and impulse-response theory. A clear “crystal” sound indicates successful osseointegration, whereas a “dull” sound indicates osseointegration failure. However, this method has limitations as a standardized test because it depends on the clinician's experience and infeasibility of quantification. To measure implant stability, resonance frequency analysis (RFA) and damping capacity analysis have been used.
RFA is a non-invasive diagnostic tool that measures implant stability at various time points. A transducer vibrating with a sinusoidal signal is connected to measure the implant stability. The resonance peak of the received signal represents the first flexural resonance frequency of the implant, which can quantitatively evaluate implant stability. Resonance frequency values between 3500 and 8500 were presented in a study,69 which were later used to develop a device that resulted in implant stability quotient (ISQ) values between 0 and 100.70 Manufacturer guidelines indicate that successful implants typically have an ISQ of 65 or more, according to scientific evidence.71
However, the use of this RFA analysis method for root analog implants is limited because of the infeasibility of transducer connections. Most root analog implants are manufactured as a one-piece and cementation type without internal screw connections, making it impossible to connect the transducer to the implant.
Damping capacity analysis is a quantitative evaluation of the percussion test and is measured by striking the implant with an electromagnetically actuated and electrically controlled metal-tapping rod.72 The response to striking is measured using a small accelerometer included in the head, and the contact time between the object and the tapping rod is measured to analyze implant stability.73 The signal is converted into a periotest value (PTV), ranging from -8 to +50, which varies according to the damping characteristics of the surrounding teeth or implant tissue.73
Recently, a modified damping capacity analyzer with improved sensitivity was developed.74 The high implant stability test (IST) value, a measurement parameter of implant stability used in this device, ranges from 1 to 99. A safety control system is designed to immediately stop the impact test when it detects a stability below 59, and the number of tapping sequences is reduced to six times to increase user convenience compared to periotest. This device does not require specific transducers and has a reliability similar to that of RFA, which is useful for evaluating the stability of root analog implants. Recently, root analog implant studies have used this damping capacity analysis device to evaluate changes in implant stability following implant placement.75 Quantitative evaluation of root analog implant stability has not been performed in clinical studies so far (Table 4). Quantitative implant stability measurements are expected in future studies using a modified damping capacity analyzer.
10. Clinical parameters
It is important to perform a clinical examination after prosthesis delivery. The following parameters can be evaluated in root analog implants similar to that in conventional thread-type implants, according to previous study:76,77
- modified plaque index (mPII),
- modified bleeding index (mBII),
- peri-implant mucosal recession in millimeters,
- pocket probing depth (PPD) in millimeters,
- probing attachment level (PAL) in millimeters, calculated by subtracting PPD from DIM,
- bleeding on probing (BoP).
In the study, the modified plaque index was measured to identify the patient’s plaque control, and the modified bleeding index and bleeding on probing were recorded to determine the presence of periimplant inflammation. The peri-implant mucosal recession and probing attachment levels were measured to evaluate the stability of the peri-implant mucosa. Peri-implant inflammation and bone loss were assessed by measuring the pocket probing depth.
11. Perspective and future directions
Future evaluation methods for the mechanical strength, biological safety, and stability of root analog implants depend on clinical demands, including the extent of clinical convenience, patient satisfaction, aesthetics, and long-term stability. Digital workflow for fabrication is an essential aspect of the future of root analog implants, and a substantial portion of the manufacturing processes are fully established. In addition, digital workflows enable a high level of accuracy and productivity for milling or additive manufacturing.
Nonetheless, all evaluation methods should be based on previous techniques used in conventional thread-type implants, including FEA, mechanical testing, histologic evaluation, histomorphometry analysis, destructive mechanical testing for osteointegration measurement, micro-CT, periapical radiography, implant stability testing, and clinical parameters. Structural differences between root analog implants and conventional thread-type implants make several methods used in conventional thread-type implants infeasible, such as RFA. However, implant stability tests using damping capacity analysis can be used instead of RFA. When evaluating root analog implants, careful consideration should be given to whether the evaluation method for thread-type implants can be applied.
