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Influence of cortical bone thickness and implant length on implant stability at the time of surgery—clinical, prospective, biomechanical, and imaging study
Bone 37 (2005) 776 – 780 www.elsevier.com/locate/bone
Influence of cortical bone thickness and implant length on implant stability at the time of surgery—clinical, prospective, biomechanical, and imaging study Ikuya Miyamoto⁎ , Yoichi Tsuboi, Eishin Wada, Hirohiko Suwa, Tadahiko Iizuka Department Oral and Maxillofacial Surgery, Kyoto University Graduate School of Medicine, 54 Kawahara-cho, Shogoin, Sakyo-ku, Kyoto 606-8507, Japan Received 18 February 2005; revised 2 June 2005; accepted 30 June 2005 Available online 8 September 2005
Abstract This clinical study is the first to quantitatively evaluate both regional bone structure by computed tomography preoperatively and dental implant stability by resonance frequency analysis at the time of surgery to explore the relation between local bone structure and dental implant stability in humans. Implant stability at the time of installation is often difficult to achieve in lower density bone and implant stability might influence treatment efficacy. Few clinical studies have reported detailed bone characteristics obtained using computed tomography prior to surgery and comprehensive implant stability measurements at the time of surgery. We hypothesized that thicker cortical bone would improve the stability of the dental implant at the time of placement. Before radiographic examination, diagnostic radiographic templates were made by incorporating radiopaque indicators. Computed tomography scans were obtained for 50 edentulous subjects prior to surgery. Preoperatively, the thickness of the cortical bone at the sites of implant insertion was measured digitally, and then implant insertion surgery was performed. A total of 225-implant stability measurements were made using a resonance frequency analyzer. There was a strong linear correlation between cortical bone thickness and resonance frequency (r = 0.84, P b 0.0001). The implant length had a weak negative correlation with stability (r = −0.25, P b 0.0005). These results suggest that the initial stability at the time of implant installation is influenced more by cortical bone thickness than by implant length. The cortical and cancellous ratio of local bone is extremely important for implant stability at the time of surgery and determining the local bone condition is critical for treatment success. © 2005 Elsevier Inc. All rights reserved. Keywords: Cortical bone; Computed tomography; Dental implant; Implant stability; Resonance frequency analysis
Introduction Clinical and basic research indicates that dental implant stability at the time of surgery is important for therapeutic success [1–4]. There are many factors, such as implant geometry, preparation technique, and quality and quantity of regional bone, that influence primary implant fixation [5]. Dental implants for the mandible have higher survival rates than those for the posterior maxilla [6–8]. In the posterior maxilla, there is frequently thinner cortical bone combined ⁎ Corresponding author. Fax: +81 75 761 9732. E-mail address: [email protected] (I. Miyamoto). 8756-3282/$ - see front matter © 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.bone.2005.06.019
with thicker trabecular bone [9]. Thus, clinicians generally observe a poor degree of bone mineralization or limited bone resistance by tactile assessment while drilling [10,11]. This low density of bone is called “soft bone” [9,12]. It is often difficult to obtain optimum primary stability in soft bone and there are higher implant therapy failure rates [13–15]. Accordingly, preoperative examination of the host bone is important for treatment predictability. Clinically, computed tomography (CT) is currently the only diagnostic imaging technique that allows for a rough determination of the structure and density of the jawbones [16,17]. It is also an excellent tool for assessing the relative distribution of cortical and cancellous bone [18,19]. On the other hand,
I. Miyamoto et al. / Bone 37 (2005) 776–780
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problems. The purpose of the experiment was explained to the subjects, and informed consent was obtained. The Ethics Committee of Kyoto University approved the protocol. Surgical stent and computed tomography
Fig. 1. Schematic drawing of cross-sectional images illustrating the region of interest between cortical and trabecular bone (A). The guide indicator can be clearly seen. Model implant (10-mm long and 3.5-mm diameter) was superimposed on cross-sectional image to determine optimal angulations and direction. Measurement of resonance frequency by Osstell™ (B). Transducer was attached to the implant (C).
implant stability at the time of surgery has traditionally been difficult to evaluate and is often reduced to a simple assessment of mobility [20]. Implant stability can be measured by noninvasive methods using a resonance frequency analyzer [21]. The in vivo experimental findings demonstrate that resonance frequency is related to implant stiffness in the surrounding tissues, which means a higher bone to implant contact percentage [22]. Clinically, the increase in implant stability as been measured using a resonance frequency analyzer, and the increase in stability were attributed to corticalization of the surrounding bone [10]. In addition, marginal bone loss and loss of implant stability might correlate with resonance frequency [21]. There are few published studies dealing with bone and implant stability in human subjects [23]. In the present study, prior to implant placement, cross-sectional dental CT was taken with radiographic stents, to guide placement of the implant model in the optimum occlusal direction. At the time of installation, implant stability was measured using resonance frequency analysis. The aim of the study was to determine the influence of cortical bone thickness and implant length on implant stability at the time of surgery in human subjects.
Materials and methods
Prior to radiographic examination, individual templates were made in accordance with optimum dental occlusion with wax-up, which indicated favorable angulations and direction of the implant models, and then gutta-percha indicators were inserted into the stents. Cross-sectional images were acquired at each bone site with multi-slice spiral CT scans (120 kVp and 300 mAs, with a slice thickness of 1.0 mm, pitch 0.4 mm, scan time 750 ms; AQUILION™, Toshiba-Japan, Tokyo, Japan). The data were analyzed with a three-dimensional volume-rendering technique using Xtension® software (Toshiba-Japan, Tokyo, Japan). To evaluate cortical bone thickness, a replica implant with the same magnification (10-mm long and 3.5-mm diameter) was superimposed on each cross-sectional CT image. The implant model was set where the implant was to be installed under the radiographic indicator (Fig. 1A). The cortical bone thickness was digitally measured on the image of the superimposed implant model (Photoshop, version 5.0 J; Adobe, Mountain View, CA). The outer cortical bone surface surrounding the implant model was measured twice along each side of the implant model, and the mean value was defined as the cortical thickness of the bone in millimeters. Surgical procedures The surgery was performed according to the manufacturer's instructions. Oral implants (Astra Tech, Mölndal, Sweden; diameter, 3.5 mm; length 8, 9, 11, 13, 15, or 17 mm) were placed in the subjects along the alveolar ridge. A total of 225 (maxilla 98, mandible 127) implants were used in this study. Table 1 shows the details of implant length distribution in the subjects. Stability measurements At the time of implant placement, the stability of each implant was tested with a resonance frequency analyzer (Osstell™; Integration Diagnostics, Gothenburg, Sweden; Figs. 1B, C). A new unit to describe implant stability was introduced: implant stability quotient (ISQ). The relationship between the ISQ value and resonance frequency value
Subjects A total of 50 Japanese patients participated in the study (25 males, 25 females; age range 24–76; mean age 52.5 years). Single, partial, or total edentulous patients were included. There were 31 mandibular sites and 28 maxillary sites. The subjects had no specific documented medical
Table 1 Implant length distribution Implant length (mm) Maxilla/mandible Total
8 11/34 45
9 12/40 52
11 17/31 48
13 42/16 58
15 13/6 19
17 3/0 3
778
I. Miyamoto et al. / Bone 37 (2005) 776–780
Table 2 Mean cortical bone thickness at the implant insertion site Mean ± SD (range) Cortical bone thickness
1.9 ± 0.56 (n = 225) (0.79–3.21)
Mean ± SD (range) Mandible (31 patients) (n = 127) Maxilla (28 patients) (n = 98)
2.22 ± 0.47 (0.79–3.21) mm 1.49 ± 0.34 (0.92–2.54) mm
is close to linear, but factors such as individual differences between transducers are also taken into account. ISQ is recorded as a number between 1 and 100, 100 representing the highest degree of stability. As each transducer can be independently calibrated, it is possible to directly compare ISQ values that originate from different transducers. The resonance frequency was observed as a peak in the amplitude-frequency plot of the response of the transducer beam. The method involves the use of a small transducer that is attached to the implant (Fig. 1C). All of the implants were inserted mono-cortically and clinically mobile implants were not used due to the increased variability in the ISQ value, as recommended by the manufacturer [23]. Statistical analysis The data were entered into a personal computer and analyzed with JMP for Windows® (Software 5.1, SAS Institute Inc., Cary, NC). Differences in the means of continuous measurements were tested by Student's t test and Mann–Whitney U test. The relationship between cortical bone thickness and stability was examined, as well as the correlation between implant length and stability using Pearson's correlation coefficient. A P value of less than 0.05 was considered to be statistically significant.
Results
Fig. 2. Correlation between cortical bone thickness and ISQ (implant stability quotient) value. There was strong statistically significant positive linear correlation (r = 0.84, P b 0.0001).
Resonance frequency analysis of implant stability The implants had a mean stability of 68.2 ± 6.6 ISQ (n = 225, range 52–82). The average ISQ value of the mandibular implants was 71.7 ± 5.23 ISQ (n = 127, range 57–82) and that of the maxillary implants was 63.5 ± 5.2 ISQ (n = 98, range 52–76). Mandibular implants had a significantly higher ISQ value than maxillary implants (P b 0.0001; Table 3). Correlations between cortical bone thickness, implant length, and implant stability Correlations between cortical bone thickness and implant stability, as well as implant length at the time of installation, were determined. Fig. 2 shows all the data pairs collected from the measurement of resonance frequency and CT, providing strong evidence of a linear relationship between ISQ value and cortical bone thickness; there was a strong correlation (r = 0.84, P b 0.0001). Fig. 3 shows the correlation between ISQ value and implant length; there was a weak negative correlation (r = −0.25, P b 0.0005).
Cortical bone thickness Mean jawbone thickness was 1.9 ± 0.56 mm (n = 225, range 0.79–3.21). The mean mandible thickness was 2.22 ± 0.47 mm (n = 127, range 0.79–3.21). Mean maxilla thickness was 1.49 ± 0.34 mm (n = 98, range 0.92–2.54). Cortical bone in the mandible was significantly thicker surrounding the implant than in the maxilla (P b 0.0001; Table 2). Table 3 Mean implant stabilities at the time of surgery Mean ± SD (range) ISQ value
68.2 ± 1.9 (n = 225) (52–82)
Mean ± SD (range) Mandible (n = 127) 71.7 ± 5.2 (57–82) Maxilla (n = 98) 63.5 ± 5.2 (52–76)
Fig. 3. Correlation between implant length and ISQ value. There was a weak statistically significant negative correlation (r = −0.25, P b 0.0005).
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Discussion The present quantitative imaging and biomechanical evaluations clearly demonstrated that dental implant stability at the time of surgery was weakly influenced by implant length, but cortical bone thickness strongly increased implant stability in humans. This finding is consistent with an experimental study demonstrating that removal torque for implants in the fibula, iliac crest, and scapula of cadavers was related to cortical bone thickness, not total bone thickness [24]. Also, the results agree with reports of in vivo experiments that the amount of cancellous bone did not increase implant stability at the time of surgery [3,25–27]. Bone quality and quantity are often more compromised in maxillary than in mandibular sites in implant dentistry [28]. It has been difficult to assess bone condition preoperatively, however, due to a lack of evidence to support the validity and reliability of measurement methods [9,29]. Bone quality is often referred to in the literature as the amount of cortical and cancellous bone in which the recipient socket is drilled, and lower bone density might compromise osteogenesis or cause excessive resorption compared with higher density bone, thereby upsetting osseous healing [30]. Increased in implant stability over time, as measured with resonance frequency, however, occurs in sites of low density [10,23]. In addition, in vivo experimental studies suggest that the implant holding properties increased with time as a result of osteogenesis in trabecular bone [10,25]. A progressive tissue response over a long period of time also occurs [6]. Therefore, although there is a range of implant stability at installation, implants eventually achieve good stability clinically. Implant stability depends largely upon cortical bone thickness and, therefore, the application of longer implants is not effective to increase primary implant stability. In the conventional surgical protocol of dental implant therapy, implants are placed in bone and completely covered by oral mucosa, so that functional loading is avoided during the initial healing period of 3 months in the mandible and 6 months in the maxilla to allow for the integration between bone and implant [31]. In low-density bone, the healing period after implant insertion should be extended for better integration of bone to implant [10]. If an implant is not stable at the time of installation, micro-motion might occur during the healing period resulting in a thin fibrous layer at the bone–implant interface [32]. Early implant failure is thought to be due to excessive mechanical load applied to the implant, coupled with lower stability at implant placement [33,34]. In contrast, by reducing micro-motion to within the critical threshold, it is possible to apply mechanical load to the implant with a reduced healing period [35]. Clinical follow-up studies have demonstrated the possibility of applying an early or immediate loading protocol to the dental implant to minimize the interval between surgery and prosthetic rehabilitation [36–39]. While the procedure is beneficial for patients, the mechanism of acceptance of mechanical loads is unknown in premature interfacial region
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between implant and bone. Taken together, implant stability at the time of surgery might be a crucial factor for the success of early or immediate loading therapy. Without sufficient implant stability, it would be difficult to shorten the loading protocol. In conclusion, CT and resonance frequency analysis might be useful techniques to explore implant-bone biology and to serve as a guide to treatment success. These medical engineering devices have potential applications for the research of implant behavior and bone condition. Implant stability at the time of surgery might largely depend on local bone conditions rather implant length. Further research is required to achieve predictable clinical success.
Acknowledgments This research was partly supported by a Research Grant for Scientific Research (B) 12558107 from the Japanese Ministry of Education, Culture, Sports, Science, and Technology.
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Influence of cortical bone thickness and implant length on implant stability at the time of surgery—clinical, prospective, biomechanical, and imaging study Ikuya Miyamoto⁎ , Yoichi Tsuboi, Eishin Wada, Hirohiko Suwa, Tadahiko Iizuka Department Oral and Maxillofacial Surgery, Kyoto University Graduate School of Medicine, 54 Kawahara-cho, Shogoin, Sakyo-ku, Kyoto 606-8507, Japan Received 18 February 2005; revised 2 June 2005; accepted 30 June 2005 Available online 8 September 2005
Abstract This clinical study is the first to quantitatively evaluate both regional bone structure by computed tomography preoperatively and dental implant stability by resonance frequency analysis at the time of surgery to explore the relation between local bone structure and dental implant stability in humans. Implant stability at the time of installation is often difficult to achieve in lower density bone and implant stability might influence treatment efficacy. Few clinical studies have reported detailed bone characteristics obtained using computed tomography prior to surgery and comprehensive implant stability measurements at the time of surgery. We hypothesized that thicker cortical bone would improve the stability of the dental implant at the time of placement. Before radiographic examination, diagnostic radiographic templates were made by incorporating radiopaque indicators. Computed tomography scans were obtained for 50 edentulous subjects prior to surgery. Preoperatively, the thickness of the cortical bone at the sites of implant insertion was measured digitally, and then implant insertion surgery was performed. A total of 225-implant stability measurements were made using a resonance frequency analyzer. There was a strong linear correlation between cortical bone thickness and resonance frequency (r = 0.84, P b 0.0001). The implant length had a weak negative correlation with stability (r = −0.25, P b 0.0005). These results suggest that the initial stability at the time of implant installation is influenced more by cortical bone thickness than by implant length. The cortical and cancellous ratio of local bone is extremely important for implant stability at the time of surgery and determining the local bone condition is critical for treatment success. © 2005 Elsevier Inc. All rights reserved. Keywords: Cortical bone; Computed tomography; Dental implant; Implant stability; Resonance frequency analysis
Introduction Clinical and basic research indicates that dental implant stability at the time of surgery is important for therapeutic success [1–4]. There are many factors, such as implant geometry, preparation technique, and quality and quantity of regional bone, that influence primary implant fixation [5]. Dental implants for the mandible have higher survival rates than those for the posterior maxilla [6–8]. In the posterior maxilla, there is frequently thinner cortical bone combined ⁎ Corresponding author. Fax: +81 75 761 9732. E-mail address: [email protected] (I. Miyamoto). 8756-3282/$ - see front matter © 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.bone.2005.06.019
with thicker trabecular bone [9]. Thus, clinicians generally observe a poor degree of bone mineralization or limited bone resistance by tactile assessment while drilling [10,11]. This low density of bone is called “soft bone” [9,12]. It is often difficult to obtain optimum primary stability in soft bone and there are higher implant therapy failure rates [13–15]. Accordingly, preoperative examination of the host bone is important for treatment predictability. Clinically, computed tomography (CT) is currently the only diagnostic imaging technique that allows for a rough determination of the structure and density of the jawbones [16,17]. It is also an excellent tool for assessing the relative distribution of cortical and cancellous bone [18,19]. On the other hand,
I. Miyamoto et al. / Bone 37 (2005) 776–780
777
problems. The purpose of the experiment was explained to the subjects, and informed consent was obtained. The Ethics Committee of Kyoto University approved the protocol. Surgical stent and computed tomography
Fig. 1. Schematic drawing of cross-sectional images illustrating the region of interest between cortical and trabecular bone (A). The guide indicator can be clearly seen. Model implant (10-mm long and 3.5-mm diameter) was superimposed on cross-sectional image to determine optimal angulations and direction. Measurement of resonance frequency by Osstell™ (B). Transducer was attached to the implant (C).
implant stability at the time of surgery has traditionally been difficult to evaluate and is often reduced to a simple assessment of mobility [20]. Implant stability can be measured by noninvasive methods using a resonance frequency analyzer [21]. The in vivo experimental findings demonstrate that resonance frequency is related to implant stiffness in the surrounding tissues, which means a higher bone to implant contact percentage [22]. Clinically, the increase in implant stability as been measured using a resonance frequency analyzer, and the increase in stability were attributed to corticalization of the surrounding bone [10]. In addition, marginal bone loss and loss of implant stability might correlate with resonance frequency [21]. There are few published studies dealing with bone and implant stability in human subjects [23]. In the present study, prior to implant placement, cross-sectional dental CT was taken with radiographic stents, to guide placement of the implant model in the optimum occlusal direction. At the time of installation, implant stability was measured using resonance frequency analysis. The aim of the study was to determine the influence of cortical bone thickness and implant length on implant stability at the time of surgery in human subjects.
Materials and methods
Prior to radiographic examination, individual templates were made in accordance with optimum dental occlusion with wax-up, which indicated favorable angulations and direction of the implant models, and then gutta-percha indicators were inserted into the stents. Cross-sectional images were acquired at each bone site with multi-slice spiral CT scans (120 kVp and 300 mAs, with a slice thickness of 1.0 mm, pitch 0.4 mm, scan time 750 ms; AQUILION™, Toshiba-Japan, Tokyo, Japan). The data were analyzed with a three-dimensional volume-rendering technique using Xtension® software (Toshiba-Japan, Tokyo, Japan). To evaluate cortical bone thickness, a replica implant with the same magnification (10-mm long and 3.5-mm diameter) was superimposed on each cross-sectional CT image. The implant model was set where the implant was to be installed under the radiographic indicator (Fig. 1A). The cortical bone thickness was digitally measured on the image of the superimposed implant model (Photoshop, version 5.0 J; Adobe, Mountain View, CA). The outer cortical bone surface surrounding the implant model was measured twice along each side of the implant model, and the mean value was defined as the cortical thickness of the bone in millimeters. Surgical procedures The surgery was performed according to the manufacturer's instructions. Oral implants (Astra Tech, Mölndal, Sweden; diameter, 3.5 mm; length 8, 9, 11, 13, 15, or 17 mm) were placed in the subjects along the alveolar ridge. A total of 225 (maxilla 98, mandible 127) implants were used in this study. Table 1 shows the details of implant length distribution in the subjects. Stability measurements At the time of implant placement, the stability of each implant was tested with a resonance frequency analyzer (Osstell™; Integration Diagnostics, Gothenburg, Sweden; Figs. 1B, C). A new unit to describe implant stability was introduced: implant stability quotient (ISQ). The relationship between the ISQ value and resonance frequency value
Subjects A total of 50 Japanese patients participated in the study (25 males, 25 females; age range 24–76; mean age 52.5 years). Single, partial, or total edentulous patients were included. There were 31 mandibular sites and 28 maxillary sites. The subjects had no specific documented medical
Table 1 Implant length distribution Implant length (mm) Maxilla/mandible Total
8 11/34 45
9 12/40 52
11 17/31 48
13 42/16 58
15 13/6 19
17 3/0 3
778
I. Miyamoto et al. / Bone 37 (2005) 776–780
Table 2 Mean cortical bone thickness at the implant insertion site Mean ± SD (range) Cortical bone thickness
1.9 ± 0.56 (n = 225) (0.79–3.21)
Mean ± SD (range) Mandible (31 patients) (n = 127) Maxilla (28 patients) (n = 98)
2.22 ± 0.47 (0.79–3.21) mm 1.49 ± 0.34 (0.92–2.54) mm
is close to linear, but factors such as individual differences between transducers are also taken into account. ISQ is recorded as a number between 1 and 100, 100 representing the highest degree of stability. As each transducer can be independently calibrated, it is possible to directly compare ISQ values that originate from different transducers. The resonance frequency was observed as a peak in the amplitude-frequency plot of the response of the transducer beam. The method involves the use of a small transducer that is attached to the implant (Fig. 1C). All of the implants were inserted mono-cortically and clinically mobile implants were not used due to the increased variability in the ISQ value, as recommended by the manufacturer [23]. Statistical analysis The data were entered into a personal computer and analyzed with JMP for Windows® (Software 5.1, SAS Institute Inc., Cary, NC). Differences in the means of continuous measurements were tested by Student's t test and Mann–Whitney U test. The relationship between cortical bone thickness and stability was examined, as well as the correlation between implant length and stability using Pearson's correlation coefficient. A P value of less than 0.05 was considered to be statistically significant.
Results
Fig. 2. Correlation between cortical bone thickness and ISQ (implant stability quotient) value. There was strong statistically significant positive linear correlation (r = 0.84, P b 0.0001).
Resonance frequency analysis of implant stability The implants had a mean stability of 68.2 ± 6.6 ISQ (n = 225, range 52–82). The average ISQ value of the mandibular implants was 71.7 ± 5.23 ISQ (n = 127, range 57–82) and that of the maxillary implants was 63.5 ± 5.2 ISQ (n = 98, range 52–76). Mandibular implants had a significantly higher ISQ value than maxillary implants (P b 0.0001; Table 3). Correlations between cortical bone thickness, implant length, and implant stability Correlations between cortical bone thickness and implant stability, as well as implant length at the time of installation, were determined. Fig. 2 shows all the data pairs collected from the measurement of resonance frequency and CT, providing strong evidence of a linear relationship between ISQ value and cortical bone thickness; there was a strong correlation (r = 0.84, P b 0.0001). Fig. 3 shows the correlation between ISQ value and implant length; there was a weak negative correlation (r = −0.25, P b 0.0005).
Cortical bone thickness Mean jawbone thickness was 1.9 ± 0.56 mm (n = 225, range 0.79–3.21). The mean mandible thickness was 2.22 ± 0.47 mm (n = 127, range 0.79–3.21). Mean maxilla thickness was 1.49 ± 0.34 mm (n = 98, range 0.92–2.54). Cortical bone in the mandible was significantly thicker surrounding the implant than in the maxilla (P b 0.0001; Table 2). Table 3 Mean implant stabilities at the time of surgery Mean ± SD (range) ISQ value
68.2 ± 1.9 (n = 225) (52–82)
Mean ± SD (range) Mandible (n = 127) 71.7 ± 5.2 (57–82) Maxilla (n = 98) 63.5 ± 5.2 (52–76)
Fig. 3. Correlation between implant length and ISQ value. There was a weak statistically significant negative correlation (r = −0.25, P b 0.0005).
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Discussion The present quantitative imaging and biomechanical evaluations clearly demonstrated that dental implant stability at the time of surgery was weakly influenced by implant length, but cortical bone thickness strongly increased implant stability in humans. This finding is consistent with an experimental study demonstrating that removal torque for implants in the fibula, iliac crest, and scapula of cadavers was related to cortical bone thickness, not total bone thickness [24]. Also, the results agree with reports of in vivo experiments that the amount of cancellous bone did not increase implant stability at the time of surgery [3,25–27]. Bone quality and quantity are often more compromised in maxillary than in mandibular sites in implant dentistry [28]. It has been difficult to assess bone condition preoperatively, however, due to a lack of evidence to support the validity and reliability of measurement methods [9,29]. Bone quality is often referred to in the literature as the amount of cortical and cancellous bone in which the recipient socket is drilled, and lower bone density might compromise osteogenesis or cause excessive resorption compared with higher density bone, thereby upsetting osseous healing [30]. Increased in implant stability over time, as measured with resonance frequency, however, occurs in sites of low density [10,23]. In addition, in vivo experimental studies suggest that the implant holding properties increased with time as a result of osteogenesis in trabecular bone [10,25]. A progressive tissue response over a long period of time also occurs [6]. Therefore, although there is a range of implant stability at installation, implants eventually achieve good stability clinically. Implant stability depends largely upon cortical bone thickness and, therefore, the application of longer implants is not effective to increase primary implant stability. In the conventional surgical protocol of dental implant therapy, implants are placed in bone and completely covered by oral mucosa, so that functional loading is avoided during the initial healing period of 3 months in the mandible and 6 months in the maxilla to allow for the integration between bone and implant [31]. In low-density bone, the healing period after implant insertion should be extended for better integration of bone to implant [10]. If an implant is not stable at the time of installation, micro-motion might occur during the healing period resulting in a thin fibrous layer at the bone–implant interface [32]. Early implant failure is thought to be due to excessive mechanical load applied to the implant, coupled with lower stability at implant placement [33,34]. In contrast, by reducing micro-motion to within the critical threshold, it is possible to apply mechanical load to the implant with a reduced healing period [35]. Clinical follow-up studies have demonstrated the possibility of applying an early or immediate loading protocol to the dental implant to minimize the interval between surgery and prosthetic rehabilitation [36–39]. While the procedure is beneficial for patients, the mechanism of acceptance of mechanical loads is unknown in premature interfacial region
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between implant and bone. Taken together, implant stability at the time of surgery might be a crucial factor for the success of early or immediate loading therapy. Without sufficient implant stability, it would be difficult to shorten the loading protocol. In conclusion, CT and resonance frequency analysis might be useful techniques to explore implant-bone biology and to serve as a guide to treatment success. These medical engineering devices have potential applications for the research of implant behavior and bone condition. Implant stability at the time of surgery might largely depend on local bone conditions rather implant length. Further research is required to achieve predictable clinical success.
Acknowledgments This research was partly supported by a Research Grant for Scientific Research (B) 12558107 from the Japanese Ministry of Education, Culture, Sports, Science, and Technology.
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