This study investigated the effects of bone stiffness (elastic modulus) and three-dimensional (3D) bone-to-implant contact ratio (BIC%) on the primary stabilities of dental implants using micro-computed tomography (micro-CT) and resonance frequency analyses. Artificial sawbone models with five values of elastic modulus (137, 123, 47.5, 22, and 12.4 MPa) comprising two types of trabecular structure (solid-rigid and cellular-rigid) were investigated for initial implant stability quotient (ISQ), measured using the wireless Osstell resonance frequency analyzer. Bone specimens were attached to 2 mm fibre-filled epoxy sheets mimicking the cortical shell. ISQ was measured after placing a dental implant into the bone specimen. Each bone specimen with an implant was subjected to micro-CT scanning to calculate the 3D BIC% values. The similarity of the cellular type of artificial bone to the trabecular structure might make it more appropriate for obtaining accurate values of primary implant stability than solid-bone blocks. For the cellular-rigid bone models, the ISQ increased with the elastic modulus of cancellous bone. The regression correlation coefficient was 0.96 for correlations of the ISQ with the elasticity of cancellous bone and with the 3D BIC%. The initial implant stability was moderately positively correlated with the elasticity of cancellous bone and with the 3D BIC%.
Clinical studies have found high success rates for dental implant treatment, but the failure rate of dental implants remains high in poor quality bone. Usually the bone quality and physical properties are better for the mandible than for the maxilla, and superior for the anterior region than for the posterior region. This has resulted in research findings that the survival rate is higher for dental implants in the mandible than in the maxilla, particularly in the anterior region.
Bone quality is clinically classified into types 1–4 according to Lekholm and Zard based on the amounts of cortical bone versus cancellous bone evident on radiograph film. There are reports of only 3% of bone being lost in implants placed in bone of type 1, 2, or 3, whereas the failure rate was 35% in type 4 bone, which has a thin cortical shell and constitutes softer cancellous bone. The implant stability is lower in type 4 bone than in bone of other qualities. Orthopaedic and dental implant research have indicated that the stiffness of cancellous bone significantly influences the holding strength of an implant. In the clinical situation, the Lekholm and Zarb classification only provides a rough assessment of the quality and quantity of jaw bone, and hence precisely how bone quality and quantity affect implant stability is still unclear.
Micro-level computed tomography (micro-CT) is now widely used for observing and analyzing the internal structure of hard tissue due to its advantages of rapidity, reproducibility, and nondestructiveness. Many studies have used micro-CT to obtain high-resolution images and assess the trabecular structure of human bone quantitatively in three dimensions (3D). Micro-CT has also been increasingly employed in research related to dental implants to evaluate the peri-implant bone, and has been validated using histomorphometric results. Micro-CT also allows for accurate 3D measurements of bone-to-implant contact (BIC). Only a few studies have assessed the 3D BIC, and no study has investigated its correlation with initial implant stability.
Resonance frequency analysis (RFA), which was introduced by Meredith et al., can measure the initial stability of a dental implant nondestructively. This is performed using the Osstell instrument (Osstell, Göteborg, Sweden), and has become a common method for determining the initial stability of a dental implant in both in vitro and clinical studies. Many studies have used histological methods to measure two-dimensional (2D) BIC and investigate its relationship to implant stability as measured by RFA. The 2D BIC obtained from a histological section may not accurately represent the entire 3D interface between implant and bone, and no previous study has used micro-CT to evaluate the relation between the 3D BIC and RFA values (usually evaluated as the implant stability quotient (ISQ)).
The present study used high-resolution micro-CT to determine how initial implant stability, as measured by RFA, is related to the 3D BIC ratio (BIC%) and bone stiffness (elastic modulus). This study also examined differences in implant stabilities between cancellous bone with and without a cellular structure.
Materials and methods
Bone sample preparation
Sawbone models of one solid (elastic modulus of 123 MPa) and four cellular (elastic moduli of 137, 47.5, 23, and 12.4 MPa) polyurethane foam blocks (models 1522-02, 1522-09, 1522-10, 1522-11, and 1522-12, Pacific Research Laboratory, Vashon Island, WA, USA) were used to mimic the cancellous bone of the jaw. The trabecular bone blocks were attached to 2 mm thick commercially available synthetic cortical shell (model 3401-01, Pacific Research Laboratories) with an elastic modulus of 16.7 GPa to represent the experimental bone models of cancellous bone with varying densities ( Fig. 1 ). All of the experimental bone models were rectangular with dimensions of 38 mm × 20 mm × 42 mm. A commercial dental implant (4 mm in diameter and 12 mm long; ATLAS Implant System, Cowell Medi, Busan, South Korea) ( Fig. 2 ) was used as the target dental implant in this study. Five implant specimens were prepared for each of the four groups (comprising artificial foam bones with four elastic moduli) for the measurements of implant stability and BIC%.
3D BIC analysis by micro-CT
For calculating the 3D BIC%, micro-CT (SkyScan-1076, Skyscan, Aartselaar, Belgium) was used to obtain 3D information from each specimen with an implant at a resolution of 17.2 μm × 17.2 μm × 17.2 μm. Scans were performed on approximately 800 slices, with 1024 × 1024 pixels for each slice. The micro-CT images of each specimen with an implant were imported into professional medical imaging software (Mimics 13.0, Materialise, Leuven, Belgium) ( Fig. 3 a ) and were segmented by using different thresholds for implant and bone to construct a 3D computer model ( Fig. 3 b). The whole exterior surface of the implant inside the specimen and the BIC area were determined, with the 3D BIC% calculated as the area of the BIC region divided by the exterior surface of the implant.
Implant stability measurement
Before measuring BIC%, the wireless resonance frequency analyzer Osstell ISQ (Osstell ISQ, Osstell AB, Gothenborg, Sweden) was used to measure the ISQ value. The smartpeg for external hex connection of Altlas implant (Type 1, Osstell AB) was placed on the top of the implants. The peg has a magnetic material attached to its upper part. When the probe of the Osstell ISQ instrument is near to the smartpeg, the peg is vibrated by magnetic pulses and then Osstell ISQ instrument can detect the resonance frequency and translate it into the ISQ value ( Fig. 2 ). The ISQ of the abutment–implant system was assigned a value between 0 and 100 to represent the ISQ, where a larger ISQ indicates a higher stability. The ISQ was measured twice for each specimen, and the mean was determined.
Correlation and statistical analyses
The ISQ values for the designed scenarios of trabecular-bone stiffnesses and the 3D BIC% were summarized as mean and SD values. One-way analysis of variance (ANOVA) and Duncan’s multiple comparisons were used to assess differences. Quadratic regression models were applied and the goodness of fit for regression models was quantified using squared correlation coefficients ( R 2 values). All statistical analysis was performed with SAS software (SAS v9.1.2, SAS Institute, Cary, NC, USA) with an alpha value of 0.05.
The 3D BIC% differed significantly between the sawbone models with and without the cellular type of bone structure ( Table 1 ) and with four types of cellular characteristics (elastic moduli of 137, 47.5, 23, and 12.4 MPa) in both the ANOVA ( p < 0.001) and Duncan’s multiple range tests ( p < 0.05) ( Table 1 ). The 3D BIC was more than twofold larger for the solid-rigid bone model (elastic modulus of 123 MPa) than for the cellular-rigid model (elastic modulus of 137 MPa). The 3D BIC% increased from 17.2% to 58.4% as the elastic modulus of the cellular type of trabecular bone increased from 12.4 to 137 MPa.
|Specimen structure||Elastic modulus (MPa)||BIC% (mean ± SD)||ISQ (mean ± SD)|
|Solid type||123.0||100 ± 0||83.7 ± 1.9|
|Cellular type||137.0||41.5 ± 0.5 a||81.1 ± 1.0 a|
|47.5||39.0 ± 1.0 b||73.4 ± 2.3 b|
|23.0||30.8 ± 1.1 c||69.1 ± 1.7 c|
|12.4||26.2 ± 1.6 d||64.1 ± 2.1 d|