Topic on Implant and Tissue Management | The Impact of Implant Macro Design on Initial Stability of Implants in Different Bone Qualities: An In Vitro Study Using a Standard Drilling Protocol (I) (German)

Background: Initial stability of implants is crucial for the success of implant therapy. This study investigates the impact of implant shape, length, and diameter on the initial stability of implants in varying bone qualities.

Methods: Using a standard drilling protocol, implants of three different lengths and diameters (two parallel-walled and one tapered implant) were placed in polyurethane foam blocks of different densities (35, 25, 15, and 10 PCF). The initial stability of the implants was evaluated through insertion torque (IT) and resonance frequency analysis (RFA). The optimal range for IT and RFA was defined as 25 to 50 Ncm and ISQ 60 to 80, respectively. Implants from different groups were compared to determine if their initial stability fell within the optimal range.

Results: The macro design, length, diameter of the implants, and the density of the bone blocks significantly influenced IT and RFA. In the two parallel-walled implant groups, 8/40 and 9/40 implants had IT within the optimal range, while in the tapered implant group, 13/40 reached optimal IT (within the 25 to 50 Ncm range). Implant diameter greatly impacted initial stability; although the tapered implants outperformed, only one-third achieved adequate stability.

Conclusion: The results of this study emphasize the need for adjusting the drilling protocol based on different bone qualities in clinical practice. The influence of modified drilling protocols on implant outcomes requires further research.

This article is divided into two parts. The first part provides an introduction to the study background and methods, while the second part, to be published in the sixth issue of the “Dental Comprehensive Edition,” focuses on discussing the results and further elaboration.

Keywords: Immediate loading, initial stability, insertion torque, implant stability quotient (ISQ), resonance frequency analysis (RFA), bone quality

Introduction

Restoring partially or fully edentulous patients with implants has become a highly reliable and predictable treatment option with substantial survival and success rates. To meet the growing demand for shorter waiting periods before implant restoration and to enhance understanding of the osseointegration process, clinicians have developed immediate loading protocols. Unlike delayed loading, which occurs two months post-implantation, immediate loading takes place within a week of insertion. Ensuring ideal initial stability of the implant is a critical prerequisite for the success of immediate loading and long-term outcomes. The application of immediate loading on the day of surgery, connecting the implant to the prosthetic restoration, and using full-arch fixed dentures for edentulous patients have evolved into a predictable process. Reports indicate that even with lower initial stability, some full-arch restorations can achieve successful outcomes. Therefore, achieving a quadrilateral interlock of implants seems beneficial for survival prognosis in immediate loading protocols.

Initial stability refers to the short-term immobility of the implant within the bone or the biomechanical interaction between the implant surface and the surrounding bone during implantation. It is essential to distinguish between initial and secondary stability, the latter arising from the bone healing process itself. Good initial stability is generally believed to promote bone cell differentiation. Conversely, reduced initial stability may lead to micromotion, fibrous tissue formation, and early implant failure. Factors influencing initial stability include implant design, surface treatment, site characteristics, and surgical drilling technique. Additionally, implant diameter and length positively correlate with initial stability.

Several quantitative methods for assessing initial implant stability have been reported, with insertion torque (IT) and resonance frequency analysis (RFA) being the most commonly used. RFA is expressed as the implant stability quotient (ISQ), widely used in the literature. Clinically, ISQ measurement serves as an indirect indicator to determine the appropriate timing for effective implant loading and as a potential predictor of implant failure. RFA involves evaluating the response of a piezoelectric ceramic element connected to the implant to vibrational stimuli composed of small sinusoidal signals in the 5 to 15 kHz range, with 25 Hz increments. The maximum amplitude of the element’s response can be converted to a parameter called the Implant Stability Quotient (ISQ), which ranges from 0 to 100. The ISQ value positively correlates with the overall mechanical stability of the implant-bone interface it reflects.

Insertion torque (IT) is an easily obtainable representative parameter for assessing initial stability during the implant insertion process. IT can be measured using a standard torque wrench or a specially designed surgical handpiece. It reflects the bone’s resistance to cutting during implant insertion, measured in Ncm. Increased insertion torque aids in achieving initial stability by reducing implant micromotion. Currently, IT and RFA are the most critical parameters for determining initial implant stability in clinical settings. Several clinical studies have utilized these stability measurement methods, attempting to establish predictive values for osseointegration probabilities in highly complex protocols such as immediate loading.

An ISQ value of 60 and an insertion torque of 35 Ncm are generally considered favorable for achieving good initial stability. However, the ideal insertion torque for immediate loading remains a hotly debated topic within the academic community. Studies by Weigl and Strangio et al. suggest no significant difference in implant survival rates between IT = 25 Ncm and IT = 32 Ncm groups. In contrast, higher torque rates, such as 40 or 50 Ncm, appear to be associated with higher implant survival rates. To ensure sufficient initial stability, especially for immediate loading, some researchers recommend an insertion torque of at least ≥ 32 Ncm or > 35 Ncm. Animal studies have shown a strong correlation between an insertion torque of ≥ 32 Ncm and implant success. In a systematic review and meta-analysis, Benic et al. concluded that immediate and conventional loading of single-crown implants show no significant differences in implant retention rates and marginal bone loss. This conclusion mainly stems from evaluating studies with insertion torques of 20 to 45 Ncm or ISQ values of 60 to 65, with no simultaneous bone grafting required. Additionally, a randomized controlled clinical trial by Degidi et al. on immediate implant placement and immediate restoration (inclusion criteria: IT > 25 Ncm, ISQ > 60) demonstrated good osseointegration and clinical stability for all implants during follow-up examinations. Although Greenstein and Cavallaro indicated in a literature review that IT > 50 Ncm does not seem to cause damage to bone tissue, some other studies suggest that excessive IT might not be suitable for all implant systems or bone types. Compared to implants inserted with conventional IT (< 50 Ncm), excessive IT might result in greater peri-implant bone remodeling and buccal soft tissue recession. Moreover, research by Marconcini et al. indicates that implants inserted in the mandible with higher IT (> 50 Ncm) result in more significant bone resorption and mucosal recession compared to those with conventional IT (< 50 Ncm). Overall, based on the current state of this research, IT > 25 Ncm and < 50 Ncm, along with ISQ > 60, seem to be reliable clinical indicators of initial stability for successful immediate loading protocols.

Considering various factors in clinical situations is crucial for achieving adequate initial stability, especially for immediate loading protocols. These factors include the implant’s geometry, length, and diameter, along with appropriate drilling protocols adjusted for bone quality. In areas dominated by cortical and dense bone tissue, slow drilling and gentle tapping are advised to avoid thermal and mechanical necrosis. Conversely, in areas with poorer bone quality, bone condensation, staged drilling, and implants requiring no tapping are recommended to enhance initial stability.

Bone density plays a significant role in selecting and applying implant systems. In softer cancellous bone, using longer implants with threaded tips positively impacts initial stability. Implants with deeper threads also seem to improve initial stability in areas with poorer bone quality. However, Makary et al. suggest that this beneficial effect may only manifest in softer bone types (i.e., D3 and D4 categories).

Regarding implant geometry, parallel-walled and tapered implants are mainly used clinically. Tapered implants generally display higher insertion torque compared to parallel-walled implants, despite no significant difference in implant failure rates between the two geometries. Due to higher initial stability, tapered implants are more likely to be applied in immediate loading protocols.

This study aims to establish clinical recommendations for implant selection to achieve optimal initial stability based on bone density and quality. Several other studies have addressed this issue, but they often use different types of implants from various manufacturers, each undergoing different surface treatment processes. Additionally, researchers have used different types of implant drilling kits. However, in actual clinical settings, bone quality assessment primarily occurs during drilling and relies on the surgeon’s expertise. The purpose of this study is to compare the impact of different implant types and macro geometries on initial stability under a standardized drilling protocol. To achieve this goal, the study employs a standardized in vitro model and uses polyurethane foam blocks of varying densities. The focus is on evaluating the initial stability of three different implant types and macro geometries, all subjected to the same surface treatment. Additionally, the study examines the impact of different implant lengths and diameters on initial stability under the same drilling protocol. Initial stability is quantified through insertion torque (IT) and resonance frequency analysis (RFA), evaluated according to the predefined successful immediate loading protocol range (IT: 25 to 50 Ncm; ISQ: ≥ 60). The first null hypothesis proposed in this study is that there is no difference in the optimal initial stability range for different types of implants in different bone qualities under a standard drilling protocol. To further comprehend implant behavior, another null hypothesis is proposed, stating that implant size (length and diameter) does not affect initial stability under varying bone density and implant type conditions.

The final null hypothesis of the study states that insertion torque is solely determined by the contact surface area between the implant and bone, irrespective of the implant’s macro geometry and dimensions.

Materials and Methods

In this study, three different types of implants with varying lengths and diameters (all procured from SIC invent, Switzerland; Table 1) were inserted into solid rigid polyurethane foam blocks of four different densities (10 PCF, 15 PCF, 25 PCF, and 35 PCF; PCF = pounds per cubic foot) from Sawbones, USA. Evaluation of the initial stability of the implants was performed using insertion torque (IT) and resonance frequency analysis (RFA).

Implant Characteristics

All implants used in this study underwent surface treatment, including sandblasting with zirconia beads followed by acid etching (SICmatrix®). This process achieves a moderate surface roughness (Sa = 1.0 μm). This study evaluated the performance of three groups of dental implants with different macro designs:

Group 1: Cylindrical Implant (SICace®)

The SICace® cylindrical screw implant has a basic cylindrical shape with a tapered apex. The apex thread is self-tapping with a wide trapezoidal design, providing significant cutting space and an aggressive thread design. The manufacturer recommends SICace® implants for all oral implant indications in D1 to D3 bone quality.

Group 2: Cylindrical Implant (SICmax®)

The SICmax® cylindrical screw implant has a two-stage cylindrical basic shape with a tapered apex. The apex thread design is self-tapping with a narrow trapezoidal shape, featuring a blunt end and relatively small cutting space. In the coronal region of the implant, there is a larger thread core diameter with double micro threads. As SICmax® implants combine a blunt apex lacking cutting capability, they are particularly suitable for soft bone tissue and the posterior maxillary region, particularly for all forms of sinus lift procedures. The manufacturer recommends SICmax® implants for all indications and immediate placements in D2 to D4 bone quality.

Group 3: Tapered Implant (SICtapered®)

The SICtapered® cylindrical screw implant has a basic tapered shape with a two-stage cylindrical coronal region. The apex thread is self-tapping with a very narrow trapezoidal design, providing significant cutting space and an aggressive thread design. In the coronal region, the implant features a larger thread core diameter. The manufacturer recommends SICtapered® implants for all indications in D1 to D4 bone quality and for immediate placements.

Preparation of Implant Sites

For implant site preparation, a CAD drawing was made, which uniformly transferred a matrix of rows and columns onto the various polyurethane foam blocks. This matrix formed the basis of the preparation axes for the predetermined 180 implant cavities on each polyurethane foam block, encompassing all implant types and sizes (n = 6 per group; four bone qualities and three implant types, totaling 720 implant sites; Figure 1). High-precision CNC milling machines were used for the preparation process (Figures 2a to e). The drills used were original SIC® drills, corresponding to the SIC® drilling protocol (drilling speed: 300 rpm). This process ensured that the cavities fully aligned with the ideal SIC® contour, free from manual errors such as angle deviations or incorrect preparation depths. Table 1 lists the different implant types, implant diameters and lengths, and the drilling sequence (final preparation and crestal bone cortical preparation).

Implant Insertion and Stability Measurement

An experienced implantology expert inserted all implants into the prepared sites using a torque-controlled surgical motor and contra-angle handpiece (Implanted Plus, WS-75L; W&H, Germany) at a rotation speed of 25 rpm. The surgical motor was capable of recording insertion torque throughout the implant insertion process. The maximum insertion torque of the surgical motor was 80 Ncm. For the analysis of insertion torque, the maximum insertion torque value reached during the insertion of each implant was recorded and used for statistical analysis. If the maximum torque of 80 Ncm was reached during implant insertion and the implant shoulder did not align flush with the surface of the polyurethane foam block (i.e., the implant failed to be seated flush with the bone surface), the length of the implant portion protruding above the foam block surface was measured and recorded in millimeters.

In addition to insertion torque, a compatible SmartPeg (model 92) was mounted on each implant for resonance frequency analysis (RFA) using an electronic device (Osstell®IDx, Osstell, Sweden). For each implant, the average implant stability quotient (ISQ) value from three RFA assessments was calculated and recorded.

Statistical Analysis

The mean maximum IT ± standard deviation (SD) and ISQ mean ± SD were calculated and further processed using statistical analysis. The sample size for each specimen was n = 6 (total N = 720). For initial implant stability, the optimal range for IT was 25 to 50 Ncm, and for ISQ it was > 60. This study defined IT values of < 25 Ncm or > 50 Ncm and ISQ values < 60 as critical ranges. To compare the three different implant macro designs, the frequency of IT and ISQ values meeting the optimal range across different bone qualities was descriptively presented.

To detect differences in the impacts of implant length and diameter on IT across different bone qualities, a general linear model was fitted with implant length and diameter as covariates, foam block type and implant type as fixed factors, and the logarithm of implant torque as the dependent variable. Samples with implant torque values ≥ 80 Ncm were excluded, as this represented the maximum torque of the surgical motor. Additionally, implants of the largest size (diameter: 5 mm; length: 14.5 mm) were excluded because they exceeded the maximum torque in most cases within the denser foam blocks. If the log of implant torque exhibited heteroscedasticity, heteroscedasticity-consistent standard errors (HC4) were estimated.

To test whether insertion torque could be adequately predicted by the contact surface, another general linear model was fitted with foam block type and implant type as fixed factors, contact surface as a covariate, and insertion torque as the dependent variable. The contact surface area between all implants and bone was provided by the manufacturer, as shown in Table 2.

Significant interactions between the two models were considered during analysis, and simple effects analysis was conducted when necessary. Main effects were analyzed where interaction effects permitted. Statistical analysis was performed using SPSS software (Version 27, IBM Corporation, USA). It was assumed that the linear model was a sufficient approximation for extracting basic conclusions about the impact of implant dimensions on IT. For paired comparisons, Bonferroni correction was applied to adjust p-values, and results were reported as estimates ([lower 95% confidence interval, upper 95% confidence interval]). GraphPad Prism 6 (GraphPad Software, USA) was used for plotting.