Influence of Bone Quality, Drilling Protocol, Implant Diameter/Length on Primary Stability: An In Vitro Comparative Study on Insertion Torque and Resonance Frequency Analysis

Dental implant stability is considered as one of the most important factors for successful osseointegration, and it must be maintained from the time of installation and throughout the healing phase.^1,2  The initial mechanical interlocking between the implant and bone, also known as primary stability, is mainly influenced by 3 factors: bone anatomy (quality and quantity), implant design, and surgical technique.³  The establishment of good primary stability is a primary focus during implant installation, because implant excessive micromotion is associated with higher failure rate.^4,5  On the contrary, it is advocated that a high level of primary stability makes immediate loading more predictable.⁶ 

Commonly, primary stability can be assessed by 2 widely used clinical parameters: insertion torque (IT) and resonance frequency quotient (RFA).⁷  IT, which is a measure of the rotational friction of the implant, is a purely mechanical factor, perceived by the clinician as a good estimator for primary stability. However, IT seems not to be an absolute parameter, especially when comparing different implant systems, as it is largely affected by implant design.^8,9  Differently, RFA is based on resonance frequency of the implant–bone complex analysis. By means of a transducer, which is directly inserted into the implant, RFA measures the stiffness and deflection of the implant into the bone.¹⁰  Thus, previous authors argued that IT and RFA are distinct parameters evaluating primary stability.^11,12 

It has been pointed out that high IT and a compression of the cortical layer could lead to the activation of osteoclastic activity.¹³  In a recent animal study a major extent of intracortical remodeling spaces was found in cortical bone subjected to compression after undersized preparation, compared with lower compression following a wider osteotomy.¹⁴  Thus, novel implant designs are currently developed to provide increased bone contact, reduced bone compression, together with an easy installation procedure.¹⁵  Aiming to this, the prototype implant presented in this research was designed to reduce bone compression and maximize the initial contact with the native bone. In particular, this implant design was provided with knife-edge threads with a progressive increase of thread depth in the apical region. The mechanical behavior in IT, and RFA of such implant design is unknown. Previous studies have demonstrated that clinical parameters, such as bone or implant features, may be the determining factors affecting the primary stability.^16–18  Thus, the primary stability of this implant design can be altered by parameters such as bone quality, implant diameter, and length and drilling technique. Therefore, the actual mechanical behavior, in IT and RFA, of this novel implant design has to be tested in different in vitro conditions before the clinical application. This preclinical experimental step is necessary to evaluate the implant mechanical performance in the absence of patient-related and operator-related factors and therefore to adopt the most appropriate surgical approach.

The aim of this in vitro study was to assess the influence of bone quality, drilling technique, implant diameter, and implant length on IT and RFA of a prototype implant specifically designed for reduced bone compression.

In this in vitro investigation, 195 new prototype implants (ProShape, Prodent Italia, Pero, Italy) were used. These non-self-tapping screw-shaped implants had a tapered implant core provided with knife-edge threads (Figure 1). A progressive increase of thread depth in the apical part was given, so that the thread crests outer profile was cylindrical in the body length and conical (taper angle: 10°) at the apical portion. Implants were characterized with a double-etched surface and conical internal connection.

Implants with a different diameter combined with different length were used. The following nominal implant diameters were considered: 3.8 mm, 4.2 mm, 4.6 mm, and 5.1 mm, having maximum diameters at the thread crest of 4.2 mm, 4.6 mm, 5 mm, and 5.5 mm, respectively. The implant lengths considered were: 7 mm, 11.5 mm and 15 mm.

In this in vitro experiment, implants were inserted into rigid polyurethane foam blocks (dimensions: 130 × 180 × 40 mm) (Sawbones AB, Malmö, Sweden). These test blocks have been previously used for dental implant mechanical investigations.^19,20  The following block densities were considered: 20, 30, 40, and 50 pcf, having a density of 320, 480, 640, and 800 mg/cm³, respectively. The blocks selected, which were not provided of cortical layer, represent a wide range of bone densities, from soft to dense bone.²¹ 

Implant bed sites were performed without irrigation using a handpiece mounted on a vertical drill press (MA TR C22B echoRD, EchoEng, Cormano, Italy) to perform a reproducible osteotomy with high precision (Figure 2). After a cortical drill and pilot drill, increasing-diameter drills were used. The surgical protocol did not include countersink drill or tap drill. Two drilling protocols were tested, according to the last drill used:

• Group A: conventional protocol. According to this preparation, the implant-bone interlocking is created at the top of the threads. The discrepancy between implant outer diameter and drill size was 0.15 mm. This preparation was performed in 480, 640, and 800 mg/cm³ blocks.

• Group B: undersized protocol. According to this protocol, an underpreparation of the implant site is achieved. Compared with the A protocol, a tighter interlocking is reached between the substrate and the threads. The discrepancy between implant outer diameter and drill size was 0.30 mm at the implant body and 0.50 mm at the implant apical part. In this manner, compression is produced at the implant neck and in the apical region. This preparation was performed in 320, 480, and 640 mg/cm³ blocks.

Specifically, group A drill was not used in 480 mg/cm³ blocks due to the impossibility to reach a rotational stability of the implant. On the contrary, group B drill was excluded from 800 mg/cm³ blocks due to the impossibility of implant placement at the correct positioning.

A comparison between each osteotomy line and the implant outline is displayed in Figure 3.

According to the manufacturer indications, implant shoulder vertical position was achieved 0.5 mm below the superficial crestal level. During implant installation, final IT value was measured using a digital torque gauge (AFG 500 N, S/N 97400, Mecmesin Ltd, Slinfold, UK). After installing a compatible smart peg, implant RFA was assessed by means of implant stability quotient (ISQ) values with Osstell ISQ S/N 3901 (Integration Diagnostics, Göteborg, Sweden). Five implants were tested for each testing condition.

Data analysis was performed using SPSS 20 (IBM Corporation, Armonk, NY). Normality distribution for IT values and RFA values could be assumed taking into account the large sample size and graphical evaluation of Q-Q plots. Correlation between IT values and RFA values was assessed with Pearson correlation coefficient. Linear regression models were estimated to assess the effect of drilling protocol, implant diameter, implant length, and substrate density on study outcomes (IT and RFA). All tests were 2-sided and a P value less than .05 was considered statistically significant. Statistical approach and results were reviewed by an independent statistician.

Mean IT value was 49.9 Ncm (SD: 34.6) and mean ISQ was 70.6 (SD: 5.8). IT and RFA values were significantly correlated (Pearson correlation coefficient: 0.54; t = 9, df = 193, P < .0001) IT and RFA results, based on drilling protocol and substrate density, are showed in Figure 4.

Higher IT values were associated with drilling protocol B vs A (mean difference: 71.7 Ncm, 95% confidence interval 66.7 to 76.7; P < .0001), implant length (3.6 Ncm increase per cm in length, 95% CI: 3.0–4.2; P < .0001) and substrate density (0.199 Ncm increase per mg/cm³ in density, 95% CI: 0.183–0.215; P < .0001), whereas implant diameter was not associated with IT (P = .23) (Table 1).

Higher RFA values were associated with drilling protocol B vs A (mean difference: 3.9, 95% CI: 2.9–4.8; P < .0001), implant length (1.0 increase per cm in length, 95% CI: 0.9–1.1; P < .0001), and substrate density (0.032 increase per mg/cm³ in density, 95% CI: 0.020–0.035; P < .0001), whereas implant diameter was not associated with IT (P = .69) (Table 2).

With the present study, we investigated the mechanical behavior of a novel implant design in primary stability in different testing conditions. Implant macro design plays a remarkable role in the establishment of primary stability.²²  The design of the prototype implant was provided with a deep thread depth, since it was demonstrated that such configuration is able to give higher primary stability.²³  Therefore, a mechanical interlocking at the top of the cutting threads is reached, while slightly compression of the bone walls is achieved at the coronal and apical third of the osteotomy. At the same time, bone chambers between the walls of the osteotomy and the implant body are formed. It is known that this type of structure would trigger a quick de novo bone formation in living bone, which would enhance the osseointegration process.²⁴  Implant primary stability was assessed by 2 broadly used clinical parameters, namely IT and RFA. There is a major difference between those variables: IT is a parameter that can be recorded only at the implant installation, while RFA can be measured at different time intervals. In such manner, RFA could provide an assessment of both primary stability, if it is measured at the time of installation, and secondary stability, if it is measured during the healing period.²⁵  The value of RFA may have a relevant clinical impact, since it was suggested that low values of RFA are predictors for implant loss.²⁶ 

Previous studies aimed to compare the mechanical behavior of IT with RFA at the time of implant installation. The relationship between those parameters is controversial in the literature.

A recent review,²⁷  including 12 clinical trials, concluded that IT and RFA are 2 independent and incomparable parameters, since no statistically significant correlation was found. The authors underlined that such result may be influenced by the different clinical conditions in which those parameters where tested and whether the instrumentation used were calibrated. Ito et al²⁸  suggested that RFA is mostly influenced by the bone-implant contact at the neck region of the implant, whereas IT is subjected by the bone interlocking at the entire implant length. This difference may explain the discrepancy found in the review.

Nevertheless, a recent clinical study showed that RFA have a positive correlation when a moderate IT was recorded when assessing implant primary stability.²⁹  The present study found a strong correlation between IT and RFA. Such robust relationship may have been favored by the highly controlled setup of this in vitro study, in which the substrate was made by polyurethane. The fine calibration of the instruments used to record the measurements may have influenced the final result as well.

According to the multivariate model, both IT and RFA were associated to substrate density and implant length. This result is not surprising, since it is known that bone anatomy and implant macro design are 2 major factors affecting primary stability.^25,30  In particular, bone density is considered as the main predictor for IT value.³¹  Implant length was also positively associated with an increased IT and RFA. It is known that one technique to increase IT before the application of immediately loaded rehabilitations is the use of a long and titled implant. IT is, in fact, directly related to the total amount of surface in contact with the bone.⁹  Differently, RFA is considered to be less influenced by the implant length in clinical conditions, in which the implant stability is mainly provided by the interlock with the cortical bone;^32,33  the present results may have been influenced by the isotropic characteristics of the substrate. Interestingly, the implant diameter did not showed any significant influence on primary stability. Future clinical studies should confirm such associations.

One of the most noteworthy findings was the role of the drilling protocol on IT and RFA. Protocol B showed increased values for both IT and RFA compared to protocol A. This result entails a relevant clinical observation. Although the increase of RFA values from dense to soft bone protocol is not particularly noteworthy (95% CI: 2.9–4.8), the increase of IT values is highly relevant (95% CI: 66.7–76.7). Such increase to high levels of IT may represent, however, an adverse condition from a biologic and a mechanical point of view.

High IT is commonly pursued by clinicians as a good predictor for osseointegration,^4,34  and there are a number of clinicians who pursue very high torque when performing immediate loading protocols. However, this belief is debatable. On the one hand, good results in clinical outcomes have been reported for implants inserted both at extremely high levels of IT (up to 176 Ncm).³⁵  On the other hand, one could argue whether such high levels of strain are necessary or beneficial to the bone biology.

The most recent research have been supporting the growing evidence that extreme IT values are harmful for the bone metabolism, causing compression damage,²⁰  decreased osseointegration,³⁶  and eventually marginal bone loss.^18,37  Rather, in implants placed with high IT (>50 Ncm) might have a worse outcome than implant placed with lower IT (<50 Ncm).^38,39 

Lastly, high torque values can permanently damage the implant prosthetic connection and components,⁴⁰  thus it is recommended to follow the manufacturer instructions on the limit of IT.

Therefore, from the present results, it can be said than the risk of having excessive compression in certain cases is too great compared to the benefit of a small RFA increase. Thus, if high values of IT are expected (ie, in dense bone), it can be recommended to avoid an underpreparation of the osteotomy.

Such experimental observations should be considered within the limitation of the present study. The in vitro design of this study is an appropriate method to explore the pure mechanical behavior of an implant. The advantage over other experimental designs is that possible additional variables, such as patient-related factors (bone macro and microarchitecture) and operator-related factors (skills and experience) are excluded. The substrates used were made by polyurethane, which are normally used in such tests. However, such a model entails some limitations: a cortical layer is lacking and the peculiar micro-architecture of the bone is not reproduced. Moreover, implant site preparation was precisely obtained with a vertical drill press; obviously, such parallel drilling procedure cannot be reproduced by a human operator. It may be recommended that future in vitro research should involve the insertion of the studied implant into fresh bone substrates (ie, porcine ribs) by a human operator.

From the present findings very robust models were constructed for the studied implant. The predictors for IT and RFA were individuated (drilling protocol, implant length, and substrate density). There is an indication that underpreparing the implant site may lead to excessive IT and a modest increase of RFA. To avoid undesirable bone compression dense bone protocol is recommended. On the other hand, it must be said that the present results should be cautiously interpreted. The clinical implications hereby presented represent a speculation on exploratory data; therefore, they might not reflect the clinical reality. Future research should clinically confirm the present biomechanical findings. Moreover, the biologic and clinical response to the implant studied is lacking and needs to be investigated.

Within the limitations of this in vitro study, it can be said that the studied implant can achieve a good level of primary stability in IT and RFA. A strong correlation was found between IT and RFA values. Both parameters are influenced by the drilling protocol, implant length, and substrate density.

Further studies are required to investigate the clinical response in primary stability and marginal bone response.

Abbreviations

ISQ:

implant stability quotient

IT:

insertion torque

pcf:

pound per cubic foot

RFA:

resonance frequency analysis

The authors wish to thank Francesco Cavallin, University of Padova, Italy, for the statistical revision of the methodology, results, and conclusions.

The authors declare that the present study was conducted in the absence of any conflict of interest.