Comparative evaluation of the stability of two different dental implant designs and surgical protocols—a pilot study

Dental implants are now a widely accepted treatment option for the replacement of missing teeth. The therapeutic goal of dental implants is to support restorations that replace single or multiple missing teeth so as to provide patient comfort, function, and esthetics as well as assist in the ongoing maintenance of remaining intraoral and perioral structures. However, anatomic limitations such as the maxillary sinus may limit the amount of bone available to place traditional length implants (>10 mm). To avoid invasive sinus elevation procedures, manufacturers have developed shorter implants (<10 mm). Multiple studies have proven that short implants are equally successful to longer implants [1–9]. Tapered implant design further enhances primary implant stability, especially in the posterior maxilla where bone quality is usually poor [10–12].

The purpose of this study was to evaluate the initial stability of the OsseoSpeed TX™¹ tapered implant (OSPTX) and to compare it to the standard OsseoSpeed™¹ parallel walled implant (OSP) as well as to compare the soft bone and standard surgical protocols. Both implants included in this study are manufactured from high-grade commercially pure titanium with surface roughness produced via a fluoride treatment process. The OSPTX and OSP implants are self-tapping implants. The implants used in this study were all of 4.0 mm in diameter and 8 mm in length. Microthreads™ characterize the coronal aspect of both implants. The OSPTX implant has the same features as OSP except the apex of the implant is tapered (Fig. 1).

Successful integration of dental implants is largely dependent on their primary stability [13]. Implants placed in the maxilla present more challenges due to the poor bone quality usually found in these areas. Another anatomic challenge in the posterior maxilla is the pneumatization of the maxillary sinus which can limit the length of implant that can be placed. To avoid invasive sinus augmentation procedures, implants have been designed in shorter lengths such as 8 mm. To further enhance short implant primary stability, a tapered design has been developed which has been proven to provide greater initial stability [10–12, 14]. Implant stability can be evaluated by different measures such as torque at the time of implant placement, resistance to reverse torque, and resonance frequency analysis (RFA). Multiple studies have established feasibility for validating implant stability in lab and animal models to justify using resonance frequency analysis in clinical trials [15, 16]. Limited literature exists on the OSPTX implant design, and to our knowledge, no clinical studies exist that compare OSP to OSPTX. A recent ex vivo comparison of two different designs of OSPTX implants in porcine mandibles demonstrated that a conical neck design presented higher primary stability (insertion torque and implant stability quotient (ISQ)) than a cylindrical neck design [17]. In our study, both the torque value and ISQ value were recorded at the time of placement. ISQ values were also recorded at implant uncovery at 6 weeks and also at 6 and 12 months when the final restoration was placed.

A recent systematic review by Stocchero et al. concluded that an undersized drilling protocol in soft bone is an effective way to enhance insertion torque but recommended that further clinical studies are needed to confirm these data [18]. Our study was designed to address this question, as it compared the standard drilling protocol to a soft bone protocol.

Our study hypothesis is that the stability of the OSPTX implant will be greater than that of the OSP implant due to the tapered design of the OSPTX implant.

To facilitate randomization, the manufacturer packaged each implant with a prescribed surgical protocol included. The surgeon was blinded to the implant type until the opening of the package when the patient was seated for the surgery.

The soft bone drilling protocol used for group A results in an underpreparation compared to the implant diameter by −0.5 mm at the body portion. Corresponding underpreparation at the apex is from the beginning of apex towards the tip of the implant −0.8, −0.4, and 0 mm, respectively. All implants were of 4 mm diameter and 8 mm length and were placed at sites coronal to the maxillary sinus where at least 8-mm bone height was available. Every patient received a cone beam computed tomography (CBCT) evaluation pre-operatively using an i-CAT®² unit. Bone quality was measured clinically by the surgeon during preparation of the osteotomy [19]. Implants were placed following a two-stage protocol. They were uncovered at 6 weeks at which time functionally loaded screw-retained provisional crown was delivered per a FDA approved protocol for this implant system. Implant stability was measured by insertion torque using a calibrated torque wrench³ at the time of implant placement and by ISQ measurements using the Osstell™⁴ unit at the time of implant placement and at 6 weeks and 6 and 12 months (Fig. 2). Standardized periapical radiographs were taken at the time of implant placement and at 6 and 12 months. Changes to the bone level heights were measured at 6 and 12 months by two blinded examiners using the ImageJ®⁵ software. The final cement-retained PFM crown (DSIGN porcelain) was delivered at 12 months.

ANOVA was used to compare the mean implant stabilities between the three groups. Post hoc testing was done via Tukey’s honestly significant differences test to calculate the differences between ISQ measurements at the time of implant placement, 6 weeks and 6 and 12 months (Fig. 2) as well as bone levels at 6 and 12 months (Fig. 3). The correlations of multiple parameters such as insertion torque, ISQ, and crestal bone level were calculated using the Pearson product-moment correlation coefficient.

Augmentation of the maxillary sinus prior to dental implant placement is routinely performed in order to help patients restore their maxillary posterior dentition. Unfortunately, not all patients are candidates for this procedure due to either health, personal, or financial concerns. An alternative treatment without the need for a sinus elevation procedure is the use of a shorter implant. Research has shown that shorter implants (<10 mm) have comparable survival and success rates to longer implants (>10 mm) [1–4, 6–9]. Primary implant stability, as measured at the time of placement, is another important factor for both short and long implants. Tapered implant designs are considered to provide greater initial stability [12, 14]. Specifically, Lozano-Carrascal et al. in a prospective clinical study compared OSP implants to tapered MIS® implants placed in human mandibles. They reported the tapered implants achieved higher primary stability measured through ISQ and insertion torque [20]. Our study did not support these findings as we did not find a statistically significant difference in primary stability between the OSP and OSPTX implant designs. However, the OSPTX implants used in our study were tapered only at the apex as opposed to the MIS® implant which is tapered throughout the body of the implant. Furthermore, the mean insertion torque value observed in our study for the OSP group was lower (27.6 Ncm) than that observed by Lozano-Carrascal et al. in the maxilla for the same implant (35.8 Ncm) [20]. This difference may be attributed to the shorter implant length and wider diameter used in our study. The mean ISQ at insertion for the OSP implants in our study presented comparable values to an ex vivo study using the same implant placed in fresh porcine mandibles [17].

Surgical protocols have been developed to overcome the poor bone quality found in the posterior maxilla, so as to increase primary implant stability. Most surgical systems recommend a soft bone surgical protocol which requires a narrower diameter osteotomy than that of the implant being placed. This can involve underpreparing the complete length of the osteotomy or only underpreparing the apical ¾ of the osteotomy when the crestal bone is denser. In the posterior maxilla, the bone quality can vary greatly. By comparing the stability between the three groups, we found that implant stability was neither statistically significantly different between the two different implant designs or between the two different surgical protocols used. These findings are in agreement with Siera-Rebolledo et al., who also found no statistically significant differences between a soft bone drilling protocol and a standard drilling protocol [21].

Insertion torque presented a moderate to strong correlation with ISQ values at 6 weeks, 6 months, and 1 year but not at time of implant insertion. This finding is in agreement with Acil et al. who reported no statistically significant correlation between insertion torque and ISQ at time of implant placement [22].

Although a strong correlation was found between insertion torque and bone loss at all time points, the mean bone loss observed was minimal (<0.5 mm).

The OSP implant system has demonstrated high survival rates ranging from 94 up to 100% in previous long- and short-term studies [23–25]. Our findings are comparable with an overall 93.3% survival rate at 1 year, despite the fact that all implants were placed in the posterior maxilla.

Survival rates and stability of OSP and OSPTX implants was comparable.

Osteotomy preparation either by the standard or by the soft bone surgical protocol had no significant effect on implant survival, success, and stability.

Insertion torque presented a moderate to strong correlation with ISQ values at 6 weeks, 6 months, and 1 year.

Insertion torque presented a weak correlation to ISQ values at time of implant insertion.