In retrospective and meta-analysis studies, it has been reported that the pterygoid implants have a high success rate in the rehabilitation of posterior atrophic maxilla [8–10]. Curi [11], Graves [3], and Rodriguez et al. [12] reported the most important factors in the success of pterygoid implants are; the learning curve of the surgeon, the anatomy of the region, and the use of the correct technique. Graves reported that the implant slot should be drilled in the maxillary second molar region at an angle of 45° to the occlusal plane [12]. So, the second molar region is the most appropriate place for pterygoid implant to reach pterygopalatine fossa. In their cadaver study, Uchida et al. found that the mean of alveolar crest-Frankfurt plane angle to the occlusal plane was 52.3+-10 = 42.3° in all samples [13]. This finding was in line with the angulation of the pterygoid implant to the occlusal plane proposed by Graves and Venturelli [3, 14]. Unlike these researchers, Rodriguez et al., suggested that this angle should be 70.8° in their radiographic study [12]. Rodriguez [15] and Balshi et al. [16, 17] were reported that the implants with a diameter of 3.75-4.00 mm and a length of 15–18 mm bring out good clinical results. In another study, Rodriguez et al. examined Cone Beam Computed Tomography (CBCT) data of 202 patients and revealed that the mean length of the alveolar ridge from the tuber region to the apical point of the pterygoid was 22.15 ± 1.56 mm. Furthermore, considering the safety distance of > 2 mm between the implant and the palatine artery, an implant length of 15–18 mm would be appropriate in the pterygomaxillary region in most cases [18]. In our study, with reference to Rodriguez et al. and Graves, the surgical intervention was carried out in the maxillary 1st and 2nd molar region at an angle of 70° to the horizontal plane of Frankfurt, an angle of 45° to the occlusal plane, and a buccopalatinal angle of 20°. All implants placed on both sides were the same diameter and length (4.3*18 mm), but also in two different macro designs (tapered and parallel). It should be noted that, as in the case of traditional implant treatments, the implants placed in the pterygoid region, the implant placement should be positioned depending on the anatomy of each patient, and the appropriate implant location, angle, diameter, and length should be determined based on the 3D evaluation with CBCT.
The primary stability plays a main role in the clinical success of implants. Bone quality significantly determines the biomechanical response and clinical outcomes of implants. The measurement of the primary stability of implants can be done by insertion torque resonance frequency analysis and perio-test methods. In some studies, it was asserted that the ISQ values obtained by RFA were sufficient to determine the reliability of implant stability [19, 20]. In case the bone quality is poor at the implant placement area, different implant designs such as tilted, zygomatic, and pterygoid implants and/or alternative surgical techniques are currently being used to increase primary stability.
It was reported that the pterygoid implants were successful in osseointegration and had a high functional stability in all the previous studies [11, 21]. Rodriguez et al. examined the CBCT data of 202 patients with atrophic maxilla and found that the bone density of the pterygoid plate was three times greater than that of the maxillary tuber and found that the bone density of the pterygoid plate was three times higher than the maxillary tuberosity [18]. In their study, Jaffin and Berman found that there was statistically significant difference between the tuberosity and pterygoid plate in terms of bone density, and asserted that placing of the apical part of implant into the pterygoid plate would increase the success [22]. There are some studies arguing that the primary implant stability is related with the cortical bone thickness [23–27]. In the light of this information, it can be suggested that pterygoid implants can be preferred in the treatment of the posterior edentulous maxilla, since they provide high primary stability through the dense cortical structure of the pterygoid bone.
There are very few studies examining the primary stability of tilted implants. In their in-vitro study on the primary stability of implants with different inclinations, Kashi et al. found that the 10-degree tilted implants had a higher primary stability than axial implants [28]. On the other hand, in their 7-year prospective study, Ayub et al. asserted that there was no statistical difference between the axial and tilted implants in terms of primary stability [29]. Considering the differences between the findings of the studies on this field, we are of the opinion that since there might be deviations in the RFA measurement of the tilted implants, there are very few studies evaluating this. To the best of our knowledge, there is no study examining the primary stability of pterygoid implants in the literature. Therefore, in this study, we aimed to contribute to the literature by means of evaluating the primary stability of tilted pterygoid implants. The primary stability of pterygoid implants was measured using RFA, as it is fast, simple and easy to perform as part of a routine clinical procedure.
Implant design is one of main factors affecting the implant primary stability and the loading forces during or after osseointegration. Implant design features can be classified as macro and micro designs. While the macro design features include the body shape, pitch, and groove design; the micro design mainly includes the surface morphology [30, 31]. The key point for comparing the primary stability of different types of implants is to prepare the socket in line with the instructions of the producing company and to use the implants having the diameter and length appropriate to the socket. In our study, while evaluating the primary stability of pterygoid implants supported by a pterygoid plate consisting of dense cortical bone, two different macro designs were compared. In accordance with the company instructions, the tapered implant was placed on one side and the parallel implant was placed on the other side randomly. In the previous studies, the bone density and the placement torque (PT) values, bone density and ISQ values, and PT and ISQ values were evaluated in different implant designs and it was asserted that there were significant correlations between the two factors in each study [32–34]. Howashi et al. compared the PT values in different implant designs and reported that the PT values in the tapered implant group were significantly higher than those in the parallel implant group. Furthermore, when they evaluated the bone quality by means of CBCT and compared the values again, they found that in general the PT and ISQ values in the tapered implant group were higher than those in the parallel implant group [35]. In our study, the ISQ values were found higher in the tapered implants than parallel implants; however, no significant difference was noted.
In the literature, the normal ISQ values for primary stability was reported at the range from 60 to 80; however, there was no consensus at the ISQ threshold [36–38]. Nevertheless, on the other side, some studies reported that ISQ value at least 55 during implant placement ensured the clinically significant stability and successful osseointegration [37, 38]. Furthermore, these values are moderately correlated with the placement torque and depend on the various variables such as the implant length and width and the bone density [36–38]. In our study in which the ISQ values were measured from three different angles for each implant and the average of the values was recorded, it was found that the average of ISQ values for the parallel and tapered implant groups were 61 (54–66) and 68 (66–72), respectively.