# The Effect of Insertion Angles and Depths of Dental Implant on the Initial Stability
## 4 Discussion  
Dental implant is one of the major treatments for patients with full or partial dentulism, and the degree of primary stability affects the survival rate of the dental implant. Numerous studies have measured the primary stability of vertical implants [24; 25; 26; 27]; however, studies on the primary stability of tilted implants are rare. In fact, this study was the first to explore the effect of insertion angle and insertion depth of tilted implants, as well as the effect of measurement direction on the PTV and ISQ of implant primary stability, aiming to provide a reference for clinical dentists during implant operations. When the dental implant fixtures were inserted at various tilt angles, the PTV of the primary implant stability depended on the measurement direction. Therefore, the tapping direction for the PTV should be carefully selected during the clinical test. In 1999, [PERSON] et al. [28] used tilted implants to treat patients with severe atrophic dentulous maxillae, noting that tilting the implants at 30\({}^{\circ}\)-40\({}^{\circ}\) could prevent bone grafting. [PERSON] et al. [29] used 4-6 tilted implants to treat 33 patients with severe atrophic dentulous maxillae, and achieved a 97% success rate in long-term follow-up (average of 10 years). [PERSON] et al. [30] used tilted implants to treat patients with severe alveolar atrophy because of edentulism in the posterior mandible, achieving a 99% success rate. The use of all-on-4 for complete denture fabrication had become increasingly popular for edentulism treatment. This surgical technique was proposed by [PERSON] et al. [18] and mainly involved inserting two dental implant fixtures axially in the anterior region and two tilted implant fixtures in the posterior region. [PERSON] noted that all-on-4 had four advantages: avoidance of bone grafting, a high success rate, superior hygienic maintenance, and immediate loading. [PERSON] et al. [31] evaluated the success rate of patients who underwent the all-on-4 treatment; 324 patients underwent mandibular reconstruction using 1296 fixtures, and the success rate of the fixtures 7 years after the surgery was 96.6%. Because of the high surgical success rate of all-on-4, an increasing number of dental implant fixtures had been inserted into the jawbone at titled angles. In laboratories, acquiring fresh human jawbones with similar bone quality and quantity is extremely difficult [32; 33]. Scholars had used fresh animal bones for biomechanical experiments with artificial implants [20; 34; 35]; however, the material properties of each bone specimen were different. Some scholars [26; 32; 36; 37], as well as the ASTM regulations [38], contended that artificial foam bones were suitable materials for evaluating the biomechanical properties of artificial implants. Therefore, this study referred to experiments by scholars [26; 36; 37] and conducted experiments that used cellular polyurethane blocks. This study referred to the literature and selected the 137-MPa elastic modulus for artificial cancellous bone and artificial cortical bones with 2-mm thickness [36; 37; 39]. The temperature during inserting implant was not measured due to the artifial bone specimens used in this study. However, several researchers indicated that the high temperature during drilling or inserting implant may induce surrounding bone thermonecrosis [40; 41]. The implant stability might be affected by the necrosis bone. The three common methods for the clinical measurement of the primary stability of implants [8; 36; 42] are ISQ, PTV, and ITV. ITV can only be measured after inserting the dental implant fixture into the jawbone, whereas PTV and ISQ can be measured repeatedly; thus, PTV and ISQ are frequently used for the clinical assessment of changes in the fixture stability of patients during follow-up visits [43]. In addition, [PERSON] et al. [44] noted that the primary stability measured using ISQ and PTV should have more clinical implications compared to ITV and removal torque value. [PERSON] et al.