# The Effect of Insertion Angles and Depths of Dental Implant on the Initial Stability
## 1 Introduction  
Dental implant is one of the mainstream approaches for treating the problem of missing teeth. Unlike a conventional bridge, dental implants do not damage the adjacent teeth, are aesthetically pleasing, and provide sufficient strength [1; 2; 3]. When the implant root is first inserted into the alveolar bone, part of the bone tissue attaches to the dental implant fixture, and the connection between the bone and the fixture becomes denser after healing and undergoing osseointegration with the fixture surface. Typically, osseointegration is a slow process that requires 3-6 months, and the primary stability of the dental implant fixture after its insertion has been confirmed to be a crucial factor affecting osseointegration [4; 5]. Several studies have noted that a high initial or primary stability of the dental implant fixture after insertion into the alveolar bone leads to superior osseointegration and a high postoperative implant survival rate [4; 5; 6]. Current clinical practice employs three measures of the primary stability of dental implant fixtures, namely the implant stability quotient (ISQ), the Periotest value (PTV), and the maximum insertion torque (ITV) of the inserted fixture [7; 8]. Both ISQ and PTV are widely used because of their postoperative implant stability tracking capabilities. At one time, dentists mostly used tissue-supported complete dentures for patients with edentulism; however, this approach caused residual ridge resorption, which reduced the retention of the tissue-supported complete denture and impaired long-term treatment [9; 10; 11]. The researchers stated that many treatment options are available for patients with edentulism [12], such as using shorterdental implant fixtures [13], performing sinus lift surgery on the maxilla to increase the height of the bone [14], or using zygomatic implants [15]. In addition, the tilted insertion of dental implant fixtures, or tilted implants, offer patients a more conservative option. In a study of patients with edentulism, [PERSON] et al. [16] evaluated the success rate of tilted and vertical insertion of the dental implant fixtures. They documented that implants could be tilted backward by 25\({}^{\circ}\)-35\({}^{\circ}\) and 30\({}^{\circ}\)-35\({}^{\circ}\), respectively, while at the same time avoiding their insertion into the mental foramen and maxillary sinus. In a meta-analysis in 2014, [PERSON] et al. [17] commented that the angulation of clinically tilted implants is typically between 11\({}^{\circ}\) and 45\({}^{\circ}\). [PERSON] et al. [18] proposed the technique of all-on-4, which offers an alternative treatment for patients with edentulism. The technique involves using four dental implant fixtures in the maxilla and mandible to support the prosthetic structure. Because of the anatomy of the jawbone, and to avoid inserting the fixture into the maxillary sinus, tilted implants must be used to acquire more implant anchorage. Regarding inserting fixtures in the mandible, care should be taken to avoid damaging the inferior alveolar nerve. In addition, tilted implants are also frequently used in clinical treatment with complete-arch prostheses to reduce the cantilever length. Bone quality and quantity, anatomical limitations, and the patient's insertion position are used for clinical references to determine the insertion depth of the dental implant fixture. [PERSON] et al. [19] contended that inserting implants in a subcrestal position can reduce the risk of future fixture exposure. [PERSON] et al. [20] inserted eight subcrestal and equicrestal fixtures at various depths on both sides of the mandibles of nine dogs. Their experiment demonstrated that subcrestally inserting implants resulted in a more aesthetically pleasing emergence profile, as well as superior treatment in the aesthetic areas. However, [PERSON] et al. [21] noted that deep fixture insertion was more likely to cause crestal bone resorption over time, and [PERSON] et al.