## 4 Discussion  
Pedicle screw fixation has been broadly used in patients with various spinal disorders in recent decades [3; 4; 5]. Screw loosening is one of the most frequently reported complications of pedicle screw fixation and is associated with the necessity of revision surgery [9; 11]. Poor bone quality followed by cyclic caudocephalad toggling and rotational stress cause micromotion between bone and screws finally results in screw loosening [26]. Several revision strategies, including various cement augmentations, extended fixations, and replacements of loosened screws with larger diameter screws have been studied [10; 13; 24; 27]. Replacing loose screws with larger diameter screws potentially increases the risk of pedicle wall violation, which reasonably suggests a pronounced trend of pedicle bone stock preservation in primary surgery [12]. Our biomechanical results showed that in the dual-thread groups, when using a smaller volume screw (D-PCo), even less bone stock was purchased, providing similar stability when compared with traditional screws (D-Cy and D-Co), which decreased the necessity of upsizing the screw diameter in revision surgery. In cases of metastatic spinal cord compression, rigid pedicle screws fixation is important. However, postoperative iatrogenic neurological deterioration due to the extravasation of the tumor or the fragility of the pedicle bone reached 0.6-7% [28; 29; 30], and complications were rare but could still be prevented. Extended clinical application could be used in these diseases, which requires immediate postoperative stability without risking the extravasation of intravertebral lesions. Synthetic bone models, such as Sawbones(r) and polyurethane foam, are widely accepted due to their regularity and reproducibility compared to the characteristics of human or porcine cadaveric vertebrae; moreover, they are well-established vertebral surrogates for biomechanical testing [15; 31; 32]. Considering the anatomical simulation and distribution of pedicle bone density, Sawbones(r), instead of a polyurethane test block, was chosen for this in vitro biomechanical testing [15]. Our biomechanical data showed that dual-thread screws led to higher insertion torques and pullout forces than single-thread screws, which was consistent with other research. Dual-thread screws had been proven to increase insertional torque and pullout force in healthy/osteoporotic human cadaveric vertebrae and testing foam blocks [22; 23]. Increased fatigue cyclic loading was observed in dual-thread screws compared with single-thread screws in human vertebrae with reduced bone mineral density [25]. Additive biomechanical performance was observed when combined with conical shape, which progressively allowed pedicle compression with advancing screws [24; 33]. An additive trend could also be observed in our data. The reasons why proximal-conical dual-thread screws (D-PCo) decreased screw volume and reached similar insertion torque and pullout force compared to the other two screws (D-Co and D-Cy) are as follows: (1) The screw pullout force was a major factor in the ability of the pedicle to achieve 57-60% performance; thus, the effect of the vertebral body was far less influential [13]. The distal portion of the screw plays a less important role in pedicle screw pullout performance. (2) The geometric matching of the proximal-conical shape to the pedicle. Conical screws have been reported to possess superior anchoring power compared to cylindrical screws [24; 33] and a higher slope conical angle (D-PCo) embedded more bone in threads when screws advanced. The proximal-conical screws tapered by 30% from the hub to the proximal 20 mm; however, the traditional conical screws tapered by 30% from the hub to the tip (50 mm); the supposedly decreased pullout force resulting from the downsized distal portion of the screw was balanced by this proximal enhancement.