The current study focused on the shape of the pedicle, and used six parameters based on CT scanning to establish a coordinate system to guide cervical spine pedicle screw placement. First D1 and D2 were used to coordinate positioning in this system, to replace the inaccurate but traditional method which relies on landmarks (articular mass, inferior articular process of the cephalad vertebra, and lateral vertebral notch) for the SP for placement of the pedicle.[1, 19–22] Unlike D1, which has gained much attention and has been the subject of extensive study, D2 was considered first in this study and deemed worthy of more attention because its value is relatively fixed in all segments. No previously published study has combined these two parameters to determine the insertion point of the pedicle. We have previously used D2 in clinical practice and found that it can be easily measured via CT reconstruction preoperatively, and during pedicle screw placement from C3 to C7.
Given that most preoperative preparations for pedicle screw placement only focus on the narrowest PW to choose the appropriate width of the pedicle screw, PW is often measured clinically. In the current study there was a linear correlation between PW and the pedicle’s narrowest PH, which can be depicted by an equation. We can incorporate PW—the most common clinical pedicle data—into the equation, obtain PH, and further guide the choice of pedicle screw size. At the same time the sagittal plane of the pedicle has a larger fault tolerance space, which means that the offset of the pedicle screw in the sagittal plane is comparatively safer than that in the transverse plane.
After confirming the SP of the pedicle screw by D1 and D2, and choosing the appropriate pedicle screw based on PW and PH, the angle of pedicle screw placement can be guided by TSA and SSA. TSA can be accurately measured via CT scanning, which is also a common and necessary parameter in surgical planning.[20, 23] Moreover, TSA was related to the sequence of the vertebra. The lower the vertebral body the larger the TSA, and D1 will also be larger. There was a positive correlation between D1 and TSA, which may be explained as a right triangle effect. D1 can be considered the right-angle edge, and TSA the opposite angle. The larger TSA is, the longer D1 is. For this reason TSA and D1 were the parameters used to confirm the transverse orientation of the pedicle screw.
The correlations between PW and PH, and D1 and SSA can be explained by examining the development of the pedicle. As the spine gradually ossifies after chondrification at the 6th week of embryonic development, three main ossification centers play an important role; one in the centrum, and one each on either side of the vertebral arch. Longitudinal and latitudinal growth of the vertebral body accompany the growth, development, and movement of the whole body. At the same time, the second primary ossification center and the mechanical load should also be considered.[24–26] However, these were not the focus of the current study.
The aim of the present study was to facilitate more individualized and accurate pedicle screw placement. With respect to individualization, the first considerations in adults are differences in gender, height, and weight. Pedicles in males and individuals of greater heights and weights were larger than those of females and individuals of smaller heights and weights. This is due to the innate proportional development of individuals.[3, 27, 28]
We considered SSA to be another essential parameter that is often overlooked relative to TSA. Routine CT examination is in the supine position, and a change to the prone position during the operation would inevitably lead to data changes with respect to the sagittal alignment of the cervical spine. Therefore, if the sagittal angulation is positioned in the horizontal plane accuracy will be affected,[3, 29] and if the lower edge of the vertebral body is used for positioning it is difficult to grasp during the operation.[30, 31] In the current study SSA was associated with each vertebral body’s D2 parameter, which can be understood as the lamina line. It is not associated with the sequence of vertebrae, rather it can be more accurately determined via direct visualization during the operation. The above-described six parameters obtained via CT scanning in the supine position to guide screw placement are accurate, and will not change regardless of how the position changes. More importantly, they are not affected by lordosis or kyphosis, degeneration and hyperplasia, imbalance or deformity of the cervical spine.
Notably we do not advocate memorizing the dates of the pedicle’s size and angle. Instead, each pedicle of each patient must be carefully measured before surgery so as to obtain 6 parameters that can be individually and accurately applied to surgery.
The current study had some limitations. The size of the sample was relatively small, and it was restricted to Chinese adults. Data measurements are likely to differ in other races and in children, but notably the same measurement methodology could be used. Secondly, the primary aim of the study was to provide an effective aid to assist freehand pedicle screw placement, and it is not appropriate to rely solely on the measurement data to place screws mechanically. The experience of the surgeon and intraoperative feel are still very important. Lastly, although we could precisely fix the SP position and the entrance angle, the system needs convenient devices with which to apply the parameters during surgery. Therefore, we are currently conducting further studies aimed at developing a new locating device based on the system, and broader clinical application of the system.