One of the key goals of ACDF is the restoration of the intervertebral disc height with vertebral distraction and graft insertion, which can indirectly decompress the neural foramina, as well as the spinal canal [6, 7]. The Caspar distractor system is widely used to achieve such distraction, as it improves the exposure of the intervertebral space, making it easier and safer to perform the decompression [2, 8].
The technique of pin insertion is very important for visualization and maintaining the working space during the operation. Malposition, such as the pin being placed too close to the operative disc, can result in interfering with the endplate preparation, while those placed too distally can cut thru adjacent endplates, especially in osteoporotic patients. In the coronal plane, the entry points of the Caspar pins should be in the midline. Off-center pins can create vertebral rotation and scoliosis. If the pins are both placed off to one side, distraction can cause asymmetrical endplate preparation. Moreover, inserting the pins off to one side can compromise plate fixation if it is in close proximity to where the screw needs to be inserted into. For the pin direction, the cranial angulation of the pin should be parallel to the disc space in the sagittal plane. Previous studies had no consensus about the appropriate trajectory for placement of Caspar pins. Only one study suggested that the cranial pins be inserted at the upper third of the vertebral body to be fused and the caudal pins be placed at the lower third of the vertebral body under fluoroscopic guidance . For an ACDF operation, the superior pin is ideally approximately 7 to 10 mm from the upper level’s inferior endplate because of the concavity of that endplate. The inferior pin can be placed approximately 5 mm below the lower level’s superior endplate (Fig. 1C). However, when performing a total disc replacement operation (TDR), the Caspar pins must be placed as far from the operative level as possible without violating the adjacent discs. In a previous study in an Asian population, Chen et al. reported that the average cranial angulation of the superior endplates ranged from 4.5 to 9.0° and the average cranial angulation of inferior endplates were between 4.5 to 7.5° . Yukawa et al. reported that the minimal disc height of the cervical vertebra was 5.8 ± 1.3 mm .
For most patients with normal anatomy, the ideal position of the Caspar pin is in the center of the anterior vertebral body in the coronal plane and parallel to the vertebral endplates. Caspar pins are typically inserted by free-hand technique with or without intraoperative imaging . However, this technique can result in nearly perfect pin placement or sometimes require multiple attempts of pin insertion that can compromise the vertebral body and necessitate increased radiation exposure.
Our study shows that the average superior and inferior endplate slopes are different from the previous study by Chen et al. (superior endplate slope; 7.4° to 11.7° vs. 4.5° to 9.0°, inferior endplate slope; 6.0° to 8.2° vs. 4.5° to 7.5°). One reason may be that the age (at the time of death) in this study is older compared to the previous study (73.5 years, range 55–88 years vs. 41 years, range 25–51 years). The purpose of our study is to develop and test a novel aiming device prototype to control the pin entry points as well as direction. The pin entry point is 7 mm. from each side of the vertebral endplate. The center in the coronal plane is found by identifying the uncinated process bilaterally and placing the aiming device in the middle. The aiming sleeves then guide the pin direction in a cephalad inclination identical to the cervical endplates. The study shows the average SE-CP was 6.2 ± 2.0° and the average IE-CP was 6.3 ± 2.2°. In the present study, we do not observe any cervical endplate violation in any of the Caspar pin insertions from C3 to C7.
The study has some limitations. First, this is the only a cadaveric study designed to be a proof-of-concept trial. Another shortcoming is the relatively small number of cadavers utilized. But these were Asian subjects and their vertebral size are smaller than other races. Therefore, if it worked in this relatively smaller necks, it should work in larger subjects. Finally, it is still unclear that the device would be effective in highly kyphotic individuals where the pins, sometimes, need to be placed independent of each other.
The ideal patients who are suitable for this device may be less kyphotic sagittal alignment and nearly-preserved disc height. We recommend checking a lateral image once the device is in place, prior to inserting the pins, which should help to ensure that the pins will not violate the adjacent endplates in small individuals. The authors believe that a clinical study is important for the next stage.