Surgical options for C2 OPLL involving anterior procedures (anterior cervical corpectomy and fusion) and posterior procedures (laminoplasty, laminectomy and laminectomy with fusion) have been established . However, the optimum method for C2 OPLL is still controversial because of the complex structures around the C2 level. Anterior procedures can provide direct spinal cord decompression and achieve good results, especially when there are few ossified segments and the occupying rate of spinal canal of > 60% . Sun, J., et al.  proposed a modified anterior controllable antedisplacement and fusion, which is a “shelter technique” to treat patients with OPLL extending to C2. However, considering the limitations of the sheltered space and the difficulty of intraoperative osteotomy, this surgical technique has not been popularized. In addition, the higher operation level, longer surgery segments, the thickness of the cervical plate, and the stretching or injury of the esophagus during exposure also affect the clinical prognosis . At this moment, posterior approach has shown its advantages, especially when OPLL involves a wide range of segments or with developmental cervical spinal stenosis. It allows adequate indirect decompression through the floating spinal cord, which is safer and easier to obtain good results .
There have been many reports on the placement technology of C2 pars screws and C2 pedicle screws, with slightly different starting points and placement trajectories [21,22,23]. Compared with the C2 pedicle screw, the starting point of the C2 pars screw is closer to the caudal side of the lateral mass, and the placement trajectory is also steeper. Hoh et al.  recommended a screw length of 14–16 mm to achieve maximum safety. When we insert the C2 pedicle screw, to avoid damage to the VA, the starting point is often more lateral, and the abduction angle is larger. At the same time, studies have shown that for patients with atlantoaxial screw rod bone graft fusion, the ideal degree of pars screw placement after postoperative CT examination is better than that of pedicle screws [4, 25]. At the same time, a previous review also showed that  the risk of VA injury with C2 pedicle screws is approximately 0.3%, while the shorter pars screws have no VA injury. Evidenced by CT, pedicle screws have a higher proportion of screw misalignment than pars screws. This is obviously because the pars screws are shorter and have a safer nail trajectory. However, there are different opinions that although the length of the pars screw is short, it may damage the transverse hole during insertion. Therefore, application of the pars screw does not necessarily reduce the risk of VA injury .
In the past 10 years, we have noticed some general trends in our hospital for C2 posterior fixation in patients with OPLL involving the C2 segment. The use of C2 pedicle screws seems to decrease with the appearance of C2 pars screws, indicating that the latter has become the preferred fixation method for patients with challenging VA anatomy. Especially in elderly patients, due to the abnormal and fragile vertebral bone caused by osteoporosis, the use of pedicle screws will have a high risk of VA injury, while short pars screws are usually used when the VA is abnormal or pedicle screw fixation is used when the risk is too high, which indicates that it has become the screw of choice for patients with challenging VA anatomy. Our study found that with the same segmental fixation, the postoperative health-related outcomes of the two groups did not show significant differences, which may be related to the fact that both groups of patients received adequate decompression. However, the operation time and blood loss related to C2 pars screws were significantly less than those of C2 pedicle screws. This may be because compared with a C2 pedicle screw, a C2 pars screw has less muscle dissection on C2 lateral mass and back of lamina and retains the C2 attachment muscle group to a large extent, which reduces partial blood loss. At the same time, the tail direction of C2 pars screws is more medial than C2 pedicle screws, so it is more conducive to the connection of the screw rod system, which saves time in bending and inserting the connecting rod. In addition, O-arm navigation is required for C2 pedicle screw placement, which undoubtedly increases the operation time and repeated adjustment of the screw placement direction also indirectly increases unnecessary blood loss. However, as long as the C2 epidural venous plexus is not damaged, there will be no more blood loss during C2 pedicle screw placement. We think this is an invasion of the muscle, but if it is separated under the periosteum, the muscle will hardly bleed. So the difference in our research may be related to exposure. In addition, for highly skilled surgeons, the implantation of C2 pedicle screws is performed without O-arm navigation and normal fluoroscopy imaging. Therefore, the advantages of pars screw technology in operation time and blood loss may be weakened. According to previous reports, the rate of VA damage and imaging screw dislocation rate of C2 pars screws were both significantly lower than those of C2 pedicle screws. None of the C2 pedicle screws or pars screws had VA, spinal cord, neurologic injury or screw looseness/failure in our study. One patient in both groups showed CSF leakage. The incidence of screw misalignment is also much lower than that reported in previous studies. The high extent of accuracy was attributed to the excellent surgical technique. The ideal and acceptable degree of screw placement in the pedicle group was 91.3%, while the ideal and acceptable degree of screw placement in the pars group was as high as 100%. Although it has not been proven, we believe it is due to the emergence of C2 pars screw technology. Further, the high rate of success also demonstrates that when we have thorough knowledge of pivotal anatomy, all C2 pars screws and C2 pedicle screws can be placed accurately and risk of VA damage or serious clinical dislocation can be avoided. However, biomechanical studies have proven that C2 pedicle screws have greater rigidity and higher load failures than C2 pars screws [28, 29]. The importance of this difference is unclear . If it is necessary to use pedicle screws instead of pars screws, we need to make the delicate trade-off between safety and rigidity, carefully check the preoperative CT, decide which type of screw to use, and consider the habits of surgeons and the actual clinical situation. Therefore, according to our experience, for patients with OPLL involving the C2 segment, we believe that C2 pars screw placement is a secure and effective technique.
However, there are still several weaknesses to be further emphasized and discussed regarding the current research. The first limitation is related to the small-sized retrospective sample. All patients were chosen based on strict inclusion and exclusion criteria. Second, the average follow-up time was too short. A prospective research with a longer follow-up period is necessary to further observe the postoperative clinical results of patients with OPLL. Third, we can only judge that the biomechanical strength of C2 pars screws is sufficient from the previous literature on atlantoaxial internal fixation. Although our study showed that there was no screw looseness or failure in either group within 2 years after the operation, it has not been proven by finite element analysis that patients with OPLL can obtain enough holding force, similar to C2 pedicle screws, after inserting C2 pars screws. A longer follow-up period (e.g., more than 5 years) to observe the screw loosening is very necessary, which is also the focus of future research. Nevertheless, despite these restrictions, our research is valuable in demonstrating the advantages of C2 pars screw placement in patients with OPLL involving the C2 segment.