Application of C2 subfacetal screws for the management of atlantoaxial dislocation in patients with Klippel-Feil syndrome characterized by a narrow C2 pedicle and high-riding vertebral artery

C2-3 congenital fusion is often associated with AAD, which results in neurological dysfunction and requires reduction and fixation to relieve ventral cervical cord compression [10, 15]. Pedicle screws insertion is the commonly used technique for C2 fixation. Pedicle screws are combined with C1 lateral mass screws to perform Goel-Harms technique, which is a widely used fixation technique in clinical practice [3, 11]. The C2 pedicle screw can directly fix the isthmus and pedicle as a result of its long implant depth, good pull-out strength, and biomechanical stability [6]. However, C2-3 fused vertebrae are different from normal vertebrae, which are often associated with HRVA and narrow pedicles [22, 23].

Preoperative measurements of the patients in this study showed a decrease in the isthmus height and pedicle width of up to 3.7 ± 1.8 mm and 3.0 ± 1.4 mm, respectively. According to the diagnostic criteria for HRVA proposed by Neo et al.[16], namely, isthmus height 5 mm or less and/or internal height 2 mm or less, all seven patients (12 lateralities) had HRVAs. In this group of patients, the isthmus height was less than 5 mm (Fig. 3A, Table 1). The risks of inserting pedicle screws and atlantoaxial transarticular screws were high because of the reduced isthmus height. Moreover, the lack of the bony structure of the pedicle made it difficult to implant 3.5 mm pedicle screws. Therefore, it was necessary to select the appropriate entry point and screw trajectory.

Fig. 3figure 3

An illustrative case. A 36-year-old man presented with weak limbs and unstable motion. A Preoperative computed tomography (CT) showed an increased atlanto dental interval, C1 occipitalization, C2-3 fusion, bilateral atlantoaxial lateral joints tilted forward, an almost vertical left articular surface. B, C High-riding vertebral arteries on both sides. The medial height of the right side is 5.5 mm. The height of the isthmus is 3 mm. The medial height of the left side is 5 mm. The height of the isthmus is 2 mm. D Bilateral pedicle stenosis. The maximum width of the pedicle measured using axial CT is 2 mm. E A sagittal magnetic resonance (MR) T2 image shows atlantoaxial dislocation, the dentate process pressing the spinal cord, and a high signal in the spinal cord. F Sagittal CT showed a reduced atlantoaxial median joint and that the cage position between the lateral atlantoaxial joints was suitable (G, H). I Both screws are in proper positions. J Postoperative T2 MR showed that the spinal cord compression was relieved. There was still a high signal in the spinal cord. K, L CT scan showing bone fusion between the bilateral lateral joints at 8 months after surgery

Presently, commonly used alternatives to pedicle screws and transarticular screws include lamina screws, which are inserted through the junction of the spinous process and lamina. This technology is widely used, but when the height of the spinous process and the thickness of the axial lamina are small, its use is limited. Moreover, this technology only fixes the posterior single column structure and has a relatively weak biomechanical performance and a low fusion rate [14]. Rusconi et al. have reported the application of inferior articular process screws, but this technology only fixes the inferior articular process, which cannot achieve long trajectory length and thus a superior biomechanical property [19]. Furthermore, some studies have reported the use of transarticular screw fixation with the lower vertebrae [2], which is expected to achieve multi-cortical fixation through the facet joint. However, this technology fixes an additional vertebra and increases the fusion segments (C4) for patients with congenital C2-3 fusion, which worsens the cervical mobility of patients with C2-3 fusion. Some studies have reported improved pedicle screw techniques. Du et al. and Lee et al. have reported the medial “in–out-in” technique, which prevents VA injury by opening a window through the wall of the spinal canal to achieve multi-cortical tri-column fixation. Although no spinal cord injury has been reported, the screw entering the spinal canal poses a certain threat to the spinal cord [3, 13]. Additionally, Goel performed the VA transposition technique, using two smaller screws for fixation. However, this technique requires a careful separation of the VA, which is difficult to perform [9].

On the contrary, Patkar et al. and Salunke have used subfacetal screws for C2 screw placement [17, 20]. We found this technique particularly suitable for the patients in this study. Although the pedicles of C2-3 fused vertebrae are slender, the superior articular processes in patients in this study were thick. We measured 14 lateralities of seven patients in this group, and the average internal height was 10.8 ± 3.2 mm. The diameter of the commonly used screw is 3.5 mm, and it is assumed that the bone on both sides of the screw is at least 0.5 mm. Therefore, it is considered safe to place the subfacetal screws and pedicle screws in a bone with a minimum width of 4 mm. The average internal height of the C2 vertebra in the study patients was more than 4 mm and there was sufficient space for C2 subfacetal screw insertion (Fig. 1B). In addition, the entry point was located 3 mm to 4 mm below the articular surface in a small area above the transverse foramen (Fig. 1C). The screw trajectory was parallel to the articular surface or slightly downward, approximately 20 degrees inward, and tilted inward as far as possible into the vertebral body. The average effective screw length inserted in the study patients was 10.8 ± 3.2 mm. There was no incidence of nerve or blood vessel injury during the procedure. Additionally, there was no screw loosening or screw breakage observed during follow-up visits. A subfacetal screw in the vertebral body from the posterior facet of the articular process, achieving dual-column fixation, may be a better alternative to pedicle screws.

All patients in this study had AAD. Intrafacet distraction and implantation of the cage were performed during the procedure. The following details were given utmost attention during the procedure. First, the screw path of the C2 subfacetal screw is located on the upper and inner sides of the VA. If the screw loosens and subsidence occurs after surgery, VA injury would occur. Thus, a cage for inter-articular fusion was inserted to reduce the stress of screw and prevent subsidence. Second, because of the entry point of the C2 subfacetal screw close to the articular surface and the caudal tilted trajectory, the cage should be inserted first, and thereafter, the screw should be inserted. If the screw is inserted first, the path of the cage entering the lateral joint may be blocked by the screw head, which is not conducive for the preservation of the C2 nerve root. We observed a large operating space around the lateral articular space after cage implantation; therefore, there was no need to cut the C2 nerve root during the procedure. Third, examination images should be carefully studied before the procedure, with special focus on the internal height measurement and design of the screw path. The direction of the screw path should not be too downward to prevent the screw from entering the transverse foramen. The tapered tap should be carefully inserted during the puncture of the screw path. After inserting the tapered tap at 1 mm to 2 mm, the operator should retreat to observe the bleeding. After entering the tapered tap at a depth of 10 mm, it can be determined to exceed the vertebral artery, and then it is safe to advance the tapered tap. In one patient, a screw invaded the transverse foramen during the procedure, but no obvious bleeding was observed when the awl was used for puncture during the procedure; however, when the screw was inserted, it burst the screw path and partially entered the transverse foramen. We adjusted the screw placement direction inward and upward, and the vertebral artery was not damaged after screw placement.

The reduction in AAD and improvement of clinical symptoms were achieved for all patients in this study (Figs. 4, 5). We consider C2 subfacetal screws to be a neglected alternative to pedicle screws for the management of patients KFS. The C2 subfacetal screw technique is simple and safe. The dural membrane can be observed directly inside the insertion point, and the risk of spinal cord injury is low. The insertion point is located above the VA and there is no risk of VA injury. Furthermore, the C2 subfacetal screw has a long insertion depth and good fixation stability.

Fig. 4figure 4

An illustrative case. A 33-year-old woman presented with occipital pain and unstable walking. A Preoperative sagittal computed tomography (CT) showed basilar invagination (BI), atlantoaxial dislocation, atlantooccipital fusion, and C2-3 fusion. Axial CT showed bilateral pedicle stenosis and the cortical pedicle (B, C). D The sagittal CT image shows a right internal height of 9.78 mm, isthmus height of 3.74 mm, left internal height of 11.5 mm, and isthmus height of 2.04 mm. Three-dimensional CT reconstruction showed that despite the high-riding vertebral arteries (HRVAs), there was still enough space above the vertebral artery to insert the subfacetal screw (E). F Sagittal magnetic resonance imaging shows atlantoaxial dislocation, compression of the spinal cord by the odontoid process, and chiari malformation

Fig. 5figure 5

A After cage implantation, screws were inserted below the bilateral C2 articular surface. B Postoperative sagittal computed tomography (CT) showed that the odontoid process had been reduced and the cage was inserted anteriorly between the bilateral joints. C, D, E, F The insertion point of the C2 screw was located below the bilateral C2 articular surface and in a small area above the vertebral artery. The screw was inserted medially as far as possible. G Postoperative sagittal T2 magnetic resonance (MR) image showing that the ventral spinal cord compression was completely relieved, and the ventral spinal cerebrospinal fluid (CSF) signal was recovered. At 13 months after surgery, CT showed that the odontoid process remained in the reduced state (H) and the atlantoaxial interarticular bone bridge was formed (I, J)

However, this study had some limitations. There were only seven patients included in the study. Studies involving more patients or a larger number of participants are needed to confirm the efficacy and safety of this procedure. In addition, during this study, the average internal height of the C2 vertebra of the seven patients was more than 4 mm and the VA was far from the superior articular surface; therefore, there was enough space for subfacetal screw insertion. However, if the internal height was small and the VA was closer to the superior articular surface of C2, then this technique would not be appropriate. Therefore, more studies are needed to confirm whether HRVA are caused by isthmus height loss in patients with KFS. Additionally, the follow-up time was short, not allowing for observation of the long-term effects of inserted subfacetal screws. Therefore, studies with longer follow-up duration are needed to sufficiently monitor possible long-term effects. The trajectory length of the subfacetal screws was smaller than that of pedicle screws; therefore, the biomechanical properties may be inferior to that of pedicle screws.

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