Various posterior fixation techniques have been introduced for the management of atlantoaxial instability.1,2 In 2003, Tan et al.3 described a revised technique for C1 screw insertion, in which they inserted the C1 lateral mass screw through the C1 posterior arch (PA). Generally, three techniques are used for C1 fixation: C1 screw insertion through the inferior lateral mass (ILM),4 insertion via the PA,3 and insertion by notching the PA.5–7 However, surgeons often encounter challenging situations in which screw insertion into the C1 lateral mass is not possible with these conventional methods.
Patients with rheumatoid arthritis or long-lasting facet arthrosis in C1–2 show various accompanying bony deformities. In such cases, a PA (or notching) screw cannot be applied to the hypoplastic PA because of the arch fracture and the possibility of vertebral artery (VA) injury.3,7–11 An ILM screw cannot be inserted when the height of the ILM is too small or the ILM is covered with a caudally tilted PA. Multiple types of bony changes are usually found together and thus compel surgeons to consider salvage techniques, such as occipitocervical fixation (OCF). However, sacrifice of about 20° of cervical flexion/extension occurs when the surgical level is extended with OCF.12–15 Moreover, complications related to the horizontal gaze have been reported.16,17
For these reasons, we applied screws at the superior lateral mass (SLM) of the C1 in patients with a deformed PA and ILM. To our knowledge, no study to date has reported the feasibility of this technique and its possible complications. Hence, we evaluated the feasibility of using the over-the-arch (OTA) screw for C1 fixation and assessed possible complications, including VA injury, occipital neuralgia, and screw malposition.
Methods
Inclusion Criteria
This study was approved by our institutional review board. Patient informed consent was waived owing to the retrospective study design. This age-, sex-, and BMI-matched case-control study included and compared patients who were treated with the OTA technique (OTA group) and those treated with conventional PA/ILM techniques (PA/ILM group). From March 2011 to July 2019, the senior author of this study (D.H.L.) performed lateral mass screw insertion into C1 using the OTA technique in 12 patients (2 males and 10 females) and conventional techniques in 103 patients (36 males and 67 females). In the current analysis, the 12 consecutive patients treated with the OTA technique were matched at a 1:4 ratio to patients treated with the conventional PA/ILM technique.
Patient Selection
The surgical decision to use the OTA technique for C1 screw insertion was made when other conventional technical options for C1 screw insertion were not feasible. We observed three types of bony deformation that hindered the use of a PA or ILM screw: 1) PA with a very small height (< 3.5 mm); 2) a caudally tilted PA blocking the inferior part of C1 lateral mass; and 3) loss of height at the ILM (< 3.5 mm). It is more difficult to insert screws with conventional techniques in such instances because all three factors are frequently combined. Long-lasting os odontoideum and rheumatoid arthritis are accompanied by severe facet arthrosis, resulting in height loss at the ILM. This not only makes it difficult to place a screw under the arch but also limits the operative field, resulting in failed bleeding control at the venous plexus and injury to the C2 nerve root.4,6,18 Bleeding of the C1–2 venous sinus is troublesome and frustrating during C1 lateral mass screw insertion, and this can even compel the surgeon to change the preoperative plan to extend to OCF.5 These bony changes combined with vertebral anomalies, including persistent first intersegmental artery, fenestration, and ponticulus posticus, may force surgeons to consider extension to an OCF. In our present study, we utilized a C1 screw in patients with these deformations by using the OTA technique (Fig. 1). However, the OTA screw was not considered in patients with 1) C0–1 joint violation with an OTA screw trajectory, 2) VA dominance at the screw insertion site, or 3) deformed SLM with cranially tilted PA.
A 63-year-old woman with long-lasting os odontoideum showed a deformed hypoplastic arch on the right side. A fenestrated VA anomaly was observed on the same side on a 3D reconstruction CT image (A). C1–2 facet arthritis on both sides and severe ILM height loss on the right side were observed on CT arteriography (B and C). On preoperative CT arteriography, the arch height was 1.41 mm and the ILM height was 0.41 mm on the right side. This patient underwent C1–4 posterior fusion. C1 fixation was achieved using a PA screw on the left side (D) and an OTA screw on the right side (E), as shown on postoperative CT sagittal reconstruction images. The PA on right side is marked with the red arrows. A hypoplastic PA, ILM height loss, and fenestrated VA under the arch made it difficult to use a PA or ILM screw for C1 fixation. Preoperative MR sagittal reconstruction image (F) shows severe compression of the cervicomedullary junction at the spinal cord due to long-lasting os odontoideum with retropulsion of the C2 body. Incomplete segmentation was observed at the C3–4 vertebral body. Immediate postoperative (G) and 1-year (H) follow-up MR images show correction of atlantoaxial subluxation and widening of the spinal canal at the C1–2 junction. Figure is available in color online only.
Surgical Technique for the OTA Screw Insertion
After exposing the C1 and C2 in the usual manner, the VA over the PA was dissected cranially. Using a curette and a Penfield dissector, gentle dissection was performed until the SLM of the C1 was exposed. Care was taken to avoid extensive dissection of the VA. The venous plexus above the PA was usually meager compared with that under the PA. A C1 screw was placed just above the C1 PA to minimize VA manipulation. While protecting the cranially retracted VA with the Penfield dissector, a pilot hole was created using a burr, followed by tapping and screw insertion. Although the SLM environment varies depending on VA anomalies and C1 nerve root anatomy, we tried to secure the entry point of the SLM by maintaining gentle retraction of the VA and C1 nerve root. The entry point was made at the midline of the SLM after identification of the lateral and medial margins of the lateral mass using a Penfield dissector (Fig. 2A and B). A 3.5-mm polyaxial screw with a length between 24 mm and 32 mm was used, depending on the preoperative measurements made using CT. The trajectory was approximately 10° and medially angulated (Fig. 2C). The aim point of the screw trajectory was between the 40% and 60% points on the C1 anterior arch to prevent violation of the C0–1 and C1–2 joints (Fig. 2D).19 The trajectory on the coronal and sagittal planes was confirmed with fluoroscopy. C2 fixation was performed using a pedicle screw, laminar screw, and pars screw on the basis of preoperative planning. The C1–2 screws were connected and fixed through bilateral longitudinal rods. A hard cervical brace was used for 2 to 3 months after surgery.
Depiction of the use of an OTA screw for C1 fixation. A: The entry point of the OTA screw is just above the PA and midline of the SLM. The VA is dissected cranially using Penfield elevators. B: The vertebral venous plexus is usually meager above the arch. A hypoplastic and caudally tilted PA and short ILM on the right side are depicted with asterisks (*). C: Medial angulation of the screw trajectory is usually about 10°. D: Under fluoroscopic guidance, the aim point of the screw trajectory is made between the 40% and 60% points on the C1 anterior arch to prevent violation of the C0–1 and C1–2 joints. Copyright Sun Joo Kim. Published with permission. Figure is available in color online only.
Clinical and Radiological Evaluation
The patients’ clinical and radiological data were collected from the electronic medical records. Neck Disability Index (NDI) and Japanese Orthopaedic Association (JOA) scale scores were collected before surgery and at the final follow-up. Preoperative CT vertebral arteriography was performed in every patient to evaluate the specific vertebral anatomy and bony deformities. Arch height, width, and ILM height were measured using preoperative CT arteriography. Caudally tilted PA blocking the ILM was also checked in all patients. We checked for the risky triad of C1 (i.e., arch height < 3.5 mm, ILM height < 3.5 mm, and blocked ILM by caudally tilted PA) in all 60 patients (120 screw sites) and recorded the number of patients with this condition. Radiological measurements were performed by two examiners (H.R.L. and S.Y.S.) who were blinded to patient information and not involved in patient treatment. VA injury, CSF leakage, and screw malposition were evaluated using postoperative CT. Ischemic signs at the VA, bony injury of the occiput and atlas, screw-related complications (i.e., loosening, pullout, and fracture), and bony union were evaluated at 1-year postoperative CT. Bony union was assessed using two different methods: 1) the presence of an obvious bone bridge on sagittal reconstruction CT, and 2) the absence of segmental motion on dynamic lateral radiography.8
Results
A total of 60 patients were included in this age-, sex-, and BMI-matched case-control study. There were no significant demographic differences between the OTA and PA/ILM groups. All patients were followed for longer than 12 months (range 12–36 months). NDI and JOA scale scores had improved significantly at the final follow-up in the OTA group (p = 0.03 and p < 0.001, respectively) and the PA/ILM group (both p < 0.001). However, NDI and JOA scale scores showed no significant differences between the two groups at both preoperation and final follow-up (Table 1).
Patient characteristics and clinical outcomes
| Characteristic | OTA Technique (n = 12) | PA/ILM Technique (n = 48) | p Value |
|---|---|---|---|
| Age, yrs | 58.9 ± 14.9 | 61.2 ± 10.7 | 0.41 |
| Sex, M/F | 2:10 | 8:40 | |
| Height, cm | 152.7 ± 8.4 | 154.9 ± 8.7 | 0.41 |
| Weight, kg | 60.7 ± 10.5 | 62.1 ± 9.2 | 0.58 |
| BMI, kg/m2 | 25.9 ± 3.2 | 26.2 ± 2.8 | 0.75 |
| Diagnosis | |||
| OO/OA/RA/other | 4:1:7:0 | 25:6:8:9 | 0.02*† |
| Follow-up period, days | 20.0 ± 4.6 | 20.1 ± 5.8 | 0.96 |
| EBL, ml | 195.7 ± 118.5 | 214.3 ± 94.2 | 0.87 |
| Op time, mins | 174.3 ± 52.4 | 159.8 ± 26.7 | 0.35 |
| Hospital stay, days | 7.5 ± 0.79 | 7.65 ± 0.75 | 0.92 |
| NDI | |||
| Preop | 19.6 ± 5.6 | 17.4 ± 6.5 | 0.22 |
| Final follow-up | 12.0 ± 8.4 | 10.4 ± 7.7 | 0.51 |
| p value‡ | 0.03* | <0.001* | |
| JOA scale score | |||
| Preop | 11.6 ± 5.7 | 11.2 ± 4.0 | 0.74 |
| Final follow-up | 15.4 ± 5.3 | 16.3 ± 5.8 | 0.62 |
| p value‡ | <0.001* | <0.001* |
EBL = estimated blood loss; OA = osteoarthritis; OO = os odontoideum; RA = rheumatoid arthritis.
Values are shown as number of patients or mean ± SD unless indicated otherwise.
p < 0.05.
The Fisher’s exact test was used.
The paired t-test was used to compare preoperative and final follow-up measurements.
The C2 root did not have to be transected during insertion of the OTA screw because the entry point was located in the SLM. The detailed results of the patients with OTA screws are presented in Table 2.
Patients included in the OTA group
| Patient No. | Age (yrs) | Sex | Diagnosis | Op Level | Arch Height (mm) | Arch Width (mm) | ILM Height (mm) | Screw Placement Method | Screw Length (mm) | Hospital Stay (days) | Follow-Up (mos) | |||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Rt | Lt | Rt | Lt | Rt | Lt | Rt | Lt | Rt | Lt | |||||||
| 1 | 63 | F | OO | C1–4 | 1.41 | 2.53 | 7.71 | 9.1 | 0.41 | 4.89 | OTA | PA | 26 | 28 | 9 | 24 |
| 2 | 64 | F | OO | C1–2 | 3.12 | 3.7 | 7.34 | 9.75 | 1.14 | 2.41 | OTA | PA | 28 | 26 | 8 | 18 |
| 3 | 38 | M | OO | C1–2 | 2.57 | 2.66 | 5.67 | 7.24 | 3.13 | 2.51 | OTA | OTA | 30 | 30 | 8 | 12 |
| 4 | 53 | M | OO | C1–2 | 3.47 | 1.76 | 8.16 | 8.5 | 4.27 | 1.24 | PA | OTA | 32 | 30 | 7 | 24 |
| 5 | 81 | F | OA | C1–2 | 1.21 | 3.9 | 7.34 | 7.9 | 2.49 | 3.63 | OTA | ILM | 26 | 26 | 7 | 24 |
| 6 | 62 | F | RA | C1–2 | 1.98 | 2.94 | 6.56 | 8.73 | 0 | 2.16 | OTA | PA | 24 | 24 | 7 | 24 |
| 7 | 76 | F | RA | C1–2 | 2.6 | 0.71 | 8.24 | 7.57 | 2.56 | 0 | PA | OTA | 24 | 26 | 7 | 18 |
| 8 | 29 | F | RA | C1–2 | 1.25 | 2.95 | 5.47 | 2.9 | 2.85 | 2.3 | OTA | PA | 28 | 28 | 7 | 24 |
| 9 | 59 | F | RA | C1–2 | 1.15 | 3.42 | 7.79 | 8.84 | 1.76 | 2.72 | OTA | PA | 26 | 26 | 7 | 18 |
| 10 | 72 | F | RA | C1–2 | 1.62 | 2.92 | 7.25 | 7.73 | 0 | 1.84 | OTA | PA | 24 | 24 | 9 | 12 |
| 11 | 49 | F | RA | C1–2 | 2.15 | 3.44 | 6.72 | 7.95 | 0.72 | 2.45 | OTA | PA | 26 | 26 | 7 | 18 |
| 12 | 61 | F | RA | C1–2 | 3.86 | 1.47 | 7.66 | 7.2 | 2.27 | 1.21 | PA | OTA | 26 | 24 | 7 | 24 |
Of a total of 24 sites in 12 patients, OTA screws were used at 13 sites and conventional PA/ILM screws were used at 11 sites.
The arch height and ILM height of the OTA screw group were significantly lower than those of the PA/ILM screw group (both p < 0.01). The atlas measurements of the OTA screw group were as follows: average (range) height of arch 1.77 (0.71–3.1) mm; average (range) width of arch 7.10 (5.67–8.5) mm; average (range) height of ILM 1.34 (0–3.13) mm. The atlas measurements of the PA/ILM screw group were as follows: average (range) height of arch 3.85 (2.23–5.25) mm; average (range) width of arch 7.90 (2.9–9.75) mm; average (range) height of ILM 3.17 (0.7–5.12) mm (Fig. 3). Table 3 shows that although most of the patients in the OTA group met the criteria for the risky triad of C1, the patients in the PA/ILM group did not. The proportions of patients who met each of the criteria for the risky triad of C1 were all significantly greater in the OTA screw group than in the PA/ILM screw group (arch height < 3.5 mm, p = 0.001; ILM height < 3.5 mm, p = 0.002; blocked ILM, p < 0.001). The intraclass correlation coefficients for intrarater and interrater reliability of grading were sufficiently high (range 0.85–0.92).
Comparisons of atlas measurements between OTA screws (n = 13) and PA/ILM screws (n = 107). The atlas measurements of the OTA screw sites (n = 13) were significantly lower than those of the PA/ILM screw sites (n = 107) in terms of arch height (p < 0.01) and ILM height (p < 0.01). The 25th, 50th, and 75th percentiles are indicated by the boxes, and the range by the whiskers. *p < 0.05. Figure is available in color online only.
Number of screw insertion sites that met the criteria for the risky triad of C1
| Criterion | OTA (n = 13) | PA/ILM (n = 107) | p Value |
|---|---|---|---|
| Arch height <3.5 mm | 13 (100) | 54 (50.5) | 0.001* |
| ILM height <3.5 mm | 13 (100) | 58 (54.2) | 0.002* |
| Blocked ILM | 10 (76.9) | 11 (10.3) | <0.001* |
Values are shown as number (%) unless indicated otherwise. A total of 120 screw insertion sites at C1 in 60 patients were included.
p < 0.05.
No patients had evidence of nonunion on both CT and dynamic lateral radiography at 1-year postoperative follow-up. There were no VA injuries recognized during screw insertion with the OTA and PA/ILM technique or during the immediate follow-up CT scan. In 1 patient, a VA injury was detected during C1–2 facet arthrodesis, and hemostasis was achieved with tamponade. In this patient, no abnormal postoperative CT arteriography findings were observed at the immediate postoperative evaluation or 1-year follow-up, nor was there any evidence of ischemic signs at the VA. Two cases of CSF leakage were observed in only the PA/ILM group, for which 1 patient underwent reoperation due to persistent CSF leakage. In the OTA group, 3 patients had preoperative occipital neuralgia with a mean visual analog scale score for neck pain of 6.3 of 10 (range 6–7). All these patients showed resolution at the 1-year postoperative follow-up (visual analog scale range 1–4). None of the remaining 9 patients without preoperative occipital neuralgia developed ipsilateral postoperative neuralgia after C1 screw fixation using the OTA technique. On the other hand, of the 27 patients without preoperative occipital neuralgia, 5 patients in the PA/ILM screw group developed new neuralgia postoperatively (Table 4).
Frequency of surgical complications
| Complication | OTA Technique (n = 12) | PA/ILM Technique (n = 48) | p Value |
|---|---|---|---|
| Nonunion | 0 | 0 | |
| VA injury during screw insertion | 0 | 0 | |
| CSF leakage | 0 | 2 (4.2) | 0.47 |
| Reop | 0 | 1 (2.1) | 0.61 |
| Arch fracture | 0 | 5 (10.5) | 0.24 |
| Occipital neuralgia* | 0/9 (0) | 5/27 (18.5) | 0.16 |
| Bony erosion | 0 | 0 | |
| Screw malposition | |||
| C0–1 violation | 0 | 2 (4.2) | 0.47 |
| C1–2 violation | 0 | 0 | |
| Medial violation | 1 | 3 (6.3) | 0.80 |
| Lateral violation | 0 | 0 | |
| Screw-related complication | |||
| Screw failure | 0 | 0 | |
| Screw pullout | 0 | 0 | |
| Screw fracture | 0 | 0 |
Values are shown as number (%) unless indicated otherwise.
Patients with new postoperative occipital neuralgia/patients without preoperative occipital neuralgia.
There were no screw-related complications (i.e., screw failure, pullout, and fracture) in the study patients. However, screw malpositioning was observed. Medial wall violation was observed for 1 and 3 screws in the OTA and PA/ILM groups, respectively. Two cases of C0–1 violation were observed in only patients with PA/ILM screws. In our study cohort, no C1–2 violations, lateral wall violations, or evidence of bony erosion in the occiput or atlas were noted on 1-year follow-up CT (Table 4).
Discussion
We have reported the technique and results of C1 screw insertion via the SLM over the arch, which we called the OTA technique, in challenging situations (e.g., patients with the risky triad of C1). We reviewed and compared the contemporary factors underlying the safety and feasibility of the OTA technique and conventional PA/ILM procedures. Despite the small sample size, our study demonstrated favorable results for the OTA technique compared with the PA/ILM technique in terms of clinical outcomes, occipital neuralgia, and arch fracture with screw malpositioning.
C1–2 posterior fixation has been the most popular method for achieving atlantoaxial stabilization.20,21 There are various possible entry points for a C1 lateral mass screw, and the PA has advantages over the ILM in terms of biomechanical strength and avoidance of occipital neuralgia and venous plexus bleeding.4,22,23 However, a PA screw is not recommended for most patients in which the height of the PA is < 4.0 mm.9,24 Although there are reports of the feasibility of using a PA screw even in patients with a small PA height,8,10 its use may lead to an arch fracture very near the VA that would result in a catastrophic situation. Lateral violation of the C1 screw is known to be related to VA injury.11,25
There are often situations in which neither a PA nor ILM screw can be applied. Some patients with long-lasting atlantoaxial subluxation show deformations from severe C1–2 arthritis that have led to a deformed ILM, in which case there is no space under the arch to insert a screw.26 Moreover, patients with RA often have a severely deformed and hypoplastic PA. We referred to the three types of atlas bony deformities that prevent the consideration of conventional PA/ILM screws as the risky triad of C1. This triad, with frequent VA anomaly coursing inferior to the arch, can sometimes require a surgeon to consider OCF as a salvage technique. However, OCF sacrifices C0–1 function, and because C0–1 contributes 15%–20% of the flexion/extension of the neck, OCF may cause deterioration of the normal horizontal gaze.14,15 Transarticular screws may be an alternative option for such challenging situations.27,28 However, insertion of a transarticular screw is technically demanding, particularly in patients in whom atlantoaxial subluxation remains irreducible preoperatively.29 Also, a biomechanical study reported that posterior fixation with a lateral mass screw is superior to transarticular screw fixation in terms of radiological nonunion.30 The surgical decision-making algorithm for various screw techniques is shown in a flowchart (Fig. 4).
Surgical decision-making algorithm for C1–2 fixation technique. The OTA screw can be considered when surgeons encounter patients with the risky triad of C1 (i.e., arch height < 3.5 mm, ILM height < 3.5 mm, and blocked ILM by caudally tilted PA).
The possibility of VA injuries has been a principal cause of avoiding screw insertion through the SLM. However, this artery is in danger during screw insertion with the PA or ILM technique, especially when the VA or its major branch courses inferiorly to the C1 arch in a patient with the risky triad of C1. If the height of the SLM is sufficient for insertion of a 3.5-mm screw, the OTA technique may be a better alternative solution. We always meticulously checked and protected the observed VA during the procedure. There may be concerns about the occurrence of ischemic damage to the VA over time considering that the VA and the screw are crowded between the atlas and occiput. However, in our patients, 1-year follow-up postoperative CT arteriography yielded no evidence of ischemic damage or indentation to the VA, and bony erosions to the occiput and atlas were also absent. This might be at least partially explained by the fact that patients would avoid extensive hyperextension that results in compression of the C1 nerve root and VA. Our results indicated that when conventional screws cannot be considered for C1 fixation, the OTA technique can be considered an alternative option.
Venous plexus bleeding between C1 and C2 is frustrating and troublesome for surgeons performing C1 screw insertion with conventional PA/ILM techniques.31,32 However, the venous distribution is usually meager in the SLM area, and surgeons can focus on protecting the VA when using our OTA technique.
Occipital neuralgia has always been an issue of concern during surgical procedures involving C1–2 posterior fixation.8,33,34 Compared with a conventional ILM/PA screw, an OA screw has lower theoretical risk of injuring the C2 nerve root. It was notable that we did not have to sacrifice the C2 nerve root during OTA screw insertion. Because of the entry point over the PA, the C1 root should be protected during insertion of the OTA screw. Although this was not the case in most patients, we had to retract the C1 nerve root for OTA screw insertion. Although Issing and Lang35 reported that the C1 and C2 dorsal roots have less than 30% connection, the C1 root predominantly carries the motor fibers. This may explain our result that no patients presented postoperatively with new-onset occipital neuralgia.
Limitations
First, our study was limited because only 12 patients with 13 OTA screws were included. However, our study is meaningful because the indication for our technique can be applied in challenging situations in which bony deformation (i.e., the risky triad of C1) combined with VA anomaly hinders conventional screw insertion. Second, the lack of biomechanical studies of the OTA technique in comparison with transarticular or conventional posterior screws (i.e., PA, notching, and ILM screws) is regrettable. Nonetheless, there have been no trials that used a technique similar to ours, and we hope that our study may lead to further studies that investigate the use of our technique. There were no signs of ischemic damage to the VA and bony erosion in the occiput at 1-year follow-up; however, these possible complications should be investigated in a long-term follow-up study. Despite the small sample size and relatively short follow-up duration, our study showed excellent results in terms of the union rate and complications.
Conclusions
To our knowledge, our current study is the first to demonstrate the feasibility of the OTA technique for C1 fixation. We showed that a surgeon can refer to our present results when treating patients with a deformed, small, and caudally tilted PA and ILM height loss (the risky triad of C1). Moreover, we demonstrated that the OTA technique may be beneficial for avoiding unnecessary extension of the surgical level, and thus preventing deterioration of the patient’s normal horizontal gaze. However, a meticulous technique conducted by experienced surgeons is a prerequisite for the successful outcome of our technique.
Acknowledgments
We thank Sun-Joo Kim and Chul-Hee Han at Medical Art Studio for their assistance in the preparation of the excellent illustrations and graphic design.
Disclosures
The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper.
Author Contributions
Conception and design: DH Lee. Acquisition of data: Hwang, Seok. Analysis and interpretation of data: HR Lee, Cho. Drafting the article: HR Lee. Critically revising the article: DH Lee, HR Lee, Hwang, Park. Reviewed submitted version of manuscript: Cho, Park. Statistical analysis: HR Lee. Administrative/technical/material support: Hwang. Study supervision: DH Lee, Seok, CS Lee.
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