Double Anchoring Technique of Occipito-Cervical Fixation Using Innovative Occipital Plate: A Preliminary Study
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0028-3886.360909
Source of Support: None, Conflict of Interest: None
Keywords: AAD, basilar invagination, CV junction, inside-out, occipital plate, occipito-cervical fixation
Occipito-cervical fixation (OCF) is an important surgical tool among other fixation methods for managing cranio-vertebral junction (CVJ) instability. Over the last two decades, advances in instrumentation have changed methods of OCF from older wires, hooks or plate constructs to more rigid and efficient screw-plate and rod constructs., Superiority of currently used screw-plate-rod constructs have been demonstrated by many clinical trials.,, But type of implant at occipital end of fixation in OCF is still a dilemma due to thin bone, irregular contour and poor bone purchase of occiput.,,, Most studies in the past describe the occipital plate with typical outside-inside screw constructs in OCF with good fusion rates though it has 7%–15% implant failure rate., Very few studies described inside-outside occipital fixation technique with similar fusion rates.,,,, Biomechanical strength and rigidity of inside-outside technique has proven to be better than conventional occipital screw constructs.,, In this study, we evaluated the safety and efficacy of newly designed occipital plate (OC) which incorporates both inside-outside and conventional occipital screw techniques in one implant (double anchoring technique), thereby improving biomechanical strength of construct and fusion rates.
It was a prospective observational study conducted in Dr. RMLIMS from January 2019 to January 2021. Thirty-two patients of cranio-vertebral junction anomalies (like AAD, BI, with or without Chiari malformation) requiring OCF were included in the study after their written and informed consent. All patients in this study had congenital CVJ anomalies with difficult anatomical factors like vertical C1-C2 joints, assimilated atlas, lateral mass/condylar hypoplasia or unfavorable vascular anatomy which precluded doing C1-C2 fixations, and hence needed OCF. Study is approved by Institutional ethics committee.
Patients were admitted and evaluated clinically, necessary radiological investigations were done, like MRI CVJ and CT CVJ with vertebral angiography along with other routine investigations. After pre-anesthetic checkup, patients underwent OCF with or without posterior decompression and newly designed OC was used in all of them for fixation, with screw-rod constructs and joint spacers used in a few cases.
Clinical outcome of patients was analyzed using mJOA score on admission and postoperatively at six months., Radiological outcome was evaluated with craniometrics indices like ADI (normal considered to be <3–5 mm) and evident correction of AAD and/or BI on CT CVJ after surgery; statistical analysis was done using paired t test to calculate P value. For evaluating bony fusion X-ray or CT CVJ was performed at three months in follow-up period.
Operative technique: Patients were operated under general anesthesia in prone position, and head supported on a horseshoe headrest. Intraoperative cervical traction using Crutchfield skull tong were done in all cases, which were irreducible on preoperative dynamic imaging. Midline vertical incision was given in suboccipital region and subperiosteal muscle dissection was done to achieve bony exposure from occipital bone to C3-C4 vertebrae as per desired lower level of fixation. Presence of high riding vertebral artery against the C1 lateral mass, vertical orientation of C1- C2 joint with overhanging occiput and hypoplastic C1 lateral mass enforced our decision to put occipital plate, instead of C1 lateral mass screws in all cases included in this study. Careful exploration of C1-C2 joints were done on both sides under microscope, taking account of regional vascular anatomy and joint surfaces were prepared with fine drill bit. We preferred to open and prepare C1-C2 joint for fusion in all cases as this provided the optimum site for bone graft placement. We did not cut C2 root ganglion on either side in cases where OCF was planned. C2 root ganglion division may lead to sensory impairment over the occiput, causing occipital scalp sore and occipital plate exposure. Surgical steps and technique was same as with other typical OCF methods apart from occipital end of fixation in which newly designed OC used. Lower end of fixation included standard C2 pedicle, pars or translaminar screws and in a few cases, C3 or lower cervical lateral mass screws were also included in construct as was thought appropriate. In a few cases with sever BI C1-C2 Goel's titanium joint spacers are used along with OC-screw rod construct for fixation. We preferred to remove the assimilated C1 posterior arch in all cases. Standard foramen magnum decompression with C1 posterior arch removal and duraplasty was performed in four cases associated with Chiari malformations.
Occipital plate fixation: The basic design of this new (titanium alloy) OC consists of two components: first, the central inside-outside screw of 1.5 mm thickness and 12 mm diameter flat head, with its washer and bolt [Figure 1]a, and second, the occipital plate which fits over central screw having three small holes for outside-inside conventional screws [Figure 1]b, [Figure 1]c and lateral most two large holes which holds inbuilt screw heads onto which the rod can be fitted [Figure 1]d. Flat head inside-outside screws were made in 10 mm and 12 mm sizes with appropriate washer sizes for use in pediatric patients [Figure 1]e.
Steps: 1. After adequate suboccipital region bony exposure, marking for central and lateral screws were made with high-speed drill holding OC in desired position, as shown in [Figure 2]a, such that the central inside-outside screw was in midline where thickness of occiput was maximum. This provided better rigidity to construct by virtue of thickness of occiput.
2. A burr hole was made with standard 14-mm perforator at the marked point and a thin trough was drilled at 12 o' clock position of burr hole which was advanced to the point of central screw mark, [Figure 2]b.
3. Central screw was placed in epidural space under occipital bone which was passed from burr hole, through the bone trough and up to the desired location [Figure 2]c and [Figure 2]d. Placement of this central inside-outside screw was done directly under vision (without the need of fluoroscopic guidance) so complications like venous sinus injury, dural tear, CSF leak or cerebellar injury was completely avoided.
4. Afterwards, specially designed OC was placed over central inside-outside screw onto which appropriately sized washer and bolt were to be fixed later on [Figure 2]e. To counteract rotational forces that may compromise stability of construct, this OC also incorporated three conventional outside-inside occipital screws around central screw. Hand drill (2.8–3.5 mm) with guard was used to make appropriate depth holes and measured screws (3.5–5.0 mm thick and 6–10 mm length) were inserted in conventional outside to inside technique. We preferred unicortical purchase of screws in cases with short posterior fossa to avoid injury of venous sinuses with this OC as these conventional screws are only for additional strength of the construct.
5. After tightening conventional screws, central inside-outside screw was fixed with washer and bolt. The washer placement covered conventional occipital screws partially, providing locking mechanism to the screws. With this double anchoring technique, the occipital bone was sandwiched firmly between flat screw head from the inside and washer-bolt from the outside which provided enhanced immediate biomechanical strength and rigidity to the construct. After final tightening, projected extra length of central screw was cut using a rod cutter and removed [Figure 2]f.
6. Lateral most part of this OC harbored two inbuilt fixed screw heads with slot for rods on either side which could be rotated 360 degrees for technical ease and fine adjustments, and could thereby be used in all types of fixation variations (C2 pedicle/pars/translaminar screws or cervical lateral mass screws) [Figure 3].
Lower end screws of construct, that is. -C2 pedicle, pars, translaminar with or without lower cervical C3, C4 etc., lateral mass screws were inserted under fluoroscopic guidance by conventional methods. C1-C2 joint spacers were used in few cases when associated with severe BI to achieve reduction. Appropriately bent rod was placed in slots of screw heads and inni was tightened over it in the desired position of the head (extension) so that reduction of AAD and or BI was achieved.,,
Adequate posterior decompression with FMD and duraplasty was done in four cases associated with Chiari malformation. Autologous bone graft mixed with bone granules were placed around the implant and in C1-C2 joint space for fusion followed by drained closure of incision. If desired reduction of AAD or BI was not attained even after OCF, then anterior transoral decompression was performed in the second stage, which was needed in only two cases.
Patients received postop analgesics and antibiotics for five days and were discharged according to recovery. They were followed up for clinical improvement, complications and fusion over variable period from 9 to 23 months. Patients were advised neck orthoses (hard collar/Philadelphia collar) for 12 weeks postoperatively.
This study included 32 patients (19 men and 13 women) with mean age of 37.8 years. The follow-up period of the study was 9–23 months (mean 18 months). According to frequency of occurrence shown in [Table 1], neck pain, restricted neck movement, and motor symptoms were the most common clinical presentations.
All patients in our study had congenital CVJ anomalies with AAD and most of them, that is 87% (28/32), had irreducible type AAD. Associated BI was present in 65.6% cases (21/32) and Chiari malformation with syringomyelia in 21% cases (7/32). Most patients in our study had difficult bony anatomy like assimilated atlas, vertical C1-C2 joints, C1 lateral mass or condylar hypoplasia, with assimilated atlas being the commonest anomaly in 93% of cases (30/32).
Types of constructs used for OCF are mentioned in [Table 2]. Newly designed OC was used for occipital end of fixation in all cases and lower end fixations included O-C2 construct in most 81% (26/32) cases (i.e., O-C2 pedicles in 16, O-C2 translaminar in 7 and O-C2 pars in 3 cases). Lower cervical (C3-C4) lateral mass screws were also included in construct in five cases and O-C1-C2 construct was used in one case. C1-C2 joint spacers were used in OCF constructs additionally in 5/32 cases to get desired correction of BI. We didn't encounter any incidence of dural or venous sinus injury while putting the inside-out screw in occipital plate.
Symptomatic improvement in form of pain relief, reduced spasticity and/or decreased paresthesia was observed in 96% of patients (31/32). Clinical outcome analysis showed statistically significant improvement in mJOA score of patients with P value of 0.002. Radiological outcome as measured by ADI showed significant reduction of mean ADI from 8.19 mm to 2.24 mm after surgery. Satisfactory correction of AAD (ADI < 3–5 mm) and or BI achieved in 94% cases (30/32) and anterior transoral decompression required in only two cases [Table 3].
Postoperative course of all patients was uneventful with no major neurological or vascular complications. One patient had wound infection that was managed conservatively and one had postoperative CSF leak as duraplasty was done (Chiari malformation) which was managed with lumbar drainage. Overall complication rate in our study was 6.2% (2/32). One patient met with an accident four weeks after surgery, had translaminar screw pullout and re-operated. On follow-up flexion-extension X-ray or CT scan, solid bony fusion was noted in 31 patients with overall fusion rate of 97%. One patient developed cranial settling after eight months and required re-operation. Implant failure or screw pullout of new occipital plate was not observed in any case in our study. Theoretical risk of EDH, CSF leak or cerebellar compression by epidural placed implant was not observed in any case.
Illustrative case: A 60-year-old male patient presented with spastic quadriparesis. CT CVJ showed AAD and BI with assimilation of atlas and unfavorable vertebral artery anatomy. Double anchoring OCF in the form of occiput-C2 pedicle screw-rod fixation with C1-C2 joint spacer placement and posterior bony decompression was performed. Satisfactory correction of AAD and BI was evident on post-operative CT [Figure 4] and patient showed symptomatic improvement during follow-up period with solid fusion across the C1-C2 joint at three months follow-up CT scan [Figure 4]f.
Nearly 4/5th of studies in literature on OCF described conventional outside-inside occipital screw-rod technique which is easy and less time consuming.,, Due to factors like thin bone, irregular surface, unicortical screws, occipital plate fixation in OCF is associated with complications like screw pullout and implant failure in about 5%–15% of cases.,, Few studies mentioned bicortical screws which showed increased complications of CSF leak and dural or cerebellar injuries. Very few studies are available on the inside-outside occipital screw technique (first described by Pait TG), despite having better biomechanical strength and fusion rates, which might be due to the relative complexity of procedure.,,,,
In our study with new OC, both the above techniques were integrated in a single implant to provide superior biomechanical strength to the construct. We compared the pullout strength of the inside-out occipital plate with conventional occipital plate on universal testing machine (UTES-40) using porous ceramic board. When we evaluated the strength of the fixation (pullout strength), the load at peak of the inside-out construct was greater (1.6 KN at 9.00 mm rod displacement) than that of the conventional occipital construct (1.220 KN at 4.20 mm rod displacement). We also found significant differences in the failure load (1.2 KN/mm2 vs 0.37 KN/mm2) and stiffness, showing superiority of inside-out occipital plate over conventional occipital plate [Figure 5]. Stress testing at lateral bending, axial rotation and flexion-extension maneuvers could not be done.
Almost all inside-outside OCF techniques described in the past used bilateral occipital plates which engaged thin lateral occipital squama with poor hold, thereby having more higher of implant failure. As the occipital plate was fixed on both sides it was more time consuming and a complex procedure.,,,, These disadvantages are neutralized in this new OC which holds the central thickest part of the occipital bone for purchase, and being a single midline implant, it is less time consuming. As inside-outside screw is placed in epidural space under direct vision, chances of any venous sinus injury, dural tear, and cerebellar injury are very low and we didn't encounter any such complication in our study.
To augment the fixation and avoid any rotational micro movements of the construct, this implant was held in place with additional three conventional occipital screws. Bicortical purchase of occipital screws as described in many studies was associated with dural tear or CSF leak in a significant number of cases (4%–6%).,, In our technique, these conventional screws were inserted with unicortical purchase and chances of dural tear, CSF leak were not present. Occipital screws in our implant had a locking mechanism which further reduced any chances of screw pullout.
96% of the patients in our study showed symptomatic improvement and ADI showed statistically significant reduction with satisfactory correction AAD and/or BI achieved in 94% (30/32) cases. Clinico-radiological outcome in our study was comparable to other studies in the past.,,,
Meta-analysis of past studies showed fusion rates in OCF of about 86%–96% and overall higher (5%–15%) rate of occipital implant failure.,,, We observed 97% fusion rate in our study without any case of occipital (plate or screw) implant failure over completion of nine months follow-up period for all cases. Only one case of implant failure noted was due to neck injury, that too at the C2 translaminar screw site. Overall, the complication rate of 6% in our study is comparable to other studies in past.
This new OC provides advantages of both inside-outside and conventional outside-inside techniques in a single implant. Unicortical purchase of conventional occipital screws can be done to avoid chances of venous sinus injury, dural tear, or CSF leak, especially in pediatric patients. Locking mechanism of washer over occipital screws further reduces chances of screw pullout and implant failure. Double anchoring technique of occipital fixation using new occipital plate implant is a safe, easy and an effective method for OCF. Since, it is a preliminary study, larger biomechanical comparative studies are needed to prove its superiority over other techniques of occipital fixation.
AAD: atlantoaxial dislocation
BI: basilar invagination
CVJ: craniovertebral junction
CSF: cerebrospinal fluid
CT: computed tomography
MRI: magnetic resonance imaging
OCF: occipito-cervical fixation
OC: occipital plate
FMD: foramen magnum decompression
mJOA: modified Japanese orthopedic association score.
It is a preliminary study, further comparative studies with other techniques needed.
Declaration of patient consent
The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2], [Table 3]